Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session K1: Poster Session II |
Hide Abstracts |
Room: LACC Exhibit Hall 2:00-5:00pm |
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K1.00001: METALS |
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K1.00002: Magnetoelectric susceptibility dispersion of ferrite - piezoelectric composites V.M. Petrov, M.I. Bichurin, O.V. Ryabkov, G. Srinivasan Ferrite-piezoelectric composites are magnetoelectric (ME) due to the interaction between magnetic and electrical subsystems through elastic deformations. In an external magnetic field, the magnetostriction of the ferrite results in an induced polarization due to piezoelectric effect. There have been several theoretical and experimental studies on ME effect in the composites [1,2]. However, the dispersion characteristics of ME parameters have not been considered in detail. Here we discuss the frequency dependence of ME parameters of nickel ferrite and lead zirconate titanate composites. The frequency spectrum shows two distinct regions: the Maxwell-Wagner relaxation region and dispersion region due to electromechanical (EMR) resonance. The study would facilitate: (i) determination of frequency range for maximum ME interactions and (ii) improve the physical parameters of composite that determine the strength of ME interactions. \begin{enumerate} \item \textit{M. I. Bichurin, V. M. Petrov, and G. Srinivasan, Phys. Rev. B }\textbf{\textit{68}}\textit{, 054402 (2003).} \item \textit{V. M. Petrov, M. I. Bichurin, and G. Srinivasan, Tech. Phys. Lett. }\textbf{\textit{30}}\textit{, 341 (2004).} \end{enumerate} - supported by grants from the Russian Ministry of Education (Å02-3.4-278), the Universities of Russia Foundation (UNR 01.01.026) and the National Science Foundation (DMR-0302254) [Preview Abstract] |
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K1.00003: Semimetallic Antiferromagnetism in the Half-Heusler Structure: CuMnSb Tae Seong Jeong, Ruben Weht, Warren Pickett The half-Heusler compound CuMnSb, the first antiferromagnet in the Mn-based class of Heuslers and half-Heuslers that contains several conventional and half metallic ferromagnets, shows a peculiar stability of its magnetic order in high magnetic fields. Density functional based studies reveal an unusual nature of its unstable (and therefore unseen) paramagnetic state, which for one electron less (CuMnSn or NiMnSb, for example) would be an (accidental) zero gap semiconductor between two sets of very flat, topologically separate bands of Mn $3d$ character. The extremely flat and narrow Mn bands result from the environment, which has four tetrahedrally coordinated Cu atoms whose $3d$ states lie well below the Fermi level and four other tetrahedrally coordinated empty sites. The AFM phase can be pictured as a heavily doped Cu$^{1+}$Mn$^{2+}$Sb$^{3-}$ compensated semimetal with heavy mass electrons and light mass holes. The ratio of the linear specific heat coefficient and the calculated Fermi level density of states indicates a large mass enhancement $m^{\ast}/m \sim10.$ [Preview Abstract] |
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K1.00004: Bipolar Effects in Hetero Electrodepositions in ZnSO$_4$ Solution Seth Ross, Nicholas Loudon, G.P. Zhang, Gregory Putman Electrodeposition is a popular method to grow materials. Normally, both electrodes and solutions consist of the same type of element, such as zinc electrodes in zinc sulfate. While depositing zinc ions onto copper electrodes, we have found a drop in the voltage across the cell. This drop is due to two competing reactions: Zn$^{2+}$ + Cu$\to $ Cu$^{2+}$ +Zn and Cu$^{2+}$ +2e$\to $ Cu. Upon further investigation we have found that the time drop is dependent on the voltage applied across the cell. We propose this could be used to build a solution diode. [Preview Abstract] |
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K1.00005: Bimodal 3D islands of InGaN self-assembled on GaN Liu Ying, Xie Maohai Initial stage InGaN alloy growth on GaN(0001) by molecular-beam epitaxy has been followed by in situ reflection of high-energy electron diffraction and scanning tunneling microscopy. It is found that the three-dimensional, Stranski-Krastanov islands evolve from the initial cone-shape to finally the pillar shape with flat-tops as they grow. The small, cone-shaped islands are inferred to be coherent to the underlying GaN, whereas for the pillar-like large islands, they are dislocated. Within a certain range of material coverage, the two types of islands coexist on surface. As the deposition proceeds, they grow with vastly different rates, leading to an overall bimodal island size distribution. [Preview Abstract] |
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K1.00006: Calculation of the latent heat at the phase transitions in magnesium Fernando Maga\~na, Gregorio Ruiz-Chavarria, Gerardo J. Vazquez From a first principles local pseudopotential we have studied the structural magnesium stability under high pressures. We found two possible phase transitions. One going from the structure hcp to bcc and another one from the structure bcc to the fcc. The first transition has been reported experimentally and agrees with our results. Another authors have obtained similar results to ours using more complicated approaches. However, our value of pressure for the second transition is usually larger than the corresponding one from other approaches. Here, we calculated the latent heat associated to these transitions. [Preview Abstract] |
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K1.00007: Thermal Activation Effect on Spin-Transfer Magnetization Reversal Toshiyuki Onogi We investigate an effect of thermal fluctuations on the spin-transfer (spin-current-induced) magnetization reversal mechanism for the tri-layer of ferromagnetic/nonmagnetic/ferromagnetic metals, by using the Langevin dynamic simulation method at finite temperatures (T). By calculating the hysteresis magnetization curves based on the current sweep process, we demonstrate that the critical current for the magnetization reversal decreases according to almost T-linear dependence, suggesting an effective thermal assistance for the magnetization switching in the magnetic recording system. We also show that the switching speed is remarkably enhanced by the thermal activation effect. This work was supported by NAREGI Nanoscience Project, Ministry of Education, Culture, Sports, Science and Technology, Japan [Preview Abstract] |
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K1.00008: The effect of M (M=Ti,Cr,V,Nb) on the transport and elastic properties of nanolayered ternary carbides M2AlC J. Hettinger, P. Finkel, T. Meehan, S. Lofland, K. Harrell, M. Barsoum, A. Ganguly, S. Gupta We report a systematic investigation of the electronic, magneto-transport, thermal and elastic properties of the family of materials M$_{2}$AlC where M is Ti, V, Cr or Nb in the temperature range 4 to 300K. The elastic constants were measured for all compounds ultrasonically. The bulk moduli and anisotropic Young's moduli were found to vary in these compounds depending on the transition metal M. The Debye temperatures were in the 640-710 K range for all materials investigated. The Seebeck coefficients for these four materials were small with differing temperature dependences. All but the Nb containing material have Seebeck coefficients that change sign. The electrical conductivity, Hall coefficient and magnetoresistances are analyzed within a two-band framework assuming a temperature-independent charge carrier concentration. We concluded that there is little correlation between the Seebeck voltage and Hall number. As with other MAX-phase materials, all these materials are nearly compensated. Comparisons between these results will be presented. Results will be discussed in relation to theoretical work and recent measurements on related systems. [Preview Abstract] |
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K1.00009: Optical Transitions in Spherical NanoCrystalline Layer in the Presence of Homogeneous Electrical Field Volodya Harutyunyan The nanospherical layer-type heterophase systems are of great interest primarily because they ``synthesize'' at the same time the properties of both quantized films and quantum dots. In this report the specifics of energy spectrum of charge carriers in the regime of ``strong'' quantization and electrooptical transitions in nanoradial spherical layer are theoretically observed, when the external electric field is present. The ``thinness'' of layer allows to suggest a simple adequate model to describe the states under which the radial and orbital motions of carriers are ``separated'', and the final results can be obtained in analytical form. Taking into account the specific features of energy structure the different cases of perturbation of orbital and radial motions of charge carriers are considered separately. For optical transitions the presence of field leads to change of selection rules by quantum numbers and an explicit dependence of the absorption from the effective mass of carriers appears. In the absorption strip along with the ``main'' resonance peak field satellites appear. The position of these satellites can be regulated by variation of geometrical sizes of sample and the field's intensity. The presence of field leads to an effective change of width of gap of the sample and consequently to short-wave shift of threshold frequency of interband and inter-subband transitions. Numerical calculations for Stark shift in the CdS/HgS/CdS structure are carried out. [Preview Abstract] |
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K1.00010: Ab Initio Hartree-Fock Calculation of Electronic Band Structure in Transition Metal Silicides Alvaro Posada-Amarillas, Jos\'e Samaniego-Reyna, Margarita Franco-Ortiz, Donald Galv\'an \textit{Ab initio} Hartree-Fock method has been used to calculate electronic properties in CoSi$_{2}$ and NiSi$_{2}$. \textit{A posteriori} DFT energy correction has been included in order to test the effect of electronic correlation using B3LYP, VWM and LDA approximations for the correlation term. No correlation and VWM correlation results are compared, finding that the electronic correlation produces band gaps at specific high symmetry points in the first Brillouin zone. Total energy results indicate that correlation energy is high compared to the Hartree-Fock energy for both Co and Ni silicides. This is an indication that CoSi$_{2}$ and NiSi$_{2}$ are highly correlated systems. [Preview Abstract] |
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K1.00011: Bound states of three- body particles (charged exciton) in a system of three perpendicular quantum wires Marcelo del Castillo-Mussot, Jaime Besprosvany, J. Adrian Reyes Advances in the fabrication of low-dimensional structures motivate the investigation of particular configurations. We study a three- body quasiparticle system made of three semiconducting quantum wires (QW) which are all perpendicular among them. There is only one quasiparticle confined in each QW and we are interested in calculating the bound states of the system when two of the quasiparticles are holes (light or heavy) and one is an electron, and when two are electrons and one is a hole. When the tranverse confinement in each QW is harmonic we obtain an analytical expression for the effective Coulomb interaction. We use a variational approach to find the groundstate energy and wave function for different values of the QW thicknesses and quasiparticle effective masses. [Preview Abstract] |
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K1.00012: Effect of Frequency and Electric Modulation on the Resistivity of a 2D Electron Gas Godfrey Gumbs, Paula Fekete We calculate the frequency-dependent quantum magnetotransport (QMT) coefficients of a two-dimensional electron gas (2DEG) with electrostatic modulation. A perpendicular magnetic field is applied and our calculations are carried out using the Kubo method. We obtain analytic expressions for the band part and impurity contributions to the longitudinal and transverse conductivity coefficients. We analyze the effect of the scattering potential and frequency on the QMT coefficients. Preliminary calculations show that at zero frequency, the scattering part of the longitudinal conductivity increases with the lattice scattering at low magnetic fields but decreases when the lattice potential is increased at large magnetic fields. The effect of finite frequency is to enhance the forward scattering significantly on the longitudinal conductivity at low magnetic fields when the scattering potential is weak. Detailed results will be presented for various values of the parameters. [Preview Abstract] |
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K1.00013: Plasma Losses to a Pair of Nanotubes from a Charged Particle Antonios Balassis, Godfrey Gumbs We use the RPA to calculate the inverse dielectric function for a pair of single-walled hollow cylindrical nanotubes of arbitrary radii. The axes of these nanotubes are parallel and their separation exceeds the sum of their radii. We have also derived a general formula for the energy loss of a charged particle moving parallel to the axes of the cylinders, given in terms of the inverse dielectric function and the impact parameter. In our formalism, we separate the contributions to the stopping power from the collective plasmon modes and the single-particle excitaions. Our analysis shows that these contributions depend on the impact parameter and their characteristics are determined by the dispersion relation of the plasma spectrum and the relation between their group velocity and that of the charged particle. [Preview Abstract] |
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K1.00014: Structural Dependence of the Optical Properties of Cr$_{1-x}$Cu$_{x}$ Alloy Films J. B. Kim, Y. J. Yoo, Y. P. Lee, J. Y. Rhee, Y. V. Kudryavtsev Cr$_{1-x}$Cu$_{x}$ ($0 < x < 1$) alloy films have been prepared by face-to-face dc sputtering of the Cr and the Cu targets on a large glass substrate (25 x 120 mm$^{2}$ in dimension), located parallel to a line connecting the Cr and the Cu targets and kept at room temperature. After deposition, the sample was cut into 12 pieces along the short side of substrate, and a set of 12 alloy films with different compositions were obtained. The composition of each alloy sample was determined at its central part by x-ray fluorescence. The structural characterization of Cr$_{1-x}$Cu$_{x}$ alloy films was performed by high-angle x-ray diffraction. The optical properties were investigated by using a spectroscopic ellipsometer at room temperature in a spectral range of 265 - 2500 nm (4.7 - 0.5 eV). It was found that the optical properties were categorized into two groups with different crystalline structures [Preview Abstract] |
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K1.00015: Elasticity as a function of grain size in polycrystalline copper Gunes Kaplan, Gunjan Mishra, T.J. Ulrich, Katherine McCall The elastic behavior of high purity polycrystalline copper samples has been investigated using Resonant Ultrasound Spectroscopy (RUS) to determine the effect of grain size on bulk elastic properties. RUS measurements were performed on ten nominally identical copper samples (1 cm$^3$, 5N purity). Standard recrystallization and heat treatment techniques were used to increase the grain size in each of the samples from $\mu$m to mm sized grains. Results of the RUS analysis can be compared to predictions of the expected elastic behavior as described by the Hall-Petch relationship. The Hall-Petch relationship is generally accepted to be applicable in determining physical properties of polycrystalline metals, however, there is some question as to its validity in the elastic regime. Work supported by the US DOE under Grant No. DE-FC52-01NV14050 at UNR. [Preview Abstract] |
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K1.00016: Identifying phase transitions and kinetics in cerium metal using resonant ultrasound spectroscopy Gunjan Mishra, Gunes Kaplan, T.J. Ulrich, Katherine McCall Resonant Ultrasound Spectroscopy (RUS) is a technique which employs the information revealed by the natural modes of vibration or resonance of solids to determine elastic moduli, ultrasonic attenuation and crystalline symmetry and is sensitive both to micro and macroscopic properties of an object. Cerium in its elemental form has been of great interest to both theoretical and experimental research, especially for its polymorphic tranformation between two fcc phases ($\gamma \rightarrow \alpha$), as well as an intermediate phase ($\beta$) at low pressures. In our present work the RUS technique is used to study the elastic behavior of pure Cerium metal during these phase changes as well as to understand more clearly the phase transformation behavior and transition kinetics within the temperature range of 4 -- 350 K and at low pressures. Work supported by the US DOE under Grant No. DE-FC52-01NV14050 at UNR. [Preview Abstract] |
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K1.00017: \emph{Ab initio} Prediction of Yield-Stress Anomalies in L$1_2$ Ni$_{3}$Ge-Fe$_{3}$Ge Psuedo-Binaries Jianbo Liu, Duane Johnson L1$_{2}$-based (Ni$_{1-c}$Fe$_{c}$)$_{3}$Ge is an ideal system to study yield-strength anomaly and its origin as it is a continuous solid-solution versus \textit{c}, and Ni$_{3}$Ge exhibits an anomaly while Fe$_{3}$Ge does not. We calculated planar-fault energies, i.e. antiphase boundaries (APB) and generalized stacking faults as a function of \textit{c}. We predict to loss of yield-strength anomaly via an energy-based, \emph{necessary} condition using APB energy anisotropy and elastic anisotropy in combination with a \emph{sufficiency} condition that APB(111) is stable against formation of a superlattice intrinsic stacking fault. We predict the transition from anomalous to normal temperature dependence of yield strength for $c\ga0.35$ (or 26 $at.\%$ Fe), as is observed. The APB energies agree quantitatively with experimentally assessed values when antisite disorder (either thermal or off-stoichiometric), which is intrinsic to the characterization data, is taken into account. *Support through the DOE at the Frederick Seitz Materials Research Laboratory (DEFG02-91ER45439), and the NSF at the Materials Computation Center (DMR-0312448) [Preview Abstract] |
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K1.00018: Fracture behaviour of ceramic – metal composites under impact loading. Rynno Lohmus, Irina Hussainova This paper covers research into tribological properties and fracture behaviour of ceramic - metal composites with the hard ceramic phase of titanium, tungsten and chromium carbides studied under the solid particle impact of controlled energy. Surface damage or material removal during particle -- target collision is the result of material response to the contact stresses. Energy dissipation under two bodies collision may be estimated by means of the coefficient of velocity restitution. Approach of impact dynamic and experimental study of dynamic coefficients have been applied to clarify the composite material behaviour under conditions of solid particle erosion. Microstructural factors such as grain boundary chemistry and grain boundary structure can play a significant role in determining the fracture behaviour of multiphase materials. The influence of the boundary parameters on the tribological performance of cermets results from the creation of internal stress state at grain boundaries by the thermal expansion anisotropy between individual carbide grains and carbide -- metallic binder. The internal state of material depends on microstructural characteristics of multiphase materials. [Preview Abstract] |
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K1.00019: Low-temperature Resonant Ultrasound Spectroscopy on Alpha-Pu Sami El-Khatib, H. Nakotte, A. Migliori, H. Ledbetter, J. Betts, S. Harrington We measured the elastic constants of polycrystalline Alpha-plutonium (16 atoms per unit cell) as a function of temperature (18-350K) using Resonant Ultrasound Spectroscopy$^{1}$. A smooth behavior in the temperature dependence of the elastic moduli curves is found, with no evidence for any electronic, magnetic or structural phase transitions. The data can be fit by the quasiharmonic Einstein-oscillator model. The high-temperature bulk modulus yields an anharmonic Gr\"{u}neisen parameter gamma of 5.1, and the low-temperature elastic constants results a Debye temperature $\Theta _{D}$ of about 205K$^{2}$. [1] Albert Migliori and John Sarrao, ``Resonant Ultrasound Spectroscopy,'' John Wiley {\&} Sons, INC 1997 [2] A. Migliori, H. Ledbetter, J. Betts, M. Ramos, S. Harrington, and S. El-Khatib, to be submitted to Physical Review B, 2004 [Preview Abstract] |
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K1.00020: Spectroscopic ellipsometry study of optical anisotropy in Gd5Si2Ge2 and comparison with reflectance difference spectra S.J. Lee , J.M. Park, J.E. Snyder, D.C. Jiles, T.A. Lograsso, D.L. Schlagel, A.O. Pecharsky, D.W. Lynch Recently, Gd$_{5}$Si$_{2}$Ge$_{2}$ has been extensively studied due to its giant magneto-caloric effect, colossal magnetostriction, and giant magnetoresistance in the region of an unusual first-order magnetic-structural phase transformation. In this presentation, we report the complex dielectric functions of single crystals of Gd$_{5}$Si$_{2}$Ge$_{2}$ obtained using spectroscopic ellipsometry (SE) in the photon energy range 1.5 to 5.0 eV. Reflectance difference (RD) spectra for Gd$_{5}$Si$_{2}$Ge$_{2}$ single crystals have been measured by reflectance difference spectroscopy (RDS). Reflectance difference spectra for the $a-b$ and $b-c$ planes of single crystals of Gd$_{5}$Si$_{2}$Ge$_{2}$ were derived from the complex dielectric functions obtained from SE measurements and compared with those obtained from RDS measurements at near normal incidence. The measured spectra agreed well. The in-plane optical anisotropy of the sample is mainly due to intrinsic bulk properties because it has large values (4$\times $10$^{-2})$ compared to surface induced optical anisotropies, with values of only of about 10$^{-3}$ for a typical cubic material. [Preview Abstract] |
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K1.00021: Atomic mechanisms of misfit dislocation nucleation in Pd/Cu and Cu/Pd heterostructures Oleg Trushin, Tapio Ala-Nissila, Enzo Granato, See-Chen Ying Generation of misfit dislocations during growth of heteroepitaxial films is a long standing problem in technology. Effective control of the relaxation processes in such systems requires understanding atomic mechanisms of defect formation. Pd/Cu and Cu/Pd systems (with misfit value about 7{\%}) represent typical model systems to study this problem. We used molecular static simulations with EAM potential and extensive saddle point search methods to estimate energetics and find possible transition paths for strain relaxation processes in these systems. FCC(111) and FCC(100) lattice structures were considered. Different transition paths for strain relaxation , which lead to formation of defects of different types were investigated. Systematic study of the process as function of film thickness was performed. We revealed strong asymmetry between compressive and tensile strain cases, which is due to asymmetry of repulsive and attractive parts of the interatomic potential. [Preview Abstract] |
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K1.00022: CHEMICAL PHYSICS |
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K1.00023: Energy Transfer in Molecular Gas-Surface Interactions Hailemariam Ambaye, J. R. Manson A theoretical study of energy accommodation, momentum accommodation and reduced force coefficients for the diatomic molecular gases O$_2$, N$_2$ and H$_2$ scattering from Au, Al and SiO$_2$ surfaces has been carried out. The theory is a mixed classical-quantum formalism which includes energy transfer between the surface and molecule through translational motion and internal molecular rotational and vibrational excitations. The translational and rotational degrees of freedom are treated classically while the molecular internal vibrational excitation is treated quantum mechanically with a semiclassical formalism. The accommodation coefficients and force constants are calculated as functions of the surface temperature, incident beam angle, incident energy, and mass ratio of the incident molecule to the surface substrate atomic mass. The calculations are calculated with recent experimental data and show good agreement in general. [Preview Abstract] |
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K1.00024: Photochemical Processing of Solid Carbon Dioxide David Cornelison, Thomas Dillingham The investigation of the photochemical processes that can occur in solids like carbon dioxide have important applications in atmospheric physics, astrophysics, and planetary astronomy. In this study, carbon dioxide ices are grown at various temperatures using a closed-cycle helium cryostat. The ices are characterized using XPS and FTIR, and the chemical changes are monitored while the solids are irradiated with x-rays for up to six hours. A quadrupole mass spectrometer was also used to monitor the gas phase species evolving from the ice surface during photoprocessing. The XPS and FTIR results are presented and correlated. It is noted that significant differences are observed for the time dependence of gas phase molecule evolution between low temperature (20 K) and high temperature (77 K) ices. [Preview Abstract] |
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K1.00025: Mechanism of Phosphine Dissociation on the Si(001) Surface Oliver Warschkow, Hugh F. Wilson, Nigel A. Marks, David R. McKenzie, Steven R. Schofield, Neil J. Curson, Michelle Y. Simmons, Phil V. Smith, Marian W. Radny The continued down-scaling of electronic devices to the atomic scale increasingly requires an atomic-level understanding of the elementary processes of semiconductor doping. We present a combined experimental and theoretical investigation into the dissociation mechanism of phosphine (PH3) on the Si(001) surface. As reported by us elsewhere in this conference, a number of prominent intermediate species of PH3 dissociation observed in STM experiments have been structurally characterized as PH2+H, PH+2H and P+3H species respectively. In this poster we present detailed quantum chemical calculations of these and other short-lived intermediates as well as the transition (kinetic) barriers between them. This leads us to formulate a step-by-step mechanism for the complete dissociation of PH3 on the Si(001) surface. [Preview Abstract] |
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K1.00026: Asymptotic Linear Scaling of Adsorption Induced Stress Raj Ganesh Pala, Feng Liu We illustrate there exists an asymptotic linear scaling functional form of adsorption induced stress (AIS) at low coverage, as predicted by continuum elastic theory, using first-principles calculations of CO adsorption on Au(100) and K(100) surface. We observe that when the lateral separations between the adsorbents on a surface is $\sim $8-10 {\AA}, the adsorption induced strain field on the surface no longer overlap. This sets a limit to indirect substrate-mediated inter-adsorbent interaction, which also corresponds to the limit beyond which the AIS scales linearly with coverage. At high coverage, AIS show non-monotonous variations and possible interpretation of these variations is suggested. [Preview Abstract] |
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K1.00027: Mechanisms Leading to Single-Atom Catalysis in Oxide Surfaces Raj Ganesh Pala, Feng Liu Recent experiments suggest that few or even a single metal adatom can be catalytically active on oxide surfaces. We propose certain general atomic-level mechanisms that can provide design criterions for these catalytic systems. The adatoms in the oxide surfaces can catalyze chemical reactions by directing reactants' surface diffusion, by altering their structural mode of adsorption and by activating the reactants in its vicinity to create a highly reactive zone with enhanced reactive collisions rate. An important additional criterion is facile removal of the products by desorption or by surface diffusion away from the catalytically active sites. We investigate a model system, CO oxidation by single Au adatom adsorbed on TiO$_{2}$ surface, using first-principles calculation to evaluate which of the above criterions are satisfied in this system. [Preview Abstract] |
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K1.00028: Full-dimensional, permutationally invariant and size-extensive potential energy surfaces for small molecules B.J. Braams, Joel M. Bowman, Zhong Jin, Xinchuan Huang, Zhen Xie, Peng Zhang, Seung Park, John F. Stanton, Mychel Varner We have constructed full-dimensional potential energy surfaces for a variety of molecular systems, among them CH5+, H3O2-, H4O2, H5O2+, C2H2O, C3H3O, CH2O, and HOONO/HONO2. The property of invariance under permutations of like nuclei is built into the basis for the least-squares fitting procedure, and we relied on the MAGMA computational algebra system to find the invariants and to help generate the codes. The use of a cluster expansion (many-body expansion), going up to five-body or at most six-body terms, caters for dissociation and reaction processes and also for extension to larger systems. The fitted potential and its gradient are evaluated on a millisecond timescale, making it possible to do molecular dynamics or quantum Monte Carlo calculations at ab initio accuracy without anywhere near the cost that is normally associated with ab initio MD, or even with a Car-Parrinello treatment. We also use the fitted surface for MULTIMODE calculations of vibrational spectrum. The poster will present the computational approach and results for the mentioned systems. [Preview Abstract] |
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K1.00029: Relaxation time of a Brownian rotator in a periodic potential with nonparabolic barriers Yuri Kalmykov, Roland Bastardis, Pierre-Michel D\'ejardin The extension of the Kramers theory of the escape rate of a Brownian particle from a potential well to the entire range of damping proposed by Mel'nikov and Meshkov [J. Chem. Phys. \textbf{85}, 1018 (1986)] is applied to the rotational Brownian motion of fixed axis rotators in a Acos 2$\phi $ + Bcos 4$\phi $ potential exhibiting parabolic wells and nonparabolic barriers (A and B are parameters determining the shape of the barriers and $\phi $ is the angle of rotation). By applying an eigenvalue approach, we calculated the Kramers escape rate valid for all values of the dissipation including the very low damping, very high damping, and turnover regimes. Analytical results so obtained agree closely with the exact matrix continued fraction solution. [Preview Abstract] |
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K1.00030: An Analysis of the Site-Site Cluster Diagram Series Kippi Dyer, John Perkyns, B.M. Pettitt The site-site cluster diagram series is examined in terms of the generating series of molecular diagrams. Two results are presented. First, the value of certain of the integrated molecular diagrams are shown to be subject to order of integration, such that, for any finite closure on the site-site distribution functions, some diagrams appear as bridge diagrams within one species labelled pair function while being simple non-bridge diagrams for alternate species labels. This results in inconsistent Kirkwood G factors calculated on different sites for heteronuclear molecules in the proper interaction site integral equation theory. Second, the direct and indirect correlation functions are shown to not be equivalent to the angular average of the molecular direct and indirect correlation functions, respectively. [Preview Abstract] |
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K1.00031: Polarizability anisotropy relaxation in liquid binary mixtures of benzene and hexafluorobenzene Branka Ladanyi, Dolores Elola The relaxation of the many-body polarizability anisotropy in liquid binary mixtures of benzene and hexafluorobenzene was studied by molecular dynamics simulations at room temperature. Strong attractive intermolecular interactions between benzene and hexafluorobenzene molecules lead to the formation of relatively long-lived heterodimers. In this work we investigate the anisotropic nuclear response and orientational dynamics of benzene-hexafluorobenzene mixtures as a function of composition. The contribution to the collective polarizability from intermolecular interactions was included using first-order perturbation theory and calculated considering both molecule-centered and distributed site-polarizabilities. As a result of mixing, both benzene and hexafluorobenzene relaxation rates in mixtures are noticeably slower than those in the corresponding pure liquids, even at low mole fractions. The results are discussed and compared with experimental measurements performed by J. T. Fourkas and co-workers, using optical Kerr effect spectroscopy. [Preview Abstract] |
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K1.00032: Laser Photodetachment Electron Spectroscopy of Zirconium Dimers Srividya Duvvuri, Aaron Covington, Vernon Davis, Erik Emmons, Richard Kraus, Jeffrey Thompson The electron affinity of the zirconium dimer has been measured using laser photodetachment electron spectroscopy (LPES). A photon beam emitted from a 532.2 nm CW laser crossed a 10 KeV beam of Zr$_2^-$ ions at a $90^\circ$ collision angle. The kinetic energies of the photodetached electrons were measured with a $160^\circ$ spherical sector energy analyzer. The electron spectrometer was operated in the constant pass energy mode.Photoelectron spectra were used to determine the electron affinity of the Zr$_2$. Moreover, studies of photoelectron angular distribution measurements were also performed. Analysis of the angular distribution data is in process, and preliminary results will be presented. [Preview Abstract] |
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K1.00033: Computer Simulations of Ice Growth from Pure and Salty Water Marcelo Carignano, Igal Szleifer A six-site water model recently proposed by H. Nada et al (J. Chem. Phys. 118, p7401, 2003) is extensively studied in relation to its ability to reproduce the growing of a crystal phase over the supercooled liquid. We perform Molecular Dynamics simulations at constant pressure and several temperatures below and above the melting temperature of the model, and the resulting kinetics is analyzed in terms of classical nucleation theory. The effect of adding dissociated sodium chloride on the kinetics of crystallization and the structure of water-ice interface is also studied. Preliminary results show that upon crystallization the ions occupy lattice position replacing Oxygen, and in all cases studied so far the ions are found in pairs. [Preview Abstract] |
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K1.00034: Ab Initio Study of Photocatalytic Activity of MIn2O4 (M = Ca, Sr) Masahiko Katagiri, Taizo Sasaki, Hidehiro Onodera, Junwang Tang, Jinhua Ye, Takehiko Matsumoto MIn$_{2}$O$_{4}$ (M = alkali earth metal) semiconductors show the novel photocatalytic activity under visible light irradiation. It has been reported that the activity, K, for Methylene Blue (MB) degradation is increased with decreasing the atomic radius of M, K(CaIn$_{2}$O$_{4}) \quad >$ K(SrIn$_{2}$O$_{4})$. We investigate the electronic state by using the ab initio pseudopotential calculations and reveal the meaning of the classical atomic radius in the activity. CaIn$_{2}$O$_{4}$ and SrIn$_{2}$O$_{4}$ have the similar electronic structure but the band gap of CaIn$_{2}$O$_{4}$ becomes larger than that of SrIn$_{2}$O$_{4}$. It is directly connected to the photocatalytic effect. The difference is attributed to the shape of the ionic core potential. Both compounds have the same number of valence electrons. However Ca with the smaller atomic radius has the sharper and deeper ionic core potential than Sr. As a result, the valence band shifts to the lower energy state, leading to the large band gap. The classical explanation from viewpoint of atomic size is resulting in the argument of the shape of the ionic core potential. It is also found that the atomic distortion is more pronounced in CaIn$_{2}$O$_{4}$. The effect of the atomic distortion is also discussed. [Preview Abstract] |
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K1.00035: A New Field Model of the Evanescent Microwave Needle Probe James Baker-Jarvis, Pavel Kabos, Michael Janezic, Mitch Wallis The evanescent microwave probe is useful for determining the permittivity of thin films down to submicrometer scales. Measuring the permittivity on the micrometer scale is becoming more important as devices shrink in size since the representative permittivity must be known at these scales. Currently, models used with the evanescent microwave probe for the determination of the permittivity from resonant frequency and quality factor (Q) data are primarily based on quasistatic approximations. In this paper we develop a full-mode model for the resonant system with attached needle probe suspended above an airgap over a material under test. We develop expressions for the fields in the different regions and then we obtain a resonant condition and solve the nonlinear equation for the resonant frequency given the permittivity of the sample. We compare the model predictions to experiment. This fundamental model allows us to study the nature of evanescent fields used as probes. [Preview Abstract] |
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K1.00036: Fabrication of single digit electrodes by e-beam lithography for coupled Raman spectroscopy and electrical conductivity measurements of single molecules Neil Smith, Jan Yarrison-Rice, Archena Jaiswal, Shouzhong Zou We have successfully fabricated Cr: Au electrodes on thermally oxidized silicon wafers with repeatable gaps of 35 nm using electron-beam lithography and thermal deposition. Next steps include closing the gap to approximately 3 nm using additional Au deposition with chemical electrolysis and attaching the dimetal complexes across the nanometer gap. The coupling of Raman spectral measurements, which probe structural and orientational changes, with electrical measurements can shed light on the current (I)- voltage (V) characteristics of molecules.[1] Initial measurements of the I-V behavior of self-assembled monolayers of 1,4-Phenylene Diisocyanide in an applied field were made between large electrodes (0.78 micron). The monolayer exhibited a symmetric I-V curve, and the Raman lines shifted and broadened under the influence of the applied field. Once the electrodes are completed, the electrical conduction of single molecules with dimetal bonds will be studied. We gratefully acknowledge the support of NSF through NER-ECS-0403669. \newline References: \newline 1. S. Loridant, M. Bonnat, and E. Siebert, \textit{Spec. Tech}., \textbf{49,} 1193-1196 (1995). [Preview Abstract] |
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K1.00037: Silver Trimer: An Interesting Coinage-Metal System with Jahn-Teller Activity Ilias Sioutis, Russell Pitzer, Terry Miller Silver trimer Ag$_{3}$ is believed to exhibit large amplitude motions away from the D$_{3h}$ configuration attributable to the single e$^{\prime }$ mode that shows linear and quadratic Jahn-Teller (JT) activity. Both laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra have been reported for Ag$_{3}$.$^{1}$ Preliminary JT analyses for the B $^{2}$E$^{\prime \prime }$and X $^{2}$E$^{\prime }$ states have been based, respectively, on the LIF and DF data. However, the interpretation has been subject to some ambiguity. For the X $^{2}$E$^{\prime }$ state, extensive \textit{ab initio} computations were carried out to calculate the linear and quadratic JT constants, the geometries and symmetries of the global minima and saddle points as well as the barrier to pseudorotation around the moat of the potential energy surface (PES). The magnitude of the spin-orbit (SO) effects for this heavy system was also determined. For the B $^{2}$E$^{\prime \prime }$state, the JT constants were obtained. For the theoretical calculations we utilized our newly constructed basis set for Ag. The JT simulations of the LIF and DF spectra were carried out by means of the SOCJT program$^{2}$ which calculates the positions of the spin-vibronic energy levels in the presence of JT and SO effects. The analysis of the JT distorted ground and excited PES's of Ag$_{3}$ and their vibronic structure will be reported. $^{1}$A. M. Ellis, E. S. J. Robles, and T. A. Miller \textit{Chem. Phys. Lett.} \underline {201}, 132, 1993. $^{2}$T. A. Barkholtz, and T. A. Miller \textit{Int. Rev. Phys. Chem. }\underline {17}, 435, 1998. [Preview Abstract] |
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K1.00038: Development of Long, Stiff DNA Tubes as Nanopatterned Substrates for Protein Binding Ashish Kumar, Axel Ekani-Nkodo, Paul Rothemund, Eric Winfree, Deborah Fygenson We describe progress towards developing DNA Nanotubes into a tool for nano-patterning and assaying protein binding. DNA nanotubes are uniquely accessible equilibrium polymers made of motifs known as double- crossovers (DX units). They are typically 10 nm in diameter, up to 100 microns in length and correspondingly stiff (persistence length longer than 5 microns). We have predicted and thereby manipulated the tube-structure to selectively decorate the tubes along the interior or the exterior surface. This ability allows us to use DNA tubes as protein-binding substrates with unusually high density of binding-sites (around 500 within a micron), arrayed along the exterior of a tube in a regular lattice of 14.5 nm x 4 nm. We describe results showing the use of DNA Nanotubes as substrates for proteins such as ligase, restriction enzymes and regulatory proteins. [Preview Abstract] |
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K1.00039: Effect of Amphiphilic Geometry on Phase Separation and Micellization: A Gibbs-ensemble Monte Carlo Study Geuorgui Bourov, Aniket Bhattacharya We are using an off-lattice Gibbs ensemble Monte Carlo method to explore the phase diagram of amphiphiles with different head-tail ratios, as well as for different sizes of the hydrophilic head. While phase diagram for lattice amphiphiles$^{\dag }$has been studied, there is hardly any simulations where geometric effects on the phase diagram has been investigated in a systematical way. We will compare our simulation results with those obtained using lattice models and with the available experimental data. Our studies are relevant to obtain controlled nanostructures using amphiphiles as templates. $^{\dag }$A. Z. Panagiotopoulos, M. A. Floriano, S. K. Kumar, Langmuir 2002, 18, 2940.$^{ }$ [Preview Abstract] |
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K1.00040: Cubic Phase Formation in Phospholipid and PEG-Lipid Mixtures Kimberly Murley, Beth Cunningham, David Wolfe, Patrick Williams Lipid systems modeling cell membranes are capable of self-assembling into various liquid crystal mesophases with varying geometry and dimensions. We have suggested that it is possible to engineer the lipid systems through the incorporation of covalently attached polymer lipids to produce unique effects. The results of this engineering process include both the stabilization of lipid phases that normally exist over very limited temperature ranges and the induction of novel phases that are not normally present in the parent lipid. In this study, we used x-ray diffraction and NMR to investigate the phase behavior of the DOPE:PEG:MO and MO:PEG:D$_{2}$O systems with varying molar ratios and PEG sizes. The phase diagram which we have generated indicates the conditions necessary to induce specific phase structures and sizes into three-dimensional cubic lipid systems. This information may be useful to create nanostructures which will be valuable in applications such as protein crystallization and protein biochip development. [Preview Abstract] |
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K1.00041: Self-Assembling of Polymer-Enzyme Conjugates at Oil/water Interfaces Bi-min Zhang Newby, Lifang Wang, Ping Wang Interface-binding enzymes are desirable for biphasic reactions in that they offer simultaneous access to substrates dissolved in both phases across the interface. It has been shown that conjugating water-soluble enzymes with hydrophobic polymers facilitated the assembling of enzymes at oil/water interfaces. In this work, the interfacial assembling of alpha-chymotrypsin conjugated with polystyrene, poly(methyl methacrylate) and poly(L-latic acid) was examined using the pendant drop method. The interface-assembling process of the conjugates from the organic phase followed a similar pattern of that of native alpha-chymotrypsin from the aqueous buffer phase, i.e., the interfacial tension decreased gradually with time. However, when the conjugates were dispersed in the form of particulates in the aqueous phase, in which the conjugate was insoluble, the assembling occurred faster and the interfacial tension approached zero quickly. It was suspected that the assembling in this case involved two steps, i.e. the adsorption of the particulates and the subsequent rearrangement, dissociation and re-dispersion of the conjugates at the interface. The effect of other factors including the polarity of organic solvent, pH value and ionic strength of the aqueous phase was evaluated. It was found that the polar solvent slightly facilitated the assembling, whereas pH and ionic strength showed minimal effects. [Preview Abstract] |
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K1.00042: A Theoretical Investigation of the Charge Transfer System TCNQ-F4 and Alpha-Sexithiophene Kai-Felix Braun, Saw-Wai Hla The electronic and geometrical structures of the charge-transfer system of alpha-sexihiophene and tetrafluorotetracyanoquinodimethane are calculated self-consistently from first principles. By means of density functional theory (DFT) methods several configurations of the free molecules are calculated within LDA and B3LYP employing a plane wave basis and different atomic orbital sets. The combined system exhibits preferential binding of the center of the TCNQ-F4 on top of a c-c bond of the sexithiophene, thereby the central configuration having the lowest energy. As opposed to the periodic arrangement in a crystal of the related system dimethylquaterthiophene and TCNQ-F4, the free system exhibits a strong interaction going along with a substantial polarization of both molecules. For comparison with scanning tunneling spectroscopy results, the molecules were adsorbed in a parallel geometry on a Au(111) slab. To take into account the voltage applied to the STM tip the system was finally calculated within an electric field. This work is financially supported by the US-DOE grant no. DE-FG02-02ER46012. [Preview Abstract] |
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K1.00043: Global optimization of free and supported metallic nanoclusters Giulia Rossi, Francesca Baletto, Christine Mottet, Arnaldo Rapallo, Riccardo Ferrando The global optimization of free and supported metallic nanoclusters is performed by two different approaches: a genetic algorithm and a basin-hopping algorithm. Optimization has been applied to a series of different systems, such as free pure clusters (Pd, Ag, Cu, Ni), free binary clusters (AgPd, AgCu, AgNi) and supported Pd clusters on MgO(100) substrates. The metals are described by a many-body potential derived within a tight-binding scheme in the second moment approximation, while the interaction between the metallic atoms and the oxide is modeled by an analytical potential fitted to \textit{ab initio} calculations. As regards free hetero-metallic clusters, the optimization has led to the location of a family of stable core-shell polyicosahedral structures. According to this geometrical configuration, clusters structures are made of several interpenetrating icosahedra, and the atoms with smaller radius (Cu, Ni, Pd) occupy volume sites while Ag atoms are placed on the cluster surface. Such a geometrical configuration agrees with the bond order-bond length correlation, that is typical of metallic interactions. Concerning supported Pd clusters, their structure modifications resulting from the interaction with the substrate are investigated as a function of clusters size. [Preview Abstract] |
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K1.00044: Optically active second-harmonic generation from a uniaxial fluid medium S. H. Han, M. A. Belkin, Y. R. Shen We have shown that optically active second-harmonic generation is allowed in a uniaxial fluid medium. A homeotropically aligned chiral smectic-A liquid crystal was used as an example. Phase matching of chiral SHG in the medium was achievable by angle tuning and the chiral nonlinear susceptibility was deduced from the results. The SHG output vanished when the LC moved into the isotropic phase as expected. This work provides a method for us to measure chiral nonlinearity of chiral LC molecules, or more generally, chiral molecules that can be uniaxially aligned in a fluid medium. [Preview Abstract] |
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K1.00045: How does thermal motion of atoms influence the rates of bridge-mediated electron transfer reactions? 2. Examination for Frank-Condon approximation. Tustomu Kawatsu, Spyros Skourtis, Ilya Balabin, David Beratan Bridge-mediated long-range electron transfer (ET) is ubiquitous in biological systems. The ET reaction rate is generally calculated using Born-Oppenheimer and Franck-Condon approximations. We focus on the validity of the Frank-Condon approximation and estimated how well it describes protein ET for Ruthenium-modified azurins. We compare the auto-correlation function decay time for the electronic coupling with the decay time of the Franck-Condon factor (computed by D. M. Lockwood, Y-K. Cheng and P. J. Rossky; Chem. Phys. Lett. 345, (2001) 159). We used molecular dynamics and tight-binding quantum chemical methods to compute the evolution of the electronic coupling. We explore the auto-correlation functions for the protein mediated electronic coupling and its decay time. We confirmed that the Franck-Condon approximation works well in ET for these Ruthenium-modified azurins. [Preview Abstract] |
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K1.00046: Quantum Delocalization and STM Image Simulation of Hydrogen Atoms Adsorbed on Pd(111) Surface Sungho Kim, Seong-Gon Kim, Steven C. Erwin Dissociative hydrogen adsorption process on noble metal is one of the most crucial catalytic processes in the fuel cell technology. Quantum delocalization model is proposed to understand the image of scanning tunneling microscopy (STM) observed during hydrogen adsorption on palladium. We use numerical simulation techniques based on the first principles density functional theory (DFT) to support our quantum delocalization arguments. As observed in a recent experiment [Nature 422, 705 (2003)] two vacances of hydrogen on palladium surface appears three loped triangle in experimental STM image and dissociative hydrogen adsorption on palladium requires aggregates of three or more vacancies. Our STM simulation and total energe calculation based on DFT using quantum delocalization model explain consistently the above experimental results. [Preview Abstract] |
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K1.00047: Femtosecond Electron Diffraction: Direct and Real-Time Probing of Coherent and Thermal Atomic Motions Xuan Wang, Zhao Hao, Hyuk Park, Shouhua Nie, Rick Clinite, Jim Cao Recent developments in time-resolved diffraction have led to the capability of directly observing the laser-induced loss of long-range order on the ps and sub-ps time scale. However, a clear picture of atomic motions during the phase transition remains obscure. Here we report a direct and real-time measurement of both coherent and thermal atomic motions in thin- film aluminum using femtosecond electron diffraction. It showed a coherent lattice vibration with a period of 8 picoseconds starting immediately after the optical excitation with a concurrent heating of the lattice, reaching its final equilibrium temperature 5 picoseconds later. These observations provide a clear atomic-level view of laser-induced lattice dynamics. [Preview Abstract] |
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K1.00048: SURFACE, INTERFACES & THIN FILMS |
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K1.00049: Atomic Force Microscopy and Quartz Crystal Microbalance Study of Surface Roughness of Polymer Films Prepared from Solutions Nanxia Rao, Yingzi Hao, Da-Ming Zhu In order to gain detailed insight on how a polymer film's surface morphology depends on the temperature as well as the thickness, we used quartz crystal microbalance to monitor the thickness of polymer films prepared from solution with different concentrations, and then use atomic force microscope to investigate the roughness of the surface as a function of film thickness and temperature. Several systems have been studied so far, which include polystyrene poly (vinyl chloride). The results will be discussed in terms of dewetting of the polymer films. [Preview Abstract] |
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K1.00050: Selective growth of nano-scaled islands A.L. Chin, M.H. Lee, H.C. Wang, F.K. Men To grow nano-scaled structures in designated regions on surfaces is essential to the success of nanotechnology. Here we demonstrate a scheme offering the opportunity of selectively growing nano-scaled islands in selected regions of a surface. By depositing sub-monolayer of Au onto a nominally flat Si(111) surface, we constructed a surface consisting of two structurally different domains: the (5×2)/Au and the (7×7) domain. We then deposited up to 0.3 monolayers of Co at room temperature followed by annealing at an elevated temperature. The resultant surface structure shows, only on (5×2)/Au domains, the growth of islands with narrow size distribution. [Preview Abstract] |
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K1.00051: Growth of Yttria-Stabilized Zirconia (YSZ) Thin Films Deposited under Various Substrate Temperatures by Pulse Laser Ablation EnGang Fu, Ju Gao Yttria-stabilized zirconia thin films were deposited on Si (001) by pulse laser ablation with YSZ target. The crystallinity and structural features of thin films were charactered by using X-ray diffraction (XRD) and rocking curve measurements, and the thickness of thin films was determined by a Dektak3ST surface step profiler. The results showed the substrate temperature is one of the most important factors during deposition process and the thin films grown under different substrate temperatures behaved diverse properties. Thin films with very good crystalline deposited at the substrate temperature of 800${^\circ}$ and oxgen gas pressure of 5$\times $10$^{-4}$ mbar were obtained. [Preview Abstract] |
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K1.00052: New Candidate for FRAM Dielectric Layer—Rare Earth Europium Doped PZT Thin Films Y.J. Yu, Y.M. Li, H.L.W. Chan Europium (Eu) doped lead zirconium titanate (PZT) ferroelectric thin films (PEZT) were grown on platinized Si substrates by a sol-gel technique with a rapid thermal process. Based on the analysis of TEM and AFM, an obvious modification of PZT films by Eu doping was observed. Compared with undoped PZT, PEZT films show 5 times larger size of grains (100 nm) but only 15{\%} increase in roughness mean square (about 1.08 nm). That is, PEZT films with high quality (large grain size and good uniformity) were fabricated under the same processing condition as pure PZT, in no charge of long-time and high-temperature. The high quality PEZT dielectric layer could greatly improve the performance, mainly the reliability and the reproducibility of FRAM units. Furthermore, increased remenant polarization and improved polarization fatigue properties were found by optimizing Eu doping content. Mechanism of Eu doping effects on the microstructure and electrical properties of PZT films was discussed from the physics of crystal growth and the defect chemistry points of view. [Preview Abstract] |
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K1.00053: Low-Energy Electron Diffraction Study of Potassium Adsorbed on Graphite Sharon Finberg, Mellita Caragiu, Nicola Ferralis, R. D. Diehl The (2x2) structure of potassium dosed on both single-crystal graphite (SCG) and highly-oriented pyrolitic graphite (HOPG) has been investigated by LEED for surface temperatures between 50 and 150K. Previous studies [1,2] observed intercalation of one or more potassium layers depending on the temperature of the substrate, and a recent photoemission study [3] suggested that a (2x2) underlayer forms before an overlayer forms. We found no evidence for an intercalated potassium layer for the same experimental conditions. A dynamical LEED study of the (2x2) structure indicates that the K atoms adsorb in hollow sites on top of the surface with a K-graphite average perpendicular spacing of 2.79$\pm $0.03{\AA}, and a graphite interlayer spacing consistent to the 3.35{\AA} bulk spacing. [1] P. Sjovall, Surf. Sci. 345, L39 (1996). [2] J. C. Barnard, K. M. Hock and R. E. Palmer, Surf. Sci. 287/288, 178 (1993). [3] M. Breitholtz, T. Kihlgren, S.-{\AA}. Lindgren, L. Walld\'{e}n, Phys. Rev. B 66, 153401 (2002). [Preview Abstract] |
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K1.00054: Raman Spectroscopic Mapping of Strain Distribution in Si/SiGe Heterostructures Y. Bao, K.A. Alim, M. Shamsa, W.L. Liu, A.A. Balandin Strain in Si/SiGe heterostructures and GeSi/Si quantum dot superlattices may strongly affect the carrier mobility [1]. In this talk we report mapping of the inhomogeneous strain field distribution in 10 nm strained Si layer using cross-sectional Raman spectroscopy. The tensile-strained Si layers were grown on a Si$_{0.8}$Ge$_{0.2}$ buffer layer by the solid-source molecular beam epitaxy in UCLA. The post-growth sample preparation allowed us to carry out Raman scans across the layers of the structure. The stress and the lattice mismatch values have been extracted from the measurements of the Raman peak shifts. The obtained experimental values of $\varepsilon _{\bot }$ and $\varepsilon _{\vert \vert }$ are about -4.24 $\times $ 10$^{-3}$ and 3.27 $\times $ 10$^{-3}$, while the stresses values are $\sigma _{\bot }$ = -0.91 GPa and $\sigma _{\vert \vert }$ = 1.18 GPa. [1] Y. Bao, A.A. Balandin, et al., Appl. Phys. Lett., 84, 3355 (2004). [Preview Abstract] |
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K1.00055: Direct evidence on the preferable growth direction of the gold stripe on reconstructed Au(111) surface Soohyon Phark, Zheong G. Khim, Seokwon Yoon We have investigated the growth of nanometer-scale gold stripes on reconstructed Au(111) surface using scanning tunneling microscopy (STM). The stripes are caused by the moving tip of the STM operated in ultrahigh vacuum at room temperature. Different from what has been reported previously, we find, by directly comparing the direction of the stripes and the orientation of the underlying lattice, that the gold stripes grow preferentially along [1,-1,0] direction and its threefold symmetric directions at (111) surface of fcc structure. We also find that the scanning direction of the STM tip does not affect the direction of the stripe growth although the growth rate is suppressed remarkably when the scanning direction is close to [1,1,-2] direction of Au(111) surface. [Preview Abstract] |
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K1.00056: Growth of Flat Au(111) Surfaces on Mica for Ellipsometric, AFM and X-ray Studies of Organic Films P. Soza, V. del Campo, E. Cisternas, M. Pino, U.G. Volkmann, H. Taub, F.Y. Hansen To produce large, atomically flat gold substrates for organic film studies, we have used the method reported by Hegner et al.$^{2}$ in which gold films grown on mica are glued onto Si(100) wafers. Atomic Force Microscopy, Energy Dispersive X-ray Spectroscopy, and x-ray diffraction measurements give evidence of the good quality of our gold surfaces. As a first check, docosane (n-C$_{22}$H$_{46})$ films were deposited from a heptane (n-C$_{7}$H$_{16})$ solution onto the gold surface. We conducted ellipsometric and stray light intensity measurements on these films in air as a function of temperature in order to determine their optical thickness and surface roughness. From our results, we have identified the bulk melting and the film wetting transitions. The wetting transition takes place about 2 K lower than in docosane films of the same thickness adsorbed on SiO$_{2}$ substrates. Further study of these flat gold surfaces is necessary to assure their suitability for alkane film studies by synchrotron x-ray scattering. $^{2 }$M. Hegner et al., Surf. Sci. 291, 39 (1993). [Preview Abstract] |
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K1.00057: Adsorption Behavior of Selected Aromatic Molecules on Cu(111) Xing Lin, Ki-Young Kwon, Greg Pawin, Erick Ulin-Avila, Ludwig Bartels The interaction of thiophenol (TP) and its halogen substituted derivatives (X-TP) with a Cu(111) surface has been studied with a combination of temperature programmed desorption (TPD) and scanning tunneling microscopy (STM). TPD data shows desorption of the intact molecules from the multilayers at temperatures that increase with the size of the halogen substituent from Fto Br. For the monolayers the carbon-sulfur bond breaks, the sulfur atom remains bound to the surface and the remainder of the molecule desorbs as. Exclusively in the case of Br-TP we observe Ullmann coupling and biphenyl desorption. In the sub-monolayer regime, STM studies reveal all molecules adsorb with the benzene ring near parallel to the surface. At 80K substituted molecules are mobile and self-assemble into ordered islands. Upon desorption of the molecules by thermally breaking the C-S bond, the remaining sulfur atoms form characteristic reconstructions of the Cu(111) substrate. Moreover, we studied the adsorption structure of 2,5-diphenyl-1,3,4-oxadiazole on Cu(111). Prior to adsorption, the rings of this molecule are coplanar. Our STM images indicate that the molecule retains this symmetry and adsorbs with the ring vertical to the substrate in marked contrast to the thiol-bound species. [Preview Abstract] |
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K1.00058: Studies of Dynamical Layering in Adsorbed Organic Films A. Diama, M. Simpson, H. Taub, F.Y. Hansen, R. Dimeo, D. Neumann, K.W. Herwig, U.G. Volkmann It is well known from experiments using a surface force apparatus that organic fluids confined between two surfaces exhibit a layered structure at the molecular level. This static layering has motivated us to consider the possibility that the individual molecular layers in fluid films may also have different dynamical properties. We have found evidence of such dynamic layering effects in computer simulations of fluid heptane ($n$-C$_{7}$H$_{16})$ films and have therefore begun investigating the diffusive motion in films of longer alkanes of relevance to lubrication. Here we report high-resolution quasielastic neutron scattering (QNS) measurements on fluid monolayer, bilayer, and trilayer tetracosane ($n$-C$_{24}$H$_{50}$ or C24) films adsorbed on exfoliated graphite (Grafoil). We believe this system is favorable since the large aspect ratio of the C24 molecule may inhibit interlayer exchanges compared to a more spherical molecule. We discuss evidence of progressively faster diffusive motion in the lowest, middle, and top layers of the trilayer film. Molecular dynamics simulations are in progress in an effort to corroborate this interpretation. [Preview Abstract] |
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K1.00059: Phonon Thermal Conductivity of the Quantum Dot Superlattices Manu Shamsa, Weili Liu, Yun Bao, Alexander Balandin Quantum dot superlattices (QDS) have been proposed for thermoelectric and other device applications [1]. In this paper we present results of our experimental and theoretical investigation of the thermal conductivity in the doped and undoped Ge/Si quantum dots superlattices [2]. We have observed an order of magnitude decrease in the room-temperature thermal conductivity compared to bulk, as well as significant shift of the thermal conductivity peak to the higher temperature values. The thermal conductivity dependence on temperature has been approximated as K$\sim$ T$^{0.7}$ -- T$^{0.9}$ in the low-temperature region. We have also carried out modeling of thermal conduction in QDS in order to elucidate the effect of thermal boundary resistance at the interfaces between Si and Ge layers. [1]. A.A. Balandin and O.L. Lazarenkova, Appl. Phys. Lett.,\textbf{ 82}, 415 (2003). [2]. W.L. Liu and A.A. Balandin, Appl. Phys. Lett, 85 (2004). This work has been supported by the NSF CAREER and NSF SGER awards to A.A.B. [Preview Abstract] |
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K1.00060: Magnetic, Optical and Magneto-optical Properties of Ni$_{2}$MnGe Alloy Films R. J. Kim, N. N. Lee, Y. P. Lee, Y. V. Kudryavtsev, K. W. Kim The influence of atomic ordering on the magnetic, the optical and the magneto-optical (MO) properties of Ni$_{2}$MnGe Heusler alloy (HA) films was investigated. The bulk Ni$_{2}$MnGe HA was prepared by arc melting, and the films were deposited by flash evaporation onto glass substrates at several substrate temperatures from 150 to 730 K. The bulk Ni$_{2}$MnGe HA exhibits the cubic $L$2$_{1}$ structure with $a$ = $b$ = $c$ = 0.5761 nm, and the annealed (at 573 K) bulk alloy is in the tetragonal structure with $a$ = $b$ = 0.5720 nm and $c$ = 0.5865 nm. While the films deposited at 720 K show a well-ordered $L$2$_{1}$ structure, the deposition at 150 K $< T <$ 710 K results in the formation of a nanocrystalline or an amorphous microstructure. It was found the structural disorder in Ni$_{2} $MnGe films induces lack of the ferromagnetic order and noticeable changes in the optical and MO response. [Preview Abstract] |
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K1.00061: Gibbs Adsorption in Binary Liquid Mixtures: Concentration Variations Beyond Surface Monolayer in BiSn alloy Reinhard Streitel, Oleg Shpyrko, Alexei Grigoriev, Peter S. Pershan, Ben Ocko, Moshe Deutsch We report the observation of a surface-induced structure of $Bi_{43}Sn_{57}$ eutectic alloy measured at $147^{\circ}C$ (eutectic melting point $141^{\circ}C$). X-ray reflectivity indicates the formation of surface layering structure extending to the top 3 surface layers. Application of resonant x-ray reflectivity measurements reveals enhancement of the lower-surface tension component in surface monolayer (92 at.$\%$ of Bi/8 at.$\%$ of Sn), in full agreement with Gibbs adsorption rule. However, we also observe a Bi depletion (24 at.$\%$ of Bi/76 at.$\%$ of Sn) in the second surface layer followed by a slight Bi enhancement (54 at.$\%$ of Bi/46 at.$\%$ of Sn) in the third surface layer. These results demonstrate the interplay of minimization of surface energy by reducing surface tension and the formation of an eutectic alloy at the surface. The propagation of a surface adsorption effect well beyond the surface monolayer will be discussed. [Preview Abstract] |
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K1.00062: Probing of wavefunctions in 2D-electronic waveguides: an exact approach Arkady Satanin, Ronald Cosby, Yong Joe Scanning probe experiments give very important information about coherent electron current flow in 2D nanostructure electronic waveguides (M.A.Topinka et al.). Recently an approach based on the recursive Green's function method has been effectively used to simulate electronic patterns. We present here analytical calculations of conductance variations in 2D-waveguides, modeling the probe experiments with a short range potential. It was shown that the wavefunction variation in a waveguide may be expressed exactly through the Green's function of an unperturbed system if the characteristic size of the probing potential is less than the Fermi wavelength. The variation of the conductance of different geometrically shaped perfect waveguides and waveguides with quantum dots (QD's) has been investigated. We have found that in general the variation of the conductance depends on the phase interference of different waveguide modes. We have investigated the changes on the plateau's threshold when the probe is placed in a perfect waveguide. In the waveguide with an attractive QD we have found that near a resonance in conductance variation, the dominant terms are proportional to a resonant wavefunction. In addition, we have studied the effects of wave interference and the probing of the wavefunction when there is a Fano resonance in the conductance. [Preview Abstract] |
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K1.00063: Physical Adsorption of Nitrogen and Xe on Ag (111) Surface: First-principles Density Functional Theory Comparative Investigation Shizhong Yang, James Phillips Nitrogen adsorption on Ag (111) surface was studied using first-principles density-functional theory method. The geometry structure was fully relaxed and for nitrogen we found that it has two preferred adsorption sites that corresponding to the incommensurate and herring bone structures respectively. The adsorption energy, diffusion barrier, electronic properties were calculated and compared to the Xe/Ag (111) results. [Preview Abstract] |
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K1.00064: STATISTICAL AND NONLINEAR PHYSICS |
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K1.00065: A quasi-dissipative and discontinuous bifurcation in an electronic circuit Yu-mei Jiang, Yue He, Jun Dai, Da-Ren He An electronic relaxation oscillator with over-voltage protection performs a transition simultaneously from conservative to quasi-dissipative and from continuous to discontinuous by varying a characteristic parameter. The bifurcation can produce a sudden transition of a conservative stochastic web to a transient one so that the iterations on the web escape to some conservative elliptic islands. We numerically show that a fat fractal forbidden network suddenly appears inside the original web at the bifurcation threshold. The remnant forms the transient web. The fractal exponent of the forbidden web displays an exponential dependence on the control parameter. This should be the principal rule describing the bifurcation. In addition, the dependence of the averaged lifetime of the iterations inside the transient stochastic web on the control parameter still follows the universal power law. We analytically and numerically deduced the scaling exponent. [Preview Abstract] |
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K1.00066: Possible misunderstandings with different Poincare sections in a kicked rotor Jun Dai, Da-Ren He In previous studies on kicked rotors, Poincare sections often were chosen by collecting the angular momentum and angular position values of the moving particle just before the kicks. By an example system, where the kicking periods are different in the upper or lower semicircle, we show that such a traditional Poincare section may lead to incorrect dynamical behavior due to the fact that one put phase points in different times onto same phase plane. In such kind of systems the suitable Poincare section may be constructed by the angular momentum (or angular position) and time step values of the moving particle just before the kicks. Such a Poincare section should absolutely avoid the possibility for putting phase points in different times onto same phase plane. We show numerically that such a Poincare section can avoid all the abovementioned mistakes. [Preview Abstract] |
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K1.00067: Quantum Monte Carlo-simulation for the conductance of one-dimensional quantum-spin systems Kim Louis, Claudius Gros We study the conductance in spin systems with a Monte Carlo approach using the stochastic series expansion (SSE). This allows the evaluation of the conductance at low temperatures in quasi one-dimensional systems. This method is applied to anisotropic Heisenberg chains, Kane-Fisher scaling for one impurity in a Luttinger-liquid, and the spin drag problem in the Hubbard model. [Preview Abstract] |
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K1.00068: Level Dynamics on a Negatively Curved Surface Sally Koutsoliotas, David Farmer We study the energy levels of a free particle on a non-compact, negatively curved surface. Changes in the energy levels are studied as the surface is deformed, and comparisons are made to predictions from Random Matrix Theory. Special behavior is observed near points where the surface takes on extra symmetry. [Preview Abstract] |
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K1.00069: Precession of nearly circular orbits in the Newtonian 2-body problem perturbed by constant positive curvature Daniel E. Shai, John F. Lindner We generalize the reduced Newtonian 2-body problem by embedding it in a space of constant positive curvature. Using perturbation theory, we demonstrate analytically that a nearly circular orbit will precess with a frequency proportional to the square root of its size. Using computer simulation, we confirm this behavior numerically by directly integrating the equations of motion. This work is part of a larger study of the nonlinear dynamics of the 2-body problem in a spherical universe and was supported by NSF Grant No. DMR-0243811 and The College of Wooster. [Preview Abstract] |
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K1.00070: Classical dynamics of the 2D hydrogen atom driven by elliptically polarized microwave field Elena Shchekinova, Turgay Uzer The behavior of the highly excited hydrogen atoms driven by the electromagnetic fields is observed to be very sensitive to the parameters of the field such as amplitude and polarization. We study classical dynamics of hydrogen atoms with quantum numbers $n_0$ in the range $31-45$ subject to $9.904$ GHz electric field. As the polarization of the field is varied from circular to linear limits the ionization yield curves show a very different behavior. This sensitive dependence can be explained by analyzing various phase space transitions that occur for different parameters of the driving field. For the elliptically polarized microwave field the Hamiltonian is time dependent and the phase space is three dimensional. There are no integrals of motion. Therefore Poincare surface of section can not be of any use. The novel technique based on the short time Lyapunov exponents computation renders us geometrical insight into the phase space structure of this high dimensional system. We show that the method gives an accurate prediction of the stability properties and ionization of trajectory for each initial condition in the phase space after an interval of time much shorter then the time necessary for the corresponding trajectory to ionize. [Preview Abstract] |
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K1.00071: Isotopic Randomness and Maxwell's Demon Alexander A. Berezin Isotopic disorder in crystals can lead to suppression of thermal conductivity, mobility variations and (weak) Anderson localization on isotopic fluctuations. The latter (AAB, J.ChemPhys.1984) is akin to polaron effect (self-localization due polarization). Possibility of isotopic patterning (IP) increases near melting point (thermally activated isotopic hopping swaps). Crystal near melting threshold become “informationally sensitive” as if its IP is operated by some external Maxwell’s Demon, MD (AAB, URAM J, 2002). At this state short range (e.g. electrostatic inverse square) forces evolve into long-range interactions (due to divergence of order parameter) and information sensitivity can be further amplified by (say) a single fast electron (e.g. beta-particle from decay of 14-C or other radioactive isotope) which may result in cascade of impact ionization events and (short time-scale) enhancement of screening by impact-generated non-equilibrium (non-thermal) electrons. In this state informationally driven (MD-controlled) IP (Eccles effect) can result in decrease of positional entropy signifying emergence of physical complexity out of pure information, similar to peculiar “jinni effect” on closed time loops in relativistic cosmology (R.J.Gott, 2001) or Wheeler’s “it from bit” metaphor. By selecting special IP, MD modifies ergodicity principle in favor of info rich states. [Preview Abstract] |
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K1.00072: Numerical solution for Nagumo's equation for the electron density in photorefractive materials Fernando Maga\~na, Laura O. Palomares, J. Adrian Reyes-Cervantes, Gerardo J. Vazquez We study the distribution of the electron density in a photorefractive material, using a set of nonlinear partial differential equations, that describes the physical response of photorefractive systems under inhomogeneous ilumination based on the band transport model, proposed by Kukhtarev et al. (Ferroelectrics, vol. 22, 949 (1979)). Assuming that the electron density only depends of x coordinate and taking a constant external electric field E in the same x coordinate we find that the electron density obeys a Nagumo's equation whose solution is soliton type. [Preview Abstract] |
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K1.00073: Compaction Bubbles in Sand Xiang Cheng, Rachel Smith, Heinrich Jaeger, Sidney Nagel We studied granular compaction by tapping a tilted tube filled with well-prepared, loosely-compacted, fine glass beads. Instead of uniformly compacting, the granular medium reduces its volume by forming a train of upward moving voids - that is, the bed bubbles after a tap. We found that the bubbling of the granular bed is robust in that the length and the velocity of bubbles are independent of the method of tapping. We investigated the properties of the bubbles as a function of the angle of the tube with respect to the horizontal and found a threshold angle below which the bubbling behavior disappears. The velocity of a bubble increases as it rises in the tube. By changing the ambient pressure of the system, we found that the interstitial gas plays an essential role in generating the bubbles. [Preview Abstract] |
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K1.00074: Experiments on Random Packings of Ellipsoids Weining Man, Aleksandar Donev, Frank Stillinger, Matthew Sullivan, William Russel, David Heeger, Souhil Inati, Salvatore Torquato, Paul Chaikin Recent simulations indicate that ellipsoids can randomly pack more densely than spheres and, remarkably, for axes ratios near 1.25:1:0.8 can approach the densest crystal packing (FCC) of spheres, with packing fraction 74{\%}. Here we fabricated about 1000 such ellipsoids and measured their packing properties in a number of different containers and using a variety of conventional and novel techniques. We find excellent agreement with the previous simulations and with new simulations for the density profile in finite size systems. We introduce a new method, using a spherical container and measuring the volume of fluid needed to fill the voids as a function of the fluid level, which greatly reduces surface effects and enables the determination of the bulk or core packing fraction $\phi _{bulk }$for rather small systems. This technique should prove useful for many granular material and packing studies. We also confirm these measurements with magnetic resonance imaging (MRI) studies which provide the most direct method in the case of non-metallic particles. From our experiments, we find the random jammed packing faction $\phi $, as $\phi _{sphere}$=.635 $\pm $ .005, $\phi _{m\& m\mbox{'}s}$=.700 $\pm $ .005, $\phi _{ellipsoid}$=.739$\pm $.005 for our 2.344:1.875:1.5 cm ellipsoids. [Preview Abstract] |
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K1.00075: Response to natural disturbance and attack in a trade model Bei-Bei Su, Tian Xu, Da-Ren He We suggest a trade model, which may describe the trade activities between the open ports along Yangzi River. The cumulative distribution of the quantities of port export and import, generalized by the model, shows a good agreement with our statistical results over 229 open ports. Randomly select a port and let its production decrease greatly by a fixed ratio (a disturbance), we find that the averaged quantities of port export and import increase obviously. This means that, in a healthy trade network, a lot of elements can make benefits when one of them is hit by a natural calamity. When select the port that has most trade relationship with others and let its production decrease greatly by a fixed ratio (an attack), we find that the averaged quantities of port export and import soon recovers and reach the original value. This healthy response will significantly change if the trade rules in the model are only slightly revised. This result may suggest a feature, which can distinguish healthy complex systems from others. [Preview Abstract] |
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K1.00076: Degree distribution in some non-social collaboration networks Pei-Pei Zhang, Yue He, Bei-Bei Su, Hui Chang, Yue-ping Zhou, Da-Ren He We suggest a notice on the systems, which are not social ones but still can be described by biparticle graph models as well as collaboration networks. As examples, we present real data statistical results about public transportation network in Yangzhou city in 2003, public transportation network in Beijing city in 2003 and the touristy traveling line system in China. We show that, by certain definitions, each transportation or traveling line is described by a complete graph (an act) in the networks. The first system shows a good power-law degree distribution, while the second one shows a nice exponential degree distribution. The third one shows a degree distribution between the two cases: the distribution shows an exponential-like curve in a double-logarithmic plane but with a good linear part (a scaling-free region) in the middle. These results are in very good consistent with their bridge coefficient (how many collaboration acts a node takes part in) distribution. A simplified model that will be presented in another abstract may explain the reason [Preview Abstract] |
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K1.00077: A dynamic model explaining degree distribution in collaboration networks Da-Ren He, Pei-Pei Zhang, Bei-Bei Su, Hui Chang, Yue-ping Zhou We suggest a simplified model and try to explain our statistical results about some collaboration networks. Following Barabasi and Albert [1] and J. J. Ramasco et. al. [2], we suppose that a new node is added in each time step. It connects with T-1 old nodes and forms a new complete graph (an act) including T nodes. The old nodes are selected by a linear-preferential rule according to the node's bridge coefficient (how many collaboration acts a node takes part in). With this model it is easy to obtain a power-law bridge coefficient distribution. Since every act contains exactly the same number of nodes, T, in this simplified model, the degree distribution must show a same power-law distribution. When the old nodes are selected randomly, one obtains an exponential bridge coefficient distribution and a same degree distribution. When the old nodes are selected part randomly and part linear-preferentially, one obtains the two degree distributions between exponential and power-law cases. Numerical simulation results show very good agreement with the analytic ones. [1] A-L Barabasi and R. Albert, Science£¬1999£¬286£º509. [2] J. J. Ramasco et. al., Phys. Rev. E 70, 036106 (2004). [Preview Abstract] |
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K1.00078: An investigation on Chinese actor-collaboration network Zhi-qiang Gong, Yu-mei Jiang Chinese actor collaboration network has been investigated by counting 5026 Chinese films and 9601 actors between 1905 and 2000. The network can be described by a biparticle graph. A kind of particles represent actors, the other kind represents films (acts). Two actors are linked by an edge if they have performed in a same film. Our statistical results show that the Chinese actor-collaboration network is a small world one. The average shortest-path length is 4.10 and the clustering coefficient is 0.80. It is very interesting that the results also show that the distribution of the multiple edge degree shows a better scaling-free property than the distribution of the single node degree. [Preview Abstract] |
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K1.00079: Reliability of Rank Order in Sampled Networks Hawoong Jeong, Pan-Jun Kim In scale-free networks, ranking the individual nodes based on their importance has useful applications, e.g. identifying hubs for epidemic control. However, in most real situations only limited sub-structures of the complete networks are available, therefore the reliability of order relationship in the sampled networks is worth to investigate. With the set of randomly sampled nodes from the underlying original networks, we rank individual nodes by the three centrality measures -- degree, betweenness, and closeness. We show that the nodes of the higher ranks from the sampled networks provide a relatively better characterization of their ranks in the original networks than the nodes with lower ranks. We also reveal that closeness-based order relationship is more reliable than any other quantities due to the global characteristic of the closeness measure. Finally, it is demonstrated that if the access to hubs is limited during the sampling process, increase in sampling fraction can even lower the accuracy of sampling. [Preview Abstract] |
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K1.00080: Simulations of grafted and ungrafted polymeric films near the glass transition Regina Barber-DeGraaff, Arlette Baljon, Rajesh Khare Glass transition behavior of ultrathin polymer films is investigated by means of molecular dynamics simulations. We study thin polymer films composed of bead-spring model chains and supported on an idealized lattice substrate surface. Experimentally, it has been found that the glass transition temperature (Tg) of such films rises sharply when a fraction of the polymer chains are grafted to the substrate. In this work, we investigate the impact of chain grafting, and the strength of polymer-surface interaction, on the film glass transition temperature. Three different methods - volumetric, energetic, and dynamic – are used to determine the Tg of the films. Our results suggest the existence of two different transition temperatures: when the temperature of the melt film is lowered, a first transition, which is characterized by an anomaly in the heat capacity, occurs. Upon decreasing the temperature further, a point is reached at which the internal relaxation time of the film (as calculated from bead diffusion, for example) diverges. The former transition temperature appears to be related to the structural changes in the film, and in qualitative agreement with the experiments, shows a dependence on the characteristics of the polymer-surface interaction. By contrast, the latter temperature is independent of the strength of the polymer- surface interaction. [Preview Abstract] |
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K1.00081: Correlated Single Particle Jumps In a Glass Elizabeth A. Baker, Katharina Vollmayr-Lee We study a three-dimensional Binary Lennard-Jones Glass below the glass transition. To study the dynamics we focus on events when a particle jumps out of its cage formed by its neighboring particles. We distinguish between irreversible jumps, where a particle successfully escapes its cage, and reversible jumps, where a particle returns to one of its previous cages within the time of the simulation. To investigate the spatial correlation of jumping particles we identify clusters of jumping particles. For differrent temperatures $T$ below the glass transition, the clusters are analyzed by size, $s(T)$; coordination number, $z(T)$; and its distribution, $P(z)$. When the jumping particles of the whole simulation run are analyzed the main temperature dependence of $s$ and $z$ is due to the increasing numbers of jumping particles with increasing temperature. To take time correlations into account, we also analyze clusters of particles which jump at the same time. We find for high temperatures that the irreversibly jumping particles form string-like clusters. We also find that aging is present because most jumps occur in the first tenth of the simulation run. [Preview Abstract] |
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K1.00082: Monte-Carlo rejection as a tool for measuring the energy landscape scaling of a simple liquid Gerardo Naumis A simple modification of the Monte-Carlo algortihm is proposed to explore the topography and the scaling of the energy landscape. We apply this idea to a simple hard-core fluid for which it is shown how the landscape topology determines the place where phase transition ocurrs. [Preview Abstract] |
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K1.00083: Experimental Investigation of an Electrostatic Brownian Ratchet Brian Long, Heiner Linke An asymmetric, periodic array of linear Pt electrodes on a SiO$_2$ substrate forms a ratchet potential for charged particles in solution. By using 0.2 micrometer diameter, carboxylate-modified, fluorescent polystyrene beads we can track an ensemble of individual particles as they diffuse and respond to the time-dependent potential of a Brownian flashing ratchet. Unlike previous experimental investigations of electrode ratchet systems (e.g. Bader, et al Electrophoresis 2000, 21, 74-80), particle tracking allows us to both quantify ensemble quantities and characterize the trajectories of individual particles. Current project status will be reported. [Preview Abstract] |
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K1.00084: Structural response in flow around vertical fault slit in a porous medium: an interacting lattice gas computer simulation Joe Gettrust, Ras Pandey Response of the density profile of constituents in a driven flow through a porous medium with a vertical fault slit is studied by an interacting lattice gas model on a cubic lattice. The porous medium with a source of fluid (particles) at the bottom and open top is generated by a random distribution of sediment barriers with a longitudinal slit of transverse width $L_s$. A set of nearest neighbor interactions between mobile particles and effective medium (empty sites and barriers) is considered. The hydrostatic pressure bias (H) is implemented probabilistically. Stochastic movements of particles are governed by the bias and the interaction energy with the Metropolis algorithm. A periodic boundary condition is used along the transverse directions. Fluid particles can leave the system from top or bottom along the longitudinal direction but can be released into the lattice only from the source at the bottom according to their lattice concentration. We examine the transport of constituents, their flow, and density profiles as a function of bias (H) at different porosities. The steady state density exhibits interesting profiles as a function of bias, as the high mobility within the slit region causes hydrostatic correlations among the fluid constituents. [Preview Abstract] |
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K1.00085: The local structural organization of metabolic networks Hawoong Jeong, Young-Ho Eom Metabolic networks share global statistical features. Their connectivity distributions follow a long-tailed power-law and they show the small-world property. In addition, their modular structure is organized in a hierarchical manner. Although the global topological organization of metabolic networks is well understood, local structural organization is still not clear. It is also necessary to understand local properties of metabolic networks for analysis of metabolism. Here, by analyzing subgraphs of metabolic networks of 43 organisms from three domains of life and using the significance profile of subgraphs, we identified the local structural organization of metabolic networks. We identified network motifs, which are statistically significant subgraphs patterns, of metabolic networks. We also showed that metabolic networks from different domains have similar local structure. And we found that local structure of each metabolic network has its own taxonomical meaning. The closer in taxonomy, the more similar local structure. In addition, we found that the common substrates of 43 metabolic networks are not randomly distributed but more likely to be constituents of more cohesive subgraph patterns. [Preview Abstract] |
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K1.00086: BIOLOGICAL PHYSICS |
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K1.00087: Destruction of Bacterial Biofilms Using Gas Discharge Plasma Nina Abramzon, Jonathan C. Joaquin, Jonathan Bray, Graciela Brelles-Mari\~no Biofilms are bacterial communities embedded in an exopolysaccharidic matrix with a complex architectural structure. Bacteria in biofilms show different properties from those in free life thus, conventional methods of killing bacteria are often ineffective with biofilms. The use of plasmas potentially offers an alternative to conventional sterilization methods since plasmas contain a mixture of charged particles, chemically reactive species, and UV radiation. 4 and 7 day-old biofilms were produced using two bacterial species: \textit{Rhizobium gallicum} and \textit{Chromobacterium violaceum}. Gas discharge plasma was produced by using an AtomfloTM reactor (Surfx Technologies) and bacterial biofilms were exposed to it for different periods of time. Our results show that a 10-minute plasma treatment was able to kill 100{\%} of the cells in most cases. Optical emission spectroscopy was used to study plasma composition which is then correlated with the effectiveness of killing. These results indicate the potentiality of plasma as an alternative sterilization method. \newline \newline Supported by CSuperb. [Preview Abstract] |
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K1.00088: Long range attraction between two different likely charged macroions B. I. Shklovskii, Rui Zhang It is known that in a water solution two likely charged macroions can attract each other due to correlations of multivalent counterions adsorbed on their surfaces [1]. This attraction is short-ranged and decays exponentially with distance between macroions. In this work, we show that a longer range attraction exists when the bare surface charge densities of the two macroions have the same sign but different in absolute values. The key idea is that with adsorbed multivalent counterions, two such macroions can be considered as conductors with fixed but different electric potentials. Each potential is determined by the difference between the entropic bulk chemical potential of a multivalent counterion and its correlation chemical potential determined by the bare surface charge density of the macroion. When the two macroions are close enough, their adjacent spots form a charged capacitor, which leads to attraction. This attraction is long-ranged: it decays with distance as a power law. This attractive force may play a important role in gene delivery, in which poly-cations are used to invert the charge of negative DNA so that it is not repelled by negative charged cell membrane. The attraction discussed above makes sure that even the charge of the membrane is also inverted, DNA may still be attracted to it. [1] I. Rouzina and V. A. Bloomfield, J. Phys. Chem. \textbf{100}, 9977 (1996). [Preview Abstract] |
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K1.00089: A survey of DNA looping and cleavage properties of different restriction enzymes using optical tweezers Rachel Millin, Gregory J. Gemmen, Douglas E. Smith Of the more than 3500 known Type II REases, a small but growing number have been identified that require two copies of the enzyme's recognition site for activity. Each site is bound to one enzyme subunit, and the two subunits come together by thermodynamic DNA looping to form an active multimer that cleaves the DNA. When Ca$^{++}$ is replaced with Mg$^{++ }$however, the multimers usually ``staple'' the recognition sites together trapping the DNA loops. Using force measuring optical tweezers, we investigate the behavior of 16 different two-site REases from the Type IIe, Type IIf, and Type IIs subsets on single DNA molecules in the presence of Mg$^{++}$, Ca$^{++}$, and EDTA. We show that one-site and two-site REases may be rapidly discerned. By measuring the force needed to disrupt the loops in the presence of Ca$^{++}$, we elucidate various binding behaviors amongst the two-site REases, probing DNA-enzyme and/or enzymatic subunit-subunit affinity. For one enzyme, HpaII, the effect of [Ca$^{++}$] on activity is studied in detail. [Preview Abstract] |
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K1.00090: Ab initio Free Energies of Enzymatic Reactions Using Model Potentials Edina Rosta, Arieh Warshel In order to reliably model biological reactions in solutions and in proteins one has to use both high accuracy ab initio models and proper averaging over the configurations that determine the free energy. In our group, previously the Empirical Valence Bond (EVB) method was often applied successfully to determine reaction free energy profiles in proteins and in water solution. In the current work we would like to combine the advantages of the fast extensive sampling with EVB method and the accuracy of the ab initio Density Functional Theory (DFT) for Quantum Mechanics / Molecular Mechanics (QM/MM) modeling. The free energy profiles of the catalytic reaction of the dehalogenase enzyme in the protein environment and the corresponding water reaction are studied by both the reference EVB approach and the ab initio QM/MM method. Ab initio QM/MM reaction free energies and activation energies are presented in both environment utilizing the model potential. [Preview Abstract] |
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K1.00091: Diffusion of dextran inside microtubule sample Camelia Prodan, Deborah Fygenson Microtubules (Mts) are the bones of the cell. Their exterior has been extensively studied but little is known about their interior. We have studied the diffusion of fluorescein labeled dextran in the presence of GDP Mts and taxol stabilized GDP Mts. The diffusion coefficient, D, of different size dextran (10 kD, 40 kD, 70 kD, 500 kD) was measured using fluorescence recovery after photo-bleaching (FRAP). If dextran was present during the assembling of Mts, D was smaller then free diffusion coefficient. When dextran was added after the assembling, D was the same as the free diffusion coefficient. For taxol stabilized Mts (0.90 fill ratio), D was also found the same as the free diffusion coefficient . [Preview Abstract] |
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K1.00092: Effects of 3-repeat tau on taxol mobility through microtubules Hyunjoo Park, Deborah Fygenson, Mahn Won Kim Both the anti-cancer drug taxol and the microtubule-associated protein tau suppress dynamics of microtubules (MT). We have observed taxol mobility with full-length 3-repeat tau, one of six tau isoforms, using fluorescence recovery after photobleaching (FRAP) on MTs and compare with earlier results on recombinant full-length adult 4-repeat tau. Taxol mobility becomes highly sensitive to taxol concentration in the presence of 3-repeat tau (up to 1:1 molar ratio) as it does in the presence of 4-repeat tau, but is 2 to 3 times faster at low taxol concentrations. Fitting to a mean-field binding reaction model [J.L. Ross \textit{et.al}, \textit{PNAS} 101:12910-5 (2004)] suggests that the presence of 3-repeat tau enhances taxol movement through pores in the MT walls. [Preview Abstract] |
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K1.00093: An optical microscopy study of the swelling of wet-spun films of CsDNA as a function of hydration and CsCl concentration Megan Schwenker, Robert Marlowe, Scott Lee, Allan Rupprecht Highly oriented, wet-spun films of DNA expand in the direction perpendicular to the helical axis as the hydration of the film is increased. CsDNA films with a high CsCl content show an unexpected shrinkage at a relative humidity of 92$\%$. Our most recent experiments have been to measure the perpendicular dimension of CsDNA as a function of both hydration and concentration of CsCl. Our preliminary results show that no shrinkage is observed at low contents of CsCl, showing that the CsCl plays an integral role in the shrinkage phenomenon. [Preview Abstract] |
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K1.00094: Observation of a phase transition in deoxycytidine at high pressures via infrared spectroscopy Scott Lee, Ian Lawson, L. Lettress, A. Anderson Crystalline deoxycytdine has been studied via infrared spectroscopy at room temperature up to 10 GPa of pressure. Samples, typically 250 microns in diameter and roughly 25 microns in thickness, were loaded into a piston-cylinder type diamond anvil cell supplied by Diacell Ltd. and fitted with type IIa diamonds. To avoid saturation of strongly absorbing modes, the deoxycytidine sample was diluted with KBr powder, which also served as an isotropic pressure-transmitting medium. A number of changes in the infrared spectra are noted near 4 GPa, suggesting a phase transition. A change in the geometry of the sugar ring, such as a different sugar pucker, is believed to be the origin of the phase transition. [Preview Abstract] |
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K1.00095: Vibrational mode degeneracy in adenosine: A Raman and infrared study at high pressures J. Li, S.A. Lee Comparison of the Raman and infrared spectra of crystalline adenosine reveals a number of coincident peaks. These coincidences can be explained by either a mode is both Raman- and infrared-active or two different modes (one Raman-active and the other infrared-active) have the same frequency. High pressure has been applied to crystalline adenosine as a perturbation to separate the frequencies of any degenerate modes. The results of these experiments are presented. [Preview Abstract] |
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K1.00096: Brownian Motion of Hybridized DNA in the Presence of Base-Pair Mismatch Kathryn Schallhorn, Katherine Freedman, Pu-Chun Ke The conformational change of hybridized DNA in solution has been characterized with single-pair fluorescence resonance energy transfer (spFRET), in the presence of base pair mismatch. It has been found that base-pair mismatch, a major form of DNA damage, can affect the flexibility of DNA, as manifested by a change of spFRET efficiency. The averaged FRET efficiency was increased from 50.061 percent to 50.778 percent and to 52.796 percent for no mismatches, 3 base-pair mismatches, and 7 base-pair mismatches, respectively. This is possibly due to the shortened end-to-end distance between the DNA, as well as the weakening of the hydrogen bonds, which are not well formed between the mismatched base-pairs. As a result, the flexibility of the DNA is affected. The implications behind understanding such changes in DNA conformation due to damage extend into major components of molecular cell biology and carcinogen studies. [Preview Abstract] |
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K1.00097: Using Measurements of Mobility, Diffusion, and Dispersion to Predict Separation Resolution in DNA Electrophoresis Roger Lo, Victor Ugaz Electrophoresis of DNA continues to be a key component in a wide variety of genomic analysis assays. In order to customize and optimize these assay systems, much effort has been directed to improve and predict separation resolution using various sieving matrices and experimental platforms. Predicting separation resolution requires a much more detailed understanding of mobility, diffusion, and dispersion phenomena of DNA fragments migrating in the sieving matrix than is currently available in literature. In this study, we address this issue by obtaining a series of systematic measurements of mobility, diffusion, and dispersion using an automated DNA sequencer. Using this data, we are able to isolate key factors governing separation performance, and make comparisons with biased reptation theory to extract information on gel structure and predict achievable resolution under each set of operating conditions. We are also able to predict the separation resolution under specific run conditions, thereby giving researchers and engineers the ability to easily tailor DNA separation systems for required separation performance. [Preview Abstract] |
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K1.00098: The influence of ergosterol on the phase behaviour of phospholipid membranes Ya-Wei Hsueh, M.-T. Chen Lipids in biological membranes form domains having distinct physical properties. Defined mixtures of lipids and sterols are of interest to ascertain the fundamental interactions governing these lipids in the absence of other cell membrane components. We study the effect of ergosterol (an important component of fungal plasma membranes) on the physical properties of phospholipid multibilayers using deuterium nuclear magnetic resonance ($^{2}$H NMR). In the $^{2}$H NMR experiments the sn-1 chain was perdeuterated and NMR spectra were taken as a function of temperature and ergosterol concentration. It is found that domains of different composition and phase state are present in membranes. The phase diagram constructed from the NMR spectra will be discussed. [Preview Abstract] |
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K1.00099: Mathematical Structure of Electrostatic Interactions Among Biomolecules: Many Interacting Charged Dielectric Objects Timothy P. Doerr, Yi-Kuo Yu The electrostatic force is one of the more important forces acting on each atom in a biomolecular system. Although the fundamental equations for electrostatics are known, the solution in low symmetry situations with a high dielectric constant solvent (e.g. water) can be difficult to obtain in an appropriate form and with an acceptable degree of accuracy and amount of computation. In order to compute this force, each atom is usually modeled as a dielectric sphere with a point charge at its center. Even the case of two spheres is non-trivial. The case of many spheres (necessary for any realistic biomolecule) can be constructed from the most general two sphere case with appropriate mathematical machinery for handling rotations between the global "laboratory" coordinate system and the local coordinate system of each pair of atoms (defined to have its z axis along the line connecting the centers of the two spheres). We provide an intuitive explanation of the machinery involved in carrying out this process. Ions can be included, as well. The solution for surfaces more general than the union of the surfaces of many spheres can be obtained numerically by choosing a tiling of the surface and solving a corresponding set of linear algebraic equations (the finite-element method). [Preview Abstract] |
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K1.00100: Bovine serum albumin adsorption on passivated porous silicon layers David Lockwood, Li-Lin Tay, Daniel Poitras, Jeff Fraser, Nelson Rowell, Rabah Boukherroub Hydrogen-terminated porous silicon (pSi) films were fabricated through electrochemical anodization of crystalline Si in HF-based solutions. The pSi-H surface was chemically functionalized by thermal reaction with undecylenic acid to produce an organic monolayer covalently attached to the silicon surface through Si-C bonds and bearing an acid terminal group. Bovine serum albumin (BSA) was then adsorbed onto the modified surface. SEM showed that the porous films were damaged and partially lifted off the Si substrate after a prolonged BSA adsorption. Ellipsometry revealed that the BSA had penetrated $\sim $ 1.3 micrometers into the porous structure. The film damage results from BSA anchoring itself tightly through strong electrostatic interactions to the acid-covered Si sidewalls. A change in surface tension during BSA film formation then causes the pSi layer to buckle and lift-off the underlying Si substrate. FTIR results from the modified pSi surfaces showed the presence of strong characteristic Amide I, II and III vibrational bands after BSA adsorption. [Preview Abstract] |
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K1.00101: A general method for preparing DNA sequences for optical tweezers manipulation Derek Fuller, Aurelie Dupont, Pierre Recouvreux, Gregory J. Gemmen, Rachel Millin, Douglas E. Smith Manipulation of single DNA molecules with nanometer-level position resolution and picoNewton-level force resolution is a powerful technique in the study of protein-DNA interactions. Here we present a general method for efficient preparation of any DNA sequence from any organism for optical tweezers manipulation. We demonstrate this method in preparing genomic DNA sequences from Bacteriophage Lambda, E. Coli, Drosophila, Arabidopsis, and Human sources. End-labeled constructs up to 40 kilobasepairs are generated by PCR and single molecules are tethered to microspheres and manipulated using optical tweezers. DNA attachment kinetics, binding strength, tether length, and elastic properties are characterized. This sample preparation method is applicable to a wide range of biophysical studies. [Preview Abstract] |
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K1.00102: Diagnosing Aorta Stiffness by Temporal Analysis of Echocardiographic Images Yu-Hsi Cheng, Tsu-Chiang Yen, Doyal Lee In the diagnosis of cardiovascular diseases, information about the intracardiac system function and blood flow can be obtained by echocardiography due to its high spatial resolution capability. However, seldom message is known about the aorta stiffness. This work investigated a method to quantitatively analyze the aorta stiffness. The aorta was modeled as a periodic-force-driven damping oscillator, in which the aorta stiffness was the damping factor. From the analysis of echocardiographic images, the delay time of the maximal aorta distention relative to the R-peak of the electrocardiographic trace was measured to reveal the aorta stiffness. A study based on 10 samples suggested that a delay time greater than 0.17 sec could be a criterion to diagnose that the aorta is quite stiff. This method could also clearly discern some abnormal cardiac performance. A large-scale study with this method should be conducted in the future. [Preview Abstract] |
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K1.00103: A mathematical model for T-cell differentiation in Asthma episodes David Quesada Asthma is a respiratory disorder resulting from a combined response of the immune system to environmental agents (allergens) and the expression of different genes (complex trait disorder). Asthma episodes are characterized at least by an increase in levels of Immunoglobulin E (IgE) and by the bronchial hyper-responsiveness (BHR). These two effects are consequences of the proliferation of different T-helper cells (T$_{h1}$ and T$_{h2})$. A coarse graining approach has been adopted for developing a mathematical model for T-cell differentiation during asthma episodes and related illness as well. The model considers the input from the thymus, two different kinds of competitions between immune cells (a Lotka-Volterra interspecies competition and a hierarchical one) and a feedback control onto the production of helper T cells on the side of cytokines. The Lotka-Volterra model has been used for the self control of T$_{h1}$ and T$_{h2}$ cells. The hierarchical model is used to account for cytokines secreted by each one of helper T cells. The obtained system of differential equations has been solved and a phase diagram has been obtained. [Preview Abstract] |
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K1.00104: Period doubling observed in the circadian photosynthetic rhythm of the prokaryotic cyanobacterium Cyanothece RF-1 Tsu-Chiang Yen, Li-Ting Wang, Da-Long Cheng The circadian rhythm is an endogenous biological clock that governs biochemical phenomena or behavior in organisms. The Cyanothece RF-1 is the first prokaryote shown to exhibit circadian nitrogen-fixing rhythm. The observation of the circadian photosynthetic rhythm of this strain was recently reported by the authors. In this work, the dissolved-oxygen variation in the culture of Cyanothece RF-1 was recorded, which would reveal the photosynthetic activity of the strain. For a culture of about 1x10$^{8}$ cells/ml in concentration, a period-doubling pattern was clearly displayed in the circadian photosynthetic rhythm signals. The mechanism corresponding to this nonlinear effect will be discussed. These results represent the first observation of the period doubling in the circadian rhythm of a prokaryotic cyanobacterium. [Preview Abstract] |
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K1.00105: Monte Carlo simulation of actin self-assembly Xinjiang L\"{u}, James Kindt Using grand canonical Monte Carlo simulations we study the equilibrium properties of actin self-assembly. The statistics of actin polymerization is described by a mechanism involving monomer activation and chain propagation with bond association constants derived from experimental free energy parameters. For efficiency in representing systems of very long, stiff chains we use a coarse-graining based on spherocylinders. We will present results pertaining to the isotropic-nematic transition in this equilibrium polymer system. [Preview Abstract] |
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K1.00106: Monte-Carlo Simulation of a Simple Biopolymer Growth Model Jenny Son, G. Orkoulas, A. Kolomeisky Rigid biopolymers, such as actin filaments, microtubules and intermediate filaments, are vital components of the cytoskeleton and the cellular environment. Understanding biopolymer growth dynamics is essential for the description of the mechanisms and principles of cellular functions. These biopolymers are composed of N parallel protofilaments which are shifted at arbitrary distances, giving rise to complex end structures. In this work, we investigate simple biopolymer growth models by Monte Carlo simulations that include the effects of such ``end properties'' and lateral interactions. The simulations reproduce the theoretical predictions for the simple N=2 model. For the biologically relevant case of N=13, we have found that the simulation results approach the approximate model predictions. The simulations indicate that polymerization events are evenly distributed among N protofilaments, which implies that both end structure effects and lateral interactions are significant. [Preview Abstract] |
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K1.00107: A concept for an artificial single-molecular motor Matthew Downton, Martin J Zuckermann, Michael Plischke, Erin Craig, Heiner Linke In this study we present and examine by numerical simulation a polymer based theoretical concept for an artificial single-molecular motor. We model the motor potential by a 'flashing ratchet' in which non-equilibrium fluctuations bias Brownian motion in an asymmetric periodic potential without macroscopic force fields. For the potential we choose either an asymmetric saw tooth model or an asymmetric periodic potential created by alternating infinite line electrodes of positive and negative charge respectively. The polymer models a DNA molecule which is assumed to have a total length much greater than its persistence length and is thus simulated by Brownian molecular dynamics on a freely jointed polyelectrolyte with Lennard-Jones potentials between all monomers. We report the results of several numerical calculations using this model. These include (a) the polymer velocity as a function of the periodic length of the potential, the polymer length and the off-time of the flashing ratchet when the potential is switched off and the polymer diffuses freely, (b) stall force and motor efficiency, (c) the effect of a load on the kinetics of the polymer, (d) the conditions for reversal of polymer motion and (e) the detailed characteristics of the polymer in the ratchet potential. We predict optimal design parameters for a related experimental project. [Preview Abstract] |
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K1.00108: Reversal of motion induced by coupling in Brownian motors Erin Craig, Martin J. Zuckermann, Heiner Linke We use numerical simulations to examine the behavior of coupled particles in a `flashing ratchet' system, in which an object undergoing free diffusion is periodically subjected to an asymmetric periodic potential that biases the motion of the object without using macroscopic force fields. We are exploring ratchet systems where the object is large enough for the finite size, the shape, and the internal degrees of freedom to play an integral role in the transport mechanism. In contrast to the behavior of a point particle, reversal of motion is observed for two or more particles forming a rigid rod constrained to move in one dimension. If the rods, however, are allowed to move in three dimensions, or if their length is much less than the spatial period of the ratchet potential, the reversal of motion disappears and the qualitative behavior of single particle motion is recovered. We will also discuss the effect of inducing partial flexibility into the rods, as well as the possibility of reversal for polymers in a ratchet. [Preview Abstract] |
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K1.00109: Thermal Fluctuations of extended DNA single molecules Rajalakshmi Nambiar, Ben Sonday, Jens-Christian Meiners Studying the thermal fluctuations of DNA molecules reveals not only a wealth of interesting statistical mechanics, but is also of importance for understanding genomic function in vivo. The thermal fluctuations are extremely sensitive to mechanical constraints, such as mechanical tension in the DNA. The force scale for this sensitivity is of the order of 100 fN, which is small for a cellular environment. We study the dynamics of single DNA molecules under tension both under equilibrium and non-equilibrium conditions using a modified scanning-line laser trap. In the equilibrium measurements, we measure the thermal fluctuations of the DNA molecule around its equilibrium extension. The time constants of the fluctuations yield the friction coefficient and thus information about the intramolecular hydrodynamic coupling as a function of the extension of the molecule. The results are compared to Brownian dynamics simulations and theory. In the non-equilibrium studies, the molecule is extended and then allowed to relax against an external force. The average relaxation trajectory is compared to a quasi-static prediction from the wormlike chain model, and the fluctuations of the individual trajectories around their mean are discussed in the framework of non-equilibrium statistical mechanics. [Preview Abstract] |
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K1.00110: Effect of nonlinear filters on detrended fluctuation analysis Zhi Chen, Kun Hu, Pedro Carpena, Pedro Bernaola-Galvan, H. Eugene Stanley, Plamen Ch. Ivanov We investigate how various linear and nonlinear transformations affect the scaling properties of a signal, using the detrended fluctuation analysis (DFA). Specifically, we study the effect of three types of transforms: linear, nonlinear polynomial and logarithmic filters. We compare the scaling properties of signals before and after the transform. We find that linear filters do not change the correlation properties, while the effect of nonlinear polynomial and logarithmic filters strongly depends on (a) the strength of correlations in the original signal, (b) the power of the polynomial filter and (c) the offset in the logarithmic filter. We further investigate the correlation properties of three analytic functions: exponential, logarithmic, and power-law. While these three functions have in general different correlation properties, we find that there is a broad range of variable values, common for all three functions, where they exhibit identical scaling behavior. We further note that the scaling behavior of a class of other functions can be reduced to these three typical cases. We systematically test the performance of the DFA method in accurately estimating long-range power-law correlations in the output signals for different parameter values in the three types of filters, and the three analytic functions we consider. [Preview Abstract] |
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K1.00111: Transcriptional Regulation by the Numbers Hernan Garcia, Lacramioara Bintu, Jane' Kondev, Rob Phillips The study of gene regulation and expression is becoming ever more quantitative. In particular, with increasing regularity the expression of genes is characterized quantitatively with respect to how much, when and where. The key argument of the present work is that such quantitative data demands quantitative models. We examine a class of models (``thermodynamic models'') which exploit the tools of statistical mechanics to compute the probability that RNA polymerase will be found at the appropriate promoter. Recent arguments have suggested that in some instances, the action of activators can be thought of strictly as agents of recruitment which increase the probability that RNA polymerase will be found at the promoter of interest. We develop an allied mathematical framework which describes the interactions of repressors, activators, helper molecules and RNA polymerase and culminates in an expression for the probability of RNA polymerase binding at the promoter of interest as a function of the concentrations of all of these regulatory agents. These ideas are applied to several case studies which illuminate the general formalism and shed light on the role of DNA looping. [Preview Abstract] |
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K1.00112: Stochastic QM/MM Models for Proton Transport in Condensed Phase: An Empirical Valence Bond (EVB) Approach Anton Burykin, Sonja Braun-Sand, Arieh Warshel Proton transport (PT) plays a major role in biophysics in general and bioenergetics in particular. In view of the crucial role of biological PT processes it is important to gain a quantitative molecular understanding of the factors that control such processes. While modeling actual time-dependent PT in biological systems one has to deal with up to microsecond time scales which are not accessible to QM/MM methods. In order to overcome this problem we have developed a new type of hybrid quantum/classical approach which combines explicit QM (EVB) representation of the chain of donor and acceptors and implicit representation (via the effective coordinates) of the environment (the rest of the protein/water system). The dynamics of the whole QM/MM system is described by stochastic (langevin) equations. This model takes into account the correct physics of proton charge delocalization and the reorganization of solvent polar groups during the PT process. The description of QM/MM langevin dynamics method is given and several applications to biological systems (PT in Gramicidin A channel and Carbonic Anhydrase) are presented. [Preview Abstract] |
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K1.00113: POLYMERIC & ORGANIC MATERIALS II POSTER SESSION |
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K1.00114: Intercalation Mechanism and Interlayer Structure of Hexadecylamine inside Layered $\alpha$-Zirconium Phosphates Bongwoo Ha, Kookheon Char Well-defined intercalated structure, either interdigitated layers or bilayers, of hexadecylamines (HDAs) in a confined space of a highly-functionalized layered material, $\alpha$- zirconium phosphate ($\alpha$-ZrP), was prepared using two-step intercalation mechanism and these two distinct intercalated structures can serve as model systems to investigate the interaction of the two monolayers whose amphiphilic tails are adjacent to each other. Interlayer structure of HDAs in $\alpha$- ZrP intergallery was characterized with XRD and TEM, and an intriguing two-step deintercalation behaviors of intercalated HDAs were observed with TGA and in-situ SAXS experiments during heating. This deintercalation behaviors are believed to be due to two different interaction force of intercalated HDAs (i.e., electrostatic interaction between host and guest molecule, and hydrophobic interaction between guest molecules) and this interaction force is found to affect interlayer structure of intercalated HDAs. We also found that the thermal hysteresis for the amphiphilic HDA molecules attached to confined geometries is stronger for the interdigitated layers, which is in reasonably good agreement between DSC and SAXS experiments. [Preview Abstract] |
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K1.00115: X-ray Standing Wave Measurements of Gold Nanoparticles of Varying Sizes Embedded in Polymer Thin Films Aleta Hagman, Kenneth Shull, Jin Wang, Xuefa Li, Suresh Narayanan Polymer/Metal interactions in a model nanocomposite have been studied with the use of total external reflection x-ray standing waves (TER-XSWs). The TER-XSW technique has been used to measure the diffusive properties of thermally evaporated and colloidal gold nanoparticles buried in polymer thin films (coated on x-ray reflecting mirrors) over distances of nanometers or less. Particle size effects become important for particles that are comparable in size to the smallest possible period of the standing wave, which for a silver mirror is $\sim $ 9 nm. Diffusive motions of colloidal particles with well-controlled sizes can be monitored by appropriately accounting for these size effects. [Preview Abstract] |
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K1.00116: The influence of chain rigidity and the degree of sulfonation on the morphology of block copolymers as nano reactor K. Hong, S. I. Yun, J. Mays, X. Zhang, R. M. Briber Polyelectrolyte block copolymer was used to form an ordered domain of ionic block as a ``nanoreactor'' due to its ability to bind oppositely charged metal ion, Zn$^{2+}$, Fe$^{2+}$ etc. The purpose of our research is to investigate the controllability of the size and morphology of domains (inorganic nano particles) by changing backbone stiffness, the charge density and the volume fraction of ionic block. Poly(styrene sulfonate) (PSS), which backbone is flexible, and poly(cyclohexadiene sulfonate) (PCHDS), which backbone is ``semiflexible'', were used as ionic blocks. We synthesized PtBS-PSS and PS-PCHDS with various degree of sulfonation and the volume fraction. Zinc oxide (ZnO) nano particles successfully formed in the ionic domain of microphase separated block copolymers. We used SANS to characterize the morphology of block copolymers and TEM for block copolymer containing ZnO nano particles. Our experimental results show that the chemistry of ``sulfonation'' of block copolymers can be successfully used to synthesize nano composite materials. [Preview Abstract] |
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K1.00117: Multiscale Modeling of Viscoelastic Properties of Polymer Nanocomposites Oleg Borodin, John Nairn, Dmitry Bedrov, Grant Smith A methodology for a simple multiscale modeling of polymer nanocomposites has been developed and applied to simulations of random polymer nanocomposites. This methodology consisted of three steps: a) obtaining viscoelastic properties of bulk-like polymer and approximating behavior of the interfacial polymer from molecular dynamics (MD) simulations; b) using bulk and interfacial polymer properties obtained from MD simulations, performing stress-relaxation simulations of nanocomposites with material point method (MPM) simulations in order to extract nanocomposite viscoelastic properties; c) performing direct validation of viscoelastic properties obtained from MPM simulations with those obtained from MD simulations for relatively small nanocomposites consisting of one nanoparticle in polymer matrix. MPM calculation of random nanocomposites with attractive and neutral interfaces indicated that turning on attraction between polymer and cylinder could increase time dependent shear modulus by multiple orders of magnitude with the increase being more substantial at longer times. [Preview Abstract] |
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K1.00118: Effects of temperature and dissolved LiClO$_{4}$ on the viscoelastic and dynamic properties of poly(ethylene oxide), (PEO) melts Radoslav Bogoslovov, James C. Selser, Shufu Peng, Greg Piet The physical properties of poly(ethylene oxide) (PEO) melts and PEO/LiClO$_{4}$ complexes are of major importance for understanding the mechanism and dynamics of lithium-ion transport in polymer electrolytes. We used a fiber-optic coupled triple-pass Fabry-Perot interferometer to study the Brillouin light scattering spectra of PEO melts and PEO melt/LiClO$_{4}$ solutions for various salt concentrations and in the temperature range from the melting point up to 180$^{\circ}$C. We report the measured Brillouin line-shifts and line-widths. A relaxation process was identified in the gigahertz frequency range and was studied in detail. The ``mapping'' of the relaxation in the frequency--temperature--concentration parameter space yields important information about the local segment dynamics of the polymer chain, which has a direct effect on the transport of the charge carriers in the polymer electrolyte. [Preview Abstract] |
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K1.00119: Microwave Induced Structural Transitions in Polymers Yuning Yang, Wei-Chi Lai, Shaw Ling Hsu Polymer chain dynamics as a function of temperature (in the range from 20 to 190 $^{\circ}$C) have been studied using dielectric spectroscopy within the microwave frequency range. The frequency of radiation was varied from 0.5 GHz to 18 GHz. These studies were conducted for poly(caprolactone) (PCL), poly(ethylene oxide) (PEO), poly(vinyl acetate) (PvAC), poly(lactic acid), polystyrene (PS), nylon 6 and poly(methyl methacrylate) (PMMA). These polymers possess glass temperatures ranging from -- 62 $^{\circ}$C (PCL) to 110 $^{\circ}$C (PMMA). One broad relaxation process was found only for low T$_{g}$ polymers (PCL and PEO) and not for the others. Results from temperature-dependent dielectric spectroscopy indicate that the relaxation process follows an Arrhenius T dependence suggesting the relaxation process is due to local motions. Moreover, the effect due to end groups was investigated by comparing results of PEO with hydroxy versus methoxy end groups. It was determined the structural transitions measured are not with end group motions. From the Debye diffusive model, the relaxation process is concluded to be associated with the short segmental motion along the backbone. [Preview Abstract] |
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K1.00120: Light Scattering Investigation of Dynamic and Viscoelastic Properties of Entangled Poly(ethylene oxide) Melts in the Presence of LiClO4 Shufu Peng, J. C. Selser, R. Bogoslovov, G. Piet Laser light scattering measurements are employed to investigate the dynamics of entangled poly(ethylene oxide) with and without lithium perchlorate in melts. The single-exponential ACF relaxation mode was observed and exhibited $q^{2}$ dependence with and without salt in melts; behavior interpreted as due to the relaxation of a transient PEO network. The results of activation energy from the dynamic light scattering and viscometry measurements were compared. Static scattering measurements indicated that the structure factor exhibits a universal power law I($q) \quad \sim \quad q^{-2.0 \pm 0.1}$ in the melts; which agree with the prediction of the percolation theory. Furthermore, the results revealed that the level of entanglement of PEO chains is different depending on the existence of LiClO$_{4}$. [Preview Abstract] |
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K1.00121: Correlation length of a near-critical, eight-arm star polystyrene in methylcyclohexane Angie Triplett, Nithya Venkataraman, D.T. Jacobs The turbidity of eight-arm star polystyrene in methylcyclohexane has been measured and used to determine the correlation length amplitude $\xi _{0}$. The turbidity in this system was determined from the measured ratio of the transmitted to incident light intensities as a function of temperature. Various systematic errors were explored and eliminated in obtaining a value of $\xi _{0}$ for this branched polystyrene with a total molecular weight of 228,000. Ornstein-Zernike theory is used to fit the turbidity data. The value of $\xi _{0}$ is comparable to that reported in the literature for a linear polystyrene of the same molecular weight in methylcyclohexane. We acknowledge the support from NSF-REU grant DMR 0243811 and to the donors of the American Chemical Society Petroleum Research Fund. [Preview Abstract] |
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K1.00122: Coexistence curve of a near-critical, eight-arm star polystyrene in methylcyclohexane Mark Lightfoot, D.T. Jacobs The coexistence curve of eight-arm star polystyrene in methylcyclohexane has been measured and used to determine the coexistence curve amplitude B. An automated measurement of the minimum deviated angle in each phase provides the refractive index and thus the composition in each phase. By exploring temperatures from a few milliKelvins to ten Kelvin below the critical temperature, the shape of the coexistence curve is determined and compared to a simple power law of amplitude B and exponent $\beta $. The exponent should be independent of molecular weight while the amplitude should vary as a power-law in molecular weight. We report the results of one molecular weight (228,000) and compare them to published values for a smaller molecular weight (74,000) and to the results for a linear polystyrene in the same solvent. We acknowledge the support from the donors of the American Chemical Society Petroleum Research Fund. [Preview Abstract] |
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K1.00123: Mapping Instabilities in Polymer Friction Charles Rand, Alfred Crosby Schallamach waves are instabilities that occur as interfaces between a soft elastomer and rigid surface slide past each other.(1) The presence of Schallamach waves can lead to drastic changes in frictional properties. Although the occurrence of Schallamach waves has been studied for the past several decades, a general map relating fundamental material properties, geometry, and operating conditions (i.e. speed and temperature) has not been established. Using a combinatorial approach, we illustrate the role of modulus, testing velocity and surface energetics of crosslinked poly(dimethyl siloxane) on the generation Schallamach waves. This knowledge will be used with polymer patterning processes to fabricate responsive coatings for applications such as anti-fouling coatings. (1)Schallamach, A.;Wear 1971,17, 301-312. [Preview Abstract] |
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K1.00124: Influence of Patterned Surfaces on Adhesion Edwin Chan, Tina Thomas, Alfred Crosby Nature has illustrated the influence of surface patterns in controlling adhesion. Despite the current experimental and theoretical work to mimic such systems (e.g. Gecko setae), little is known on how individual features as well as neighboring features affect adhesion. We address this question by utilizing a combinatorial approach to simultaneously vary the feature spacing and size of the patterns for soft elastomers such as poly(dimethyl siloxane) and poly(n-butyl acrylate) elastomers. Contact adhesion tests based on Johnson, Kendall and Roberts (JKR) theory are used to characterize the adhesion of these patterned soft elastomers to a glass hemisphere. Scaling laws for various adhesion descriptors are presented to demonstrate the relationship between the material properties and the pattern characteristics. [Preview Abstract] |
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K1.00125: Ordered Helices in Chiral $\sigma$-conjugated Polysilanes Withoon Chunwachirasiri, Michael Winokur, Josef Michl, Julian Koe Molecular modeling of studies of poly[(S)-2-methylalkyl($n$-alkyl$'$)]silanes identify a tacticity insensitive steric packing constraint that underlies the existence of highly ordered helices in these polymers. Thus this family of polymers is dominated by extended repeating sequences of the D+/g-g- (main chain deviant, side chain gauche/gauche) conformational isomer. Solid-state films of poly[(S)-2-methylbutyl(nonyl)]silane have been studied by optical spectroscopy. Sharp photoabsorption and photoluminescence peaks are observed at both ambient and reduced temperatures. The observed photoabsorption and photoluminescence spectra are interpreted using H{\" u}ckel molecular orbital theory. [Preview Abstract] |
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K1.00126: Segment Orientation and Optical Birefringence of Amorphous Polymers Under Tensile Deformation: Novel Computational Method applied to Different Glassy Polycarbonates Upendra Natarajan, M.S. Sulatha Orientation dependent optical properties of Bisphenol A polycarbonate and two aliphatic substituted polycarbonates in glassy phase have been studied by atomistic modeling using molecular mechanics simulations under tensile deformation. Probability distributions and orientation functions show that phenylene rings and carbonate groups vectors along the main chain orient towards stretching direction following deformation. Interchain packing of rings and carbonates become ordered with strain. Efficient computational approach for calculation of optical birefringence of amorphous polymers is presented and applied to the polycarbonates in detail. Polarizability anisotropy of the polymer segments and chain as a function of deformation is calculated by combining information on the conformations and group polarizabilities, and used to estimate birefringence during deformation. Simulated and experimental values for segment orientation and bulk birefringence are in very good agreement. Effect of the optical properties of atomic groups on bulk birefringence is brought forth for the first time by molecular simulation for polymers other than polyethylene. [Preview Abstract] |
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K1.00127: Adsorption of Proteins to Cu(II)-IDA and Ni(II)-IDA Functionalized Langmuir Monolayers and Insertion Processes by Grazing Incidence Neutron and X-ray Techniques Michael Kent, Hyun Yim, Darryl Sasaki, Sushil Satija, Young-Soo Seo, Ivan Kuzmenko, Thomas Gog, Jaroslaw Majewski The adsorption of myoglobin, lysozyme, and several synthetic helical peptides to Langmuir monolayers of a metal-chelating lipid in crystalline phase was studied using neutron and X-ray reflectivity and grazing incidence X-ray diffraction. In this system adsorption is due to the interaction between chelated divalent copper or nickel ions and the histidine moieties at the outer surfaces of the proteins. Adsorption was examined under conditions of constant surface pressure ranging from 30 to 40 mN/m, and at constant area at an initial pressure of 40 mN/m. Layer characteristics were obtained as a function of time during the adsorption process by neutron and X-ray reflectivity. Insertion of segments into the lipid membrane was detected by a strong increase in surface pressure for constant area experiments, and by disruption of lipid crystalline packing for constant pressure conditions. The conditions affecting both processes will be discussed. [Preview Abstract] |
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K1.00128: Design and Synthesis of pH Sensitive Polymeric Sensor for Potential Molecular Imaging using Fluorescence Resonance Energy Transfer Sung Woo Hong, Keon Hyeong Kim, June Huh, Cheol-Hee Ahn, Won Ho Jo Molecular imaging in tumor biology is becoming one of the most active scientific and clinical research areas. Especially, optical imaging based on fluorescence resonance energy transfer (FRET) phenomena has emerged as an important technique to describe clinical targets in vivo. The goal of this study is the development of polymeric biosensor which emits specific fluorescence upon responding to lowering pH. In this work, sulfadimethoxine is chosen for pH sensitive moiety and is conjugated with methacrylol chloride to synthesize methylmethacrylate based sulfadimethoxine monomer (MbSDM). Coumarin and pyrene are selected as FRET donor and acceptor. Modified coumarin derivative is used as atom transfer radical polymerization (ATRP) initiator of MbSDM to prepare pH sensitive polymeric backbone with coumarin at one end. The sequential ATRP of methylmethacrylate based pyrene is performed to position pyrene at the other end of polymeric backbone. This polymeric backbone could be reversibly changed in end-to-end distance in response to the variation of pH condition, which results in the pH-switching photonic property. [Preview Abstract] |
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K1.00129: Theory of End-Labeled Free Solution Electrophoresis: Using Branched Polymeric Labels with ssDNA Sorin Nedelcu, Martin Kenward, Laurette McCormick, Gary W. Slater End-Labeled Free Solution Electrophoresis allows one to sequence DNA molecules without the need for a sieving matrix. As the name suggests, a label is attached to the DNA molecules in order to overcome their free-draining property by changing the balance between the friction and the electrophoretic forces. Recent experimental results have shown that the friction coefficient of short branched labels increase linearly with their total molecular weight, thus suggesting a new strategy to design labels for long ssDNA read lengths. In our work, we study the use of branched labels for this purpose using two different approaches. First, we use an exact analytical theory that neglects excluded volume interactions, and conclude that the friction coefficients increase almost linearly with molecular weight, with strong correction factors that increase rapidly with the length of the label. We then examine the same problem using extensive Molecular Dynamics simulations. Our results indicate that the correction factor is smaller than predicted by the analytical theory, and that the linear regime observed experimentally can extend to fairly large molecular weights. [Preview Abstract] |
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K1.00130: Counteracting the electrophoretic motion of a polyelectrolyte: a Molecular Dynamics study Martin M. Bertrand, Gary W. Slater For a polyelectrolyte undergoing electrophoretic motion, it is predicted (D. Long, A. Ajdari, Electrophoresis 1996, 17:1161) that the mechanical force necessary to counteract the electrical force exerted on the molecule should be substantially smaller than the latter. In our work we examine this prediction using coarse grained Molecular Dynamics simulations in which we explicitly include the polymer, the solvent, the counterions and added salt. We measure the mechanical force required to counteract the electrophoretic motion and observe and quantify the resulting polymer conformations. One end of the polyelectrolyte is tethered to a fixed point in space via a harmonic potential. We measure the average counteracting force by monitoring the distance between the fixed point and the end of the molecule. Our simulations are carried out in a long cylindrical capillary with periodic boundary conditions. [Preview Abstract] |
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K1.00131: Diffusion in a system of vibrating obstacles: Exact numerical results Smaine Bekhechi, Gary W. Slater We present a generalized version of our lattice model of diffusion that allows us to treat the case of particles diffusing in arrays of obstacles that can vibrate around a mean position. The diffusion problem is modeled as a random walk in the conformational space of the combined obstacles-particle system. We show that it is possible to obtain exact diffusion coefficients as a function of the frequency of vibration of the obstacles on a two dimensional square lattice. We examine the smooth transition between the quenched system (zero-frequency) and the annealed system (infinite frequency). [Preview Abstract] |
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K1.00132: DNA Molecules for Single Polymer Dynamics and Rheology Studies Rae Robertson, Stephan Laib, Douglas E. Smith A major topic in complex fluids research is to understand how macroscopic properties of polymeric fluids arise from microscopic molecular dynamics. Traditional experiments measuring bulk rheological properties often only test microscopic theories indirectly. Manipulation and visualization of single polymers using optical tweezers and fluorescence microscopy allows direct determination of microscopic dynamics and testing of molecular theories. DNA is employed as a model polymer in these experiments and here we report on the preparation of monodisperse DNA samples covering a wide range of chain lengths in circular and linear forms. In addition to having biological relevance, DNA molecules have many advantages as a model for polymer physics and rheology studies. Using techniques in molecular biology, exactly monodisperse solutions of DNA molecules are produced with a precise control of polymer length and topology and replicated exactly and limitlessly. These samples will expand research possibilities in this area and will be provided as a resource to researchers in polymer physics, rheology and biophysics. [Preview Abstract] |
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K1.00133: Charged surface Induced Diblock Copolymer Micellization Monica Olvera de la Cruz, Hao Cheng Diblock copolymers with one charged block A and one hydrophobic block B are mainly single chains in solution when the concentration is lower than the critical micellization concentration (cmc. Electrostatic interaction between copolymers and a negatively charged surface decreases the non-ideal part of the micelle chemical potential and induce the micelle formation at the surface when the bulk copolymer concentration is still lower than the bulk cmc. The effects of surface charge density \textit{$\sigma $, }monomer number of charged block$ N_{A}$, monomer number of hydrophobic block$ N_{B }$ and the fraction of charged monomers in the charged block $f$, on the number of chains in micelles $p, $micelle radius \textit{Ls} and critical surface micellization concentration (csmc) are studied. When \textit{Ls} is smaller than the size of the micelle Wigner-Seitz cell $R$, $p$ is very small. When \textit{Ls} is equal to $R$, $p$ does not depend on the number of charged monomers in hydrophilic block. The size of the micelle has the same dependence on $N_{A}$ and $N_{B}$ as uncharged system. The effect of electrostatic is reflected by $R\sim (f$/\textit{$\sigma $})$^{1/8}$ and $p \quad \sim (f$/\textit{$\sigma $})$^{-3/4}$. The csmc rises with the increase of salt concentration when the copolymer layer charge density is lower than the surface charge density. However the cscm decline with the increase of salt concentration when the layer charge density is higher than the surface charge density. [Preview Abstract] |
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K1.00134: Effect of Beta-Hairpin Peptide Strand Length on the Dynamics of Semiflexible Networks Bulent Ozbas, Darrin Pochan, Karthikan Rajagopal, Joel Schneider Semiflexible behavior of fibrillar networks formed by the intramolecular folding and consequent self-assembly of beta hairpin peptides was characterized using rheological, electron microscopy, and neutron scattering measurements. The peptide molecules used are locally amphiphilic with two linear strands of alternating hydrophobic valine and hydrophilic lysine amino acids flanking a central turn sequence. The effect of number of amino acid residues of the beta-hairpin molecules (24, 20, 16, 12) on the folding, beta-sheet and self-assembly were studied by CD and FTIR spectroscopy. The network properties of the hydrogels and the nanostructure of the fibrillar assemblies were studied by TEM and SANS. The results show that local nanostructure of the fibrils is similar. The cross-sectional diameter of the fibrils, and thus, the bending modulus of the chains vary with the number of amino acids of strands of the molecules. Dynamic oscillatory and transient rheological measurements were employed to probe the differences in the dynamics of the fibrillar network structure. Rheological results reveal that rigidity of the hydrogels differ with strand length of the molecules. An increase in the relaxation times of the network was observed with decreasing cross-section diameter of the fibrils. [Preview Abstract] |
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K1.00135: Biodegradable Poly(L-Lactic Acid) (PLLA) Thin Films by Plasma Polymerization Arnold Yang, Yi-Hsing Chang, Hsun Li, Tai-Wei Chang, Yin-Chang Liu, Cheng Kung Cheng By using high concentration monomer vapor as the source for plasma generation, poly(L-lactic acids) (PLLAs) was successfully prepared in the form of thin films for the first time via plasma polymerization. The radiation damages commonly incurred from plasma processes were minimized and hence essentially all the characteristic functional groups of the monomer were retained in the resulted polymer films. The polymer demonstrated similar hydrolytic and biocompatibility properties as that of the conventional PLLA and showed significant improvements in mechanical properties, surface roughness, and cell adhesion capability. The plasma films was amorphous and nanoscopically smooth. The Young's modulus and hardness increased linearly with plasma power and could be as high as approximately 5 times of the conventional PLLA. In addition, chain crosslinking took place during the process of polymerization, and was controllable by adjusting the plasma parameters, which also strongly affected hydrolytic degradation rate of the plasma polymer. [Preview Abstract] |
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K1.00136: Dielectric and a.c conductivity relaxation processes of ion conducting amorphous polymer. Baskaran Natesan, Naba Karan, Ram Katiyar The dielectric and a.c conductivity relaxation processes of lithium ion conducting amorphous polymer (PEO) films have been investigated using dielectric spectroscopy. The purpose of this work was to explore the effects of repeated heating/cooling treatments and the addition of nano- sized TiO$_{2}$, on the relaxation processes. The dielectric measurements ($\varepsilon $' and $\varepsilon $'') were carried out as function of frequency at various temperatures. The temperature dependence of the $\varepsilon $' exhibited two slopes. The first slope break observed at --35\r{ }C was attributed to the onset of the glass-rubbery transition and the second at 60\r{ }C was due to the melting of PEO. From the frequency dependence of dielectric loss, a loss peak corresponding to segmental relaxation associated with the glass transition ($\alpha $- process) was noticed from 30\r{ }C onwards. The $\alpha $-relaxation was found to slow down during repeated heating/cooling cycles. Further, the skewness and width of the dielectric loss peak increased upon TiO$_{2}$ addition. The a.c conductivity relaxation was analyzed using electrical modulus formalism. The electrical modulus spectrum showed two relaxation peaks associated with the relaxation of lithium in two different polymer environments. The shape and the nature of the modulus spectrum reveal that the conductivity relaxation process follows a stretching exponential functional form, $\phi $(t)=$\phi _{o}$ exp (-t/$\tau )^{\beta }$, typical of disordered system. [Preview Abstract] |
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K1.00137: Conformation and dynamics of chained molecules in nanoscopic cylinders Kyusoon Shin, Jiun-Tai Chen, Duyeol Ryu, Amanda Leach, Pappannan Thiyagarajan, Thomas Russell So far thin film has been the main repertoire of study to understand the structure of chained molecules in confined environment. Recently, we have explored the physical properties of chained molecules confined in nanopores that have different dimensionality from thin films. We used anodized aluminum oxide membrane with hexagonally packed, regular-sized nanopores with the pore diameter of 15 nm. Via strong capillary action, we have successfully filled different size polystyrene (molecular weight range 20,000 to 3,000,000) in these membranes. In order to characterize the conformation and the dynamics of polystyrene chain in the anodized aluminum oxide nanopores, we are doing neutron and light scattering etc., and they will be discussed on the presentation. [Preview Abstract] |
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K1.00138: Thermal Analysis, X-ray and Electron Diffraction Studies on Crystalline Phase Transitions in Solvent-Treated Poly(hexamethylene terephthalate) Ming-Chien Wu, Eamor M. Woo, Taiyo Yoshioka, Masaki Tsuji Crystal polymorphism, transformation, and morphologies in poly(hexamethylene terephthalate) (PHT) samples produced by solvent-casting from chloroform were examined by using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and in-situ transmission electron microscopy (TEM). Solvent-induced crystallization of PHT at room temperature yielded an initial crystal of $\gamma $-form, as confirmed by WAXD. Upon DSC scanning, the original $\gamma $-form in PHT exhibited three endothermic peaks, whose origins and association were carefully analyzed. The $\gamma $-form could be transformed to $\beta $-form at 125$^{\circ}$C via a solid-to-solid transformation mechanism. In addition, WAXD showed that $\gamma $- and $\beta $-forms co-existed in the temperature range of 100-125$^{\circ}$C. These mixed crystal-forms were further identified using TEM, and the selected-area electron diffraction patterns revealed that both $\gamma $- and $\beta $-form crystals co-existed and were packed within the same spherulite. Evidence of solid-solid transformation from $\gamma $-form to $\beta $-form in PHT was presented and discussed. [Preview Abstract] |
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K1.00139: Electrospinning of nanocomposite fibers Vahik Krikorian, Darrin Pochan Physical properties of a novel biocompatible nanocomposite fiber are investigated. The fibers are fabricated by incorporation of organically modified clay in a fiber electrospinning process. Commercially available Montmorillonite type organoclays with different extent of miscibility with the polymer matrix are employed to study the effect of organic modifier/matrix interactions. The nanocomposite fibers are prepared by electrospinning a suspension of organoclay/dichloromethane with poly(L-lactic acid), PLLA, a widely used biodegradable synthetic polyester. Effect of clay incorporation on fiber diameter, crystallinity and mechanical properties are studied. A high degree of birefringence in polarized light microscopy suggested that the polymer chains in as-spun fibers are highly aligned. However, wide angle x-ray scattering (WAXS) data revealed no crystalline peaks in as-spun fibers. Annealing the samples above the glass transition temperature induces high degree of crystallinity. Based on Scanning electron microscopy (SEM), spun fibers are highly porous, which may be beneficial in biomedical applications, membranes, and reinforcement matrices. Transmission electron microscopy (TEM) data show the ordering of silicate platelets along the fiber axis, consistent with the d-spacings obtained from WAXS. Cold crystallization behavior of as spun nanofibers studied via \textit{in-situ} Fourier Transform Infrared spectroscopy (FTIR) will also be presented. [Preview Abstract] |
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K1.00140: Can short alkyl chain fold in lamellar crystals? Jianjun Miao, Li Cui, Lei Zhu, Igors Sics, Benjamin Hsiao By coupling a hydroxyl-terminated polyethylene-\textit{block-} poly(ethylene oxide) (PE-$b$-PEO or EEO) diblock oligomer and isocyanatopropyldimethylsilylcyclopentyl-polyhedral oligomeric silsesquioxane (CP-POSS), an asymmetric ABC coil-coil-sphere triblock oligomer was successfully synthesized. The structure and morphology of the supramolecular self-assembly in bulk EEO-POSS was studied by differential scanning calorimetry (DSC), synchrotron small- and wide-angle X- ray scatterings (SAXS and WAXD) and transmission electron microscopy. The melting temperature of the POSS in triblock copolymer was at ca.156\r{ }C as determined by DSC. The POSS crystallized into ABCA four-layer trigonal structure and sandwiched between the amorphous EEO layers. Below 100\r{ }C the PE block crystallized into two types of crystals, namely, extended and once-folded chain crystals with overall d-spacing of 14.0nm and 10.9nm, respectively. During a cooling process, the once folded-chain grew first and then extended chain grew at higher supercoolings. In a heating process, extended chain first melted and once folded chain disappeared at higher temperature. [Preview Abstract] |
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K1.00141: Stereocomplex Formation in Racemic Chiral Polylactide Block Copolymers Lu Sun, Lei Zhu Stereocomplexes can be formed for chiral polymers, $e.g$., isotactic and syndiotactic poly(methyl methacrylate) (PMMA) and poly(L- lactide) (PLLA)/poly(D-lactide) (PDLA). Stereocomplexes in racemic chiral polylactide (PLA) block copolymers have not been widely studied. In this work, we successfully synthesized PLLA and PDLA containing block copolymers using stannous octoate as catalyst for ring opening polymerization from hydroxyl-terminated oligomers. After fractionation by preparative size-exclusion chromatograph, narrow polydispersity samples (Mw/Mn = 1.1$\sim $1.2) were obtained. Stereocomplex of PLA block copolymer blends was prepared by growing stereocomplex crystals either in solution or from the melt. The formation of stereocomplexes was confirmed by differential scanning calorimetry and wide-angle X-ray diffraction (WAXD). The evolution kinetics of stereocomplexes in the bulk was investigated by time-resolved synchrotron small-angle X-ray scattering (SAXS) and WAXD. The morphology of these stereocomplex crystals was studied by transmission electron microscopy. [Preview Abstract] |
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K1.00142: Single Crystal Engineering of Diblock Copolymer Brushes Huiming Xiong, Joseph X. Zheng, Stephen Z.D. Cheng, Ya Guo, Roderic P. Quirk, Bernard Lotz In the past two decades, research of polymer brushes have been substantially progressed not only due to scientific interest but also because of its potential application. Usually, physical absorption, ``grafting to'' and ``grafting from'' methods have been used to fabricate them on solid substrate. During these processes, how to achieve uniform tethering density and narrow molecular weight distribution of the tethered chains are always challenges for the experimental efforts. Recently we have proposed a novel method by using amorphous-crystalline block copolymers. Here I would like to report another system of triblock copolymer with one crystalline block at one end, PMMA-PS-PLLA. By using self-seeding technique, we can create tethered diblock copolymer PMMA-PS brushes on the basal planes of PLLA single crystal. The morphology changes of the diblock copolymer brushes with treatments by different solvents and tethering densities which can be precisely controlled by changing crystallization temperature and quality of solvents have been studied. Furthermore, the surface topologies can be enhanced by etching PMMA blocks away using UV light. [Preview Abstract] |
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K1.00143: Spectroscopic analysis of poly(lactic acid) crystals and their formation Kaoru Aou, Shuhui Kang, Shaw Ling Hsu Vibrational spectroscopic analyses (both experimental and normal coordinate analysis) have been used to analyze the various crystalline forms (stereocomplex and alpha crystal) associated with poly(lactic acid). Spectroscopic features reflecting the different symmetry species in chain conformation and interchain packing have been characterized. Normal coordinate analyses for the single chain and crystalline state have been carried out. In addition to the intramolecular bonded and non-bonded interactions, transition dipole coupling of the carbonyl groups have been incorporated. From experimental and symmetry arguments, it is possible to explain the origin of the unexpected features associated with either homopolymer or stereocomplex. It was also possible to clarify during the crystallization process that chain conformation order was established first, followed by packing order. [Preview Abstract] |
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K1.00144: Equilibrium Fold Thickness in Polymer Crystals Buckley Crist, Herve Marand It has been accepted for nearly four decades that the limited thickness of folded chain polymer crystals results from secondary nucleation barriers. In recent years, however, simulations of polymer crystallization indicate that the observed crystal thickness is a consequence of free energy minimization, not kinetic pathways. An analysis in terms of classical (homogeneous) nucleation theory indeed gives a column-like equilibrium crystal shape (thickness/width $>$ 1) that results from anisotropic surface energies. All dimensions increase commensurately as the crystal grows, with the equilibrium thickness approaching the extended chain length when an unlimited number of chains are available to crystallize. Beyond a certain point in the growth process, however, one expects nucleation barriers to favor less thick crystals that have non-equilibrium, plate-like shapes. For simulations done with a limited number of chains the equilibrium crystal thickness is observed because the crystal is small. But one should not conclude that equilibrium dictates the dimensions at later stages of crystal growth. [Preview Abstract] |
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K1.00145: Crystallization Behavior of Inter-Chain H-Linked Isotactic Poly(propylenes) from their Quiescent Melts Anindya Ghosal, Rufina Alamo Copolymerization of propylene with 1,9 decadiene using an isospecific metallocene catalyst leads to isotactic poly(propylenes) with H-type intermolecular linkages and enhanced melt strength. In this work we discuss the effect of very small concentrations of diene (0 - 400 ppm) on the crystallization behavior of poly(propylenes) from their quiescent melts. The H-linkages shift the high end of the molecular weight distribution to higher values leading to a significant enhancement of the primary nucleation density, as a consequence, the overall crystallization rates increase up to 25 times with respect to the homopolymer. However, except for a small molecular mass effect, the spherulitic linear growth rates that are led by secondary nucleation, are basically unaffected by the presence of the diene, reflecting the small changes in the overall fractional content of defects induced by copolymerization at these levels. In addition, structural changes caused by the H-linkages led to 10 --15 {\%} higher contents of gamma crystals at any isothermal crystallization temperature studied. This increase cannot be associated with the nucleating activity of the diene because iPPs with common nucleating agents display the same content of gamma crystals as the non-nucleated material. [Preview Abstract] |
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K1.00146: Induced PEO Crystal Orientation within the Inversed Cylindrical Morphology of PEO-b-PS Block Copolymer Ping Huang, Stephen Z. D. Cheng, Ya Guo, Roderic P. Quirk, Benjamin S. Hsiao, Carlos A. Avila-Orta, Igors Sics A poly(ethylene oxide)-\textit{block}-polystyrene (PEO-$b$-PS) diblock copolymer with a $M_{n}$ for the PS and PEO blocks being 7.7k and 20.1k g/mol, respectively, was studied by small and wide angle X-ray scattering. An inversed cylinder phase morphology was observed with the PS cylinders hexagonally distributed within the PEO matrix. Since the $T_{g}$ of the PS blocks ($\sim $30 \r{ }C) is lower than the $T_{m}$ of PEO crystal ($\sim $ 62 \r{ }C), the d-spacing of the inversed cylinder phase increased with increasing temperatures greater than 30 $^{o}$C. In the inversed cylinder morphology, the PEO is the major phase, and PS cylinders cannot confine the crystallization of PEO. However, $T_{c}$ dependent PEO crystal orientation changes are induced by the PS cylinders. Furthermore, different from the one-dimensionally preferred crystal orientations when $T_{c}$ is lower than 30 $^{o}$C, the PEO crystals are preferentially oriented in two dimensions when the $T_{c}$ is higher than 30 $^{o}$C in the inversed cylindrical morphology. [Preview Abstract] |
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K1.00147: SANS Study of Polyethylene Crystallization from Solution Howard Wang, Boualem Hammouda Crystallization of low molecular weight polyethylene from solution has been studied using small angle neutron scattering. The primary focus is on concentrated solutions that resemble some features of polymer melts. Both the static structure and kinetics of structure evolution are illustrated. In the limit of current detection sensitivity and spectrum resolution, the high-Q cut-off during the initial crystal growth indicates the length scale of the lamellar thickness. This observation is discussed in the light of the critical nucleus dimension at the early stage of crystallization. [Preview Abstract] |
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K1.00148: Structure and Morphology of PEO-b-PLLA Diblock Copolymer Single Crystal Lingyu Li, Kishore Tenneti, Christopher Li Poly (L-lactide) (PLLA) is an important biodegradable synthetic polymer of interest for medical applications such as controlled drug delivery, resorbable sutures, medical implants, and scaffolds for tissue engineering. Combining PLLA with Poly (ethylene oxide) (PEO) to form a block copolymer PEO-b-PLLA has attracted the interests of material scientists because modifications of physical and chemical properties lead to an accelerated biodegradability. Generally, the rate of degradation strongly depends on the solid state structure of the material therefore clear understanding of crystallization behavior of PEO-b-PLLA is important. Crystallization of PEO-b-PLLA primarily depends on crystallization temperature (Tc). Solution cast thin film crystallization method was used to obtain the PEO-b-PLLA single crystals. At temperatures above Tm of PEO and below that of PLLA, PLLA crystallizes and forms lozenge-shaped single crystal .When cooled to room temperature, PEO begins to crystallize and form fractal-like single crystal on the top of already formed PLLA crystals. However, at temperatures below Tm of PEO, only the fractal-like PEO single crystals were observed. Structure and morphology of this novel single crystal was explored using TEM and AFM. [Preview Abstract] |
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K1.00149: Tracer Diffusion of Polystyrene in Lightly Sulfonated Polystyrene Chen Xu, Nancy Zhou, Wesley Burghardt, Karen Winey, Russell Composto The tracer diffusion coefficient $D^{\ast }$ of deuterated polystyrene (d-PS) (M$_{w}$ = 65,900 g/mol) in lightly sulfonated polystyrene (P(S-SS$_{x}))$ (M$_{w}$ = 65,000 g/mol) as a function of sulfonation mole fraction (x) was measured by forward recoil spectrometry (FRES). For x $<$ 0.7{\%}, d-PS undergoes Fickian diffusion; however, as x increases beyond 0.7{\%}, partial miscibility and eventually immiscibility occurs. For x $<$ 1{\%}, $D^{\ast }$ exponentially decreases with sulfonation, according to $D^{\ast }$ = $D_{o}$ exp(-0.14 N$_{s})$, where N$_{s}$ is the number of sulfuric acid groups per chain. This slowing-down is attributed to an increase in the monomeric friction coefficient which increases with sulfonation. The diffusion mechanism includes both reptation and constraint release. The monomeric friction coefficient for d-PS in P(S-SS$_{x})$ is compared with the coefficient for P(S-SS$_{x})$ measured by rheology. [Preview Abstract] |
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K1.00150: Molecular Dynamics Simulations of Liquid, Gel and Polymer Electrolytes Oleg Borodin, Grant Smith Molecular dynamics simulations have been performed on PEO-based comb-branched polymer electrolytes and liquid electrolytes (ethylene carbonate, propylene carbonate, dimethyl carbonate, oligoethers) doped with LiTFSI and dry and gel single ion conductors with TFSI anions affixed to side chains. Development of the quantum chemistry-based polarizable force field will be discussed. Structural, thermodynamic and transport properties of pure solvents and electrolytes were found in good agreement with experimental data. Transport mechanism, structural and dynamic properties of dry, gel and liquid electrolytes will be discussed. [Preview Abstract] |
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K1.00151: Semicrystalline Ionomer-Metal Carboxylate Composite: Phase Behavior and Mechanical Properties Katsuyuki Wakabayashi, Richard A. Register We have shown previously that the thermal and mechanical behavior of ethylene-methacrylic acid (E-MAA) ionomers can be tuned by the addition of certain magnesium carboxylates, such as magnesium stearate (MgSt). The property modifications result from coassembly of the two components, both co-aggregation of the ionic groups and co-crystallization of the methylene sequences, as revealed by X-ray scattering. When MgSt is replaced by sodium stearate (NaSt), a different suite of mechanical properties is obtained. NaSt, with its high melting and clearing (288~$^{\circ}$C) points, readily crystallizes out of solution in the molten polymer and forms an effective composite upon cooling from a single-phase melt. The NaSt crystals in the composite resemble the rectangular polymorph in pure NaSt, though with some differences in lattice parameters and transition temperatures due to interaction with the acid groups of the copolymer. The different physical properties of the NaSt $\it{vs.}$ MgSt modified ionomers are traced to these microstructural differences, elucidated through a combination of X-ray scattering and microscopy. [Preview Abstract] |
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K1.00152: Effect of pH on Swelling Behavior of Polyelectrolyte Brushes Produced via Surface Confined Atom Transfer Radical Polymerization. Amit Sankhe, Scott Husson, Michael Kilbey Surface-tethered polyelectrolyte brushes comprised of poly (itaconic acid) (PIA) and poly(methacrylic acid) (PMAA) were grown using surface-confined atom transfer radical polymerization (ATRP). The surface- tethered initiator monolayer was formed by self-assembling 2-bromoisobutyryl bromide terminated thiol molecules on gold coated silicon substrates. This polymerization initiator molecule and a copper-based organometallic catalyst allowed tethered polyelectrolyte chains to be grown via radical polymerization at room temperature in aqueous solutions. The behavior of these polyelectrolyte brushes as a function of pH was studied using a phase modulated ellipsometery. The presentation explains how the brushes are affected by external conditions such as the pH of the contacting solution. As the polymer brushes already exist in the charged state, addition of neutral water or salt solution did not affect the polymer brush height, however a decrease of thickness with pH is found. [Preview Abstract] |
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K1.00153: Can Nanorods Emulsify Immiscible Polymer Blends? Michael J.A. Hore, Mohamed Laradji The addition of nanoscale rods to immiscible binary polymer blends has a pronounced effect on the dynamics of phase separation. The results of computer simulations of the Dissipative Particle Dynamics (DPD) model in three dimensions (3D) indicate that when nanorods prefer one of two components in an immiscible binary polymer blend, the rate of phase separation decreases when the volume fraction of nanorods is increased, or as the aspect ratio (L/D) of the rods is increased. Interestingly, anisotropic nanoparticles have a much more pronounced effect on phase separation dynamics in the system when compared to the effects of spherical nanoparticles, which, generically, do not alter the characteristics of the kinetics in the system. In particular, it may be the case that at high volume fractions -- or alternatively, low volume fractions and large aspect ratio of nanorods -- the system may undergo micro-phase separation only, indicating that the nanorods may be excellent emulsifying agents. [Preview Abstract] |
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K1.00154: Viscoelastic Properties of PBX-9501 by Material Point Method (MPM) Simulation Liping Xue, Oleg Borodin, Grant Smith Viscoelastic properties of polymer bonded explosive material PBX-9501 were studied by Material Point Method (MPM) for the first time. The homogenization problem was studied thoroughly. The material point resolution issue, system size effect and time step problem that related to the MPM simulation were also studied and determined. The simulation results were compared with the experimental data and the third-order upper- and lower bounds. The particle shape and distribution effect, as well as temperature effect to the viscoelastic properties of PBX-9501 were also studied. [Preview Abstract] |
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K1.00155: Conformation and Dynamics of a Flexible Sheet in Solvent Media by Monte Carlo Simulations Ras Pandey, Kelly Anderson, Hendrik Heinz, Barry Farmer Flexibility of the clay sheet is limited even in the ex-foliated state in some solvent media. A coarse grained model is used to investigate dynamics and conformation of a flexible sheet to model such a clay platelet in an effective solvent medium on a cubic lattice of size $L^3$ with lattice constant $a$. The undeformed sheet is described by a square lattice of size $L_s^2$, where, each node of the sheet is represented by the unit cube of the cubic lattice and $2a$ is the minimum distance between the nearest neighbor nodes to incorporate the excluded volume constraints. Additionally, each node interacts with neighboring nodes and solvent (empty) sites within a range $r_i$. Each node execute their stochastic motion with the Metropolis algorithm subject to bond length fluctuation and excluded volume constraints. Mean square displacements of the center node and that of its center of mass are investigated as a function of time step for a set of these parameters. The radius of gyration ($R_g$) is also examined concurrently to understand its relaxation. Multi-scale segmental dynamics of the sheet is studied by identifying the power-law dependence in various time regimes. Relaxation of $R_g$ and its dependence of temperature are planned to be discussed. [Preview Abstract] |
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K1.00156: Block Copolymer Surfactancy: Swollen Micelles and Interfacial Tensions in Immiscible A/B Blends with AB Copolymer Kwanho Chang, David Morse We consider the interfacial tension between immiscible A and B homopolymers mixed with an AB diblock copolymer, in which the copolymer forms micelles in the A-rich phase, and in which the micelles generally swell with B homopolymer in the micelle core. We have carried out self-consistent field theory (SCFT) calculations of the free energy ofswollen micelles and of the macroscopic interface between A- and B-rich phases, while equating the chemical potentials used both calculations. Nearly balanced copolymer surfactants, which form saturated interfaces with small spontaneous curvatures, tend to form highly swollen micelles, and also lead to small macroscopic interfacial tension. Previous theoretical analyses of the macroscopic interfacial tension obtained when micelles are present have completely neglected swellingof the micelles, and made some qualitative errors as a result. The relationship between interfacial tension and micelle structure is most easily understood within the context of the Helfrich theory of interfacial bending elasticity, with bending elastic parameters obtained from SCFT. [Preview Abstract] |
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K1.00157: Watching Molecules Near Surfaces: Vibrational Spectroscopy in a Confined Geometry Shan Jiang, Jeff Turner, Sung Chul Bae, Malgorzata Graca, Steve Granick It is a long standing interest to know the structure of liquid near a surface. Seeking more direct information than available from force measurements, and more time-resolved information than available from scattering measurements, we have developed an experimental platform that combines the Surface Forces Apparatus (SFA) with in situ vibrational spectroscopy. Using confocal Raman and surface enhanced Raman (SERS) spectroscopy, new direct information has been obtained about how polymers align in molecularly-thin films, when confined to thickness less than the unperturbed radius of gyration. In another set of experiments, we use nonlinear broadband sum frequency generation (SFG) spectroscopy to probe the structure of aqueous systems near hydrophobic and electrochemical surfaces, without background from the bulk. [Preview Abstract] |
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K1.00158: Water at a Hydrophobic Surface Wina Tjen, Adel\'e Poynor, Steve Granick It has been proposed that a low-density water layer forms at the hydrophobic/water interface, yet the evidence is mixed. We have investigated this using phase-modulated ellipsometry and many experimental conditions. Using water of various compositions and a wide assortment of hydrophobic surfaces prepared by different methods, we find that the proposed effect under some circumstances, and fail to find it under other circumstances. This provides a pleasing resolution to controversies in the literature. [Preview Abstract] |
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K1.00159: Optical interferometry and refractive index measurement at sub-angstrom resolution Minsu Kim In this laboratory, the confinement of fluids between atomically-smooth crystals of muscovite mica is presently being combined with single-molecule fluorescence spectroscopy and high-resolution confocal Raman spectroscopy. Furthermore, the traditional methods of optical interferometry can be enhanced by using a high-resolution CCD camera to analyze many more interference fringes than is customary using the surface forces technique, thereby achieving a resolution an order of magnitude better than the customary 1-4 angstroms. This allows us to consider the molecular alignment of chain fluids through measurement of the local refractive index. Concerning aqueous salt solutions, it allows us to consider the local refractive index within the Stern layer itself. Different findings are obtained depending on the method of mica cleavage. This provides a pleasing resolution to some inconsistencies in prior literature. [Preview Abstract] |
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K1.00160: Spin-orbit effects on reflectance anisotropy spectroscopy of aclean CdTe(001) surface Ra\'{u}l A. V\'{a}zquez-Nava, B.S. Mendoza, N. Arzate The spectroscopical reflectance anisotropy (RA) response of a clean (001) surface of CdTe, which exhibits a $c(2 \times 2)$ surface reconstruction, is studied using a microscopic formulation based on a semi-empirical tight binding approach (SETB) which includes the spin-orbit (SO) interaction. Following Ref. 1, we apply an unitary transformation to the usual SETB $sp^3s^*$ basis to describe the electronic states in terms of a set of atomic states which are eigenstates of the total angular momentum (TAM). These states are better suited to treat the SO interaction in this model, and their use in the computation of the RA signal is straightforward [1]. We show how the RA changes when SO is taken into account and compare our theoretical results with experimental measurements of Ref. 2. [1] R.A. V\'azquez-Nava, B.S. Mendoza and C. Castillo, Phys. Rev. B {\bf 70}, 165306 (2004). [2] J. R. Molina and R. Espinosa-Luna, J. Phys. D: Appl. Phys. (2004), accepted. [Preview Abstract] |
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K1.00161: Formation and properties of silicon elastomer-based responsive surfaces Julie Crowe, Kirill Efimenko, Jan Genzer, Dwight Schwark Responsive surfaces facilitate prompt response to an external stimulus, such as electrical, chemical, or mechanical; the ultimate responsive surface is one that instantaneously responds to its environment with a measurable property change. We report on forming responsive surfaces from model silicon elastomer (SE) networks containing vinyl functionalities, which can be chemically or physically modified to attach various functional groups. Rapid responsiveness of such SE surfaces stems from the cooperative effects due to high flexibility of the siloxane backbone and the enthalpic interactions of the SE functionalized surface with the outside medium. We present several examples of responsive SE materials, including those in which the vinyl bond conversion has been achieved by chemical grafting of alkane chains and carboxylic acids. The responsiveness of such materials is tested with respect to the nature of the contact medium (hydrophilic vs. hydrophobic) and the temperature. We also discuss the reversibility of these phenomena. [Preview Abstract] |
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K1.00162: Scaling roughness and transport properties correlation in manganite thin films Juan Ram\'irez, Maria Elena G\'omez, Wilson L\'opera, Pedro Prieto A scaling roughness study was done on digitized Atomic Force Microscope (AFM) images of La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ thin films grown on (100) oriented SrTiO$_{3}$ substrates. The films were grown via sputtering technique at high oxygen pressures (mbar) and at substrate temperatures of 850 $^{\circ}$C. The films were characterized by resistivity measurements to determine the Curie temperature by the transition temperature from insulating to metallic phase. From digitized AFM-images and by using a specific self-designed algorithm, we statistically study the scaling roughness properties. We obtain quantitative values for the roughness parameters: interface width ($\sigma )$, correlation length ($\xi )$, and roughness exponent ($\alpha )$. The calculated $\alpha $-values are 0.85 $\pm $ 0.05, indicating a highly oriented growth mechanism. We analyzed the dependence of layer thickness (d) and image size (D) with the parameters describing roughness and founded that Curie-Temperature is correlated with the lateral correlation length whereas there is no correlation with the saturation roughness. [Preview Abstract] |
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K1.00163: Positron annihilation induced Auger electron spectroscopic studies of oxide surfaces Manori Nadesalingam, J.L. Fry, N. Fazleev, A.H. Weiss Defects on oxide surfaces are well known to play a key role in catalysis. TiO$_{2}$, MgO, SiO$_{2}$ surfaces were investigated using Time-Of-Flight Positron induced Auger Electron Spectroscopy (TOF-PAES). Previous work in bulk materials has demonstrated that positrons are particularly sensitive to charged defects. In PAES energetic electron emission results from Auger transitions initiated by annihilation of core electrons with positrons trapped in an image-potential well at the surface. Annealed samples in O$_{2}$ environment show a strong Auger peak of Oxygen. The implication of these results will be discussed [Preview Abstract] |
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K1.00164: Formation and applications of multifunctional polymer brush gradients Michael Tomlinson, Rajendra Bhat, Jason Stone, Jan Genzer, Tao Wu Surface confined polymer brushes prepared via surface-initiated controlled/”living” radical polymerization represent an effective way of tuning the physico-chemical properties of surfaces. We have recently developed methods leading to assemblies of surface-tethered polymers with continuously varying grafting density, molecular weight, and composition. We will demonstrate how these assembling concepts can be utilized to systematically study the performance of block copolymer polyampholyte brushes, comprising blocks of poly (dimethyl aminoethyl methacrylate) and poly(acrylic acid), on flat substrates. We also introduce a method of forming hydrophobic/hydrophilic polymer gradients by preparing poly (pentafluoro benzoyloxyethyl methacrylate)-block-poly(acrylic acid) polymer brushes. We will discuss how these amphiphilic surface-anchored polymer assemblies can be used to control transport of fluids on surfaces. [Preview Abstract] |
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K1.00165: Drying Mediated Pattern Formation From a Restricted Geometry Jun Xu, Zhiqun Lin There is much interest in causing patterns (of dyes, nanoparticles, or polymers) to emerge spontaneously on surfaces. A main characteristic pattern known as the ``coffee ring'' formed when the contact line of an evaporating drop becomes pinned, ensuring that liquid evaporating from the edge is replenished by liquid from the interior, so that outward flow carries the nonvolatile dispersion to the edge. Here we report the remarkable observation that a complex structure consisting of a periodic family of hundreds of concentric rings with definite spacing can be achieved when solvent evaporates irreversible from a restricted geometry. Each ring is approximately nanometers high and micron wide. The observed micron size rings are governed by the imposed geometry, the solution concentration and the solvent properties. The mechanism, which is believed to be a series of successive pinning and depinning of the contact line as solvent evaporates, will be discussed. This simple yet novel approach affords a means to produce and organize surface patterns in a well-ordered gradient fashion. [Preview Abstract] |
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K1.00166: Roughness in a kinetic film growth of hydrophobic and polar components in aqueous solution - spectroscopic studies and Monte Carlo simulations Otts Daniel, Luis Cueva-Parra, Ras Pandey, Marek Urban Using Monte Carlo simulations, we investigate the film formation from mobile constituents of multi-component mixtures in order to understand the spectroscopic measurements of polyurethane thin film. The mixture consists of water (A), hydrophobic (H) and polar groups represented by particles of appropriate molecular weight and interactions on a discrete lattice. The empty sites of the host lattice matrix are used to describe effective medium for the mobile constituents. Metropolis algorithm is used to move particles. Periodic boundary condition is used along transverse directions with open top for water evaporation and impenetrable adsorbing substrate at the bottom. Stable film morphology emerges as system precipitate, equilibrate, evaporate, and covalent bonds form from the kinetic reactions. Density profile and roughness of the film are examined in detail as a function of temperature and water concentration. Increase in roughness due to phase partitioning on increasing the water concentration is found to be consistent with the experimental observations. [Preview Abstract] |
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K1.00167: Tunneling and fatigue properties of SRO/PZT/Pt structures P. Prieto, A. Cortes, E. Delgado, J. Realpe, W. Lopera Tunneling and fatigue measurements at different temperatures were carried out in capacitor structures based on Pb(Zr$_{0.52}$Ti$_{0.48})$O$_{3}$ (PZT) ferroelectric thin films with bottom electrodes of SrRuO$_{3}$ (SRO) and top electrodes of Platinum (Pt). SRO electrodes were deposited on (100) SrTiO$_{3}$ single crystal substrates using a high oxygen pressure on-axis dc-sputtering technique. PZT films were grown by rf magnetron sputtering in pure oxygen atmosphere. Surface roughness and morphology were studied by atomic force microscopy. Electrical characterization has been realized by P-E hysteresis loops and fatigue measurements. Current-voltage (I-V) characteristics showed a slight hysteretic behavior while the bias voltage dependence of the dynamic conductance measurements presented a parabolic characteristic indicating electron tunneling. I-V curves and conductance measurements also show an asymmetric shape that can be explained by the different work functions at the interfaces. A Brinkman fit of the normalized conductance curves as function of the temperature gives barrier thicknesses below 2 nm. [Preview Abstract] |
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K1.00168: Determination of Physical Aging in Thin Polymer Films via Fluorescence: Effects of Confinement and Attractive vs. Neutral Polymer-Substrate Interactions Rodney D. Priestley, Linda J. Broadbelt, John M. Torkelson The effect of nanoconfinement on physical aging of polystyrene (PS) and poly(methyl methacrylate) (PMMA) is studied by fluorescence. Rotor dyes are used as probes ($<$ 0.2wt{\%}) dispersed in the polymer or as labels covalently attached ($<$ 1 label/400 repeat units) to the polymer. Fluorescence intensity increases as local specific free volume/local mobility surrounding the dye decreases, exhibiting a nearly linear change with logarithmic aging time. Thin (500-nm-thick) and ultrathin (20-nm-thick) films supported on silica are annealed above and below the bulk glass transition temperature (Tg,bulk). With PMMA, which has attractive polymer-substrate interactions, ultrathin films exhibit aging at Tg,bulk + 7 K while thin films do not. With PS, which lacks polymer-substrate interactions, thin films exhibit physical aging at Tg,bulk - 10 K while ultrathin films do not. These results are explained by an enhancement (reduction) in Tg of the ultrathin PMMA (PS) film, relative to bulk. Structural recovery is dramatically reduced in ultrathin PMMA films, indicating that attractive interactions significantly affect structural recovery near the substrate. [Preview Abstract] |
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K1.00169: Influence of curing temperature on properties of GPS adhesion promoter layers Elisabeth Pavlovic, Edward J. Kramer, Shigeo Nakamura, Michael Kent, Hyun Yim Adhesion promoter layers of glycidoxypropyltrimethoxysilane (GPS) were cured at various temperatures ranging from room temperature to 250 degC. The degree of cross-linking was investigated using infrared spectroscopy combined with deuterium/hydrogen exchange. The swelling of the GPS layers by solvent (d-nitrobenzene) and water was investigated using x-ray and neutron reflection. Curing temperatures higher than 90 degC produced highly cross-linked GPS layers, with a loss of epoxy groups. A complete cross-linking was reached at a curing temperature of 250 degC. We expect that the cross-link density of the GPS layer, and subsequently its swelling ability, as well as its remaining epoxy functionality should have a major impact on the fracture energy of the interface with epoxy. These questions are investigated by asymmetric double cantilever beam fracture experiments on interfaces between GPS layers cured at these different temperatures on silicon wafers and epoxy resin beams. [Preview Abstract] |
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K1.00170: Phase Transitions in Triblock Copolymer Thin Films A. Hexemer, G.E. Stein, V. Khanna, E.J. Kramer, X. Li, J. Wang Using scanning force microscopy, X-ray reflectivity and grazing incidence small angle X-ray scattering we investigate a phase transition in a thin film poly(styrene-b-ethylene-r-butylene-b-styrene) SEBS triblock copolymer (M$_w$ = 54kg/mol; f$_{PS}$ = 0.24). It has been shown for similar systems that the morphology of thin triblock copolymers is dependent on film thickness.\footnote{A. Knoll et al. Phys. Rev. Lett. 89, 035501 (2002)} A 37nm thick SEBS film on SiO$_2$ annealed at 180$^o$C shows a perforated lamella (PL) structure of PS embedded in an EB matrix with no islands. We investigate the phase transition of the PL into a cylindrical structure by further annealing the film at a higher temperature of 210$^o$C. In contrast to the flat PL film, the cylindrical phase shows islands of cylinders surrounded by a cylindrical monolayer. After a third annealing step at 180$^o$C, the islands return the PL morphology while the surrounding monolayer remains in the cylindrical morphology. [Preview Abstract] |
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K1.00171: Self-folding membranes Galen T. Pickett I consider an elastic membrane treated so that one side is solvophobic, and the other is solvophillic. The membrane crumples itself into a complex, compact form under specific, strong solvent interactions. The form of the collapsed membrane is strongly affected by scoring a tessellating network of ordered creases into the fabric of the membrane. The ordered tessellated collapsed structure exhibits several interesting mechanical properties, such as a traction-induced spontaneous curvature and a negative Poisson ratio. [Preview Abstract] |
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K1.00172: Chemical Functionalization of Silicone, Quartz and Mica: Soldiers Report from the Trenches Yan Yu, Liang Hong, Adele' Poynor, Steve Granick Methods of organic surface functionalization are widely reported yet in implementing them, different research groups encounter varying degrees of success. This laboratory has been involved in subsequent physical studies that are exceptionally sensitive to defects in organic monolayers: (a) single-molecule fluorescence studies of polymer diffusion, where chemical defects provide `hot spots' to which polymers adsorb too strongly; (b) surface forces studies of molecularly-thin films, where topographical defects prevent atomically-smooth mica surfaces from contact; (c) ellipsometry experiments of how water dewets hydrophobic surfaces, where topographical surface heterogeneity appears to negate this effect. This has also motivated us to perform systematic studies concerning monolayers formed by different chemical reactions. We have explored in depth which experimental details are most critical and the criteria by which `good' monolayers can be distinguished from `bad'. [Preview Abstract] |
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K1.00173: Autophobic Dewetting of PS/dPS-\textit{b}-PVP Blend Thin Films Huiman Kang, Bumjoon Kim, Seung-Heon Lee, Kookheon Char, Edward J. Kramer The stability of polystyrene (PS) films on silicon oxide surface was improved by adding a symmetric deuterium-labeled polystyrene- block-poly(2-vinylpyridine) (dPS-\textit{b}-PVP). Above 3 wt\% loading of dPS-\textit{b}-PVP in the blend thin films, autophobic dewetting, however, has occurred. The autophobic dewetting of PS thin films was investigated by optical microscopy (OM) and atomic force microscopy (AFM). Since the PVP block interacts with the silicon oxide substrate more favorably, the dPS-\textit{b}-PVP is segregated at the interface between the PS film and the silicon oxide layer. Dynamic secondary ion mass spectrometry is also employed to obtain the normalized interfacial excess ($\xi$ = z$^{*}$$_{PS}$/R$_{g,PS} $) of the segregated deuterium-labeled block copolymer, yielding a quantitative criterion for the chain stretching of polymer brushes. The value of $\xi$ initially increases with increase in the dPS-\textit{b}-PVP content of the blend films and it levels off at 0.365 above 3 wt\% of block copolymer. The segregation of dPS-\textit{b}-PVP toward the silicon oxide substrate causes the entropy difference between PS homopolymers and dPS blocks in the dPS-\textit{b}-PVP, which eventually leads to the autophobic dewetting of the PS films from the segregated dPS-\textit{b}-PVP layer. [Preview Abstract] |
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K1.00174: Confinement and the Glass Transition Temperature in Supported Polymer Films: Molecular Weight, Repeat Unit Modification, and Cooperativity Length Scale Investigations Manish K. Mundra, John M. Torkelson It is well known that the glass transition temperatures, Tgs, of supported polystyrene (PS) films decrease dramatically with decreasing film thickness below 60-80 nm. However, a detailed understanding of the cause of this effect is lacking. We have investigated the impact of several parameters, including polymer molecular weight (MW), repeat unit structure, and the length scale of cooperatively rearranging regions in bulk. There is no significant effect of PS MW on the Tg-confinement effect over a range of 5,000 to 3,000,000 g/mol. In contrast, the strength of the Tg reduction and the onset of the confinement effect increase dramatically upon changing the polymer from PS to poly(4-tert-butylstyrene) (PTBS), with PTBS exhibiting a Tg reduction relative to bulk at a thickness of 300-400 nm. PTBS also shows a Tg reduction relative to bulk of 47 K in a 21-nm-thick film, more than twice that observed in a PS film of identical thickness. Characterization of the length scale of cooperatively rearranging regions has been done by differential scanning calorimetry but reveals at best a limited correlation with the confinement effect. [Preview Abstract] |
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K1.00175: Layered glass transition temperature in polymer thin films Haobin Luo, Dilip Gersappe We use a nanofiller particle as a probe in Molecular Dynamics simulations of the glass transition temperature in thin polymer films. We find that the supported surface effect persists for many Rg's into the polymer film, while the free surface effect dissappears within a few monomer distances. We also report on calculations in which the probe particle is moved in an oscillatory manner. [Preview Abstract] |
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K1.00176: Polyelectrolyte Spin-Assembly: Effect of Ionic Strength and Spinning Rate on the Growth of Multilayered Thin Films Christophe Lefaux, Pritesh Patel, Junhwan Jeon, Andrey Dobrynin, Patrick Mather Polyelectrolyte spin assembly (PSA) of multilayers is a sequential process featuring adsorption of oppositely charged polyelectrolytes from dilute solutions undergoing spin-coating flow. We investigated the dependence of multilayer film build-up of poly(sodium-4-styrene sulfonate) and poly(allylamine hydrochloride) on solution ionic strength and spin rate. We observed that at fixed spin rate the PSA growth rate shows a non-monotonic dependence on salt concentration. It first increases and then decreases with increasing the solution ionic strength. This observation is a manifestation of two competing mechanisms driven by electrostatic interactions and shear flow, which control film build up. This non-monotonic behavior is explained in the framework of a Flory-like theory of multilayer formation from polyelectrolyte solution under shear flow. We performed molecular dynamics simulations to further elucidate the factors controlling the film structure. [Preview Abstract] |
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K1.00177: Resonant soft x-ray reflectivity of polymer bilayers Cheng Wang, Tohru Araki, Shane Harton, Jeff Kortright, Gary Mitchell, Harald Ade Hard x-ray reflectivity is a very successful and common tool to measure thin film thickness, roughness, and interfacial widths in bilayers. However, hard x-rays have low or even insufficient contrast for a number of polymer species. For this reason, neutron reflectivity is often used in which the contrast can be enhanced enormously by deuterating one species. Alternatively, x-ray reflectivity capabilities could be extended by enhancing the contrast between layers through resonant methods near absorption edges. We are in the process of evaluating if soft x- ray resonant reflectivity is a valuable complement to hard x-ray and neutron reflectivity. Measurements were performed on PS/PMMA and PS/P2VP bilayer structures near the carbon-1s and nitrogen-1s or oxygen 1s K absorption edges on samples with individual layer thicknesses of 10-40 nm. At these photon energies the scattering factors f$_{1}$ and f$_{2}$ can be varied substantially, amounting to ?turning on and off? PS and PMMA or P2VP selectively. Large differences in reflected intensity have indeed been observed as a function of photon energy in $\theta $-2$\theta $ measurements. We are in the process of characterizing the intrinsic limitation of interfacial width measurements and the limitations on film thickness due the longer wavelength and lower penetration power of soft X-rays. - Data acquired at the X-ray Calibrations and Standards beamline 6.3.2 at the Advanced Light Source at the Berkeley National Laboratory. Research supported by U.S. DoE. [Preview Abstract] |
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K1.00178: Nanoporous Polymer Films via the Self-Assembly of Triblock Copolymers Seung Hyun Kim, Joona Bang, Eric Drockenmuller, Matthew J. Misner, Craig J. Hawker, Thomas P. Russell Diblock copolymer films of polystyrene-$b$-poly(ethylene oxide) (PS-PEO) exhibiting cylindrical microdomains, where PS is the major component, demonstrate a high degree of long range lateral order by solvent casting or solvent annealing. However, to generate nanoporous films of these materials we have incorporated a third, center block that is photodegradable by ultra violet radiation. This provides for a robust route to generating highly ordered, nanoporous arrays polymer films. Triblock copolymers of polystyrene-$b$-poly(methyl methacrylate)-$b$-poly(ethylene oxide) (PS-PMMA-PEO) were synthesized by ATRP. Three different molecular weights and compositions were used, where PS is the major component, and PMMA and PEO are the minor components. After casting and solvent annealing, all three copolymers exhibit similar cylindrical microdomains. Subsequently, PMMA block can be UV degraded and by rinsing in acetic acid and water, only the PS block remains in the nanoporous films. [Preview Abstract] |
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K1.00179: Orientation and Lateral Order in Block Copolymer Thin Films Matthew J. Misner, Seung Hyun Kim, Thomas P. Russell Diblock copolymers self-assemble on length scales of a few tens of nanometers into several morphologies. By solvent casting or solvent annealing thin films of polystyrene-b-poly(ethylene oxide), we have demonstrated methods to produce diblock copolymer films with highly oriented, close-packed arrays of nanoscopic cylindrical domains with a high degree of long-range lateral order with few defects. The solvent imparts a high degree of mobility in the microphase-separated copolymer that enables a rapid removal of defects and a high degree of lateral order. Moreover, lateral confinement by topographically patterned surfaces has been utilized to improve long-range order and direct the grain orientation macroscopically. Additionally, we have observed large effects on microdomain orientation expressed by varying relative humidity and salt concentrations. At very low humidity the copolymer exhibits parallel orientation relative to the substrate and at moderate to high humidity perpendicular orientation is observed. Similar results were observed by increasing the salt concentration on the order of a few ions per chain such that at relatively high salt concentrations perpendicular orientation is observed over a full range of humidity. [Preview Abstract] |
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K1.00180: Dynamics at a Buried Polymer Interface Xuesong Hu, Xuesong Jiao, Zhang Jiang, Suresh Narayanan, Alec Sandy, Sunil Sinha, Laurence Lurio, Jyotsana Lal We present a further development of X-ray Photon Correlation Spectroscopy (XPCS) technique to study polymer interfaces. We have probed capillary wave dynamics not just at a free surface, but also at buried polymer/polymer interface within a bilayer film. The bilayer was chosen so that the critical angle of the top layer is smaller than that of the bottom layer. When X-rays are incident below the critical angle of the top layer, only the structure and dynamics of the top surface are probed. When X-rays are incident above the critical angle of the top layer but below that of the bottom layer, a standing wave is set up. The phase of this standing wave can be adjusted to have a high intensity at the polymer/polymer interface and simultaneously a node at the polymer/air interface. Consequently, one can isolate for the first time the static and dynamic scattering from a single buried layer. Results on a system consisting of a 100nm polystyrene film on top of an 100nm poly(4-bromo styrene) film, supported on a Si substrate will be discussed. [Preview Abstract] |
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K1.00181: Electric Field-Induced Dewetting and Structure Formation in Thin Polymer-Polymer-Air Trilayers K. Amanda Leach, Suresh Gupta, Thomas P. Russell, Michael D. Dickey, C. Grant Willson The size scale of electrohydrodynamic instabilities in thin polymer films can be controlled by manipulating electrostatic pressure and surface tension. It has been shown that electric field-induced fluctuations can also drive polymer-polymer dewetting in trilayers and that concentric structures can be formed, in which the lower polymer layer surrounds the structures in the upper polymer film. We show that an array of encapsulated or ribbed cage-type structures can be formed when the height of the structures is increased. Alternatively, when the upper polymer layer preferentially wets the substrate, the lower layer is encapsulated. These methodologies have the capability to produce arrays of structures, with controlled size scale and spacing, that cannot otherwise be produced. [Preview Abstract] |
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K1.00182: Self assembled monolayers of rigid thiols on Gold Svetlana Stoycheva, Joerg Fick, Alexander Kornviakov, Avi Ulman, Michael Himmelhaus, Michael Grunze Self assembled monolayers (SAM) of $\omega $-functionalized mercaptobiphenyls ($\omega $-MBP) are stable, molecularly engineered ultrathin organic films, which can be used for the design of model surfaces with well-defined chemical and physical properties. For example, the number and distribution of terminal hydroxyl groups can be tailored to study water-surface interactions or the chemisorption of organic molecules from solution. We have prepared and studied SAM of mercaptobiphenyls (MBP), 4-Methyl-4'-MBP, and 4-Hydroxy-4'-MBP on Au (111). The film quality of the SAM with respect to coverage and structure was characterized by contact angle measurements, spectral ellipsometry, infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, and sum frequency generation. By means of a frequency analysis based on ab initio calculations, the different vibrational modes could be assigned and used for the study of chemisorption processes at the surface. [Preview Abstract] |
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K1.00183: Interaction of Self-Assembled Monolayers of Oligo(ethylene glycol)-Terminated Alkanethiols with Water studied by Vibrational Sum Frequency Generation (VSFG) Joerg Fick, Rongyao Wang, Sascha Herrwerth, Wolfgang Eck, Michael Himmelhaus, Michael Grunze Conformational changes in oligo(ethylene glycol)-terminated (OEG) self-assembled monolayers (SAMs) have been a topic of interest in recent years because of their crucial role for the protein repulsion properties of these SAMs. In our study, VSFG was used to investigate the conformational changes of hexa(ethylene glycol) (EG6OMe)-terminated SAMs on gold, when exposing these films successively to liquid water, to the ambient, and to vacuum. The spectra show that the OEG films in water contact are in a disordered, solvated state. This disorder is partially maintained after removing the samples from direct water contact. It is not until evacuation of the samples that the molecules revert to their original, ordered state. Our study is the first to give experimental evidence for the presence of bound water molecules in the SAM, thereby supporting recent ab initio calculations and Monte Carlo simulations. [Preview Abstract] |
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K1.00184: The effect of substrate temperature and annealing time on the morphology of metallophthalocyanine thin films: an AFM study Amos Sharoni, Corneliu Colesniuc, Casey Miller, Ge Liu, Bernd Fruhberger , Ivan K. Schuller Atomic force microscopy (AFM) measurements were preformed on Iron-phthalocyanine (FePc) thin films in order to investigate their topographical features as a function of preparation parameters. FePc was grown on A-plane sapphire in an organic molecular beam epitaxy system with a base pressure of $\sim $ 5x10$^{-10 }$Torr, using a low temperature Knudsen cell. Samples were deposited with different substrate temperatures, ranging from ambient temperature and up to 250 C, and then annealed \textit{in situ} up to 8 hours without modifying the temperature. All other growth parameters were unchanged. Upon increasing temperature, the AFM measurements reveal a transition from a granular morphology, with grain size of $\sim $ 50 nm, to a flat terrace structure with monolayer high steps and clear pinholes. Annealing results in different degrees of morphological reorientation. The size of the pinholes grows with elevating the temperature further, and the FePc film attains rod-like crystal shapes (100 nm wide and 1 micron long), with no preferred direction. Schemes towards obtaining pinhole-free films will be discussed. This work was supported by AFOSR MURI{\#} F49620-02-1-0288. [Preview Abstract] |
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K1.00185: Compatibilization of Immiscible Polymer Blends via Gradient Copolymer Addition during Melt Processing: Stabilization of the Dispersed Phase against Coarsening Jungki Kim, Maisha K. Gray, John M. Torkelson The first compatibilization an immiscible polymer blend by addition of gradient copolymer during melt processing is demonstrated by complete suppression of coarsening during annealing. As compared to block copolymer addition, this strategy has the advantage of much higher theoretical critical micelle concentrations, making gradient copolymer more available to locate at blend interfaces. Compatibilization was achieved in an 80/20wt{\%} polystyrene (PS)/poly(methyl methacrylate) blend upon addition of 5wt{\%} gradient copolymer (59/41mol{\%} styrene (S)/methyl methacrylate (MMA)) made by controlled radical polymerization. In contrast, a blend made with 5wt{\%} of a S/MMA random copolymer of identical overall composition as the gradient copolymer exhibited coarsening as did a blend with no copolymer. The success of this novel compatibilization strategy depends on the gradient copolymer composition and the blend incompatibility. [Preview Abstract] |
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K1.00186: New Strategy for Compatibilization of Immiscible Polymer Blends: Block Copolymer Addition during Solid-State Shear Pulverization Ying Tao, Andrew H. Lebovitz, John M. Torkelson Hundreds of studies have investigated block copolymer addition during melt mixing as a strategy for compatibilizing immiscible blends. While a few small-scale, academic studies have found that compatibilization may be possible by this route, it has not been commercialized due to thermodynamic and kinetic stumbling blocks, e.g. block copolymers have extremely low critical micelle concentrations in homopolymer leaving few free block copolymers to reach interfaces, and block copolymer diffusion in homopolymers in slow. We have used solid-state shear pulverization (SSSP) to mix a commercially available styrene/ethylene-butylene/styrene block copolymer with a polystyrene (PS)/polyethylene (PE) blend. This approach was demonstrated to be effective by comparing the coarsening rate constant, K, for the pure 90/10wt{\%} PS/PE blend, the PS/PE blend melt mixed with 3.5wt{\%} block copolymer, and the PS/PE blend mixed by SSSP with 3.5 or 5wt{\%} block copolymer. The K values decreased as follows: 0.037 0.022, 0.0039, 0.0012 microns$^{3}$/min. When 10wt{\%} block copolymer was mixed via SSSP with an 80/20wt{\%} PS/PE blend, the average dispersed-phase particle size ceased coarsening during annealing when it reached a size of 1.6 microns. [Preview Abstract] |
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K1.00187: Influence of Copolymer Composition on Morphology Development in Blends Jayaraman Krishanmoorthy, Young Gyu Jeong, Tomoko Hashida, Thomas J McCarthy, Shaw Ling Hsu Blends involving a crystallizable polyester [poly(hexamethylene adipate) (PHMA)] with an amorphous polyether has been studied. These systems have hydroxyl end groups which can react with diisocyanate in the presence of nucleophilic catalysts. These functionalized (isocyanate-terminated) polymer blends form the fundamental structure of polyurethanes. These prepolymers can then be cured at a later stage due to ambient water molecules. Thus the migration of water molecules through the morphology formed needs to be understood well. The morphology development (i.e. phase separation occurring concurrently with crystallization) of nascent polymer blends has been characterized. The morphology of functionalized polymers exhibit very different kinetics due to the presence of the small amount of copolymers formed. The ultimate morphology formed can be ``tuned'' by varying blend composition and crystallization conditions. The structures of prepolymers will be reported. The presence of this copolymer can significantly alter the structural transformation in either binary or ternary blends. [Preview Abstract] |
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K1.00188: Porod Scattering Study of Coarsening in Immiscible Polymer Blends Kristin Brinker, Wesley Burghardt Studies of immiscible blend compatibilization often involve laborious microscopy methods to characterize the changes in the droplet size distribution with time in the melt. Here we explore a simple alternative approach based on Porod scattering from the two phase structure. Although micron-sized droplets in immiscible polymer blends are too large to be fully characterized by small-angle x-ray scattering, Porod scattering measurements of the interfacial area combined with knowledge of the blend volume fraction allows determination of an average droplet diameter from a single scattering measurement. This technique is illustrated in experiments monitoring coarsening of droplet sizes in immiscible PS/PMMA blends prepared either by melt blending or by solid-state shear pulverization. [Preview Abstract] |
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K1.00189: Determining the melt miscibility of commercial polyolefin blends by Small-Angle Light Scattering Ping Peng, Yvonne Akpalu The melt miscibility of high-density polyethylene (HDPE) ($M_{w}$ = 52 kg/mol, PDI = 2.9) and linear low-density polyethylenes (LLDPEs) based on homogeneous ethylene-1-butene copolymers (EB0187: 18.7 mol{\%} butene branches, $M_{w}$ = 58.1 kg/mol and EB0059: 5.9 mol{\%} butene branches, $M_{w}$ = 70 kg/mol) is determined by Small-Angle Light Scattering (SALS) under parallel-polarized optical alignment. Blends with branching density differences ($\Delta $y) ranging from 5.9 to 18.7 mol{\%} were studied. For the HDPE/EB0187 blend, ($\Delta $y = 18.7{\%}), cooling and isothermal measurements (2 h) in the melt can be used to determine the miscibility and the phase boundary. When this blend is rapidly cooled room temperature, the resulting SALS patterns show that the size of phase separated domain ($\sim $ 1 $\mu $m) is much smaller than the average spherulite size (68 $\mu $m). These results consist with our microscopic observations and the transmitted light measured simultaneously with the $V_{V}$ scattering in the melt. From the compositions studied, we can conclude that this blend exhibits UCST behavior. For blends with lower $\Delta $y, isothermal measurements provide a sensitive means for determining the melt miscibility. Based on the above observations, we can conclude that SALS can be used to determine the melt miscibility of polyolefin blends. [Preview Abstract] |
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K1.00190: Effect of molecular architecture on the phase diagram of multi-arm acrylate and nematic liquid crystal mixture Scott Meng, Hatice Duran, Thein Kyu The effect of molecular architecture on the phase behavior of a binary mixture was investigated by varying the number of functional arms in one component. A new free energy expression was derived from the first principle to account for the entropic correction, when the molecular topology changes from a linear structure to a star shape. In particular, the mixtures of nematic liquid crystal and multi-arm acrylate were selected as our reference systems, owing to their importance in the fabrication of switchable photonic crystals via photolithography. Theoretical phase diagrams have been calculated based on the combination of Flory-Huggins free energy of isotropic mixing, Maier-Saupe free energy of nematic ordering, and the entropy correction term. Good accordance was found between the model predictions and the cloud point curves. [Preview Abstract] |
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K1.00191: Photopolymerization induced phase ordering in confined regions Thein Kyu, Rushikesh Matkar, Scott Meng, Soojeoung Park, Greg Yandek Phase ordering in confined spaces such as stratified layers or columnar domains is of interest for a variety of optical and electronic applications. We have investigated various possibilities of creating photonic crystal structures using an optical wave interference technique subjected to holographic photopolymerization of acrylate systems blended with nematic liquid crystals or conductive crystalline polymers. Melting point depression is known to occur between acrylate monomers and polyethylene oxide or succinonitrile. During the course of photopolymerization, the increase in molecular weight of acrylate and subsequent network formation have shifted the melt point upward above the reaction temperature, which in turn drives PEO to undergo crystallization; hereafter called polymerization-induced crystallization. A variety of conditions have been investigated in the framework of phase field theory for crystallization by coupling with the extended Flory-Huggins theory for polymer blend demixing. The phenomenon of polymerization-induced crystallization during photolithography has been investigated with emphasis on the effect of intensity gradient on the emerging morphology. Additionally, crystallization in confined regions has also been explored. [Preview Abstract] |
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K1.00192: Diffusion of low molecular polymers through tunable phase separated morphology Jake Ferguson, Shuhui Kang, Kaoru Aou, Shaw Hsu Based on phase diagrams measured for ternary blends, specific phase separated morphologies have been prepared. These morphologies are useful for controlling migration of molecules or low molecular weight polymers. The applications in our laboratory include drug delivery and curing behavior of reactive prepolymers. This talk will focus on blends including crystallizable polyesters, e.g poly(lactic acid), with a number of small molecules or low molecular weight polymers. Generally the mixtures can also include a solvent of various quality. The morphology formed is dependent on the equilibrium phase diagram and kinetics associated with solvent removal. The migration of specific components is highly dependent on chain dynamics. The stability of such morphology as a function of time and temperature is also an important consideration and relates to chain dynamics. Based on our previous studies, we provide an explicit molecular model to explain PLA chain dynamics. The diffusion behavior of low molecular weight polymers is found to depend on both single chain dynamics and macroscopic morphologies. A relationship between the phase separated morphology and their formation to the overall migration dynamics will also be discussed. [Preview Abstract] |
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K1.00193: How Chemical Reactions Take Place in Reactive Ternary Blends Young Gyu Jeong, Tomoko Hashida, Shaw Ling Hsu Reaction mechanism and kinetics of reactive ternary blends, consisting of a crystallizable polyester, an amorphous polyether, and an acrylate with high glass transition temperature, have been investigated using time-resolved Fourier transform infrared spectroscopy. Since the polyester and polyether have reactive isocyanate units, they are able to react with water vapor in the environment. A specially-designed cell was constructed to obtain reaction kinetics for reactive ternary blends with a variety of thickness at different relative humidity and temperatures. The reaction kinetics obtained are significantly slower than expected for a diffusion-limited mechanism of a homogeneous medium, indicating that curing reactions in the thin dimension are primarily controlled by the reaction-limited mechanism. Morphological features (phase-separated structure, degree of crystallinity, and crystalline domain size), which have been characterized by vibrational spectroscopy, optical microscopy, and atomic force microscopy, are dependent on composition, thermal history, and initial phase behavior. Reaction rates were shown to be highly dependent on sample morphology and composition, being faster with smaller phase-separated domains and lower degrees of crystallinity. [Preview Abstract] |
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K1.00194: Nucleation Induced Molecular and Micron-Scale Dual Ordering of Flourinated Dendron Monolayer Farhan Ahmad, Kwanwoo Shin, Dong Ki Yoon, Hae Tae Jung Langmuir monolayer structures of a partially fluorinated dendron molecules is studied at interfaces using Langmuir balance technique, scanning probe microscopy, X ray reflectivity, and X- ray photoelectron spectroscopy. We have achieved simultaneous nano and micro length scale ordering in the Langmuir-Blodgett films of dendron molecules. The ordering process was observed to be initiated by the nucleation process on the water surface, which resulted in hexagonally arranged microsized domains. Further, characterization of these micro domains by SPM showed the perfect hexagonal molecular chain arrangement. Furthermore, molecular thickness and orientation was cross checked using XPS and X-ray reflectivity. The immobilization of bio- macromolecules on the engineered thin films is limited due to the defined molecular architecture and their spacing till now. Now, these ultra thin films of dendrons can provide a control over the surface density and molecular spacing, without varying the molecular architecture of the dendron molecules. [Preview Abstract] |
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K1.00195: Determination of the molecular parameters and studies of the aggregation behavior of polybenzimidazole in solution. Christopher Shogbon, Jean-Luc Brousseau, Haifeng Zhang, Brian Benicewicz, Yvonne Akpalu Static light scattering using a seven angle BI-MwA instrument has been used for the first time to obtain the molecular parameters as well as study the aggregation behavior of previously synthesized $m$-PBI samples in DMAc/LiCl solvent mixtures. Depolarization ratios for the samples were obtained using a BI-200SM goniometer system. Molecular parameters obtained were $m-$PBI(118) M$_{w}$: 179,300 g/mol; R$_{g}$: 36.4 nm; A$_{2}$: 0.0016 cm$^{3}$mol/g$^{2}$ and $m-$PBI(159) M$_{w}$: 206,500 g/mol; R$_{g}$: 46.6 nm; A$_{2}$: 0.0020 cm$^{3}$mol/g$^{2}$. The aggregation behavior of $m$-PBI(159) was studied as a function of salt concentration (0 to 7 {\%}) at a fixed polymer concentration of 0.3 mg/mL. With increasing salt concentration, R$_{g}$ decreases initially and goes through three maxima with a minimum R$_{g}$ of 31.0 nm at 4 {\%} LiCl. This result parallels the previously observed aggregation behavior of corn amylase in DMAc/LiCl. Thus our results suggest a general mechanism for the aggregation of semiflexible polymers which can be controlled by varying polymer or salt concentration. [Preview Abstract] |
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