Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session W25: Polymer Melts and Solutions |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Kyusoon Shin, Seoul National University Room: Colorado Convention Center 203 |
Thursday, March 8, 2007 2:30PM - 2:42PM |
W25.00001: Dynamic Light Scattering Studies of Light Absorbing Solutions Thomas Seery, Maria DeMesa Dynamic light scattering (DLS) is a powerful tool for characterizing polymers in solution and is especially useful for observing polymer-polymer interactions. The ease of sample preparation and \textit{in situ}, non-perturbative nature are powerful advantages to a technique capable of probing 9 decades of dynamic behavior in a single measurement. However, application of scattering methods is greatly restricted when the sample absorbs light at the scattering wavelength. Absorption leads to sample heating, deflection of the probing laser beam and convection in otherwise quiescent solutions. Convective flow contributes an oscillating component to the smoothly decaying correlation functions that are normally obtained from DLS. These effects have limited the application of DLS in studies of conducting polymers, nanotubes, heme containing proteins and noble metal nanoparticles. We have a theoretical model that provides broad agreement with the behavior observed for light absorbing solutions of polyaniline, carbon nanotubes, cytochrome-C and silica coated gold nanoparticles and we have a physical model system that produces oscillating correlation functions using non-absorbing scatterers and absorbing dyes. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W25.00002: Small Angle Neutron Scattering Study of Oligo(ethylene glycol) Grafted Polystyrene in Aqueous Solutions G. Cheng, Y.B. Melnichenko, G.D. Wignall, F. Hua, K. Hong, P.F. Britt, J.W. Mays Considerable efforts have been made to understand the cooperative interactions of living organisms using synthetic polymers. A typical example is poly(ethylene glycol) (PEG) in water where two fundamental interactions (hydrophobic and hydrophilic) are present and control its conformation and phase behavior. By manipulation these interactions in a systematic way, one gains better understanding of many phenomena of water soluble macromolecules. To this end, polystyrene grafted with short EG side chains have been synthesized by nitroxide-mediated radical polymerization and temperature induced phase separation and conformation changes of (EG)$_{4}$ densely grafted polystyrene in aqueous solutions have been investigated by Small Angle Neutron Scattering (SANS) and Dynamic Light Scattering (DLS). While large clusters are detected by DLS in the solutions, SANS data indicate individual polymer assumes a prolate ellipsoid. With increasing temperature, the attractive interaction between ellipsoids increases while slight intra-chain contraction occurs. The particle however stays the same near the LCST and collapse into a sphere at temperatures far above the LCST. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W25.00003: Steady state structure factor and stress in sheared semi-dilute polymer solutions Prasanth Jose, Grzegorz Szamel We use Brownian dynamics simulations to elucidate the contributions of intra and inter chain correlations to anisotropic scattering patterns observed in light scattering experiments on sheared semi-dilute polymer solutions (Wu et al., Phys. Rev. Lett. 66, 2408 (1991)). We find that while in equilibrium solutions the contributions to structure factor from intra and inter chain correlations cancel each other almost completely, under shear these contributions are modified in different ways resulting in incomplete cancellation and the formation of anisotropic scattering patterns. We also investigate the contribution of intra and inter chain correlations to shear stress and viscosity, and to the shear stress relaxation. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W25.00004: Structure of polydisperse star branched polymers grown by diffusion Guillermo Ramirez-Santiago, Carlos I. Mendoza We present a numerical algorithm to construct polydisperse star branched polymers in two and three dimensions whose morphology is fully determined by diffusion. We analyze the monomer-monomer correlation function to calculate the fractal dimension of the structures. In addition, we carry out a finite-size analysis to determine the scaling properties of the radius of gyration. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W25.00005: Modeling the liquid-solid transition in saturated triglycerides C.B. Hanna, D.A. Pink, A.J. MacDonald, K. Thillainadarajah, R. Corkery, D. Rousseau Corkery \textit{et al.} have proposed that the high-temperature state of the triglyceride trilaurin (TL) is a Y-conformer, in which the three hydrocarbon chains are dynamically twisted with an average angle of $\sim $120\r{ } between them. Using computer simulations, we first show that the high-temperature state is indeed the Y conformation. We then develop a theory of the liquid-solid transition of this system, in which TL molecules are in a chair (h) conformation, with extended, possibly all-trans, chains at low-temperatures, and are in a Y conformation in the liquid phase at temperatures higher than the transition temperature, T*$\approx $319K. We map this ``h-Y model'' onto an Ising model in a temperature-dependent field, perform a mean-field approximation, and calculate the transition enthalpy, which is in good agreement with experiment. We also predict the temperature-dependence of the 1132 cm$^{-1}$ Raman band. Our results support the proposal that the liquid state is made up of molecules in the Y conformation. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W25.00006: Kinetics of Anionic Polymerization of Polybutadienyl Lithium in Benzene: An Osmotic Effect on Propagation Process. Hiroshi Watanabe The anionic polymerization (propagation) kinetics of polybutadienyllithium (PBLi) in benzene was examined with H-NMR. The PBLi chains formed aggregates with an average aggregation number f = 4 through their Li ends. The residual monomer fraction y(t) did not rigorously exhibit the single-exponential decay expected for the conventional propagation mechanism through the dissociated chains, suggesting a competing propagation mechanism through the transiently fused 2f-mer aggregates (detected with Li NMR): This fusion-aided propagation should have been osmotically suppressed on a decrease of y (increase of the PB concentration) to give the deviation from the conventional behavior. The propagation became slower in the presence of chemically inert, neutral PB chains that just tuned the osmotic environment for the aggregates, lending support to molecular picture. Furthermore, the y(t) data in the absence/ presence of the neutral PB chains were semi-quantitatively described by a simple model considering the competition of the propagation through the fused 2f-mer aggregates and dissociated chains. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W25.00007: Conformational studies of conjugated polymers substituted with different side chains Yunfei Jiang, Uwe H. F. Bunz, Dvora Perahia The conformation of conjugated poly(\textit{para}phenylene ethylene) (PPE), in different complex fluids states including molecular solutions, aggregates and gels, has been studied by Nuclear Magnetic Resonance and Molecular Mechanics simulations. PPEs are inherently semiconductors. Their electro-optical response depends on their degree of conjugation, determined by the degree of torque applied on the backbone by the substituents and the association mode of the polymer. Two types of substituents were compared: a nine carbons linear chain nonyl group and a space occupying tri-\textit{iso}propylsilyloxy (TIPS). In molecular solution the side chains are fully flexible assuming random conformations while in any aggregation mode a well-ordered stretched out conformation due to the dense packing between polymeric molecules, is detected. TIPS substituted PPEs results in a non correlated configuration in all of the association modes. These findings are consistent with our independent fluorescence studies of this system. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W25.00008: High-Pressure Vibrational Spectroscopy of Polymers E.D. Emmons, R.G. Kraus, J.S. Thompson, A.M. Covington Polymers are often subjected to extreme conditions of high pressure and temperature in shock compression experiments and in their use as binders in high explosives. We have begun a program to examine polymeric materials at high-pressures in a diamond anvil cell using infrared and Raman vibrational spectroscopies. There is a significant lack of measurements of basic spectroscopic data on scientifically and technologically interesting polymeric materials at high pressure. Data for different materials, including pressure-dependent FTIR absorption spectroscopy of poly (methyl methacrylate) (PMMA) will be presented. The data were analyzed to determine mode Gr\"uneisen parameters and vibrational anharmonicities. Such measurements are useful for interpreting experimental studies of shock compression of polymers as well as benchmarking theoretical models of the behavior of polymers under pressure. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W25.00009: Conformational Heat Capacity of Liquid Biodegradable Polymers in the Absence and Presence Water Marek Pyda, Elzbieta Nowak-Pyda The conformational heat capacity of biodegradable polymers such as amorphous poly(lactic acid) PLA and starch with and without water have been evaluated from a fit of experimental data to a one-dimensional Ising-like model for two discrete states, characterized by parameters linked to stiffness, cooperativity, and degeneracy. For the starch-water system the additional changes in the conformational heat capacity arise from the interaction of the carbohydrate chains with water. The liquid heat capacities at constant pressure$ C_{p}$, of amorphous PLA and partially liquid state of starch, starch-water have been computed as the sum of vibrational, external, and conformational contributions. The vibrational contribution was calculated as the heat capacity arising from group and skeletal vibrations. The external contribution was estimated from experimental data of the thermal expansivity and compressibility in the liquid state. The experimental liquid $C_{p}$ agrees with these calculations to better than $\pm $3{\%}. The calculated liquid $C_{p}$ with the solid $C_{p}$ was employed in the quantitative thermal analysis of the experimental $C_{p }$of biodegradable polymer PLA, starch, and starch-water. Supported by European Union, grant (MIRG-CT-2006-036558), Cargill Dow LLC [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W25.00010: Diffusion in Polypropylene Melts: Role of Stereochemistry. Ernst von Meerwall, Numan Waheed, Wayne Mattice We have performed numerical simulations and experiments at 180$^{o}$C to study the effect of stereochemical composition on the diffusion (D) of linear polypropylene melts of moderate polydispersity. The coarse-grained Monte-Carlo (MC) simulations were based on the rotational isomeric state model and repulsive Lennard-Jones potentials. For the pulsed-gradient $^{1}$H NMR diffusion measurements the three specimens used had probabilities of meso diad P$_{m}$ = 0.02, 0.23, and 0.89. The conversion factor between MC steps and real time was obtained by comparison with the measured D; no dependence on stereochemistry was evident. Using a molecular-weight (M)-scaling known from earlier work on n-alkanes, results were normalized to a common M after accounting for differences in experimental polydispersity. Results agreed closely with the monodisperse simulations. D at high P$_{m}$ was found to be several times faster than at low P$_{m}$, but the simulation also showed a maximum in D at P$_{m}$ near 0.75, an effect attributed to quenched randomness. Likely for similar reasons the experimental D-distribution for the P$_{m}$ = 0.89 sample greatly exceeded that expected from the known polydispersity. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W25.00011: An examination of the whipping instability of viscoelastic jets in electrospinning. Pradipto Bhattacharyya, Jian Yu, Gregory Rutledge, Gareth McKinley The whipping instability of a viscoelastic jet is examined for a number of different test fluids. The whipping motion of the jet during electrospinning is generally conjectured to initiate the large drawdown required to generate polymeric nanofibers with diameters less than 100 nanometers. The phenomenon plays a dominant role in determining the characteristic strain rate at which a fluid filament is deformed during the drawdown. In order to study the whipping motion, a laser beam is passed through the whipping region such that the beam is intercepted by the jet at intervals of time. We analyze and compare both mechanically-forced and natural (unforced) jets. The beam intersection events are manifested as regular drops in the detector voltage and subsequent analysis of the detector signals combined with high-speed digital videomicroscopy provides information on the periodicity of the jet's motion. It is demonstrated that the dynamics transition from a periodic to more complex behavior depending on the viscoelastic nature of the fluid used. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W25.00012: The role of extensional stress in the formation of electrospun fibers Jian Yu, Sergey Fridrikh, Gregory Rutledge Electrospinning is a process that employs electrostatic forces to form polymer continuous fibers. Although the process of making electrospun fibers is easy to implement, many polymer solutions are not readily electrospun into uniform fibers. Here we present a study on the role of fluid elasticity in the formation of fibers from polymer solution by electrospinning. For some dilute polymer solutions without elasticity, the electrospinning jet breaks up into droplets due to the Rayleigh instability driven by surface tension. On the other hand, solutions with some degree of elasticity can generate a stress that retards the growth of Rayleigh instability, forming a ``beads-on-string'' structure. In the extreme case, where a large stress on the jet can suppress the Rayleigh instability completely, uniform fibers are obtained. We calculate a theoretical critical stress on the jet required for complete suppression of the instability. This calculated value is in agreement to experimental observation. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W25.00013: First observation of the first-order transition in ultra-filtration of flexible linear polymer chains Chi Wu, Fan Jin Using a special double-layer membrane to avoid interaction among flow fields generated by different pores, we have, \textit{for the first time}, observed the predicted discontinuous first-order transition in ultra-filtration of flexible linear polymer chains. Namely, the chain could pass through a pore much smaller than its unperturbed radius only when the flow rate is higher than a certain value. When only one chain and one pore considered in theory, such a threshold is surprisingly independent of both the chain length and the pore size. Our results reveals that for a membrane with many pores and at a microscopic flow rate (q) lower than the threshold, the inevitable blocking of some pores by longer non-stretched coiled chains increases q in those non-blocked pores because the macroscopic flow rate (Q) is a constant. Long chains have two populations, coiled and stretched, in a real ultra-filtration experiment when q is lower than the threshold. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W25.00014: First-order Conformation Transition of Single Polyelectrolyte Molecules in Aqueous Solutions Shengqin Wang, Jiang Zhao The molecular conformation of a weak polyelectrolyte, poly 2- vinylpyrindine (P2VP), as a function of charge density and electrostatic screening was studied by single molecule fluorescence techniques. By fluorescence correlation spectroscopy (FCS), the diffusivity of the P2VP in solution was studied at single molecule level. It was found that the diffusion coefficient (therefore the hydrodynamic radius) of P2VP experienced a first-order transition at varying pH values (charge density), while it underwent a continuous transition at different electrostatic screening. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W25.00015: Influence of the solvent size on the behavior in polymer solution Lijia An, Yunqi Li, Tongfei Shi The effects of solvent size on properties of homopolymer solution have been investigated by Monte Carlo Simulation. Increasing the solvent molecular size leads to shrinkage of the polymer chains and increase of the critical overlap concentrations. The root-mean-square radius of gyration of polymer chains ($R_{g})$ is less sensitive to the variation of polymer concentration in solutions of larger solvent molecules. In addition, the dependency of $R_{g}$ on polymer concentration under normal solvent conditions and solvent molecular size is in good agreement with scaling laws. When the solvent molecular size approaches the ideal end-to-end distance of the polymer chain, an extra aggregation of polymer chains occurs, and the solvent becomes the so-called medium-sized solvent. When the size of solvent molecules is smaller than the medium size, the polymer chains are swollen or partially swollen. However, when the size of solvent molecules is larger than the medium size, the polymer coils shrink and segregate, enwrapped by the large solvent molecules. [Preview Abstract] |
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