Bulletin of the American Physical Society
Spring 2011 Meeting Ohio-Region Section of the APS
Volume 56, Number 3
Friday–Saturday, April 15–16, 2011; University Heights, Ohio
Session D2: Polymer Physics and Polymeric Materials |
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Chair: Clemens Burda, Case Western Reserve University Room: Dolan Science Center E134 |
Saturday, April 16, 2011 8:10AM - 8:22AM |
D2.00001: Effect of chain stiffness on structural and thermodynamic properties of polymers Kiran Khanal, Jutta Luettmer-Strathmann The stiffness of the chains affects many properties of polymers. We investigate structural and thermodynamic properties of a bond- fluctuation lattice model for semiflexible polymer chains. Monte Carlo simulations for polymer melts for a range of values of the bending penalty, density, and temperature show elongation of the polymer conformations with increasing chain stiffness but no transition to a nematic phase. Results for average bead-bead interaction energy and bending energy were studied separately and showed that the bending energy is almost independent of the filling fraction, suggesting that the stiffness of the chains can be treated as a single chain property. We calculated the bending energy from the partition function of a pair of neighboring bonds and find excellent agreement between theory and simulation. Equation of state properties of the lattice model were determined from insertion methods and thermodynamic integration. We develop a theoretical description of these properties and use it to determine model parameters for real polymer melts from a comparison with experimental data. [Preview Abstract] |
Saturday, April 16, 2011 8:22AM - 8:34AM |
D2.00002: Tethered chains in good and poor solvent - effects of lateral confinement on adsorption and chain collapse Jutta Luettmer-Strathmann The grafting density of a polymer brush affects the response of the system to changes in solvent quality and surface interactions. In this work, we focus on low (mushroom) and intermediate (semi-dilute regime) grafting densities and model a polymer chain in a brush as a single tethered chain subject to an applied force field of cylindrical symmetry that pulls the chain segments toward an axis through the tethering point and normal to the surface. The polymer chain is represented by a bond-fluctuation model with extended range attractive bead-bead interactions and variable bead surface interactions. Monte Carlo simulations with a Wang-Landau type algorithm are performed to determine the density of states in the state space of monomer- monomer contacts, monomer-surface contacts, and lateral chain extension. We present results for the effect of lateral confinement on conformational transitions such as chain adsorption in good and poor solvent. [Preview Abstract] |
Saturday, April 16, 2011 8:34AM - 8:46AM |
D2.00003: Surface Dynamics of Partially Tethered Layers Jin Kuk Lee, Bulent Akgun, Sushil Satija, Zhang Jiang, Suresh Narayanan, Mark Foster Thermally stimulated fluctuations on a polymer melt surface are important for adhesion, wetting, and tribology. We have found [1] that the characteristic time scale of these surface dynamics can be slowed by orders of magnitude by densely tethering the polymer chains to the underlying substrate. Thus, we are interested in understanding how the surface dynamics can be engineered by tailoring the degree of tethering of the chains. To achieve ``partially tethered'' polymer layers, sparsely grafted layers have been swollen with untethered chains and the surface dynamics studied as a function of the tethering details using X-ray Photon Correlation Spectroscopy. How the tethered chains are distributed through the layer has been determined using neutron reflectivity. Both grafting density and thermodynamic interaction between the tethered and untethered chains play roles in determining the surface dynamics.\\[4pt] [1] American Physical Society, APS March Meeting 2010, March 15-19, 2010, abstract \#Z19.009 [Preview Abstract] |
Saturday, April 16, 2011 8:46AM - 8:58AM |
D2.00004: Conformational Dependence of Charge Transport and Band Gap in Poly (3-Hexyl Thiophene) Oligomers Ram Bhatta, Yeneneh Yimer, Mesfin Tsige, David Perry Structural defects will affect the charge transport properties and the band gap in the Poly (3-Hexylthiophene) (P3HT) polymer, a promising electron donor for organic solar cells. In the present work, such effects are modeled by density functional theory (DFT) calculations on P3HT oligomers up to 12 monomer units in planar and non-planar conformations. DFT calculations were performed at B3LYP/6-31++G(d,p) treating both backbone and hexyl chains explicitly. The structural properties of the oligomers change significantly for 2 to 8 unit isolated oligomers but reach asymptotic values by a 10 unit P3HT chain. The dependence of charge transfer integral on chain length and on the backbone torsional angle is reported. The band gap approaches $\sim $2.0 eV asymptotically as the chain length is increased, which is close to the experimental value. Comparison of the charge transport integral for P3HT with polythiophene (PT) shows that the hexyl chains enhance electronic coupling across the P3HT chain. The charge transport integral declines exponentially as the oligomers are lengthened with a half-length of 4.4 units. [Preview Abstract] |
Saturday, April 16, 2011 8:58AM - 9:10AM |
D2.00005: Cavity-Broadened Absorption in Organic Photovoltaics Brent Valle, Stephen Loser, Jonathan Hennek, Kenneth D. Singer, James Andrews, Tobin Marks One method of increasing power conversion efficiency in organic photovoltaics is to extend absorption to longer wavelengths, capturing a larger portion of the solar spectrum. In this work we use optical transfer matrix formalism to model the absorption spectra of organic photovoltaic devices. The thin Bulk Heterojunction (BHJ) layer in these devices is sandwiched by an aluminum cathode and Transparent Conducting Oxide (TCO) anode forming an optical cavity. Interference effects result in enhanced absorption for wavelengths resonant with cavity modes, and due to the large dispersion of the BHJ materials near their absorption peaks, these enhancements have interesting spectral aspects. By finely controlling the thickness of the TCO and BHJ layers absorption can be extended to longer wavelengths, absorption features can be broadened, and peak absorbance can be increased. Because these changes to the optical cavity do not require modification of the chemical structure of the photovoltaic materials, open-circuit voltage and other material-dependent device parameters remain unchanged. [Preview Abstract] |
Saturday, April 16, 2011 9:10AM - 9:22AM |
D2.00006: Well-Defined Comb Polymer for Surface Segregation Study Boxi Liu, Roderic Quirk, Mark Foster, David Wu Blending polymers with different chain architectures may prove useful in controlling interfacial properties by controlling interfacial segregation. A linear response theory by Wu \textit{et al.} predicts that a long-chain branched polymer blended with its linear analog will be preferentially segregated to the surface and interface of the blend film. The comb architecture is promising for achieving substantial surface segregation. Its high degree of branching, which makes it more prone to separate from linear analogs in the bulk, also provides a substantial driving force for surface segregation when chain ends prefer the surface. In this study comb polystyrenes with well-defined architectural details were prepared by living anionic polymerization. DSC indicates that phase separation occurs for some blends of the linear and comb analogs, and this is also seen by SANS study of these blends. Surface segregation of comb polystyrene when blended with its linear analog has been quantified using the complementary techniques of neutron reflectivity and SIMS. [Preview Abstract] |
Saturday, April 16, 2011 9:22AM - 9:34AM |
D2.00007: Fabrication of Microlens Arrays on Layered Polymers Dustin Horvath, Tom Oder Microlens arrays have numerous applications they offer in many areas, such as optical communication, optical computing, and for collecting light power needed in photovoltaics and laser beam shaping. Microlens made out of polymer Gradient Refractive INdex (GRIN) materials can eliminate spherical aberration, which is generally reduced by designing the lens with aspheric surfaces. At the micro and nano scale, aspheric surfaces are hard to fabricate. The goal of this project was to fabricate arrays of microlens on GRIN polymer materials by photolithography and plasma dry-etching, creating lenses with axially-terminating edges. Lenses produced in this manner ideally correct for spherical aberration. We were able to successfully fabricate microlens arrays in multilayer polymer, though dry etching with Ar/O$_{2}$ gas. The lenses were characterized using atomic force microscopy and scanning electron microscope. The diameters of these microlenses ranged from 20 -- 80 $\mu $m, a height of 3-5 $\mu $m and an estimated focal length of about 50 $\mu $m. In this presentation, methods for improving out fabrication process as well as optically characterizing the microlens arrays will be discussed. [Preview Abstract] |
Saturday, April 16, 2011 9:34AM - 9:46AM |
D2.00008: Protein-like folding and free energy landscape of a homopolymer chain Mark Taylor, Wolfgang Paul, Kurt Binder Many small proteins fold via a first-order ``all-or-none'' transition directly from an expanded coil to a compact native state. We have recently reported an analogous direct coil-to-crystallite transition for a flexible homopolymer [1]. Wang-Landau sampling was used to construct the 1D density of states for square-well chains up to length 256 and a microcanonical analysis shows that for short-range interactions the usual polymer collapse transition is preempted by a direct freezing transition. A 2D configurational probability landscape, built via multi-canonical sampling, reveals a dominant folding pathway and an inherent configurational barrier to folding. Despite the non-unique homopolymer ground state, the thermodynamics of this direct freezing transition are identical to those of two-state protein folding. Homopolymer folding proceeds over a free energy barrier via a transition state folding nucleus, displays a protein-like Chevron plot, and satisfies the van't Hoff two-state criterion.\\[4pt] [1] Phys. Rev. E 79, 050801(R) (2009); J. Chem. Phys. 131, 114907 (2009). [Preview Abstract] |
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