Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session A17: Focus Session: Relaxation Dynamics of Polymeric Glasses I |
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Sponsoring Units: DPOLY Chair: Jack Douglas, NIST Room: B116 |
Monday, March 15, 2010 8:00AM - 8:36AM |
A17.00001: Molecular simulations of polymer glasses under active deformation Invited Speaker: Many of the technologically important mechanical properties of polymers depend crucially on their properties in the glass state. Despite their tremendous importance, our understanding of many of the commonly-observed phenomena in the glass state (e.g. aging and the changes in mobility with deformation) remains primitive. In this talk, I will describe recent molecular simulations examining how the segmental dynamic properties evolve during aging and deformation of polymer glasses. When a constant stress is applied to our materials, we find that the mobility is immediately enhanced by a substantial amount and continues to increase until the material begins to strain harden. After strain hardening begins, the mobility gradually decreases. However, when we deform at a constant strain rate, we find that the mobility is essentially constant after the yield point even when strain hardening occurs. Although comparable experimental measurements of the mobility in polymer glasses examine time scales many orders of magnitude longer than those observable from our simulations, the trends in the changes in mobility agree very well between the experiments and the simulations. In addition, we will explore how the dynamic heterogeneity can be tuned with various additives such as antiplasticizers, which render our polymer a stronger glass former with more homogeneous dynamics, and how this dynamic heterogeneity evolves with deformation. [Preview Abstract] |
Monday, March 15, 2010 8:36AM - 8:48AM |
A17.00002: Four-Point Analysis of Molecular Fluctuations in Sucrose Benzoate Near the Glass Transition by Fourier Imaging Correlation Spectroscopy James Utterback, Jasper Cook, Andrew Marcus Glass-forming liquids exhibit broad heterogeneous distributions of relaxations. It is of considerable importance to determine the detailed forms of these distributions in order to understand how fast and slow processes are partitioned, and how they couple over time. Such information is available through four-point correlation and distribution functions. We present initial fluorescence fluctuation measurements of probe molecule rotation and translation in the glass forming liquid sucrose benzoate. These measurements are performed using a unique method called polarization-modulated Fourier imaging correlation spectroscopy (PM-FICS), which can simultaneously measure molecular center-of-mass motions and optical anisotropy fluctuations. By combining PM-FICS with single-molecule imaging techniques, we construct two-dimensional spectral densities and joint distribution functions that establish temporal correlations of microscopic coordinates over successive time intervals. [Preview Abstract] |
Monday, March 15, 2010 8:48AM - 9:00AM |
A17.00003: Distribution of relaxation times in miscible polymer blends close to the glass transition Paul Sotta, Gareth Royston, Didier R. Long It is known that the dynamics in molecular and polymeric glass formers become strongly heterogeneous as they approach the glass transition. Here we present data recently acquired in non-polar, dynamically asymmetric, miscible blends of poly(alpha-methylstyrene) / poly(cyclohexylmethacrylate) and in blends of the molecular liquids glyceryltriacetate / toluene. The various sources of dynamical heterogeneities are discussed. We show that the Long-Lequeux model (Long and Lequeux, EPJ E 2001, 4, 371), based on density fluctuations, describes quantitatively the broadening of the relaxation time distributions on the low frequency side in the pure components as temperature comes close to Tg. The model has been extended in order to include both the concentration fluctuations in blends and the high frequency broadening. The extended model provides a quantitative description of the observed broadening of the relaxation time distributions on the whole frequency range. [Preview Abstract] |
Monday, March 15, 2010 9:00AM - 9:12AM |
A17.00004: Glass Transition Temperature of Isolated Polymer-A Chains Dispersed within a Bulk Polymer-B Phase: Novel Characterization by Fluorescence Robert Sandoval, John Torkelson Since the publication of ``Self-concentrations and effective glass transitions in polymer blends'' (Macromolecules 2000) by Lodge and McLeish, many studies have focused on characterizing the glass transition temperature (Tg) of particular components in miscible blends. However, the lack of sensitivity via conventional techniques to a minor component in asymmetric blends has hindered direct characterization of Tgs associated with trace levels of one polymer dispersed in a second polymer. We have developed an intrinsic fluorescence technique to determine the Tg of styrene-containing components dispersed at trace levels in a polymer blend where the blend is miscible and the minor component is dissolved in the bulk phase. The bulk phase dramatically affects the Tg of the dispersed PS chains; a 62 K PS Tg reduction is observed when PS is dispersed within poly(n-butyl methacrylate) at trace levels. This study is the first to allow direct Tg measurements of isolated chains dispersed in a bulk phase and allows for a critical test of the Lodge-McLeish model at highly asymmetric blend ratios. [Preview Abstract] |
Monday, March 15, 2010 9:12AM - 9:24AM |
A17.00005: Application of the entropy theory of glass formation to poly(alpha-olefins Karl Freed, Evgeny Stukalin, Jack Douglas The lattice cluster theory for the configurational entropy is combined with the Adam-Gibbs model to describe the dependence of the glass transition temperature, structural relaxation time, and fragility of polymer melts on the monomer molecular structure, the molecular weight, pressure, and molecular parameters, such as the backbone and side chain stiffness, cohesive energy, and the side chain length. Specific applications provided to poly(alpha-olefins) where agreement with experiment ranges from quantitative to qualitative depending on the particular property. [Preview Abstract] |
Monday, March 15, 2010 9:24AM - 9:36AM |
A17.00006: Reaction rates in hydrophilic glass Marcus Cicerone, Qin Zhong Sugar-based glasses are known to stabilize proteins against aggregation and chemical degradation. It has long been supposed that, due to the long timescales involved in protein aggregation and chemical degradation in the glass, relaxation processes essentially control the rate of degradation. We have shown that, although the degradation processes occur on timescales of the alpha relaxation process, secondary relaxation processes, and not alpha relaxation seems to dominate in influencing both chemical and physical degradation. In this presentation we will discuss results from time-resolved fluorescence studies in sugar-based glasses designed to help understand the physics underlying these puzzling observations. [Preview Abstract] |
Monday, March 15, 2010 9:36AM - 9:48AM |
A17.00007: A simple bead necklace model to study secondary relaxations in glassy polymer melts and blends Dmitry Bedrov, Grant Smith We demonstrate that molecular dynamics simulations using a simple bead necklace polymer model are capable to show distinct primary and secondary (or Johari-Goldstein) relaxation processes of backbone segments in linear polymer melts and blends. Despite the simplicity of the utilized polymer model, chains are comprised of only one type of beads and bonds, the observed secondary relaxations show characteristics defining the true Johari-Goldstein process, therefore allowing fundamental study of underlying mechanisms of secondary relaxations as well as their correlation with primary relaxation in polymer melts and blends as a function of temperature and pressure. [Preview Abstract] |
Monday, March 15, 2010 9:48AM - 10:00AM |
A17.00008: Dynamic Heterogeneity in Highly Cross-linked Epoxy in the Vicinity of Glass Transition Po-Han Lin, Rajesh Khare Cross-linked epoxy has been widely used in aerospace and electronics industries. The highly cross-linked nature of these systems leads to different chain dynamics as compared to the linear polymeric systems. In this work, we have used molecular dynamics (MD) simulations to study the dynamic heterogeneity in cross-linked epoxy near the glass transition temperature. Well-relaxed atomistic models of cross-linked epoxy were first created by employing the simulated annealing polymerization approach. The specific epoxy system studied consisted of diglycidyl ether of bisphenol-A (DGEBA) as the epoxy monomer and trimethylene glycol di-p-aminobenzoate (TMAB) as the cross-linker. The glass transition temperature of these model structures was determined from MD simulation by monitoring their volume-temperature behaviour in a stepwise cooling run. The chain dynamics of these systems were characterized by their local translational and orientational mobility. Furthermore, dynamic heterogeneity was studied by analyzing the spatial distribution of the mobile and immobile atoms in the system near the glass transition temperature. [Preview Abstract] |
Monday, March 15, 2010 10:00AM - 10:12AM |
A17.00009: Antiplasticization of polymer melts diluted by low molar mass species Evgeny Stukalin, Jack Douglas, Karl Freed An analysis is made of glass formation in polymers diluted by molecular additives by combining the Adam-Gibbs model and computations of the configurational entropy using the lattice cluster theory. Plasticization and antiplasticization of polymer melts is shown to depend on the molecular properties of the additive. Antiplasticization is accompanied by a toughening of the glass mixture and occurs when the diluent is a small species having strongly attractive interactions with the polymer chains. Plasticization leads the softening of the host polymer and emerges for small additives with weakly attractive interactions. Shifts in T$_{g}$ and changes in compressibility and density of polymer melts with a diluent are evaluated to characterize the efficiency of (anti-)plasticizers. The general reduction in the T$_{g}$ and fragility of polymers by molecular additives is rationalized by analyzing the influence of the diluent's properties (cohesive energy, chain length, and stiffness) on glass formation. We also study the vitrification of mixtures at fixed temperature due to increasing polymer concentration. The zero mobility concentration is found to scale linearly with the inverse degree of polymerization. [Preview Abstract] |
Monday, March 15, 2010 10:12AM - 10:24AM |
A17.00010: Relaxation Dynamics of the Glass Transition in PMMA+SWCNT Composites by Temperature-Modulated DSC Nihar Pradhan, Germano Iannacchione Temperature Modulated Differential Scanning Calorimeter (TMDSC) used to investigate the thermal relaxation dynamics of PMMA-Single wall carbon nanotubes (SWCNTs) through the glass transition as a function of frequency. A strong dependence of the temperature dependence peak in imaginary part of complex heat capacity ($T_{max}$) was found during the transition from glass like to liquid like region and can be described by Arhenius law. The activation energy shows increases while the charactersistic time decreases with increasing mass fraction ($\phi_m$) of SWCNTs. Decreasing of enthalpy, while heating and slowly increasing while cooling at 2.0~K/min scan rate was observed and as frequency of temperature modulation increases. There is no relative change of enthalpy in heating and cooling observed at sufficiently slow scan rate. The glass transition temperature ($T_g$) shows increases as a function of frequency of temperature modulation, $\phi_m$ of SWCNTs and with increasing scan rate. From imaginary part of heat capacity, it obvious that $T_{max}$ is not the actual glass transition temperature of pure polymer but $T_{max}$ and $T_g$ values can be superimpose when $\phi_m$ of SWCNT increases in polymer. [Preview Abstract] |
Monday, March 15, 2010 10:24AM - 10:36AM |
A17.00011: Specifics of nanostructure stability patterned polystyrene under high electric fields Sergei Lyuksyutov, Victoria Nedashkivska, Mindaugas Rackaitis The stability of nanostructures patterned in annealed polystyrene films exposed to electric fields of 10$^{9}$ -10$^{10}$ V m$^{-1}$ suggests slow exponential relaxation ($\tau $ = between 55 and 160 hours) while relaxation in non-annealed surface occurs on a scale of minutes. A lithography protocol was designed for fast formation of nanostructures 30-100 nm in width and 1-10 nm in height in the films thicker than 100 nm. After more than 300 hours of observation under humidity varied between 25 and 27{\%}, it was suggested that the major factor of slow dynamical changes of the nanostructures attributed to electric charge dissipation. It is suspected that slow relaxation of the nanostructures is not related to the glass transition in 150-nm thick polymer films. [Preview Abstract] |
Monday, March 15, 2010 10:36AM - 10:48AM |
A17.00012: Structural Characterization of Polymers of Intrinsic Microporosity Using X-ray Scattering Methods Amanda McDermott, Nhamo Chaukura, Christopher Mason, Peter Budd, Kadum Msayib, Mariolino Carta, Bader Ghanem, Neil McKeown, James Runt Polymers of intrinsic microporosity (PIMs) are a novel class of materials with a large inner surface area and large free volume, which make them suitable for hydrogen adsorption. In addition, they exhibit a combination of high permeability and high selectivity useful for filtration applications. A series of polymers with high glass transition temperatures based on PIM-1, a polybenzodioxane with a ladder-type structure combining stiff segments with sites of contortion, as well as a series of triptycene-based network PIMs, are characterized via small-angle and wide-angle X-ray scattering. Scattering features, including multiple amorphous halos and a characteristic power-law slope, are related to polymer chemistry, hydrogen adsorption performance, and gas permeation parameters. [Preview Abstract] |
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