### Session H19: Solid Amorphous Polymers

 Tuesday, March 17, 2009 8:00AM - 8:36AM H19.00001: Polymer Physics Prize Symposium Break Tuesday, March 17, 2009 8:36AM - 8:48AM H19.00002: Paradigms for the Glass Transition Gregory McKenna Several paradigms of the glass transition are commonly used to either characterize the behavior of glass-forming materials or as targets'' for theoretical outcomes. The purpose of the work and discussion was motivated by a view that some of the paradigms currently used to frame the glass transition event, while potentially useful, may also have limitations that we often do not fully consider. Discussion focuses on isochoric glass formation paths, thermodynamic and dynamic fragilities and how dynamic fragility in many systems (especially polymers, metals, ionic liquids and hydrogen bonding systems) seems to vary primarily with the glass transition temperature, Tg, itself. This leads to the conclusion that such systems have an apparent activation energy that varies as the square of the glass temperature and consequently a fragility index m that varies linearly in Tg. The work concludes with a presentation of evidence that the apparent super Arrhenius divergence of viscosity or relaxation time as temperature decreases towards the Tg is precarious, suggesting that the common expectations of a diverging relaxation time or viscosity as the glass temperature is approached are not met. Tuesday, March 17, 2009 8:48AM - 9:00AM H19.00003: Twinkling Fractal Theory of the Glass Transition: Applications and Insights Richard Wool The new perspective on the Glass Transition of amorphous materials offered by theTwinkling Fractal Theory (TFT). [R. P. Wool, J. Polym. Sci, Part B: Polym Phys. 46, 2765 (2008)] is examined in several applications. The TFT describes Tg in terms of the autocorrelation relaxation function for the spatio-temporal solid-liquid fluctuations which are related to the vibrational frequencies (twinkles'') described by the Orbach vibrational density of states for a fractal. The twinkling frequencies for solid-liquid interchange are due to Boltzmann energy populations of interatomic oscillators interacting through anharmonic potentials U(x) with energy D$_{o}$ of order 1-5 kcal/mol. T$_{g}$ occurs when the activation energy for the solid-liquid transition goes to zero at the inflection point of U(x) and is given by T$_{g }$= 2D$_{o}$/9k. The applications include: (a) group contributions to Do, (b) the rate and temperature dependence of yielding and fracture, (c) shear thickening fluids, (d) rate dependence of dynamical mechanical properties, particularly the tan delta damping peak used to measure Tg, (e) derivation of the empirical WLF time-temperature superposition empirical relation, (f) thermal expansion and (g) physical aging. Tuesday, March 17, 2009 9:00AM - 9:12AM H19.00004: Cooperativity and fragility in glass forming systems: not a simple relationship Liang Hong , Alexander Kisliuk , Alexei Sokolov Understanding the sharp increase of the main structural relaxation time $\tau$ on approaching the glass transition temperature (Tg) remains a great challenge. Traditionally this relaxation is considered as a cooperative process, with larger cooperativity leading to a steeper temperature dependence of $\tau$ around Tg, i.e. higher fragility. On the other hand, the boson peak, a collective vibration in the pico-second time region, is also described as a cooperative motion. In this study we estimate the structural correlation length for various glass forming systems from the collective vibration. The obtained values are in good agreement with the dynamic heterogeneity length estimated by 4 dimensional NMR for the main structural relaxation. Thus the two different motions appear to have very similar length scale for cooperativity. Direct comparison of cooperativity to fragility reveals no correlation. However, we discover that cooperativity correlates with the pure volume contribution to fragility. This result explains why many earlier attempts to find direct relationship between fragility and cooperativity fail. A possible origin for the observed correlation is discussed. Tuesday, March 17, 2009 9:12AM - 9:24AM H19.00005: Molecular Interpretations of Observed non-Fickian Penetrant Transport Behavior in Glassy Polymers Adam Ekenseair , Richard Ketcham , Nicholas Peppas The relative rates of the diffusional and relaxational processes during the absorption of penetrant molecules in glassy polymers determine the nature of the transport process and lead to Fickian, Case II, and anomalous absorption behavior. While previous models account for anomalous behavior, there is still a disconnect between theory and experiment, as data must be fit to the model with previously determined independent parameters. With trends leading to smaller device scales and increasingly complex polymer structures, there is a need for a quantitative understanding of the manner in which a polymer's network structure alters both the rate and the mode of penetrant transport. To this end, the effects of the basic network parameters of PMMA, including the degree of crosslinking, polymer mesh size, and the crosslink size, on the integral sorption of methanol were studied utilizing both gravimetric data and \textit{in-situ} ultra-high-resolution X-ray computed tomography studies. The effects of sub-Tg annealing/aging, temperature, and the presence of un-reacted monomer were also investigated. Controlling the relative timescale of the relaxational process by altering the polymer network structure is shown to directly influence the Case II front propagation velocity and the nature of the observed transport behavior. Tuesday, March 17, 2009 9:24AM - 9:36AM H19.00006: On the Nature of Gas Transport of Ethylene Vinyl Alcohol Copolymers Sergei Nazarenko , Justin Brandt , Brian Olson , Alexander Jamieson Historically, all the approaches describing gas diffusion in polymers can be roughly divided in two categories, based on free volume models and the activation molecular models, which take into account the cooperative penetrant-polymer chain motions, chain rigidity and intermolecular forces. Although gas transport characteristics exhibit a general correlation with free volume, alone free volume can not adequately describe gas barrier. The chain rigidity and the strength of intermolecular interactions are two additional important factors which are manifested via activation energy. The main objective of this work was to develop fundamental understanding of oxygen transport in a broad range of EVOH copolymers as it is related to free volume characteristics probed by positron annihilation lifetime spectroscopy and hydrogen bonding interaction. Tuesday, March 17, 2009 9:36AM - 9:48AM H19.00007: Photochemical Crosslinking of Preformed Glassy and Amorphous Polymers Through bis-Benzophenone Mediated Covalent Bridging Nicholas Carbone , Mary Dickson , Jeffrey Lancaster , Greg Carroll , Jeffrey Koberstein We show that bis-benzophenone (bis-BP) is an effective method to photochemically crosslink essentially any solvent-free glassy or amorphous preformed polymer system that contains abstractable hydrogen atoms. When bis-BP is mixed into a polymer and exposed to UV radiation, it abstracts hydrogen atoms from any chains in proximity, thereby initiating a cascade of free radical reactions that include several mechanisms that can lead to covalent polymer crosslinking. Herein we study the early stages of branching reactions that precede gelation by following molecular weight changes in bis-BP modified glassy polystyrene (PS) and amorphous poly(n-butyl acrylate) (PnBA) thin films on silicon wafers by Gel Permeation Chromatography. Quantitative molecular weight changes in PS:bis-BP and PnBA:bis-BP thin films are studied as a function of irradiation time, polymer:bis-BP molar ratio, and film height. Increases in molecular weight and polydispersity are quantified and model equations are developed. Tuesday, March 17, 2009 9:48AM - 10:00AM H19.00008: Evolution of Entanglements During Craze Formation Ting Ge , Mark Robbins , Robert Hoy , Stefanos Anogiannakis , Christos Tzoumanekas , Doros Theodorou Craze formation occurs during fracture of many polymers and leads to a substantial increase in the fracture energy. Models of craze formation usually assume that entanglements act like permanent chemical crosslinks.This model is tested by following the evolution of entanglements using the Contour Reduction Topological Analysis (CReTA) algorithm. The CReTA algorithm shortens each chain until further shortening would require chains to pass through each other. The contacts between chains that limit further shortening are identified as entanglements or topological constraints. Unlike related algorithms, the chain shortening has little effect on the craze structure, allowing the entanglements to be followed in real space, as well as along chains. CReTA is applied to molecular simulations of crazing using a coarse-grained bead-spring polymer model. The number of beads in each chain N and the entanglement length Ne are varied. Our results show that entanglements do not act like fixed chemical crosslinks. There is a systematic loss in entanglements during craze formation that does not occur when chains are deformed affinely and is nearly independent of N/Ne.The role of chain length, N, Ne, interchain friction and other parameters in determining the degree of entanglement loss is discussed. Tuesday, March 17, 2009 10:00AM - 10:12AM H19.00009: Dynamics of a polymer nanocomposite during active deformation Robert Riggleman , Gregory Toepperwein , Juan de Pablo , Hau-Nan Lee , M. D. Ediger Recent molecular simulation and experimental studies have explored the effects of stress on the dynamics of polymer glasses and both have demonstrated that relaxation times can decrease by more than two orders of magnitude. However, many questions on the origins of the changes in the dynamics remain unaddressed. In this study, we have performed extensive molecular dynamics and Monte Carlo simulations of a polymer glass and a polymer nanocomposite undergoing active deformation. We measure the dynamics during both constant stress and constant strain rate deformations and provide a detailed comparison of the two modes of deformation. The nanoparticles impart mechanical reinforcement onto the polymer, requiring larger stresses to achieve the same deformation. In both systems, the dynamics correlate very well with the instantaneous strain rate whether we deform at constant stress or constant strain rate. Additionally, we explore the effects of each mode of deformation on the potential energy landscape and find qualitatively different behaviors when we deform at constant stress versus constant strain rate. Finally, we provide a brief comparison of our simulation results to recent experiments and demonstrate that the simulations are capable of reproducing all of the behaviors observed in the experiments. Tuesday, March 17, 2009 10:12AM - 10:24AM H19.00010: Rheological Scaling Relation for an Out-of Equilibrium Colloidal Solid H. Henning Winter , X. Wang , G. Xue , P. Sun We explore scaling relations for the slow ripening of an out-of-equilibrium model colloidal solid that consists of clay particles that swell and exfoliate into randomly oriented clay sheets through the action of end-functionalized (sticky'') polymer molecules. A freshly mixed sample quickly forms a sample-spanning network structure that gradually approaches its equilibrium. The ripening process accelerates at elevated temperature. After rescaling (Rheol Acta 45:331-338, 2006), the complex modulus data $G$',$G$''(\textit{$\omega$}, $t_{r})$ from time-resolved mechanical spectroscopy (Rheol Acta 33:385-397, 1994) shows that, surprisingly, the growth function of the elastic modulus is the inverse of the decaying characteristic relaxation time. Parameter of the isothermal ripening process is the ripening time'', $t_{r}$. A single scaling function with two pronounced powerlaw regions, a fast ripening process ($\sim \quad t_{r}^{-2})$ followed by slow ripening ($\sim \quad t_{r}^{-1/2})$, defines the state of ripening and projects the time necessary to reach equilibrium. Tuesday, March 17, 2009 10:24AM - 10:36AM H19.00011: Strain Hardening in Bidisperse Polymer Glasses Mark O. Robbins , Robert S. Hoy The connections between glassy and rubbery strain hardening have been a matter of great controversy in recent years. Recent experiments and our earlier simulations have suggested that the hardening modulus $G_R$ is proportional to the entanglement density in glasses, as it is to the crosslink density in rubbers. In this work we present more extensive studies of strain hardening in bidisperse glasses and its relation to microscopic conformational changes. The mixtures contain chains of very different lengths but equivalent chemistry. $G_R$ does not scale simply with the entanglement density. Instead it obeys a simple mixing rule, with $G_R$ equal to the volume fraction weighted average of the moduli of the two pure components. As in recent studies of monodisperse systems (R. S. Hoy and M. O. Robbins, Phys. Rev. Lett. \textbf{99}, 117801 (2007)), the stress is directly correlated to the degree of chain orientation. Chains of a given length undergo almost the same degree of alignment in pure systems and mixtures, explaining why the simple mixing rule applies. The connection to recent analytic theories by K. Chen and K. S. Schweizer (PRL, in press) will be discussed. Tuesday, March 17, 2009 10:36AM - 10:48AM H19.00012: A Model of Glassy Polymers that Includes both Spatial and Temporal Fluctuations Grigori Medvedev , James Caruthers Glass forming polymers near and below Tg are dynamically heterogeneous as has been found via a number of experimental techniques, where the dynamic heterogeneity is the probable cause of the non-exponential decay of the orientation correlation function of probe molecules embedded in polymer matrix as well as breaking'' of the Stokes-Einstein relations for rotational and translational diffusion. Although dynamic heterogeneity in glassy polymers is well established, constitutive models for describing the mechanical behavior employ quantities that ignore fluctuations. Consequently, the mechanical implications of dynamic heterogeneity are largely unexplored. In this talk we report on a finite element type model, where the local relaxation times in the material experience fluctuations, i.e. both the temporal and spatial nature of the fluctuations is explicitly acknowledged. The stochastic force between neighboring domains is assumed to be uncorrelated; however, since neighboring domains tile space, there is spatial and temporal correlation in the stochastic response of the system. The mechanical response of the sample under different deformation histories, including constant strain rate tensile and compressive loading as well as creep under constant load, will be presented. Tuesday, March 17, 2009 10:48AM - 11:00AM H19.00013: Anomalous crack propagation in reinforced natural rubber Paul Sotta , Brice Gabrielle , Didier Long , Loic Vanel , Pierre-Antoine Albouy , Francesca Peditto In reinforced natural rubber, crack propagation in mode I exhibits rotation of the tear in a direction perpendicular to the usual one. Our objective is, first, to understand the impact of this phenomenon on fracture toughness of the material, and, secondly, to understand how this phenomenon is related to the specific properties of reinforced natural rubber. To this aim, we combine measurements of ultimate properties, measurements of the number and length of tear rotations as a function of loading velocity and temperature, and investigation of material heterogeneities at sub-micrometric scales, originating both from fillers and strain-induced crystallites (strain-induced crystallinity is measured up to failure by X ray diffraction), in natural rubber samples reinforced by nanometric aggregates. Observations suggest that tear rotation is related both to the mechanical anisotropy induced by strain-induced crystallinity and to the dissipative properties of the material at high strain.