Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H17: Focus Session: Relaxation Dynamics of Polymeric Glasses II |
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Sponsoring Units: DPOLY Chair: Greg Mckenna, Texas Tech University Room: B116 |
Tuesday, March 16, 2010 8:00AM - 8:36AM |
H17.00001: Polymer Physics Prize Break |
Tuesday, March 16, 2010 8:36AM - 9:12AM |
H17.00002: Nanoscale fluctuations and responses in equilibrium and nonequilibrium glassy polymers Invited Speaker: Recent developments have have made it clear that finding a dynamical correlation length, and its spatio-temporal behavior on approach to the glass transition are the keys to explaining the dramatic growth of viscosity near the glass transition, and testing leading theory candidates. Several lines of indirect evidence (e.g. Berthier et. al. Science 2005) point to a weak but growing temperature-dependence of this length on approach to the glass transition. In this talk I will discuss an approach based on scanning probe microscopy, for probing and imaging spontaneous dipolar noise and dielectric response. We used these methods to study nanoscale spatio- temporal dynamics in polymer glasses and polymer blends. Various space-time correlation functions are analyzed in an effort to search for growing correlation length scales near the glass transition. We are working to extend the instrumental resolution closer to these intrinsic length scales thought to control the glass transition. The ability to quantitatively measure on the nanoscale both fluctuations and responses also allows us to test the validity of the global and local fluctuation-dissipation-relation (FDR) in equilibrium and out- of equilibrium. Using this approach, we have been able to study, for the first time, FDR violations in the strong aging regime in structural glasses (Oukris, et. al. Nature Physics, 2009). The results give insight into the equilibrium glass order parameter. [Preview Abstract] |
Tuesday, March 16, 2010 9:12AM - 9:24AM |
H17.00003: Molecular cooperativity in the dynamics of glass-forming systems Liang Hong, Purushottam Gujrati, Vladimier Novikov, Alexei Sokolov The mechanism behind the sharp slowing down of the main structural relaxation in a glass-forming liquid upon approaching the glass transition remains a great puzzle. Most of the theories relate this mechanism to the cooperativity in molecular motion. On the other hand, the collective vibration in the pico-second time region, the so-called boson peak, is also described as a cooperative process. In our recent work (L. Hong, \textit{et al.}, J. Chem. Phys., in print), we demonstrated that the collective vibrations and the main structural relaxation involve a similar length scale of cooperative molecular motions. More importantly, we found that the cooperativity length scale in different materials directly correlates to only one part of the mechanism of slowing down the structural relaxation, i.e., the dependence of the structural relaxation on volume. In this presentation, we will further demonstrate that this correlation holds true not only for different chemical species, but also for the same chemical specie with different molecular weight or under pressure. The results are compared to predictions of theoretical models. [Preview Abstract] |
Tuesday, March 16, 2010 9:24AM - 9:36AM |
H17.00004: Ellipsometric Studies of the Structure and Transformation Kinetics of Ultra Stable Glasses into the Supercooled Liquid Zahra Fakhraai, Gregory K. Pribil, Mark D. Ediger Exceptionally stable glasses of low molecular weight organic materials have been recently made by means of physical vapor deposition. Compared to the ordinary glass made of the same liquid, these glasses have lower enthalpy, higher density and slow transformation kinetics. Wide angle X-ray scattering (WAXS) measurements show that these glasses exhibit a distinct packing structure not observed in the ordinary glass structure. We used ellipsometry to study the density and structure of the as deposited stable glasses of tris-naphthylbenzene and indomethacin as a function of deposition temperature and film thickness. Ellipsometric measurements indicate that these glasses have a slight structural anisotropy in the direction of the deposition, providing a potential explanation for the observed peak in the WAXS measurements. These glasses were then annealed above the ordinary glass transition temperature either at a constant heating rate or isothermally to investigate the transformation kinetics into the ordinary supercooled liquid state. [Preview Abstract] |
Tuesday, March 16, 2010 9:36AM - 9:48AM |
H17.00005: Molecular motion confinement after physical aging Xiaoliang Wang, Qiang Gu, Pingchuan Sun, Chao Teng, Dongshan Zhou, Gi Xue As a structural relaxation of the glassy state toward the metastable equilibrium amorphous state, physical aging plays a very important role in the design, manufacture, and use of glassy polymer materials and devices. Combined with several solid state NMR techniques, we first illuminate the mechanism of structure relaxation of polystyrene at molecular level. $T_{1}$ relaxation of $^{1}$H and $^{13}$C showed no difference between aged and quenched sample. The amplitude and geometry of aromatic ring motion was detected by two-dimensional separated local field experiments (PISEMA). We found the amount of 180 degree flip motion decreased after aging, while asymmetry parameter increased. And the wobbling amplitude of both 180 degree flip and static motion decreased after aging. The results showed physical aging make the aromatic side rings stacking closer and tighter. However no change of phase happened at scale larger than 20 nm. This result well explained that why physical aging make polymer material brittle while most of other characterizing methods could not show the difference before and after aging. [Preview Abstract] |
Tuesday, March 16, 2010 9:48AM - 10:00AM |
H17.00006: On the Triviality of Aging Stefan Boettcher, Paolo Sibani The dynamics of complex systems collectively known as glassy share important phenomenological traits. Whether structural or quenched, either in low-$T$ magnets or in dense colloids, physical changes occur in an intermittent fashion and, on average, at a decreasing rate. Despite their microscopic differences, generally, a transition is observed from a time-homogeneous regime to one which is likewise homogeneous, but in terms of the \emph{logarithm} of time: A global change of the independent variable \emph{trivializes} the dynamics. Focusing here on experimental data from dense colloids\footnote{Courtland \& Weeks, {\it J. Phys.: Condens. Matter} {\bf 15}, S359 (2003).}, the crucial importance of record-size fluctuation for the aging dynamics is revealed. A model with a generic stochastic dynamics is introduced\footnote{see also: Boettcher \& Sibani, http://arxiv.org/pdf/0910.5470.} which relies on the growth and collapse of strongly correlated clusters (``dynamic heterogeneities''). In the limit where large clusters dominate the dynamics, intermittency in terms of record-size events occurs with rate $\propto1/t$, implying a homogeneous, $\log$-Poissonian process that qualitatively reproduces the experimental results for colloidal dynamics.\footnote{See also http://www.physics.emory.edu/faculty/boettcher/.} [Preview Abstract] |
Tuesday, March 16, 2010 10:00AM - 10:12AM |
H17.00007: Deformation perturbs heterogeneous dynamics and position on the energy landscape Mark Ediger, Hau-Nan Lee, Benjamin Bending Optical photobleaching experiments were used to investigate the interaction between physical aging, heterogeneous segmental mobility, and deformation in polymer glasses. Experiments were performed on lightly crosslinked poly(methyl methacrylate) glasses with systematically varying aging histories. By directly measuring the molecular mobility of polymer glasses under deformation, we observe that stresses in the pre-flow regime and flow regime are qualitatively different. In the pre-flow regime, aging and stress apparently act as two independent influences on segmental mobility; deformation causes an increase in segmental mobility but does not erase the influence of previous aging. In contrast, as a sample enters the flow regime, plastic deformation takes the glass into a high mobility state with quite homogeneous dynamics; this state is high on the energy landscape and independent of any pre-deformation aging history. Changes in dynamic heterogeneity provide information about the portions of the energy landscape visited during deformation. [Preview Abstract] |
Tuesday, March 16, 2010 10:12AM - 10:24AM |
H17.00008: Impact of Stress Present During the Thermal Quench on the Subsequent Physical Aging in Polymer Films Connie Roth, James Davidheiser, Suk Yoon Physical aging studies in the research literature over the past 15 years have observed large increases in physical aging rate with decreasing film thickness. Surprisingly these effects are observed for micron thick films, an order of magnitude or two larger than thicknesses where nanoconfinement effects on the glass transition and modulus are typically observed. We present physical aging measurements using ellipsometry on polystyrene (PS) films thermally quenched in different states suggesting that the increased physical aging rate with decreasing film thickness may be attributable to residual stresses in these films. For example, we find that 1400 nm thick PS films quenched in a free-standing state exhibit physical aging rates comparable to those for bulk PS measured by dilatometry, while 600 nm thick PS films quenched in a free-standing state exhibit a much faster aging response more comparable to those observed for supported PS films of any thickness. We present a discussion of the different stresses resulting from the geometry of the film and support, and its impact of the subsequent physical aging of the material. [Preview Abstract] |
Tuesday, March 16, 2010 10:24AM - 10:36AM |
H17.00009: Simple Model of Polymeric Viscoplasticity Robert Hoy I develop a simple theory for polymeric viscoplasticity and test it using molecular dynamics simulations. The evolution of stress at large strains is controlled by the large scale orientation $\epsilon_{eff}$ of chains, which serves as a mesoscale internal state variable. The evolution of $\epsilon_{eff}$ is decribed by a Maxwell-like model. The strain rate, temperature and chain length dependence of the key relaxation times are surprisingly simple. Relaxation in actively deformed systems is faster than in nondeforming systems because the former is coherent and strain activated. Entanglements serve to limit the coherence of this relaxation. Results are briefly compared to recent theoretical and experimental work. [Preview Abstract] |
Tuesday, March 16, 2010 10:36AM - 10:48AM |
H17.00010: A microscopic view of deformation-accelerated dynamics in polymer glasses Mya Warren, Joerg Rottler When amorphous polymers are deformed, the slow glassy dynamics resulting from broad distributions of relaxation times becomes accelerated and permits plastic flow. We use molecular dynamics simulations as a computational microscope to obtain insight into the origin of the deformation-accelerated dynamics and its relationship to aging in a model polymer glass. Segmental trajectories are analyzed to identify individual relaxation events, and the full distribution of relaxation times is obtained under three deformation protocols: step stress (creep), step strain, and constant strain rate deformation. As in experiments, the dynamics are accelerated by several orders of magnitude by the deformation, and a narrowing of the distribution of relaxation times during creep is directly observed. Additionally, the acceleration factor describing the transformation of the relaxation time distributions was computed and found to obey a universal dependence on the strain, independent of the age and deformation protocol. [Preview Abstract] |
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