Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session R32: Focus Session: Polymer Liquids and Glasses |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Rodney Priestley, Princeton University Room: 340 |
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R32.00001: Scattering and Physical Aging in High-Free-Volume Polymeric Glasses Amanda G. McDermott, Peter M. Budd, Neil B. McKeown, Coray M. Colina, James Runt Polymers of intrinsic microporosity (PIMs) form glassy, rigid membranes featuring a large concentration of pores smaller than 1 nm, large internal surface area, and high gas permeability and selectivity. Porosity in these materials---equivalent to free volume---arises from an unusual chain structure combining rigid segments with sites of contortion. Like other glasses, PIMs are subject to physical aging, which reduces the permeability of films over time. Although it is possible to derive useful information such as surface areas and pore sizes from the scattering patterns of many porous materials, scattering from PIMs includes some unusual features. A robust interpretation of these features is presented with support from molecular dynamics simulations. The sensitivity of PIM SAXS/WAXS patterns to time, temperature and film thickness is shown to be qualitatively consistent with physical aging. Models for extracting quantitative information about changes in the sizes and volume fraction of pores are also discussed. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R32.00002: Stress Applied during Vitrification Influencing the Subsequent Physical Aging of Polymer Glasses Laura A.G. Gray, Connie B. Roth How stress and mechanical deformation impart mobility to glasses is an active area of study across a range of glassy systems from polymers and small molecules, to colloids and granular materials. Conceptual frameworks such as the jamming phase diagram have been proposed to investigate if stress acts as another independent variable similar to temperature and density (volume fraction). Existing studies have focused primarily on applying stress or strain to a glassy state that has been formed stress free. Here, we investigate the stability of polymer glasses when stress is applied during the formation of the glassy state. We have constructed a jig to apply a known stress to free-standing polymer films during the thermal quench. Ellipsometry is used to measure the physical aging rate of these stress-quenched polystyrene films transferred onto silicon wafers by quantifying the time-dependent decrease in thickness that results from an increase in average density during aging. We observe a transition to a faster aging rate for stresses applied above a critical threshold. We hypothesize that increased stresses may trap the glassy state into higher, less stable potential energy minima resulting in faster aging rates. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R32.00003: Quench, equilibration, and subaging in structural glasses Joerg Rottler, Mya Warren In the glassy state, structural relaxations become increasingly sluggish with the wait time $t_w$ since vitrification. While most theoretical models of aging predict that the relaxation times $t_\alpha$ should increase linearly with the wait time, results from both experiments and simulations are frequently better described by a sublinear scaling: $t_\alpha \sim t_w^\mu$, with an aging exponent $\mu<1$. We show with molecular dynamics simulations of a Lennard-Jones glass former at various temperatures that this apparent ``subaging'' behavior may be explained by crossover effects from the freshly quenched state at short $t_w$, and into the equilibrated state at long $t_w$. Additionally, the aging behavior on the molecular level is quantitatively reproduced by a coarse-grained continuous time random walk description over the entire range of temperatures and wait times. Since this model is formally equivalent to the well known trap model of aging, this suggests that the Lennard-Jones glass belongs to the ``full'' aging class $t_\alpha \sim t_w$. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:42PM |
R32.00004: Dynamics and thermodynamics of polymer glasses Invited Speaker: Daniele Cangialosi The dynamics and thermodynamics of glass-forming systems have been the subject of intense research in the last decades. Among the variety of aspects that have been analyzed, the following can be included: i) the dramatic slowing down of the dynamics when decreasing temperature often described by a Vogel-Fulcher-Tammann (VFT) law; ii) the possible connection between such slowing down and the thermodynamics of the glass-former. These aspects have been deeply investigated above the laboratory glass transition temperature ($T_g$). It has been speculated that mere extrapolation of the dynamics and thermodynamics to low temperatures produces a singularity at a finite temperature. In particular, extrapolating the behavior above $T_g$ to low temperatures would imply that: (i) the relaxation time associated to the glassy dynamics shows a divergence; (ii) the entropy of the glass equals that of the crystal. Experimental as well as theoretical efforts in the sub-$T_g$ regime are required to clarify whether this scenario really exists. Recent experimental studies indicate deviations of the relaxation time from the VFT behavior to a milder temperature dependence [1,2] and several theoretical approaches provide a rationale to such deviations [3-7]. In this contribution the temperature range of dynamics and thermodynamics is extended to temperatures as low as $T_g-$40 K by performing enthalpy recovery experiments on glassy polymers for times up to 10$^7-$10$^8$ seconds. We find a single stage recovery behavior for temperatures larger than about $T_g-$10 K. Interestingly, a double stage recovery is observed for $T <$ $T_g-$ 10 K. In all cases the enthalpy recovered after the two-stage decay approximately equals that extrapolated from the melt. Time-temperature superposition close to each plateau in the enthalpy delivers shift factors containing information on the dynamics below $T_g$. The following scenario emerges analyzing the temperature dependence of the shift factors: i) In both stages of recovery, Arrhenius temperature dependence of the shift factor is observed; ii) The shift factor corresponding to the first stage recovery exhibits relatively low activation energy (several times smaller than that of the $\alpha$ process at $T_g$); iii) The second stage exhibits activation energy similar to that of the polymer $\alpha$ relaxation at $T_g$. These results indicate that divergence of the relaxation time at a finite temperature is likely avoided, whereas the question of a thermodynamic singularity remains open.\\[4pt] [1] P. O'Connell and G. B. McKenna, J. Chem. Phys. {\bf 110}, 11054 (1999).\\[0pt] [2] S. Simon, J. Sobieski, and D. Plazek, Polymer {\bf 42}, 2555 (2001).\\[0pt] [3] I. Avramov and A. Milchev, J. Non-Cryst. Solids {\bf 104}, 253 (1988).\\[0pt] [4] K. S. Schweizer and E. Saltzman, J. Chem. Phys. {\bf 119}, 1181 (2003).\\[0pt] [5] T. Hecksher, A. I. Nielsen, N. B. Olsen, and J. C. Dyre, Nat. Phys. {\bf 4}, 737 (2008).\\[0pt] [6] J. C. Mauro, Y. Yue, A. J. Ellison, P. K. Gupta, and D. C. Allan, Proc. Natl. Acad. Sci. U. S. A. {\bf 106}, 19780 (2009).\\[0pt] [7] Y. S. Elmatad, D. Chandler, and J. P. Garrahan, J. Phys. Chem. B {\bf 113}, 5563 (2009). [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R32.00005: Molecular mobility measurement during constant strain rate deformation of polymer glasses Benjamin Bending, Kelly Christison, M.D. Ediger We use a dye reorientation method to measure the segmental mobility in poly (methyl methacrylate) during active deformation. During constant strain rate deformation at 14 K below the glass transition we observe that mobility initially increases by up to a factor of 500, as compared to the starting undeformed mobility. After the softening regime the mobility remains constant as the strain is increased. Similar qualitative trends have been seen in simulations by Riggleman et al. and in the model of Govaert et al. Comparison of these simulations and model to our experiment will be the focus of this talk. In our previous studies of poly (methyl methacrylate) and polystyrene glasses deformed with a constant stress protocol (creep), at 16 K below the glass transition of the polymers we have seen a hundred-fold enhancement of mobility. Results from all these systems can be plotted on a master plot of mobility as a function of the local strain rate during creep deformation. We have found that this correlation holds for multiple glassy polymer systems, thermal and temporal histories, and with different deformation protocols. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R32.00006: Origin of mechanical stress from tensile extension of polymer glasses Panpan Lin, Shi-Qing Wang During uniaxial extension, polymer glasses undergo elastic deformation, yielding, strain softening, neck propagation, and ``strain hardening''. Both plasticity and anelasticity emerge under the large deformation, making the origin of the mechanic stress elusive to identify. The present work employs an IR camera to make \textit{in situ} temperature measurements on the extending specimen along with the conventional force measurements. To demonstrate the generality of our findings we studied the ductile polycarbonate as well as two brittle polymers, i.e., PS and PMMA, which can be made ductile by melt extension [1]. We found that the rate of heat generation is only a small fraction of the mechanical power involved in the uniaxial extension of these polymer glasses. Thus, it seems that the origin of the tensile stress is largely intrachain, stemming from straining of the chain network. \\[4pt] [1] Zartman, G. D.; Cheng, S. W.; Li, X.; Lin, F.; Becker, M. L.; Wang, S. Q. \textit{Macromolecules} \textbf{2012}, \textit{45}, 6719-6732. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R32.00007: Observation of yield in a triaxial deformation of a glassy thermoset polymer Grigori Medvedev, Jae-Woo Kim, James Caruthers Yield and post-yield behavior of amorphous polymers in a glassy state have been extensively studied in uniaxial tension and compression. In such geometry, the volume change is relatively small reaching the maximum value of approximately 0.5\% at the yield point. To study the role of the volumetric contribution a different geometry is needed. Here we report on the first observation of yield behavior in the longitudinal deformation, where the volume change is an order of magnitude higher than in the uniaxial test. The experiment is performed sufficiently close to Tg to ensure that yielding occurs before brittle failure. To characterize the evolution of the mobility/relaxation rate induced by deforming the glassy material through yield, a series of stress relaxation experiments are carried out at various pre-yield and post-yield strains. These experiments are done in uniaxial tension, compression and, for the first time in a longitudinal deformation. Implications of the observation of yield in a dilatation dominated deformation in addition to the traditional uniaxial and shear yield for the theories of glassy behavior and the development of constitutive models are discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:54PM |
R32.00008: Evidence for non-diverging time-scales in glass-forming liquids Invited Speaker: Gregory McKenna One perceived important signature of the ``ideal'' glass transition and of the complex fluid nature of glass-forming liquids remains the apparent divergence of the dynamics at temperatures above zero Kelvin. Recently, however, this perception has been increasingly challenged both through experiments and in new theories of the dynamics of glass forming systems. In this presentation we summarize some of the prior evidence suggesting that time scales actually do not diverge in glasses that are aged into equilibrium, perhaps 15 K below the conventional glass transition temperature $T_{g}$. We then show new results from an extremely densified glass, 20 Ma old Jamaican amber, in which we were able to obtain the upper bound to the relaxation times through a step-wise temperature scan in which the stress relaxation response of the amber was measured both below and above the fictive temperature $T_{F}. $We find that in the case of the upper bound responses at \textit{T\textgreater T}$_{F}, $there is a strong deviation of the response from the Super-Arrhenius Vogel-Fulcher behavior and this persists to the fictive temperature which is some 33.8 K below $T_{g}$. The results are compared to the parabolic model of Chandler and co-workers and we find the model to be consistent with our results if the value of $T_{x}$ in the model is taken to be the calorimetric glass transition temperature. The significance of the results will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R32.00009: Collective effects on activated segmental relaxation in supercooled polymer melts Stephen Mirigian, Kenneth Schweizer We extend the polymer nonlinear Langevin equation (NLE) theory of activated segmental dynamics in supercooled polymer melts in two new directions. First, a well-defined mapping from real monomers to a freely-jointed chain is formulated that retains information about chain stiffness, monomer volume, and the amplitude of thermal density fluctuations. Second, collective effects beyond the local cage scale are included based on an elastic solid-state perspective in the ``shoving model'' spirit which accounts for longer range contributions to the activation barrier. In contrast to previous phenomenological treatments of this model, we formulate an explicit microscopic picture of the hopping event, and derive, not assume, that the collective barrier is directly related to the elastic shear modulus. Local hopping is thus renormalized by collective motions of the surroundings that are required to physically accommodate it. Using the PRISM theory of structure, and known compressibility and chain statistics information, quantitative applications of the new theory to predict the temperature and chain length dependence of the alpha time, shear modulus, and fragility are carried out for a range of real polymer liquids and compared to experiment. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R32.00010: The Relationship of Dynamical Heterogeneity to the Adam-Gibbs and Random First-Order Transition Theories of Glass Formation Francis Starr, Jack Douglas, Srikanth Sastry We examine measures of dynamical heterogeneity for a bead-spring polymer melt and test how these scales compare with the scales hypothesized by the Adam and Gibbs (AG) and random first-order transition (RFOT) theories. We show that the time scale of the high-mobility clusters and strings is associated with a diffusive time scale, while the low-mobility particles' time scale relates to a structural relaxation time. The difference of the characteristic times naturally explains the decoupling of diffusion and structural relaxation time scales. We examine the appropriateness of identifying the size scales of mobile particle clusters or strings with the size of cooperatively rearranging regions (CRR) in the AG and RFOT theories. We find that the string size appears to be the most consistent measure of CRR for both the AG and RFOT models. Identifying strings or clusters with the``mosaic'' length of the RFOT model relaxes the conventional assumption that the``entropic droplet'' are compact. We also confirm the validity of the entropy formulation of the AG theory, constraining the exponent values of the RFOT theory. This constraint, together with the analysis of size scales, enables us to estimate the characteristic exponents of RFOT. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R32.00011: Enthalpy Recovery of Polystyrene: Is the Liquid Equilibrium Line Reached? Yung P. Koh, Sindee L. Simon Glasses are not in thermodynamic equilibrium below the glass transition temperature (T$_{\mathrm{g}})$, and consequently, their properties such as enthalpy, volume, and mechanical properties evolve toward equilibrium in a process known as structural recovery or physical aging. However, several recent studies have suggested that the equilibrium liquid line is not reached even when properties have ceased to evolve. In this work, we present measurements of the enthalpy recovery of polystyrene at the aging temperature of 15$^{\circ}$C below the nominal T$_{\mathrm{g}}$, for aging times up to 1 year. The results are analyzed in the context of the TNM model of structural recovery. The results show that the equilibrium liquid enthalpy line is indeed reached at temperatures below T$_{\mathrm{g}}$ when enthalpy recovery ceases to evolve. Our results will be discussed and compared to results from works leading to different conclusions. We also use our results to probe the issue of whether or not equilibrium relaxation times diverge from super-Arrhenius behavior below T$_{\mathrm{g}}$. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700