Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q43: Focus Session: Manipulating Glasses, Theory and Experiment |
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Sponsoring Units: DPOLY GSOFT Chair: Connie Roth, Emory University Room: 214C |
Wednesday, March 4, 2015 2:30PM - 2:42PM |
Q43.00001: Dynamic Correlation Length Scales under Isochronal Conditions at High Pressures Riccardo Casalini The origin of the dramatic changes in the behavior of liquids as they approach their vitreous state -- increases of many orders of magnitude in transport properties and dynamic time scales -- is a major unsolved problem in condensed matter. These changes are accompanied by greater dynamic heterogeneity, which refers to both spatial variation and spatial correlation of molecular mobilities. The question is whether the changing dynamics is coupled to this heterogeneity; that is, does the latter cause the former? To address this we carried out the first nonlinear dielectric experiments at elevated hydrostatic pressures on two liquids, to measure the third-order harmonic component of their susceptibilities. We extract from this the number of dynamically correlated molecules for various state points, and find that the dynamic correlation volume for non-associated liquids depends primarily on the relaxation time, sensibly independent of temperature and pressure. We support this result by molecular dynamic simulations showing that the maximum in the four-point dynamic susceptibility of density fluctuations varies less than 10{\%} for molecules that do not form hydrogen bonds. Our findings are consistent with dynamic heterogeneity serving as the principal control parameter for the slowing down of molecular motions in supercooled materials. [Preview Abstract] |
Wednesday, March 4, 2015 2:42PM - 2:54PM |
Q43.00002: Stochastic model prediction of the Kovacs' ``expansion gap'' effect for volume relaxation in glassy polymers Grigori Medvedev, James Caruthers The classic series of experiments by A. Kovacs on volume relaxation following temperature jumps for poly(vinyl acetate), PVAc, in the Tg region revealed the richness and complexity of the viscoelastic behavior of glassy materials. Over the years no theoretical model has been able to predict all the features of the Kovacs data, where the so-called ``expansion gap'' effect proved to be particularly challenging. Specifically, for a series of up-jump experiments with different initial temperatures, Ti, but with the same final temperature, as the relaxation approaches equilibrium it would be expected that the effective relaxation time would be the same regardless of Ti; however, Kovacs observed that the dependence on Ti persisted seemingly all the way to equilibrium. In this communication we will show that a recently developed Stochastic Constitutive Model (SCM) that explicitly acknowledges the nano-scale dynamic heterogeneity of glasses can capture the ``expansion gap'' as well as the rest of the Kovacs data set for PVAc. It will be shown that the success of the SCM is due to its inherent thermo-rheological complexity. [Preview Abstract] |
Wednesday, March 4, 2015 2:54PM - 3:06PM |
Q43.00003: A hybrid Brownian Dynamics model for yielding, aging, and rejuvenation in deforming polymeric glasses Weizhong Zou, Ronald Larson We describe the rheology of polymeric glasses by combining a simple constitutive equation for the fast segmental modes, borrowed from Fielding, et al.[1], with Brownian dynamics (BD) simulations of the slow polymer modes. The BD simulations determine the polymeric stress from ensembles of finitely extensible bead-spring chains, where the bead drag coefficient is governed by solutions to the equation for segmental relaxation. Thus the model treats the short glassy segmental mode as ``solvent'' for the polymer modes. With rubbery modulus for the slow-relaxing polymer modes as one of our model parameters, stress-dependent relaxation, physical aging, flow rejuvenation as well as strain-hardening and recovery can be successfully accounted for in uniaxial extension and steady shear, without the use of an artificial ``crinkle factor'' used to account for recoil dynamics in previous work [1]. Our simulation results remarkably agree with the experimental data from Lee et al.[2] A comparison between our model and the barrier-hopping theory [3] is also made.\\[0pt] [1] Fielding, S.M.; Larson, R.G.; Cates, M. E. Simple model for the deformation-induced relaxation of glassy polymers. Physical Review Letters, 2012, 108, 048301. [2] Lee, H-N.; Paeng, K.; Swallen, S.F.; Ediger, M.D. Direct measurement of molecular mobility in actively deformed polymer glasses. Science, 2009, 323, 231-234. [3] Chen, K.; Schweizer, K.S. Theory of relaxation and elasticity in polymer glasses. The Journal of Chemical Physics, 2007, 126, 014904. [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:42PM |
Q43.00004: Structural Recovery of Glass-Forming Materials Invited Speaker: Sindee Simon The glass transition and structural recovery of glass-forming materials, including polymeric, small-molecule, and inorganic network glasses, have been studied in our laboratory using dilatometry and calorimetry. Of particular interest have been the relative timescales to reach equilibrium for different properties, whether the extrapolated liquid line is reached at equilibrium, the ability of phenomenological models to describe the structural recovery process, and behavior at the nanoscale. Recent work using a commercial rapid-scanning calorimeter has extended the time and temperature range to times as short as 0.01 s and temperatures up to 15 K above the nominal T$_{\mathrm{g}}$. Our results will be discussed in the context of unanswered questions in the field. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q43.00005: Quantitative relaxation dynamics of supercooled liquids from first principles Liesbeth Janssen, Peter Mayer, David Reichman Understanding the liquid-to-glass transition remains one of the deepest unsolved problems in condensed matter science. Here we present a novel first-principles framework, referred to as generalized mode-coupling theory (GMCT), which can predict the microscopic dynamics of glass-forming systems with near-quantitative accuracy using only simple static information as input. The theory is based on the well-established standard mode-coupling theory (MCT) of the glass transition, but rigorously incorporates higher-order dynamic density correlations neglected in standard MCT. We demonstrate that GMCT can accurately describe the dynamics of quasi-hard spheres over an unprecedentedly large time and density domain, supporting the view that activated glassy behavior is inherently dynamic in origin. Finally, our schematic results show that GMCT is capable of predicting novel types of glass transitions, including type-A, type-B, and avoided transitions, as well as different types of relaxation-time scaling behaviors. This suggests that GMCT may constitute the first microscopic, first-principles theory that can account for different fragilities in glass-forming materials. [Preview Abstract] |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q43.00006: Using s-ensemble to probe glasses formed by cooling and aging David Chandler, Juan P. Garrahan, Aaron S. Keys Space-time phase transitions can be studied within the context of large-deviation formalism. Numerical implementations of this formalism to lattice models and atomistic models have demonstrated the existence of such transitions between liquid-like (i.e., dynamically active) and glassy (i.e., dynamically inactive) phases. Here, in terms of an emergent nonequilibrium correlation length and formulas derived from dynamical facilitation theory [1,2], we describe how glassy states obtained from the large-deviation formalism (i.e., the s-ensemble) are equivalent to the glassy states obtained from nonequilibrium cooling protocols. We test the formulas with lattice models, and we demonstrate that the formulas are consistent with nonequilibrium calorimetry experiments [3].\\[4pt] [1] J. P. Garrahan {\&} D. Chandler, PNAS 100, 9710 (2003)\\[0pt] [2] A. S. Keys et al. Phys. Rev. X 1, 021013 (2011)\\[0pt] [3] A. S. Keys, J. P. Garrahan {\&} D. Chandler, PNAS 110, 4482 (2013) [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q43.00007: Nontrivial correlation length distinguishes melt from glass in a large-scale atomistic non-equilibrium simulation of a glass transition Kranthi Mandadapu, Alexander Hudson, David Chandler Dynamical facilitation theory [1,2] predicts the emergence of a non-trivial correlation length from the cooling process that transforms the reversible melt to the irreversible glass [3]. A decrease in cooling rate produces an increase in correlation length, and an increase in correlation length coincides with an increase in stability and aging rate of the glass. Here, we present results from a large-scale non-equilibrium numerical simulation that provide the first demonstration of the emergent nonequilbrium correlation length for an atomistic model. The study also tests the ability of the theory to predict the value of the nonequilibrium length and its corresponding glass transition temperature in terms of material properties and cooling protocols. \begin{enumerate} \item J. P. Garrahan {\&} D. Chandler, PNAS 100, 9710 (2003) \item A. S. Keys et al. Phys. Rev. X 1, 021013 (2011) \item A. S. Keys, J. P. Garrahan {\&} D. Chandler, PNAS 110, 4482 (2013) \end{enumerate} [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q43.00008: Thwarting Crystallization through Hydrogen Bonding in Triazine Molecular Glasses Audrey Laventure, Armand Soldera, Olivier Lebel, Christian Pellerin Using irregular shaped molecules interacting weakly with each other is the most intuitive choice to generate amorphous molecular materials. In contrast, H-bonds are commonly used in crystal engineering to create predictable ordered and well-packed structures. In spite of this fact, Lebel et al. have demonstrated that H-bonds can be used efficiently to thwart crystallization by inducing the self-assembly of aggregates that pack poorly. Since 2006, libraries of triazine derivatives with a variety of different substituents capable of forming molecular glasses have been synthesized and studied. Their outstanding glass-forming ability (with critical cooling rates lower than 0.5 $^{\circ}$C/min) and their wide range of Tg (from below ambient temperature up to 160 $^{\circ}$C) make them an attractive amorphous model system to deepen our understanding of the relationship between microscopic features and macroscopic behavior of glasses. In this presentation, we will show that variable-temperature infrared spectroscopy is a tool of choice to probe the vitreous state of these compounds. We take advantage of the selectivity of this technique to correlate their molecular features to their thermal properties. Quantitative monitoring of hydrogen bonds during vitrification will be addressed. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q43.00009: Theory of the Role of Attractive Forces in the Dynamics of Supercooled Liquids under Isochoric and Isobaric Conditions Zachary E. Dell, Kenneth S. Schweizer Microscopic, force-level, dynamical theories of supercooled liquids (e.g., mode-coupling, elastically cooperative nonlinear Langevin equation (ECNLE)) employ a projection approximation that replaces the real forces by a single effective force determined by the equilibrium pair structure. Recent isochoric simulations by Berthier and Tarjus suggest that under isochoric conditions such theories fail to capture the effect of attractions which might not change structure but can strongly slow relaxation. We propose a hybrid dynamical theory where the ECNLE approach with effective forces is employed for repulsions, but for attractions a new projection-less description is developed where the attractive forces directly enter. The theory is applied to the Lennard-Jones and repulsive WCA fluids in the supercooled regime. Under isochoric conditions, we find qualitative agreement with the recent simulations, where attractive forces are important depending on the specific density. Under isobaric conditions, attractive forces are found to be unimportant due to the combined effect of thermal contraction and increasing effective particle size upon cooling. The new theory exhibits density scaling, and realistically predicts the relaxation time of molecular liquids over 14 decades in time. [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q43.00010: Relation of dynamics and local structure to glass-formability in a crystallizable bead-spring polymer model Hong Nguyen, Tyler Smith, Robert Hoy, Nikos Karayiannis We relate the dynamics and local structure of equilibrium and supercooled polymer melts using a model wherein a single parameter (bending stiffness) controls the morphology of the equilibrium, low-temperature crystal. The dynamical slowing down in strongly glassforming systems correlates directly to the increasing presence of microstructural features that are incompatible both with each other and with crystalline order. Systems which more readily crystallize also exhibit rich behavior since their solid-state morphology can be varied from nearly amorphous to highly crystalline by varying their thermal preparation protocol. We tie the ``critical'' cooling rates, across which this behavior varies, to the lifetimes of structural features such as small crystalline nuclei and stable liquid-like clusters. The role such structures play is analogous to that recently demonstrated for colloidal systems [S. R. Williams, arXiv:0705.0203, 2007], but is considerably enriched both by the dynamical constraints imposed by covalent connectivity and by the presence of a second characteristic length scale (the polymer Kuhn length) controlled by chain bending stiffness. [Preview Abstract] |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q43.00011: Connecting Thermodynamic Trends to the Polymer Glass Transition Ronald White, Jane Lipson In studying glassy behavior the utility of measuring dynamic properties has been well established, while relating to thermodynamic properties is still a question. Which thermodynamic properties of a system have the potential to reveal its glassy nature? Are there patterns for example, in free volume, or pressure-related behavior, or molecular stiffness or other characteristics? If so, could trends in any of these properties predict the temperature ($T_{\mathrm{g}})$ of the glass transition? In this work we apply our simple locally correlated lattice (LCL) model equation of state to a sizeable sample of different polymers. Using the model, we require just a minimal amount of experimental data to deduce the underlying molecular parameters, and from this we generate a complete thermodynamic description of that system for analysis and comparison. As a result a number of interesting trends have emerged and in this talk we will discuss how they correlate with $T_{\mathrm{g}}$. [Preview Abstract] |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q43.00012: Dynamic Odd-Even Effects in a Network-Forming~Ionic Glass Homologue Ke Yang, Madhusudan Tyagi, Jeffrey Moore, Yang Zhang Odd-even effects, the non-monotonic dependency of physical properties on odd/even number of structural units, are widely observed in homologous series of crystalline materials. However, such alternation is not expected for amorphous molecular materials because absence of periodic packing. Herein, we report the synthesis and characterization of a class of stable network-forming ionic glasses with specific structure to frustrate crystallization. We performed incoherent elastic neutron scattering measurements of the nanosecond mean squared displacement and quasi-elastic neutron scattering measurements of the nanosecond relaxation time. The results indicated that the even-numbered cations showed much slower dynamics than neighboring odd-membered cations, i.e. one-methylene unit structural difference causes an abnormal difference (about five times) in cations' diffusional dynamics. The observed difference in mobility exists both around T$_{g}$ and extends to the liquid temperature regime well above T$_{g}$. The observed dynamic odd-even effect serves as another intriguing example of significant dynamic differences induced by insignificant structural changes, as is common in glass transition. [Preview Abstract] |
Wednesday, March 4, 2015 5:18PM - 5:30PM |
Q43.00013: Glass formation behavior of an isolated polymer chain Weston Merling, Jack Mileski, David Simmons A single polymer chain in isolation logically represents the extreme limit of nanoconfinement with respect to segmental dynamics and glass formation. Work in thin polymer films suggests that one should expect a large Tg suppression in such systems. However, recent dielectric relaxation measurements of isolated chains of P2VP on a silica substrate found bulk-like Tg in this system, apparently raising questions about the nature of observed nanoconfinement effects on the glass transition. Here we describe simulations of glass formation in an isolated polymer chain, both free-floating and deposited on a substrate. Results indicate that free-floating isolated polymer chains exhibit a depression in the dynamic glass transition temperature equivalent to more than 100K in polystyrene units. However, when a chain is deposited on a substrate with sufficiently favorable surface interactions, bulk-like dynamics can be recovered due to competition between the free-surface and adsorbed interface. When this compensation effect is taken account, these results indicate that the observation of bulk-like dynamics in isolated P2VP chains on a silica substrate is consistent with observations of large Tg suppressions in polymer films supported by less attractive substrates. [Preview Abstract] |
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