Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Z19: Supercooled Polymer Liquids and Glasses |
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Sponsoring Units: DPOLY Chair: Zahra Fakhraai, University of Pennsylvania Room: 404 |
Friday, March 7, 2014 11:15AM - 11:27AM |
Z19.00001: Nano-composites obtained by phase separation of polymer blends close to and below Tg Gregoire Julien We propose a model for describing the dynamics in polymer blends close to and below Tg. The model is solved on a 2D lattice corresponding to spatial scales a few 100 nm and a resolution corresponding to the scale of dynamical heterogeneities. In the course of spinodal decomposition at low temperatures we observe slow structures building, which coexist with faster ones. Simultaneously, the distribution of relaxation times evolves toward longer times, corresponding to an ageing process. Domains are found to grow like the logarithm of the time. By tuning the physical parameters of the systems (polymer mass, polymer Tg's and volume fraction) it should help making nano-composites with controlled morphologies on a scale typically of order 10 nm. We study also the melting dynamics of these systems, after the temperature is increased again in the totally miscible range. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z19.00002: Effect of Counteranion on Caged-like Dynamics of 1-alkyl-3-methylimidazolium-based Ionic Liquids Jenny Kim, Cheol Jeong, Madhu Tyagi, Christopher Soles Understanding physicochemical properties of ionic liquids (ILs) is essential to realize task-specific ILs. To better understand the structural and dynamic heterogeneity in ILs, we conducted quasielastic neutron scattering (QENS) measurements that cover time ranges from picosecond to nanosecond. Series of 1-alkyl-3-methylimidazolium-based ILs is chosen to explore the relationship between local dynamics and long-range translational dynamics. Two distinct dynamical processes have been examined: caged-like dynamics and jump-diffusion processes. Size and shape of a dynamic cage can be obtained by fitting elastic incoherent structure factor (EISF) with spherical and cylindrical Bessel functions. The cage geometry turns out to be strongly dependent on the counteranion. Residence time of ions or molecules attained from jump-diffusion model increased by several factors for the ILs with smaller and more isotropic cage which will lead to slower diffusion. Mean square displacements coupled with viscosity and conductivity by generalized localization model also confirms the relevance of caging to the ion transport and structural relaxation of ILs. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z19.00003: Dynamical Heterogeneities in Glasses from Fluctuating Mobility Generation and Transport: Two Equilibration Mechanisms in Glasses Apiwat Wisitsorasak, Peter G. Wolynes In the random first order transition theory, fluctuating mobility generation and transport are explored from an extended mode coupling theory of the glass transition that includes activated events. We numerically solve the continuum equations of mobility and temperature fields and find that the fluctuations which account for spatiotemporal structure in aging and rejuvenating glasses lead for dynamical heterogeneity in glass. Non-Gaussian distribution of free energy, stretch exponent $\beta$, and growing characteristic length are presented along with the four-point correlation function. Our results demonstrate that two equilibration mechanisms that have been observed in aged polymer glasses are the results of the heterogeneity and out-of-equilibrium behavior of glasses below the glass transition temperature. Numerical results of distribution of relaxation time agree with experimental observations. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z19.00004: Plastic Flow of Polymer Chains below $T_{\mathrm{g}}$ Induced by Jamming Transition Chao Teng, Gi Xue Polymer chains begin to flow when they are heated above $T_{\mathrm{g}}$. Other glassy systems, such as colloidal suspensions and granular materials, begin to flow when subjected to sufficiently large stresses. The equivalence of these two routes to flow is a basic tenet of jamming theory. However, a full understanding of jamming transition for polymer chains remains elusive. In this work, we adjust the polymer chain packing density by spry-drying and some other methods, and then apply shear stress at temperature far below its $T_{\mathrm{g}}$. The resulting pellet shows very similar features as the hot processed or solution casting samples, which strongly indicates that the plastic flow of polymer chains ever happened below $T_{\mathrm{g}}$. We found that the packing density and shear stress play important roles during the plastic flow process at low temperature, which is according with the jamming theory. This kind of plastic flow at low temperature shows its advantages when processing materials with bioactivity, which will be deactivated at high temperature. Furthermore, these findings also suggested an approach to understand the high mobility of surface layer of polymer thin film and nanoparticles. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z19.00005: Dynamical heterogeneity and structural relaxation in periodically deformed polymer glasses Nikolai Priezjev The dynamics of structural relaxation in a model polymer glass subject to spatially-homogeneous, time-periodic shear deformation is investigated using molecular dynamics simulations. We consider a coarse-grained bead-spring model of short polymer chains below the glass transition temperature. It is found that at small strain amplitudes, the segmental dynamics is nearly reversible over about 10,000 cycles, while at strain amplitudes above a few percent, polymer chains become fully relaxed after a hundred cycles. At the critical strain amplitude, the transition from slow to fast relaxation dynamics is associated with the largest number of dynamically correlated monomers as indicated by the peak value of the dynamical susceptibility. The analysis of individual monomer trajectories indicated that mobile monomers tend to assist their neighbors to become mobile and aggregate into relatively compact transient clusters. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z19.00006: Dramatic alteration of Tg of polystyrene confined in cylindrical nanopores Gi Xue, Chao Teng, Jie Xu, Linling Li Vitrification of polystyrene melt infiltrated in AAO template was greatly altered by geometrical confinement. DSC detected two distinguish Tgs for PS slowly cooled from the melt in nanopores. One was at a lower temperature than the bulk Tg and the other was at a higher temperature. The deviation between the two Tgs could reach as big as 60 $^{\circ}$C and increased with decreasing pore size. Surprisingly, Tg for the PS nanorodes recovered back to the bulk value when the template was removed, indicating the importance of the interfacial interaction. The major factor which induced such a strong confinement is the cooling rate. The coefficient of thermal expansion (CTE) of AAO template is about one order lower than that for PS. The mismatch of CTEs causes a strain induced stress during cooling: the PS chains tend to shrink and to de-wet from the walls; meanwhile the interfacial interaction tends to hold the chains back. The chains are subjected to a high residual stress by a fast cooling and would peel off from the wall. When the cooling rate is sufficiently slow, and the stress was relaxed during cooling. Then the chains were still stick on the wall, resulting in a strong interfacial constraints for chains. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z19.00007: Structural and Dynamical Heterogeneities in Thin FIlms of a Generic Glass-forming Liquid Amir Haji-Akbari, Pablo G. Debenedetti Recent discovery of stable glasses by Ediger and coworkers [1] has spurred an interest in structural and dynamical properties of atomic and molecular thin films. Here, we use molecular dynamics simulation to study thin films of a model glass-forming liquid, the Kob-Andersen binary Lennard Jones system [2], and compute profiles of structural properties such as densities, potential energies, stresses and lateral radial distribution functions, as well as dynamical properties such as relaxation times across the film. We observe the liquid to be stratified in the vicinity of the wall, but this stratification is not always accompanied by long-range order. We also observe two distinct dynamical regimes close to the liquid/solid interface. For weakly-interacting walls, a highly mobile region emerges with relaxation times smaller than the bulk, while for strongly-interacting walls, relaxation times can be several orders of magnitude larger in the same region than in the bulk. We are able to establish correlations between density modulations and normal stress modulations, and between relaxation time modulations and lateral stress modulations in the disordered regions of the film. \\[4pt] [1] Swallen SF, et al., Science 315: 353 (2007).\\[0pt] [2] Kob W, Andersen HC, Phys. Rev. E 51: 4626 (1995). [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z19.00008: Study of physical vapor deposited glasses of tris-naphthyl benzene based organic molecules Yue Zhang, Tianyi Liu, Ethan Glor, Guoyu Yang, Yi-Chih Lin, Zahra Fakhraai Stable glasses can be prepared by physical vapor deposition method (PVD), and these stable glasses will show greatly different properties compared to ordinary glasses prepared upon cooling a liquid, including higher density, higher thermal stability, increased charge transport ability and so on. Different organic molecule structures are also responsible for different glass structures and related glass properties. These properties strongly depend on the deposition temperature. We work with small organic molecules tris-naphthyl benzene (TNB) and molecules based on TNB that have slightly different substituents. These molecules will have different molecular weight and chemical structure such as $\pi $ stacking, which will have an influence on the structure of stable glasses, their glass transition temperature, Tg and density. In our study, we can vary both the physical and chemical properties of these organic molecules in a systematical way to have a better understanding of the relationship between molecule structure, glass structure and related properties. These studies allow us to probe whether the stability of these glasses correlate with their glass transition temperature, Tg or their chemical structure. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z19.00009: Glasses and Liquids Low on the Energy Landscape Prepared by Physical Vapor Deposition Shakeel Dalal, Zahra Fakhraai, Mark Ediger The lower portions of the potential energy landscape for glass-forming materials such as polymers and small molecules were historically inaccessible by experiments. Physical vapor deposition is uniquely able to prepare materials in this portion of the energy landscape, with the properties of the deposited material primarily modulated by the substrate temperature. Here we report on high-throughput experiments which utilize a temperature gradient stage to enable rapid screening of vapor-deposited organic glasses. Using ellipsometry, we characterize a 100 K range of substrate temperatures in a single experiment, allowing us to rapidly determine the density, kinetic stability, fictive temperature and molecular orientation of these glasses. Their properties fall into three temperature regimes. At substrate temperatures as low as 0.97T$_g$, we prepare materials which are equivalent to the supercooled liquid produced by cooling the melt. Below 0.9T$_g$ (1.16T$_K$) the properties of materials are kinetically controlled and highly tunable. At intermediate substrate temperatures we are able to produce materials whose bulk properties match those expected for the equilibrium supercooled liquid, down to 1.16T$_K$, but are structurally anisotropic. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z19.00010: Resolving the Puzzle of Two Glass Transitions in Miscible Polymer Blends Jacek Dudowicz, Jack Douglas, Karl Freed The existence of two glass transitions has widely been observed in experiments for miscible polymer blends. Qualitative explanations postulate models of local concentration enhancements (depletions) of component 1 (2) in the neighborhood of a chain of species 1 (2). The occurrence of two glass transition temperatures is analyzed and explained by using a merger of three statistical mechanical theories: the generalized entropy theory for glass-formation in binary homopolymer blends, the lattice cluster theory for the thermodynamics of binary polymer blends, and Kirkwood-Buff theory for concentration fluctuations in binary mixtures. Specific computations of glass transition temperatures are provided for blends of semi-flexible linear chains with varying stiffness. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z19.00011: Unified Theory of Activated Relaxation in Cold Liquids over 14 Decades in Time Kenneth Schweizer, Stephen Mirigian We formulate a predictive theory at the level of forces of activated relaxation in thermal liquids that covers in a unified manner the apparent Arrhenius, crossover and deeply supercooled regimes (J.Phys.Chem.Lett.4,3648(2013)). The alpha relaxation event involves coupled cage-scale hopping and a long range cooperative elastic distortion of the surrounding liquid, which results in two inter-related, but distinct, barriers. The strongly temperature and density dependent collective barrier is associated with a growing length scale, the shear modulus and density fluctuations. Thermal liquids are mapped to an effective hard sphere fluid based on matching long wavelength density fluctuation amplitudes. The theory is devoid of fit parameters, has no divergences at finite temperature nor below jamming, and captures the key features of the alpha relaxation time in molecular liquids from picoseconds to hundreds of seconds. The approach is extended to polymer liquids based on the Kuhn length as the key variable. The influence of chain length and backbone stiffness on the glass transition temperature and fragility have been studied where degree of polymerization enters via corrections to asymptotic conformational statistics. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z19.00012: Effects of hydrophobic aggregation on the charge transport mechanism of quaternary ammonium ionic liquids Philip Griffin, Adam Holt, Yangyang Wang, Vladimir Novikov, Joshua Sangoro, Alexei Sokolov Aprotic quaternary ammonium ionic liquids (ILs) are an important class of ILs due to their large electrochemical window and hydrophobicity. However, many of these ILs suffer from relatively low conductivity at room temperature which limits their use in electrochemical applications. In order to understand the nature of this low conductivity and its relation to the chemical structure of the alkyl ammonium cation, we have measured the charge transport properties and structural dynamics of the room temperature ionic liquid methyltrioctylammonium bistriflimide [m3oa][ntf2] over a broad temperature range using dielectric spectroscopy, dynamic light scattering, rheology, and pulsed field gradient nuclear magnetic resonance. We demonstrate that the low values of dc conductivity are due to the combined effects of significantly reduced ion mobility as well as reduced free ion concentration relative to other types of ILs. Secondly we find evidence for a mesoscopic scale structural relaxation process that we attribute to the reorientational motion of nanometer sized alkyl nanodomains. These two findings indicate that hydrophobic aggregation plays an important role in the charge transport mechanism of aprotic ammonium ionic liquids with long aliphatic side chains. [Preview Abstract] |
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