Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session B18: Polymer Glasses: Formation, Aging, and Nonlinear Response |
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Sponsoring Units: DPOLY Chair: Laura Gray, Princeton University Room: 277 |
Monday, March 13, 2017 11:15AM - 11:27AM |
B18.00001: Insights into Polymer Normal Mode Dynamics During Glass formation from Efficient In-Equilibrium Molecular Dynamics Simulation Jui-Hsiang Hung, Tarak Patra, Jayachandra Hari Mangarala, David Simmons The Rouse model of chain dynamics is the foundational model for the normal mode dy-namics of unentangled polymer melts. A central prediction of this model is that the tem-perature dependence of a chain's normal mode relaxation time is coupled to the tempera-ture dependence of its segmental relaxation time. However, studies of polymers near their glass transition temperature Tg have sometimes observed a failure of this coupling. Most commonly, end-to-end dynamics and viscosity are observed to exhibit a weaker tempera-ture dependence than segmental dynamics in this temperature range, signaling a break-down in time-temperature superposition. Here we describe long-time molecular dynamics simulations of chain normal mode dynamics in the supercooled regime. Results indicate that a decoupling of chain normal modes leads to an incipient `crisis' at which whole chain relaxation is extrapolated to occur more rapidly than segmental relaxation at tempera-tures below Tg. We compare this behavior to the predictions of several established models of glass formation in order to elucidate its physical origins. [Preview Abstract] |
Monday, March 13, 2017 11:27AM - 11:39AM |
B18.00002: Comparative molecular simulation study of low and high density polymer glasses: A competition between attractive and repulsive interactions Jalim Singh, Prasanth Jose Results of molecular dynamics simulations of a system of Kremer and Grest linear polymer melts are presented at moderate and high number density. A detailed study of molecular pair distribution function shows that potential of mean force between the molecules has form of Gaussian with an attractive tail at number density $\rho$ = 0.85 (in Lennard-Jones units), which is due to the dominating attractive interactions from temperature $T$ = 0.7. This system shows gelation assisted glass transition, which is interpreted from peaks of molecular structure factor at small wave-numbers. At low temperature, this system phase separate to form dense domains whose local density is high; these domains show many dynamical features of glass transition in monomer and molecular level of relaxation indicating glass transition is assisted by gelation in this system. In the same system, at $\rho$ = 1.0, repulsive interactions dominate, structure does not change even at low temperatures; the system exhibits dynamic heterogeneity and known to undergo glass transition. In this work, we compare and contrast the structure and dynamics of the system near its glass transition. Also, we computed correlation length of systems from the peak value of four-point structural dynamic susceptibility. [Preview Abstract] |
Monday, March 13, 2017 11:39AM - 11:51AM |
B18.00003: liquid-solid transitions in a bridging system with short-ranged attractive interparticle potential Guangcui Yuan, Junhua Luo, Chuanzhuang Zhao, Charles C. Han we approach to the liquid-solid transitions problem from a very fundamental point---build a model system with simple and tunable inter-particle potential, then investigate the effect of the inter-particle potential (mainly the attractive part) on the transitions, which includes gelation at low packing density and glass formation at high packing density. A peculiar way to control inter-particle attraction by using mixed suspensions of large hard colloid and adsorptive small soft microgel will be introduced, in which the small particle can serve as a bridge to connect neighbouring large particles thus to introduce the bridging attraction. We determined the positions of the state-transition boundaries and describe the characters of these transitions, from structural, dynamical, and thermodynamic point of views. Our results indicate that the attraction force between the added small polymers and the large particles (or the origin of effective inter-particle potentials, or maybe the very details of attractive potentials) have a fundamental impact on the mechanism of liquid-solid transition. Under this direction, we will give our interpretation on the glass transition of structural materials. [Preview Abstract] |
Monday, March 13, 2017 11:51AM - 12:03PM |
B18.00004: String-like Collective Motion in the $\alpha $- and $\beta $- Relaxation of a Coarse-Grained Polymer Melt. Jack Douglas, Beatriz Betancourt, Francis Starr The relaxation of glass-forming liquids occurs as a two-stage process- a $\beta $-relaxation process having a relaxation time $\tau_{\beta \thinspace \thinspace }$on the order of \textit{ps,} followed by an $\alpha $-relaxation process having a relaxation time $\tau_{\alpha \thinspace \thinspace }$that ranges from \textit{ps} to \textit{min} as the fluid is cooled towards its glass transition temperature. Of course, the dramatic change of $\tau_{\alpha \thinspace }$with temperature garners the most attention because the impressive changes in $\tau_{\alpha \thinspace }$ and direct relevance of these changes to applications of glassy materials, but there has also been much interest in $\beta $-relaxation observed in neutron and other high frequency measurement methods. We investigate a model glass-forming polymer melt and establish that collective motion has a large influence on relaxation in both the $\beta $- and $\alpha $-relaxation regimes where in both regimes the collective motion takes the form string-like exchange motion of the polymer segments. The temperature dependence of the average string length is \textit{inverted} in the $\beta $- and $\alpha $-relaxation regimes where we see a progressive suppression of collective motion upon cooling in the $\beta $-relaxation regime leads to a corresponding increase in the scale of collective motion in the $\alpha $ relaxation regime. We are able to model the string formation in both regimes in terms of equilibrium polymerization models. [Preview Abstract] |
Monday, March 13, 2017 12:03PM - 12:15PM |
B18.00005: Why many polymers are so fragile: a new perspective Vladimir Novikov, Cecile Dalle –Ferrier, Alexander Kisliuk, Liang Hong, Giovanni Carini Jr, Giuseppe Carini, Giovanna D’Angelo, Christiane Alba-Simionesco, Alexei Sokolov Many polymers exhibit much higher fragility than liquids of small molecules. Its mechanism remains a puzzle. We analyzed correlation of many properties of polystyrene to its fragility for samples with various molecular weights (MW). We demonstrate that these correlations work for short chains, but fail with increase in MW. Fragility of the viscosity that is determined by chain relaxation follows the correlations at all molecular weights. These results suggest that the molecular level relaxation follows the behavior usual for small molecules even in polymers, while segmental relaxation has unusually high fragility. We speculate that many polymers cannot reach an ergodic state on the time scale of segmental dynamics due to chain connectivity and rigidity. This leads to decrease in accessible configurational entropy upon cooling and results in higher fragility of segmental relaxation. This scenario provides a new insight in polymer dynamics: the role of ergodicity time and length scale. [1] C. Dalle –Ferrier, et al, J. Chem. Phys. 145, 154901 (2016). [Preview Abstract] |
Monday, March 13, 2017 12:15PM - 12:27PM |
B18.00006: How Free Volume Controls Polymer Segmental Relaxation Times Ronald White, Jane Lipson In this talk we calculate the free volume in polymer melts and map out the underlying connection with temperature- and pressure- dependent segmental relaxation times. Free volume has had a long and controversial history in the polymer physics community. Historical "free volume models" have failed in explaining pressure-dependent dynamics. A problem with some of these models has been that they assumed an $a$ \textit{priori} connection between dynamics and free volume. We do not make this assumption. Instead we use our locally correlated lattice (LCL) model equation of state to determine free volume in polymer melts first, and then, we look for correlations with the experimentally measured dynamics data. In our discussion we will propose predictive relationships for dynamics wherein free volume plays an important role. [Preview Abstract] |
Monday, March 13, 2017 12:27PM - 12:39PM |
B18.00007: Evidence for a universal localization transition underlying the glass transition David Simmons, Jui-Hsiang Hung, Tarak Patra, Venkatesh Meenakshisundaram, Jayachandra Hari Mangalara The glass transition is a ubiquitous pathway to the development of solid-like character, occurring in materials ranging from polymers to metals. Despite its technological and fundamental importance across diverse materials, the underlying nature of the glass transition remains a durable open question. Here we describe results from high-throughput simulations of the glass transition in metals, polymers, small organic molecules, and organics, indicating that a universal particle localization transition underlies the dynamic glass transition. We find that a single adjustable parameter is sufficient to describe the nonuniversal growth in relaxation time resulting from this localization event. These results point to an opportunity to advance the modern understanding of the glass transition by refocusing attention on the onset of localization rather than the growth in relaxation time as the key experimental observable. [Preview Abstract] |
Monday, March 13, 2017 12:39PM - 12:51PM |
B18.00008: Dynamical heterogeneity of star-polymers Hamed Emamy, Alexandros Chremos, Jack Douglas, Francis Starr The formation of a glass is one of the most vital features of amorphous polymers. While this subject has been exhaustively studied for linear chain polymers, comparatively little is known about the glass formation of star polymers, one of the most important classes of branched polymers. Using molecular dynamics simulation methods, we study the dynamical heterogeneity of star-polymers. We characterize the cooperative nature of the dynamic properties melts via the non-Gaussian nature of displacements, four-point correlations, clusters of highly mobile monomers, and subsets of string-like monomer motion. We contrast the behavior to that of ordinary linear chains, considering the role of both number of arms and molecular weight. In doing so, we quantify the degree to which the topology of star polymer plays a role in dynamical heterogeneity. [Preview Abstract] |
Monday, March 13, 2017 12:51PM - 1:03PM |
B18.00009: Retarded Local Dynamics of Single Fluorescent Probes in Polymeric Glass due to Interaction Strengthening Hao Zhang, Jingfa Yang, Jiang Zhao The effect of strengthening of interaction between single fluorescent probes and polymer matrix to the probes’ dynamics is investigated using single molecule fluorescence defocus microscopy. By introducing multiple hydroxyl groups to the fluorescent probes, which builds up hydrogen bonds between the probe and polymer matrix, the dynamics is discovered to be retarded. This is evidenced by the lowering of the frequency of the vibrational modes in the power spectra of the rotation trajectories of individual fluorescent probes, and also by the lowering of population of rotating probes. The results show that by strengthening the probe-matrix interaction, the local dynamics detected by the probes is equivalent to that detected by a bigger probe, due to the enhanced friction between the probe and the polymer matrix. [Preview Abstract] |
Monday, March 13, 2017 1:03PM - 1:15PM |
B18.00010: Dynamical heterogeneities, shear banding and internal stress in polymer melts Robin Masurel, Pierre Gelineau, Sabine Cantounet, Hélène Montes, Didier Long, Alain Dequidt, Francois Lequeux As evidenced since about two decades, amorphous polymers present important dynamical heterogeneities at the scale of a few nanometers close in the vicinity of their glass transition. This means that they can be represented as a tiling of nanometric domains, each one with a dynamics very different from its neighbors. We show that these heterogeneities that are known to govern the linear response dynamics, are also responsible for various features of the non-linear mechanical response. For that purpose, we just assume that each domain follows an Eyring law i.e. its relaxation time depends on the stress it undergoes. We show that this simple idea permits to describe the formation of shear bands, the narrowing of relaxation times distribution during loading observed experimentally, and the existence of an unrecoverable elastic energy after unloading. The two last properties are predicted quantitatively without adjustable parameters. [Preview Abstract] |
Monday, March 13, 2017 1:15PM - 1:27PM |
B18.00011: Features of Structural Relaxation in Diblock Copolymers Yunlong Guo, Mingchao Ma, Tianju Xue Time- and temperature-dependent structural relaxation (physical aging) of poly(styrene-b-methyl methacrylate) (PS-b-PMMA) block copolymers was investigated by calorimetry. Our study reveals the interplay of the relaxation responses of the two components of the copolymer in an intermediate temperature regime. That is, when the testing temperature is closely below the glass transition temperatures of PS and PMMA, structural relaxation in these polymer phases takes place concurrently, the corresponding thermogram displays partially superposed dual endothermic peaks as a feature of physical aging in the diblock copolymers. The aging response for each component is identified from a curve fitting method and analyzed by the relaxation of enthalpy. Comparing with the homopolymer analogs, the PS and PMMA in diblock copolymers show enhanced aging rate. [Preview Abstract] |
Monday, March 13, 2017 1:27PM - 1:39PM |
B18.00012: Abstract Withdrawn
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Monday, March 13, 2017 1:39PM - 1:51PM |
B18.00013: Determination of the Nonlinearity Parameter in the TNM Model of Structural Recovery Rozana Bari, Sindee Simon Structural recovery of non-equilibrium glassy materials takes place by evolution of volume and enthalpy as the glass attempts to reach to equilibrium. Structural recovery is nonlinear, nonexponential, and depends on thermal history and the process can be described by phenomenological models of structural recovery, such as the Tool-Narayanaswamy-Moynihan (TNM) and the Kovacs-Aklonis-Hutchinson-Ramos (KAHR) models. The goal of the present work is to analyze methods to determine the nonlinearity parameter x and activation energy $\Delta $h/R. The methods to determine x includes the inflectional analysis, time-temperature superposition, and two-step temperature jump methods. The activation energy $\Delta $h/R can also be obtained by the first two methods. The TNM model is used to simulate structural recovery data, which are then used to test the accuracy of the methods to determine x and $\Delta $h/R, with a particular interest in data obtained after cooling at high rates as can be obtained in the Flash DSC. The nonlinearity parameter x by the inflectional analysis and two-step temperature methods are accurate for exponential recovery. However, for real systems with nonexponential relaxation, methods to determine x are not reliable. The activation energy is well estimated by both the time-temperature superposition and inflectional analysis methods, with the former being slightly better. [Preview Abstract] |
Monday, March 13, 2017 1:51PM - 2:03PM |
B18.00014: New Insight in Understanding the~mechanical responses of polymer glasses using molecular dynamic simulation Yexin Zheng, Shi-Qing Wang, Mesfin Tsige The Kremer-Grest bead-spring model has been the standard model in molecular dynamics simulation of polymer glasses. However, due to current computational limitations in accessing relevant time scales in polymer glasses in a reasonable amount of CPU time, simulation of mechanical response of polymer glasses in molecular dynamic simulations requires a much higher quenching rate and deformation rate than used in experiments. Despite several orders of magnitude difference in time scale between simulation and experiment, previous studies have shown that simulations can produce meaningful results that can be directly compared with experimental results. In this work we show that by tuning the quenching rate and deformation rate relative to the segmental relaxation times, a reasonable mechanical response shows up in the glassy state. Specifically, we show a younger glass prepared with a faster quenching rate shows glassy responses only when the imposed deformation rate is proportionally higher. [Preview Abstract] |
Monday, March 13, 2017 2:03PM - 2:15PM |
B18.00015: Explanation of the Exothermic Enthalpy Peak Exhibited by Glassy Polymers Following Loading-Unloading. Grigori Medvedev, James Caruthers When a glassy material is heated at a constant rate, an endothermic peak is observed. In contrast, when a glass is subjected to a large loading-unloading deformation prior to heating, an exothermic peak emerges well below Tg and the conventional endothermic peak located near Tg disappears. The deformation induced exothermic peak is extremely broad where, depending on the material, it may begin more than 100 degrees below Tg. It has been speculated that the effect of deformation is similar to that of hyper-quenching, where the latter is known to also produce an exothermic peak in the heat capacity vs temperature curve. However, no existing model of glass contains a mechanism by which a loading-unloading cycle would produce the experimentally observed enthalpic response. In this communication we will show that the recently developed Stochastic Constitutive Model (SCM) does predict emergence of the exothermic peak following large loading-unloading deformation below Tg. According to the SCM, during a large deformation that takes the material into the post-yield flow state, the work of deformation is partially converted into excess non-equilibrium entropy which does not relax immediately upon unloading. During subsequent heating this excess entropy manifests as an exothermic peak. [Preview Abstract] |
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