Percolation of Immobile Domains in Supercooled Thin Polymeric Films

We present an analysis of heterogeneous dynamics in molecular dynamics simulations of a thin polymeric film, supported by an absorbing structured surface. Near the glass transition "immobile" domains occur throughout the film, yet the probability of their occurrence decreasing with larger distance from the surface. Still, enough immobile domains are located near the free surface to cause them to percolate in the direction perpendicular to surface, at a temperature near the glass transition temperature. This result is in agreement with a recent theoretical model of glass transition

The Aggregation Kinetics of a Simulated Telechelic Polymer

We investigate the aggregation kinetics of a simulated telechelic polymer gel. In the hybrid Molecular Dynamics (MD) / Monte Carlo (MC) algorithm, aggregates of associating end groups form and break according to MC rules, while the position of the polymers in space is dictated by MD. As a result, the aggregate sizes change every time step. In order to describe this aggregation process, we employ master equations. They define changes in the number of aggregates of a certain size in terms of reaction rates. These reaction rates indicate the likelihood that two aggregates combine to form a large one, or that a large aggregate splits into two smaller parts. The reaction rates are obtained from the simulations for a range of temperatures. Our results indicate that the rates are not only temperature dependent, but also a function of the sizes of the aggregates involved in the reaction. Using the measured rates, solutions to the master equations are shown to be stable and in agreement with the aggregate size distribution, as obtained directly from simulation data. Furthermore, we show how temperature induced variations in these rates give rise to the observed changes in the aggregate distribution that characterizes the sol-gel transition.Comment: 9 pages, 10 figure

Microstructural Origins of Nonlinear Response in Associating Polymers under Oscillatory Shear

The response of associating polymers with oscillatory shear is studied through large-scale simulations. A hybrid molecular dynamics (MD), Monte Carlo (MC) algorithm is employed. Polymer chains are modeled as a coarse-grained bead-spring system. Functionalized end groups, at both ends of the polymer chains, can form reversible bonds according to MC rules. Stress-strain curves show nonlinearities indicated by a non-ellipsoidal shape. We consider two types of nonlinearities. Type I occurs at a strain amplitude much larger than one, type II at a frequency at which the elastic storage modulus dominates the viscous loss modulus. In this last case, the network topology resembles that of the system at rest. The reversible bonds are broken and chains stretch when the system moves away from the zero-strain position. For type I, the chains relax and the number of reversible bonds peaks when the system is near an extreme of the motion. During the movement to the other extreme of the cycle, first a stress overshoot occurs, then a yield accompanied by shear-banding. Finally, the network restructures. Interestingly, the system periodically restores bonds between the same associating groups. Even though major restructuring occurs, the system remembers previous network topologies

Computational Study of the Structure and Rheological Properties of Self-Associating Polymer Networks

Utilizing a novel, hybrid molecular dynamics, Monte Carlo simulation, we report on microstructural changes in a polymer network that arise in response to oscillatory shear deformation. We model telechelic self-associating polymers as a course-grained, bead–spring system. The stress response of the system is obtained from rheological experiments and is reported as a function of frequency and amplitude in both the linear and nonlinear regimes. The frequency-dependent material properties are then correlated with observed changes in the topological network structure. While only minimal structural variations are observed in the elastic regime, a substantial rearrangement occurs in the low frequency, large amplitude viscous regime. Aggregates tend to break apart, resulting in an increased density of free chains. Additionally, the network tends to break and form larger structural elements with an increase multiplicity of chains bridging between the same two aggregates

Glass transition behavior of polymer films of nanoscopic dimensions

Glass transition behavior of nanoscopically thin polymer films is investigated by means of molecular dynamics simulations. We study thin polymer films composed of bead-spring model chains and supported on an idealized FCC lattice substrate surface. The impact on the glass transition temperature of the strength of polymer-surface interaction, and of chain grafting to the surface is investigated. Three different methods -volumetric, energetic, and dynamic -are used to determine the glass transition temperature of the films. Based on these, we are able to distinguish two different transition temperatures. When the temperature is lowered, a first transition occurs when the beads order locally. This transition is characterized by an anomaly in the heat capacity. Upon decreasing the temperature further, the point is reached at which internal relaxation times diverge, as calculated, using for instance mode coupling theory. In qualitative agreement with the experiments, the former temperature depends on the characteristics of the polymer-surface interaction. By contrast, the latter temperature, is independent of these