Multiscale modeling of polyisoprene on graphite.

The local dynamics and the conformational properties of polyisoprene next to a smooth graphite surface constructed by graphene layers are studied by a multiscale methodology. First, fully atomistic molecular dynamics simulations of oligomers next to the surface are performed. Subsequently, Monte Carlo simulations of a systematically derived coarse-grained model generate numerous uncorrelated structures for polymer systems. A new reverse backmapping strategy is presented that reintroduces atomistic detail. Finally, multiple extensive fully atomistic simulations with large systems of long macromolecules are employed to examine local dynamics in proximity to graphite. Polyisoprene repeat units arrange close to a parallel configuration with chains exhibiting a distribution of contact lengths. Efficient Monte Carlo algorithms with the coarse-grain model are capable of sampling these distributions for any molecular weight in quantitative agreement with predictions from atomistic models. Furthermore, molecular dynamics simulations with well-equilibrated systems at all length-scales support an increased dynamic heterogeneity that is emerging from both intermolecular interactions with the flat surface and intramolecular cooperativity. This study provides a detailed comprehensive picture of polyisoprene on a flat surface and consists of an effort to characterize such systems in atomistic detail.

Entanglement network in nanoparticle reinforced polymers.

Polymer nanocomposites have been widely studied in efforts to engineer materials with mechanical properties superior to those of the pure polymer, but the molecular origins of the sought-after improved properties have remained elusive. An ideal polymer nanocomposite model has been conceived in which the nanoparticles are dispersed throughout the polymeric matrix. A detailed examination of topological constraints (or entanglements) in a nanocomposite glass provides new insights into the molecular origin of the improved properties in polymer nanocomposites by revealing that the nanoparticles impart significant enhancements to the entanglement network. Nanoparticles are found to serve as entanglement attractors, particularly at large deformations, altering the topological constraint network that arises in the composite material.

Molecular plasticity of polymeric glasses in the elastic regime.

We examine the plastic deformation of a model polymeric glass under tension. Local plastic events are found at extremely small strains, well below the yielding point, in a regime where the material is traditionally described as perfectly elastic. A distinct relationship is identified between these irreversible displacements (plastic events), the amplitude of segmental motion, local structure, and local elastic moduli. By examining the motion during the deformation of individual sites, we arrive at a mechanistic explanation for how these events arise. It is shown that, upon deformation, polymer sites that exhibit relatively small but positive elastic moduli are prone to failure. Such sites are also found to exhibit a large vibration amplitude, as indicated by their respective Debye-Waller factors. Sites that fail first tend to have a higher degree of nonsphericity than the rest of the material. We also find the collective occurrence of local plastic events where percolation or concerted action leads to global failure of the material above a certain strain.

Calculation of local mechanical properties of filled polymers.

A study is presented on the effects of smooth nanoparticles on the structure and elastic moduli of a polymer matrix. Structural changes between the unfilled polymer matrix and the nanocomposite give rise to the formation of a glassy layer that surrounds the nanoparticles. Results for the effects of particle size and concentration on the local and overall mechanical properties of the polymer are consistent with experimental macroscopic observations. At the molecular level, it is found that dispersed, attractive nanoparticles alter the nonaffine displacement fields that arise in the polymer glass upon deformation, thereby rendering the nanocomposite glass less fragile.

Local mechanical properties of polymeric nanocomposites.

The inclusion of a nanoparticle into a polymer matrix is studied by efficient Monte Carlo simulations. The resulting structural changes in the melt and glass exhibit a strong dependence on the strength of the polymer attraction to the surface of the filler. The mechanical properties of the nanocomposite are analyzed in detail through a formalism that permits calculation of local elastic constants. The average shear and Young's modulus of the nanocomposite are higher than those of the pure polymer for neutral or attractive nanoparticles. For repulsive particles, these moduli are lower. Simulation of local properties reveals that a glassy layer is formed in the vicinity of the attractive filler, contributing to the increased strength of the composite material. In contrast, a region of negative moduli emerges around repulsive fillers, which provides a mechanism for frustration relief and a lowering of the glass transition temperature.