Molecular Simulations of Polymer Nanocomposites*

In this talk I will describe molecular simulations of two different polymer nanocomposites. A common strategy to control the nanoparticle (NP) distribution in composites is to graft polymer chains onto the NP surfaces. I will describe the use of a new method, theoretically-informed Langevin dynamics (TILD) simulations, to calculate equilibrium phase diagrams for grafted NPs in polymer melts. The phase diagram for NPs densely grafted with short A chains and blended with long B homopolymer chains is significantly shifted compared to that for an equivalent AB homopolymer blend. Adding additional A homopolymer leads to an increase in miscibility of the gNPs on the gNP-rich side of the phase diagram. The extra A homopolymer helps to compatibilize the interface between the gNPs and the matrix B chains. Our results are consistent with both experiments and modeling of poly(methyl methacrylate) (PMMA)-grafted silica NPs in poly(styrene-ran-acrylonititrile) (SAN) and PMMA-NP/SAN/PMMA composites.

A second polymer nanocomposite system of current interest is that of rubber filled with silica nanoparticles. It is important to understand the effects of high-pressure hydrogen gas on rubber materials used in hydrogen infrastructure. I will describe atomistic molecular dynamics simulations of the interface between a silica nanoparticle and a hydrogen gas-saturated, crosslinked rubber. The hydrogen gas preferentially adsorbs to the silica-rubber interface and has higher mobility at the interface than in the bulk. Hypothesizing that the excess gas may impact the strength of the weak, non-covalent interaction across the rubber-filler interface, we calculated the work of adhesion (WOA) in the presence of H2 gas.  A linear decrease in the WOA was found with increasing gas concentration.