Molecular Dynamics Simulation of Nanostructure of High Free Volume Polymers with SiMe3 Side Groups

Abstract

A group of four highly permeable glassy polymers, Si-substituted addition polynorbornenes, and polytricyclononenes were simulated using molecular dynamics (MD) method. For each polymer 32 model samples were prepared by virtual “polymerization” with periodic boundary conditions in cubic boxes (edge of about 6.7 nm). In contrast to standard MD approach sample density was not used as an input parameter but was calculated independently for each sample. The obtained range of density was in agreement with experimental values. Visualization of free volume showed that for the polymers with greater permeability larger clusters of free volume are observed. The average accessible volume fraction decreases with penetrant radius r and its limiting value for zero r is 0.316. Size distribution of free volume elements was obtained for the probes with widely varying size from o-positronium to Xe. It was shown that such size distribution is shifted to smaller radii for gaseous probes (from H2 to Xe) and becomes continuous.