A molecular dynamics study on the local mobility of cis-polyisoprene within the mixed glassy/rubbery states induced by graphene planes

Abstract

The glass transition behavior of a simulated model consisting of cis-1,4-polyisoprene nanofilm confined within parallel graphene planes was visualized through chain mobility statistics. Firstly, simulated annealing molecular dynamics (MD) simulations were performed for a system of bulk polyisoprene and the polyisoprene nanofilm within parallel graphene planes at two different gap sizes. The glass transition temperature (T g) and the critical density of bulk polyisoprene were then calculated. The critical density was used to compare with the local density of the confined polyisoprene segments. The mixed glassy/rubbery states were found at temperatures higher than the bulk T g. It was also shown that bulk modulus was increased when graphene planes are present and was further increased when the gap between graphene planes became narrow, and a ‘glassy bridge’ was formed by the highly confined nanofilm with highly-ordered polymer chains. Then, local mobility of each polyisoprene chain segment was quantified through the square displacement (SD) after a 1 ns motion. Local glass transition within any region was determined by whether local SD was lower than the critical mean square displacement (MSD0) of bulk polyisoprene, which was in the ‘glassy regime.’ The analysis results showed that decreasing the temperature and reducing the gap between graphene planes increased the proportion of chain segments in the glassy regime and were in concurrence with the increased bulk modulus.