MOLECULAR PACKING AND SMALL-PENETRANT DIFFUSION IN POLYSTYRENE - A MOLECULAR-DYNAMICS SIMULATION STUDY

Molecular dynamics (MD) simulations have been used to study bulk atact ic polystyrene (aPS). A united-atom non-bonded potential is calibrated for the aromatic-ring carbons, which, along with previously determine d non-bonded functions, results in a good representation of pressure-v olume-temperature relations for aPS. Experimental X-ray scattering dat a for glassy aPS are well reproduced in simulation. Packing features i n the glass are discussed in terms of various site-site radial distrib ution functions. Diffusion coefficients for methane as an example of a small-molecule penetrant are determined as a function of temperature in the range 380-550 K. The values from simulation when extrapolated t o room temperature via an Arrhenius plot are found to be consistent wi th experimental values for the similar gas CO2 at that temperature, th us implying that the glass transition in the matrix has little effect on the diffusion. The temperature behaviour of the diffusion coefficie nts as well as the detailed jump behaviour of the penetrant indicate t hat the diffusion mechanism corresponds to hopping from site to site i n a solid-like medium over the temperature range studied. The lack of effect of the glass transition on diffusion is rationalized in terms o f the mechanism already being hopping in a solid-like medium well abov e T-g. Diffusion is the slowest in aPS of any of the polymeric matrice s studied so far by MD simulation. This correlates well with the fract ional free volume found, which is also the lowest yet found in the pol ymeric matrices. Copyright (C) 1996 Elsevier Science Ltd.