SMALL PENETRANT DIFFUSION IN POLYBUTADIENE - A MOLECULAR-DYNAMICS SIMULATION STUDY

Molecular dynamics (MD) simulations have been used to study the diffus ion of methane as a small molecule penetrant example in cis-1,4-polybu tadiene (PBD). The non-bonded potential for an 'anisotropic' united at om (AUA) representation of the -CH=group was calibrated by adjusting t he constants to fit experimental volume temperature data for the polym er melt. This potential was used along with an already available AUA f unction for the -CH2-group. The diffusion coefficient of methane was d etermined via MD simulation over a wide range of temperature. The resu lts agree well with near room temperature experimental values for N2 i n cis-PBD. The latter gas is similar in diffusion behaviour to methane . The mechanism of diffusion in terms of the nature of the diffusive j ump process and its response to temperature is found to be similar to that in atactic polypropylene and polyethylene melts. At low temperatu res the penetrant is trapped for relatively long periods in a cage of surrounding polymer and makes occasional large jumps. As temperature i ncreases the size of the jumps increases further. The quiescent trappe d periods disappear and a liquid-like scattering regime prevails. This change in mechanism as temperature increases is accompanied by a decr ease in activation energy. The diffusion in PBD is faster than in othe r hydrocarbon polymers studied so far by simulation (polyethylene, pol yisobutylene and atactic polypropylene). The order of diffusion coeffi cients correlates with the free volumes available in the polymer hosts .