Molecular packing and diffusion in polyisobutylene

Polyisobutylene (PIB) is an elastomer that is notable for low rates of diffusion of small molecules. This has been suggested to be due to efficient intermolecular packing as evidenced by relatively low specific volume. These questions were studied via simulations. Polyethylene (PE), which is chemically isomeric with PIB, was studied as a comparison material. Monte Carlo simulations of bulk PIB were carried out at constant temperature and pressure, and molecular dynamics simulations of methane diffusion in both polymers were made. The MC simulations result in lower specific volumes for PIB compared to PE, as observed experimentally. The more efficient packing in PIB is attributed to the unoccupied volume in the systems being largely at the intermolecular interfaces and thus a molecular surface phenomenon. The thicker cross-sectioned PIB molecules have less surface area/carbon atom. The MD simulations show a slower diffusion rate in PIB. Further simulations at different densities show the diffusion to be very sensitive to small volume changes. For example, diffusion in PIB at the experimental density of polyethylene is calculated to be faster than for PE at this density.