Atomistic Simulation of the Diffusion of Small Gas Molecules in Polyisobutylene

Abstract

Computer simulation approaches to explain the markedly low permeability properties of polyisobutelene (PIB) to small-molecule penetrants have led in the past to predictions that, unfortunately, deviate significantly from the experimental values [1-3]. We have recently introduced a new simulation strategy [4] based on a more accurate and detailed united-atom (UA) model for PIB, which has opened up the way towards understanding the molecular origin of the superior barrier properties of this polymer. For example, by borrowing well-equilibrated representative PIB structures obtained with the new UA forcefield and using them as input to explicit, all-atom sorption simulations with the COMPASS force-field has led to solubility predictions for small molecules such as He, Ar, N2 and O2 in PIB that compare extremely favourably with available experimental data. In this work, we will present additional simulation results for the diffusion coefficient of these penetrants in PIB, obtained by detailed, brute-force molecular dynamics simulations in the NPT statistical ensemble at small enough loadings. The simulations cover a wide range of temperatures (from 500K down to room temperature) and, in addition to the values of the diffusion coefficients of the small penetrants, provide extremely useful information for the local packing in PIB and the free volume and its distribution, all as a function of temperature. A comparison against other elastomers with similar chemical structure (like polyethylene PE) will also be presented. How the new molecular-based permeability results can be integrated into grand hierarchical design methodologies for quantifying structure-property relationships in polymer science will also be analyzed and discussed.

References

[1] A.A. Gusev, U.W. Suter, J. Chem. Phys. 1993, 99, 2228.

[2] F. Müller- Plathe, S.C. Rogers, W.F. Vangunsteren, J. Chem. Phys. 1993, 98, 9895..

[3] G.J. van Amerongen, J. Polym. Sci. 1950, 5, 307.

[4] G. Tsolou et al., Macromolecules, 2008, submitted.