Diffusion of Small Penetrants in Polybutadienes

The diffusion coefficient $D$ in the dilute limit for three different penetrants--- oxygen, water, and methanol---in three different conformations of polybutadiene (all cis-1,4, all trans-1,4, and a random copolymer containing 50\% trans-1,4, 40\% cis- 1,4, and 10\% vinyl-1,2 repeat units) has been computed using molecular dynamics simulations for temperatures in the range $T=300$--$400$\,K. Simulations runs of 25 and 50 ns made for each of the 45 combinations of penetrant, conformation, and temperature studied. Over this temperature range the density of the all-cis-1,4 conformation is higher than that of the all-trans-1,4 and random copolymer conformations, which are approximately equal. For all three conformations, $D$ for oxygen and water are comparable and larger than that of methanol. However, for a given penetrant, strong differences were observed in the rate of increase of $D$ for the three conformations. We find that the activation barriers for the three penetrants are generally between 20 and 30 kJ/mol, in agreement with experimental results. The magnitude of the activation energy is directly proportional to the size, rather than the mass, of the penetrant molecule. (Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.)