Transport of noble gases in poly(methyl acrylate)

The permeability, diffusivity, and solubility of He, Ne, Ar, and Kr were determined in poly(methyl acrylate) (PMA), in a temperature range encompassing the glass transition temperature, Tg. Activation energies for diffusion, ED, were higher above Tg than below Tg for all four penetrants in PMA and in the structural isomer of PMA, poly(vinyl acetate) (PVA). For all penetrants studied, the Tg, the magnitude of the enthalpy of mixing (ΔHm), as well as the ED were all larger for PVA than for PMA. These differences were attributed to the stronger dipole-dipole interactions possible in PVA where the dipolar carbonyl group is separated from the chain backbone by an oxygen atom. The carbonyl group in PMA is immediately adjacent and presumably sterically hindered by the chain backbone which suggests that PMA might be a stiffer molecule than PVA. Entropy considerations suggest that molecularly stiff polymers should be associated with small values of v Δα, ΔED, ΔED, ΔHp, and (β), where v is the specific volume, Δα is the change in thermal expansivity about Tg, ΔHp is the change in activation energy for diffusion about Tg, ΔED is the enthalpy of polymerization, and (β) is the logarithmic bulk relaxation rate constant of specific volume below Tg. These predictions appear to be satisfied for the systems of argon in PVC, PMA, PVA, and poly(ethyl methacrylate) (PEMA). The data suggest that PMA is a stiffer molecule than PVA. The weaker intermolecular forces of attraction and more hindered molecular rotations of PMA are consistent with a priori considerations of steric effects which have their origin in the fundamental structure of the PMA and PVA molecules. The fraction of volume, φ, not occupied by polymer, estimated by several independent means for PMA at Tg, appears to be greater than that of PVA. Each of these two comparisons suggests that the molecular packing of PMA is more dense than the molecular packing of PVA. The observed slow decrease in specific volume for PMA below Tg suggests that there could be regions throughout the polymer below Tg where the density is less than the equilibrium value. These predictions are consistent with the analyses of penetrant solubility below Tg which suggest that the molal volumes of the penetrants below Tg are larger than those above Tg. Also, analyses of the solubility data suggest that the enthalpies of mixing the noble gases in PMA tend to be exothermic below Tg. This suggests that below Tg, specific interactions occur between PMA and noble gas penetrants. The interactions are apparently manifested by decreases in free energy which are larger than expected for inert penetrants. The solubility and volumetric data appear to be consistent with the hypothesis that the polymer below Tg contains expanded, high-energy regions which preferentially absorb the diffusing penetrants.