The Effects of CO2-Conditioning of Polymers on Positron Annihilation and Gas Permeation Properties

Abstract

Abstract The effects of CO2-conditioning, i.e. exposing a polymer film to high pressure CO2 and degassing it in vacuo, on positron annihilation and gas permeation properties of polyimides and polyethylene were investigated. Both lifetime and intensity of ortho-positronium (o-Ps) (τ3 and I3, respectively) increased for polyimides by the CO2-conditioning. The size distribution of free space holes, which was evaluated from annihilation rate distribution of o-Ps, became broader and shifted to larger size. Permeability, diffusion, and solubility coefficients of gases for polyimides increased by the CO2-conditioning. The increases in diffusion coefficient are attributed to increases in volume fraction of free space holes, VF. The increases in τ3 and in I3 were much larger than those expected from the increases in VF. This may be explained by a model based on an interplay between electron affinity and size of free space holes for Ps formation. In the CO2-conditioned films, permeability and diffusion coefficients, VF, and I3 gradually decreased with lapse of time but after five months they were still larger than those in the as-cast films, whereas τ3 remained almost constant. No effect of the CO2-conditioning was observed for polyethylene.