The sorption and diffusion of small molecules in amorphous and crystalline polybutadienes

Abstract

Abstract Diffusion coefficients of methylene chloride, chloroform, benzene, and 2,2,4‐trimethyl‐pentane in 1,4‐polybutadiene having trans contents between 2 and 100% have been obtained from vapor sorption and desorption kinetics. For amorphous polymers (trans content less than 70 to 80%) the diffusion coefficient at zero penetrant concentration decreases with increasing trans content and with increasing size of the diffusant. The activation energies for diffusion are independent of temperature and increase with trans content and diffusant size. Comparison with the activation energies for viscous flow give values for the B factor in Fujita's free volume theory and this increases with diffusant size towards a value of unity. At trans contents above about 80% a sudden drop in the diffusion coefficient and in the equilibrium vapor sorption occurs accompanied by a large increase in the concentration dependence of the diffusion coefficient. These changes result from the onset of crystallinity but the absence of any sudden change in the activation energy suggests that the main effect is a geometric one due to the inaccessibility of the crystalline regions with little change in the basic mechanism of diffusion in the amorphous regions. Vapor sorption enables calculation of the accessibility of the crystalline polymers and this is found to decrease with increasing size of the diffusant. Variations in crystallinity produced by different heat treatments give the expected variations in the diffusion coefficient and equalibrium vapor sorption.