TOLUENE DIFFUSION IN BUTYL RUBBER

The sorption isotherm and the polymer mass-fixed diffusion coefficient s, D, for toluene in butyl rubber have been measured by the incrementa l sorption method to concentrations of 130%, corresponding to a solven t volume fraction of 0.578. The increase in D with concentration is st rongly exponential to a concentration of 30% and then begins to level out. Since the nature of the dimensional change occurring in vapor sor ption was not known, the values of D were converted to solvent self-di ffusion coefficients, D1, assuming both swelling in the thickness dire ction (1D) and isotropically (3D). The free volume (FV) theory of Fuji ta was fitted to the resulting D1 with the zero concentration diffusio n coefficient and the self-diffusion coefficient of toluene as limitin g values leaving only a single arbitrary parameter. In this form the F V theory was able to describe the trend of the experimental D1 for the 1D and 3D cases equally well. Values of D were back-calculated from t he FV relations for the 1D and 3D cases for comparison with the experi mental results and with the diffusion coefficient determined by immers ion in toluene. These comparisons favor the assumption that swelling i s isotropic. It appears that the simple free volume relation is capabl e of providing a satisfactory representation of the experimental data with only a single fitting parameter, although there are moderate quan titative discrepancies. (C) 1994 John Wiley & Sons, Inc.