The diffusion of dichloroethane in phase-segregated polyurethanes

Abstract

The sorption and diffusion of 1,2-dichloroethane (DCE) has been studied in a series of polyurethanes based on diphenylmethane diisocyanate and butanediol with 1000 and 2000 molecular-weight poly(tetramethylene oxide) (PTMO). DCE is a strongly swelling solvent with solubilities as large as 450% in the commercial Estane sample (63% soft segment). The sorption isotherms for the Estane sample, determined on sorption and desorption cycles and for three sample thicknesses, coincide. Hysteresis is seen in the sorption isotherms of other samples with lower soft-segment content, the isotherm on desorption being higher than that on sorption. The sorption rate curves are generally Fickian at low concentrations but become sigmoidal at higher concentrations for sorption and desorption in the Estane and the PTMO-2000 sample with the highest soft-segment content (68%). For other samples the curves are two-stage for sorption but Fickian for desorption. Diffusion coefficients ( D) were calculated by application of the analysis of coupled Fickian diffusion and first-order relaxation due to Joshi and Astarita. The resulting D versus C curves showed a maximum, attributable to the competing contributions of the solvent mobility and the thermodynamic factor. In addition, it was found that a correction to D for heating effects was required. There was evidence of significant solvent-induced structural relaxation in the higher hard-segment samples leading to higher than expected solubilities and diffusion constants. A pronounced thickness dependence of the diffusion constant could not be explained in terms of current diffusion models.