Sorption and diffusion in crystalline elastomers. Part II. Permeation of isomeric hydrocarbons in stretched rubber

Abstract

AbstractThe permeabilities and diffusion coefficients of propane, n‐butane, and isobutane in stretched peroxide‐cured natural rubber have been measured at simple elongations of 0 to 400% in the temperature range 30 to 50°C. To minimize the effect of penetrant on the degree of crystallinity, only regions of low penetrant concentration were chosen. At elongations not sufficient to induce crystallization the permeability did not alter significantly on stretching. At higher elongations the permeability decreased with increasing crystallization. Over a long period of time and at a given temperature the permeability of a crystalline membrane decreased slowly toward an equilibrium value. No such behavior was observed with noncrystalline membranes either stretched or unstretched. Part of the decrease is accounted for by a corresponding slow increase with time in the degree of crystallinity and the remainder is attributed to a partial reorientation or redistribution of the crystallites as the system tends toward an equilibrium state. The data are analyzed in terms of a model in which the crystalline rubber is presented as a rectangular array of small cylindrical crystallites impermeable to penetrant and embedded in an amorphous matrix. To a first approximation the dependence of the permeability on crystallinity and temperature is represented satisfactorily by expressions based on the model. The data indicate that the crystallites may impose slight restrictions on the segmental mobilities in the amorphous regions, but the effect is not pronounced. Some differences between steady and transient state diffusion coefficients are explained in terms of “dead volume” which is present in the steady state of flow to a greater extent than in the transient state.