Heterogeneous membranes: Diffusion in filled rubber

The diffusion and solution of propane and of benzene has been studied over a range of temperatures in a series of heterogeneous membranes of natural rubber and zinc oxide filler. The volumes of filler were 0, 5, 10, 20, 30, and 40%. Evidence was obtained of the interaction between rubber and filler, and that some of the membranes involved three phases: polymer, filler, and open gaps between filler particles. No rigorous treatment of diffusion in heterogeneous media exists, but the model of two interdispersed phases was developed and discussed in relation to permeation rates and diffusion coefficients. Differences between steady state and time‐lag diffusion coefficients were ascribed to different structure factors in steady and transient states of flow. Concentration dependences of the interdiffusion and intrinsic diffusion coefficients of benzene were measured. The influence of concentration dependence, filler, and structure factor was combined in one relationship relating the intrinsic diffusion coefficient in the filler rubber with that for diffusion in unfilled rubber, containing the same volume fraction of benzene. The resultant expression appears able to account qualitatively for some of the observed complex diffusion behavior.