SIMULATION AND MODELING OF TRANSIENT PERMEATION OF ORGANIC-SOLVENTS THROUGH POLYMER-FILMS IN THE CASE OF A CONCENTRATION-DEPENDENT DIFFUSION-COEFFICIENT

The measured transient permeation kinetics of acetic acid from a water -acetic acid mixture through poly(vinyl alcohol) films could not be ac counted for by the Pick law with a constant diffusion coefficient. A n ew calculation procedure was developed on the basis of simulation resu lts of the Fickian kinetics in which the diffusion coefficient was ass umed to increase exponentially with the local permeant concentration. A fast and reliable fitting procedure, which was set up on a Personal Computer, involves an iterative numerical adjustment of (a) the value of the limiting diffusion coefficient D (i.e., difusion of the permea nt in dry polymer) using the onset part of the permeation rate followe d by (b) the value at the upstream face of the plasticization term (ar gument of the exponential function) using the shape of the experimenta l curve. The values obtained from fitting the model to the transient k inetics showed that the limiting diffusion coefficient increases drast ically, but the plasticization term changes little, with increasing te mperature.