A new model on the diffusion of small molecule penetrants in dense polymer membranes

A new diffusion model describing the diffusion behaviors of small molecule penetrants in dense polymer membranes was proposed on the basis of the percolation theory, the fractal theory, the free-volume theory, and the random-walk theory. The concept of “diffusion passage” and the diffusion partition zone were defined in this model. The diffusion equation, the sorption equation in the initial sorption stage, and the steady permeation equation for the whole diffusion zones were derived. The dynamic sorption and permeation data of CH2Cl2, CH3OH, CH3CH2OH, and H2O in polyimide (PI) membranes were measured. The anomalous diffusion factor dw and the fractal dimension df in these equations were calculated and discussed.