**2. Diffusion process**

An enormous number of scientific attempts related to various applications of diffusion equation are presented for describing the transport of penetrant molecules through the polymeric membranes or kinetic of sorption/desorption of penetrant in/from the polymer bulk. The mass transfer in the former systems, after a short time, goes to be steady-state, and in the later systems, in all the time, is doing under unsteady-state situation. The first and the second Fick's laws are the basic formula to model both kinds of systems, respectively (Crank and Park, 1975).
