Permeation with plasticization.
Example 3, Same example as example 2 but the downstream pressure P2 is not nil (P2 = P1 / 2 in this case) :

All the membrane is swelled even if the concentrations of molecules on upstream and downstream faces are different. The stationary regime is reached more rapidly and the permeation coefficient is higher then previously. In this case, the time-lag will allow to give a rough estimation of the mean diffusion coefficient because the membrane is already plasticized in its whole thickness at the beginning of the kinetics.