Sorption with plasticization.
It may be the case of aliphatic solvents in polymers just under the glass transition. The kinetic sorption curve drawed with the square root of time scale has not got an initial linear part as it would be the case in fickian conditions. It is then not right to estimate the equilibrium diffusion coefficient by measuring the initial slope of the curve. The diffusion profile has also not got the shape as in fickian case during the beginning of the sorption. The sorption isotherm is also not linear and it is curved to the top.
Example 2, case of low speed of plasticization, moderate life time of plasticized sites and low dry diffusion speed :