B.E.T. Sorption mode.
It may be the case of solvents and water in some polymers which have a great affinity for these solvents at temperatures just under the glass transition. The number of strong interaction sites is high. The diffusion in dry parts of the matrix is low and a light plasticization process occurs with an increase of the diffusion speed near the surfaces of the sample where the concentration of absorbed and condensed molecules is high. The sorption kinetics is slow and the sorption equilibrium is then long to be reached. The internal part of the sample far from the surface is almost dry of solvent until the sorption equilibrium is reached.
Example 3, case of high trapping, high condensation and low dry diffusion speed :