“…In the sorption description, the polymer membrane is treated either as a homogenous amorphous surrounding where solute molecules are dissolved, or as surrounding including amorphous and the crystalline phases, where molecules of solutes are dissolved only in the homogenous amorphous phase and crystalline phase behaves as an inert phase [28]. Examples of the models used to describe these systems include: (i) the Flory-Huggins equation [29][30][31][32]; the Flory-Rehner equation [28,[33][34][35][36][37], which is a modified form of the original Flory-Huggins equation; (iii) the UNIFAP model [28,[38][39][40][41][42], which is based on the UNIFAC model [43,44]. A different approach for highly concentrated polymer solutions and swelled membranes immersed into liquid solutions was given by van de Witte et al [45] and Young et al [46], in which the ternary interaction parameter in the Flory-Huggins equation was used.…”