Mixed diffusion and relaxation kinetics model for hydrogels swelling

Hydrogels have a unique ability to retain moisture due to the three-dimensional (3D) polymeric network. Both swelling capacity and swelling rate of hydrogels depend on the various parameters such as solution concentration, temperature, degree of cross-linking in the gel structure, type of hydrophilic groups, pH, etc. Various kinetics models were presented for swelling rate, however, most of them only describe diffusion or polymer relaxation as the predominant process on the swelling rate. In the present work, a new mixed diffusion and relaxation kinetics model is presented by simultaneously considering both diffusion and polymer relaxation. The mixed model is able to cover the whole of swelling and provides insight into the gelling mechanism. By the presented kinetics model, it is possible to indicate the contribution of solvent molecules diffusion and polymer relaxation on the swelling rate and also determine the predominant process at different extent of swelling. It is shown that this new mixed kinetics model is a general equation and covers most of the previously reported equations. The presented mixed kinetics equation was used for modeling experimental swelling kinetics data at different conditions very well.