Water Sorption and Hindered Diffusion with Different Chain Stiffness of Superabsorbent Polymer

Water sorption and diffusion are essential physicochemical properties of a high-performance super absorbent polymer (SAP). We combine the Park model with the obstruction-scaling model and the water clustering in confined spaces of the polymer hydrogel. Special attention is focused on elucidating the effect of chain stiffness by considering the conformation of the polymer chain. Theoretical model parameters are determined from the best fits by simultaneous nonlinear regressions for both sorption and corrected diffusion data based on previous experiments with poly(acrylamide-co-sodium acrylate) hydrogel. Predictions show that the hindered water diffusivity leads to a sigmoid curve with relative humidity a(w), where it increases monotonically up to a(w) approximate to 0.6 due to the swelling but decreases at higher aw caused by water clustering. Water diffusion decreases with increasing chain stiffness, and the larger persistence length provides a smaller opening radius in void spaces regarding the weak elastic deformation of hydrogel under the applied stress.