Hydrolysis-induced large swelling of polyacrylamide hydrogels

Polyacrylamide (PAAm) hydrogels are widely used and studied. Their feature of hydrolysis is often ignored. When PAAm hydrogels are stored under alkaline conditions, they can undergo a hydrolysis reaction, which changes them from neutral hydrogels to polyelectrolyte hydrogels, resulting in significant volumetric increases. In this paper, we establish a non-equilibrium thermodynamic theory to describe hydrolysis-induced large swelling of PAAm hydrogels. In particular, a thermodynamically consistent reaction kinetics is proposed by accounting for auto-retardation of the hydrolysis reaction. As an example, hydrolysis-induced homogeneous swelling under free and constrained boundary conditions is modeled, and we show that mechanical constraints can significantly influence the swelling and reaction of the hydrogels. Our theoretical model is validated by comparing with experiments. This work provides guidelines for understanding and predicting the hydrolysis-induced swelling behavior of PAAm hydrogels under alkaline conditions, and is important for their utilization.