Rate-Dependent Fracture of Polyacrylamide Hydrogels Induced by Interior Water Migration

Abstract

The deformation of hydrogels is accompanied by water migration, a process that plays a crucial role in their fracture behaviors. Previous investigations primarily focus on how water migration between the environment and hydrogel affects the fracture. Herein, a novel mechanism of rate-dependent fracture of hydrogels induced by interior water migration is uncovered. We stretched notched polyacrylamide (PAAm) hydrogels at various stretch rates in both oil and deionized (DI) water environments, and observed that the critical stretches to crack propagation are positively correlated with the stretch rates in both environments. This rate-dependent fracture is attributed to the crack tip swelling of PAAm hydrogels. Delayed fracture tests conducted in oil further verify the co-existence of delayed fracture and rate-dependent fracture resulted from interior water migration in PAAm hydrogels. The experimental findings are interpreted by considering the imperfection of a real polymer network, in which the scission of short chains in the region neighboring the crack tip reduces the average crosslinking density locally, thereby greatly amplifying the degree of crack tip swelling and its influence on the fracture. A constitutive model coupling the evolution of polymer network and diffusion of water molecules is proposed, and the crack tip swelling of notched PAAm hydrogels under stretch is predicted by the constitutive model using the finite element method. Assuming that the decrease in fracture toughness is positively related to the swelling along the crack propagation surface, the predicted nominal fracture toughness matches the experimental results of PAAm hydrogels stretched in water well, and satisfies those in oil environment qualitatively. This work highlights the significant influence of interior water migration on the fracture of hydrogels and provides insights that may guide the design of hydrogels with enhanced fracture resistance.

Keywords: Hydrogel, Water migration, Rate-dependent fracture, Chain scission, fracture toughness