Rate-dependent fracture of hydrogels due to water migration

When a hydrogel deforms, the water molecules inside the hydrogel move together with the polymer network. At a particular time scale and length scale, the water migration within the hydrogel as well as the water exchange with the environment can greatly affect the deformation of the hydrogel. In this work, we study the rate-dependent fracture of hydrogels due to water migration both experimentally and theoretically. We build an experimental setup that can apply mechanical loads to hydrogel samples immersed in a liquid environment, water or oil. We apply monotonic load to hydrogel samples under different loading rates and find that the fracture stretch of hydrogel is highly dependent on the loading rate when the hydrogel is immersed in water, and is almost independent of the loading rate when the hydrogel is immersed in oil. We also apply step load to hydrogel samples and find that the hydrogel immersed in water is more susceptible to delayed fracture than that immersed in oil. We adopt the large deformation theory incorporating water migration developed by Hong et al. (2008) and use finite element software to calculate the deformation and fracture of hydrogels under the two types of environmental conditions. The numerical calculations show that at the time scale and length scale used in the experiment, the water migration satisfies the small-scale swelling condition of fracture. When the hydrogel is immersed in water, the water molecules can exchange with the environment under different loading rates. When the hydrogel is immersed in oil, the water molecules can hardly move in or out of the hydrogel. The theoretical predictions explain the experimental results.