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.

当水凝胶变形时，水凝胶内的水分子与聚合物网络一起移动。在特定的时间尺度和长度尺度上，水凝胶内的水迁移以及与环境的水交换可以极大地影响水凝胶的变形。在这项工作中，我们从实验和理论上研究了由于水迁移引起的水凝胶的速率依赖性断裂。我们建立了一个实验装置，可以对浸入液体环境、水或油中的水凝胶样品施加机械载荷。我们对不同加载速率下的水凝胶样品施加单调加载，发现水凝胶的断裂拉伸高度依赖于水凝胶浸入水中的加载速率，而几乎与水凝胶浸入油中的加载速率无关。我们还对水凝胶样品施加阶跃载荷，发现浸入水中的水凝胶比浸入油中的水凝胶更容易发生延迟断裂。我们采用了 Hong 等人开发的结合水迁移的大变形理论。(2008) 并使用有限元软件计算了两种环境条件下水凝胶的变形和断裂。数值计算表明，在实验所采用的时间尺度和长度尺度上，水运移满足裂缝小尺度膨胀条件。当水凝胶浸入水中时，水分子可以在不同的负载率下与环境进行交换。当水凝胶浸入油中时，水分子几乎不能移入或移出水凝胶。理论预测解释了实验结果。