Polyacrylamide hydrogels. I. Network imperfection

A real polymer network is never perfect, but the quantification of network imperfection has been elusive. Here we quantify the network imperfection of a polyacrylamide hydrogel through its mechanical properties. We find that the ultimate properties—strength, extensibility, and work of fracture—have narrow scatter, comparable to the scatter of modulus. Despite the narrow scatter, the work of fracture is about four orders of magnitude lower than that of the perfect network. This reduction in work of fracture is a measure of network imperfection. The toughness of the hydrogel is about two orders of magnitude higher than that of the perfect network. When the effects of other inelastic processes are minimized, this amplification in toughness is likely due to distributed chain scission, and is another measure of network imperfection. We find that the ultimate properties of the hydrogel are insensitive to cuts up to about 1 mm. We trace this cut insensitivity to network imperfection.