Swelling behavior of polymer gels with built-in anisotropy near the volume-phase transition point

The swelling behavior of gels with built-in anisotropy is investigated near the volume-phase transition. The anisotropic gels consist of two interpene trating polymer networks. One of them (N-isopropylacrylamide) is prestresse d before the gelation of the other one (polyacrylamide) takes place, result ing in built-in anisotropy. It is found that these gels have a preferred di rection for swelling: above the phase transition point, the gel swells more along the prestressed direction. The ratio of gel length (non-pre-stressed ) to its diameter (prestressed) exhibits a stepwise change at T-c. The ampl itude of such a change is proportional to the degree of initial stress. A t heoretical model based on the free-energy consideration has been proposed a nd can be used to explain the experimental results.