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 interpenetrating polymer networks. One of them ( N -isopropylacrylamide) is prestressed before the gelation of the other one (polyacrylamide) takes place, resulting in built-in anisotropy. It is found that these gels have a preferred direction 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 amplitude of such a change is proportional to the degree of initial stress. A theoretical model based on the free-energy consideration has been proposed and can be used to explain the experimental results.