PHASE-TRANSITION IN CONSTRAINED POLYMER GELS

Equilibrium diameter and stress change of cylindrical N-isopropylacrylamide gels of submillimeter diameter were measured as a function of temperature under mechanical constraint along the uniaxial direction. As a result, the transition temperature increased up to 1°C with increasing the strain, which indicates the possibility of the uniaxial strain-induced phase transition of gels at selected temperatures. The diameter and stress change were quite successfully described by the phenomenological model on the basis of the extended Flory-type free energy. The time course of the change in diameter and stress was also studied after the large steplike elongation was rapidly applied. A gradual increase in diameter during decrease in stress was detected clearly in the swollen phase, which is due to the relaxation of polymer segmental units to approach the favored conformation under the strong deformation. © 1994 American Institute of Physics.