Coexistence of phases and the nature of first-order phase transition in poly-N-isopropylacrylamide gels

The occurrence and the nature of first-order transition of gels are analyzed and discussed. First, the condition for the first-order transition and the crossover between the continuous and discontinuous transitions are reviewed within the Flory-Rehner and Erman-Flory phenomenological theories. Then, experimental investigations related to this subject are reviewed and compared with the predictions of the above theories. Our attention is focused on poly(N-isopropylacrylamide) (NIPA) gel and its ionized copolymers. It is shown that the continuous as well as slightly discontinuous transitions in neutral NIPA gels can be understood semiquantitatively on the basis of the above theories. On the other hand, it is pointed out that the strongly discontinuous transitions in ionized NIPA gels are less well understood, and that the real nature of these transitions has not been clarified at all. There is a summary of the results of new observations on the discontinuous transition process in NIPA gels, which have been undertaken to shed some light on this transition. From these results, an entirely new concept of the first-order transition of gels has emerged, in which the transition between the swollen and the shrunken phases proceeds via an intermediate state of coexistence of two phases. This state is considered to be not a transient, but a stable or metastable equilibrium one and has a number of unusual features. A close similarity is pointed out between this state and the coexisting-phases state observed in some metallic alloys around their order-disorder transition.