Shrinking pattern and phase transition velocity of poly(N-isopropylacrylamide) gel

We have studied the shrinking phase transition of cylindrical poly(N-isopropylacrylamide) gels with submillimeter diameter. The macroscopic conformation change and the phase transition velocity were obtained during the heating process by two different methods. One is a continuous heating process with a constant temperature drift rate, and the other is an isothermal process after a steplike temperature increase beyond the transition point. In the former measurement, the phase transition can be controlled by the nucleation mechanism in the smaller temperature drift rates; at the transition point, after the fine pattern appears and disappears on the surface, for instance, the gel gradually and uniformly shrinks while keeping a smooth surface. On the other hand, at the larger temperature drift rates, the phase transition comes into the unstable region before being completed; after the fine pattern disappears, a coarse pattern appears on the surface, and the entire gel becomes opaque. The gel gradually becomes transparent with time from the surface layer to the core portion. These two processes, characterized by two types of surface pattern as well as the growth of a collapsed surface skin layer, can be clearly observed in the latter measurements, which depend on the degree of super-heating (how far the final temperature is from the transition point). The results are discussed qualitatively on the basis of the classical phase separation model of nucleation and spinodal decomposition, as well as the phase diagram of the present gel system. (C) 1999 American Institute of Physics. [S0021-9606(99)70125-X].