Phase separation of weakly ionized polymer gels during shrinking phase transition

We have investigated the shrinking phase transition of weakly ionized poly(N-isopropylacrylamide) gels prepared in a cylindrical shape with submillimeter diameter. The macroscopic conformation changes were obtained on heating processes in 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. It was found that the macroscopic behavior can be characterized by several conformation changes; the phase coexistence, the grain pattern, the bubble pattern, and the opaque phase. On a continuous heating process, the phase transition can be characterized by the phase coexistence of completely collapsed and swollen states for the smaller temperature drift rates; the selected portions on the surface can start to collapse at the transition point, which develops with time and finally becomes a completely collapsed phase. For the larger temperature drift rates, the phase transition starts many places on the surface, and the whole gel with surface bubble pattern gradually shrinks with time. These different processes can be clearly observed in the latter measurements, which depend on the degree of the super-heating (quench); for the shallow quenches, the number of the completely collapsed states correspondingly increased with increasing the super-heating. For the deep quenches, the gel becomes opaque, and the transparent surface skin (collapsed phase) develops with time. The stability of the phase coexistence and the relationship with the transition velocity were qualitatively discussed in terms of the classical phase-separation model based on the nucleation and the spinodal decomposition. (C) 1999 American Institute of Physics. [S0021-9606(99)50146-3].