We have investigated the shrinking phase transition of weakly ionized poly(
N-isopropylacrylamide) gels prepared in a cylindrical shape with submillime
ter diameter. The macroscopic conformation changes were obtained on heating
processes in two different methods. One is a continuous heating process wi
th a constant temperature drift rate, and the other is an isothermal proces
s after a steplike temperature increase beyond the transition point. It was
found that the macroscopic behavior can be characterized by several confor
mation changes; the phase coexistence, the grain pattern, the bubble patter
n, and the opaque phase. On a continuous heating process, the phase transit
ion can be characterized by the phase coexistence of completely collapsed a
nd swollen states for the smaller temperature drift rates; the selected por
tions on the surface can start to collapse at the transition point, which d
evelops 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 shrin
ks with time. These different processes can be clearly observed in the latt
er measurements, which depend on the degree of the super-heating (quench);
for the shallow quenches, the number of the completely collapsed states cor
respondingly increased with increasing the super-heating. For the deep quen
ches, the gel becomes opaque, and the transparent surface skin (collapsed p
hase) develops with time. The stability of the phase coexistence and the re
lationship with the transition velocity were qualitatively discussed in ter
ms 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].