Nucleation and spinodal decomposition of neutral poly(N-isopropylacrylamide) gels

The shrinking phase separation of polymer gels was investigated using the neutral poly(N-isopropylacrylamide) gels prepared in a cylindrical shape with submillimeter diameter. The macroscopic conformation change was obtained on the isothermal processes after a step-like temperature increase beyond the transition point. The gel was quenched (super-heated) from three different initial temperatures to several final temperatures. It was observed that there exist three characteristic regions of transition velocity, which depend on the final temperature. The phase transition could be controlled by the nucleation mechanism in the shallow quenches (smaller super-heating; Stage I), which is characterized by a fine pattern: the gel remains transparent during the transition. On the other hand, in the deep quenches (larger super-heating; Stage In), the phase transition reaches the unstable region, which is characterized by a coarse pattern: the gel becomes transparent with time, from the surface layer to the core opaque portion. The transition velocity in Stage I or III does not depend on the initial temperature. The kinetics of the quenching into the intermediate temperature range (Stage II), however, is strongly affected by the initial temperature. The results were qualitatively discussed in terms of the macroscopic conformation changes resulting from the microscopic phase separation: nucleation and its growth (coarsening), spinodal decomposition and the transition-induced strain of networks.