Dependence of shrinking kinetics of poly(N-isopropylacrylamide) gels on preparation temperature

Preparation temperature dependence of equilibrium swelling degree and shrinking kinetics of poly(N-isopropylacrylamide) gel has been investigated by optical microscopic measurements. The degree of swelling, d/d0, at 20 °C was found to be strongly dependent on the preparation temperature, Tprep, where d and d0 are the diameter of gel during observation and preparation, respectively. The value of d/d0 was about 1.2 for Tprep=20 °C, but steeply increased by approaching the phase separation temperature ≈32.0 °C. Above 32.0 °C, d/d0 decreases stepwise to 1.46. This upturn in d/d0 was correlated with spatial inhomogeneities in gels. That is, the gel became opaque by increasing Tprep. Though the shrinking half-time, t1/2, of gel was on the order of 500 min for Tprep≤20 °C, t1/2 decreased to 2 min for Tprep≥26 °C. Hence, a rapid shrinking was attained by simply increasing Tprep. The physical implication of this rapid shrinking in gels was discussed in conjunction with the gel inhomogeneities and a thermodynamic theory of swelling equilibrium.