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/d(0), at 20 C was found to be strongly dependent on the preparation temperature, T-prep, where d and d(0) are the diameter of gel during observation and preparation, respectively. The value of d/d(0) was about 1.2 for T-prep = 20 degreesC, but steeply increased by approaching the phase separation temperature approximate to 32.0 degreesC. Above 32.0 degreesC, d/d(0) decreases stepwise to 1.46. This upturn in d/d(0) was correlated with spatial inhomogeneities in gels. That is, the gel became opaque by increasing T-prep. Though the shrinking half-time, t(1/2), of gel was on the order of 500 min for T-prep less than or equal to 20 degreesC, t(1/2) decreased to 2 min for T-prep greater than or equal to 26 degreesC. Hence, a rapid shrinking was attained by simply increasing T-prep. The physical implication of this rapid shrinking in gels was discussed in conjunction with the gel inhomogeneities and a thermodynamic theory of swelling equilibrium. (C) 2002 Elsevier Science Ltd. All rights reserved.