Discontinuous crossover between fast and slow kinetics at the volume phase transition in poly-N-isopropylacrylamide gels

The kinetics of the volume phase transition in N-isopropylacrylamide gels has been studied as a function of crosslinker concentration. The shrinking kinetics at the transition is extremely slow in gels with a standard composition which have been used widely in various experiments. A discontinuous crossover from slow to much faster kinetics were observed in both high-and low-crosslinker-concentration regions, where the characteristic time of the shrinking process changes abruptly by two to four orders of magnitude with a minute change of the crosslinker concentration. In spite of these marked changes in the kinetics, no anomaly was observed in the degree of equilibrium swelling in these regions. A mechanism leading to sudden changes in kinetics is discussed in terms of an inhomogeneous network structure which is dependent on crosslink density.