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

The kinetics of the volume phase transition in N-isopropylacrylamide gels h as been studied as a function of crosslinker concentration. The shrinking k inetics at the transition is extremely slow in gels with a standard composi tion which have been used widely in various experiments. A discontinuous cr ossover from slow to much faster kinetics were observed in both high-and lo w-crosslinker-concentration regions, where the characteristic time of the s hrinking process changes abruptly by two to four orders of magnitude with a minute change of the crosslinker concentration. In spite of these marked c hanges in the kinetics, no anomaly was observed in the degree of equilibriu m swelling in these regions. A mechanism leading to sudden changes in kinet ics is discussed in terms of an inhomogeneous network structure which is de pendent on crosslink density.