Kinetics of volume phase transition in poly(N-isopropylacrylamide) gels

Kinetics of volume phase transition in poly(N-isopropylacrylamide) (NIPA) gels jumped from a low-temperature swollen phase to a high-temperature shrunken phase was studied as functions of NIPA monomer and crosslinker concentrations. We found for the first time a clear kinematical boundary at which the shrinking relaxation time of gels changes discontinuously by ${10}^2{\text{–10}}^4$ times, and that the profile of the boundary correlates with the sol-gel transition line and the contour line of turbidity of gels. A “morphological” boundary which characterizes the emergence of the bubble formation on gel surface was also determined. The theoretical calculation of the phase diagram on the basis of the mean field theory shows qualitatively that the shrinking speed of gels could be connected with the depth of the thermodynamic region of the spinodal instability $\text{(K+4μ/3=0)}$ into which they are transferred where $K$ and μ are the bulk and the shear moduli, respectively. A mechanism of discontinuous change of the shrinking speed is discussed in connection with the thermodynamic properties as well as the inhomogeneity of network structure.