Volume phase transition of swollen gels: Discontinuous or continuous?

Poly(N-isopropylacrylamide) (PNIPAM) linear chains and spherical microgels were studied using laser light scattering. The volume change associated with the temperature dependence of the hydrodynamic radii of both the linear chains and the spherical microgels is continuous, in contrast to a discontinuous volume phase transition observed for a bulk PNIPAM gel with an identical composition. A reexamination of the volume phase transition theory shows that the phase transition temperature depends on the length of the subchain between two neighboring cross-linking points inside the gel. A comparison of the results of the linear chains and the spherical microgels suggests that the volume change of a polymer gel should be practically continuous because the inhomogeneity of the subchain length smears the volume phase transition. A possible explanation is also presented to rationalize the discontinuous volume phase transition observed in certain bulk PNIPAM gels, which is based on the bulk shear module of the gel and the microscopically inhomogeneous shrinking generated inside the gel in the volume phase transition.