Simple thermodynamics of macroscopic phase separation in shrinking gels
Abstract
The shrinking process induced in spherical gels by an abrupt change in temperature has been investigated qualitatively. The phase diagram of the gel system has been found to be helpful in classifying a variety of shrinking processes. When the solvent quality is lowered within the region between the volume transition and coexistence temperatures, the local swelling ratio of the inner portion, which is divided by a moving interface from the outer shrunk phase, declines in the course of the shrinking process. On the contrary, the local swelling ratio of the inner portion of the shrinking gel is enlarged when the solvent quality is lowered into the region between the coexistence and spinodal temperatures. In this latter case, owing to large local swelling of the inner portion in the vicinity of the interface, spherical symmetry will imply mechanical instability. This instability will be the origin of transient spatial patterns on the surfaces of shrinking gels.