Theoretical Study of the Postgel Regime in Thermoreversible Gelation

Abstract

Phase diagrams of thermoreversible gels are derived by the use of two different conventional postgel treatments, i.e., Flory's treatment and Stockmayer's treatment. Phase diagrams by Flory's treatment, which allows cycle formation in the gel network, often show two critical points for intermediate association constants, one in the sol region and the other in the gel region. In such a case three-phase equilibrium temperature appears. Existence of a critical solution point in the gel region suggests the possibility of phase separation into a dilute gel and a concentrated gel. One of these two critical points is a conventional type that lies on the stable coexistence curve (binodal), but the other one may lie on the metastable binodal. In contrast, phase diagrams by Stockmayer's treatment, which strictly excludes cycle formation in the gel network, often lead to the appearance of a tricritical point (TCP) where sol/gel transition curve intersects with the binodal line. This treatment gives no critical solution point, and hence gel/gel phase separation, in the postgel regime.