PECULIAR DYNAMIC VISCOELASTIC BEHAVIOR OF THERMOREVERSIBLE PVC GELS

Dynamic viscoelastic measurements of PVC-based aged gels reveal the existence of two pseu-doequilibrium moduli independent of temperature, frequency, and strain amplitude: Gelat low temperatures and Ge2 at intermediate temperatures (Gei > Ge2). A maximum in tan δ spectra is observed at intermediate temperatures. From these results and DSC results (which reveal the existence of a low-temperature endotherm, besides the gel-melting endotherm) a transition, T8, fixed in the range 45-65 °C, is defined. The results of thermal analysis are coincident with those reported in recent literature for PVC gels in several solvents. Diffusion coefficient versus temperature data seems to confirm the existence of the transition only in the case of PVC/Reofos gels. Clark and Ross-Murphy's cascade model is applied to the concentration dependence of Gei and G62, using functionalities of/= 3-5. These values of “f are obtained assuming that crystals of an average of 40-Ä length produce the connectedness required for gelation. In order to analyze the hypothesis of liquid-liquid phase separation as responsible of the relatively high values of the modulus Ge2, the effect of an increment of concentration in Ge2 is evaluated. The fact that the values of Gei are higher than those that can be obtained from a simple increment of concentration of polymer (polymer-rich phase) lead us to think that the participation of another type of link, besides crystallites, in the formation of the structure of aged gels is an open hypothesis. © 1990, American Chemical Society. All rights reserved.