Rheological images of poly(vinyl chloride) gels. 1. The dependence of sol-gel transition on concentration

Dynamic viscoelastic properties of poly(vinyl chloride) (PVC)/bis(2-ethylhexyl) phthalate (DOP) solutions and gels have been studied as a function of polymer concentration and molecular weight. All PVCs were linear polymers and had a similar distribution in molecular weight. Increasing the polymer concentration gradually from the liquid state to the solid state allowed the rheological observation of the sol-gel transition. The gel point c(g), which depends on the polymer concentration, was accurately determined by the frequency independence of loss tangent in the vicinity of the sol-gel transition and was found to be inversely proportional to the molecular weight as expressed by c(g) proportional to M-w(-1). The scaling exponent n obtained was constant (n = 0.75) and independent of the molecular weight of PVC. The gel strength S-g at the gel point decreased with increasing PVC molecular weight. A constant gel strength (S-g/c(g) = 1.27 x 10(-2) Pas(0.75) g(-1) L) was obtained by normalizing the gel strength S-g with c(g) to eliminate the effects of M-w. These results suggest a unique character and structure at the gel point of the gelling system.