Rheological Images of Poly(vinyl chloride) Gels. 5. Effect of Molecular Weight Distribution

Abstract

Three blends were prepared from a high molecular weight of poly(vinyl chloride) (PVC) (Mw = 173 000, Mw/Mn = 2.0) and a low molecular weight PVC (Mw = 39 400, Mw/Mn = 1.7). Dynamic viscoelastic properties of these PVC blends in bis(2-ethylhexyl) phthalate (DOP) were measured at 40 °C as a function of polymer concentration, and the effect of long chains on gelation has been studied. The scaling exponent n at the gel point was found to be constant (=0.75), independent of molecular weight and molecular weight distribution. The critical concentration cg for the sol−gel transition still followed the relation cg ∝ Mw-1, which was unchangeable with the molecular weight distribution and was also in good agreement with the previous results. As a result, cg was well expressed by a mixing rule, 1/cg = w1/cg1 + w2/cg2, where wi is the weight fraction of the component polymer i. The gel strength Sg at the gel point did not obey the relation Sg ∝ Mw-1, but scaled as Sg ∝ Mz-1 to show the effect of long chains on gelation. In the postgel state, the gel elasticity determined by the quasi-equilibrium modulus Ge still followed the scaling law, Ge ∝ εz, where ε is the relative distance to the gel point and z = 2.6 for the (PVC blend)/DOP samples, but the Ge values at the same ε were observed to be dominated by the long chains of PVC.