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

Three blends were prepared from a high molecular weight of poly(vinyl chloride) (PVC) (M w = 173 000, M w/M n = 2.0) and a low molecular weight PVC (M w = 39 400, M w/M n = 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 c g for the sol−gel transition still followed the relation c g ∝ M w -1, which was unchangeable with the molecular weight distribution and was also in good agreement with the previous results. As a result, c g was well expressed by a mixing rule, 1/c g = w 1/c g1 + w 2/c g2, where w i is the weight fraction of the component polymer i. The gel strength S g at the gel point did not obey the relation S g ∝ M w -1, but scaled as S g ∝ M z -1 to show the effect of long chains on gelation. In the postgel state, the gel elasticity determined by the quasi-equilibrium modulus G e still followed the scaling law, G e ∝ ε z , where ε is the relative distance to the gel point and z = 2.6 for the (PVC blend)/DOP samples, but the G e values at the same ε were observed to be dominated by the long chains of PVC.