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 degrees 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) proportional to 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 proportional to M-w(-1), but scaled as S-g proportional to 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) proportional to epsilon(z), where epsilon 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 epsilon were observed to be dominated by the long chains of PVC.