Effects of polymer concentration and cross-linking density on rheology of chemically cross-linked poly(vinyl alcohol) near the gelation threshold

The evolution of viscoelasticity during the gelation reactions of semidilute solutions of poly(vinyl alcohol) (PVA) in the presence of cross-linker (glutaraldehyde) has been monitored through the sol-gel transition with dynamic mechanical experiments. The gelation time of the system decreased with increasing PVA and cross-linker concentrations. At the gel point, a power law frequency dependence of the dynamic storage modulus (G' proportional to omega(n')) and loss modulus (G '' proportional to omega(n '')) was observed with n' = n '' = n. The power law exponent is lower than that predicted (0.7) from the percolation model. The value of n decreases with increasing polymer and cross-linker concentrations. The critical gel strength parameter (S) rises with increasing polymer and cross-linker concentrations. Some stress relaxation measurements on gelling PVA systems were also carried out. At the gel point, a power law time dependence of the relaxation modulus was detected, but the relaxation exponent was lower than the corresponding one obtained from oscillatory shear measurements. The fractal dimension (d(f)) was determined for the incipient PVA gels. The value of df increased with increasing polymer concentration and cross-linking density, This trend of d(f) suggests a more ''tight'' structure of the network at higher polymer concentrations and cross-linking densities.