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‘ ∝ ω n ‘ ) and loss modulus ( G‘‘ ∝ ω 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 d f 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.