Effects of Polymer Concentration and Cross-Linking Density on Rheology of Chemically Cross-Linked Poly(vinyl alcohol) near the Gelation Threshold

Abstract

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‘ ∝ ωn‘) and loss modulus (G‘‘ ∝ ωn‘‘) 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 ...