Effect of Chain Structure on the Rheological Properties of Vinyl Acetate–Vinyl Alcohol Copolymers in Solution and Bulk

Abstract

Bulk and solution studies revealed a strongly pronounced effect of chain structure on the rheological and relaxation behavior of well-characterized vinyl acetate–vinyl alcohol copolymers of similar composition and polymerization degree. The frequency–temperature superposition principle is fully applicable to the random copolymers, which demonstrate all expected relaxation states, whereas a divergence of the reduced dynamic moduli–frequency dependences is observed for the multiblock copolymers. In the latter case, the terminal zone is sensitive to the self-assembling of vinyl alcohol blocks into (depending on the copolymer composition) crystalline or amorphous microstructures. The monomer unit distribution particularly affects properties of the copolymer solutions in N,N-dimethylformamide (DMF). 5% solutions behave as simple viscoelastic liquids at 20 °C and show viscoplastic behavior at −20 °C, where more blocky chains are characterized by up to 4 orders of magnitude higher yield stress values. The multiblock copolymer solutions demonstrate a pronounced viscosity hysteresis in the heating–cooling cycle, being absent in the random copolymers. 10% solutions of multiblock copolymers are practically gelatinous even at room temperature. The observed effects are explained by examining the peculiarities of hydrogen bonding in vinyl acetate–vinyl alcohol copolymers using FTIR spectroscopy. The multiblock copolymers are characterized by stronger hydroxyl–hydroxyl H-bonds and greater fraction of interchain hydroxyl–acetyloxy H-bonds providing aggregation of chains and high viscosity of the corresponding samples, whereas the random copolymers more strongly interact with the residual solvent. Dynamic light scattering studies prove that the relaxation of concentration fluctuations is completely diffusive, being bimodal in the random copolymers and trimodal in the multiblock ones. The fast mode in the latter case demonstrates anomalous concentration behavior. In the dilute regime, up to very low concentrations, multiblock copolymer chains form stable aggregates, and this fact correlates with an unusual growth of the reduced viscosity in the corresponding rheological experiments.