Determination of equilibrium surface energy of adsorbed polyvinyl alcohol layers in water at 25°C

Abstract

Contact angle measurements using octane and air have been performed on polyester surfaces coated with poly(vinyl alcohols) (PVAs) of different molecular weights and degrees of hydrolysis, immersed in water at 25°C. The contact angle was time-dependent. Using the instantaneous contact angle of octane and air as probing fluids, one can estimate the equilibrium dispersion and polar components, and the total equilibrium surface free energy of the PVA-coated polyester surfaces under water. The results show that at low polymer concentrations, the contact angle of the PVA-coated polyester surface, measured through the water phase, decreases with decreasing hydrolyzed content at the same molecular weight. In other words, the surface becomes more hydrophilic or its total surface energy is increased. Of the PVAs tested, the PVA with the highest molecular weight (125 000) and lowest degree of hydrolysis (87%) was found to be the most effective for lowering the interfacial free energy of the polyester substrate and water. The method presented is useful for studying the polymer chain conformation that can occur when such surfaces are immersed in water. A comparison of polar/dispersion ratios of the same surfaces in air and under water suggests that hydrophilic groups on polymeric surfaces undergo significant conformational changes to minimize the surface free energies, when such surfaces are immersed in water. The work of cohesion is also calculated for PVA-treated and untreated polyester surfaces.