Modification of exterior and intraporous surfaces of polyvinylidene fluoride membranes using KOH/water/alcohol ternary: Effects of wettability, polarity, and OH− activity

Abstract

Alkaline treatment is a cost-effective and easy-to-operate method that can effectively alter the hydrophilicity of polyvinylidene fluoride (PVDF) membranes. It makes the surface of PVDF membranes chemically active by introducing hydrophilic groups, e.g., hydroxyl and carboxyl groups, and therefore could be used as an effective pretreatment for membrane surface engineering including the grafting of other target chemical molecules onto to membrane surface. This treatment enhances the hydrophilicity of the membrane surface and could therefore also improve membrane permeability and reduce fouling. In this research, we systematically studied the alkaline treatment using KOH/alcohol/water ternaries at different conditions. Results indicate that the inclusion of alcohol in the reaction system could impact the efficiency of membrane surface modification through altering the chemical reaction mechanism as well as affecting the reacting mixture's properties such as OH⁻ activity, polarity, reactivity, and membrane wettability. The effectiveness of different alcohols was in the order of methanol > ethanol > isopropanol in terms of increasing the surface hydrophilicity, which is in accordance with the orders of their polarity and reactivity. In the meantime, the coloration of treated membranes followed a reversed order. It is evident that increasing treatment temperature, duration, OH⁻ activity, reactivity and polarity in the tested range would lead to the increase of both the surface hydrophilicity and membrane color. On the other hand, increasing a ternary's ability to wet the PVDF membrane had none or little impact on membrane hydrophilicity but strong impact on membrane color. Results suggest that coloration reactions did not contribute to the improvement of surface hydrophilicity and should therefore be minimized.