One-Step Modification of Poly(vinyl alcohol) with Benzophenone and Sulfonic Acid Groups for Waterborne and UV-Curable Humidifier Membrane Coatings

The water-soluble polymer poly(vinyl alcohol) (PVA) is a well-known coating material for the fabrication of composite membranes. In order to limit swelling and prevent dissolution in aqueous environments, PVA layers must be cross-linked before use. For this purpose, in this work, water-soluble, photoreactive PVA derivatives were synthesized and spin coated onto silanized silicon wafers, and their cross-linking was investigated. Moreover, composite membranes were formed via doctor blading, followed by cross-linking the resulting polymer layers and determination of the water vapor permeance. In a one-pot synthesis, PVA was reacted with 1,3-propane sultone and 4-fluorobenzophenone, yielding sulfopropyl and benzophenone-modified PVA (sPVA-BP). The modification degrees were successfully controlled via the reaction conditions and amounted to values between 6 and 26% for benzophenone modification and 6 and 12% for sulfopropyl modification. All synthesized polymers had a solubility of at least 10% (w/w) in water. Thin layers of sPVA-BP with thicknesses of around 30 nm on silanized silicon wafers formed insoluble networks upon irradiation. The gel contents (Y) of the cross-linked networks were approximately 80%. The equilibrium degree of swelling (EDS) strongly depended on the modification degrees of the polymers and decreased while cross-linking. After 30 min of irradiation with UV light, EDS plateaus between 4 and 12 were reached, indicating a completed network formation. In order to prove the applicability of sPVA-BP layers for membrane applications, polypropylene/sPVA-BP composite membranes with an sPVA-BP layer thickness of 3.3 μm were tested for their water vapor permeance. The membranes reached a water vapor permeance (Jw) of approximately 2000 GPU, which is comparable to those of other PVA-based composite membranes. We therefore conclude that sPVA-BP is a promising photoreactive coating material, e.g., for the fabrication of composite humidifier membranes