Poly(vinyl alcohol)-based thin films for optical humidity sensing

The optical and humidity-sensing properties are studied of hydrophobically modified PVA, namely poly(vinyl alcohol-co-vinyl acetal)s of varied copolymer composition. Copolymers are synthesized by reacting PVA with acetaldehyde in aqueous solution, thus introducing cyclic acetal functionalities in the polymer chain. Thin polymer films are deposited by spin-coating and their optical properties are studied as a function of the copolymer composition, i.e., degree of acetalization. Reflectance measurements at different relative humidities in the range 5 - 95 % RH are conducted in order to probe the sensing behavior. The comparison with neat PVA thin films confirms the influence of the copolymer's acetal fraction on the optical and sensing properties and degree of hysteresis. The feasibility of applying poly( vinyl alcohol-co-vinyl acetal) thin films for optical sensing of humidity is demonstrated and discussed.