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

Abstract

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.

The Influence of Annealing on Optical and Humidity Sensing Properties of Poly(Vinyl Alcohol-Co-Vinyl Acetal) Thin Films

“…So, in this case, the highest n is observed for the copolymer of intermediate modification degree, unlike samples annealed at 60 °C, in which case the refractive index was found to decrease gradually with increasing acetal content [13]. Figure 1b shows the percentage decrease in thickness of films annealed at 180 °C compared to films annealed at 60 °C [13]. From the calculated values, it can be seen that the thicknesses of all films decreased when they were annealed at 180 °C.…”

“…In the literature, a number of papers report on the application of PVA-based composite films as active media for humidity sensing [9][10][11][12], but to the best of our knowledge PVA copolymers were not tested in optical sensors. We have already shown that films of poly(vinyl alcohol-co-vinyl acetal)s of varied copolymer compositions annealed at 60 °C can be used as active media for optical sensing of humidity [13]. The comparison with pure PVA thin films confirms the positive influence of acetal fraction in the copolymer on sensing properties.…”

“…It is seen that the values of n are in the range 1.41-1.47 with the highest value of 1.47 observed for the sample of acetal content of 24%. So, in this case, the highest n is observed for the copolymer of intermediate modification degree, unlike samples annealed at 60 °C, in which case the refractive index was found to decrease gradually with increasing acetal content [13]. Figure 1b shows the percentage decrease in thickness of films annealed at 180 °C compared to films annealed at 60 °C [13].…”

“…where Rup and Rdown are reflectance values measured for increasing and decreasing humidity, respectively, ∆ is the reflectance change in the whole range ∆ of measured humidity and ∆ is the humidity range where hysteresis is observed. Recently, on the basis of the calculated values of H for samples annealed at 60 °C it was concluded that thin films of pure PVA annealed at 60 °C are not suitable for optical sensing of humidity because of very high hysteresis levels [13]. The comparison of values of H for samples annealed at both temperatures is presented on Figure 2.…”

“…Hydrophobically modified PVA copolymers were synthesized by reacting a portion of hydroxyl groups of PVA (average polymerization degree 1600) with acetaldehyde in aqueous solution as already described elsewhere [13]. The copolymer composition was controlled by PVA-toacetaldehyde molar ratio and determined by 1 H NMR spectroscopy.…”