**1. Introduction**

Poly(vinyl alcohol) (PVA) is a hydrophilic and very water-soluble polymer due to pendant hydroxyl groups, which are mainly responsible for its reactivity and crystallinity [1]. Due to its outstanding mechanical and film-forming properties PVA is used in a variety of areas such as membranes, adhesives, coatings, etc. [2]. Because PVA can absorb and desorb water quickly, an increasing interest in the research of PVA-based humidity sensors has been observed recently and PVA is implemented as a humidity-sensitive medium in various sensor types [3–8].

However, development of PVA-based optical humidity sensors with high sensitivity, wide dynamic range and linearity, stability, and low hysteresis is still a challenge. This is probably due to the highly water-soluble nature of PVA that limits its stable sensing properties in a form of nanometer-thick polymer film. One possible approach to overcome these drawbacks is to use different composites consisting of PVA as a matrix. Mixture of PVA and graphene quantum dots (GQDs) [9–11] and crosslinked PVA/functionalized graphene oxide nanocomposite films [12] were used for humidity-sensing using optical fiber technology [9–11] and attenuated reflectance measurements [12]. Composites consisting of PVA and nanosilica particles were used for humidity sensing through crystal microbalance [13] or for depositing sensitive opal structures [14]. Polyaniline/poly(vinyl alcohol) composites [15] and silver–polyaniline/polyvinyl alcohol composites [16] were used as sensitive media for acoustic wave-impedance humidity sensors and resistive humidity sensors, respectively. Thick PVA substrates

(around 80 microns) doped with silver nanoparticles were applied for humidity sensing through transmittance measurements [17].

Another possible approach to enhance PVA sensing performance is to use hydrophobically modified PVA copolymers such as poly(vinyl acetal)s [18]. In general, poly(vinyl acetal)s are class of polymers obtained by reaction of PVA with aldehydes, especially formaldehyde, acetaldehyde, and butyraldehyde finding advanced application as structural adhesives in the aircraft industry, as the interlayer in automotive safety glass, etc. [19]. The contents of unreacted hydroxyl groups along with the acetal rings and the molecular weight determine the polymer properties. Hydrophilicity of partially acetalized PVA is reduced while maintaining its inherent response to humidity.

Our previous studies [20,21] have shown that hydrophobically modified PVA copolymers, namely poly(vinylalcohol-*co*-vinylacetal)s (PVA–Ac) of acetal content in the range 18%–28% prepared as a single films on opaque substrate are suitable for optical sensing of humidity. In this work, the influence of film thickness, post-deposition annealing and substrate type/configuration on humidity-sensing properties is studied for partially acetalized PVA films and comparison with neat PVA is made. The possibility to improve the reaction toward humidity of the polymer thin films via doping with SiO2 particles is explored and polymer–silica hybrids are obtained. Enhancement of sensing behavior through this approach is demonstrated and discussed.
