<p>Casting aqueous solutions of certain water-soluble polymers with heteropolyacids (cluster compounds of early-transition metal oxides) gives photochromic nanocomposites that are currently attracting considerable attention as functional materials for inkless and erasable printing, sensors, smart windows. Here, with the goal of improving the performance of such nanocomposites in these applications, effects of low-volatile polyol co-solvent (ethylene glycol, EG) addition to aqueous poly(vinyl alcohol)-heteropolyacid (PVA-HPA) solutions on the photochromic response of PVA-HPA nanocomposite films cast from these solutions are investigated. The main and unexpected finding of this study is that although the films from EG-containing solutions are EG free (due to its complete evaporation from the films during drying) and thus compositionally identical to those from solutions without EG, their photochromic efficiency increases (up to 50%) with increasing EG concentration in the casting solution. It is concluded that this phenomenon is caused by the molecular plasticization of PVA by low-volatile polyol co-solvent in the course of evolution of the cast film from a liquid to a solid state. Namely, polyol molecules prevent intra- and inter-chain hydrogen bonding and preserve the mobility of PVA chains in the cast solution during water evaporation, and thus facilitate the formation of a larger number of PVA-HPA hydrogen bonds on the surface of HPA nanoparticles needed for the photoinduced PVA-to-HPA electron transfers causing photochromism. Overall, this work shows the importance and means of controlling the polymer chain dynamics and intra-/inter-chain interactions for the fabrication of polymer matrix-based nanocomposites with improved properties via solution casting techniques.</p> Graphical abstract <p></p>