Structure and photochromic properties of poly(vinylalcohol)/phosphotungstic acid nanocomposite films

Poly(vinylalcohol)/phosphotungstic acid (PVA/PWA) nanocomposite films were studied using Fourier-transform infrared and electron absorption spectroscopy. It was found that entrapping PWA into PVA leads to the formation of hydrogen bonds between OH groups of PVA and bridging oxygen atoms of PWA. Terminal oxygen atoms of PWA do not participate in hydrogen-bonding interactions with the polymer. Exposure of the nanocomposite film to UV radiation results both in the photo-induced transfer of protons from PVA to PWA with their attachment to the bridging oxygen atoms of PWA and in the formation of a PVA-PWA complex with the participation of deprotonated oxygen atoms of PVA and the terminal oxygen atoms of PWA. The UV irradiation causes the films to turn blue and a band of d-d transitions of W5+ ions (480 nm) and two bands of intervalence W5+ → W6+ charge-transfer transitions (740 and 1250 nm) to appear in their electronic spectrum. (authors)