Structure and properties of poly(vinyl alcohol)/tungsten trioxide hybrids

Abstract

PVA/WO 3 hybrids were produced by using ion exchange process of Na 2WO 4·2H 2O. Microstructures of this hybrid was estimated by SAXS measurement as follows; the WO 3 rich domains having radius of gyration of about 5.7 nm (estimated in Guinier regime), in which PVA chains were partly incorporated, were dispersed evenly in PVA matrix with the correlation length of 20–30 nm (approximated by Bragg equation). The electron density difference between WO 3 and matrix, Δ ρ, was rather high and sharp, and this indicates that the domains dispersed in PVA matrix is rich in WO 3 component. The physical properties of the hybrids were affected by the morphology. The mechanical properties of the hybrid were improved markedly, e.g. the tensile strength increased from 35 MPa (PVA) to 55 MPa at 27 wt% of WO 3 content, and the modulus also increased above T g with increasing amount of WO 3. A tan δ peak appeared at 75 °C shifted to the higher temperature side with WO 3 content. These were caused by strong interaction between PVA chains and WO 3 domains, which was formed by incorporation of PVA chain into WO 3 domains having hydrogen bonding. This hybrid was transparent and showed the photochromism especially at low concentration of WO 3. When UV–visible light of wavelength above 350 nm was irradiated on the hybrid, a minimum occurred at 630 nm in the UV–visible spectrum, and color changed from yellow to blue. Bleaching occurred in the dark in 100% relative humidity. Consequently, the ion exchange process was suitable to obtain PVA/WO 3 hybrid, since this procedure was simple and economical, and hybrid thus produced had excellent mechanical and photochromism properties.