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

Abstract

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 W 5+ ions (480 nm) and two bands of intervalence W 5+ → W 6+ charge-transfer transitions (740 and 1250 nm) to appear in their electronic spectrum.Introduction. Phosphotungstic acid (PWA) is one of many heteropolyacids, which in turn include a variety of polyoxometalates (POM) that are nanoclusters based on transition metal oxides [1]. POM have a broad spectrum of important practical applications including chemical catalysis, membrane and sensor technologies, electrochemical fuel elements, and medicine [2]. These compounds exhibit photochromic properties (reversibly acquire or change color under the action of UV radiation). This enables them to be viewed as promising materials for optical and opto-electronic technologies and information recording and storage media [3]. Lately the number of publications dedicated to studies of hybrid nanomaterials obtained by dispersion of POM nanoclusters in matrices of organic polymers has been increasing rapidly [4-9]. Our goal was to prepare materials combining the advantages of polymers (low specific weight with high mechanical strength, thermal stability, long service life, and ease of fabrication into the required shape) and the photochromic properties of POM incorporated into them. The photochromic additive must be highly dispersed in the polymeric matrix in order to obtain polymer/POM nanocomposites. This is possible only if strong non-bonding interactions occur between the polymer and POM molecules. In practice, this is achieved by using polymers such as polyvinyl alcohol (PVA) [4,5,7], polyvinylpyrrolidone [6], polyacrylamide [8], and dextran [9]. However, strong nonbonding interactions between the polymer and POM can affect the structure of the polymeric matrix and/or the POM nanoclusters and cause substantial changes in the overall properties of the material, including its photochromic properties. Therefore, detailed studies of the molecular structure and non-bonding interactions in these materials with respect to their photochromic properties are needed in order to fabricate polymer/POM hybrid nanocomposites with given and controlled photochromism.The goal of our work was to study the structure and photochromic properties of PVA/PWA nanocomposite films using IR and electronic absorption s...

Poly(vinyl alcohol)/heteropolyacid nanocomposites as new materials for radiochromic dosimetry in radiation processing

“…The intensities of the bands rise steadily with increasing dose while their positions do not change, and the blue color of the films becomes progressively darker. The spectra of gamma‐irradiated PVA/H 3 PW 12 O 40 films shown in Figure 1 coincide in the position, shape, and relative intensities of the absorption bands with the spectra reported previously for H 3 PW 12 O 40 in PVA matrices and in solutions of low molecular weight alcohols irradiated with UV light 13,14,16 . This implies that the mechanism of coloration and the bands assignment in the case of gamma irradiation are similar to those already reported for the case of UV exposure 16,17 .…”

“…The nanocomposite nature of PVA/H 3 PW 12 O 40 films obtained by solution casting has been demonstrated previously by us 15 and others 13 with the use of transmission electron microscopy: the heteropolyacid is present in the films in the form of nanoparticles that are uniformly distributed in the polymer matrix and have spherical shape and size about 25–50 nm. FTIR studies of PVA/H 3 PW 12 O 40 films carried out by us earlier indicated that the Keggin structure of PW 12 O 40 3− heteropolyanion retains in the nanoparticles and that the hydroxyl groups of PVA form hydrogen bonds with the bridging (W– O –W) oxygen atoms of heteropolyacid 14,15 . Original (non‐irradiated) PVA/H 3 PW 12 O 40 films are colorless, homogeneous and have the same high transparency in appearance as pure PVA films.…”

“…PVA is one of the polymers of choice in the fabrication of dosimetry films 3,6,12 due to its outstanding properties such as elasticity, processability, high thermal, chemical and radiation stability. Another key reason for choosing the PVA–heteropolyacid combination is that heteropolyacids display photochromic properties (i.e., become colored upon exposure to UV light) in some organic matrices including PVA 13,14 . This paper reveals that the PVA/heteropolyacid systems possess also radiochromic properties.…”