The characterization and photochromism of copolymer composite films containing phosphotungstic acid

• VPMA copolymer was copolymerized with VP and MA monomer.
• PWA/VPMA was synthesized by introduced PWA into the VPMA copolymer.
• The photo-reduction processes occurred according to the proton transfer mechanism.

The composite film (PWA/VPMA) was synthesized by introduced phosphotungstic acid (PWA) into the poly(vinylpyrrolidone and methyl acrylate) (VPMA), which was copolymerized with vinylpyrrolidone (VP) and methyl acrylate (MA) monomer. The microstructure, thermal stability, photochromic properties and mechanism were investigated via atomic force microscopy, Fourier transform infrared spectroscopy, differential thermal analysis, differential scanning calorimetry, ultraviolet–visible spectra and electron resonance spectra. The results indicated that the Keggin geometry of PWA and the basic structure of VPMA were not destroyed in the composite process. After PWA particles were introduced into the VPMA, the thermal stability of composite film was lowered due to the interaction between PWA and VPMA. Irradiated with UV light, PWA/VPMA composite film changed from colorless to blue and showed reversible photochromism. The appearance of W5+ in ESR spectra indicated that the photo-reduction process between PWA and copolymer matrix occurred according to the proton transfer mechanism.