PRIMARY PHOTOPROCESSES OF CR(VI) IN REAL-TIME HOLOGRAPHIC RECORDING MATERIAL - DICHROMATED POLY(VINYL ALCOHOL)

The primary photoprocesses of Cr(VI) in poly(vinyl alcohol) have been studied under various experimental conditions. Aqueous solution and polymer films of dichromated poly(vinyl alcohol) (DCPVA) have been irradiated, and the quantum yield values were determined for the photoreduction of Cr(VI) to Cr(III) at different pH values. Thermal and photochemical evolution of Cr(V), Cr(III), and polymer radical in DCPVA films was monitored at room temperature using real-time ESR spectrometry. Spin-trapping experiments with N-tert-butyl-alpha-phenylnitrone (PBN) gave evidence for the formation of radical adduct from the polymer matrix. Effect of pH of the coating solution and concentration of dichromate on the photochemical evolution of intermediates were observed. Possible involvement of all intermediate species from Cr(VI) has been discussed. On the basis of our experimental findings, a suitable mechanism has been suggested, involving the electron transfer from the polymer matrix to Cr(VI) leading to the formation of Cr(V) and polymer radical, which further undergoes reaction to produce Cr(III) and a cross-linked polymer matrix.