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A comparative investigation of the photochemical behavior of dichromated gelatin (DCG), dichromated polyacrylic acid (DCPAA) and dichromated polyvinylalcohol (DCPVA) gave evidence for the similarity of the photochemical behaviour involved when recording holograms in DCG and DCPVA. Chromium (V), the chromium species resulting from the
photoredox process between the starting chromium (VI) (dichromate) and the polymeric chain, was stable likely strongly complexed in gelatine and polyvinylalcohol. The quantum yields of the reaction are high and similar for both reactions. On the contrary, even though chromium (V) was also formed during the primary process in DCPAA, this species was quite unstable and disappeared as soon as the irradiation was stopped. The behavior observed in DCG can be assigned to the presence of OH groups on the polymeric chain, these groups being the only groups present in polyvinylalcohol. Actually, the stability and the state of complexation of the different chromium species was completely opposite in the two matrices: the stabilization of chromium (V) resulting from the photochemical charge transfer in DCPVA, by complexation with PVA is in contrast to what was observed in DCPAA where chromium (V) was highly instable. Regarding chromium (III), the final reduction chromium species, it was complexed in PAA and not in PVA. The experiments performed on films of DC (PVA+PAA) with only a few percents of PAA gave evidence for the strong influence of the presence of the carboxylic groups on the photochemical behavior: the rate of the formation and the
stability of chromium species that reflects the chelating properties of the polymeric matrix. The photochemical results were correlated to the results obtained when recording holograms in the corresponding material: a very nice agreement appeared between the two sets of results. An innovative approach combining the monitoring of the structural modification of the polymeric matrix and the fate of the various chromium species ((VI), (V) and (III)) was then implemented. For the first time, it was established that chromium (V) was at the origin of the cross-linking implied in the hologram formation by acting as a bridge between hydroxyl groups of the polymeric chains. A second unanswered question was also elucidated. The improvement brought by ammonium dichromate with respect to potassium dichromate involves amide groups as additional chelating sites for chromium (V) resulting in the increase of the matrix cross-linking.
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