KINETIC-STUDY OF THE PHOTO-CROSS-LINKING OF 1,6-HEXANEDIOL DIACRYLATE GRAFTED ONTO CELLULOSE IN THE PRESENCE OF PHOTOCATALYTIC SYSTEMS

The rate of photocrosslinking of photografted layers of 1,6-hexanediol diacrylate (I) (10.5 +/- 0.5-mu-mol cm-2) on cellulose was investigated kinetically at 30 +/- 2-degrees-C, as a function of the residual unsaturation of grafted I, in the presence of 1,2-diphenyl-2,2-dimethoxyethanone as photoinitiator. The impinging polychromatic photoenergy was varied in the range (2.8 +/- 0.3) x 10(-8) to (23.2 +/- 0.9) x 10(-8) einstein s-1 cm-2. The study was carried out with or without the addition of catalytic amounts (20-150 ppm) of three vanadium(V) alkoxides. Although the initial concentration of the photoinitiator added to the reacting monomer did not exert any influence on the rate of photocrosslinking (which was generally controlled by the absorbed radiation intensity and was significantly decreased with increasing degree of conversion of the double bonds), the vanadium photocatalysts were particularly active (an acceleration factor of about 75 times was reached with 150 ppm vanadium (synergic mixture of vanadium(V) isopropoxide and isobutoxide)). The kinetics of photocrosslinking of grafted I did not follow any simple kinetic law. A multi-relaxation spectrum was observed and interpreted in terms of two parameters (width of spectrum beta and mean lifetime tau). The beta-values (0.38 +/- 0.02) were almost the same in all the experiments and were attributed to the thermal decay of trapped radicals produced photochemically in bulk I. In contrast, the tau-values were particularly useful as a measure of the activity of the photocatalytic systems investigated. Some kinetic features of a physical model are discussed, which can explain the photocrosslinking and the possible role of photocatalysts in facilitating the transport of radical sites by hydrogen transfer.