Thermal decay of trapped radicals produced during the photopolymerization of diacrylates

Direct evidence of the presence of extremely long-lived radicals produced in the photopolymerization of 1,6-hexanediol diacrylate and tetraethyleneglycol diacrylate has been obtained by EPR spectroscopy. As radical decay in thermal after-treatment is accompanied by additional polymerization and cross-linking, kinetic studies were carried out at different temperatures (40-120°C) both by EPR spectroscopy and by analysis of the parallel disappearance of double bonds. Apart from EPR data at 120°C, all experimental results show that both radical decay and double-bond conversion are first-order reactions, and kinetic constants evaluated for the two processes are practically coincident at the same temperature. Activation parameters for thermal after-treatment of the two diacrylates are almost the same, owing to the similarity in their chain length and mobility. The behaviour of the system at 120°C, accounted for by a multiple relaxation model, is interpreted as being due to prevailing cross-linking reactions in the vitrified network of trapped radicals.