Dark reactions of free radicals crosslinked polymer networks trapped in densely after photopolymerization

摘要

Difunctional methacrylate monomers containing, stiff or flexible spacers were combined with a vinyl-cyclopropane derivate in a copolymerization to form densely crosslinked networks. The high rate of visible light photoinitiation at 20-22degreesC was used for starting the polymerization. By switching off the light after 40 s, the long-lived free radicals, trapped in the polymer network, retain radical processes, however, at substantially retarded rates. The postcuring development of the network structure in the investigated samples lasting at least 12 weeks was proved by the increasing density of the crosslinks, by the increasing amount of the gel fraction, and by the decreasing of the residual unsaturation. The relaxation spectra from dynamic mechanical thermal analyses (DMTA) show curves with two evident maxima. The first one belonged the glass transition (T,) of original network and increases in the postcuring period. The second one belonged to the final structure to which the crosslinked polymer approaches slowly during aging. The final state develops fast during a DMTA thermal scan to 200degreesC. All procedures proceeded in the dark, but in the presence of air oxygen. The chemiluminescence (CL) response of the postcured samples proved the accumulation of hydroperoxides. The oxidation chain reaction took place in the postcuring time and competed with the building up of additional crosslinks. The majority of postcuring changes were observed during the first 3 weeks of aging. Nine weeks later, the long-lived radicals were still present in the network and maintained the sensibility of the crosslinked polymer to free-radical reactions. (C) 2003 Wiley Periodicals, Inc. [References: 40]