Light emission on heating polymer networks formed by radical polymerization

Organic-inorganic hybrid monomers based on alkoxysilanes bearing methacrylate groups, or the pure organic dimethacrylate monomers were photo-polymerized to densely cross-linked networks. Storage of the polymer samples in nitrogen and at ambient laboratory temperature was associated with a slow decay of trapped free radicals and formation of new cross-links. In air, reactions with oxygen lead competitively to formation of peroxides due to easier diffusion in the polymer matrix. The formation and existence of peroxides in samples stored in air was shown by the chemiluminescence (CL) induced at elevated temperatures. The peroxide accumulation proceeds in three stages. At first, approximately up to 6 weeks, a high rate of accumulated peroxides activated by long living radicals is assumed. During the next 20 weeks a steady state oxidation stage is observed before another accumulation of peroxides occurs. The glass transition temperature (Tg) changes were used to estimate variation of cross-link density during ageing the networks. As far as the entirely organic network is concerned the main portion of precursors giving rise to cross-links in the post-curing period was deactivated within 9 weeks under the influence of oxygen. Contrary to this finding in networks based on organic-inorganic hybrid monomers and a copolymer of the inorganic and organic monomers the additional cross-linking is considerably less influenced by oxygen. It is anticipated that a pronounced network structure favours further cross-linking over oxygen addition.