The stability of free radicals in densely crosslinked acrylate polymer networks

The four types of crosslinked polymer samples based on pentaerythritol tetr aacrylate (PETA) and propylene glycol-425-diacetoacetate (PGDAA) were I pre pared in two steps. At first the Michael dark addition of L-vinyl groups of PETA to PGDAA results in polymer net, work (cf. Scheme 1). The higher conv ersion of residual vinyl monomer and pendant double bonds built Lip in the Michael network was achieved by a photo-initiated free radical polymerizati on in the second step of polymer net work synthesis. The lifetime of trappe d radical was estimated by following the decay of radicals in the dark peri od of intermittent illumination directly in the resonator -1 cavity of ESR spectrometer. The determined values, of bimolecular termination rate consta nts k(t) are of order,10(-1) to 10(2) kg/mol (.) s in dependence on composi tion, structure, gen of network, and environment. It was observed that oxy, facilitates the decay of free radicals. The k(t), values are of about 1 to 2 decimal orders higher than that determined in nitrogen. Moreover, as in each subsequent light-dark cycle the k(t) was increased, we believe that it is a consequence of enhancement the diffusion of free radicals by assisted oxygen chain reaction with formation of-hydro-peroxides. The exception is the sample B with the highest final crosslinks density and conversion of mo nomers to network. More tightly trapped free radicals were present in polym er networks photo-cured in nitrogen. The k(t) values in all four types of n etworks decreased in each consecutive light-dark cycle. [GRAPHICS] The dependence of free radicals accumulation or decay on time in illuminati on and respective dark periods. The Michael network B-1 at room temperature in nitrogen (full points), or in air (open points).