Studies in radiation-induced polymerization of vinyl monomers at high dose rates. II. Methyl methacrylate

The radiation-induced polymerization of methyl methacrylate was investigated with radiation sources of cobalt 60 and accelerated electrons at dose rates up to 3 Mrads/sec. Extrapolation of previous rates of polymerization at dose rates of 0.01–200 rads/sec coincided with the present results, the rates being approximately proportional to the square root of the dose rate throughout the entire set of dose rates measured. The molecular weights seemed to be independent of dose rate at the highest dose rates investigated. A combination of high polymer with a much higher molecular weight than expected was formed, together with a substantial portion of low molecular weight polymer. The reason for this behavior is not clear at this time. The G(M·) calculated from the molecular weights and fraction of polymer and resin was 6.0, which approaches that reported in previous investigations at low dose rates. There was no significant effect of air on the polymerization kinetics of methyl methacrylate at above 1 Mrad/sec. Nitrogen also did not influence the measured rates. Conversions to polymer were not substantially reduced by the presence of inhibitor at above 1.26 × 105 rads/sec. Water did not influence the rates of polymerization, except at the highest temperature (50°C) investigated. A large posteffect was observed in sealed degassed ampoules after 25% conversion to polymer. Only 3.4% additional polymer was formed in 24 hr after irradiation in the presence of air. The activation energy for the electron beam polymerization of methyl methacrylate was about 7.0 kcal/mole. This value, considering the complications in technique such as beam heating, did not differ from literature data enough to suggest any mechanistic difference in the polymerization at high dose rates.