The effect of molecular motion on radiation-induced chain scission and racemization of iso-poly(methyl methacrylate)

Abstract

In this paper, the radiation-induced chain scission and racemization of isotactic poly(methyl methacrylate) (iso-PMMA) in amorphous, solid, crystalline state and in solution have been studied with nuclear magnetic resonance and molecular weight determination. It is shown that the chain scission is the most prior effect for iso-PMMA in its dilute solution and the racemization reaction is the inferior one in this case. However, on the contrary, the racemization is the most prior effect when iso-PMMA was irradiated in its crystalline state and chain scission is the most inferior effect. Such experimental results could well be explained with the view of point from molecular motion and “cage effect”. The hypothesis we proposed, that chain scission, recombination and racemization were in competition to each other and the result of priority depends on the mobility of macromolecules or the state at which iso-PMMA was irradiated, were verified once again. In our previous papers [1, 2], the temperature dependence of radiation-induced chain scission of poly(methyl methacrylate) (PMMA) with various stereospecific configuration as well as the radiation-induced racemization of isotactic PMMA have been studied. We proposed the mechanism of chain scission as a dynamic equilibrium of chain scission and recombination to elucidate the temperature dependence of the process. During recombination of the chain fragments, stereospecificity change due to steric hindrance effect, mm triads content in iso-PMMA gradually change into mr(rm) and rr ones. As a result, racemization takes place. In order to study further the mechanism of chain scission and racemization processes, the radiation effect on polymer chain were studied in its various states, i.e., amorphous solid, crystalline state and that in solution. In solution, especially in dilute solution, the polymer chain and its segments are much more mobile than those in their solid states. In their solid state, especially in crystalline state, the chains and their segments are restricted to be not easy to move away. Such a study is significant for the study of the mechanisms of radiation-induced chain-scission and racemization.