Influence of C60 Fullerene on the Oxidative Degradation of a Free Radical Poly(Methyl Methacrylate)

Abstract

The results of studies of thermal and oxidative degradation of a free-radical poly(methyl methacrylate) (PMMA) and the influence of a fullerene C60 on these processes are considered. The best characteristic of the change of the thermal stability is the temperature shift of the entire thermal gravimetric (TG) curve (or the middle part of it), but not the shift of onset or the starting temperature of the mass loss. During oxidative degradation in air of PMMA, three processes occur practically simultaneously: thermal degradation, or scission of chains; oxidation, or insertion of oxygen atoms in the polymer chains and in products of the degradation; and a sublimation of degradation products. The competition between exothermic and endothermic processes results in sharp changes in the balance of heat energy in the system and the sign of the total heat effect. In accord with these changes all of the process of PMMA oxidative degradation can conditionally be divided into three stages. The introducing of small quantities of C60 in a system results in replacement of oxygen by a fullerene or to a partial avoidance of oxidation, as a result of which, at the first stage, a smaller quantity of degradation products is formed and oxidized. There is a “transfer” of degradation products and heat generation from the first, lowest temperature stage, to the third, highest one. Simultaneous application of DSC and TG techniques has shown that during oxidative degradation of PMMA, especially in the presence of a fullerene, there is a latent degradation manifested in exothermal effects on DSC curves without mass losses on the TG curves. The introducing of C60 results in narrowing of the peaks (by ∼1.5 times) on DTG and MTA curves that may be explained by the narrowing of a set of possible degradation reactions. The C60 molecules are divided during oxidative degradation of PMMA into two groups: linked (∼20%) and nonlinked with macroradicals of PMMA. The nonlinked molecules of C60, in turn, are subdivided into oxidized and volatilizing without oxidation and degradation. The inhibiting effect of C60 on the thermal oxidative degradation reduces basically to two processes: to formation of fullerene-containing chains of PMMA and to nonchain inhibition. The matrix of PMMA also has a significant influence on the thermal behavior of C60.