REACTION DYNAMICS IN GLASS-TRANSITION REGION - PROPAGATING RADICALS IN ULTRAVIOLET-IRRADIATED POLY(METHYL METHACRYLATE)

The decay kinetics of propagating radicals in UV-irradiated poly(methy l methacrylate) (PMMA) was followed at the irradiation temperatures in the range 300-400 K. At the highest temperatures, in the glass transi tion region of PMMA the radical decay patterns approached that of clas sical kinetics. Below the glass transition region, increase of reactio n dispersivity with temperature decrease was found to be accompanied b y a marked decrease, of about two orders of magnitude, of activation e nergy for the reaction. Both these phenomena are rationalized in terms of the stochastic model [A. Plonka and A. Paszkiewicz, J. Chem. Phys. 96, 1128 (1492)], which includes structural reorganization of the hos t matrix by imposing upon the static disorder model for radical reacti on the additional assumption that matrix relaxation randomly reassigns the reactivity of guest species. Effective relaxation is shown to lea d to the classical pattern of radical decay and the accompanying marke d increase of activation energy is seen to be due to the contribution of activation energy for matrix structural relaxation. (C) 1996 Americ an Institute of Physics.