| Abstract: | There have been several investigations dealing with the influence of the glass transition on the rate of radiation-induced reactions, especially the crosslinking reaction. On the other hand, no correlation has been reported between secondary transitions and the variation of a reaction rate. In the present work, the degradation of poly(methyl methacrylate) in the temperature range of ?196 to 160°C has been studied. Two breaks in the Arrhenius plot were found. The first one is located at ?5°C and can be correlated with the γ transition, which has been assigned to the hindered rotation of the ester group whose scission initiates the degradation. It is concluded that the thermal excitation of the bond rotation can either increase the scission rate or decrease the recombination rate of reactive intermediates by expanding the cage of the surrounding molecules. If the second mechanism occurs, the rotation of the α-methyl group should also be expected to favor the degradation. However, a second break in the Arrhenius plot was found at ?130°C, and therefore significantly above the δ transition. The influence of this motion is therefore not clear. Certain radiation-induced reactions of poly(vinyl chloride), known from literature, seem to be influenced by a secondary transition. It is concluded that secondary transitions may be significant for scission reactions. This suggests, with respect to the known significance of the glass transition, that the various types of molecular motions in polymers may influence radiation-induced reactions. |
