Influence of cooperative alpha dynamics on local beta relaxation during the development of the dynamic glass transition in poly(n-alkyl methacrylate)s

The development of the dynamic glass transition in poly(n-alkyl methacrylate)s is investigated with broad-band dielectric spectroscopy in the frequency range from 10(-4) to 10(9) Hz. The experimental data were analyzed by adjustment with one or a sum of two Havriliak Negami functions. Upon decreasing the temperature, the high-temperature relaxation (a) changes into the local beta relaxation (Johari Goldstein mode), and the cooperative a relaxation sets in close to this a beta transition. For poly(n-butyl methacrylate) a separate onset (zero intensity) of the alpha process and a parallel course of both traces in the Arrhenius diagram were observed. The activation energy of the beta process does not change in spite of the parallel development of the alpha process. On the other hand, for poly(ethyl methacrylate) the alpha onset is close to a bend in the local process, i.e., the activation energy of the latter changes after the alpha onset. In both materials the intensity of the alpha process linearly increases with falling temperatures but with different intensity. Several scenarios for the alpha beta-splitting region are suggested.