Influence of alkyl chain length on dielectric relaxations in poly(n-alkyl methacrylates)

Relations between chain structure and the dynamic of molecular movements are studied in the glass transition temperature range for a series of amorphous model polymers: the poly(n-alkyl methacrylates). Two dielectric spectroscopies are used in order to enlarge the accessible frequency range: the dynamic dielectric spectroscopy and the thermo-stimulated currents technique. We show the existence of three dipolar relaxation modes: the α mode assigned to the glass relaxation, the β mode, a secondary relaxation mode, below Tg and the α´ mode, above Tg, dielectric manifestation of the liquid-liquid transition. We follow the shift of these relaxations with frequency, determine the behaviour laws of relaxation times and the fine structure of these complex modes. From the comparison of these results, we identify the effect of the alkyl group length on the relaxations and we, propose a phenomenological model to describe the molecular mobility through the glass transition.