Kinetics of phase transition and dielectric relaxations in poly-bis(trifluorethoxy-phosphazene) (PBFP)

Abstract

The kinetics of the high temperature (isotropic mesophase) and low temperature (mesophase crystalline) transition of PBFP at Tm and T(1), respectively, and its local dynamics were studied by differential scanning calorimetry (DSC) at variable heating and cooling rate and by dielectric relaxation spectroscopy (DRS). While on heating no substantial effect of the rate on both transition temperatures was found, T(1) and Tm showed a strong dependence on the cooling rate. The high temperature transition revealed thermally activated behaviour with an activation energy of Ea = 400 kJ/mol, whereas the transition at T(1) showed the typical signature of supercooling. This finding is consistent with the picture that the isotropic/mesophase transition at Tm is controlled by long-range, strongly hindered, chain diffusion that leads to the long-range orientational mesomorphic order. In contrast, the low temperature transition, the enthalpy of which is far larger and the temperature of which depending linearly on the cooling rate, indicates a kinetical control by nucleation as characteristic for a (2D 3D) crystallization process.
Complementary information about the molecular dynamics in the various phases was obtained by DRS that revealed three relaxation processes, which were assigned to the dynamic glass transition (), local motions of CF3¿groups in the glassy state ( -process) and cooperative fluctuations of side-groups both in the mesophase and crystalline state ( -process). Interestingly, both the Arrhenius-type - and -relaxation are characterised by a non-zero activation entropy, which indicates the cooperative nature of the underlying molecular dynamics.