Kinetics of phase transitions 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 T m 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 T m showed a strong dependence on the cooling rate. The high temperature transition revealed thermally activated behaviour with an activation energy of E a =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 T m 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 linearly depends 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 (α), local motions of CF 3 -groups in the glassy state (β-process) and cooperative fluctuations of side-groups both in the mesophase and crystalline state (λ-process). Interestingly, both the Arrhenius-type β-and λ-relaxation are characterised by a non-zero activation entropy, which indicates the cooperative nature of the underlying molecular dynamics.

Kinetics of Fast and Slow Transitions in a Liquid Crystalline Polyimide

“…The value of 360 kJ mol 31 as obtained for the case of non-isothermal solidification of PMDA/BACB polyimide (I), peak temperatures varying from 253 to 234 2 C, fits in the following range: 125 kJ mol 31 for HDPE [8], 160 kJ mol 31 for PHBA (derived from the data given by Chuah [11]), 290 kJ mol 31 for PA6 (present study), 328 kJ mol 31 for nylon 11 [8] and 400 kJ mol 31 for PBFP [5]. Apparently, steric hindrance and H-bonding contribute to the activation energy.…”

“…Such differences were recently observed by us for poly(bistrifluorethoxy-phosphazene) (PBFP1 II), which also shows LC-like transitions [5].…”

“…For polyphosphazene (PBFP1 II), the peak temperature of the transition from the 2Dmesophase to the fully crystalline phase appeared to be linearly related to the cooling rate, whereas the peak temperature of the transition from the isotropic melt to the 2Dmesophase showed a non-linear dependency on the cooling rate that could be interpreted as a thermally activated transition [5]. From the foregoing discussion we conclude that instantaneous transitions with peak temperatures linearly depending on the cooling rate are LC transitions, whereas non-LC transitions are linked to gradually evolving DSC peaks with peak temperatures non-linearly depending on the cooling rate.…”

“…For PA6 the temperature range was selected to be between 250 and 125 2 C. No mass loss could be observed after performing the DSC measurements. Details of the measurement procedure were described elsewhere [5].…”

Semifluorinated Methacrylate Random Copolymers: Phase Transitions and Molecular Dynamics

“…The copolymers where the side chains are organized follow the additive rule, however the values in these samples are lower than those expected taking into account the value observed for the 11 of dielectric strength due to cancellation of dipole moments. A relaxation with similar characteristics has already been observed in systems having fluoroalkanes as side chains and has been ascribed to motions of CF 3 -groups [19,20,22,29].…”