P(VDF-TrFE) Copolymer Dynamics as a Function of Temperature and Pressure in the Vicinity of the Curie Transition

We report on the phase behavior and the respective dynamics in random P(VDF-TrFE) copolymers using standard and temperature-modulated differential scanning calorimetry, X-ray diffraction, and a combination of temperature- and pressure-dependent dielectric spectroscopy measurements. Depending on the copolymer composition, the coexistence of three/four weakly ordered phases was identified in the vicinity of the Curie transition (Tc). With respect to the dynamics, we demonstrate that the segmental dynamics associated with the relaxation of constrained amorphous VDF segments at the crystal/amorphous "phase"can be used as a marker of the Curie transition. The corresponding segmental relaxation freezes at about 50 K above the lower liquid-to-glass temperature associated with the freezing of amorphous segments away from the interface. Pressure-dependent dielectric measurements provided quantitative insight into (i) the molecular origin of the segmental processes (by employing the pressure sensitivity of relaxation times and the pressure coefficient of the respective Tg's), (ii) the nature of the phase transition at Tc, and (iii) information about the stability of phases under the variation of temperature and pressure (through the T-P phase diagram). We show that Tc increases linearly with pressure and is accompanied by small volume changes, implying a weakly first-order thermodynamic transition. Furthermore, pressure stabilizes the ferroelectric phase over a broader temperature range. This could extend the operating temperature range of ferroelectric devices based on P(VDF-TrFE) copolymers.