Phase Transitions and Ferroelectric Relaxor Behavior in P(VDF−TrFE−CFE) Terpolymers

Abstract

We show that the microstructures and phase transition behavior of vinylidene fluoride−trifluoroethylene−chlorofluoroethylene terpolymer, P(VDF−TrFE−CFE), with composition of 61.5/30.3/8.2 mol %, can be varied dramatically via different processing conditions. In the well-annealed sample, the polymer exhibits typical relaxor ferroelectric behavior with the nonpolar crystalline phase having a chain structure of random sequence of TT/TG/TG‘ conformations. In contrast, by lowering the crystallization temperature in the solution-cast films, the polar crystal phase with all-trans (Tm>4) planar zigzag conformation (similar to the ferroelectric phase in the corresponding copolymer) can appear or even become a dominating phase. Four types of transitions can be detected in this terpolymer, besides the crystal melting, the glass transition in amorphous region, and the transition between the ferroelectric-like and paraelectric crystals (30−90 °C), an additional transition appears near 10 °C which can be attributed to the crystalline domains with all-trans sequence randomly disrupted by defects, and the coherence length of these domains is estimated to be less than 4.7 nm (polar nanodomains). The large dielectric response of the terpolymer comes from the two crystalline phases: the ferroelectric-like crystals and polar nanodomains. The former is responsible for the frequency-independent dielectric maxima while the latter generates the strong frequency dependence in the dielectric spectra, a typical feature of relaxor ferroelectrics, which we suggest to come from the short-range molecular motion in the polar nanodomains.