Study of the thermal and dielectric behavior of P(VDF-TrFE) copolymers in relation with their electroactive properties

On the basis of the pyroelectric spectra—pyroelectric coefficient p 3 versus temperature—of β-PVDF and P(VDF-TrFE) 75/25 mol.%, recorded between -100 and 70°C, we have investigated transition and relaxation phenomena involving both the crystalline and amorphous phases of P(VDF-TrFE) copolymers for TrFE unit content in the range 0–50 mol.%. To this aim, complementary techniques, DSC and TSC were used. The classical glass transition of the copolymers (Tgl) manifests by a clear change in slope in the p 3 versus T diagram. From its variation in DSC as a function of the weight fraction of TrFE units, we deduced a Tg of-12°C for PTrFE. An upper component of the glass transition, Tgu, has been found at ≈10–20°C in the pyroelectric spectrum. It has been associated to the amorphous phase constrained by crystallites. The pyroelectric coefficient increases strongly in the glass transitions range due to the large thermal expansion coefficient of the amorphous phase in the rubber-like state. The crystalline α-relaxation appears in the range 35–60°C, and displays a complex behavior, considering both DSC and pyrocurrent thermograms. This process plays a key role in the stability of the electroactive properties, and in the mobility of the crystalline phase. It has been ascribed to an ordering/disordering process at the lamellae surface. Finally, we proposed two methods for determining the crystallinity of the copolymers from DSC data, and we have drawn a phase diagram of these polymers, which is extended to the low temperature region.