Secondary crystals in piezoelectric P(VDF-co-TrFE) copolymers

ABSTRACT. Poly(vinylidene fluoride) (PVDF) and its copolymer P(VDF-co-TrFE) present increasing interest due to their remarkable piezoelectric properties. However, their properties can be affected by the formation of secondary crystals during storage or annealing, that may reduce the mobility of the amorphous fraction. This study aims to deepen the understanding of the mechanisms underlying the formation of secondary crystals in P(VDF-co-TrFE) copolymers with VDF/TrFE molar ratios of 80/20 and 55/45, after various annealing times and temperatures. Differential scanning calorimetry (DSC) analyses revealed the presence of a low-temperature endothermic peak during the heating ramp, attributed to the melting of secondary crystals. The temperature of this peak and the associated enthalpy variation are directly dependent on the annealing conditions. Subsequently, the impact of secondary crystals on dielectric relaxations was evaluated using broadband dielectric spectroscopy (BDS). These analyses showed that secondary crystals mainly influence the αc relaxation, associated with the mobility of chains in the crystalline regions and their surfaces. These results contribute to a better understanding of the structure-property relationships in P(VDF-co-TrFE) films, which is a key challenge for the design of future piezoelectric devices.