Structural and Morphological Study of Thermally Fractionated P(VDF- <i>co</i> -TrFE) Ferroelectric Copolymers

Thermal fractionation of P(VDF-co-TrFE) copolymers with VDF/TrFE ratios ranging from 85/15 to 75/25 was achieved by applying successive self-nucleation and annealing (SSA) protocol. This method, scarcely applied to VDF-based semi-crystalline polymers, involves multiple annealing isotherms at decreasing temperatures, initiated within the self-nucleation domain. Fractionation was observed for all the samples, but its expression on the differential scanning calorimetry (DSC) scans varied depending on the copolymer chemical composition. The Curie transition was found to be significantly influenced by the fractionation of 80/20 and 75/25 samples. A structural study revealed an increase in both the crystallinity and the ferroelectric phase fraction after SSA, consistent with the enhancement of the remanent polarization in polarization-electric field loops. Furthermore, the formation of a distribution of crystalline lamellae with different thicknesses was highlighted by in situ small-angle X-ray scattering (SAXS) analysis, which revealed the progressive melting of increasingly thicker crystalline lamellae during the heating ramp.