Determination of equilibrium heats of fusion for polymorphic poly(vinylidene fluoride) homopolymers and poly(vinylidene fluoride-co-trifluoroethylene) random copolymers

This study addresses the long-standing discrepancy regarding fundamental thermodynamic constants of poly(vinylidene fluoride) (PVDF) and poly(VDF-co-trifluoroethylene) [P(VDF-TrFE)] random copolymers. Despite their critical role in flexible electronics, researchers have traditionally relied on an oversimplified equilibrium heat of fusion (∆Hfo) of 104.6 J/g for the α-PVDF, leaving the thermodynamic limits for β and γ phases, as well as P(VDF-TrFE) copolymers, largely unresolved. By employing robust density and wide-angle X-ray diffraction analysis of crystallinity, we established a definitive energetic map for these ferroelectric polymers. For common PVDF resins with 4–4.5% head-to-head and tail-to-tail defects, the ∆Hfo values were found to be 120, 161, and ~140 J/g for the α, β, γ phases, respectively. Thermodynamic analysis confirmed that while the α phase was kinetically favored during nucleation, the β phase was the most stable with the lowest Gibbs free energy below the equilibrium melting temperature. Finally, the maximum heat of Curie transition for P(VDF-TrFE) was found to scale with the VDF content, whereas the ΔHfo remained nearly constant at approximately 39 J/g, ultimately leading the total enthalpies of copolymers to converge with the value of the β-PVDF. This work provides a comprehensive enthalpy reference, resolving existing ambiguities in calculating crystallinity and providing a reliable baseline for the study of ferroelectric fluoropolymer