Semicrystalline Organization of VDF- and TrFE-Based Electroactive Terpolymers: Impact of the trans-1,3,3,3-Tetrafluoropropene Termonomer

Abstract

In the search for fluorinated polymers with new electroactive properties, the radical polymerization of vinylidene fluoride (VDF), trifluoroethylene (TrFE), and trans-1,3,3,3-tetrafluoropropene (1234ze) was achieved. The crystalline organization and the electroactive properties of semicrystalline poly(VDF-ter-TrFE-ter-1234ze) terpolymer films with 1234ze molar contents ranging from 0 to 6%, obtained by solvent-casting, were investigated using a combination of structural, dielectric, and electromechanical techniques. For 0 mol % 1234ze, poly(VDF-co-TrFE) copolymers exhibited a Curie transition from the ferroelectric (FE) phase to the paraelectric (PE) phase at the Curie temperature (TC ∼ 95 °C). DSC, dielectric spectroscopy, and FTIR experiments revealed the preservation of this Curie transition (TC ∼ 75 °C) while increasing the termonomer content. WAXS measurements confirmed the persistence of the FE phase as the main phase at room temperature but also showed the appearance of a secondary ZFE phase (expanded FE phase incorporating 1234ze units) in lower proportion. The WAXS crystallinity was halved (42 to 23%) from 0 to 6 mol % 1234ze terpolymer annealed films. Furthermore, SAXS and WAXS experiments highlighted a decrease of the crystalline lamellae thickness and a significant modification of the electronic density distribution upon increase of 1234ze units. A new model for this particular arrangement of crystalline lamellae was thus proposed with the location of the ZFE phase at the interface between the FE crystalline lamellae and the amorphous phase. Finally, reduced remnant polarization (Pr ∼ 15 mC/m2) and transverse piezoelectric coefficient (d33 ∼ −5 pC/N), compared to those of the reference poly(VDF-co-TrFE) copolymer (Pr ∼ 50 mC/m2 and d33 ∼ −20 pC/N), were mainly assigned to the decrease of the FE phase content in terpolymer poled films.