Ferroelectric fluorinated copolymers with improved adhesion properties

Abstract

Modified fluorinated electroactive poly(VDF-co-TrFE) copolymers with improved adhesion properties, on glass or metal substrates, are presented. Vinylidene fluoride (VDF) and trifluoroethylene (TrFE) were terpolymerized with 1 to 8 mol% of 2-(trifluoromethyl)acrylic acid (MAF) as an adhesion-promoting monomer. Monitoring the evolution of the reactor pressure and of the ¹⁹F NMR spectra of the terpolymer over the course of solution polymerization revealed the very pronounced compositional heterogeneity of the resulting terpolymer. The terpolymerization was scaled-up and implemented under aqueous suspension conditions in a 3 L-autoclave. This aqueous process, more industrially viable, was successful in spite of the undesirable addition reaction of water onto MAF producing 3-hydroxy-2-(trifluoromethyl)propanoic acid. The structure of the resulting polymers was characterized in detail by ¹H, ¹⁹F NMR and FTIR spectroscopies, and their physical properties (thermal, adhesion and electroactive) were examined. The decarboxylation of MAF, observed by TGA to occur at ca. 250 °C, is not a limitation for the targeted applications. A MAF content of 1 mol% was shown to be sufficient to provide very good adhesion onto glass and metal substrates, and also to decrease the water contact angle from 104° (reference poly(VDF-co-TrFE) copolymer) to 86° (terpolymer containing 1 mol% of MAF). Polarization studies revealed that the terpolymers exhibited identical ferroelectric behavior as unmodified poly(VDF-co-TrFE) copolymers, characterized by a wide hysteresis (E_c = 60 MV m⁻¹) and a high saturation polarization (P_sat = 77 mC m⁻²). This study reports the first electroactive fluorocopolymers with improved adhesion, and paves the way for better incorporation of fluorinated electroactive thin films in multilayer films for flexible electronic devices.