Importance of Microstructure Control for Designing New Electroactive Terpolymers Based on Vinylidene Fluoride and Trifluoroethylene

A new family of electroactive fluorinated terpolymers of vinylidene fluoride (VDF), trifluoroethylene (TrFE) and 3,3,3-trifluoropropene (TFP) is presented. Statistical poly(VDF-ter-TrFE-ter-TFP) terpolymers with a VDF/TrFE molar ratio of ca. 65/35 and a TFP composition ranging from 0 to 10 mol % were prepared in high yields by free radical terpolymerization in dimethyl carbonate (DMC), initiated by a symmetrical peroxydicarbonate initiator. The choice of TFP as a termonomer was driven by the potential property of the CF3 side groups to limit crystal growth and potentially favor the formation of nanodomains known to enhance electrostrictive properties. For the first time, the reactivity ratios of the TrFE/TFP (r(TrFE) = 0.13 and r(TFP) = 3.72 at 48 degrees C) couple were determined using the Kelen-Tudos linear method, and used in combination with VDF/TrFE and VDF/TFP reactivity ratios to better understand the structures of the terpolymers. Detailed H-1 and F-19 solution NMR spectroscopic studies were performed and afforded the in-depth characterization of the terpolymers microstructures. The examination of the terpolymer's composition as a function of the three monomers conversions revealed a strong structural heterogeneity where a 62/33/5 VDF/TrFE/TFP initial monomer composition resulted in a 47/11/42 poly(VDF-ter-TrFE-ter-TFP) terpolymer at low conversion. It was indeed found that TFP preferentially homopolymerizes despite its low initial concentration. The influence of the TFP units on the thermal transitions (T-Curie = 65 degrees C and T-m = 148 degrees C for a 67/28/5 poly(VDF-ter-TrFE-ter-TFP) terpolymer), thermal stability and electroactivity (E-c = 63 MV/m at 150 MV/m) was also examined. The combination of the determination of the monomers' reactivity ratios of the terpolymer microstructures and of the assessment of the physical properties of the terpolymers provided insights on the structure-property relationship of the poly(VDF-ter-TrFE-ter-TFP) terpolymers.