Structure and dielectric properties of vinylidene fluoride copolymers

Abstract

: Molecular chain structures of vinylidene fluoride (VDF)/trifluoroethylene (TrFE) copolymers were investigated by high resolution F NMR. It was found that the arrangement of comonomers in the VDF/TrFE copolymer chain is statistically random, and the head-to-tail connection of monomer units is predominant. The experimental results on DSC, MNR and X-ray diffraction indicated that copolymer chains are in isomorphically crystalline state in the whole range of copolymer compositions. In particular, 50–80 mol% VDF copolymers shows a ferroelectric character associated with a strong remanent polarization at room temperature. In addition, these copolymers exhibit a crystal phase transition associated with appearance of an anomalous specific heat peak, abnormal peak of dielectric constant, and disappearance of remanent polarization. These results are consistent with the fact that VDF/TrFE copolymers are ferroelectric. The dielectric constant ε1 of 55 mol% VDF copolymer is 18 at 1 KHz and 24°C. The d31 piezoelectric constant of a roller-elongated 55 mol% VDF copolymer is 51 pC/N at 11 Hz and 24°C. The electromechanical coupling factor kt of 75 mol% VDF copolymer in the thickness direction is 0.3. The pyroelectric constant of 55 mol% VDF copolymer is 50 x10-5C/M2K. Accordingly, VDF/TrFE copolymers can have wide range applications as transducers. © 1989 IUPAC