Synthesis and properties of ferroelectric fluoroterpolymers with Curie transition at ambient temperature

This paper discusses a new family of ferroelectric polymorphic fluoroterpolymers comprosed of vinylidene difluoride (VDF), trifluoroethylene (TrFE), and a chloro-containing third monomer, such as chlorodifluoroethylene (CDFE), and chlorotrifluoroethylene (CTFE), which feature relatively narrow molecular weight and composition distributions. The terpolymers were prepared by a bulk reaction process with a low temperature free radical initiator, i.e., the oxidation adducts of trialkylborane and oxygen. The slightly bulky chlorine atom serves as a kink in the polymer chain, which spontaneously alters the chain conformation and crystalline structure. Compared to the corresponding VDF/TrFE copolymer (> 20 mol % of TrFE), the slowly increasing chlorine content (< 8 mol % of termonomer) gradually changes the all-trans (t(m>4)) chain conformation (beta-phase) to tttg(+)tttg(-) conformation (gamma-phase) without significant reduction of overall crystallinity. The Curie (ferroelectric-paraelectric) phase transition temperature between the mixed ferroelectric beta- and gamma-phases and paraelectric alpha-phase (tg(+)tg(-) conformation) also gradually reduced to near ambient temperature with very small activation energy. Consequently, the terpolymers show high dielectric constant (> 70), slim polarization hysteresis, and large electrostrictive response (>4%) at ambient temperature and exhibit common ferroelectric relaxor behaviors with a broad dielectric peak that shifted toward higher temperatures as the frequency increased.