Synthesis of PVDF Homo- and Co-Polymers with Well-Defined Molecular Structure; Using Borane/Oxygen Coordination Free Radical Initiator

摘要

Several important progresses have been made in the past three-year ONR program. (i) We have developed a new polymer chemistry that allows the polymerization of fluoro-monomers in bulk condition at ambient temperature. (ii) A new family of ferroelectric polymorphs fluoro-terpolymers have been prepared, which comprise vinylidene difluoride (VDF), trifluoroethylene (TrFE), and a chloro-containing third monomer, including vinyl chloride (VC), 1, 1- chlorofluoroethylene (CFE), chlorodifluoroethylene (CDFE), chlorotrifluoroethylene (CTFE). The new terpolymers show narrow molecular weight and composition distributions and good processibility. (iii) The terpolymers exhibit excellent electric properties. Comparing with the known VDF/TrFE copolymer, the slightly bulky chlorine atoms randomly distributed along the polymer chain spontaneously alter the crystalline phase from Beta-phase (all- trans conformation) to gamma-phase (tttg+ttg- conformation), without significant reduction of overall crystallinity. The Curie (F-P) transition from gamma-phase to alpha-phase (tg+tg- conformation) occurs at near ambient temperature, with a very small activation energy and slim hysteresis loop during heating and cooling cycle. The terpolymers show high dielectric constant (>50) and large electrostrictive response ((caret)5%) at ambient temperature, and exhibiting common ferroelectric relaxor behaviors with a broad dielectric peak that shifted toward higher temperatures as the frequency increased, and a slim polarization hysteresis loop at near the dielectric peak (around ambient temperature).