Prof. Zhicheng Zhang, Department of Applied Chemistry, School of Science, Xi’an Jiaotong University, China

Speech Title:Synthesis of PVDF Based Ferroelectric Polymers with Tunable Ferroelectric Performances

. Abstract:Poly(vinylidene fluoride) (PVDF) based relaxor ferroelectric polymers show great potential application in transbucers, sensors and artificial muscles for their excellent electrostrictive properties. In state-of-the-art relaxors, trifluoroethylene (TrFE) units have to be incorporated to maintain the TTTT conformation of PVDF polymer chain, which is crucial to achieve large polarization under electric field. To accelerate the polarization relaxation of ferroelectric domains, defects have to be designed to tailor the sequence length of PVDF chain in TTTT conformation by either exposing the P(VDF-TrFE) films to electron beams or inserting bulky comonomers into the polymer chains. Terpolymers of VDF and TrFE with the third monomer including chlorotrifluoroethylene (CTFE) and chlorofluoroethylene (CFE) with optimized composition show the best properties including the largest strain of 6% and the maximum polarization of 10Ccm2 under 400MV/m field. However, a high driven electric field over 50 MV/m (Ec) is required to achieve the polarization of ferroelectric domains thus the high electrostrictive properties. Suffering from the high driving voltage required in thicker films and the low strength realized in thinner films, terpolymer relaxors are significantly limited in the practical application as electro-mechanical transformation devices. To reduce the driving electric field, for the first time, a series of relaxor ferroelectric polymer films have been fabricated from uniaxial stretching crosslinked amorphous PVDF films. The amorphous PVDF bearing no TrFE units, named as P(VDF-DB), was synthesized facilely from a full dehydrochlorination of P(VDF-CTFE) copolymer with 20 mol% CTFE. The optimized P(VDF-DB) film with an extension ration of 500% exhibits the best relaxor ferroelectric behaviors including a dielectric constant of 70 at 100Hz, a Curie temperature at 20-40 oC, a maximum polarization of 12Ccm2 at 375 MV/m and a remnant polarization of 2.8Ccm2. Most noticeably, a lowest coercive field (Ec) of 18 MV/m is observed, which is much lower than current terpolymer relaxors. The crystalline and ferroelectric phase transitions induced by the uniaxial stretching in P(VDF-DB) films have been carefully characterized. The stretching induced chain conformation switching from TGTG’ to TTTT conformation and the extended-chain crystal generated among the aligned high polar polymers are disclosed to address for the excellent relaxor ferroelectric performances. The work provides an alternative strategy to fabricate relaxor ferroelectric polymers beside of giving a deeper insight of the stress induced chain conformation and phase transition in PVDF based ferroelectric polymers.