氟乙烯对聚偏氟乙烯基铁电聚合物的相变及介电性能调控机理研究

Poly(vinylidene fluoride) (PVDF) based ferroelectric polymers have attracted considerable interests for their excellent and tunable dielectric, ferroelectric, piezoelectric and actuation properties, which allows them to be fabricated into massive devices for sensing, energy harvesting, energy converting and storage. Currently, the PVDF based ferroelectric polymers are mostly constructed onto the beta-PVDF and copolymers of VDF and trifluoroethylene (TrFE). However, the relatively large steric bulk of TrFE leads to the serious disadvantages in the applications of different ferroelectrics. Normal ferroelectrics, limited by the low processing temperature of beta-PVDF and ferro- to para-electric (F-P) transition temperature of P(VDF-TrFE)s, could hardly be utilized under elevated temperature. The relaxor ferroelectrics, synthesized from the modification of P(VDF-TrFE) chemically or physically, suffer from the high driving electric field (over 100MV/m) when they are utilized as soft actuators. Besides, the anti-ferroelectric like copolymers, prepared by grafting polystyrene or poly(methyl methacrylate) side chains onto P(VDF-TrFE)s and designed for high pulse energy storage capacitors, could hardly overcome the large energy loss induced by the relaxation of ferroelectric phase together with the high leakage current. All of those problems are induced by the ferroelectric domains established on P(VDF-TrFE) chains, since the insertion of relatively large steric bulk of TrFE units into PVDF inevitably leads to the enlarged polymer chain distance, the quickly reduced crystallinity and F-P temperature. Therefore, in present project, we would like to design and synthesize a family of novel ferroelectric polymers based on the copolymer of fluoroethylene (VF) and VDF to solve the problems observed in P(VDF-TrFE) based ferroelectrics. Instead of direct copolymerization of VDF and VF, which is rather difficult for their different reactivity ratios, indirect polymerization processes based on the reduction of PVDF and poly(vinylidene fluoride-co-chlorofluoroethylene) (P(VDF-CFE)) would be investigated for the synthesis of P(VDF-VF) copolymers. Followed by the living radical polymerization, a family of P(VDF-VF) based copolymers with tunable composition, topology and electroactive performances would be synthesized and finely characterized with F-H decoupled nuclear magnetic resonance (NMR). By adjusting their composition and processing conditions, the crystalline and ferroelectric phase and F-P transition could be finely tuned in a wide range. By systematically investigating the electric performances of copolymers with normal ferroelectric, relaxor ferroelectric and anti-ferroelectric like characters, the influence of composition and the aggregation structure onto the electroactive properties of copolymers would be deeply illustrated. With the assistance of molecular simulation technique, the mechanism about how VF units could tune the chain conformation, crystal form, F-P transition along with electroative performances of P(VDF-VF) based copolymers would be clearly disclosed. The achievement of this project would develop novel methods for the synthesis and modification of PVDF based fluoropolymer, offer new ferroelectric polymers with striking performances, and improve the PVDF based ferroelectric polymers related dielectric theories.

聚偏氟乙烯（PVDF）基铁电聚合物因具有优异可调的铁电、压电特性而备受关注，现有铁电聚合物主要利用三氟乙烯（TrFE）调节链段及铁电相，由于TrFE单体位阻大，导致P(VDF-TrFE)链段间距大及电性能的缺陷，难以满足在耐高温、低驱动电场及高储能低损耗等方面的应用需求。本项目拟采用位阻较小的氟乙烯（VF）代替TrFE，首先通过对P(VDF-CFE)（CFE，氟氯乙烯）的C-Cl键、PVDF的C-F键氢化还原等新方法研究，合成P(VDF-VF)基典型铁电体、弛豫铁电体以及类反铁电体聚合物；通过调节化学组成和加工工艺对其结晶及电相结构进行调控；系统研究各类铁电聚合物的电性能，明确组成及聚集态结构对其电活性的影响规律；通过构效关系的深入研究，结合模拟计算，最终揭示VF单体对P(VDF-VF)基铁电聚合物链构象、F-P相变以及电活性的调控机制。为高性能电活性聚合物的研发奠定物质和理论基础。

聚偏氟乙烯(PVDF)基铁电聚合物因其具有优异可调的铁电、压电特性而备受关注，现有铁电聚合物主要利用三氟乙烯(TrFE)调节链段及铁电相，由于TrFE单体位阻大，导致P(VDF-TrFE)链段间距大及电性能的缺陷，难以满足在耐高温、低驱动电场及高储能低损耗等方面的应用需求。本项目首先通过P(VDF-CFE)(CFE，氟氯乙烯)的氢化反应，制备出P(VDF-VF)(VF，氟乙烯)，所得共聚物的介电行为受VF含量的影响，当VF含量接近12 mol%时，共聚物表现为典型铁电体特征；接着通过对VF单元上C-F键的活化研究，在其侧链引入刚性的聚甲基丙烯酸甲酯链段(PMMA)，接枝聚合物逐渐转变为线性电介质，能量损耗得到大幅降低；随后，通过C-F键的活化研究，对PVDF进行接枝反应，阐明了C-F键的引发、增长机理及反应可控性，并在此基础上，对比了三种不同引发剂的接枝聚合反应，证实C-F键的活性与其所处的化学环境有关；最后，详细测试了不同组分P(VDF-TFE)(TFE，四氟乙烯)的介电性能，结果发现，随着TFE含量的增加，共聚物的矫顽电场降低，压电系数升高。通过对各类铁电聚合物的电性能进行系统研究，明确组成及聚集态结构对其电活性的影响规律，为高性能电活性聚合物的研发奠定物质和理论基础。