压力诱导偏氟乙烯-三氟氯乙烯无规共聚物弛豫铁电共晶生成及性能调控

In terms of significantly lowering the cost of poly(vinylidene fluoride) (PVDF)-based relaxor ferroelectric films and extend their application window, it would be highly beneficial to induce isomorphic crystalline structures in trifluoroethylene-free PVDF copolymers to obtain desired relaxor ferroelectric behavior. However, except for trifluoroethylene, second co-monomers are usually excluded from PVDF crystal lattice under normal film preparation condition. Therefore, relaxor ferroelectricity has not been reported in any PVDF-based copolymers so far. In this proposal, commercially available poly(vinylidene fluoride-co-chlorotrifluoroethylene) [P(VDF-CTFE)] was purposely selected. By manipulating the pressure and temperature during crystallization, less ordered and defect-tolerant hexagonal phase could be achieved in P(VDF-CTFE), so that CTFE co-monomer could easily sneak into PVDF crystal, and thus, form isomorphic crystals. Once CTFE is included in isomorphic crystals, it is expected to observe relaxor ferroelectric behavior in copolymer. Specifically, detailed studies will be carried out in this proposal to cover all the following points of interest: a) the temperature-pressure (T-P) phase diagrams (triple point to reach hexagonal phase) of P(VDF-CTFE) of different chemical composition; b) the evolution of isomorphic crystalline structure as pressure changes and their correlation; c) the crystallographic, morphological and conformational structures of the formed isomorphic crystals (e.g. lamella thickness, unit cell parameters, chain conformation and local defects concentration in crystal lattice); d) ferroelectric domain size and its relationship with the relaxor ferroelectric behavior. We expect that the outcome of this proposal could deepen our understanding of the structural modulation of P(VDF-CTFE) isomorphic crystals and provide experimental tools on exploiting the isomorphic crystals of PVDF copolymers to prepare cost-efficient relaxor ferroelectric polymer films for real applications.

在不含三氟乙烯单体的二聚物中形成共晶获得弛豫铁电行为，可显著降低偏氟乙烯基弛豫铁电薄膜的成本，扩宽其应用范围。但由于在常规薄膜制备条件下，二聚物中除三氟乙烯外的共聚单体被排除出晶格之外，因此，迄今为止，尚未有在偏氟乙烯基二聚物中获得弛豫铁电行为的报道。本课题拟选择已商品化的偏氟乙烯-三氟氯乙烯共聚物，通过调控压力和温度，使各组分链段均处于有序性差、对缺陷容忍度高的六方相，从而促进CTFE单元进入晶区形成共晶，在二元共聚物中获得弛豫铁电行为。拟研究不同组分比共聚物获得六方相的温度-压力相图、压力诱导共聚物共晶结构的演变规律及共晶多层次结构与相畤尺寸和弛豫铁电性能之间的关系，为实现P(VDF-CTFE)的共晶结构调控及利用二元共聚物制备弛豫铁电薄膜提供实验支持和理论指导。

本项目立足于偏氟乙烯基高分子电介质薄膜的结构调控，结构性能关系研究，开展了加工外场（压力场、拉伸外场及剪切流动场）下含偏氟乙烯基高分子的晶体结构生成规律，以及通过特殊结构设计提升含偏氟乙烯高分子复合薄膜介电和耐高温性能的相关工作。项目通过高压、拉伸等外场调控偏氟乙烯-三氟氯乙烯共聚物P(VDF-CTFE) 中三氟氯乙烯（CTFE）单体在异质晶体（共晶）中的分布和极化行为的联系，证实了大体积第三单体进入晶区形成缺陷获得2-3nm的纳米相畴是获得弛豫铁电行为的关键。同时，项目研究了压力以及压力-流动场共同作用下聚偏氟乙烯（PVDF）及其共混物的结晶行为及拉伸场下PVDF的晶体结构演变。研究发现，压力下高剪切流动场诱导PVDF铁电β相，聚甲基丙烯酸甲酯（PMMA）与PVDF之间的氢键相互作用稳定了剪切诱导的长反式（T）构象，诱导更高含量的铁电β相。项目还研究拉伸场下不同晶体结构的PVDF的固态相转变，虽然拉伸过程中α/γ-PVDF发生相似的宏观形态变化，仅诱导少量（18%）α相转变为β相，而γ相可完全转变为β相，上述工作均为高含量铁电β相的生成提供了途径。在上述研究的基础上，本项目进行了耐高温介电膜研究。在PC/PVDF共混膜中构建取向片晶结构的PVDF纳米片，纳米片和PC基体提供的受限空间提高了PVDF薄膜的常温介电性能，且PC/PVDF共混膜在高达150 ℃的高温下也不会导致介电损耗的指数增加。构建了新型PVDF与PMMA三层纳米复合膜结构（TNF），PMMA诱导PVDF和PMMA之间的界面区域形成取向α晶体，限制了杂质离子，使得TNF能够耐受120℃的高温，远远超过其对应的单层膜结构MNF（约65℃），比商业BOPP（85℃）高35℃，同时TNF的介电损耗相比于MNF低。上述工作为聚偏氟乙烯基聚合物的结构调控和高性能化提供加工和理论指导。