功能高分子材料的铁电行为调控及机制研究

This project will focus on dipole and domain switching mechanisms that determine the properties and behavior of ferroelectric polymers.Highly coupled dipole interactions and large ferroelectric domains in the ferroelectric polymers results in high energy barriers of switching the dipole direction and strong polarization hysteresis, which in turn reduces the electric field sensitivity and incurs high dielectric loss. We plan to conduct molecular design, synthesis and characterization of novel ferroelectric polymers with tunable hysteresis properties and ferroelectric switching characteristics. The introduction of rationally designed low-polarizable or inorganic segments as defects in the poly(vinylidene fluoride) based ferroelectric polymers breaks up the long-range polarization correlation and reduces ferroelectric domain sizes, which destabilizes the polar ordering, diminishes the energy barrier for phase transformation between polar- and non-polar phases, and allows reversible phase transformation with little and even no hysteresis at ambient temperature. The proposed work involves systematic development of synthetic and assembly strategies to novel organizations of the ferroelectric polymers based on our newly developed chemistries for ferroelectric polymers.The prepared materials will be studied intensively by a combination of dielectric spectroscopy and high-field polarization and electrical characterization to gain fundamental insight into how the molecular characteristics and supramolecular morphology of the assemblies influence the cooperative polarization and ferroelectric hysteresis. The proposed strategies open an exciting new research arena where electroactive nanostructures can be assembled with predetermined architecture, compositions and properties. The assembly approach and cross-linking chemistry will be utilized for 3-dimensional nanoscopic confinement and isolation the ferroelectric domains in the ferroelectric polymers. The ferroelectric copolymers with well-defined side chains will be prepared for systematic investigation of the ferroelectric confinement effect. The switching characteristics and polarization hysteresis of the ferroelectric polymers will be adjusted by judiciously controlling both the sizes of the dipolar regions and their coupled correlations via the proposed approaches.The success of this project will open a new direction in the development of novel ferroelectric polymers and have profound implications for rational design of nanostructured electroactive polymers with new architectures and superior electrical and dielectric properties. Moreover, it will shed light on our fundamental understanding of complicated polarization hysteresis and phase transition mechanisms in the ferroelectric polymers which are relevant for a range of advanced technologies such as large-strain actuators, high-precision sensors and high-energy-density capacitors.

设计并合成由铁电聚合物和低极化度聚合物组成的精确可调的交替嵌段共聚物以降低铁电畴的尺寸并分离协同的铁电效应；开发基于铁电聚合物的交联网络，使它具有可控的分子结构、可调的极化滞后特征以及更高的介电强度；设计并合成含有全氟侧链和烷基侧链的二元铁电共聚物以及三元铁电共聚物，洞悉铁电限制效应；将分子尺度的无机组分引入到铁电聚合物基体中来分离协同的铁电效应并调节体系的介电性能；探明自组装结构和无机组分对铁电聚合物的分子构象、结晶度以及结晶结构的影响； 探明在不同电场下铁电聚合物及其分子杂化材料的极化滞后和相转换行为对分子结构和形貌结构的依赖关系；通过介电谱和高场极化及电性能表征手段系统研究所制备的各种材料的介电响应以及它们对频率、温度的依赖关系、高场下的介电强度及铁电损耗，建立聚合物分子结构、结晶结构以及形貌结构与铁电性能之间的关键联系，全面揭示聚合物中新型铁电行为的本质。

铁电高分子不仅具有介电强度高、质量轻、易加工、成本低以及结构可调等特点，而且还拥有独特的柔软、易成膜等物理机械性能，因此广泛应用于压电传感器、电容器储能、制动器等领域。与无机铁电陶瓷不同，铁电高分子具有多级结构，其铁电行为受结晶度、结晶尺寸、分子构象等多方面影响。因此，探明铁电高分子极化特性与相转换行为与分子结构及形貌结构的依赖关系，对设计具有全新铁电性能的高分子材料、拓展铁电高分子材料的应用具有深远的意义。.本项目通过悬浮聚合法合成了一系列偏氟乙烯 (VDF) 和三氟溴乙烯(BTFE) 的共聚物，研究不同的链构象、分子内缺陷、结晶结构、晶粒尺寸等对铁电行为的影响。研究发现，通过引入BTFE能有效减小铁电聚合物的晶粒尺寸，并且形成稳定的γ相，从而实现较高的极化强度和较低的损耗。以偏氟乙烯（VDF）和氯三氟乙烯（CTFE）组成的二元铁电共聚P(VDF-co-CTFE)为基体，采用二氧化硅包覆的石墨烯纳米片为无机纳米填料，制备了渗流型铁电复合材料。对不同组分材料进行了系统的结构与性能表征。结果显示，以经过二氧化硅包覆的石墨烯为填料得到的复合体系具有更高的电击穿强度、更低的介电损耗以及更高的介电常数。.采用化学剥离法制备了氮化硼纳米片（BNNS），将制备的BNNS与自制的三元无规共聚物 P(VDF-TrFE-CFE)复合，制备了P(VDF-TrFE-CFE)/ BNNS薄膜复合材料，研究了BNNS对P(VDF-TrFE-CFE)结晶度和结晶结构的影响，探明了铁电高分子的铁电行为与结晶结构的依赖关系。研究发现，BNNS能有效提高基体的结晶度，减小晶粒尺寸。当BNNS添加量为12 wt.%时，P(VDF-TrFE-CFE)的结晶度从31%提升到了35%，同时晶粒尺寸从49.4 nm减小至29.0 nm，能量密度从纯聚合物的~9.2 J/cm3 提升为20.3 J/cm3。基于前述得到的高击穿强度复合材料进行多层结构设计，分析无机高介电填料对聚合物铁电储能行为的影响，以期望能够同时提高材料的击穿场强和介电常数，进而提高铁电聚合物的储能密度。采用热压法制备了由上下层为聚偏氟乙烯/六方氮化硼纳米片复合材料，中间层为聚偏氟乙烯/钛酸锶钡纳米线复合材料构成的三明治结构纳米复合材料。在中间层BST含量为8 vol.% 时，复合材料表现出最大击穿场强与能量密度。