【作者】 陈相仲；

【导师】 沈群东；

【作者基本信息】 南京大学 ， 高分子化学与物理， 2013， 博士

【摘要】 铁电聚合物的卓越性能来自极性晶体结构及其在外加电场中的反转或重新定向。因此,铁电聚合物的极性晶畴是构筑其优异性能的基本单元。而作为一种半结晶型的聚合物,铁电聚合物在晶体生长过程中链的排列和堆砌会受到多重因素的影响,从而造成了其结构的复杂性、多样性和不完备性。而正是因为其复杂性,为人们调制其微结构提供了可能性,使不同晶相的晶畴在几纳米至几百纳米的尺寸范围内可以被调制。而PVDF基铁电高分子与其他铁电材料一样,在多场耦合下具有多功能性。其复杂的结晶微结构与其不同的功能之间有着千丝万缕的联系。如何调制其微结构,从而获得某一功能的优化,是目前研究的热点之一本论文以PVDF基多功能铁电共聚物为研究对象,以优化其功能为导向,针对其不同的功能,采取不同的方法和手段进行了处理,并对调制后的共聚物的微结构进行了细致的研究,主要研究内容如下。1.针对铁电共聚物在信息存储领域的应用,我们利用简便、高效的纳米压印方法成功的制备了高密度的表面平滑的自支撑铁电共聚物P(VDF-TrFE)纳米点阵。纳米点中聚合物链的择优取向,即：聚合物链平行于基底,聚合物中偶极方向垂直于基底排列,使得单个纳米点中的极化翻转变得更容易,从而使每个纳米点的压电响应都很强而且很均匀。利用基于探针的压电力显微镜技术能够在纳米点阵上直接写入所需要的信息,写入的分辨率小于10nm,存储密度高达75Gb/inch2。我们还以光栅状的压印模板对P(VDF-TrFE-CFE)三元弛豫铁电共聚物进行了纳米压印。压印同样也造成了纳米结构中聚合物链的择优取向。作为一个易变的体系,压印对聚合物的影响还体现在限制加热条件下的分子运动,将弛豫铁电性部分地转变为铁电性。2.针对铁电共聚物在储能电容中的应用,我们发展了交联改性铁电共聚物的方法。交联的P(VDF-CTFE)既具有提高的极化值,又保持了其较高的击穿场强,这两方面共同的贡献使其放电能量密度高达22.5J/cm3,约为同等场强下未交联样品的两倍。方面,交联有利于极性构象的生成,将原先稳定存在的α相大尺寸晶体转变为几个相混合存在的多缺陷的极性纳米晶体。另一方面,交联样品的内比表面要大过未交联样品的,同时,界面相的厚度也要比未交联样品小。在性能上则表现为同等场强下更高的极化值和放电能密度,以及更少的能量损耗。3.以调制铁电共聚物的电卡效应为目的,我们对P(VDF-TrFE-CFE)三元弛豫铁电共聚物用共混复合的方法进行处理,发现三元弛豫铁电共聚物P(VDF-TrFE-CFE)与二元铁电共聚物P(VDF-TrFE)的共混物的结构和性能有着很强的可调制性。在二元共聚物含量小于15%的时候,三元共聚物中的缺陷通过与二元共聚物的界面相互作用影响其结晶,将其铁电相晶体转变成了具有类似于三元共聚物结晶结构的弛豫体。这种转变改变了体系的低场介电和高场极化的响应特征,使共混物的电卡效应相对于三元共聚物最高提高了30%以上。当二元共聚物含量高于20%之后,铁电相将会逐渐产生,最终导致介于两者中间的铁电性能。共混物体系提供了一个研究无规缺陷如何影响共聚物结构与极化响应的模型体系,并且有助于我们进一步理解共混物中极化响应与电卡效应之间的相互关系。进一步对共混物交联后的研究结果证明了共混物中的能量损耗确实能够降低,但是效果并不明显。相反,交联降低了聚合物的电卡效应。将共混物极化的实验表明,二元共聚物P(VDF-TrFE)在被极化后,其自发极化能够保持下来,从而构建一个相对稳定的内建电场。内建电场的存在可以使聚合物在较低的场下产生相对更高的极化从而提高低场下的电卡效应。4.制备了弛豫铁电三元共聚物P(VDF-TrFE-CFE)与氧化锆纳米粒子的纳米复合物。纳米粒子作为结晶成核剂,提高了聚合物的结晶度。更重要的是,纳米粒子与聚合物基体之间的界面效应提高了聚合物的极化强度,提供了额外的熵变。因此纳米复合物比纯的聚合物具有更高的电卡效应。该结果第一次证明了铁电聚合物中的电卡效应可以通过纳米复合的方式进行增强。更多还原

【Abstract】 The superior electric properties of PVDF-based ferroelectric polymers originate in the polar crystalline structures and re-orientation of the dipoles in them caused by electric field. Therefore, the polar crystalline domains are the basis of the superior electric properties. As a kind of semi-crystalline polymer, the arrangement and packing of polymer chains during crystallization is easily affected by many factors, which results in the complication, diversity, and imperfectness of the polymer structure. And it is the complication and diversity that provide us opportunities to tune the microstructure widely and precisely ranging from several nanometers to hundreds of nanometers or even larger. Similar to other ferroelectric materials, PVDF-based ferroelectric polymers are attracting due to its rich cross-coupling phenomena, which is closely related to the complicated microstructure. How to manipulate its microstructure and thus its related electric properties is one of the concerns.In this thesis, we aim to optimize certain electric properties of PVDF-based polymers by employing different treatment methods to fine-tune the microstrucutre, and try to establish the structure-property relationships by investigating the microstructure and related electric properties. The main results are listed below.1. To facilitate the application of ferroelectric polymers in memory devices, we fabricated ferroelectric polymer poly(vinylidene difluoride-trifluoroethylene), P(VDF-TrFE), nanodot arrays with ultrahigh data storage density through a facile, high-throughput, and cost-effective method of nano-imprinting. The nanodots show a large-area smooth surface morphology, and the piezoresponse in each nanodot is strong and uniform. The preferred orientation of the copolymer chains, which are aligned parallel to the substrate with dipoles perpendicular to the substrate, in the nanodot arrays is favorable for polarization switching of each single nanodot. This approach allows nanometer electronic feature to be written directly in two dimensions by PFM-probe based technology, and can reach a resolution in the order of sub-10nm with storage density as high as75Gb/inch2. We also fabricated ferroelectric relxor polymer poly(vinylidene difluoride-trifluoroethylene chlorofluoroethylene), P(VDF-TrFE-CFE) nano-gratings using the same imprinting method, and also found preferred orientation of the polymer chains. The influence of nanoimprint is also featured by constrained movement of polymer chain segments, which partially retains ferroelectricity in the polymer relaxor.2. To facilitate the application of ferroelectric polymers in high energy density capacitors, we developed a photo-crosslinking method to modify the polymer. Photoinitiated cross-linking of poly(vinylidene fluoride-co-chlorotrifluoroethylene), P(VDF-CTFE), can offer a significant increase in polarization while at the same time maintaining high electric breakdown field, both of which contribute a lot to the enhanced electric energy storage capacity (-22.5J/cm3). This improvement is related to the structure changes in the copolymer crystals brought by cross-linking. Cross-linking favors formation of polar crystalline phase, converting the large a crystals into defective polar nanosize crystals with mixed crystallograhpy. Cross-linking also increase inner interface area of copolymers and reduce their interface thickness. This copolymer case demonstrates the greatly enhanced energy storage behavior, including increased discharge energy density at reduced field strength, and improved capacitor efficiency at relatively high degree of cross-linking.3. In order to investigate the tunability of electrocaloric effect (ECE) and ferroelectric responses, blends of ferroelectric relaxor P(VDF-TrFE-CFE) terpolymer and normal ferroelectrics P(VDF-TrFE) copolymer are studied. At low copolymer content (<15wt%), the coupling between the relaxor terpolymer and the nano-phase copolymer converts the copolymer into relaxor and causes an increase in the crystallinity compared with neat terpolymer. As a result, the blends exhibit an enhanced relaxor polarization response and a significant increase in the electrocaloric effect (～30%) compared with those in the neat terpolymer. At high copolymer content (>20wt%), the blends exhibit mixed structures of the two components. By varying composition, the dielectric and ferroelectric properties of blends can be tuned in the range between the copolymer and terpolymer. This blend system provides a model system to study how random defects influence the polarization response in the normal ferroelectric copolymer, and to understand the relationship between the polarization response and ECE in the blends.4. The electrocaloric effect is enhanced in ferroelectric relaxor terpolymer P(VDF-TrFE-CFE)/ZrO2nanocomposites. It is observed that the interface effects between the polymer matrix and nano-fillers enhance the polarization response and provide additional electrocaloric entropy changes. As a consequence, the nanocomposites exhibit a larger ECE than that of the neat terpolymer. The results, for the first time, demonstrate that ECE can be tailored and enhanced through nanocomposite approach in the ferroelectric polymers.更多还原