Manipulating Microstructures And Electric Properties In PVDF-Based Multifunctional Ferroelectric Polymers

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.