Controlling Semi-Crystalline Structure And Ferroelectric Property Of PVDF-Based Polymers

Poly(vinylidene fluoride) (PVDF)-based polymers have been widely investigat-ed for their potential electroactive applications in high-energy-density capacitors, da-ta storage, electrocaloric refrigerators, self-power-generated devices, and photovoltaic cells etc., due to their good piezoelectric and ferroelectric properties. Their perfor-mances appear as very much related with the semi-crystalline structures of VDF se-quences for switching polar domains as a response to the external electric field. For example, P(VDF-CTFE) copolymer exhibits high energy density and charge-discharge efficiency due to their paraelectric phase, while P(VDF-TrFE) copolymer appears use-ful in the nonvolatile memory devices due to their good ferroelectric phase. Therefore, it is worth to further study the relationship between semi-crystalline structure and elec-tric property of PVDF-based polymers.In this thesis, we controlled the microstructures of PVDF-based polymers by var-ious treating methods, such as fast cooling, chemical cross-linking, nano-composite, and nano-imprinting. Our results provide many insights to guide the designing of high-energy-density dielectric materials and nonvolatile memory with minimized en-ergy consumption and high writing speed. The main results are listed below.In the first section, in order to analyze the microstructure of PVDF-based poly-mers, we employed the commercial chip-calorimeter (Flash DSC1) and piezo-response force microscope (PFM) to study the crystallization and melting behaviors of PVDF-based polymers under three magnitudes of cooling and heating rates. The evolution of crystalline phase could be well observed due to ultra-fast heating and cooling. We made systematic investigation of PVDF, P(VDF-TrFE) and P(VDF-TrFE-CFE) on their generating of ferroelectric phase as well as the switching to paraelectric phase. The chemical confinement of comonomers on their crystallization and the resulted semi-crystalline structures has been discussed. Our observations facilitate a systematic un-derstanding of structural optimization for various electroactive performances of PVDF-based polymers.In the second section, in order to understand PVDF-based polymers for their practical applications in high-energy-density capacitors and energy storage, we se-lected P(VDF-CTFE) as a model system. To control its energy release behavior, a small loading of PS-COOH nanoparticles and setting up the cross-linking network ef-fectively enlarges the interface amount, including interfaces between nano-fillers and polymer matrix as well as crystalline-amorphous interfaces. It results in a high di-electric permittivity in P(VDF-CTFE). Most importantly, the cross-linking effectively scales down the sizes of polar domains, and thus leads to the improved discharge en-ergy density. Furthermore, we observed energy release process with nanometer spatial resolution using dielectric polymer like PS-COOH/P(VDF-CTFE) nano-composites. Time-resolved piezo-response force microscopy (TR-PFM) was employed to directly image the nanoscale dynamics of local polarization switching in the cross-linked nano-composites. Both fast and slow processes of energy release were identified. Moreover, the slow process could be divided into two processes. The spatial map of the first slow decaying process shows that the sizes of polar domains are tailored down by the cross-linking. The reduction in the sizes of polar domains is beneficial to the high electric displacement, and affords high energy density. Most importantly, the static reconstruc-tions of the slow energy release process reveal the presence of thick interfaces. From the dynamic reconstructions, the energy release rate of the nanoparticles is much faster than that of the matrix. It is interesting that the decay curve of the interface resem-bles that of the matrix at initial stage, and finally gets close to that of the antiparticles. Time-resolved PFM technique is power for nano-materials applied in such systems as capacitors, batteries, and fuel cells, etc.In the third section, ferroelectric nano-cages opened up the new active field of nano-materials for capacitors due to the large surface-to-volume ratio. Firstly, ferro-electric barium titanate (BaTiO3-PVDF) nanoparticles with different sizes were first synthesized by exploiting amphiphilic unimolecular star-like poly(acrylic acid)-block-polystyrene (PAA-b-PS) diblock copolymers as nano-reactors. Furthermore, PVDF nano-particles and BaTiO3-PVDF nano-cages were synthesized by self-assembly in- duced by hydrophobic interaction. As for ferroelectric nano-cages, the crystal structure of BaTiO3 was transformed from cubic phase into tetragonal phase with the increasing diameter of BaTiO3 nanoparticles. In contrast, the melting point of PVDF decreased with the increasing diameter of BaTiO3 due to the nano-confinement effect. BaTiO3-PVDF ferroelectric nano-cages possess high energy density as well as high discharge rate.In the fourth section, to facilitate the application of PVDF-based polymers in non-volatile memory devices, we fabricated poly(vinylidene difluoride-trifluoroethylene) nano-dot arrays (60 nm high and 400 nm period dot) with different components of TrFE units. In comparison to P(VDF-TrFE) 68:32 nano-dots, P(VDF-TrFE) 50:50 nano-dots exhibit lower coercive voltage and higher writing speed because TrFE units effectively scaled down the sizes of polar domains. When the same voltages are ap-plied on a single nano-dot, the writing speed of P(VDF-TrFE) 50:50 nano-dot is 10 times higher than P(VDF-TrFE) 68:32 nano-dot. Furthermore, when the writing time are the same, P(VDFTrFE) 50:50 nano-dot can save 30% energy loss. P(VDF-TrFE) 50:50 nano-dots also show a rewritable capability and the stability of data storage.In summary, by using four different methods, such as fast-cooling, cross-linking, hybrid nanoparticles and nano-imprinting, we controlled the semi-crystalline structures of PVDF-based polymers, and correlated the latters with their ferroelectric properties. Our systematic studies provide a theoretical guide for the promised applications of PVDF-based polymers in high-energy-density capacitor, electric actuators, large-scale sonar sensors, electrocaloric refrigerators, and ferroelectric random access memories (Fe-RAM), etc..