The Physical Properties Of The PVDF-Based Ferroelectric Polymer Nanostructures

Ferroelectric materials have attracted much attention due to their outstanding properties, such as ferroelectricity, piezoelectricity, and pyroelectricity etc. With the increasing demand for device miniaturization, people fabricated the ferroelectric nanostructures successfully and wanted to explore the new devices based on these nanostructures. Poly(vinylidenefluoride)(PVDF) based ferroelectric polymer became one kind of the most widely researched polymers for their good chemical stability and ease fabrication. In this dissertation, PVDF, P(VDF-Tr FE) and P(VDF-TrFE-CFE) films have been prepared by Langumir-Blodgett(LB) method. P(VDF-TrFE) nanotubes and P(VDF-TrFE-CFE) nanowires have been fabricated by porous Anodic Aluminum Oxide(AAO). We have investigated the optical and electrical properties of the nanostructures mentioned abve. The main results in this dissertation are listed as following:1. The PVDF and P(VDF-TrFE) films have been fabricated by LB method. The crystal structure of the samples was analyzed by the standard x-ray diffraction(XRD) method. It can be seen that PVDF LB films show the typical(110, 200) reflection, suggesting the all-trans ? phase formed in the LB films. The ferroelectric properties of the PVDF LB film and P(VDF-TrFE) film with different thickness have been studied. The PVDF homopolymer films show better ferroelectricity with higher polarization and higher breakdown electric field than that of the P(VDF-Tr FE) film. Inspection on the thickness dependence of the coercive field of the polyvinylidene fluoride films suggests an extrinsic polarization switching occurs in the thickness range from 40 to 200 nm, and a non-extrinsic switching was observed in the range between 10 and 40 nm, which is ascribed to the transition range from extrinsic to intrinsic switching.2. Capacitor with asymmetric electrodes(Au/P(VDF-TrFE)/Al) have been fabricated based on P(VDF-TrFE) LB films. Through the C-V test,an asymmetry of capacitance in the two polarization orientations under zero-bias was observed at room temperature region. The capacitance asymmetry decays with decrease of temperature and disappears when the temperature is below 230 K. The phenomena are explained considering that a pinned dipole layer exist between P(VDF-TrFE) films and Al metal electrode.3. The temperature dependence of the dielectric and optical properties and crystal structure of the relaxor P(VDF-TrFE-CFE) terpolymer films obtained from LB method have been comprehensively investigated. All the results suggest that there exists a peculiar point at 370 K, which is called the intermediate temperature T*. Supported by Fourier transform infrared spectroscope,T* has been proved to be resulted from the change in the growth rate of the trans gauche T3 G chain conformations with temperature. As the T3 G chain conformations in relaxor polymers playing a role similar to that of the so-called polar nano-regions(PNRs) in the inorganic relaxors,the changes happened in P(VDF-TrFr-CFE) film at 370 K is similar to the transformation from dynamic to static behavior of PNRs in inorganic relaxors.4. P(VDF-Tr FE) nanotubes were fabricated by solution-wetting and melt-wetting of the anodized aluminum oxide(AAO) templates. Compared with nanotubes prepared by the solution-wetting AAO templates, the nanotubes prepared by the melt-wetting AAO templates get a higher crystallinity and more compact microstructures with a rough surface, which was supported by SEM. Confirmed by the Fourier transform infrared spectroscope, the ? structure was still found in P(VDF-TrFE) nanotubes even through a high temperature of melting point.5. P(VDF-TrFE-CFE) terpolymer nanowires with different sizes were fabricated by melt-wetting of the terpolymer in the anodized aluminum oxide(AAO) templates. The crystallinity in terpolymer nanowires with various diameters was checked by XRD. It was found that the terpolymer nanowire with a relatively smaller size has lower crystallinity. The coercive field in relaxor ferroelectric P(VDF-TrFE-CFE) terpolymer nanowires with different diameters was studied using piezoelectric force microscope. It was found that the coercive field increases significantly as the diameter of terpolymer nanowires decreases. Specially, EC in the nanowire with its diameter of 30 nm is up to 18 MV/m, which is almost 4 times of that(4.4 MV/m) in the nanowire with 170 nm in diameter. The results can be explained by the conversion of molecule conformation from T3 G to Tm>4 in the terpolymer nanowires, which is supported by Fourier transform infrared spectroscope.