Preparation And Characterization Of Thin Film And Ultrathin Film Of Polyvinylidene Fluoride

Poly(vinylidene fluoride) (PVDF) is one of the most widely used polymer materials, due to its good piezoelectricity, ferroelectricity and pyroelectricity. Nowadays, PVDF piezoelectric thin films and pyroelectric ultrathin films have gained more and more attention of materials science, which have been applied in the large-area flexible hydroacoustic transducer and infrared imaging array, respectively. At present, most of PVDF ultrathin films are prepared by cast coating; but they are fabricated by electrostatic self-assembly (SA) method is not reported.This dissertation is aimed to deal with the microstructures and electrical properties of PVDF as well as the relations between them of PVDF thin films and ultrathin films. At the same time, the variational uptrend and intrinsic matter of phase transition have been discussed in detail, while PVDF films are treated under the condition of varible poling temperature and varible electric field.The ferroelectric PVDF films made up of droplets with determinate size have been prepared by ultrasonic atomization method under a series of frequency. The parameters on the ultrasonic atomization were discussed, such as solution concentration, atomizing frequency and depositional velocity. The results show that ultrasonic atomizing effect is decided by the concentration, the density, the surface tension and the viscosity of PVDF solutions. The higher ultrasonic frequency is oscillated; the small size droplets are produced. On the other hand, the lower ultrasonic frequency is oscillated, the larger amount of atomization is obtained. At the understanding of the independence between the ferroelectricity and atomizing frequency, the thickness and uniformity PVDF films can be controlled by adjusting the depositional conditions of ultrasonic atomization processing.In order to improve the piezoelectricity and pyroelectricity of PVDF films, two polarizing methods have been proposed. In the treated PVDF films, the crystalline degree, piezoelectric coefficient and remanent polarization increase. However, the different influence is carried on by poling temperature, poling electric field and poling time. The results show that the characteristics including piezoelectric coefficient, ferroelectric polarization of PVDF treated by thermally poling are higher than them treated by corona charging, and a new carbon couple bond has been found during thermally poling.The phase transition of PVDF film under poling electric field has been studied. The results show that the process of phase transition occurred as a formation ofα-PVDF→δ-PVDF→β-PVDF. The applied electric field force on the film results in alignment with average orientation angle of 79°of the dipoles inβphase, and the effective polarized electric field is obtained as about 60 MV/m. On the assumption of chemical kinetics equilibrium for phase transition of polarized PVDF, the model has been successfully built to simulate the dependence on the polarization and poling electric field. It is found that a new organocomplex compound of F—C—Ag has been formed between the Ag electrode and PVDF thin films with the effect of high electric field at high temperature after thermally poling treatment.PVDF/PDDA and BaTiO₃-PVDF/PDDA compound ultrathin films are fabricated by alternant electrostatic layer-by-layer (LbL) self-assembly (SA) with polarized PVDF films for the first time. It is found that the PVDF/PDDA ultrathin films display the amorphous structure. Due to the alternant deposition between ferroelectric layers and non-ferroelectric layers, the ferroelectric performance of PVDF ultrathin films fell obviously. The ferroelectricity of BaTiO₃-PVDF/PDDA ultrathin films is improved by BaTiO₃ ferroelectric ceramic nano-powders, which are modified by silane coupling agent.The PVDF monolayer and the multilayer ultrathin films have been prepared by Langmuir-Blodgett self-assembly method. The results show that the polar groups of PVDF monolayer films accordantly arranged with the same orientation, which have an excellent ferroelectric performance resulted in polarβphase. However, the sequence and polarity of ultrathin films are destroyed with the number of LB layer increasing, which leads to a andβphases coexist in the PVDF multilayer ultrathin films. The ferroelectric hysteresis indicates the remanent polarization (P) and coercive field (E_c) of PVDF multilayer ultrathin film are 0.7μC/cm² and 200 MV/m, respectively. The conductive mechanism of PVDF multilayer ultrathin film has been investigated. It is found that, theâ… -â…¤characteristic of the PVDF ferroelectric ultrathin films under the low electric field accords with Ohm's law, and under middle strong electric field, its conductive mechanism is the space charge limited current (SCLC) model due to the charges immitted into films from electrode, while under the strong electric field, the free carriers cause by Pool-Frenkel effect participated in the conductive process.Summarily, the PVDF thin films have been prepared by solution casting and ultrasonic atomization, and PVDF ultrathin films have been fabricated by electrostatic self-assembly and Langmuir-Blodgett. After treated by thermally poling and corona charging, the piezoelectricity, ferroelectricity and pyroelectricity of PVDF films are improved.