Preparation Of Piezoelectric Poly (Vinylidene Fluoride) Film And Investigation Of Its Properties

Poly(vinylidene fluoride)(PVDF)is a kind of semicrystalline functional polymer material,containing at least four different crystal structures,namely ?,?,?and ?phase,respectively.Among them,the ? and ? crystals both containing all-trans(TTT)conformation and exhibiting excellent ferroelectric and piezoelectric properties.Moreover,PVDF has the advantages of soft light,good flexibility,wide frequency response,high sensitivity of power electrical conversion and impact resistance.Therefore,PVDF thin films are widely used in electronic devices.However,the preparation of these two kinds of phase is more severe.Mechanical strentching,high temperature and pressure,strong electric field and other complex experimental conditions are required.Moreover,the surface roughness and the leakage currentof the PVDF film are high.Therefore,it is important to design a simple and effective method for the preparation of high content ? and ? crystals.In this paper,a new method for the fabrication of PVDF films with high content of ? and ? crystals was developed.The change of ? and ? phases under different temperature was quantitative investigated by means of the wide angle X-ray dieeraction(WAXD),Fourier-transform infrared spectroscopy(FTIR),and Raman spectroscopy(Raman).The surface roughness and the orientation of the molecular chain of the sheared film were investigated using polarization microscope(POM)and atomic force microscopy(AFM).The effect of annealing temperature on the sheared film was qualitative investigated by in-situ thermal stage and differential scanning calorimetry(DSC).Using piezoresponse force microscopy(PFM)and ferroelectric testing system investigated the electrical properties of sheared films,provided valuable information for its application in the electronic device in the future.Results show that high content of?-phasewas formed when the shear force of the blade was applied to the moten PVDF film,and with the increase of the shear temperature,the content of ?-phase increases gradually,wheares the content of?-phase was gradually reduced.In addition,the PVDF film showed better orientation,the molecular chain was oriented along the shear direction,wheares the lamellae were perpendicular to the shear direction.At the same time,the surface of sheared film was very smooth,and the content of?-phase was uniform.It was founded that the content of ?-phase of the sheared film did not change after in-situ annealing at high temperature,which means that the crystal structure of the sheared film is steady,annealing is beneficial to the crystal structure without crystal conformation changes.The uniform oriented transcrystalline structure was formed on both sides of the polyamide fiber by pulling the fiber to apply the shear force to the molten PVDF.At lower temperature,the transcrystallinewas ?-phase,and the????phase transition occurred when the ?-transcrystalline was annealed at high temperature.However,at higher temperature,with prolonging the crystallization time,the growth of ?-transcrystallineand ????phase transition was happened simultaneously.Compared with the phase transition of?-spherulite,the phase transition in ?-transcrystalline was uniform and controllable.By controlling the temperature,in the system that fabricated an outer layer transcrystallinein the inner transcrystalline obtained first,crystallization temperature and the crystal structure of inner transcrystallinehad different influence on the crystal structure of the outer transcrystalline.In the system that crystallized at lower temperature first and at higher temperature latter,the crystal structure of the inner transcrystalline has a greater influence on the outer transcrystalline.In the system that crystallized at higher temperature first and at lower temperature latter,the crystallization temperature has a greater influence on the outer transcrystalline.