Study On The Crystal Structure And Orientation Property Of Poly(vinylidene Fluoride)and Its Copolymers

Polyvinylidene fluoride(PVDF)is a kind of polycrystalline and semicrystalline polymer.The most common crystal types are α,β and γ.Among them,β and γ are polar phases with Ferroelectricity,which is one of the research focuses.To obtain them,the usual methods are phase inversion and polymerization.Generally,the phase inversion method transforms a phase into polar phases by mechanical stretching,electric field polarization,high temperature and high pressure,but the transformation efficiency is low and the operation is difficult.The polymerization method combines vinylidene fluoride and trifluoroethylene(P(VDF-TrFE)),which can form β crystal directly.When the molecular chains of polar phases are parallel to the substrate,the ferroelectric properties of polar phases will be retained,the ferroelectric properties of polar phases under common substrate will be lost after melting and recrystallization.Therefore,it is of great significance to find a new method to obtain polar phases and to control the structure of polar phases at high temperature.In this experiment,polyimide fiber was used to shear PVDF melt film to explore the crystallization behavior of PVDF under shear,and t the relationship between the shear conditions and the orientation degree was determined by rheometer.Besides,the melting and crystallization behavior of P(VDF-TrFE)ultrathin film on graphene substrate was studied,and the device performance was further tested.The main results are as follows:1.By changing the shear temperature,shear time and shear speed,the crystal form of PVDF transcrystallization at the interface of heterogeneous fiber was controlled.It is found that the longer the shear time and the faster the shear speed,the easier the formation of a crystal.When the shear time and temperature reach a certain value,the pure α transverse crystal is formed.Low shear rate and short shear time are beneficial to the formation of γtranscrystallization,but it is difficult to obtain pure γ transcrystallization,which is generally the mixture of γ and α.Under the shear condition of 240℃and 0.06 mm/s,pure γ transcrystallization can be obtained.The experimental conditions for obtaining pure α and γ crystals are calculated quantitatively by using the plane drag flow shear model.It is proved that the orientation degree required for the formation of α crystal by PVDF molecular chain in shear is much higher than that required for the formation of y crystals.2.P(VDF-TrFE)films with graphene substrate were prepared by solution spin coating.It is found that the film is still an edge-on crystal structure after melting and recrystallization.However,the crystal structure of the films were slightly different at different heat treatment temperatures.Below 150℃,the b and c axes of the molecular chain formed by recrystallization are randomly distributed in the substrate plane,and above 150℃,the b and c axes are locally oriented in the plane.The difference is due to the memory effect of P(VDF-TrFE)melt.At the same time,two kinds of P(VDF-TrFE)devices based on indium tin oxide(ITO)and graphene were prepared,and their on-off ratio and stability were compared.It was found that the graphene based P(VDF-TrFE)film devices have higher stability and on-off ratio.