Research On The Influence Of Annealing Temperature And Stretching On Poly(Vinylidene Fluoride) Crystal Transformation

Polyvinylidene fluoride(PVDF)is a typical polymorphic polymer,and there are mutual conversions between different crystal forms,among them,y,β and other polar phase crystals have attracted much attention due to their excellent electrical properties.How to prepare PVDF with high content of polar phase in a simple and rapid way is currently a hot research topic.In this paper,we studied the effects of different annealing temperatures,uniaxial stretching ratios and stretching temperatures on the crystal form transformation of PVDF and prepare high content of γ and β phase PVDF films.1.A blend film of PVDF and polybutylene adipate(PBA)was prepared.After isothermal crystallization,the samples coexist with α,γ,γ’ crystal forms.Using X-ray diffraction(XRD),Fourier Transform Infrared Spectroscopy(FTIR),and Differential Scanning Calorimeter(DSC)to study the transformation of a crystal to y crystal and γ’ crystal to γ crystal under different annealing conditions,and the polarizing microscope(POM)and scanning electron microscope(SEM)were used to study the morphological changes before and after the crystal form transformation.The results show that annealing at 178℃ for a shorter time can obtain pure γ-crystalline PVDF film.Annealing at a lower temperature,the a crystal will transform into a y crystal during the cooling process after melting,this y crystal has basically no birefringence under a polarizing microscope,and it can be observed as a small spherulite structure under a scanning electron microscope,and the volume of spherulites increases with increasing annealing temperature;The γ crystal melts and nucleates to regenerate γ crystal,the platelet morphology has a low degree of curl,and the higher the annealing temperature,the more flat-on platelets and the weaker the birefringence.Annealing at a higher temperature,the γ’ crystals will be transformed into γ crystals after melting,forming a unique groove-shaped ring-shaped spherulite.2.Preparation of high molecular weight PVDF film by solution casting,uniaxial stretching at different temperatures and stretching ratios,and using DSC,FTIR,XRD to explore the crystal transformation during stretching.The results show that the transition from α to β is easy to occur at low temperature,while the transition from γ to β is easy to occur at high temperature,and high-temperature stretching can achieve a higher draw ratio and orientation Stretching γ-P VDF at 140℃,when the stretching ratio reaches 3,almost pureβ-phase PVDF film can be obtained.