Syudy On Crystallization Properties And Dielectric Relaxations Of Polyvinylidene Fluoride Composite Materials

Due to the rapid development of electronic and electric information industry,there comes growing market demand for high energy storage capacitor materials in recent years,accompanied with the leading of new energy hybrid and electric vehicles such as Tesla.Most of traditional film capacitor materials are polypropylene,polyethylene,polyester,polystyrene and polyimide,from which polyvinylidene fluoride(PVDF)stands out with higher permittivity(about 4.5 ~ 6.0 at 60 Hz and room temperature)and acts as a hot research subject.In this paper,analyses of different relaxation processes in PVDF series materials were conducted to connect the crystallization properties with dielectric relaxations and dielectric properties,which are helpful to the deeper understanding of ferroelectric-paraelectric phase transition mechanism,and provide a theoretical basis for the investigation of PVDF composite materials with high energy storage and high discharge efficiency.FTIR,XRD and DSC were adopted to analyze the crystallization properties of PVDF,PVDF-g-MAH,P(VDF-CTFE)and PVDF/PMMA blends,respectively.It is found that different chemical and physical modification methods have a major impact on crystalline structure and phase transition of PVDF.Grafting with MAH and copolymerization with CTFE decrease the crystallinity of PVDF significantly.For PVDF/PMMA blends,the crystallinity decreases with PMMA increasing from 0 wt% to 40 wt%,accompanied with the decrease of ? phase and increase of amorphous phase.Furthermore,when the content of PMMA reaches to 50 wt%,crystallinity reduces sharply and ? phase almost disappears,instead of a small amount of ? phase.According to the frequency and temperature dependence of dielectric relaxation spectra,three different relaxation processes were generally observed in PVDF series materials,?s relaxation related with the hopping transport process,?c relaxation contributed by the interphase between crystalline phase and amorphous region,and ?a relaxation caused by segment movements in amorphous region.The PVDF interphase leeches on to crystalline region,and serves as obstacles to both the hopping transport process and segmental motions.On the one hand,the interphase possesses a lattice structure in spatial distribution.However,on the other hand,the segmental orientation in interphase is disordering and random.Arrhenius energy and HN relaxation time for different(?s and ?c)relaxation processes of PVDF,PVDF-g-MAH,P(VDF-CTFE)and PVDF/PMMA blends were also calculated by Havriliak-Negami model function in this paper,and activation mechanisms of ?s and ?c relaxation processes were discussed,respectively.Moreover,impacts of branch points and comonomer units on crystalline structure and relaxation behaviors in PVDF-g-MAH and P(VDF-CTFE)were investigated.The content of PMMA also matters much to activation mechanisms of ?s and ?c relaxation processes in PVDF/PMMA blends,which indicates the special morphology of PVDF interphases.