Study On Preparation And Properties Of Grafted Polyvinylidene Fluoride Film

At present, the low dielectric breakdown voltage and the low dielectric constant of polymers at high frequency limit their wide application in high energy storage density capacitor. Enhancing dielectric constant and breakdown strength is crucial to improve the properties of dielectric materials. To enhance the dielectric constant and reduce the dielectric loss, we grafted polar groups(such as carbonyl) or the rigid molecules containing benzene onto the main chain of PVDF; the interaction between grafted side chain and PVDF main chain can restrict the movement of the main chain.In this paper, PVDF film was chosen as the polymer matrix, and monomers containing polar groups were grafted onto the surface of PVDF films, then PVDF-g-MC,PVDF-g-PEMA, PVDF-g-PMMA and PVDF-g-MAH grafted films were obtained. The grafting method was ozone pretreatment-thermal initiated grafting. The reactive groups,such as –O–O–, –O–O–H, C=O, or free radicals etc. were produced on PVDF molecule,because of the strong oxidizability of ozone. These reactive groups could serve as active site for further reactions with grafting monomers.All these films were analyzed with XRD, FTIR and DSC. It is found that the crystal type transformed from nonpolar alpha phase to gamma phase to some extent after grafting.The thermal stability is slightly lower than pure PVDF.The breakdown strength and dielectric properties of grafted films were tested and analyzed.Compared with the PVDF film, the dielectric strength and the dielectric constant of grafted membranes were improved. PVDF-g-MC film was better than the other three.When the grafting rate was 7.4%, the breakdown strength was 585MV/m. The frequency was 20 Hz the dielectric constant was 12.5, and when the frequency is 1MHz, the dielectric loss was only 0.23. The discharged energy density was 7.76J/cm3(325MV/m). When the electric field was 50MV/m, its discharge efficiency was 70.9%. The electric field reaching up to 325MV/m, the discharge efficiency still remained at 64.5%. For the other three kinds of grafted films, the breakdown strength was under 400MV/m, and the dielectric constant was increased, but the dielectric loss resulting from conductivity was high when thefrequency below 103 Hz.