The Graphene And Shear Effects On The Crystallization Behaviors And Crystalline Structures Of PVDF

Poly(vinylidene fluoride)(PVDF)/graphene composites were prepared through the melt blending method.The crystallization behaviors and crystalline structures of PVDF and composite samples under quiescent conditions and shear flows at various temperatures were studied by means of polarized optical microscope(POM),wide-angle X-ray scattering(WAXS)and small-angle X-ray scattering(SAXS).The subsequent melting behaviors were investigated with differential scanning calorimetry(DSC).The POM images showed that the spherulites morphology of PVDF during isothermal crystallization presented the well-defined Maltese-cross texture.Banded spherulites and the smaller and darker γ spherulites emerged with increasing crystallization temperature.The number of spherulites large increased and the crystallization time decreased dramatically under shear field.The radial growth rates of spherulites in all PVDF samples decreased exponentially with increasing crystallization temperature and almost have the same value at certain temperature.WAXS results indicated that the PVDF/graphene composites have the same crystal forms after isothermal crystallization under different conditions.However,the grain size presented various change trends with increasing crystallization temperature.The grain size in the(021)diffraction direction increased constantly with crystallization temperature increase.But the change of grain size in the(100)(020)(110)diffraction directions inversed at 160℃.Meanwhile,the grain size in the(100)(110)diffraction directions under shear fields were much higher than that crystallized at static conditions,and the pure samples increased more than composite samples.This may be due to the crystal orientation in spherulites were affected by the shear flow,and the additive graphene weakened shear effects.SAXS dates demonstrated that the tendency change of lamellae structure in the PVDF/graphene composites inversed at 160℃.The long periods and the lamellar thickness increased at first and then decreased with increasing crystallization temperature.The long periods and the lamellar thickness along the meridian were higher than those along the equator under shear fields crystallization,and the pure samples had a larger value than composite samples with 0.1wt% graphene.The chains of PVDF extended along the shear direction resulted in the thickening of lamellae,but the shear effects were suppressed by the graphene.DSC analyses indicated that the adding graphene has no obvious influence on the melting behaviors of PVDF crystals.Double equilibrium melting temperatures of 183℃ and 195℃ for PVDF were obtained by Hoffman-Weeks(HW)extrapolation.The zero growth temperature of crystallizing PVDF in the pure and composites was 185.8℃,far below the equilibrium melting temperature 195℃.The adding graphene accelerated the rates of α→γ phase transformation during isothermal crystallization at 170℃,and the rates increased with increasing the contents of graphene.