Preparation And Properties Of Polypyrrole/Multi-Walled Carbon Nanotubes/Polyvinylidene Fluoride Dielectric Composites

With the rapid development of the electronic and electrical power industry,materials with higher dielectric constants,dielectric losses,miniaturization,easy processing,flexibility and other excellent properties have attracted considerable attention.Therefore,the method to prepare materials with higher dielectric constants by adding conductive filler into polymer matrix has attracted much attention.The mechanism is to improve the dielectric properties of the material by using the seepage effect.This method can effectively make up the defects of the traditional ceramic dielectric materials.In this thesis,using polyvinylidene fluoride?PVDF?with excellent ferroelectric properties as the matrix,polypyrrole?PPy?with different morphologies that prepared by using HCl,HNO₃,citric acid or p-toluene sulfonic acid?TsOH?as dopant were selected as conductive fillers.The results showed that the HCl-doped PPy/PVDF composites exhibited best dielectric properties.Finally,PPy/MWCNTs conductive fillers were prepared by using different content of PPy and MWCNTs,and PPy/MWCNTs/PVDF dielectric composites were prepared by mixing PPy/MWCNTs with PVDF.Through charactering the dielectric properties of the materials,the specific experiment contents are as follows:?1?The cetyltrimethylammonium bromide?CTAB?and ammonium persulfate?APS?were used as the oxidation template,PPy with paperclip and spherical morphology which were prepared by using HCl,HNO₃,citric acid or TsOH as dopant were selected as filler,PPy/PVDF dielectric material.were prepared by the solution blending method.The characterization results of the structure,morphology and properties showed that with the contant of PPy increase,the mechanical properties and dielectric properties of the composites increased significantly.When the content of PPy reached to 8%,the percolation threshold appeared.When PPy was doped by HCl,the dielectric constant of PPy/PVDF composites was the highest,the dielectric constant of composite was90.89,the dielectric loss was 0.069,and the breaking elongation reached222.23%at the frequency of 1 kHz.?2?Carboxylated MWCNTs which were prepared by acid mixture?volume ratio of H₂SO₄ and HNO₃ is 3:1?were used as filler,MWCNTs/PVDF dielectric composites were formed by solution blending into PVDF.The structures and morphologies of the materials were characterized by XRD,Raman,TEM and SEM.The effects of the amount of MWCNTs on the thermal stability,mechanical properties and dielectric properties of MWCNTs/PVDF composites were investigated by using TG,tensile test and LCR impedance analyzer.The results showed that with the increase of carboxylated-MWCNTs content,the thermodynamic and dielectric properties of the composites were significantly higher than those of pure PVDF.At room temperature 100 Hz frequency,w?MWCNTs?=8%had a percolation threshold,and the dielectric constant of the composites reached 163.5,and the dielectric loss was 0.06;w?MWCNTs?=8%,the mechanical properties of the composites were best,elongation at break were 255.97%.?3?Firstly,CTAB and APS were used to form an oxidation template.Then,a double template was formed with carboxylated MWCNTs,PPy/MWCNTs conductive composites was prepared by polymerizing pyrrole?Py?monomer on the surface of carboxylated MWCNTs.PPy/MWCNTs/PVDF three-phase dielectric composites were formed by adding PPy/MWCNTs conductive fillers into PVDF matrix.The structure,morphology and properties of PPy/MWCNTs were investigated by using FTIR,Raman,TEM,SEM,XPS,TG and four probe conductivity gauge,respectively.The effect of carboxylated MWCNTs content on the structure,morphology and conductivity and thermal properties of the PPy/MWCNTs conductive fillers was studied.The effect of PPy/MWCNTs conductive fillers on the crystallization behavior,dielectric properties and mechanical properties of PPy/MWCNTs/PVDF dielectric composites was investigated by XRD,LCR impedance analyzer and tensile test.The results showed that the diameter of PPy/MWCNTs was larger than carboxylated MWCNTs,smaller than PPy,and the area of imino?=N-?decreased with the increasing of carboxylated MWCNTs.It was showed that PPy conjugate fragment and carboxylated MWCNTs had?-?coconversion when the content of carboxylated MWCNTs was 25%,the thin tubular carboxylated MWCNTs were encapsulated into a coaxial core-shell tube by the return tubular PPy,and the conductivity and thermal stability were the best.The addition with conductive fillersofPPy/MWCNTscanincreasethedielectricconstantof PPy/MWCNTs/PVDF composites more than 1000 at 100 Hz.At 1 KHz,when the conductive fillers content of PPy/MWCNTs was 4%,the seepage threshold of PPy/MWCNTs/PVDF three-phase dielectric composites occurred.The dielectric constant of PPy/MWCNTs/PVDF three-phase dielectric composites was up to 179.6,the dielectric loss of PPy/MWCNTs/PVDF three-phase dielectric composites was only 0.032,and the breaking elongation of PPy/MWCNTs/PVDF three-phase dielectric composites was maximized256.91%.