Studies On Dielectric Performance Of MWNTs/PI Composite Films

Polyimide film(PI) has excellent thermal, electronic, and mechanicalperformance which is widely used in many fields. However, with thedevelopment of science and technology, the pure polyimide film can not meet themarket demand for its corona resistance. Therefore, domestic and foreignscholars begin to improve the corona resistance of polyimide film. Among a largenumber of methods, using carbon nanotubes modified polyimide film is the mostpopular research topic in recent years. Existing research shows that the additionof carbon nanotubes can not only improve the corona resistance of polyimidecomposite film, but also can give attention to both the improvement of themechanical properties of polyimide film at the same time. However, carbonnanotubes is a conductive filler，when it is added into polyimide film, thedielectric substrate material will change. The draw ratio of the conductive fillersare important factors affecting composite dielectric properties, therefore thispaper will study the different draw ratio of carbon nanotubes on the dielectricproperties of PI/MWNTs composite film.This paper used the PMDA and ODA as raw materials, takes the differentdraw ratio of MWNTs treated by mixed as the filler, PAA/MWNTs was preparedby in-situ polymerization with hybrid adhesive solution, and simulated industrialtape casting membrane, finally heated the amination, then the thin film wasprepared. Using SEM to observe the processing conditions on the influence ofdraw ratio of carbon nanotubes. Using IR to analyze the MWNTs functionalizedfunctional groups on the grafting. Using Dielectric spectrometer to test thedielectric properties and volume resistivity of MWNTs/PI composite film, andBreakdown Tester to analyze the disruptive strength. The test results showed that: the longer time of acidification treatment, theshorter the average length of carbon nanotubes, the smaller the draw ratio,MWNTs was grafted on the functional groups after processing. The percolationthreshold of composite film was effected by the draw ratio of carbon nanotubes.When the diameter of MWNTs were the same, the larger the draw ratio of carbonnanotube, the lower the percolation threshold. And when the average length ofcarbon nanotubes were the same, the greater the draw ratio, the greater thepercolation threshold. Closed to the percolation threshold, the relative dielectricconstant and dielectric loss of composite film which were doped with differentdraw ratio of MWNTs occurred jump phenomenon, but the breakdown voltagedecreased quickly. The dielectric constant and dielectric loss of composite filmwere affected by the draw ratio of carbon nanotubes. Under the doping amount of0.2wt%and1.0wt%, when the diameter were the same, the larger the draw ratioof carbon nanotube, the lower the dielectric constant and dielectric loss washigher. And when the average length of carbon nanotubes was same, the greaterthe draw ratio, the higher the dielectric constant, the lower the dielectric loss.Doping carbon nanotubes composite film breakdown strength was lower than theunmodified PI films, under the doping amount of1.0wt%, three kinds of filmsbreakdown strength fell more than50%. The interface overlapping area sizes ofdifferent draw ratio of carbon nanotubes were different, the smaller the drawratio, the greater the interface overlapping area, the better the compositemembrane conductivity, the lower the breakdown strength.