Studies On Preparation And Properties Of Polyimide/Multi-walled Carbon Nanotubes Composite Films

Polyimide films (PI) are mainly used in the field of electrical insulation due totheir extraordinary electrical, mechanical and thermal properties. Thedevelopment of nanotechnology provides a new direction for the developmentand research of polymer insulating materials. Compared with other inorganic nanoparticles, multi walled carbon nanotubes (MWNTs) have remarkable characteristics:large length diameter ratio, high thermal and electrical conductivity. Hence domesticand foreign scholars had carried out many related studies on modification ofpolyimide films. So we choose the MWNTs as reinforcement of the PI films in orderto get the high performance of the PI/MWNTs composite films.At first, MWNTs were purified and functionalized, then were attended intothe synthesis of PI/MWNTs composite films via In Situ polymerization process.By heating process the polyamide acid was further imidized. The dispersion ofMWNTs in solvent was characterized by means of Scanning ElectronicMicroscopy (SEM) before and after functionalization. At the same time themorphology of the surfaces and the cross section of PI/MWNTs films wereinvestigated by SEM, thermal and electrical properties of nano composite flimswere tested. Experimental results showed that functionalization of MWNTsmake the dispersion of MWNTs in water and PI matrix improved. The heatresistance of the composite films was improved and the thermal decompositionrate was slowed due to the MWNTs were added. When the doping content is4.33wt%, the temperature difference of Td5and Td10was20.3℃, compared withthe pure membrane, it was improved by48.2%. When the doping content is lessthan1wt%, the dielectric constant increased as the MWNTs content increased，at the same time decreased with the increased frequency. But at low frequency, dielectric constant decreased relatively flat. Dielectric loss will increase after afirst reduction process. Overall, the dielectric loss increased with the increasedfrequency. Slower increase in the low frequency region, however the highfrequency region increases rapidly. Breakdown field strength of the overalldownward trend with the increase of MWNTs content, two turning pointsappeared with the carbon nanotube content on0.7wt%and0.9wt%. Thebreakdown field strength decreased significantly with the doping contentimproved.