Preparation Of Thermoplastic Polyimide And Study Of Properties On Its Dielectric Composites

Dielectric energy storage in high-temperature poses new challenges to the new generation of capacitor dielectric material.Combining dielectric ceramics or conductive fillers with polymers can greatly increase dielectric constant of polymer dielectric composites,but this increase comes at the cost of a sharp increase in dielectric loss and a decrease in breakdown strength.This problem can be effectively solved through grafting of multifunctional organic shells,building layered structures and satellite structures on the dielectric fillers as well as composite macro-interface design.Polyimide(PI)has very high glass transition temperature and thermal decomposition temperature,as well as low dielectric loss and high breakdown strength,which is an excellent high temperature resistant polymer dielectric matrix.In this paper,the newly synthesized thermoplastic PI was used as the matrix.Besides,a new type of"satellite structure"filler was prepared with micro-filler structure design,combined with composite macro-interface design to prepare thermoplastic PI-based dielectric composites with excellent comprehensive properties.The main research contents are as follows:A series of novel quaternary copolymerized thermoplastic polyimides with high heat resistant were successfully prepared with 2,3,3',4'-biphenyltetracarboxylic dianhydride(a-BPDA)and 4,4'-oxydiphthalic anhydride(s-ODPA)as dianhydrides,4,4'-diaminodiphenyl ether(ODA)and 2,2-bis[4-(4-aminophenoxy)phenyl]propane(BAPP)as diamines with the introduction of twisted non-coplanar structure,flexible ether bond and pendant methyl groups.Compared with traditional thermoplastic PI-6(YS20),this series of quaternary copolymerized thermoplastic PI films show good solubility in DMF,DMAc and NMP.The twisted non-coplanar structure destroyed the regularity of PI molecular chain and pendant methyl group increased the free volume of PI molecular chain,which reduced the molecular chain stacking density of PI,endowing PI excellent thermoplasticity.At the same time,the glass transition temperature,thermal decomposition temperature and the moisture resistance of the quaternary copolymerized PI-2—PI-5 films gradually increased.A new type of"satellite structure"SiC@PDA@Ag nano-filler was prepared by modifying the surface of silicon carbide(SiC).A series of SiC/PI and SiC@PDA@Ag/PI composites were prepared with thermoplastic PI-5,SiC and SiC@PDA@Ag by the solution blending method.The influence of filler micro-interface design on the microstructure,heat resistance,dielectric properties,breakdown strength and energy density of PI composite films was investigated.Although the dielectric constant of SiC@PDA@Ag/PI was slightly lower than SiC/PI,its dielectric loss was greatly reduced.At 1k Hz,the dielectric constant of 7wt%SiC@PDA@Ag/PI increased from 3.44(pure thermoplastic PI)to 5.15,and its dielectric loss was only 0.0125.In addition,the"satellite structure"SiC@PDA@Ag effectively improved the breakdown strength and energy density of the composites compared with SiC.This phenomenon was particularly obvious under high filler content.Among them,the breakdown strength and energy storage density of 1wt%SiC@PDA@Ag/PI were 266.08k V/mm and1.278J/cm~3,which were respectively higher 7.8%and 11.8%than 1wt%SiC/PI(246.77kv/mm and 1.143J/cm~3).In addition,the SiC@PDA@Ag/PI composites still maintained a relatively high glass transition temperature and thermal decomposition temperature."Satellite structure"SiC@PDA@Ag was used as the filler,and a series of sandwich structure-SiC@PDA@Ag/PI composite multilayer films were prepared by layer-by-layer coating through macro-interface design.The influence of fillers content and sandwich structure on the heat resistance,dielectric properties,breakdown strength and energy density of composite multilayer films were investigated.The pure thermoplastic PI in the middle layer effectively impedes the carrier transport between the interlayer of the composite multilayers,so that the sandwich structure-SiC@PDA@Ag/PI still had lower dielectric loss and higher breakdown strength under high fillers content.At 1k Hz,the dielectric constant of TSP-20 was7.44,while the dielectric loss was 0.0351.Similarly,the sandwich structure significantly improved the breakdown strength and energy density of the composite films.For example,the breakdown strength and energy density of TSP-5 were 273.39kv/mm and 1.343J/cm~3,which are respectively higher 14.6%and 14.7%than 5wt%SiC@PDA@Ag/PI(238.55kv/mm and1.173J/cm~3)respectively.At the same time,the energy density of TSP-5 was 10.9%higher than that of pure thermoplastic PI film(1.214J/cm~3).In addition,the sandwich structure-SiC@PDA@Ag/PI composite multilayer film also had excellent heat resistance.