Molecular Design And Performance Tuning Of Intrinsically High Energy Storage Density Polyimide Dielectrics

With the continuous development of society and technology,the demand for film capacitors continues to grow in applications such as propulsion systems,hybrid electric vehicles（HEVs）and natural gas exploration.As the basis of capacitors,dielectric materials need to continuously develop properties such as dielectric constant and thermal stability.Polyimide has a wide range of applications in aerospace,optoelectronic information,and gas separation because of its high dielectric constant,excellent high temperature resistance and flexible molecular designability.In this paper,a series of high energy storage density polyimide are prepared and characterized by molecular design using polyimide as the substrate and introducing different polar groups to the molecular chain.The research in this paper consists of two main aspects as follows.（1）Polyimides（EPI-1 to EPI-5）with ester groups in the main chain were synthesized by using p-phenylene bis（phenylene trimellitate）dianhydride（TAHQ）as the dianhydride part of the monomer in combination with the diamine components 4,4’-diaminodiphenyl ether（ODA）,4,4’-methylenediphenylamine（MDA）,4,4’-bis（4-aminophenoxy）biphenyl（BAPP）,1,4-bis（4-aminophenoxy）benzene（144APB）and bis[4-（4-aminophenoxy）phenyl]sulfone（BADS）.The structures were characterized using Fourier infrared spectrometer（FTIR）and X ray diffractomer（XRD）,while the physical and chemical properties of the prepared films were systematically characterized and the link between the macroscopic dielectric strength and the microstructure of the films was verified using theoretical simulations.When the diamine fraction of the polyimide（PI）system is chosen to be 144APB（EPI-4）,the dielectric constant at 1 k Hz is 3.8,which is a substantial increase compared to pure PI.the dissipation factor of EPI-4 is low（less than 1%at 1 k Hz）.EPI-4 has both high breakdown strength and energy storage density,with breakdown strengths up to 519MV/m.EPI-4 has a high dielectric strength at EPI-4 maintains a storage density of 2.73J/cm³ and a charge/discharge efficiency of 93%at room temperature,and still has a storage density of 2.20 J/cm³ and a charge/discharge efficiency of 92%at 150°C.EPI-4 has excellent thermal stability,with a glass transition temperature（T_g）of 247°C and a thermal decomposition temperature(T_5%)of 458°C.Meanwhile,polyimide films with ester groups in the main chain show good solubility in a variety of polar solvents.（2）The diamine monomer 1,4-bis（（2-（methylsulphonyl）methyl-4-aminophenoxy）methyl）benzene（MAMB）with a side chain containing a sulphonyl group was synthesised by using 2-chloromethyl-4-nitrophenol as the starting point for the reaction and a control diamine（AMB）with the same main chain as MAMB but without the side chain.A series of PI films（SPIs）with both strong flexible and strong polar sulphonyl side chains and control（0PIs）films without side chains were subsequently condensed with common commercial dianhydrides,including homophthalic dianhydride（PMDA）and diphenyl ether tetracarboxylic dianhydride（ODPA）,to obtain a series of PI films（SPIs）and control（0PIs）films without side chains,with both strong flexible and strong polar sulphonyl side chains.Theoretical simulations were used to verify the link between the macroscopic dielectric strength and the microstructure of the films.When PMDA was selected as the dianhydride component（SPI-1）,the dielectric constant at 1 k Hz was 5.89 compared to only 3.45for the corresponding control 0PI-1,demonstrating a significant increase in the dielectric constant of the PI with the introduction of the side chain sulfone group.the dielectric loss of SPI-1 was only 1%at 1 k Hz.Compared to the 0PIs,the breakdown strength of the side chain sulfone containing PI decreased,but still maintained a high breakdown strength of 377 MV/m as well as 477 MV/m.Despite the presence of more flexible groups such as ether bonds in the molecular chain,the side chain sulfone containing PI maintains a high thermal stability with T_5%at 323°C and 327°C and T_gat 192°C and 208°C.The side chain sulfone containing PI also maintains a high thermal stability with T_5%at 323°C and 327°C and T_g at 192°C and 208°C.At the same time,the side chain containing sulfone PI has good solubility in a variety of polar solvents.