Synthesis And Performance Analysis Of High Temperature Resistant Spiro Polyimide Dielectric Polymers

With the continuous advancement of technology,dielectric polymers with good dielectric properties have been used in many applications such as energy storage,digital memory devices,pulsed power systems,and signal processing,and so on.However,some harsh operating environments have raised new challenges for dielectric polymers.For example,film capacitors are mainly used in power converters for electric vehicles to drive motors by converting DC to AC,where the operating temperature has exceeded 150°C.Obviously,many conventional polymer dielectrics with low operating temperatures cannot meet these requirements.In recent years,high performance polyimide has been used as a high temperature dielectric for research because of its high glass transition temperature.Based on these,two types of polyimides（including PI and PEI）with hydroxyl groups were designed and synthesized.In order to meet the high temperature conditions（≥150℃）,twisted spiro monomers were introduced backbone,and the thermal and dielectric properties of these two types of polyimide polymers were investigated.First,homopolymerized polyetherimide（BAP-ODPA）was obtained by conventional thermal imidization method using bis（3-amino-4-hydroxyphenyl）propane（BAP）and 4,4’-biphenyl ether dianhydride（ODPA）as raw materials.Thermal analysis results showed that this PEI has a high glass transition temperature（～233°C）and an initial decomposition temperature(T_d5%)of 401.5°C.The results of the dielectric tests implied that,dielectric constant are improved（4.3～5.1）compared to commercial polyimide,and the energy release efficiency is higher at room temperature and 150℃,but when the temperature increased to 200℃,the released efficiency decreased significantly（η<50%）.To meet the application in high-temperature environment,a spiro monomer SPDD with highly rigid structure was first synthesized by a three-step reaction,and then copolymerized with BAP and ODPA（SPDD-BAP-ODPA）.XRD results showed that the intermolecular spacing increased with the increase of SPDD content,in which the intermolecular spacing of BAP-ODPA with 50%SPDD content reached 6.32（?）.The results of thermal analysis showed that the glass transition temperature of the polymers increased with SPDD content,where the glass transition temperature of BAP-ODPA with 50%SPDD content was 302°C.The dielectric results show that these copolymers have relatively high dielectric constants（4.2～5.1）with very low dielectric losses（tanδ<0.002）,while BAP-ODPA with 50%SPDD content exhibited a high energy storage density of 2.241 J/cm³ at 200°C and 350 MV/m,with a discharge efficiency above 90%.Finally,9,9-bis（3,4-dicarboxyphenyl）fluorene dianhydride（BPAF）with high rigidity was selected to replace ODPA,and polyimide（BAP-BPAF）was obtained by thermal imidation condensation with BAP,which was also modified by SPDD to obtain the copolymer（SPDD-BAP-BPAF）.The test results showed that the copolymerized polyimide has a larger intermolecular spacing and better heat resistance.Meanwhile,their dielectric constants are relatively high（4.1～5.2）and also have very low dielectric loss（tanδ<0.002）,among which the BAP-BPAF with 30%SPDD content can maintained a high release efficiency of 91.36%at200℃and 350 MV/m,with a high U_e,dis of 2.393 J/cm³,which is better than the 50%SPDD.It can be seen that both of those two polymers are suitable for high temperature energy storage applications.