Study On The Preparation And Properties Of Polyimides Containing Dibenzo-18-Crown-Ether

Polyimide?PI?is a kind of high-performance organic polymer material with imide ring structure in the main chain.Due to its excellent comprehensive performance,it has been widely used in microelectronics,new energy and other fields.However,the high melting temperature and low dielectric constant greatly restricts their applications in several fields.Therefore,on the basis of maintaining excellent performance,designing and preparing novel,functional,high-dielectric and high-performance PI materials and broadening their application remains a huge challenge.In this study,the crown ether rings were introduced into the main chain of PI molecules through molecular design.After that a series of PI materials with excellent performance were prepared.The relationship between the structure and properties of PI was discussed.Besides,it's electrochemical performance are also investigated,when it's serviced as a lithium ion battery separator material applied in the field of energy storage and conversion.The main contents are as follows:1.High purity diaminodibenzo-18-crown-6 was synthesized by nitration and reduction reaction using dibenzo-18-crown-6 as a raw material;then,a polyimide precursor-polyamic acid was prepared by polycondensation of diamine monomer?diaminodibenzo-18-crown-6?and three kinds of dianhydride monomers with different structures through a low-temperature solution.Polyimide nanofiber with a crown-containing ether ring in the main chain was prepared by casting PAA solution,followed by thermal imidization.The structures and properties of monomers and polymers were characterized by ¹H-NMR,FT-IR,TGA and tensile test.The results show that the diaminodibenzo-18-crown-6 has an expected structure and higher purity.All of the glass transition temperature?T_g?of the polyimides with crown ether structure in the main chain are up to 342°C,and the 5%weight loss temperature(T_5%)of PI are in the range of 415⁴32°C under nitrogen atmosphere.The tensile strength of PI film are in the range of 110.8¹147 MPa,and the elongation at break are9.8¹3.6%;the dielectric constant of PI is between 6.1 and 6.9?100Hz?.The dielectric loss is less than 0.03.2.Polyamicacids?PAA-a?werepreparedwithusing diaminodibenzo-18-crown-6 and 3,3',4,4'-biphenyl dianhydride?BPDA?as monomers through low temperature polycondensation.Polyimide nanofibers?PI-a?containing a crown ether structure in the main chain was obtained followed by two steps:polyamic acid nanofiber membranes were prepared by an electrostatic spinning technique followed by a high-temperature imidized procedure.The PI-a was used as a lithium ion battery separator to explore its application in the energy field.Various methods were used to evaluate the electrochemical performance of the PI-a separator.The results showed that the porosity,liquid uptake and room temperature ionic conductivity of PI-a are 79.98%,958%and 2.03?10^-3 S cm^-1,respectively.The discharge capacity retention of PI-a membranes remains 92.3%even after 100charge/discharge cycles at 0.1 C;The battery discharge capacity is 123 mAh g^-1 at 1 C.When compared with commercially available Celgarde separators,crown ether-functionalized PI nanofiber separator materials have a good aswantage in heat resistance,heat-resistant dimensional stability and cycle stability.Moreover,the electrolyte absorption of the resulted PI nanofiber membranes containing crown ether ring structures has a larger improvement than the corresponding unfunctionalized PI nanofiber film.The crown ether ring is introduced into the polyimide molecular structure;it can not only improve the dielectric properties of PI,but also improve the electrochemical properties,while maintaining an excellent comprehensive performance.The successful preparation of the polyimide material containing crown ether ring in the main chain is expected to provide a new way for the preparation of high-performance functionalized PI materials.It can be used as high-dielectric film material and lithium-ion battery separator material in the microelectronics electronics industry and new energy fields.