Preparation And Properties Of Polyimide With Low Glass Transition Temperature

With the rapid development of science and technology,there is a growing demand for light cable insulation materials with excellent dielectric properties and flame retardancy properties in the fields such as aerospace,high-speed rail,nuclear power and so on.Polyimide(PI)can better meet the special performance requirements of materials in the above fields due to its excellent performance.However,PI is very difficult to process because of its high glass transition temperature(T_g)(close to 400?),then its application is therefore limited.This article uses monomers containing aliphatic/alicyclic to prepare polyimide with lower glass transition temperature,which provides a certain basis for the research and application of polyimide cable materials.Modified copolyimide is prepared with pyromellitic dianhydride(PMDA)and 4,4'-diaminodiphenyl ether(ODA),adding 1,6-hexanediamine(DAH)or 4,4'-diaminodicyclohexylmethane(DCHM).The two-step synthesis method with thermal imidization is utilized to prepare aliphatic-containing polyimide(HPI)and alicyclic-containing polyimide(DPI).Futhurmore,the effect of DAH and DCHM on the various properties of copolyimide is studied,including condensed structure,glass transition temperature(T_g),solubility,thermal stability,mechanical properties and the relative dielectric constant,dielectric loss factor,volume resistivity and breakdown field strength.The results are as follows:1)X-ray diffraction analysis shows that more and more significant change of the condensed structure of PI is observed with the increase of molar content of DAH and DCHM.The chain flexibility gets enhanced,while the effect of DAH on the condensed structure of PI is more significant than that of DCHM;2)Dynamic mechanics analysis results show that the introduction of DAH and DCHM can both reduce the glass transition temperature of PI.The glass transition temperature of HPI and DPI is 290.4?and 310.5?respectively and the decreasing range is 88.4?and 68.3?respectively when the molar content of DAH and DCHM in total diamine is both50%;3)Both HPI and DPI can not dissolve in solvents like DMAc;4)Thermal gravity analysis results show that the initial decomposition temperature of HPI and DPI and the final residual mass both decrease with the increase of DAH and DCHM molar content.When the molar content of DAH and DCHM is both 50%,the initial decomposition temperatures of HPI and DPI are 406.9?and 360.7?and the residual mass is 41.5%and 30.9%,respectively.Futhurmore,the maximum decomposition rate temperature of HPI in both stages will shift as the molar content of DAH changes,which is not observed in the DPI;5)The mechanical properties test shows that the introduction of DAH and DCHM both lead to a decrease in the tensile strength of PI,while the fracture elongation and fracture energy both increased first and then decreased.When the molar content of DAH is 20%,the tensile strength,fracture elongation and fracture energy of HPI are 97.4 MPa,33.2%and 1.15 J respectively.When the molar content of DCHM is 10%,the tensile strength,fracture elongation and fracture energy of DPI are 104.9 MPa,28.4%and 1.04 J respectively;6)The dielectric spectrum tests show that both DAH and DCHM are beneficial to reduce the relative dielectric constant.When the molar content of DAH and DCHM is both 50%,the relative dielectric constant of HPI and DPI at 50 Hz is reduced form 3.43 to3.24 and 3.15 respectively,while the dielectric loss factor is 1.22×10^-3and2.19×10^-3,the volume resistivity are 5.51×10¹⁴?·m and 3.02×10¹⁴?·m,the breakdown field strength is 138.9 k V/mm and 134.6 k V/mm respectively.