Effect of crosslinking method on the microstructures and properties of polyimide-based graphite fibers

Different polyimide (PI) fibers with and without carboxyl groups on the polymer chain were prepared by dry-jet wet spinning using 1,2,4,5-benzene tetracarboxylic dianhydride (PMDA), p-phenylenediamine (p-PDA) and 3,5-diaminobenzoic acid (DABA) as the PI monomers with a PMDA/(DABA+ p-PDA) mass ratio of 1.02:1 and different DABA/p-PDA mass ratios. The PI fibers were chemically crosslinked with 1,4-butanediol to improve their thermal stability. The PI fibers before and after chemical crosslinking were carbonized at 1400℃ for 1 h and graphitized at 2800℃ for 1 h to prepare graphite fibers. Results indicate that the crosslinking by hydrogen bonding between carboxyl groups significantly increased the thermal stability and increased the carbonization and graphitization yields of the PI fibers. The graphitization degree and thermal conductivity of the graphite fibers from the PI with carboxyl groups are higher than those from the corresponding chemically crosslinked ones. The graphitization degree and thermal conductivity increase with the DABA/p-PDA ratio. The highest degree of graphitization (97.9%) was obtained at a DABA/p-PDA ratio of 5:95 while the highest thermal conductivity (245.6 W/(m·K)) was obtained at a DABA/p-PDA ratio of 10:90.