Study on the thermal imidization process of the PI fibers containing BIA monomers

This study selected three monomers, 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BPDA), p-phenylenediamine (PDA), and 2-(4-amino-phenyl)-5-amino-benzimidazole (BIA), to prepare BPDA-PDA and BPDA-PDA-BIA (mole ratio PDA:BIA=3:7) polyimide (PI) fibers through a two-step method and dry-jet wet-spinning process. The study investigated the effect of BIA monomer on the imidization process of polyamic acid (PAA) fibers and the mechanical properties of the final PI fibers. The evolution of fiber structure during the thermal imidization process was analyzed through TGA, TMA, DMA, FTIR and XRD. Kinetic analysis was made to provide the apparent activation energies, the reaction orders and the model functions of the imidization reaction. The research showed that the copolymerization of BIA introduced significant hydrogen bonding between the macromolecules. In the early stage of the imidization reaction, effect of hydrogen bonding was not significant. As imidization conversion increases, hydrogen bonding not only significantly hindered the imidization reaction but also hindered the formation of crystals inside PI fibers. The crystallization process had to be carried out at much higher temperatures. Mechanical properties of the fibers can be significantly improved as the BIA introduced into the polymer chains.