Comparison of high-performance polyimide copolymer fibers containing pyrimidine moieties based on coplanar structures. (28th September 2021) - Bao, Feng
Abstract: The effects of molecular coplanar structure on crystallinity, orientation, and overall performances of polyimide (PI) copolymer fibers, especially mechanical properties, were investigated. In detail, three diamines with similar structures, including 4, 4′-diaminobenzophenone (DABP), 4, 4′-diaminobenzanilide (DABA), and benzidine (Bz), were introduced as third monomers into the classical heterocyclic 3, 3′, 4, 4′-biphenyltetracarboxylic dianhydride (BPDA)/2, 5-bis(4-aminophenyl)pyrimidine (PRM) system, eventually the high-performance PI fibers being fabricated successfully via dry-jet wet spinning process. The conformation simulations illustrated the different coplanar levels found in three similar diamines: DABP (worst), DABA (better), and Bz (best). Thus, the BPDA/Bz dimer showed excellent coplanar structures, and the dihedral angle of the farthest benzene rings was only 4.42°. The wide-angle X-ray diffraction patterns indicated that the crystallinity and orientation of fibers increased with the increase of the coplanarity degree and molecular packing. Interestingly, the Bz-PI fiber possessed the strongest mechanical properties, i.e., fracture strength of 4.01 GPa and initial modulus of 137.34 GPa, which was primarily contributed by optimal molecular coplanarity and highest orientation. Graphical abstract: Image 1 Highlights: High-performance polyimide copolymer fibers were fabricated via molecular coplanar structure design. Both H-bonding interactions and molecular coplanarity effectively enhance the packing coefficient of molecular chains. Better molecular coplanar structures are beneficial to increase the crystallinity and orientation of fiber. The optimal fracture strength and initial modulus reach 4.01 and 137.34 GPa, respectively, corresponding to Bz-PI fiber.