Regulating Cu(II)-benzimidazole coordination structure in rigid-rod aramid fiber and its composites enhancement effects

Coordination crosslinking, as the kind of strongest supramolecular interaction, has been widely introduced into flexible polymer chains, including rubbers and aerogels, while that in rigid-rod polymer is much rare, due to the difficulty in rigid-rod macromolecules' configuration change. Moreover, the exact coordination structure inside those polymers is not clear. Herein, this coordination interaction was incorporated into a traditional rigid-rod polymer, aramid fiber, by Cu2+ -benzimidazole coordination reaction, through the careful control of the dynamic reaction conditions, and the structural/performance relationships of fiber and its composites are detailed studied. It is seen that two different coordination structures, S-coordination style (Cu2+ coordination with single benzimidazole unit) and M-coordination style (Cu2+ coordination with multiple benzimidazole units from different macromolecular chains), could be both obtained inside fiber, using different solvents as the reaction medium. Moreover, using reaction kinetics equilibrium, we presented a mathematical method to investigate those two coordination structures in detail, including their average coordination numbers of Cu2+ and reaction equilibrium constant (K). Further, the experimental results show that the fiber with average coordination number of 2 exhibits a comprehensive improved performance in heat/solvent resistance, fluorescence emission as well as fiber's transverse properties, with the corresponding increase of 54.2% and 47.1% for its composites interfacial properties and compressive strength, compared with untreated fiber.