The introduction of asymmetric heterocyclic units into poly(p-phenylene terephthalamide) and its effect on microstructure, interactions and properties

The asymmetric 5(6)-amino-2-(4-aminobenzene)benzimidazole (PABZ) was introduced into the main chain of poly(p-phenylene terephthalamide) (PPTA) in order to modify the solubility and properties. Introduction of more than 40% PABZ into PPTA main chain can effectively improve solubility and intrinsic viscosity ([eta]) of poly(p-phenylene-benzimidazole-terephthalamide)s (PBIAs) in DMAC/LiCl solvent. Two-dimensional wide angle X-ray diffraction results indicate introduction of asymmetric poly(benzimidazole-terephthalamide) (PABI) segment disturbs the packing order along lateral direction, especially pi-pi stacking order, and when the content of PABI segment exceeds 50%, crystalline phase in PBIAs fibers is disappeared and crystalline domain orientation gradually decreases with increasing PABI segment content. However, high degree of order along fiber axis is retained due to the relative high rigidity and linearity of PABI segment. Fourier transform infrared results indicate the introduction of PABI segment strengthens hydrogen bonding interactions due to the presence of benzimidazole groups. The combined effects between chain order and hydrogen bonding interactions result in the highest tensile strength in PBIA-30 fiber as the content of PABI segment is 30%. Moreover, strong hydrogen bonding interactions are also favorable for enhancing interfacial shear strength (IFSS) and compressive strength. Meanwhile, homo-poly(benzimidazole-terephthalamide) fiber shows an IFSS 30.6% and compressive strength 33.3% higher than Kevlar 29 fiber.