The Role of H₂O-Stabilized Benzimidazole Tautomerism in the Anomalous Wet-Aging Behavior of Heterocyclic Aramid Fibers

Most high-performance organic polymers are susceptible to wet-aging and degradation in mechanical properties, affecting their long-term service. Herein, we unexpectedly discover that the tensile strength of heterocyclic aramid fibers poly(p-phenylene-benzimidazole-terephthalamide) (PBIA) with the benzimidazole structure increases over time in a humid environment, showing a 17.2% enhancement even after 8 years, which is absent in poly(p-phenylene terephthalamide) (PPTA) without the benzimidazole structure. To explore the reasons, the monomer structure characterization results indicate that 6-amino-2-(4-aminobenzene)benzimidazole (6-PABZ) exhibits lower potential energy and superior planarity compared to those of 5-amino-2-(4-aminobenzene)benzimidazole (5-PABZ). But the potential energy difference is small, allowing for configuration tautomerism under ambient conditions. When crystallization or H2O complexation restricts their tautomerism, the predominant 6-configuration becomes stabilized. Subsequently, fresh PBIA fibers are prepared and subjected to hot-drawing and hydrothermal vapor treatment. Fiber structure and property characterization results show that hot-drawing leads to some crystalline regions with a lattice-stabilized six-configuration benzimidazole structure, yet free-converting benzimidazole 5/6-configurations remain in amorphous regions. After hydrothermal treatment, H2O infiltrates the amorphous regions and partly stabilizes the benzimidazole structure to the superior 6-configuration through interaction, enhancing the orientation of molecular chains in the amorphous regions. The reinforcement of weak amorphous regions after hydrothermal treatment results in a rapid 18.9% increase in tensile strength, along with long-term stability. The increase rate is correlated with the aging time, as well as the content of the benzimidazole structure and amorphous regions.