Chemical modification of poly(p-phenylene benzobisoxazole) with naphthalene moieties for enhanced ultraviolet aging resistance

Poly(p-phenylene benzobisoxazole) (PBO) fibers are known for their outstanding mechanical and thermal properties, which stem from their fully aromatic rigid-rod molecular chains and highly ordered multi-scale architecture. Nevertheless, the presence of oxazole rings in the backbone renders it susceptible to photochemical oxidative degradation under ultraviolet (UV) irradiation, resulting in a marked deterioration of mechanical performance and restricting its long-term use in outdoor environments. To address this issue, in this work, we introduce a molecular design strategy by substituting conventional phenolic monomers with naphthalene-based ones. This substitution preserves the intrinsic rigid conjugated backbone of PBO while replacing the vulnerable oxazole moiety with a more stable six-membered ring structure. The resulting naphthalene-based PBO (Naph-PBO) fibers retain high strength, modulus, and thermal properties comparable to those of pristine PBO fibers, while demonstrating enhanced UV stability. After 240 h of UV exposure, the Naph-PBO fibers retained approximately 85 % of their mechanical properties in comparison to the 65 % retention observed for PBO fibers. Comprehensive characterization via X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (¹H NMR), and scanning electron microscopy (SEM) verified the successful synthesis of naphthalene-based monomers and the structural integrity of the corresponding fibers. This study presents a new intramolecular structure-modification approach to reconcile the trade-off between UV stability and comprehensive performance in PBO fibers.