Surface modification of PBO fibers with random copolymer containing benzoxazole for improving surface activity, and enhancing interfacial bonding strength with cyanate ester resins

Poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers present excellent comprehensive properties, attracting considerable attention and favor from researchers. However, the surface chemical inertia of PBO fibers limits their application in advanced composites. In this work, random copolymers (P(S-co-BCB-co-prePBO)) containing benzoxazole precursor in side chains are synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, which are then utilized to form a polymer coating on PBO fibers by thermally cross-linking and thermal cyclization processes (PBO@PM fibers). Due to the strong π-π interactions between the random copolymer and PBO fibers, the surface roughness and surface activity of PBO@PM fibers are significantly improved, and the crystal structure are not damaged before and after modification. The surface roughness is increased from 10 nm (PBO fibers) to 68 nm, while the surface activity is increased from 39.2 mN∙m⁻¹ (PBO fibers) to 53.7 mN∙m⁻¹. Meanwhile, PBO@PM fibers present excellent interfacial bonding strength with bisphenol A cyanate (BADCy) resins. The single fiber pull-out strength of PBO@PM fibers/BADCy microdroplet composite is 5.1 MPa, 70% higher than that of PBO fibers/BADCy microdroplet composite. In general, the modification method effectively improves the surface activity of PBO fibers, further expanding their applications in the industrial sector.