Synergistic molecular design via dynamic covalent bonds and π–π stacking in waterborne polyurethane sizing coating for high–performance carbon fiber composites

Waterborne polyurethane (WPU) coatings have emerged as a preferred alternative to epoxy–based coatings for carbon fiber (CF) sizing, owing to their superior surface adhesion and environmental sustainability. However, conventional WPUs often suffer from insufficient mechanical strength, limited thermal stability, and inadequate adhesion. To address these limitations, molecular design and modification of WPU are imperative. Herein, a polyetheramine diol (PHBA–D1000) was synthesized via aldol condensation and Schiff base formation between 4–hydroxybenzaldehyde and Jeffamine D1000. This diol, along with co–monomers 4–amino–1–butanol, o–anisidine, and 2–aminophenol, was employed to fabricate modified WPUs (MPWPUs). The molecular architecture of the resulting MPWPUs incorporates dynamic reversible imine bonds, π–π conjugated stacking, aromatic ether linkages, polyurea, carbamate groups, and long alkyl chains. The synergistic interplay between the polar and rigid segments endowed the MPWPUs with enhanced thermal stability (T10 % at 331.2 °C), tensile strength (36.4 MPa), elastic modulus (14.2 MPa), and adhesion strength (21.9 kPa). Applying MPWPU as sizing coatings on CF significantly improved the fiber bundling capability, surface reactivity, wettability, and mechanical properties. The surface energy and tensile strength of MPWPU–coated CF monofilaments increased to 57.1 mN/m and 6.121 GPa, respectively, representing remarkable improvements of 57.3 % and 18.1 % compared to unsized CF. MPWPUs effectively repaired defects and microcracks on CF surfaces, thereby enhancing surface roughness. These findings underscore the exceptional performance of the developed MPWPUs and their considerable potential for advanced CF composite applications.