Temperature-dependent interfacial adhesion between polyetherimide and carbon fiber: Mechanistic insights and optimization strategy

The objective of this study was to elucidate the interfacial adhesion and its underlying mechanism between carbon fiber (CF) and polyetherimide (PEI), which is employed either as a sizing agent to enhance the interfacial adhesion between polyaryletherketone (PAEK) and CF, or as a matrix in carbon fiber-reinforced thermoplastics (CFRTPs). It was revealed that the interfacial adhesion is highly sensitive to the bonding temperature, and that a specific thermal history at an optimal temperature (∼370 o C) leads to maximum adhesion. The tensile properties of PEI/CF composites prepared using a twin-screw extruder approximately followed the same trend as the interfacial shear strength with respect to molding temperature. Rheological analysis and electron spin resonance (ESR) of PEI, X-ray photoelectron spectroscopy (XPS) of CF surfaces extracted from PEI/CF composites, and interfacial adhesion tests using model CFs with low surface chemical activity collectively demonstrated that radicals generated in molten PEI contribute to the formation of covalent bonds with functional groups on the CF surface. At bonding temperatures exceeding 390 o C, these radicals preferentially underwent recombination within PEI chains, thereby reducing their availability for interaction with the CF surface. The findings of the present study underscore the crucial role of adequate temperature regulation at each stage of the manufacturing process—namely, compounding and fusion, impregnation, and consolidation—in achieving PEI/CF composites with superior mechanical performance. • PEI/CF interfacial strength exhibits a convex dependence on bonding temperature. • Wettability of PEI on CF decreases as bonding temperature increases. • π– π and radical interactions govern interfacial strength on CF surfaces. • An optimal processing temperature yields superior mechanical properties in PEI/CF.