Comparative study of chemically grafted vs. physically coated CNTs on fracture toughness of BMI/CF laminates

This study investigates the interlaminar toughening of bismaleimide/carbon fiber (BMI/CF) composite laminates using modified carbon nanotubes (CNTs) deposited via spray coating technology. Pristine CNTs were functionalized through chemical modification (grafting 5-aminoisophthalic acid small molecules to yield CNT-COOH) and physical modification (coating with a layer of polydopamine to produce CNT@PDA) to enhance dispersion within the composite matrix. While both modification methods improved the dispersion of CNTs, their thermal stabilities during composite molding led to contrasting toughening outcomes. Chemically grafted CNT-COOH demonstrated superior performance, achieving a 20 % increase in mode I fracture toughness (G IC ) and a 32.37 % increase in mode II fracture toughness (G IIC ). In contrast, physically coated CNT@PDA exhibited reduced toughening efficacy compared to the baseline in both mode I and mode II fracture toughness. This can be attributed to the poorer thermal stability of the physically coated CNT@PDA than that of chemically grafted CNT-COOH. On one hand, the coating material PDA itself has poor thermal stability which is highly prone to decomposition during high-temperature molding. On the other hand, it can be influenced by the methods of CNT modification. The bonding mechanisms of chemical grafting (covalent bonds) and physical coating (van der Waals forces and other physical interactions) differ, leading to varying stability under high temperatures. The decomposition of the coating material and the high-temperature failure of van der Waals forces diminished surface functional groups and introduced defects around the CNTs, significantly compromising interlaminar fracture toughness. The findings underscore the critical role of thermal stability in determining the effectiveness of nanomodification strategies for advanced composite materials.