Influence of Carboxylation on Structural and Mechanical Properties of Carbon Nanotubes: Composite Reinforcement and Toxicity Reduction Perspectives

Carboxylation of carbon nanotubes (CNTs) is an important process that is applied routinely for various applications, in particular for biomedical usage and the manufacturing of next-generation composite materials. This study investigates the influence of carboxylation on the structural and mechanical properties of CNTs. Ab initio calculations were performed within the density functional theory framework for metallic and semiconducting single- and multiwalled CNTs, imperfect and carboxylated at various concentrations, including disorder. The morphologies were analyzed, the stabilities of the carboxylated CNTs were determined, relevant electronic properties were evaluated, and elastic moduli were calculated (Young’s, shear, and bulk moduli, as well as Poisson’s ratio). The properties of grafted (—COOH, —OH, ═O) and imperfect (vacancy defects) CNTs were compared with those of carboxylated CNTs. In particular, both the structural and elastic properties were found to exhibit significant differences between —COOH-grafted and carboxylated systems, with the dependence of the Young’s modulus on the concentration of functional groups or vacancies being the most striking. Experimental observations showing that small concentrations of —COOH groups improve the strength of CNTs in comparison to that of defected CNTs are explained. The critical concentration above which the carboxylation of CNTs leads to significant structural changes of the nanotubes was determined. These changes were found to result in a significant decrease of the Young’s modulus, making the CNTs unsuitable as composite reinforcements. Moreover, mechanisms governing the toxicity reduction of carboxylated CNTs are explained.