Study on Post Treatment to Improve Tensile Strength of Carbon Nanotubes Fiber from Carbon Nanotubes Aerogel

Abstract

Assembling individual carbon nanotubes (CNTs) into a long CNT fiber provides a promising way to exploit the exceptional mechanical and electrical properties of CNT in a macroscopic world. By fabricating short CNTs into a long CNT fiber, outstanding properties of CNTs can be utilized in various applications. Since the first introduction of CNT fiber, there has been increasing research interest on the synthesis and application of CNT fiber. However, as of now, the mechanical properties of CNT fiber are lower than that of carbon fiber, which is a front-runner in the next-generation material market. Thus, researches for the improvement of mechanical properties of CNT fiber are imperative. Among the various methods to synthesize CNT fiber, spinning CNT fiber directly from CNTs aerogel is widely preferred over other methods in terms of productivity due to the simplicity of spinning process form aerogel. However, CNT fiber from this method intrinsically contains impurities such as catalyst and promotor. To reduce such impurities without loss of productivity, CNT fiber directly spun from CNTs aerogel was treated by acid mixture. However, the acid treatment not only reduced impurities in the CNT fiber, but also generated functional groups such as carboxylic acid on the CNT wall. It is known that these imperfections result in the reduction of the tensile strength of the CNT fiber. A rational approach to circumvent this drawback of the acid treatment is to use the functional groups for a cross-linking reaction. The cross-linking reaction which uses carboxylic acid created during acid treatment was applied to introduce covalent bonding between individual CNTs for the improvement of tensile strength of CNT fiber. As a result of the cross-linking reaction, the specific strength increased from 0.43 to 1.12 N/tex, and Young’s modulus was enhanced from 30.70 to 47.57 N/tex. Supporting weak shear interaction of untreated CNT fiber by adding covalent bonding through the cross-linking reaction led to the improvement of mechanical properties of the CNT fiber. The mechanical properties of CNT fiber were also enhanced by diminishing empty space between CNT bundles. CNT fiber reinforced by polyimide (PI) was synthesized from the process of impregnation of poly(amic acid) (PAA) and imidization of PAA to PI. Reinforced CNT fiber showed 1.7 and 5.3 fold increase of specific and tensile strength, respectively. Both densification effect by solvent evaporation and adhesive property of PI contributed to the enhancement of mechanical properties of CNT fiber.