Effect of acidification conditions on the properties of carbon nanotube fibers

•Mixture acidification significantly tuned the properties of CNT fiber.•A strong, high conductive CNT fiber is obtained under optimal acidifying time and mix ratio.•The acid densified the surface of the CNT fiber.•Acidification resulted in two competitive effects on tensile properties of the fiber. Carbon nanotube (CNT) fibers prepared by dry-spun method were functionalized by mixture of nitric and sulfuric acids. The effects of acidification conditions on the electrical conductivity and tensile properties of CNT fibers were investigated. A strong, high conductive CNT fiber was obtained under the optimal mixture ratio and processing time, with a electrical conductivity and tensile strength up to 3.2×104S/m and 1103MPa, respectively. It showed that the acids densified the surface of the CNT fiber and introduced functional groups onto the tubes, both of which contributed to the conductivity improvement of the CNT fiber. The infrared spectroscopy, Raman and fracture analysis indicated that acidification process resulted in two competitive effects on the tensile properties of CNT fibers, one was the positive contribution by the enhancement of interactions between CNTs through the densification and functional groups, and the other was the negative effect due to the structural destruction of the tubes.