Hierarchical structure of carbon nanotube fibers, and the change of structure during densification by wet stretching

Carbon nanotube (CNT) fibers were strengthened by wet stretching with 1-Methyl-2-pyrrolidinone (NMP) or chlorosulfonic acid (CSA) as the wetting liquid, and the change of hierarchical structure of CNT fibers was observed at a nanometer level. Two kinds of voids were present in a CNT fiber: inter-bundle void and intra-bundle void. Wet stretching was a very effective method to aggregate CNT bundles and to remove the voids in a CNT fiber. CNT bundle thickness and the number of CNTs in a bundle increased and the bundle-to-bundle distance decreased as a result of the aggregation of the bundles. After infiltration of liquids, CNTs with elliptical and non-circular cross-sections appeared and the distance between CNTs in a bundle i.e., intra-bundle voids, decreased. It was confirmed that the stretch alone did not affect the intra-bundle void. CSA infiltrated CNT bundles better than NMP did, so CSA yielded better densification than NMP. Load at failure, specific strength (SS) and tensile strength of CNT fibers increased with wet-stretching. SS of the CNT fiber increased from 0.95 to 2.19 N/tex (CSA13%). Understanding structure and morphology of CNT fibers in nanoscale will be helpful to build better strengthening strategies. (C) 2018 Elsevier Ltd. All rights reserved.