Tailored twisted CNT bundle with improved inter-tube slipping performances

The exceptional mechanical properties of carbon nanotubes (CNTs) have encouraged the development of high-performance synthetic fibers in composite materials. To understand the effect of twisting on the mechanical and slipping performances of CNT bundles, molecular dynamics simulation is applied to investigate the tensile performances, failure modes, and pull-out responses of twisted CNT bundles. A molecular model comprising nineteen parallel aligned single-walled carbon nanotubes (SWCNTs) is twisted into bundles at angles of 0 degrees, 10 degrees, and 20 degrees, and further tensile and pull-out simulations are performed. The tensile simulation indicates that compared with the non-twisted CNT bundle showing a tensile strength of about 82 GPa with obvious inter-tube slipping, the 10 degrees-twisted bundle exhibits a tensile strength of approximately 70 GPa with SWCNTs fracture as the main failure mode, which indicates that twisting improves the inter-tube slipping performance without causing excessive strength reduction. Comparatively, when the twisting angle is 20 degrees, no inter-tube slipping is observed and the tensile strength of the CNT bundle is measured to be 55 GPa, which decreases by approximately 32.9 %. The pull-out simulations further reveal that the pull-out forces increase as the twisting angle increases and the weakened bundle strength of twisted bundle is attributed to the repulsive van der Waals forces caused by the reduced distances between inter-tubes. Essentially, twisting is unfavorable for the overall mechanical strength while torsional densification mitigates the inter-tube slipping, which indicates that a trade-off need to be achieved. This paper provides insights into the tensile and slipping performances of twisted CNT bundles and forms a basis for enhancing the assembled CNTs bundle as the next-generation reinforcing phase in composite materials.