Measuring the radial elasticity of carbon nanotube yarns.

Carbon nanotube yarn, one of the most important macroscopic architectures of carbon nanotubes, displays ultrahigh stiffness in axial direction yet remarkable softness in the radial deformation. However, in contrast to the axial mechanical behavior that has been extensively studied, the mechanical response to load applied in the radial direction, which is of fundamental importance to perform the material functions, was less explored. In this work, an experimental approach is developed to measure the radial elastic modulus of the carbon nanotube yarns with microscopic mechanisms elucidated by theoretical analysis. It is found that the measured radial elastic modulus increases with the increasing of the cycle loading number and the twist density of the carbon nanotube yarns, suggesting the tunability of the radial elasticity of carbon nanotube yarn by mechanical loading. Furthermore, a hierarchical model is proposed to explain the abovementioned structure-property relationship, revealing that the curvy, entangled, locally bundling nature strongly modulates the radial elastic modulus of the carbon nanotube yarns. This work not only provides essential insight in the radial deformability of carbon nanotube yarns, but also develops a method that could be applied to measure the radial mechanical properties of other micro-fibers