Giant Stretchability and Reversibility of Tightly Wound Helical Carbon Nanotubes

There
is a surging interest in 3D graphitic nanostructures which
possess outstanding properties enabling them to be prime candidates
for a new generation of nanodevices and energy-absorbing materials.
Here we study the stretching instability and reversibility of tightly
wound helical carbon nanotubes (HCNTs) by atomistic simulations. The
intercoil van der Waals (vdW) interaction-induced flattening of HCNT
walls prior to loading is constrained by the defects coordinated for
the curvature formation of helices. The HCNTs exhibit extensive stretchability
in the range from 400% to 1000% as a result of two distinct deformation
mechanisms depending on the HCNT size. For small HCNTs tremendous
deformation is achieved by domino-type partial fracture events, whereas
for large HCNTs this is accomplished by stepwise buckling of coils.
The formation and fracture of edge-closed graphene ribbons occur at
lower temperatures, while at elevated temperatures the highly distributed
fracture realizes a phenomenal stretchability. The results of cyclic
stretching-reversing simulations of large HCNTs display pronounced
hysteresis loops, which produce large energy dissipation via full
recovery of buckling and vdW bondings. This study provides physical
insights into the origins of high ductility and superior reversibility
of hybrid CNT structures.