Graphyne is a planar two-dimensional carbon allotrope formed by atoms in sp, sp2, and sp3 hybridized states. Topologically graphyne nanotubes (GNTs) can be considered as cylindrically rolled up graphyne sheets, similarly as carbon nanotubes (CNTs) can be considered rolled up graphene sheets. Due to the presence of single, double, and triple bonds, GNTs exhibit porous sidewalls that can be exploited in many diverse applications. In this work, we investigated the mechanical behavior of GNTs under torsional strains through reactive molecular dynamics simulations. Our results show that GNTs are more flexible than CNTs and exhibit “superplasticit”, with fracture angles that are up to 35 times higher than the ones reported to CNTs. This GNT “superplastic” behavior can be explained in terms of irreversible reconstruction processes (mainly associated with the triple bonds) that occur during torsional strains.
The structures considered in the present work were armchair and zigzag α-GNT and γ-GNT , with tube lengths varying from 55 up to 193 Å, and with diameter values from 9 up to 69 Å. These structures are representative of the diverse structural and electronic GNT behaviors. For comparison purposes, similar CNT structures (in terms of length, diameter, and chirality values) were also considered. The torsional tube properties were obtained from the analysis of molecular dynamics simulations carried out using the LAMMPS code, and with the atomistic interactions described by the reactive force field ReaxFF. Charge distributions were calculated based on geometry and connectivity using the electronegativity equalization method. The behavior of the GNT under mechanical twisting and the torsional stiffness values were obtained from the analysis of the second derivative of the torsional strain energy as a function of the torsion angle θ, calculated around the equilibrium angles. The energy variation was calculated subtracting the current total energy of the twisted tube from the total energy of the original non-twisted tube.