The mechanical behaviour of confined carbyne chain, the thinnest structure in nature

Carbyne chains are the thinnest structures found in nature. The synthesis of long and stable inside carbon nanotubes has recently drawn renewed attention to this linear one-dimensional carbon allotrope. Carbyne’s mechanical properties have been predicted to exceed that of carbon nanotubes and graphene, making it a very suitable structural component for many nanoscale applications. While carbyne’s mechanical behavior under tensile loading is superlative, this linear chain of bonded carbon atoms readily buckles due to thermal fluctuations, showing minimal strength under compressive loading. Here we present a detailed study on the enhancement of carbyne’s mechanical properties under compression by confinement in small diameter carbon nanotubes. First, we develop a new classical empirical forcefield based on ab-initio calculations. Molecular dynamics (MD) simulations are then carried, using this forcefield, to determine the mechanical properties of carbyne under tensile loading, namely to assess their dependence on chain length and temperature. The bending rigidity of carbyne and its persistence length are also calculated. After the validation of the new forcefield, MD simulations are employed to study the mechanical behavior of carbyne under compressive loading, when confined inside (5,5), (6,6) (7,7) and (8,8) CNTs. It is found that the mechanical behavior of confined carbyne chains depends on the confinement radius and the chain length, and can be described in three stages. For “tight” confinements, carbyne chains assume a rigid rod behavior, allowing the calculation of compressive mechanical properties and attesting an effective “bracing” effect. For “looser” confinements, a spring like behavior arises which can be modeled by Hooke’s law. In the third stage confined carbyne chains tend to buckle into a large local bend, losing the elastic flexibility that characterizes the previous stage.