Recent studies of single-walled carbon nanotubes (CNTs) in aqueous media have showed that water can significantly affect the mechanical properties of the tubes [1, 2]. CNTs under hydrostatic compression can preserve their elastic properties up to large pressure values, while exhibiting exceptional resistance to mechanical loadings.
It was experimentally observed that CNTs with encapsulated carbon linear atomic chains (CLACs), when subjected to high hydrostatic pressure values, present irreversible red shifts in some of their vibrational frequencies. In order to address this phenomenon, we have carried out fully atomistic reactive (ReaxFF)  molecular dynamics (MD) simulations.
We have considered the cases of finite and infinite (cyclic boundary conditions) CNTs with encapsulated CLACs of different lengths (from 9 up to 40 atoms). Our results show that for pressure around 10 GPa causes the coalescence of two chains. The calculated vibrational spectrum shows that the longer chains (after coalescence) present a smaller resonant frequency, consistent with experimental observations. Also we show that for extreme high pressures (up to 10 times higher than the experimental one) the CNTs are deformed in an inhomogeneous way due to the CLAC presence. The CLAC/CNT interface regions exhibit convex curvatures, which results in more reactive environments , thus favoring the formation of covalent bonds between CLACs and CNTs. This process is irreversible with the new formed covalent bonds continuing to exist even when the external pressure is released. A red shift from 1850 to 1750 cm−1 in the C-C mode of the chain was calculated, thus suggesting new interaction between chains and carbon nanotubes.
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