  Recent studies of single-walled carbon nanotubes (CNTs) in aqueous media have
showed that water can significantly affect the tube mechanical properties. 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 linear
carbon chains (LCCs), when subjected to high hydrostatic pressure values,
present irreversible red shifts in some of their vibrational frequencies. In
order to address the cause of this phenomenon, we have carried out fully
atomistic reactive (ReaxFF) molecular dynamics (MD) simulations for model
structures mimicking the experimental conditions. We have considered the cases
of finite and infinite (cyclic boundary conditions) CNTs filled with LCCs (LCC
inside CNTs) of different lengths (from 9 up to 40 atoms). Our results show
that increasing the hydrostatic pressure causes the CNT to be deformed in an
inhomogeneous way due to the LCC presence. The LCC-CNT interface regions
exhibit convex curvatures, which results in more reactive sites, thus favoring
the formation of covalent chemical bonds between the chain and the nanotube.
This process is irreversible with the newly formed bonds continuing to exist
even after releasing the external pressure and causing an irreversibly red
shift in the chain vibrational modes from 1850 to 1500 cm$^{-1}$.
