Computer simulations of the early stages of crystal nucleation of linear and short chain branched polyethylene on carbon nanotubes

Abstract

We have used molecular dynamics simulations to study the process of macromolecular organisation of single chain linear and branched polyethylene on the surface of carbon nanotubes. These systems can be considered as good models for the study of the mechanism of polymer folding at the early stage of crystal formation on carbon nanotubes. The mean-squared radius of gyration, the stem length, the bond orientation order parameter and the radial distribution have been used to describe the organisation process and the structure of the chains at the carbon nanotube surface. When compared to the process of organisation of isolated single chains, interesting results are observed: (i) two different mechanisms of chain organisation give rise to different morphologies; lamellar thickening and lateral crystal growth lead to mono- and multilayered structures, independently of the presence of short chain branching; (ii) lamellar thickening is however hindered, but still present, in the case of polymer chains with short chain branches; (iii) both the stem length and the order parameter increase in the nanocomposites with respect to those obtained for isolated chains under the same conditions; (iii) the reorganisation process of thickening is accelerated by the presence of the carbon nanotubes, which act as nucleating agents; and (iv) the presence of short chain branching in polymer chains delays the onset of nucleation and growth of the crystalline structure, suggesting that the process is quite sensitive to the local chain chemistry.