Molecular Dynamics Simulation of a Polyamide-66/Carbon Nanotube Nanocomposite

Abstract

Atomistic molecular dynamics simulations are performed on oligomeric polyamide-6,6 chains, composed of 10 chemical repeat units, at a carbon nanotube (CNT) interface. The effect of surface curvature on the structure and dynamics of polymer is studied by simulating systems containing CNTs of various diameters. It is shown that polymer at the CNT interface organizes into layered structures. The hydrogen bonding in the polymer is influenced by the CNT surface. In proximity to the CNT surface, the hydrogen bonds (HBs) are weaker than the corresponding unperturbed bonds and their density is lower than that of the bulk sample. On the other hand, over the region where organized layered structures are formed, stronger HBs with a higher density than that of the bulk sample are found. An analysis of chain orientation at the interface shows that the monomers (repeat units) very close to the CNT surface wrap around the tube. However, at distances corresponding to higher densities than the bulk density, the segments orient parallel to the CNT axis (along the CNT). The wrapping costs higher energies in the case of smaller diameter CNTs (more curved surfaces). It is shown that while the CNT surface perturbs the local chain properties up to a distance of ∼2 nm from the surface, perturbation in the global chain properties, such as the radius of gyration, extends to farther distances (a few times the radius of gyration of the unperturbed chain, R 0 g ). The chain translation at the interface is found to be anisotropic, depending on the surface proximity and surface curvature. This is due to the formation of extended conformations (along the CNT), facilitating smoother chain translation parallel to the CNT surface, compared to that in the radial direction. The magnitude of dynamics deceleration caused by the CNT surface depends on the surface proximity, surface curvature, and the time scale of the unperturbed dynamical property of interest. The dynamics decelerates more in the case of long-time dynamical properties (in the bulk) for chains at closer distances to flatter CNT surfaces. While the ratio of relaxation time to the corresponding bulk quantity for HB formation/rupture is increased by a factor of 3 in a cylindrical shell of thickness 0.6 nm around the flatter surface studied in this work, the above ratio for the decorrelation of chain's end-to-end vectors is increased by 3 orders of magnitude in a cylindrical shell of thickness 1.0 nm on the same surface. In this respect, the interphase thickness depends on the time scale of the dynamical property of interest. Our observations show that the surface effect on a short-time dynamical property, like the HB dynamics, extends to distances as long as 1.5 nm, while it extends to a few R 0 g (from the surface) for a long-time dynamical property, like the relaxation of the entire chain expressed in terms of chain's end-to-end vector relaxation.

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