Characterizing elastic properties of carbon nanotubes/polyimide nanocomposites using multi-scale simulation

This research is aimed at characterizing the elastic properties of carbon nanotubes (CNTs) reinforced polyimide nanocomposites using a multi-scale simulation approach. The hollow cylindrical molecular structures of CNTs were modeled as a transverse isotropic solid, the equivalent elastic properties of which were determined from the molecular mechanics calculations in conjunction with the energy equivalent concept. Subsequently, the molecular structures of the CNTs/polyimide nanocomposites were established through molecular dynamics (MD) simulation, from which the non-bonded gap as well as the non-bonded energy between the CNTs and the surrounding polyimide were evaluated. It was postulated that the normalized non-bonded energy (non-bonded energy divided by surface area of the CNTs) is correlated with the extent of the interfacial interaction. Afterwards, an effective interphase was introduced between the CNTs and polyimide polymer to characterize the degree of non-bonded interaction. The dimension of the interphase was assumed equal to the non-bonded gap, and the corresponding elastic stiffness was calculated from the normalized non-bonded energy. The elastic properties of the CNT nanocomposites were predicted by a three-phase micromechanical model in which the equivalent solid cylinder of CNTs, polyimide matrix, and the effective interphase were included. Results indicated that the longitudinal moduli of the nanocomposites obtained based on the three-phase model were in good agreement with those calculated from MD simulation. Moreover, they fit well with the conventional rule of mixture predictions. On the other hand, in the transverse direction, the three-phase model is superior to the conventional micromechanical model since it is capable of predicting the dependence of transverse modulus on the radii of nanotubes.