Characterization of a graphene oxide/poly(acrylic acid) nanocomposite by means of molecular dynamics simulations

Abstract

A graphene oxide/poly(acrylic acid) nanocomposite in the melt state was studied by means of fully atomistic molecular dynamics (MD) simulations. The mixture was characterized in a wide temperature range in terms of its thermal behavior and the static and dynamic properties of the polymeric material. In addition, the formation of the intra- and intermolecular hydrogen bonding network was examined in a quantitative manner, and the longevity of the formed hydrogen bonds was estimated. It was found that part of the graphene oxide (GO) flakes formed oligomeric clusters that were dispersed in the polymeric matrix. The presence of GO at the examined composition resulted in a moderate shift of the glass transition temperature of the composite with respect to that of the pristine polymer. The physical adsorption of the polymer chains onto the GO surface was found to be driven mainly by a specific type of hydrogen bond, while the presence of GO and the temperature change affected the amount of hydrogen bonding among the poly(acrylic acid) (PAA) chains. Polymer dynamics were slowed down appreciably due to the presence of GO, both at the local and at the entire molecular scale. The degree of slowing down for local reorientational motions was examined as a function of the distance from the GO flakes, assessing thus the effect of the physical adsorption and of the effective confinement of polymer chains within the GO clusters. Elucidation of the microscopic characteristics of the resulting morphology and of the correlation between structure and dynamic response of the components, offers a first step for the interpretation of the observed macroscopic behavior of such nano-hybrid materials.