Molecular Mechanics Investigations on Interfacial Properties in Nano-Materials due to Van der Waals and Electrostatic Coulombic Interactions

Abstract

Recently, it is a hot topic about the interfacial properties in various nano-materials, especially, the pull-out process of various nanofillers (e.g., carbon nanotubes (CNTs)) from various matrix phases (e.g., polymers), and the corresponding interfacial shear strength. Although some previous studies based on molecular mechanics or molecular dynamics simulations have been carried out to explore this problem, it has not been comprehensively or even correctly understood to the authors’ knowledge. In general, the pull-out force is mainly contributed by the frictional sliding force caused by various pre-formed statistical defects or mechanical interlocking, and capillary interfacial force due to van der Waals (vdW) and electrostatic Coulombic interactions. In this work, without consideration of the frictional effect, we focus on the interfacial properties and pull-out process of various nano-materials using molecular mechanics simulations due to vdW and electrostatic Coulombic interactions. Three objects, i.e., the pull-out process of some outer walls against other nested inner walls in a multi-walled carbon nanotube, the pull-out processes of a single-walled carbon nanotube from a polyethylene polymer matrix and from an alumina (α-Al2O3) matrix, respectively, are studied. The effects of some geometrical parameters of CNT, i.e., nanotube length, chirality and diameter, on this pull-out behavior are explored systematically for the first time. It is found that the energy increment between two adjacent pull-out steps, which corresponds to the pull-out force, is independent of nanotube length, but is proportional to nanotube diameter at the sliding interface. This unique characteristic indicates that the interfacial shear stress in this problem exists only at the two ends of CNT, and the conventional definition of interfacial shear strength based on the total length of nanotube is inapplicable here. Finally, a simple theory is proposed to predict this pull-out force based on the information of the nanotube diameter at the sliding interface.