Molecular dynamics modeling and characterization of graphene/polymer nanocomposites.

摘要

The current work focuses on the characterization of graphene based nanocomposites using molecular dynamic simulation and multiscale modeling approaches. Both graphene-epoxy and graphene-cellulose nanocomposites were considered in this study. A hierarchical multiscale modeling approach has been proposed using peridynamics and molecular dynamics simulation.;Firstly, the mechanical properties of crosslinked graphene/epoxy (G-Ep) nanocomposites were investigated by molecular mechanics (MM) and molecular dynamics (MD) simulations. The influence of graphene's weight concentration, aspect ratio and dispersion on stress-strain response and elastic properties were studied. The results show significant improvement in Young's modulus and shear modulus for the G-Ep system in comparison to the neat epoxy resin. It appears that the RDF, molecular energy and aspect ratios are influenced by both graphene concentrations and aspect ratios. The graphene concentrations in the range of 1-3% are seen to improve Young's modulus and shorter graphenes are observed to be more effective than larger ones. In addition, the dispersed graphene system is more promising in enhancing in-plane elastic modulus than the agglomerated graphene system. The cohesive and pullout forces versus displacements data were plotted under normal and shear modes in order to characterize interfacial properties. The cohesive force is significantly improved by attaching the graphene with a chemical bond at the graphene-epoxy interface.;In the second part of the work, cellulose was considered to study the mechanical properties of graphene-cellulose bionanocomposite. Similar to graphene-epoxy systems, the effect of graphene dispersion and agglomeration were studied in the stress-strain plots of graphene-cellulose system. A pcff forcefield was used to define intermolecular and intramolecular interactions. The effect of graphene's aspect ratio and weight concentration on the structural property of each unitcell was analyzed in terms of the radial distribution function (RDF), molecular energy, pairwise bond stretch and angle bending. The interfacial properties between graphene and cellulose were studied by analyzing both cohesive and pullout separation of graphene from cellulose matrix. Finally, the Young's modulii calculated from the MD simulation was compared with the tensile test data. The MD results showed a reasonable agreement with the tensile test results. It was addressed that incorporating graphane in cellulose matrix enhances the mechanical property of the cellulose based bio-polymer systems.;In the third part of the work, a hierarchical multiscale modeling framework was established between peridynamics and molecular dynamics simulation using an intermediate coarse grained atomic model. The peridynamics formulation is based on continuum theory implying nonlocal force based interaction. It means, continuum points are separated by a finite distance and exert force upon each other. Peridynamics applies integral equations rather than partial differential equations as used in the classical continuum mechanics. Hence, the peridynamics (PD) and the molecular dynamics (MD) have similarities since both use a nonlocal force based interaction. In this work PD based continuum model of graphene-epoxy (G-Ep) nanocomposite is defined by the Lagrangian PD particles. Atomistic model is coupled with PD model through a hierarchical multiscale framework. The PD particles at a coarse scale interact with the fine scale PD particles by transferring pressure, displacements and velocities among each other. Based on the same hierarchical coupling method, a fine scale PD model is seamlessly interfaced with the atomistic model through an intermediate mesoscale region i.e. coarse-grain model. At the end of this hierarchical downscaling, the information such as the deformation, energy and other important parameters were captured in the atomistic region under the applied force at micro and macro regions. The results from peridynamic based framework for hierarchical multiscale modeling showed reasonable agreement between PD and atomistic models. (Abstract shortened by UMI.).