Molecular Simulations Of Distribution Of Graphene In Composites And Effect On Mechanical Properties

Incorporating graphene in polymer materials as nanofiller can enhance the mechanical,electrical,and thermal properties of the polymer.Graphene-based composites have found applications in many industrial fields.However,graphene sheets tend to irreversibly agglomerate,or restack to form graphite,due to the strong interactions between two graphene sheets,including van der Walls forces and strong π-π interactions.The poor graphene dispersion in a matrix strongly limits the performance and the application of graphene-base composites.Studies on the micro-distribution behavior of graphene are scarce,and the simulation models and methods are usually simple.The effects of graphene dispersion on the structures and properties of composites are not clear yet.In the present work,molecular dynamics simulations are used to explore the micro-behavior of graphenes in polyethylene matrix and effects on the composite structures.In addition,uniaxial tensile simulations are used to investigate the influence mechanism of graphene dispersion on the composite mechanical properties.Firstly,a polyethylene matrix containing two graphene sheets is built.Three initial graphene structures are considered to investigate the micro-behavior of graphenes in host matrix,including parallel graphene structures,a half-contact graphene structure,and overlapped graphene structures.The stable distributions of graphenes are also obtained.The results show that the graphene sheets aggregate in host matrix when portions of one graphene sheet are in contact with another sheet.The adsorption layer formed on the graphene surface leads to an energy barrier which inhibits the graphene aggregation process when the two sheets are totally separated.The aggregation rate for two parallel graphene sheets in half contact with one another is constant,about 0.94 ?/ns,where host molecules are being displaced during aggregation.Two partially overlapped graphene sheets of equal length in close contact with one another will translate parallel to the plane of each sheet until the edges of both align and the two sheets are completely stacked.The rate of sheet realignment decreases as the overlap region between plates increases.The uneven ends of graphene sheets play an important role in the dispersion of graphene sheets,since the ends govern when host molecules enter into or diffuse out of the interlayer spacing.The models of the lateral plane ends of graphene sheets are studied.The results are consistent with the results of the former models with two graphene sheets.When the distance between two graphene sheets is short,the sheets can expel the host molecules between them and aggregate.However,when the distance between two sheets is large,the graphene ends keep separated.The host molecules form layers in the spacing between the sheets,where the molecules align parallel to the graphene surface.The average distance between the graphene sheets can only take on specific discrete values,because only an integer number of adsorption layers can form between the sheets.The interval between the discrete values is around 4.4 ?,because the thickness of a single adsorption layer is around 4.4 ?.In order to study the effects of graphene distribution on the composites,models with different graphene dispersions are simulated,using two graphene sheets with different interlayer distances.The micro-structures and properties of graphene/polyethylene composites are investigated.In order to facilitate comparison,the pure polyethylene model and the composites containing single graphene sheet are also studied.The results show that multi adsorption layers of host molecules form near the graphene surface in the equilibrium structure of the graphene/polyethylene composite model.These adsorption layers are dynamic stable,where the polyethylene molecules can move through the adsorption layers during the simulation progress.The “adsorption curing” of the polyethylene in the adsorption layers occurs.The polyethylene molecules in the adsorption layers become more extended,and ordered.The movements of these molecules are also inhibited in the direction perpendicular to the graphene surface.The increase of graphene dispersion improves the “adsorption solidification” degree of host molecules.As the graphene dispersion increases,more host molecules become oriented in layers parallel to the graphene surface.The radii of gyration for the host molecules are both more uniform and larger in the models with higher graphene dispersion.Also,the movement of host molecules becomes more inhibited.Finally,the composite models are subjected to uniaxial tensile simulations to investigate their mechanical properties.The yield strain is about 7.3 %.In the elastic and yield region,the non-bond interactions for the polyethylene matrix change greatly with the increasing of the strain,while the increasing of the strain mainly leads to the change of the intermolecular bonding energy in the composite after the softening region.In the elastic region and yield region,the graphene inhibits the compression deformation of the composite models in the directions perpendicular to the strain,and the “adsorption curing” structures keep stable.The more additive amount of the graphene gives the higher yield stress.During the failure process,the damage always occurs in the region with the weakest “adsorption solidification”.The yield stress of the composites increases with the increase of graphene dispersion,since the incidence of graphene is larger and the “adsorption solidification” is stronger when the graphene sheets are more dispersed.