Molecular Dynamics Simulation Of Solvent-exfoliation And Dispersion Of Graphene With The Assistance Of Supercritical CO₂ And Pyrene-polymer

Graphene,one of the most promising carbon nanomaterials,has been widely used in many fields due to its ultra-thin 2-D structure,fascinating optical,electronic,thermal,mechanical and chemical properties.However,the key challenges that limit its application involve exfoliating,dispersing the layered graphene sheets with high stability.So far,the liquid-phase exfoliation method shows great advantages for bulk production of graphene with high quality.The supercritical CO₂ has been used to intercalate and delaminate graphite because of its high diffusivity,low surface tension and low toxicity.Furthermore,functionalization of the graphene sheets has been widely considered a promising method to improve their dispersion and stability and pyrene-polymer has been used to disperse graphene sheets through robust π-πnon-covalent interactions.Although the experimental methods provide useful information about exfoliating and dispersing graphene in organic solvent with the assistance of supercritical CO₂ and pyrene-polymer,they fail to provide a complete and direct picture for understanding the detailed exfoliation and dispersion mechanism.Here,molecular dynamics simulations have been carried out to explore the mechanism of solvent-exfoliation and stabilization of graphene in the presence of supercritical CO₂ and pyrene-polymer.Based on the mentioned background,the research contents are as followings.Firstly,a series of molecular dynamics simulations are performed to probe the effect of the scCO₂ concentration and the influence of the pressures.And we also study the specific role of the scCO₂ and the solvent molecules during the exfoliation progress under two variable initial separating distance of the two graphene sheets.The results show that the realization of the exfoliation and dispersion of graphene is due to the synergistic action of the scCO₂ and solvent molecules.The confined CO₂ can afford sufficient repulsive barrier to help the solvent molecules to insert into the confined region only when the scCO₂/PX mole ratio is higher than 2.5.The scCO₂ molecules play a “wedge” role in the exfoliation process,inserting into the interlayer firstly,and then helping the solvent molecules inserting into the interlayer.The confined scCO₂ and solvent molecules weaken the interaction between the graphene sheets and broaden the separation distance,thus promoting the exfoliation and dispersion of the graphene sheets.Secondly,molecular dynamic simulations are performed to study the role of the pyrene-polymer by setting two variable initial distances of the two graphene sheets.In addition,for a deeper understanding the function of the pyrene-polymer,we calculate the PMF of graphene exfoliation using steered molecular dynamic simulation and perform graphene dispersion simulations including multi-layer graphene sheets.When the initial distance between the graphene sheets is narrow the pyrene-polymer functions a little during the exfoliation progress,decreasing the interaction between the graphene sheets slightly,and mainly play a role of modifying graphene.With the separation between the graphene sheets becoming larger,more pyrene-polymers could intercalate the graphene sheets to facilitate the exfoliation.The pyrene-polymers mainly act as stabilization medium to prevent the exfoliated graphene sheets from aggregation.