A Molecular Dynamics Simulation Study On The Tribological Properties Of Polymer Composites By Incorporating Of Functionalized Carbon Nanomaterials

Polymer nanocomposites(PNCs)are widely used in aerospace and petrochemical industry owing to their outstanding mechanical and tribological capacities as well as corrosion resistance.With the flourishing development of nanotechnology,carbon nanotubes(CNTs)and graphene(GNS)are considered as potential nano-reinforcements because of their excellent physical,mechanical,and self-lubricating performances.However,due to the strong van der Waals(vd W)interaction between nano particles,agglomerations can always be observed in the preparation process of PNCs,which limits the reinforcing effect of carbon nanomaterials to a certain extent.Further experimental research shows that surface modification of carbon nanomaterials can significantly improve their dispersion in the polymer matrix,and then effectively heighten the reinforcing effect.Simultaneously,with the vigorous development of computational science,molecular dynamics(MD)simulation method has been widely developed in the multi-scale simulation and cross-scale analysis of PNCs.This calculation method can make up for the shortcomings of traditional experimental research methods,analyze and reveal the structural characteristics and micro mechanism of PNCs from an atomic scale,and provide theoretical basis for the structural design of PNCs.In this paper,the acrylonitrile-butadiene rubber(NBR)with perfect processability and extensive application background,was adopted as the polymer matrix.The MD calculation method was used to qualitatively and quantitatively compare and analyze the reinforcement differences of CNTs and GNS with different surface functionalization in the tribological properties of the NBR matrix from the atomic scale.The enhancement mechanisms of functionalized CNTs and GNS on the friction and wear properties of the NBR matrix were then explored.The concrete research contents are as follows:(1).The molecular models of pristine graphene(PGNS),hydroxyl-functionalized graphene(OH-GNS),carboxyl-functionalized graphene(COOH-GNS),and ester-functionalized graphene(COOCH₃-GNS)reinforced NBR nanocomposites were established respectively.The mechanical and tribological properties of the systems were determined by means of MD method,and the reinforcement differences of GNS with different surface functionalization treatments in terms of NBR matrix performance were compared and analyzed.The detailed information of interfacial binding energy,free volume,friction interface temperature,concentration and so on during MD simulations were extracted to study the strengthening mechanisms of functionalized GNS to the performances of NBR matrix.The results show that owing to a rougher surface,higher in-plane stiffness,less curling and agglomeration of the functionalized GNS,the stability,dispersion,and interfacial properties between the functionalized GNS and the NBR matrix are significantly improved for enhancing the resistance under tensile and shear loadings as compared to those of the PGNS/NBR composite.Meanwhile,among the functionalized GNS,the COOH-GNS has the best reinforcing effect on the mechanical and tribological properties of NBR composites.This is attributed to the formation of strong interfacial interactions,such as hydrogen bonding and strong vd W interactions,between the COOH-GNS and the NBR matrix.In contrast,hydrogen bonding and weak vd W interactions are formed between the OH-GNS and the NBR matrix,while the COOCH₃-GNS interacts with the NBR matrix only through strong vd W interactions.(2).In order to study the effects of functional group type,sidewall functionalization degree and further chemical modification of CNTs on the properties of NBR matrix,the corresponding molecular models of the composites were constructed respectively.The mechanical and tribological properties of the composites were evaluated using MD method.The interfacial binding energy,fractional free volume and dipole autocorrelation function were extracted and analyzed to reveal the strengthening mechanism of functionalized CNTs on the mechanical and friction and wear properties of NBR matrices.The results indicate that the additions of functionalized CNTs are able to significantly improve the mechanical and tribological properties of NBR nanocomposites.Meanwhile,compared with the carboxyl-,hydrazide-,and ethylenediamine-functionalized CNTs,the amide-functionalized CNT has a preferable enhancement effect owing to the formation of stronger dipole-dipole interactions,hydrogen bonding interactions,vd W interactions,and better interfacial compatibility.Additionally,appropriate degree of sidewall functionalization(?10%)of CNTs can significantly improve the reinforcement effect of CNTs on the mechanical properties of NBR composites without destroying the perfect structure of CNTs and reducing their mechanical properties.