Investigation On The Interfacial Mechanical And Thermal Properties Of Graphene/polymer Nanocomposites By Molecular Dynamics

Graphene(GR)is considered as a two-dimensional(2-D)carbon nanofiller witha one-atom-thick planar sheet of sp~2 bonded carbon atoms that are densely packed in a honeycomb crystal lattice.GR possesses outstanding mechanical,thermal,electrical and optical properties.The discovery of GR not only enriched carbon materials,but also proved the existence of the 2-D material can be stable in the real environment.A lot of attentions have been attracted since the discovery of GR,and GR becomes the focus of research.Therefore,some material properties and applications of GR have been discovered.Due to the excellent performances of GR,it is usually used as nanofillers to enhance the mechanical,thermal,electrical as well as tribological performances of matrices.As research continues,more characteristics and functions of GR have been found;however,compared with the research about the material properties of GR,the interfacial mechanical properties and interfacial thermal properties of GR/polymer nanocomposites have been rarely reported.It is well known that the interfacial mechanical properties and interfacial thermal properties between the nanofillers and matrices are critical to the mechanical and thermal properties of nanocomposites.Study on the interfacial mechanical properties and interfacial thermal properties between the GR and matrices can be useful for understanding the interfacial and macroscopic properties of nanocomposites,and also can afford some helpful data and information for the design and development of GR/polymer nanocomposites.In order to reveal the interfacial mechanical properties between GR and polymer matrix,the interfacial mechanical properties between GR and polyethylene(PE)matrix were studied in Section 3.The computational results show that the interfacial mechanical properties can be affected by the system temperature,and the higher the temperature is,the weaker the interfacial mechanical properties are.Although the mechanical properties of GR can be reduced by the introduction of defects,the interfacial mechanical between the defective GR and polymer matrix may not always decrease.Analyses indicate that the GR with a small number of single atomic vacancy defects can enhance the interfacial mechanical properties.Meanwhile,the pull-out velocity of GR has great influence on the interfacial strength,and the larger pull-out velocity is used,the higher interfacial strength is obtained.Based on the thermal motion and structure characteristics of GR,a flat monolayer GR is unstable under the natural environment;hence,some wrinkles are produced in the GR to maintain its steady state.The wrinkles will affect the interfacial mechanical properties between the GR and matrix.Therefore,in Section 4 the MD simulations were used to pull out wrinkled GR from polymer matrix,and the effect of wrinkles on the interfacial mechanical properties was investigated,as well as the molecular chain length of matrix,matrix type and pull-out velocity.The computational results show that the wrinkles can effectively enhance the interfacial mechanical properties.The interfacial mechanical properties become stable when the molecular chain length of PE matrix is larger than 40 repeated unit length.However,for the PMMA matrix,the interfacial mechanical properties decrease and then increase with the molecular chain length.The effect of pull-out velocity on the interfacial mechanical properties can be ignored when the polymer molecular chain is long enough and the pull-out velocity is smaller than 0.005 A/fs.In Section 5,by separating functionalized GR from polymer matrix in normal direction and tangential direction,the interfacial cohesive strength and interfacial shear strength were studied,as well as the interfacial Mode-? and Mode-? fracture toughnesses.Some PE molecular chains were grafted on the GR to functionalize the GR;the influences of grafting density and PE molecular chain length on the interfacial mechanical properties were analyzed.The results indicate that grafting density and PE molecular chain length can significantly influence the interfacial mechanical properties.Meanwhile,the Mode-I fracture toughness is weaker than the Mode-? fracture toughness for interfacial of functionalized GR/PE nanocomposites.The covalent bonded 3-D hybrid GR-CNT and ?-? stack hybrid GR-CNT were built by combining the 1-D CNT and the 2-D GR using C-C covalent bonds and ?-?non-covalent bonds,respectively.The reinforcement effects of two types of hybrid GR-CNT were evaluated by using MD simulations,and the CNT's alignment,length and radius were also considered as factors.With the same parameters of CNT,the interfacial mechanical properties of ?-? stack hybrid GR-CNT is superior to the covalent bonded hybrid GR-CNT.For the covalent bonded hybrid GR-CNT,the reinforcement effects increases with the CNT length and radius,and multi-walled carbon nanotube(MWCNT)is more efficient than single-walled carbon nanotube(SWCNT).The 3-D hybrid GR-CNT structures possess the excellent thermal properties of CNT and GR.However,the interfacial thermal properties between this structure and polymer matrix have been rarely reported.Therefore,the enhancement mechanism of interfacial thermal transport of hybrid GR-CNT structure was explored by applying non-equilibrium molecular dynamics(NEMD)simulations.Three different types of hybrid GR-CNT structures were built.The influences of CNT radius and CNT type for the hybrid GR-CNT on the interfacial thermal properties were also analyzed.Compared with GR,the covalent bonded hybrid GR-CNT has better interfacial thermal transport capacity.The main reason is that the out-of-plane vibration power spectrum(VPS)of the covalent bonded hybrid GR-SWCNT structure is redistributed,leading to more VPS overlap of the covalent bonded hybrid GR-SWCNT structure with that of PE matrix.Analysis also shows that the hybrid GR-SWCNT structures have better interfacial thermal properties than those of hybrid GR-MWCNT structures.