Molecular Dynamic Simulations Study Of Polymer Microstructure And Interfacial Thermal Conductance

Polymers are wildly used in daily life and engineering due to their excellent properties,such as their light weight,low cost,and excellent chemical stability.However,the intrinsic low thermal conductivity limits their application in the areas where desire efficient thermal transport.Hydrogen bond,as an interaction about 10-100times stronger than the normal van der Waals linking(vdW)between polymer chains,has great potential in enhancing the thermal transport properties of the polymer matrix and their composites.In this paper,we study the effect of introducing the hydrogen bonds to the thermal conductivity of pure polymer matrix and interfacial thermal conductance between fillers.The main research contents of this work are listed below:(1)Effect of hydrogen bonds on thermal transport derived from simulations of a precisely branched polyethyleneHydrogen bonds are considered to be bridges in forming networks for heat transfer pathways in polymers.However,a large number of polymer systems with fulfilled hydrogen bonds have very low thermal conductivity(TC).The role of hydrogen bonds in regulating TC of polymers is still far away from fully understanding.Here we systematically investigate the effect of hydrogen bonds on the intrinsic TC of polymers by performing molecular dynamics simulations to a precise polyethylene with-COOH groups bonded to every 21st carbon atoms along the backbone,termed ar-P21AA.It has lamella structure with stacked layers alternatively composed of aligned polymer chains and hydrogen-bonded functional groups,which provides us an ideal model for studying hydrogen-bond-mediated heat transfer.Our calculation shows that the ar-P21AA has high TC of 1.65 W/(m·K)along the aligned backbone direction,while only about 0.20 W/(m·K)in the other two directions normal to the backbones.We find that the interfacial thermal resistance(ITR)at the interface composed of hydrogen-bonded carboxyl groups is about 2 times smaller than that of a typical van der Waals interface.Increasing the strength of the hydrogen bonds at the interface decreases the ITR,which mainly originates from the enhancement of van der Waals interactions between the oxygens in the hydrogen bonds.Nevertheless,for the amorphous P21AA,increasing the hydrogen-bonding strength does not lead to the higher TC because of the induced decrease of density of system and shrinkage of polymer chains.This demonstrates that introducing hydrogen bonds has significant impact on the morphology of chains in addition to acting as thermal bridges,which may compensate each other for thermal transport in the polymer.Our work provides important insights into the design of thermally conductive polymers by introducing hydrogen-bonding interaction.(2)Thermal transport between graphene nanosheets connected through hydrogen bondsThe high thermal conductivity of graphene makes it an important filler material for thermally conductive composite materials.However,the interfacial thermal resistance between the fillers greatly hinders the heat transfer between the fillers.The functionalization of the edges of graphene can introduce covalent or hydrogen bonding between the graphene edge interfaces to replace van der Waals interaction connections,which is expected to enhance the interfacial thermal t.In this paper,the interfacial heat transfer properties of the graphene chemically functionalized with-COOH groups are systematically studied,and the effects of the distance between graphene sheets and the functionalized density on the interface thermal resistance are discussed.Our results show that,as the distance between the edges of the two graphene sheets decreases,the interfacial thermal conduction between the interfaces of the functionalized graphene gradually converges to~5E8 W/(m~2·K),which is an order of magnitude higher than that between the H-terminated graphene.The interfacial thermal conduction shows a non-linear monotonous increase with increasing the functionalization density.Further results show that there is a synergistic effect between the functionalized density and the graphene edge interface spacing.Increasing the functionalized density improves the interaction between the interfaces and reduces the interface spacing between the graphene edges,thereby enhancing the interfacial thermal conduction.The results of this paper provide an important reference for the study of effects of hydrogen bonding on the interfacial heat transfer between graphene fillers.