Molecular Simulation Study Of Theeffect Of Interfacial Hydrogen Bonds On The Thermal Conductivity Of Graphene/Polyvinyl Alcohol Composite Materials

With the rapid development of energy,electrical,and electronic technologies,the demand for thermally conductive polymer materials such as graphene nanocomposites continues to grow.However,the high interfacial thermal resistance between graphene fillers and polymers can reduce the thermal transport efficiency of composite materials.Therefore,how to reduce the interfacial thermal resistance is a challenging problem.Due to the small interface size,this study aims to use molecular simulation methods to investigate the effects of interfacial hydrogen bonds and interface modification on the thermal conductivity of graphene/polyvinyl alcohol composite materials.The main research content includes the following two parts:(1)By modifying graphene with four common functional groups(methyl,hydroxyl,amino,and carboxyl),the effects of functionalization and interfacial hydrogen bonds on the thermal conductivity of graphene/polyvinyl alcohol composite materials were studied.The results show that functionalization can effectively reduce interfacial thermal resistance,with polar groups having a better effect.At the maximum degree of functionalization,non-polar methyl functional groups could only enhance interfacial thermal conductivity by 58.5%,while polar hydroxyl functional groups could enhance it by 110.0%.This was because polar groups could form hydrogen bonds with the polymer chains,thereby increasing the Coulomb energy between the polymer and functionalized graphene.The characterization of the number and lifetime of hydrogen bonds was consistent with the change in Coulomb energy.The calculation of vibrational density of states and correlation factor indicated that the grafting groups increased the phonon coupling degree between graphene and the polymer.The analysis of the integrated autocorrelation of interfacial heat power also confirmed this.The roughness of the graphene surface,as well as the radial distribution function of grafting groups and polymer chains,were calculated to reasonably demonstrate that hydroxyl modification could enhance interfacial thermal conductivity better than methyl,amino,and carboxyl modifications.However,the calculated thermal conductivity of functionalized graphene indicated that functionalization reduced the intrinsic thermal conductivity of graphene.Therefore,the study further used an effective medium model to analyze the effects of functionalization and interfacial hydrogen bonds on the thermal conductivity of graphene/polyvinyl alcohol composites.The results showed that graphene had a critical size.When the length of graphene was below the critical size,functionalization could effectively enhance the thermal conductivity of the composite.Finally,the influence of the dispersion state of graphene on the thermal conductivity of the composite was discussed using finite element analysis.(2)By modifying graphene with polar molecular chains of different grafting densities and lengths,the effects of grafting molecular chains modification and interfacial hydrogen bonds network on the thermal conductivity of graphene/polyvinyl alcohol composite materials were investigated.The experimental results showed that grafting molecular chains could more effectively enhance the interfacial thermal conductivity than functional groups.As the grafting density and length of the molecular chains increased,the temperature difference across the interface decreased from 35 K to 6.6 K,and the corresponding interfacial thermal conductivity increased up to 446.1% of the initial interfacial thermal conductivity.This was because the grafting molecular chains significantly enhanced the interface interaction between the functionalized graphene and the polymer,promoting heat transfer.In addition,a multi-layer hydrogen bonds network was formed at the interface,which also facilitated the transport of heat at the interface.Moreover,the results of vibrational density analysis showed that grafting molecular chains increased the phonon coupling degree between the functionalized graphene and the polymer,reducing the interface phonon scattering.The integrated autocorrelation of interfacial heat power between graphene and the grafting chains proved that the grafting molecular chains provided an efficient thermal pathway for interfacial heat transfer.These results suggested that grafting molecular chains could effectively promote interfacial heat transfer.However,the calculation of the thermal conductivity of graphene modified by grafting molecular chains showed that modification reduced the intrinsic thermal conductivity of graphene.Therefore,an effective medium model was used to analyze the effects of grafting molecular chains and interfacial hydrogen bonds network on the thermal conductivity of graphene/polyvinyl alcohol composite materials.The results showed that graphene also had a critical size.If the length of the graphene was less than the critical size,grafting molecular chains could be a feasible way to improve the thermal conductivity of the composite material.