Molecular Dynamics Simulation On The Thermal Transport In Polymer Chains

For more than one hundred years,polymer materials have been developed to be one of the three major materials in the world and are widely used in various fields.Compared with inorganic materials,polymer materials have the advantages of light weight,low production cost and good chemical stability.However,the thermal conductivity of polymer materials is usually less than 1 Wm^-1K^-1,which has largely limited their applications in the field of thermal management,such as heat exchangers,electronic packaging and so on.In the past two decades,enhancement of the thermal conductivity of polymer materials has attracted great research interests.The results show that the thermal conductivity of polymer materials can be comparable to that of metals after modification.Although high thermal conductivity fibers and thin films can be fabricated right now,bulk polymer materials cannot be made yet.Polymer materials consist of long polymer chains.In the process of preparing high thermal conductivity polymer materials,the key step is to stretch them so that the polymer chains are aligned in one direction.At present,the mechanism of heat transport in polymer chains and polymer materials is still not fully clear.In this paper,molecular dynamics is used to study the thermal transport in polymer chains from the nano-scale.First of all,we investigate the heat transfer in a single polymer chain.In order to simplify the model of thermal transport,we choose polyethylene as the research object,and use the Non-equilibrium Molecular Dynamics?NMED?simulation method to calculate the thermal conductivity.The simulation results show that the temperature distribution along the chain length of polyethylene single chain with curvature is almost the same as that of straight chain,while there is a significantly increased temperature drop across the kink.Furthermore,we find that curvature has no significant effect on the thermal transport along the polymer chain by changing the end-to-end length of polyethylene chain..The effects of multiple kinks on the thermal transport in a single chain of polyethylene is also studied.The experimental results and corresponding physical models show that the existence of kinks results in an additional thermal resistance,which enhances the scattering of phonons and result in a temperature drop across the kinks.Then,the heat transport between the polyethylene chain-chains by van der Waals force is studied.For double-chain model,it can be divided into overlapping part and non-overlapping part.Heat transfer along the chains in the non-overlapping part.In the overlapping part heat transport not only along the chain but also between the chains,so there is a smaller temperature gradient.First,we define a thermal conductivity between chains.Then,by changing the length of overlap,we find that the thermal conductance of overlap does not increase with the increase of length,but has a peak value.At the same time,the peak value increases with the increase of the single chain length of polyethylene.The optimal overlap position corresponding to the peak value decreases with the increase of the thermal conductivity between chains.From the above research,we have deeper understanding of the heat transport in polymer.In order to fabricate polymers with high thermal conductivity,the design of polymer chains is the key,which is summarized as:1)The covalent bond in polymer backbone cannot fold;2)The unit cell structure of the main chain should not be complicated;3)The heat transfer between the polymer chain-chain should be enhanced by hydrogen bond or the like.