Numerical Simulation Of Nanoscale Heat Transport In Polymer Chains

Polymer materials and polymer composite materials have many advantages such as low cost,light weight,insulation,easy processing and corrosion resistance.However,the thermal conductivity of traditional polymer materials is usually distributed in 0.1-0.5 Wm^-1K^-1,which is much lower than the thermal conductivity of metals(generally higher than 100 Wm^-1K^-1).The lack of heat transport capacity hinders the application of polymer materials in electronic equipment thermal packaging,lighting equipment and heat exchange equipment.Improving the thermal transport capacity of polymer materials has always been a hot topic in the subject field.How to effectively improve the thermal conductivity of polymer materials has become an urgent problem to be solved.In fact,polymer materials have extremely high heat transport potential.In the past research,it was found that the low-dimensional polymer structure has excellent heat transport properties.For example,the thermal conductivity of polyethylene single chain is equivalent to that of metal,and the thermal conductivity of graphene is even an order of magnitude higher than that of metal.Based on the high thermal conductivity of polymers in chains and layers,a series of technologies have been developed to improve the thermal conductivity of polymer materials.However,these technologies can only be used to produce polymer fibers or films,and cannot be applied to industrial production for the time being.In order to further promote the development of thermally conductive polymer materials,research on the material microstructure and microscopic thermal conduction mechanism is indispensable.First of all,this article carried out molecular dynamics research on the heat transport process between molecular chains,which is common in polymer materials.Polyethylene molecules are used as the research object,two polyethylene molecules are partially overlapped to form an inter-chain heat transport structure.In simulation calculations,statistical atomic temperature and heat flow data are used to calculate the thermophysical properties of materials and build a mathematical model of heat transport.The calculation results show that the overall thermal conductivity of the overlapping area of molecular chains increases first and then decreases with the increase of the overlap length,and will reach the maximum value at a certain overlap length.In the overlapping area,heat flows through the molecular chains as well as between molecules.The thermal conductivity of the molecular chains increases as the overlap length increases,and the thermal conductivity between the molecular chains decreases as the overlap length increases.The total thermal resistance of the overlapping area can be decomposed into two parts:the thermal resistance within the chain and the thermal resistance between the chains.The mathematical theoretical model of heat transport between molecular chains is constructed through the theorem of energy conservation and heat transfer,and the expression of thermal resistance in the overlap zone is given.The results showed that the thermal resistance within the chain increased with the increase of the overlap length,and the thermal resistance between the chains decreased with the increase of the overlap length,explaining the reason why the thermal conductivity first increased and then decreased.As the overlap length increases,the thermal resistance between chains slows down due to the increase in the coefficient?,and it always accounts for more than 20%of the total thermal resistance.It shows that the thermal resistance between chains will not disappear due to the increase of overlapping length,and reducing the thermal resistance between chains is the key to improve the thermal conductivity of polymer materials.Then,this article explores the phenomenon of dimensional effects in polymer materials.The thermal conductivity of common polymer materials generally decreases with the increase of the structural dimension,while the thermal conductivity of the poly(p-phenylene)molecule increases with the increase of the structural dimension.This work constructed poly-p-phenylene single-chain,planar multi-chain and three-dimensional crystal structures,and obtained the thermal conductivity data of different structures along the chain direction through the non-equilibrium molecular dynamics simulation method.The results show that the thermal conductivity of the poly(p-phenylene)molecule increases monotonically with the increase in dimensionality.In order to explore the reason behind this abnormal dimensional effect,this article counts the rotation dihedral angle distribution of the rotatable ring structure in the poly(p-phenylene)molecule.The statistical results show that single-chain poly-p-phenylene has molecular structure order only in the short range,and disorder in the long range.Multi-chain poly-p-phenylene has both short-range order and long-range order,and the degree of order increases with the increase in the number of molecular stacked chains.The thermal conductivity curves with temperature of poly-p-phenylene single-chain and double-chains showed the characteristics of amorphous structure and crystal structure,indicating that the molecular chain stacking of poly-p-phenylene led to the crystalline transformation of the structure.In summary,the constructed thermal transport model between molecular chains has deepened the understanding of the thermal transport process at the microscopic scale of polymer materials,and filled a part of the gap in the field of thermal transport research on polymer materials.The abnormal dimensional effect of poly(paraphenylene)shows that the interaction of molecular chains can also enhance the thermal conductivity,which provides inspiration for the development of practically valuable thermally conductive polymer bulk materials.In the future,we will continue to conduct in-depth research on the microscopic thermal conduction mechanism of polymers and the preparation of thermally conductive polymer materials.The specific research directions are mainly:(1)Reduce the thermal resistance of heat transport between molecular chains through hydrogen bonds or??bonds and other molecular structures,and eliminate the anisotropy of thermally conductive polymer materials;(2)Research on the dimensional effect of the polymers which have skeletons with rotating structure and improve the research on the phenomenon of crystallization caused by molecular chain stacking.