聚乙烯单链量子热输运的同位素效应

Isotope effect on quantum thermal transport in a polyethylene chain

高分子导热材料的有效调控受到了日益广泛的关注.应用密度泛函理论(DFT)、中央插入延展(central insertion scheme,CIS)方法及非平衡格林函数(NEGF)理论,对包含432个原子、长18.533 nm的聚乙烯单链量子热输运的同位素效应进行了研究.计算结果表明,室温下长100 nm的纯12C聚乙烯单链的热导率理论上限高达314.1 W·m^(-1)·K^(-1);对于^(12)C聚乙烯单链,其他条件一定时,^(14)C掺杂引起的热导同位素效应比^(13)C更为显著;室温下纯^(12)C聚乙烯单链中^(14)C掺杂原子百分数为50%时同位素效应最显著,此时平均热导比未掺杂时下降了51%.这对探索聚乙烯材料热输运的同位素影响机理具有十分积极的意义.

Effective control of thermal conductive property of polymeric materials is now attracting more and more attention from both the theoretical and the experimental aspects. Bulk polyethylene is regarded as a thermal insulator because its thermal conductivity is typically on the order of 0.35 W.m-1.K-1. However, recent studies demonstrate that a polyethylene chain has an extremely high thermal conductivity and the reported thermal conductivity of ultra-drawn polyethylene nanofibers is as high as 104 W m-1.K-1, about 300 times higher than that of bulk polyethylene. In order to cast off this dilemma, several simulation methods are used to detect the unusually high thermal conductivity of a polyethylene chain. Molecular dynamics （MD） simulation results are highly sensitive to the choice of empirical potential or simulation method. Even using the same potential （AIREBO potential）, the obtained thermal conductivity of a polyethylene chain is different. By combining the Green-Kubo method with a modal decomposition approach, equilibrium molecular dynamics （EMD） indicates that the thermal conductivity is able to exceed 100 W.m-1.K-1 while the polyethylene chain is longer than 40 nm at room temperature. Compared with the simulation result obtained by equilibrium molecular dynamics, the simulation result provided by using the non-equilibrium molecular dynamics （NEMD） method is only 57 W.m-1.K-1 for a 160-nm-long polyethylene chain at room temperature. We use the first-principles method to calculate the force constant tensor, and the characteristics of quantum thermal transport in a polyethylene chain can be revealed. In our algorithm, several shortcomings of molecular dynamics, i.e., different potential functions or simulation methods may lead to obviously different results for the same quantum thermal transport system, are overcome. Based on the density functional theory （DFT）, the central insertion scheme （CIS） combined with nonequilibrium Green＇s function （NEGF） is used to evaluate the isotope effect on quantum thermal transport in a polyethylene chain, which includes 432 atoms in scattering region and has a length of 18.533 nm. It is found that the upper limit of thermal conductivity of a 100-nm-long pure12C polyethylene chain reaches a high value of 314.1 W.m-1.K-1 at room temperature. Moreover, for the case of a pure polyethylene chain of 12C, with other conditions unchanged, the reduction of average thermal conductance caused by 14C impurity is more remarkable than that by 13C. The most outstanding isotope effect on quantum thermal transport can be detected in the polyethylene chain. When the doping concentration of 14C in 12C is 50% at room temperature, the average thermal conductance will be reduced by 51%. It is of great significance for studying the mechanism of isotope effect on thermal transport in polyethylene.