小分子气体在烯烃共聚膜中扩散行为的分子动力学研究

Study on the Gas Diffusion in Olefin Copolymer Membrane by Molecular Dynamics Simulation

采用分子动力学模拟方法,研究了小分子气体(氧气分子和氢气分子)在烯烃共聚物乙烯/1-己烯膜中的扩散性能。主要探讨了分子主链结构中共聚物单体的比例,以及气体分子大小对扩散系数和自由体积的影响。模拟过程中采用Compass力场,对共聚物模拟体系进行NPT系综动力学优化,计算得到其密度,并与实验值进行对比,使模拟体系接近于真实体系;随后进行NVT模拟,得到500ps时间的小分子运动轨迹,由爱因斯坦方程计算出气体分子在烯烃共聚物中的扩散系数,并与文献报道的实验数据进行对比。结果表明,随着共聚物中1-己烯含量的增加,氧分子和氢分子的扩散系数也呈增大趋势,与共聚物中分子间相互作用力的变化分析一致。说明分子动力学模拟方法是一个有效计算、预测小分子在不同比例烯烃共聚物中计算扩散系数的方法。

The paper studies diffusion of gases （O2 , H2） in poly（ethylene-co-1-hexene） with different content of 1-hexene by molecular dynamics simulation. The effects of the size of the small gases and the co-monomer content on diffusion of gases and the changes of free volume are discussed. The whole simulations were processed under Compass force field. The calculated densities refined by NPT ensemble simulationa are in good agreement with the experimental data. The diffusion coefficients were determined by NVT ensemble simulation in 500 ps. After molecular dynamic simulation, the trajectories of the small molecules in the polymer matrix were obtained. Then the diffusion coefficients were calculated from the Einstein relation. The results show that the diffusion coefficients of oxygen and hydrogen increase with the content of 1-hexene, which agrees with the analysis of the interaction between the copolymer chains. It is concluded that molecular dynamics simulation is an effective method to predict the gas diffusion coefficient of small molecules in olefin copolymers.