高分子链运动对氧气扩散行为的影响

Molecular Dynamics Simulation on the Relationship of Oxygen Diffusion and Polymer Chains Activity

以聚乙烯(PE)为研究对象,基于分子动力学模拟从温度、结构和扩散激活能等角度来研究高分子链长、主链段活跃性、端基运动和分子链取向对氧气分子(O_2)在聚乙烯中扩散行为的影响。基于COMPASS力场构建具有三维周期性边界的扩散模型(O_2/PE),对该模型优化得到的稳定构型进行分子动力学模拟。结果表明,高分子链运动对气体的扩散行为有较大的影响。对高分子链端基均方位移(MSD)以及末端距与扩散系数的关联分析表明,主链活跃性提高,有利于形成更多的"扩散通道",使分子扩散速率提高。短链活跃性高于长链,柔顺性低于长链,分子链越短自身碰撞频率越低,因此分子跃迁的自由体积增大,扩散速率升高。此外,施加拉伸载荷会使高分子链沿载荷方向取向,降低晶胞对称性,使分子扩散速率显著提升。

Taking polyethylene as the research object,based on molecular dynamics simulation,the effects of molecular chain length,main-chain segment activity,terminal group activity and molecular chain orientation on the diffusion behavior of oxygen molecules(O2)in polyethylene were studied from the aspects of temperature,structure and diffusion activation energy.Based on the COMPASS force field,a diffusion model(O2/PE)with three-dimensional periodic boundary is constructed,optimize the model and then take molecular dynamics simulation on the stabi-lity configuration.The results show that the polymer chain movement has a great influence on the diffusion behavior of the gas.The correlation analysis between the polymer chain terminal group MSD and end-to-end distance and the diffusion coefficient shows that,the"diffusion channel"increases and the molecular diffusion rate increases when the activity of the main chain and the movement ability are enhanced.Short chain acti-vity is higher than long chain,and the flexibility is lower than that of long chain.The shorter the molecular chain is,the lower the collision frequency is,and the free volume of molecular transition increases and the diffusion rate increases.Further,a tensile load is will make the polymer chains oriented in the load direction,reduce the symmetry of the unit cell and increase the molecular diffusion rate significantly.