分子动力学模拟温度对PE–LD零剪切黏度的影响

Effect of Temperature on Zero Shear Viscosity of Low Density Polyethylene by Molecular Dynamics

采用分子动力学方法模拟分析温度(375~550 K)对低密度聚乙烯(PE–LD)零剪切黏度的影响,通过Material Studio 8.0软件构建分子"位能模型",施加原子水平模拟研究凝聚态优化的分子力场(COMPASS)力场,经过几何优化、退火处理和动力学弛豫后,运行NPT系综的动力学模拟,进行数据处理。结果表明,PE–LD熔体的黏度随温度升高呈指数递减。从分子链自扩散和自由体积来分析其机理,结果显示,温度的升高,加剧了分子链之间的运动,减少了分子间的缠结,增大了熔体的自由体积,减少了分子间的内摩擦,从而导致黏度下降。

Molecular dynamics was applied to estimate the effect of temperature on zero shear viscosity of low density polyethylene (PE–LD). Frist,potential model was set up by the software of Material Studio 8.0,and Condensed-phase Optimized Molecular Potential for Atomistic Simulation Studies (COMPASS) force field was applied. Then,cell has been optimized by geometry optimization,annealing and dynamic relaxation. Finally,dynamic simulation was running and extracted data was analyzed.The results show that viscosity of PE–LD melt and temperature have exponential relation,and viscosity drops as temperatures rise.Moreover,the mechanism was analyzed from the micro perspective of molecular self-diffusion and free volume. The results show that increasing temperature increase molecular motion and reduce the tangle of molecular chain. Meanwhile,the free volume of PE–LD melt increase and internal friction of molecular chains reduce. Thus,viscosity of PE–LD drops as temperatures rise.