分子动力学方法研究单链聚乙烯的结晶过程

MOLECULAR DYNAMICS SIMULATION ON CRYSTALLIZATION\=OF FULLY EXTENDED SINGLE\|CHAIN POLYETHYLENE

用分子动力学方法（ＭＤ）研究了单链聚乙烯在不同温度（１００Ｋ、２００Ｋ、３００Ｋ、４００Ｋ、５００Ｋ）下的结晶过程，并用能量和结构参数进行了描述．结果表明伸直分子链的结晶过程都经历了三个阶段，首先是伸直链的卷曲与聚集，然后通过链段的排列形成规整的片晶结构，最后是结晶形成的片晶在结构与能量上的涨落变化。研究表明，结晶温度越高，分子链的内聚速度越快．研究发现，分子链在内聚阶段经历一个局部凝聚的中间状态，在结晶温度很低（１００Ｋ）时，局部凝聚的结构是有序的．而在５００Ｋ时，该结构为无规线团．结晶温度的差异，一般来说，将导致得到的片晶厚度的不同．对于模拟的单链，随着结晶温度的降低而形成了较厚的片晶．该行为与聚乙烯本体结晶中片晶厚度对结晶温度的依赖性相反．在有序化阶段和之后的片晶调整运动阶段，分子链线团的回转半径基本保持不变．这与宏观多链体系的结果相同．另外，在模拟中发现。

Molecular dynamics (MD) simulation has been performed on the crystallization process of a single polyethylene chain from fully extended state in the vacuum at different temperatures (100500K with an interval of 100K). The results indicated that each of the simulated crystallization processes consists of three stages. The first stage is a self\|cohesion stage, in which the extended chain collapsed to a coil with a sharp decrease of energy and size of the chain. The second stage is an ordering stage in which a lamella was formed by the chain in the way of the adjustment of the chain segments. The last stage is an adjusting stage, where the energy and the order keep almost constant, but the lamella shape varies to some extent. Even though there is a big gap in the time scale between MD simulation and laboratory experiments, the fratures of the simulated three stages were very similar to the three stages for bulk polymers in usual crystallization (the nucleation, the crystal gtrowth and the perfection stage). The collapse process of extended chain proceeded via a two\|step mechanism at 100K and 500K: a local collapse, forming compact domains, which subsequently coalesce to yield a fully collapsed state. It was found that the segments in the domains formed in local collapse step were ordered packed at 100K, while the randomly packed at 500K. The self\|cohesion rate of the chain is found intensively dependent on the crystallization temperature. The higher temperature resulted in the faster cohesion process. The crystallization temperature also influenced the thickness of the final lamellae. The thicker lamella structure is likely formed at lower temperature. This is different from the crystallization process of polyethylene bulk.\