聚乙烯单链松驰过程的分子模拟

MOLECULAR DYNAMICS SIMULATION OF POLYETHYLENE\=CHAIN RELAXATION PROCESS

利用分子模拟方法研究了不同链长聚乙烯单链折叠过程和相关的松弛和坍塌机理．发现在链长短于１０００ＣＨ２单元时，聚乙烯的链段主要通过整体塌缩机理进行折叠和取向；而链长超过１０００ＣＨ２单元时，可以明显地观察到局部取向团簇的形成，聚乙烯单链通过局部塌缩机理进行折叠和取向．通过对各阶段团簇数目，体系取向链段长度的表征，说明体系在模拟时间范围内表现了很强的松驰特性．

Molecular dynamics simulations of model polyethylene chains were performed. The time evolutions of chain conformation for a range of molecular weights are studied.The process of chain relaxation and collapse includes many stages. Local clusters formed and were relatively stable for a long time, in addition, the local order structure still exists in the last compact coil. The results verified that the modification of force field for demonstration of stages in the folding process is not necessary.When the number of chain units is less than 1000 CH\-2 units, the local collapse stage is difficult to be observed, just as the results reported by Kavassalis et al.. Nevertheless, as the chain became longer, the stage of collapse is clearly characterized by the average number of clusters. The following feature are found: first, the transition between different stages of chain folding became sharper as the chain becme longer; second, the clusters inclused the local order structure became more stable as the ch ain became longer.Based on our analysis, local collapse stages can be characterized by the number of clusters. The cluster structure was the essential feature of the local collapse, and such structure will be maintained in the following collapse structure.There is an interesting increase of the torsion energy for system during the coalescence of local clusters in the relatively long chains. The increase of the torsion energy may lead to the expansion of the radius of gyration.\=