中国物理B ›› 2008, Vol. 17 ›› Issue (8): 3115-3122.doi: 10.1088/1674-1056/17/8/058

• 8000 CROSSDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY • 上一篇    下一篇

The phase behaviour of single polyethylene chains with and without fixing one end

章林溪1, 苏加叶2   

  1. (1)Department of Physics, Wenzhou University, Wenzhou 325027, China; (2)Department of Physics, Zhejiang University, Hangzhou 310027, China
  • 收稿日期:2008-01-01 修回日期:2008-02-28 出版日期:2008-08-20 发布日期:2008-08-20
  • 基金资助:
    Project supported by the National Natural Science Foundation of China (Grant Nos 20574052 and 20774066), the Program for New Century Excellent Talents in University (Grant No NCET-05-0538) and the Natural Science Foundation of Zhejiang Province (Grant No R404047).

The phase behaviour of single polyethylene chains with and without fixing one end

Su Jia-Ye(苏加叶)a) and Zhang Lin-Xi(章林溪)b)   

  1. a Department of Physics, Zhejiang University, Hangzhou 310027, China; b a Department of Physics, Wenzhou University, Wenzhou 325027, China
  • Received:2008-01-01 Revised:2008-02-28 Online:2008-08-20 Published:2008-08-20
  • Supported by:
    Project supported by the National Natural Science Foundation of China (Grant Nos 20574052 and 20774066), the Program for New Century Excellent Talents in University (Grant No NCET-05-0538) and the Natural Science Foundation of Zhejiang Province (Grant No R404047).

摘要: The phase behaviour of a single polyethylene chain is studied by using molecular dynamics simulations. A free chain and a chain with fixing one end are considered here, since the atomic force microscope (AFM) tip can play a significant role in polymer crystallization in experiment. For a free chain, it is confirmed in our calculation that the polymer chain exhibits an extended coil state at high temperatures, collapses into a condensed state at low temperatures, i.e. the coil-to-globule transition that is determined by a high temperature shoulder of the heat capacity curve, and an additional liquid-to-solid transition that is described by a low temperature peak of the same heat curve. These results accord with previous studies of square-well chains and Lennard-Jones homopolymers. However, when one of the end monomers of the same chain is fixed the results become very different, and the chain cannot reach an extended coil-like state as a free chain does at high temperatures, i.e. there exists no coil-to-globule-like transition. These results may provide some insights into the influence of AFM tip when it is used to study the phase behaviour of polymer chains. If the interaction force between AFM tip and polymer monomers is strong, some monomers or one of them can be seen as being fixed by the tip, which is similar to our simulation model, and it is also found that AFM tip could induce polymer crystallization.

Abstract: The phase behaviour of a single polyethylene chain is studied by using molecular dynamics simulations. A free chain and a chain with fixing one end are considered here, since the atomic force microscope (AFM) tip can play a significant role in polymer crystallization in experiment. For a free chain, it is confirmed in our calculation that the polymer chain exhibits an extended coil state at high temperatures, collapses into a condensed state at low temperatures, i.e. the coil-to-globule transition that is determined by a high temperature shoulder of the heat capacity curve, and an additional liquid-to-solid transition that is described by a low temperature peak of the same heat curve. These results accord with previous studies of square-well chains and Lennard-Jones homopolymers. However, when one of the end monomers of the same chain is fixed the results become very different, and the chain cannot reach an extended coil-like state as a free chain does at high temperatures, i.e. there exists no coil-to-globule-like transition. These results may provide some insights into the influence of AFM tip when it is used to study the phase behaviour of polymer chains. If the interaction force between AFM tip and polymer monomers is strong, some monomers or one of them can be seen as being fixed by the tip, which is similar to our simulation model, and it is also found that AFM tip could induce polymer crystallization.

Key words: molecular dynamics simulations, coil-to-globule transition, polyethylene chains

中图分类号:  (Solid-liquid transitions)

  • 64.70.D-
61.41.+e (Polymers, elastomers, and plastics) 65.40.Ba (Heat capacity) 68.37.Ps (Atomic force microscopy (AFM))