Molecular dynamics modeling of polymer crystallization; from simple polymers to helical ones.
Molecular dynamics modeling of polymer crystallization; from simple polymers to helical ones.
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DOI：
10.1039/b406069e
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发表时间：
2005-11
影响因子：
3.4
通讯作者：
Takashi Yamamoto;Naohiko Orimi;Naohiko Urakami;Kaoru Sawada
中科院分区：
文献类型：
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作者：
Takashi Yamamoto;Naohiko Orimi;Naohiko Urakami;Kaoru Sawada
螺旋聚合物的结晶是分子模拟的一大挑战。它涉及许多重要的问题，如生物分子的折叠和晶体生长过程中的分子识别。虽然这个过程的直接分子模拟仍然涉及非常困难的问题，但我们在这里报告了我们最近为更好地理解螺旋聚合物的结晶所做的努力。本文首先简要回顾了以往对简单类聚乙烯聚合物的研究，然后介绍了几种系统地使其更加复杂的螺旋聚合物模型。我们已经报道了一种简单的类聚乙烯聚合物从熔体中快速结晶成链折叠片。通过引入合适的键角和二面角电位，对这个简单的聚合物模型进行了轻微的修改，得到了螺旋聚合物的现有模型之一。这种螺旋聚合物模型被设计成相对刚性但可移动，易于螺旋反转，并且对间扭式键有明确的偏好。我们发现这种高流动性的螺旋聚合物表现出快速链折叠结晶，形成近似4/1的螺旋结构。分子内秩序和分子间秩序几乎同时发展，表明这种现象具有高度合作的性质。进一步完善螺旋模型，为螺旋逆转提供了垂侧基和更高的能量势垒，使我们得到了一个现实的iPP统一原子模型。采用传统的和多正则蒙特卡罗模拟方法来寻找链折叠和基态构象的可能模式。虽然一个很短的链很容易形成一个规则的3/1螺旋交替的反式和间扭式键，但在规则折叠构象中，30-和50-丙烯单位的更长的链没有发现有能量基态。
Crystallization of helical polymers is a very big challenge for molecular simulation. It involves many significant issues, such as folding in biomolecules and molecular recognition during crystal growth. Though direct molecular simulations of the process still involve very difficult problems, we here report our recent efforts toward better understanding of the crystallization in helical polymers. We begin with a brief review of our former studies on simple polyethylene-like polymers, and then we introduce several helical polymer models which are systematically made more complicated. We have already reported that a simple polyethylene-like polymer crystallizes very fast into chain folded lamellae from the melt. A slight modification of this simple polymer model by introducing proper bond angle and dihedral angle potentials gives one of the present models of the helical polymer. This helical polymer model is devised to be relatively rigid but mobile, to show easy helix-reversals, and to have a definite preference for gauche bonds. We find that this highly mobile helical polymer shows quick chain folded crystallization and forms approximate 4/1 helical structure. The intra- and the intermolecular order grow quite simultaneously suggesting highly cooperative nature of the phenomena. Further elaboration of the helical model, giving pendant side groups and higher energy barrier to the helix reversals, leads us to a realistic united atom model of iPP. The conventional and the multi-canonical Monte Carlo simulations are applied to find probable modes of chain folding and the ground state conformations. Though a very short chain readily forms a regular 3/1 helix of alternating trans and gauche bonds, much longer chains of 30- and 50-propylene units are not found to have energetic ground states in the regularly folded conformations.