MACROMOLECULAR ORDERING; MOLECULAR DYNAMICS OF CHAIN FOLDING

合成聚合物、蛋白质和DNA等大分子由于其独特的性质而引起了人们的极大关注，而这些性质在通常的低分子物质中从未被研究过。特别是大分子的有序化是一个重要的研究方向，因为它是高分子结晶、蛋白质折叠等过程的基础。本文通过实验和计算机模拟研究了大分子有序化的分子过程，首先研究了高分子晶体生长表面二次成核的分子机制。采用500个弹簧连接的聚乙烯分子的简化模型，忽略周围分子，考虑从气相结晶。在真空条件下，将一个强塌缩的链置于聚合物晶体的无限宽的侧表面附近，用分子动力学模拟研究了分子的吸附和有序化过程.发现T 关于我们 该聚合物链以逐步的方式被快速吸附，以形成层结构：在较低温度下为多层，在较高温度下为单层。吸附到表面上的链段彼此平行排列，并逐渐在几ns内生长成一个整齐的链折叠片层，主要是相邻的折返折叠。层状结构的厚度明显地依赖于结晶温度，并在熔点附近迅速增加。最近发现链状分子，如正构烷烃，在液体的自由表面上呈现单层或双层结构。我们还用分子动力学模拟研究了表面结晶、单分子层的形成、单分子层内以及单分子层与维持熔融相之间的链扩散，除了这些关于聚合物有序化的工作外，我们还用实验方法和计算机模拟研究了聚合物晶体中的结构相变。少

Macromolecules, such as synthetic polymers, proteins, and DNAs, have attracted great attention due to their novel properties that have never been studied for usual low molecular mass substances. Especially the macromolecular, ordering is of great interest since it is a fundamental process involved in polymer crystallization, protein folding, etc. We have here investigated the molecular processes of macromolecular ordering by experiments and computer simulations.We have first investigated the molecular mechanism of secondary nucleation on a growth surface of the polymer crystal. We adopt a simplified model of polyethylene molecule made of 500 beads connected by springs, and consider the crystallization from a vapor phase neglecting surrounding molecules. A strongly collapsed chain in a vacuum was placed near the infinitely wide lateral-surface of the polymer crystal, and the molecular processes of adsorption and ordering were investigated by molecular dynamics simulation. It was found t … More hat the polymer chain is quickly adsorbed, in a stepwise manner, to form a layer structure : a multilayered at lower temperature and a monolayered at higher temperature. The chain segments adsorbed to the surface align parallel with each other and gradually, within several ns, grow into a neat chain folded lamella with predominantly adjacent-reentry folds. The thickness of the lamella sensibly depends on the crystallization temperature and shows a rapid increase around the melting point.The chain molecules, such as n- alkanes, are recently found to show monolayers or bilayers on free surface of the liquid. We have also investigated, by molecular dynamics simulation, the surface crystallization, monolayer formation, and chain diffusion within the monolayer and between the monolayer and the sustaining melt phase.Beside these works on polymer ordering, we have made investigations on the structural phase transitions in polymer crystals both by experimental methods and by computer simulation. Less