嫁接于平行板的受限紧密高分子链末端距分布函数的研究

采用剪除增加Rosenbluth方法(Pruned-enriched-Rosenbluth method,PERM)算法计算了嫁接于平行板的受限紧密高分子链的末端距分布函数.由于受限紧密高分子链具有各向异性,重点研究了平行板方向x轴上的分布函数P(x),发现P(x)可以表示为ln[P(x)/Pm(x)]/ND-5/3=a0+a1u+a2u2+a3u3(其中u=x/ND-2/3).这里N为链长,Pm(x)为分布函数P(x)的最大值,两平行板的间距为D+1.通过计算P(x)的Shannon熵发现末端距分布函数P(x)的Shannon熵可以用来描述高分子链受限的程度,Shannon熵对平行板间距的变化非常敏感,对于同一链长N,P(x)的Shannon熵会随着D的增大而迅速减小,超过临界值Dc会趋向一个定值,即当D≥Dc时Shannon熵将趋于稳定,也说明了此时受限条件对紧密高分子链影响非常小.同时临界值Dc与链长N有关,Dc~Nλ,其中λ=0.543,并进行了一定的理论分析.     

紧密高分子链从无规线团到球形相变的蒙特卡洛模拟：链单体的相互作用

本文采用蒙特卡洛模拟研究紧密高分子单链的无规线团到球形转变过程. 首先,研究某一固定链单体的相互作用下的有效尺寸效应,并发现短链表现出一次的坍塌相变,而长链则出现两次,这从热熔曲线上可以明显看出. 随着链单体相互作用的减小,热熔曲线上峰值对应的转变温度朝着低温转移. 从体系的能量、均方回转半径和形状因子可以进一步看出,高分子相变发生在更低的温度区间. 这些结果有助于加深理解高分子坍塌相变过程中链单体相互作用的影响.     

The collapse of polyethylene rings on an attractive surface

Polymers exhibit extended structures at high temperatures or in good solvents and collapsed configurations at low temperatures or in poor solvents. This fundamental property is crucial to the design of materials, and indeed has been extensively studied in recent years. In this paper, the collapse of polyethylene rings on an attractive surface was investigated by using molecular dynamics simulations. It is found that the collapse of ring chains on the attractive surface is of distinct difference from their free counterparts, where the collapse becomes more continuous and a one-stage instead of two-stage collapse can be identified by the specific heat. Some hairpin-like crystal structures are found at low temperatures, which are induced by the adsorption interaction of polymer-surface. For a given chain length, the results were further compared with those of the adsorbed linear chains. Due to the topological constraint of ring chains, the number of hairpin structures is clearly less than that of the linear chains. These numerical simulations may provide some new insights into the folding of ring polymers under adsorption interactions.     

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

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.