Huang Aiqun, Hsu Hsiao-Ping, Bhattacharya Aniket, Binder Kurt
Department of Physics, University of Central Florida, Orlando, Florida 32816-2385, USA.
Institut für Physik, Johannes Gutenberg-Universität Mainz, Staudinger Weg 9, D-55099 Mainz, Germany.
J Chem Phys. 2015 Dec 28;143(24):243102. doi: 10.1063/1.4929600.
The conformations of semiflexible polymers in two dimensions confined in a strip of width D are studied by computer simulations, investigating two different models for the mechanism by which chain stiffness is realized. One model (studied by molecular dynamics) is a bead-spring model in the continuum, where stiffness is controlled by a bond angle potential allowing for arbitrary bond angles. The other model (studied by Monte Carlo) is a self-avoiding walk chain on the square lattice, where only discrete bond angles (0° and ±90°) are possible, and the bond angle potential then controls the density of kinks along the chain contour. The first model is a crude description of DNA-like biopolymers, while the second model (roughly) describes synthetic polymers like alkane chains. It is first demonstrated that in the bulk the crossover from rods to self-avoiding walks for both models is very similar, when one studies average chain linear dimensions, transverse fluctuations, etc., despite their differences in local conformations. However, in quasi-one-dimensional confinement two significant differences between both models occur: (i) The persistence length (extracted from the average cosine of the bond angle) gets renormalized for the lattice model when D gets less than the bulk persistence length, while in the continuum model it stays unchanged. (ii) The monomer density near the repulsive walls for semiflexible polymers is compatible with a power law predicted for the Kratky-Porod model in the case of the bead-spring model, while for the lattice case it tends to a nonzero constant across the strip. However, for the density of chain ends, such a constant behavior seems to occur for both models, unlike the power law observed for flexible polymers. In the regime where the bulk persistence length ℓp is comparable to D, hairpin conformations are detected, and the chain linear dimensions are discussed in terms of a crossover from the Daoud/De Gennes "string of blobs"-picture to the flexible rod picture when D decreases and/or the chain stiffness increases. Introducing a suitable further coarse-graining of the chain contours of the continuum model, direct estimates for the deflection length and its distribution could be obtained.
通过计算机模拟研究了限制在宽度为D的条带中的二维半柔性聚合物的构象，研究了实现链刚度的两种不同机制模型。一种模型（通过分子动力学研究）是连续介质中的珠簧模型，其中刚度由允许任意键角的键角势控制。另一种模型（通过蒙特卡罗研究）是方形晶格上的自回避行走链，其中仅可能存在离散键角（0°和±90°），然后键角势控制沿链轮廓的扭结密度。第一个模型是对类DNA生物聚合物的粗略描述，而第二个模型（大致）描述了像烷烃链这样的合成聚合物。首先证明，在本体中，当研究平均链线性尺寸、横向涨落等时，尽管两种模型在局部构象上存在差异，但它们从棒状到自回避行走的转变非常相似。然而，在准一维限制中，两种模型出现了两个显著差异：（i）当D小于本体持久长度时，晶格模型的持久长度（从键角的平均余弦提取）会被重整化，而在连续介质模型中它保持不变。（ii）对于半柔性聚合物，在排斥壁附近的单体密度与珠簧模型情况下Kratky-Porod模型预测的幂律兼容，而对于晶格情况，它在条带中趋于一个非零常数。然而，对于链端密度，两种模型似乎都出现这种恒定行为，这与柔性聚合物中观察到的幂律不同。在本体持久长度ℓp与D相当的区域中，检测到发夹构象，并且当D减小和/或链刚度增加时，根据从Daoud/De Gennes“串珠”图像到柔性棒图像的转变来讨论链的线性尺寸。通过对连续介质模型的链轮廓进行适当的进一步粗粒化，可以获得挠曲长度及其分布的直接估计。
