Department of Physics, Faculty of Science, Ege University, Bornova, 35100 Izmir, Turkey.
J Chem Phys. 2013 Mar 21;138(11):114902. doi: 10.1063/1.4794920.
The connection between the molecular structure of liquid crystals and their elastic properties, which control the director deformations relevant for electro-optic applications, remains a challenging objective for theories and computations. Here, we compare two methods that have been proposed to this purpose, both characterized by a detailed molecular level description. One is an integrated molecular dynamics-statistical mechanical approach, where the bulk elastic constants of nematics are calculated from the direct correlation function (DCFs) and the single molecule orientational distribution function [D. A. McQuarrie, Statistical Mechanics (Harper & Row, New York, 1973)]. The latter is obtained from atomistic molecular dynamics trajectories, together with the radial distribution function, from which the DCF is then determined by solving the Ornstein-Zernike equation. The other approach is based on a molecular field theory, where the potential of mean torque experienced by a mesogen in the liquid crystal phase is parameterized according to its molecular surface. In this case, the calculation of elastic constants is combined with the Monte Carlo sampling of single molecule conformations. Using these different approaches, but the same description, at the level of molecular geometry and torsional potentials, we have investigated the elastic properties of the nematic phase of two typical mesogens, 4'-n-pentyloxy-4-cyanobiphenyl and 4'-n-heptyloxy-4-cyanobiphenyl. Both methods yield K3(bend) >K1 (splay) >K2 (twist), although there are some discrepancies in the average elastic constants and in their anisotropy. These are interpreted in terms of the different approximations and the different ways of accounting for the structural properties of molecules in the two approaches. In general, the results point to the role of the molecular shape, which is modulated by the conformational freedom and cannot be fully accounted for by a single descriptor such as the aspect ratio.
液晶的分子结构与其弹性性质之间的关系，控制着与电光应用相关的指向矢变形，这对理论和计算来说仍然是一个具有挑战性的目标。在这里，我们比较了两种为此目的提出的方法，这两种方法都具有详细的分子水平描述。一种是集成的分子动力学-统计力学方法，其中各向异性体的体弹性常数是从直接相关函数（DCF）和单分子取向分布函数[D. A. McQuarrie，统计力学（Harper & Row，纽约，1973）]计算得出的。后者是从原子分子动力学轨迹与径向分布函数一起获得的，从径向分布函数中，可以通过求解奥恩斯坦-泽尔尼克方程来确定 DCF。另一种方法基于分子场理论，其中在液晶相中介晶所经历的平均扭矩势根据其分子表面进行参数化。在这种情况下，弹性常数的计算与单分子构象的蒙特卡罗抽样相结合。使用这些不同的方法，但在分子几何形状和扭转势的水平上使用相同的描述，我们研究了两个典型介晶体，4'-n-戊氧基-4-氰基联苯和 4'-n-庚氧基-4-氰基联苯的向列相弹性性质。这两种方法都得到 K3（弯曲）>K1（展曲）>K2（扭曲），尽管平均弹性常数及其各向异性存在一些差异。这些差异可以用两种方法的不同近似和不同方式来解释分子的结构性质。一般来说，结果表明了分子形状的作用，分子形状由构象自由度调制，而不能仅由单一描述符（如纵横比）来完全描述。
