Alexe-Ionescu A L, Barbero G, Komitov L
Liquid Crystal Group, Department of Physics, University of Gothenburg, SE-412 96 Gothenburg, Sweden.
Phys Rev E Stat Nonlin Soft Matter Phys. 2009 Aug;80(2 Pt 1):021701. doi: 10.1103/PhysRevE.80.021701. Epub 2009 Aug 6.
A generalization of the functional form of the anchoring energy of a nematic liquid crystal in contact with a structured surface is proposed. The structured surface is represented by side-chain polymer, containing only aliphatic or aliphatic and mesogenic side groups, deposited as a thin alignment film onto a solid substrate. The basic assumption in the model developed in the present work is that the coupling between the polymer side groups and the nematic liquid crystal molecules is responsible for the elastic deformation and compression of the orienting groups taking place in the alignment film. We show that in the limit of small deformations, the surface energy is the same as the one given by the Rapini-Papoular approximation. In this limit, the effective anchoring strength is weak and is due to the elastic deformation of the orienting side groups, which is proportional to the inverse of the length of the aliphatic groups and the length of the linkage group of the mesogens, respectively. In the limit of large deformation, however, the term accounting the film compression is predominant which results in a nonzero torque facilitating the relaxation process of an imposed deformation when the applied electric field is switched off. Good agreement is found between the prediction of our model and the experimental data of the anchoring energy of a nematic liquid crystal aligned by means of Langmuir-Blodgett monolayers of fatty acids as a function of the length of their alkyl chains.
提出了向列型液晶与结构化表面接触时锚定能函数形式的一种推广。结构化表面由侧链聚合物表示，该聚合物仅包含脂肪族或脂肪族和介晶侧基，作为薄取向膜沉积在固体基板上。本工作中建立的模型的基本假设是，聚合物侧基与向列型液晶分子之间的耦合导致了取向膜中发生的取向基团的弹性变形和压缩。我们表明，在小变形极限下，表面能与拉皮尼 - 帕普洛拉尔近似给出的表面能相同。在此极限下，有效锚定强度较弱，这是由于取向侧基的弹性变形，其分别与脂肪族基团的长度和介晶的连接基团的长度成反比。然而，在大变形极限下，考虑膜压缩的项占主导，这导致当施加的电场关闭时，存在一个非零扭矩，促进了施加变形的弛豫过程。我们的模型预测与通过脂肪酸的朗缪尔 - 布洛杰特单分子层排列的向列型液晶的锚定能实验数据（作为其烷基链长度的函数）之间发现了良好的一致性。
