Kityk Andriy V, Busch Mark, Rau Daniel, Calus Sylwia, Cerclier Carole V, Lefort Ronan, Morineau Denis, Grelet Eric, Krause Christina, Schönhals Andreas, Frick Bernhard, Huber Patrick
Materials Physics and Technology, Hamburg University of Technology (TUHH), D-21073 Hamburg, Germany.
Soft Matter. 2014 Jul 7;10(25):4522-34. doi: 10.1039/c4sm00211c.
Optical polarimetry measurements of the orientational order of a discotic liquid crystal based on a pyrene derivative confined in parallelly aligned nanochannels of monolithic, mesoporous alumina, silica, and silicon as a function of temperature, channel radius (3-22 nm) and surface chemistry reveal a competition of radial and axial columnar orders. The evolution of the orientational order parameter of the confined systems is continuous, in contrast to the discontinuous transition in the bulk. For channel radii larger than 10 nm we suggest several, alternative defect structures, which are compatible both with the optical experiments on the collective molecular orientation presented here and with a translational, radial columnar order reported in previous diffraction studies. For smaller channel radii our observations can semi-quantitatively be described by a Landau-de Gennes model with a nematic shell of radially ordered columns (affected by elastic splay deformations) that coexists with an orientationally disordered, isotropic core. For these structures, the cylindrical phase boundaries are predicted to move from the channel walls to the channel centres upon cooling, and vice-versa upon heating, in accord with the pronounced cooling/heating hystereses observed and the scaling behavior of the transition temperatures with the channel diameter. The absence of experimental hints of a paranematic state is consistent with a biquadratic coupling of the splay deformations to the order parameter.
基于芘衍生物的盘状液晶在整体介孔氧化铝、二氧化硅和硅的平行排列纳米通道中的取向有序性的光学偏振测量，作为温度、通道半径（3 - 22纳米）和表面化学的函数，揭示了径向和轴向柱状有序性之间的竞争。与本体中的不连续转变相反，受限系统的取向有序参数的演变是连续的。对于通道半径大于10纳米的情况，我们提出了几种替代的缺陷结构，这些结构既与本文中关于集体分子取向的光学实验兼容，也与先前衍射研究中报道的平移径向柱状有序性兼容。对于较小的通道半径，我们的观察结果可以通过Landau-de Gennes模型进行半定量描述，该模型具有径向有序柱的向列壳层（受弹性展曲变形影响），与取向无序的各向同性核心共存。对于这些结构，预计圆柱形相界在冷却时会从通道壁移动到通道中心，加热时则相反，这与观察到的明显的冷却/加热滞后现象以及转变温度与通道直径的标度行为一致。不存在顺磁态的实验迹象与展曲变形与有序参数的双二次耦合一致。
