Optical characterization of the nematic lyotropic chromonic liquid crystals: Light absorption, birefringence, and scalar order parameter
Optical characterization of the nematic lyotropic chromonic liquid crystals: Light absorption, birefringence, and scalar order parameter
复制标题
DOI：
10.1103/physreve.72.041711
复制
发表时间：
2005-10-01
影响因子：
2.4
通讯作者：
Lavrentovich, OD
中科院分区：
文献类型：
--
作者：
Nastishin, YA;Liu, H;Lavrentovich, OD
我们报告了在排列良好的平面样品中由溶致发色液晶 (LCLC) 形成的向列 (N) 相的光学特性。 LCLC属于由一维分子自组装形成的一大类材料，与“活性聚合物”和“蠕虫状胶束”等其他系统类似。我们研究了三种水溶性 LCLC 形成材料：色甘酸二钠、阴丹酮衍生物 Blue 27 和苝衍生物 Violet 20。单个分子具有板状形状，一旦浓度超过约 0.1M，就会组装成棒状聚集体，形成 N 相。 N 相的均匀表面排列是通过抛光的聚酰亚胺层实现的。根据光吸收各向异性数据，分子平面平均垂直于聚合轴，从而垂直于向列指向矢。我们确定了这些材料在 N 和双相 N-各向同性 (I) 区域的双折射率，发现其为负值，且与典型热致低分子量向列型材料的双折射率相比，其绝对值明显较低。在吸收材料蓝色 27 和紫色 20 中，由于吸收带附近反常色散的影响，双折射的波长依赖性是非单调的。我们描述了作为双相 N-I 区域新相核形成的正和负类晶团簇（该区域在所研究的所有三种材料中都很宽）。最后，我们确定了 Blue 27 N 相的标量阶次参数，发现其相对较高，在 0.72-0.79 范围内，这使得该发现处于 Onsager 模型的一般有效性范围内。然而，观察到的标量序参数的温度依赖性表明了非热 Onsager 模型中未考虑的因素的重要性，例如聚集体间的相互作用和聚集体长度的温度依赖性。
We report on the optical properties of the nematic (N) phase formed by lyotropic chromonic liquid crystals (LCLCs) in well aligned planar samples. LCLCs belong to a broad class of materials formed by one-dimensional molecular self-assembly and are similar to other systems such as "living polymers" and "wormlike micelles." We study three water soluble LCLC forming materials: disodium chromoglycate, a derivative of indanthrone called Blue 27, and a derivative of perylene called Violet 20. The individual molecules have a planklike shape and assemble into rodlike aggregates that form the N phase once the concentration exceeds about 0.1M. The uniform surface alignment of the N phase is achieved by buffed polyimide layers. According to the light absorption anisotropy data, the molecular planes are on average perpendicular to the aggregate axes and thus to the nematic director. We determined the birefringence of these materials in the N and biphasic N-isotropic (I) regions and found it to be negative and significantly lower in the absolute value as compared to the birefringence of typical thermotropic low-molecular-weight nematic materials. In the absorbing materials Blue 27 and Violet 20, the wavelength dependence of birefringence is nonmonotonic because of the effect of anomalous dispersion near the absorption bands. We describe positive and negative tactoids formed as the nuclei of the new phase in the biphasic N-I region (which is wide in all three materials studied). Finally, we determined the scalar order parameter of the N phase of Blue 27 and found it to be relatively high, in the range 0.72-0.79, which puts the finding into the domain of general validity of the Onsager model. However, the observed temperature dependence of the scalar order parameter points to the importance of factors not accounted for in the athermal Onsager model, such as interaggregate interactions and the temperature dependence of the aggregate length.