Enhancement of the orientational order parameter of nematic liquid crystals in thin cells
Enhancement of the orientational order parameter of nematic liquid crystals in thin cells
复制标题
薄盒中向列液晶取向有序参数的增强
DOI：
10.1140/epje/i2003-10084-8
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发表时间：
2004
期刊：
影响因子：
--
通讯作者：
N. V. Madhusudana
中科院分区：
文献类型：
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作者：
S. Dhara;N. V. Madhusudana
我们报告了几种向列液晶的双折射测量，这些向列液晶在薄（1.4 to）和厚（7 to cell）中具有横向和纵向偶极矩。摩擦的聚酰亚胺涂层玻璃板用于获得这些单元中向列指向矢的平面对准。我们发现所有具有芳香核的化合物的薄单元中的（6 to）（其中Si是取向顺序参数）显着增强，即使在远低于向列相各向同性转变点的温度下也是如此。在具有多个苯环的化合物中，如果苯环的数量减少，则没有观察到显着的增强，这意味着表面电势的强度取决于核的芳香性，基于Landau-de Gennes理论的计算表明，随着向列相中温度的降低，测量的有序参数S会进一步增强。然而，弹性常数的硬化减少了向列指向矢的热波动，但在低温下，综合效应太小，无法解释实验数据。
We report measurements of birefringenceof several nematic liquid crystals having transverse as well as longitudinal dipole moments in thin (1.4 to) and thick (7 tocells. Rubbed polyimide-coated glass plates are used to get planar alignment of the nematic director in these cells. We find significant enhancement (6 to) of(, whereSis the orientational order parameter) in thin cells in all compounds witharomatic coreseven at temperatures far (C) below the nematic-isotropic transition point. The enhancement is larger in compounds having several phenyl rings and lower if the number of phenyl rings is reduced. In a compound that does not have an aromatic core no significant enhancement is observed, implying that the strength of the surface potential depends on the aromaticity of the cores. Assuming a perfect orientational order at the surface, calculations based on the Landau-de Gennes theory show that the thickness averaged enhancement ofSis sharply reduced as the temperature is lowered in the nematic phase. Themeasuredorder parameterSis further enhanced in thin cells because of the stiffening of the elastic constant which reduces the thermal fluctuations of the nematic director. The combined effect is however too small at low temperatures to account for the experimental data.