Lau Y G J, Richardson Robert M, Cubitt R
Institut Laue-Langevin, 6 Rue Jules Horowitz, BP 156-38042 Grenoble, Cedex 9, France.
J Chem Phys. 2006 Jun 21;124(23):234910. doi: 10.1063/1.2203067.
Neutron reflection was used to measure the buildup of layers at a solid surface as the smectic phase is approached from higher temperatures in a nematic liquid crystal. The liquid crystal was 4-octyl-4'-cyanobiphenyl (8CB), and the solid was silicon with one of five different surface treatments that induce homeotropic alignment: (i) silicon oxide; (ii) a cetyltrimethylammonium bromide coating; (iii) an octadecyltrichlorosilane monolayer; (iv) an n-n-dimethyl-n-octadecyl-3- aminopropyltrimethyloxysilyl chloride monolayer; and (v) a lecithin coating. The development of surface smectic layers in the nematic phase of 8CB was followed by measuring specular reflectivity and monitoring the pseudo-Bragg peak from the layers. The scattering data were processed to remove the scattering from short-ranged smecticlike fluctuations in the bulk nematic phase from the specular reflection. The pseudo-Bragg peak at scattering vector Q approximately 0.2 A(-1) therefore corresponded to the formation of long-range smectic layers at the surface. The amplitude of the smectic density wave decayed with increasing distance from the surface, and the characteristic thickness of this smectic region diverged as the transition temperature was approached. It was found that the characteristic thickness for some of the surface treatments was greater than the correlation length in the bulk nematic. The different surfaces gave different values of the smectic order parameter at the surface. This suggests that the interaction with the surface is significantly different from a "hard wall" which would give the same values of the smectic order parameter and penetration depths similar to the bulk correlation length. Comparison of the different surfaces also suggested that the strength and range of the surface smectic ordering may be varied independently.
在向列型液晶中，当从较高温度接近近晶相时，利用中子反射来测量固体表面层的堆积情况。液晶为4-辛基-4'-氰基联苯（8CB），固体为经过五种不同表面处理之一以诱导垂直排列的硅：（i）氧化硅；（ii）十六烷基三甲基溴化铵涂层；（iii）十八烷基三氯硅烷单层；（iv）n - n - 二甲基 - n - 十八烷基 - 3 - 氨丙基三甲氧基硅烷氯化物单层；以及（v）卵磷脂涂层。通过测量镜面反射率并监测来自这些层的伪布拉格峰，跟踪8CB向列相表面近晶层的形成过程。对散射数据进行处理，以从镜面反射中去除体向列相中短程近晶状涨落的散射。因此，散射矢量Q约为0.2 Å⁻¹处的伪布拉格峰对应于表面长程近晶层的形成。近晶密度波的振幅随着距表面距离的增加而衰减，并且当接近转变温度时，这个近晶区域的特征厚度发散。发现某些表面处理的特征厚度大于体向列相中的关联长度。不同表面在表面给出不同的近晶序参量值。这表明与表面的相互作用与“硬壁”有显著不同，“硬壁”会给出相同的近晶序参量值且穿透深度类似于体关联长度。不同表面的比较还表明，表面近晶有序的强度和范围可能独立变化。
