Malone Stephanie M, Schwartz Daniel K
Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309-0424, USA.
Langmuir. 2008 Sep 2;24(17):9790-4. doi: 10.1021/la801322x. Epub 2008 Aug 8.
Alkylsilane self-assembled monolayers (SAMs) on oxide substrates are commonly used as liquid crystal (LC) alignment layers. We have studied the effects of alkyl chain length, photolytic degradation, and mechanical rubbing on polar and azimuthal LC anchoring. Both gradient surfaces (fabricated using photolytic degradation of C18 SAMs) and unirradiated SAMs composed of short alkyl chains show abrupt transitions from homeotropic to tilted alignment as a function of degradation or chain length. In both cases, the transition from homeotropic to tilted anchoring corresponds to increasing wettability of the SAM surfaces. However, there is an offset in the critical contact angle for the transition on gradient vs unirradiated SAMs, suggesting that layer thickness is more relevant than wettability for LC alignment. Mechanical rubbing can induce azimuthal alignment along the rubbing direction for alignment layers sufficiently near the homeotropic-to-planar transition. Notably, mechanical rubbing causes a small but significant shift in the homeotropic-to-tilted transition, e.g., unrubbed C5 SAMs induce homeotropic anchoring, but the same surface after rubbing induces LC pretilt.
氧化物衬底上的烷基硅烷自组装单分子层（SAMs）通常用作液晶（LC）取向层。我们研究了烷基链长度、光解降解和机械摩擦对极性和方位角LC锚定的影响。梯度表面（通过C18 SAMs的光解降解制备）和由短烷基链组成的未辐照SAMs都显示出从垂直取向到倾斜取向的突然转变，这是降解或链长度的函数。在这两种情况下，从垂直锚定到倾斜锚定的转变对应于SAM表面润湿性的增加。然而，梯度SAMs与未辐照SAMs转变的临界接触角存在偏移，这表明对于LC取向，层厚度比润湿性更重要。对于足够接近垂直取向到平面取向转变的取向层，机械摩擦可沿摩擦方向诱导方位角取向。值得注意的是，机械摩擦会导致垂直取向到倾斜取向的转变发生小但显著的偏移，例如，未摩擦的C5 SAMs诱导垂直锚定，但摩擦后的相同表面会诱导LC预倾斜。
