Order and Disruption of Order Induced by Self-Assembled Monolayers

Self-assembly is defined as objects spontaneously and reversibly arranging themselves in the absence of outside direction. It involves a delicate balance between interparticle and intermolecular attraction and repulsion forces, thus resulting in a lower thermodynamic energy for an ordered state. In this work, the effect of self-assembly has been studied in both thermotropic and lyotropic liquid crystals. The utility of liquid crystals lies in the synergistic alignment that arises from the inherent elasticity of the material. Liquid crystals orient themselves based on the surface chemistry or topography of a substrate, and the bulk of the material will then align in concert with that surface layer. This allows amplification of any surface interactions, or as studied in this work, surface order, which can then be easily characterized with polarized light. Thermotropic, nematic liquid crystals were used to probe the relationship of the chain length of alkylsilane self-assembled monolayers (SAMs) to surface energy. It was found that the liquid crystals interact with only the outermost 4 molecules in a SAM. Additionally, it was found that rubbing can not only induce azimuthal orientation on chains longer than 5, but that it can alter the surface energy and impart polar liquid crystal tilt. Thermotropic liquid crystals were also used to further understand the extent of surface passivation of glycidoxypropyltrimethoxysilane (GPTMS) SAMs. While it was expected that this surface would have low azimuthal anchoring energy, this property was exploited for the detection of double-stranded DNA (dsDNA). The chirality of the molecule, combined with negligible anchoring energy, combined to allow propagation of the unique chiral alignment of -32±4 when exposed to extended dsDNA Finally, it was found that not only do linactants self-assemble into aggregates in 2D Langmuir-Blodgett films, but that linactants also followed the model of 3D micelles, despite being fundamentally different than micelles or hemimicelles. The size scale of these molecules are ~37 nm, rather than limited to the size of two molecules as with a micelle, despite an apparent CMC, as well as eventual onset of lyotropic phases