Date of Award
Doctor of Philosophy (PhD)
Materials Science & Engineering
Liquid crystals (LCs), owing to their anisotropy in molecular ordering, are of interests not only in the display industry, but also in the soft matter community, e.g., to direct colloidal assembly and phase separation of surfactants, and to actuate two-dimensional (2D) sheets into three-dimension (3D). The functionality and performance of LC materials extensively rely on the molecular ordering and alignment of LCs, which are dictated by LC anchoring at various boundaries. Therefore, this thesis focuses on the study of LC anchoring from both small molecule LCs and liquid crystal monomers (LCMs), which in turn guides my design of surface topography and surface chemistry to control formation of uniform LC defect structures over cm2 samples under complex boundary conditions. The ability to precisely embed defect structures in a LC material also allows me to exploit the responsiveness of LCs to create actuators and scaffolds to (dis)assemble nano- and micro-objects. Specifically, by exploiting the bulk disclinations formed in the nematic phase of 4-octyl-4’-cyanobiphenyl (8CB) surrounding the micropillar arrays, we demonstrate (dis)assembly of gold nano-rods (AuNRs) for dynamic tuning of surface plasmon resonance (SPR). Due to the highly temperature-sensitive elastic anisotropy of 8CB, the bulk disclinations and consequently the AuNR assemblies and SPR properties can be altered reversibly by heating and cooling the LC system. Then we design and synthesize a new type of nematic LCMs with a very large nematic window. Therefore, they can be faithfully aligned at various boundary conditions, analogous to that of small molecule LCs. After crosslinking LCMs into liquid crystal polymers (LCPs), we are able to study the LC assembly, director field, and topological defects using scanning electronic microscopy (SEM) at the 100 nm resolution. We then turn our attention to direct LCM alignment through controlling of surface chemistry and topography. We demonstrate the essential role of surface chemistry in the fabrication of liquid crystal elastomer (LCE) micropillar arrays during soft lithography. A monodomain LCM alignment is achieved in a poly(2-hydroxyethyl methacrylate) coated polydimethylsiloxane (PDMS) mold. After crosslinking, the resultant LCE micropillars display a large radial strain (~30%) when heated across the nematic-isotropic phase transition temperature (TNI). The understanding of surface alignment in LCMs is then transferred to LCEs with embedded topological defects. On micron-sized one-dimensional channels with planar surface chemistry, LCMs can be faithfully oriented along the local channel direction. After crosslinking, the 2D LCE sheets show pre-programmed shape transformation to complex 3D structures through bending and stretching of local directors when heated above TNI. Last, we control LC alignment and defect formation on a flat surface simply by using chemical patterns. Planar anchored SU8 is photopatterned on homoetropically anchored dimethyloctadecyl[3-(trimethoxysilyl)propyl] ammonium chloride (DMOAP) coated glass. By exploiting the pattern geometry, thus, boundary conditions, in combination with anisotropy of LC elasticity, we show that LC orientation can be precisely controlled over a large area and various types of topological defects are generated. Such defect structures can be further used to trap micro- and nanoparticles.
Xia, Yu, "Liquid Crystal Anchoring Control and its Applications in Responsive Materials" (2016). Publicly Accessible Penn Dissertations. 2110.