Several series of reversible physical networks based on polydimethylsiloxane (PDMS) chains and liquid crystalline (LC) structural units were synthesized and studied, which belong to three different architecture types: (1) LC-grafted PDMS (with LC quartets at the grafting sites), (2) LC-end-capped linear PDMS, (3) and linear 'infinite' LC-PDMS copolymers (with alternating LC and PDMS segments). PDMS spacers of different lengths of were tested, as well as 6 different polyaromatic mesogens of azo type and azo-free type. Hydrosilylation coupling of vinyl-functional mesogens, obtained as part of an international cooperation, with commercial Si-H-functional PDMS was employed to synthesize all the studied materials. The copolymers were physically crosslinked by the nano-aggregation of the LC units contained in their macromolecules. The thermotropic properties of the LC-nano-aggregates lent interesting physical properties to the whole material, making some of the copolymers attractive as potential smart materials. The PDMS spacer segments were selected for the sake of their extreme flexibility, which should provide elastic properties to the physically crosslinked copolymers, and also because of their (desired) incompatibility with the mesogens. This latter effect was highly helpful for achieving the...

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Liquid crystal devices are attractive for many applications such as information displays, spatial light modulators and adaptive optics, because their optical properties are electrically tunable. However, response time of liquid crystal devices is a serious concern for many applications especially for those who require large phase modulation (≥2π). This is because a thick LC layer is usually needed to achieve a large phase shift while the response time of a nematic LC is highly determined by the cell gap.

This dissertation discusses cholesteric liquid crystals (CLCs) and polymers based photonic devices including one-dimensional (1D) photonic crystal lasers and broadband circular polarizers. CLCs showing unique self-organized chiral structures have been widely used in bistable displays, flexible displays, and reflectors. However, the photonic band gap they exhibit opens a new way for generating laser light at the photonic band edge (PBE) or inside the band gap. When doped with an emissive laser dye, cholesteric liquid crystals provide distributed feedback so that mirrorless lasing is hence possible. Due to the limited surface anchoring, the thickness of gain medium and feedback length is tens of micrometers. Therefore lasing efficiency is quite limited and laser beam is highly divergent. To meet the challenges, we demonstrated several new methods to enhance the laser emission while reducing the beam divergence from a cholesteric liquid crystal laser. Enhanced laser emission is demonstrated by incorporating a single external CLC reflector as a polarization conserved reflector. Because the distributed feedback from the active layer is polarization selective, a CLC reflector preserves the original polarization of the reflected light and a further stimulated amplification ensues. As a result of virtually doubled feedback length, the output is dramatically enhanced in the same circular polarization state. Meanwhile, the laser beam divergence is dramatically reduced due to the increased cavity length from micrometer to millimeter scale. Enhanced laser emission is also demonstrated by the in-cell metallic reflector because the active layer is pumped twice. Unlike a CLC reflector, the output from a mirror-reflected CLC laser is linearly polarized as a result of coherent superposition of two orthogonal circular polarization states. The output linear polarization direction can be well controlled and fine tuned by varying the operating temperature and cell gap. Enhanced laser emission is further demonstrated in a hybrid photonic band edge - Fabry-Perot (FP) type structure by sandwiching the CLC active layer within a circular polarized resonator consisting of two CLC reflectors. The resonator generates multiple FP modes while preserving the PBE mode from the active layer. More importantly this band edge mode can be greatly enhanced by the external resonator under some conditions. Theoretical analysis is conducted based on 4×4 transfer matrix and scattering matrix and the results are consistent with our experimental observations. To make the CLC laser more compact and miniaturized, we have developed a flexible polymer laser using dye-doped cholesteric polymeric films. By stacking the mirror reflecting layer, the active layer and the CLC reflecting layer, enhanced laser emission was observed in opposite-handed circular polarization state, because of the light recycling effect. On the other hand, we use the stacked cholesteric liquid crystal films, or the cholesteric liquid crystals and polymer composite films to demonstrate the single film broadband circular polarizers, which are helpful for converting a randomly polarized light into linear polarization. New fabrication methods are proposed and the circular polarizers cover ~280 nm in the visible spectral range. Both theoretical simulation and experimental results are presented with a good match.

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Part 1. Discotic liquid crystals are a relatively new class of mesogens in which the molecules self assemble in the melt state to form highly ordered columnar stacks. The ability of the molecules to display this type of mesomorphic behavior is a function of their shape; a semi-rigid core with flexible "arms" gives the necessary flatness and broad diameter conducive to columnar stacking. We first set out to make discotic liquid crystals by synthesizing a series of three-armed aryloxy-s-triazines with aromatic Schiff's base moieties at the molecular periphery and investigate the thermal and optical behavior of these compounds. We discovered that these molecules were in fact rigid rod, or calamitic, liquid crystals based on the optical textures and X-ray diffraction patterns in the mesophase. This is in direct conflict with published but unsubstantiated reports of the "discotic” behavior of similar compounds. The failure of these compounds to give crystals suitable for X-ray crystal structural analysis prompted us to utilize electron microscopy to look at the microstructures formed when dilute solutions were evaporated onto different substrates. Surprisingly, these aryloxy-s-triazines in several different solvents formed well defined microtubules of varying dimensions on both copper and polymeric substrates. Hole diameters of up to 10³ Å and lengths of up to 0.5 cm were commonly seen using both transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Finally, we understood via molecular modeling studies that the aryloxy-s-triazines adopted a rod shape in the mesophase due to the inherent flexibility of the ether linkages at the triazine core. By substituting 1,3,5-triphenylbenzene cores in place of the s-triazine we hoped to rigidify the molecules and prompt them to stack in a discotic or columnar fashion in the melt state. This plan was successful based on the X-ray diffraction patterns and optical textures observed with these compounds in the mesophase. Part 2. α-Aminonitriles and their derivatives have played an important role in the synthesis of enantiomerically pure and racemic α-amino acids for almost ninety years. Much less studied is the alkylation behavior of this particular class of compounds. The ability of the aminonitrile moiety to be deprotonated with a base and reacted with various electrophiles allows for the placement of carbonyl functionalities virtually anywhere in a synthetic system through hydrolysis of this aminonitrile group after alkylation. Using this "umpolung”, or reversed polarity, approach we have demonstrated the utility of this class of compounds by reacting them with several activated aromatic dihalides and aliphatic dihalides to produce high molecular weight poly(bis-α-aminonitrile)s which were in turn hydrolyzed under mild conditions to afford the corresponding polymeric ketones. This ability to form both wholly aromatic and mixed aliphatic/aromatic polyketones is extremely powerful and unprecedented in the literature to date. During the course of this research, it was also discovered that some of these α-aminonitriles underwent side reactions which were undesirable for polymerization but which produced interesting compounds in their own right. These enaminonitriles and quinodimethanes which resulted from dehydrocyanation were studied extensively in order to exploit the possible polymerization of these reactive intermediates. Finally, another route to ketones is through the reaction of enamines with appropriate electrophiles followed by acid hydrolysis. Research towards polymeric ketones was Carried out using monomeric di(enamine)s and aromatic diacid chlorides with the hope of producing high molecular weight polymeric 1,3-diketones. Unfortunately, the extent of reaction was not high enough to produce high molecular weight polymers. / Ph. D.

We study minimizers of the Landau-de Gennes energy in the exterior region around a smooth 2-manifold in R3 with a constant external magnetic field present. Uniaxial boundary data and a strong tangential anchoring are imposed on the surface of the manifold and we consider the large particle limit in a regime where the magnetic field is relatively weak. Before studying the general manifold, we analyze a more simple case in which the manifold is spherical. After deriving a lower bound for the energy in this limiting regime, we prove that a director field on the boundary which maximizes its vertical component yields a minimal lower bound. We then construct a recovery sequence to show that this lower bound is in fact the optimal energy bound. These steps are later repeated in more generality for a larger class of smooth manifolds. / Thesis / Master of Science (MSc)

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This thesis presents an experimental study of the influence of an external field and alignment upon a colloid of a liquid crystal (octycyanobiphenyl denoted 8CB) and a silica gel of aerosil nano-particles. The first techniques used was an AC-calorimetry (alternating current heating) and the systems under investigation were firstly put under the influence of a magnetic field at John Hopkins University in Baltimore by professor Leheny's group. The experiments revealed changes in transition temperatures, nematic range and critical coefficient that could account for what we called a 'memory' of the above mentioned structures. The second technique, dielectric spectroscopy, was applied to the same very densities of mixture s mentioned in the first paragraph. The samples were applied in one procedure an increasingly higher alternating electric field. An overall increase of the capacitance of the sample was measured. The second experiment was to reproduce the application of the magnetic field from the AC-calorimetry experiment now with an electric field. In dielectric spectroscopy case, an increase in transition temperature after the application of the procedure was revealed.

A liquid crystal (LC) at its boundary surface adopts a preferential alignment, which is referred to as anchoring. The direction of this alignment (i.e., anchoring direction) may be perpendicular, parallel or tilted with respect to the surface. Transitions from one anchoring condition to another may occur when the parameters (e.g., temperature) charactering the surface change, as referred to as anchoring transitions. In the LC-polymer composite systems under our study, the anchoring and temperature- driven anchoring transitions of nematic fluids is very sensitive to the structure of the side chain of poly (alkyl acrylate) matrixes that encapsulate the LC. We have shown that the anchoring transition temperature of these systems can be tuned far below the nematic-to-isotropic transition temperature, by varying either the length, branching structure of the side chains of homopolymers, or the composition of copolymer of two dissimilar monomers. Both sharp and broad anchoring transitions with respect to the temperature range over which a transition occurs were observed. It is postulated that microscopic interactions between the polymer side chains and LC molecules play an important role in determining the anchoring. In particular, the conformation of the polymer side chain is proposed to have important control over the anchoring. Anchoring strength and tilt angle as a function of temperature during the anchoring transitions were also experimentally investigated, which contribute to understanding of the microscopic mechanism for such transitions. Based on the LC-polymer composites with controlled anchoring, a LC display with reverse switching mode and a novel electrically switchable diffraction grating have been demonstrated. The advantages of these devices are ease of manufacturing, low operation voltage, and mechanical stability offered by polymer matrix. Moreover, a detailed study of the director configuration of wall defects found in these composite films was carried out using fluorescence confocal polarized microscopy.