Department: College of Arts and Sciences / Department of Chemical Physics ![[clear]](close-x.png "Remove this limiter")
41 matches in the database.
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1.
Bailey, Christopher Allen.
Structure and Rheology of Some Bent Core Liquid Crystals.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2008, Kent State University
► Liquid crystal phases composed of bent core molecules have been a fascinating…
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▼ Liquid crystal phases composed of bent core molecules have been a fascinating topic in soft matter physics over the last decade. These materials which can form spontaneous polarizations (ferroelectricity) and chiral symmetry breaking with non-chiral molecules incited much interest in the scientific community. Since these early days, much research has been done on the characterization of these phases and their physical properties. In this work, we studied some of these properties by measuring the structural and viscous behavior using small amounts of material. The non-Newtonian flow characteristics of several bent core liquid crystals are characterized using a nanoliter viscometer. We show that many of these phases have viscoelastic behaviors which are fundamentally different from those observed in classical rod like liquid crystals. Finally, we studied the elasticity and dynamics of freestanding filaments which are stable structures in some bent core smectic phases. Characterization of their elasticity by resonance measurements and other techniques are in agreement with models designed to explain their microscopic smectic structures. We show that their viscosity, filament structure and stability, and microscopic structures can be understood by considering the role of steric interactions between molecules and the frustration thereof.
Advisors/Committee Members: Jakli, Antal.
Subjects: Physics
Keywords: liquid crystals, bent core liquid crystals, filaments, rheology, nanoliter viscometer
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2.
Braganza, Clinton Ignatuis.
High Dielectric Constant Materials Containing Liquid Crystals.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2009, Kent State University
► Mixtures are prepared from a thermotropic nematic liquid crystal, E7, surfactant and…
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▼ Mixtures are prepared from a thermotropic nematic liquid crystal, E7, surfactant and organic fluid for the purposes of creating a high dielectric constant, low loss material. The mixtures prepared have much better dielectric properties than the individual components. The surfactants studied included the traditional surfactant Didodecyldimethyl ammonium bromide DDAB and triblock polymers. The two organic fluids used are N-Methyl Formamide (NMF) and 1-Methyl 2 Pyrrolidione (NMP). The complex dielectric permittivities of these mixtures are studied at room temperature at frequencies between 0.01Hz and 1MHz. The mixtures and composites of these mixtures with polymers have low loss and high permittivity. The effect of conductor-dielectric interface, surfactant chain length, and surfactant mixing are also reported. Representative permittivies of 280 for mixtures containing low molecular weigh surfactants and 217 for mixtures containing high molecular weight surfactants.
Advisors/Committee Members: Chien, Liang.
Subjects: Materials science; Physics
Keywords: High Dielectric Constant; liquid crystals and surfactant
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3.
Buyuktanir, Ebru Aylin.
Electro-optical Characterization of Bistable Smectic A Liquid Crystal Displays.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2008, Kent State University
► My dissertation focuses the characterization and optimization of the electro-optical properties of…
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▼ My dissertation focuses the characterization and optimization of the electro-optical properties of smectic A (SmA) based liquid crystal (LC) displays. I present the development of robust and flexible bistable SmA LC displays utilizing polymer dispersed liquid crystal (PDLC) technology. The SmA PDLC displays produced on plastic substrates present electrically reversible memory, high contrast ratio, paper-like sunlight readability, and wide viewing angle characteristics. In order to optimize the SmA PDLC display, I investigated polymerization conditions, such as polymer concentration effect, polymerization temperature, and UV-light intensity variations. I characterized the electro-optical responses – such as static-response, time-response, threshold characteristics, and contrast ratio values – of the optimized SmA PDLC display and compared them to those of the pure SmA LC. The best electro-optical performance of SmA PDLC formulation was obtained using the combination of low (<10 mW/cm2) and high (>20 mW/cm2) UV-light curing intensity. The contrast ratio of the optimum SmA PDLC at a 5o collection angle was 83% of that of the pure SmA material on plastic substrates. I fabricated 2.5 x 2.5 in., 4 x 4 in., and 6 x 6 in. sized monochrome flexible SmA PDLC displays, as well as red, yellow, and fluorescent dyes colored SmA PDLC displays on plastic substrates. The electro-optic performance of the bistable SmA LC display consisting of a patterned field-induced polymer wall infrastructure was also studied and compared to those of pure SmA material. I found that the contrast ratio of the SmA LC encapsulated between polymer walls was much greater than that of the SmA PDLC system, approaching the contrast ratio value of the pure SmA material. I also improved the electro-optical characteristics of bistable SmA LC displays by adding ferroparticles into the system. Finally, I illustrated the unique capabilities of polarized confocal Raman microscopy (CRM) to resolve the orientational order of SmA LCs in three-dimension by investigating the characteristic vibrational bands of LC molecules. CRM provides a precise characterization of the molecular order at different depths of the LC films. I examined the director patterns of focal conic defects of smectic A LC, colloidal smectic A LC systems, and the field-oriented nematic LC in the horizontal and vertical planes.
Advisors/Committee Members: West, John.
Subjects: Materials science; Physics
Keywords: flexible bistable displays; confocal Raman imaging; PDLC; smectic liquid crystals; focal conic domains; ferroelectric colloids
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4.
Chen, Cheng.
Anchoring Transitions of Liquid Crystals on Large Angle Deposited SiOx Thin Films.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2006, Kent State University
► We have observed anchoring transitions of nematic liquid crystal mixtures on obliquely…
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▼ We have observed anchoring transitions of nematic liquid crystal mixtures on obliquely evaporated SiOx by varying the relative abundance of two components in the mixture. Of these two components, one has a longitudinal cyano group and another has lateral cyano groups. Also found is that both temperature and SiOx thickness variations are able to shift the anchoring transitions. We believe that the anchoring on SiOx has two origins: long range van der Waals potential and short range surface interactions. Since the two origins have opposite preference in liquid crystal director orientation the observed transition is caused by the change of their relative strength. We found that thermal absorption-desorption of molecules on SiOx surface is important in determining the strength of short range interactions while the layer thickness and optical properties of SiOx are important to the van der Waals potential. Based on previous work by de Gennes we proposed a model to describe the observed phenomenon.
Advisors/Committee Members: Bos, Philip J.
Subjects: Physics, General
Keywords: Liquid crystal alignment; SiOx; inorganic alignment layer; anchoring transition; temperature dependence; thickness dependence
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5.
Chen, Jia Ning.
Seed-Mediated Synthesis, Functionalization, Alignment and Characterizations of Gold Nanorods.
Degree: MS, College of Arts and Sciences / Department of Chemical Physics, 2012, Kent State University
► Gold nanoparticles possess distinguish optical properties depend on the size, shape and…
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▼ Gold nanoparticles possess distinguish optical properties depend on the size, shape and coating of the materials. In the case of gold nanorods (AuNR), the particle length difference can cause the surface plasmons resonances induce a birefringence from the longitudinal and transverse modes. My study is to find the optical properties connection of gold between bulk material and nanorod particles. I will use the seed mediated AuNR method to manipulate the particle size by changing the parameter of the method. Also, I would study the AuNR coating and alignment method, and finally characterize the optical properties of the Au nano materials.
Advisors/Committee Members: Palffy-Muhoray, Peter.
Subjects: Chemistry
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6.
Dorjgotov, Enkh-Amgalan.
Tunable Liquid Crystal Etalon and Photonic Devices.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2010, Kent State University
► Liquid crystal (LC) etalon and photonic devices are explored for applications in…
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▼ Liquid crystal (LC) etalon and photonic devices are explored for applications in high tunability wavelength select switches and display devices. A new approach to polarization independent LC etalon with twisted LC structure is proposed for applications in portable projection displays and a demo device exhibiting high transmittance, and millisecond switching speed is presented. In addition, designs for LC etalon based switchable mirror with 30 nm spectrum bandwidth and high tunability 1D photonic-crystal are numerically demonstrated.
Advisors/Committee Members: Bos, Philip.
Subjects: Optics; Physics
Keywords: Polarization Independent Liquid Crystal Etalon, Portable Projector, 1D Photonic Crystal
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7.
Fontana, Jacob Paul.
Self-assembly and characterization of anisotropic metamaterials.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2011, Kent State University
► This work focuses on experimentally synthesizing, characterizing and self-assembling gold nanorod composites…
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▼ This work focuses on experimentally synthesizing, characterizing and self-assembling gold nanorod composites for optical metamaterial applications. A method for phase transferring gold nanorods from aqueous to organic suspensions was developed. This enabled the gold nanorods to be aligned by a external electric field, achieving high order parameters. By aligning the nanorods this enabled us to develop a method to measure the real and imaginary phase shifts of light transmitted through the suspensions allowing an experimental characterization technique to be developed to determine the principal values of the electric susceptibility tensor for gold nanorods at optical frequencies. Once the optical susceptibilities of the individual nanorods were determined self-assembled densely-packed films of small aspect ratio, orientationally-ordered, silica-encased gold nanorods in square millimeter domains were created, via solvent evaporation and applied electric field.
Advisors/Committee Members: Palffy-Muhoray, Peter.
Subjects: Physics
Keywords: metamaterials; negative index materials; self-assembly; nanoparticles; susceptibility; alignment
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8.
Geng, Jun.
Self-Assembly, Elasticity, and Orientational Order in Soft Matter.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2012, Kent State University
► In this dissertation, I present four studies of soft matter: tetratic and…
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▼ In this dissertation, I present four studies of soft matter: tetratic and nematic phases of two-dimensional liquid crystals, elasticity of asymmetric thin films, morphology of vesicles with nematic in-plane order, and defect pair annihilation in vesicles with xy in-plane order. Computer simulations and analytical calculations are used to study the problems. In the first project, we propose a model for the statistical mechanics of particles with almost four-fold symmetry, which is weakly broken down to two-fold. We introduce a coefficient to characterize the symmetry breaking, and find that the tetratic phase can still exist even if this parameter is up to a substantial value. Through a Landau expansion of the free energy, we calculate the mean-field phase diagram, which is similar to the result of a previous hard-particle excluded-volume model. To verify our mean-field calculation, we develop a Monte Carlo simulation of spins on a triangular lattice. The results of the simulation agree very well with the Landau theory. In the second part, we study the deformation of a thin polymer film by a change in the target metric tensor. In this problem, unlike earlier work by other researchers in this field, the target metric is asymmetric between the two sides of the film. Changing this metric induces a curvature of the film, which may be curvature into a partial cylinder or a partial sphere. We calculate the elastic energy for each of these shapes, and show that the sphere is favored for films smaller than a critical size, which depends on the film thickness, while the cylinder is favored for larger films. In the third section, we develop a coarse-grained (CG) model to study the shape of vesicles with tangent-plane nematic order. The CG particles can self-assemble to form three dimensional vesicles and carry a tangent-plane nematic order at low temperature. As the strength of the coupling between nematic order and vesicle curvature increases, the vesicles show a morphology transition from spherical to prolate and finally to a tube. We also calculate the interesting shape and defect arrangement around the tips of the prolate vesicle. Finally, we use a modified CG model to study the role of topological defects in the formation of complex morphologies in giant unilamellar vesicles during the transition from the in-plane isotropic phase (Lα) to in-plane xy order phase (Lβ'). The studies show that such defects induce local membrane curvature which in turn inhibits their diffusion, arresting defect pair-annihilation and trapping the resulting complex morphology in a deeply metastable state. These simulation results suggest that kinetic competition between defect motion and annihilation and the evolution of membrane curvature can produce stable complex vesicle morphologies quite different from those predicted by previous theories.
Advisors/Committee Members: Selinger, Jonathan.
Subjects: Condensed Matter Physics; Physics
Keywords: Liquid Crystals; Monte Carlo; Coarse-Grained Simulation; Mean Field Theory; Elasticity; Orientational Order
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9.
Green, Lisa M.
Synthesis and Characterization of Photochemically Tunable Chiral Materials for Optically Addressed Cholesteric Displays.
Degree: MS, College of Arts and Sciences / Department of Chemical Physics, 2008, Kent State University
► Liquid crystal displays are widely used in many electronic devices. To produce…
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▼ Liquid crystal displays are widely used in many electronic devices. To produce an image the displays are electronically addressed through complex electrode patterning. The electrode patterning is costly due to the development of drive chips and software and adds to the bulk of the display in order to house the control circuitry and power supply. In effort to reduce the cost of cholesteric displays, the development of a photocontrolled cholesteric state was undertaken. Such a state would eliminate the need for electronic control, enabling the display to be optically addressed by the addition of a photochemically tunable chiral dopant. This work focuses on the synthesis of 4-({2,2'-[(4-hydroxyphenyl) diazenyl]-1,1'-binaphthalenyl} diazenyl)phenol and modified derivatives for use as photochemically tunable chiral dopants in cholesteric displays. In detail, the synthesis, characterization and preliminary evaluation of the materials as dopants are reported. Some doped mixtures exhibit a tunable Bragg reflection of over 2,500 nm, while others have been optimized to only have tunable reflection within the visible spectrum. The details of a prototype display based on the properties of these materials are also included.
Advisors/Committee Members: Li, Quan.
Subjects: Chemistry; Physics
Keywords: Photochemical; Tunable; Chiral Materials; Optically Addressed; Cholesteric Displays; Liquid Crystals; Azo; Dopant
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10.
Gu, Mingxia.
Effects of Dielectric Relaxation on Director Dynamics in Uniaxial Nematic Liquid Crystals.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2009, Kent State University
► Director reorientation dynamics is essential to most of today’s liquid crystal technologies.…
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▼ Director reorientation dynamics is essential to most of today’s liquid crystal technologies. Advances in research have allowed one to reduce the response times from millisecond to microsecond and even to nanoseconds. For these small time scales, the description of the director dynamics requires a proper account of the dielectric dispersion effects. However, the first analysis of the role of dielectric dispersion in the director dynamics has been performed only very recently. Previous investigations were limited to dual frequency nematic liquid crystals, and considered only a single dielectric relaxation process. In this thesis, we lift these limitations and propose a model that accounts for multiple processes of dielectric relaxations in both parallel and perpendicular dielectric permittivities. The model yields a number of counterintuitive predictions such as the polar contribution to the dielectric coupling between the field and the nematic liquid crystal. The most important results are summarized below: First, we developed a general model to describe the director orientation dynamics of nematic liquid crystals under an applied electric field that accounts for multiple dielectric relaxations in both parallel and perpendicular dielectric permittivities. This model presents the contribution of dielectric relaxation as a “memory” effect; the director dynamics depends not only on the present electric and director fields, but also on their pre-histories. Second, we discovered an unusual contribution to the dielectric torque as a result of dielectric memory effect. This torque is linear in the present applied electric field and is thus sensitive to its polarity. The phenomenon is accompanied by a spectacular but counter-intuitive effect: the director relaxation during the “switch-off” stage can be accelerated if instead of an abrupt vertical back edge of the voltage pulse, we apply a pulse with a non-instantaneously vanishing back edge. Third, we demonstrate the director dynamics under different “shapes” of the front edge of the voltage pulses, taking into account the dielectric relaxation. Our model allows us to optimize the front edge of the pulse to improve switching efficiency. Finally, we studied the dynamics of electrically induced isotropic-nematic (I-N) phase transition by accounting for the finite rate of the polarization dynamics, which has previously been considered an instantaneous process in the classic theory. Consequently the electrically induced nematic order parameter does not interact directly with the applied electric field, but is mediated by the dynamics of polarization. We present a model based on the Langevin equation that describes the dynamics of two crucial parameters: the polar and non-polar order parameters during the electrically induced I-N phase transition.
Advisors/Committee Members: Lavrentovich, Oleg.
Keywords: liquid crystal, nematic, dielectric relaxation, dispersion, dynamics, phase transition, fast switching, induced, dual frequency
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11.
Harden, John E.
Electro-Mechanical Couplings in Liquid Crystals.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2009, Kent State University
► With the energy demands of today, exploration for new sources is of…
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▼ With the energy demands of today, exploration for new sources is of the utmost importance. This work explores possible roles in which liquid crystals can produce electrical signals (electricity) via mechanical distortions. For example, bent core molecules have been found to have high flexoelectric coefficients when directly flexed.Typical Giant Flexoelectric coefficients are 10nC/m order. This is verified by measuring the current produced from a oscillatory bend of the sample and by measure the bending produced by application of an applied electric field. These materials may be used to produce small scale power generators. Naturally these materials need to be studied and their flexoelectric coefficients measured. Also, phospholipids, which make up a large percentage of the cell membrane, are not well studied for their liquid crystalline properties. This work found that shearing these cell membranes can produce a piezoelectric signal. The cell membrane is basically a chiral smectic A (SmA*) liquid crystal. However, a shear will produce tilt which would make it a chiral smectic C (SmC*). This symmetry breaking induces a piezoelectric signal within the cell membrane. In essence, the cell membrane is itself a generator of electricity. This newly discovered membrane piezoelectricity may play important roles in cellular processes such as ion channels, mechano-reception and magneto-reception.
Advisors/Committee Members: Jakli, Antal.
Subjects: Physics
Keywords: flexoelectric, flexoelectricity, piezoelectric, lipid, phospholipid, liquid crystal, bent-core, bentcore, bent, core, cell membrane, magnetoreception
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12.
Hicks, Sarah Elizabeth.
Polymer-Dispersed and Polymer-Stabilized Liquid Crystals.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2012, Kent State University
► This dissertation reports five specific experiments involving in liquid crystal and polymer…
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▼ This dissertation reports five specific experiments involving in liquid crystal and polymer composites. Before progressing onto the five experimental chapters, the first two introductory chapters cover the fundamentals of liquid crystals, such as liquid crystal phases, order, optics, electrical properties, and applications, as well as a background on liquid crystal polymer composites. First, nematic liquid crystal droplets, doped with liquid crystal monomers, adopted a twisted radial configuration. These droplets were suspended in silicone oil and were subjected to applied electric fields as well as temperature changes before and after polymerization. Second, a nematic liquid crystal exhibited a frozen polarization when a polymer network was present inside the liquid crystal bulk and a strong DC field was applied to the liquid crystal during polymerization. Polymer concentration and DC field strength during curing were varied in order to find the relationship between these variables and polarization effect. Finally, a dielectric study was conducted on these polymer stabilized liquid crystal samples before and after curing. Third, a polymer network was introduced inside a vertical alignment (VA) mode liquid crystal display to reduce the turn-off switching time. The turn-off time as well as contrast ratio was observed in polymer stabilized VA mode displays as the polymer concentration and rubbings along substrate alignment layer were varied. Light scattering studies on these displays and an independent study on reverse mode polymer stabilized liquid crystal displays were also performed and discussed. Fourth, a polymer-stabilized reflective cholesteric liquid crystal displays were created. These displays, consisting of a nematic liquid crystal with chiral dopants and monomers, were light reflecting at zero field, and the liquid crystal helical director was stabilized by the polymer network. When a field was applied and increased, the reflection band shifted from the initial state. The degree of shifting was observed and from that, we were able to get a picture on how the liquid crystal director responded to the increasing fields when held by the polymer network. Fifth, microstructures of different sizes and shapes were imprinted on liquid crystal elastomer films. Two lithography techniques were used in creating these structures and the structures were compared by using a polarizing microscope and scanning electron microscopy. Also the structures were studied when heated and cooled. Finally, a concluding chapter summarizes results from the experimental chapters and the future of liquid crystal polymer composites was also explored in the closing chapter.
Advisors/Committee Members: Yang, Deng-ke.
Subjects: Chemistry; Materials Science; Physics; Polymers
Keywords: polymer-stabilized liquid crystals; nematic liquid crystals; reactive mesogens; droplet dispersions; electro-optics; VA mode displays; cholesteric; elastomers; photolithography
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13.
Hurley, Shawn Patrick.
Liquid Crystal Displays for Pixelated Glare Shielding Eyewear.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2010, Kent State University
► In human vision, glare is caused by a significantly high ratio of…
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▼ In human vision, glare is caused by a significantly high ratio of luminance between the source of bright light and a nearby area of interest. Glare impairs visibility by reducing contrast from light scattering in the eye, reducing brightness due to pupil restriction, and reducing sensitivity to dark scenes due to a saturated chemical response. As the intensity of glare increases its visual effect can range from discomforting to damaging. Although polarized sunglasses can selectively reduce most reflected glare, traditional sunglasses have the undesirable effect of reducing the brightness of the entire field of view. Newer sunglasses can alter their transmission with a passively or a manually switchable device. Photochromic lenses are one passive design which reversibly decreases transmission after exposure to ultra-violet (UV) light. One manual design uses the reorientation of liquid crystal molecules to change the absorption of an incorporated dye. However, both designs uniformly darken just like traditional sunglasses. The solution is an actively switchable pixelated design where an individual region of the lens is automatically darkened to shield the source of glare. This dissertation presents two distinct pixelated glare shielding methods using a liquid crystal display (LCD). The first method uses light absorption performed by attached polarizers which dims the field of view to a constant baseline and pixels between the eye and glare will reduce or eliminate it by darkening further. The mode using this method is the super-twisted nematic (STN). The second glare shielding method uses light scattering performed by a polymer stabilized cholesteric texture (PSCT) liquid crystal configuration which has a transparent field of view and pixels between the eye and glare will reduce or eliminate it by scattering light away from the pupil. This dissertation presents the results of experimental measurements and numerical simulations that characterized both designs for the best performance as defined by driving voltage, brightness and color, contrast ratio, and response time as driven by passive matrix addressing with regard to changes in mixture formulation, cell construction and fabrication procedures.
Advisors/Committee Members: Yang, Deng-Ke.
Subjects: Physics
Keywords: liquid crystal displays; polymer stabilized cholesteric texture; normal mode; reverse mode; super twisted nematic; LCD; PSCT; STN
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14.
Iglesias, Wilder G.
Surface Interaction of Bent-Core Liquid Crystals "Slipping on a Banana Peel".
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2012, Kent State University
► The main emphasis and focus of this talk revolves around liquid crystalline…
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▼ The main emphasis and focus of this talk revolves around liquid crystalline molecules with frustrated symmetry, molecules with a kink in the core resembling the shape of a banana. These novel materials are not only suitable and interesting for the common liquid crystal display field as fast switching candidates, but for a whole wide range of potential applications, such as: power generation, microscale actuators, optical storage devices, to name a few. Understanding surface interactions of these mesogens is a key factor in controlling and unveiling the vast potential capabilities of such liquid crystals.
Advisors/Committee Members: Jakli, Antal.
Subjects: Materials Science
Keywords: bent-core; banana; liquid; crystals; surfaces; biosensor; alignment; w-flex
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15.
Jang, Yong-Kyu.
ADVANCED UNDERSTANDING OF THE OPTICAL PROPERTIES IN PHASE COMPENSATED LIQUID CRYSTAL DEVICES.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2007, Kent State University
► We investigate the optical properties of liquid crystal devices whose dark states…
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▼ We investigate the optical properties of liquid crystal devices whose dark states are optically compensated to give minimum transmittance. Firstly, the effect of multiple internal reflections on the extinction ratio of perfectly compensated liquid crystal devices is studied. We find the previously unexplained wavelength dependence of light leakage in an ideally compensated device is caused by the interference of the internally reflected ordinary and extraordinary waves. While these effects have not been previously made clear, they can place a limit on the performance of liquid crystal devices used as displays, optical switches and optical attenuators. Secondly, we study the off-axis light transmission characteristics of the bright state of common liquid crystal device modes. Our research shows there is an unexpected universal shape of the off-axis light transmittance in its bright state, regardless of what liquid crystal mode is used. To understand this surprising fact, we consider simple dark and bright state models in terms of phase retardation and transmittance. We also investigate the luminance and color properties of bright state common LCDs such as ECB, VA, Pi-cell, and TN modes. According to the results, the universality of the optical properties of a bright state is maintained not only in the transmittance but also in the luminance and color properties. Thirdly, we have studied the off-axis light transmission properties of the bright state in Pi-cell devices as a function of the white state director configuration. Above a critical pretilt angle or white state voltage, the light transmittance is a much stronger function of the incident angle of light. To understand the facts, we develop a model that explains this result and provides a description of the basic issues affecting the optics of these types of devices. Finally, we explore the phase compensation of the dark states in LCDs, and find that there is a limitation for the compensation of a uniaxial layer even in the most simple director configuration.
Advisors/Committee Members: Bos, Philip J.
Subjects: Physics, Optics
Keywords: multi-reflection, liquid crystal, liquid crystal device, liquid crystal display, LCD, residual birefringence, Pi-cell, phase compensation, bright state, viewing angle, simple model, color shift
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16.
Kim, Sang Hwa.
Fast Switching Polymer Stabilized Liquid Crystal Devices: Morphological and Electro-Optical Properties.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2004, Kent State University
► The main objective of the dissertation is to develop polymer stabilized liquid…
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▼ The main objective of the dissertation is to develop polymer stabilized liquid crystals applications with the aim of fast switching liquid crystal devices. We have scrutinized the design of the experimental setup and the optimization of the polymerization condition. We evaluate and discuss various PSLC systems, including the study of morphology of polymer networks and the electro-optical characteristics. Optically compensated bend (OCB) nematic or pi-cell is known to exhibit a fast response time and wide viewing angle with compensated films. But this bend mode has a splay-bend transition problem. These splay and bend states are topologically distinct and for real display modes it should be operated with bend mode without splay state. We introduced a polymer stabilization of the bend nematic mode in which we used polymer network acts as the volume surfaces to stabilize the bend state of the nematic liquid crystal. The switching time of a polymer-stabilized bend nematic display shows a fast rise time of 1.2ms and a 8.8ms decay time, which is sufficiently fast for video display applications. A liquid crystal blazed grating having a prismatic polymer microstructure has been developed using polymer-stabilized optical pattern forming state of a cholesteric liquid crystal. The prismatic polymer structure is formed by photo-induced localization and polymerization of a small concentration of monomer onto one substrate nearest to the UV The light incident at different angles from the normal. Using these method periodical one-dimensional patterns with a prismatic shape of polymer can be structured at the surface. The optical diffraction properties of the gratings were evaluated by the application of electric field and light incident angles, corroborate the blazed grating of asymmetric reflective index modulation. The dissertation continues on the exploration of an application of the short pitch cholesteric liquid crystals. Uniformly lying helix (ULH) of a cholesteric liquid crystal is formed with an applied electric field and temperature to exhibit the flexoelectric effect. For flexoelectro optic device applications, a problem arises in that the unwinding of the cholesteric helix is generally not completely reversible, and can produce degradation of alignment. As a result, the electro optic device does not fully return to the ULH texture after the applied electric field is removed. To address such a problem, we employ the polymer stabilization technique to stabilize the uniform lying helix texture. By using a small concentration of polymer we are able to stabilize ULH texture without the application of an electric field and lead to a device with two switching modes, amplitude (flexoelectric, at low field) and phase (dielectric, at high field) modulation, exhibiting a sub-millisecond response time.
Advisors/Committee Members: Chien, Liang-Chy.
Subjects: Physics, Condensed Matter
Keywords: LIQUID CRYSTAL; POLYMER; Cholesteric; Polymer Network; CRYSTAL; LIQUID; ULH
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17.
Liao, Guangxun.
Mechanical and Electro-Optical Properties of Unconventional Liquid Crystal Systems.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2005, Kent State University
► Four types of unconventional liquid crystal systems - amphotropic glycolipids; novel bent-core…
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▼ Four types of unconventional liquid crystal systems - amphotropic glycolipids; novel bent-core liquid crystals, bent-core liquid crystal and glycolipid mixtures, and colloidal crystal-liquid crystal systems - were studied and characterized by polarizing microscopy, electrical current, digital scanning calorimetry, and dielectric spectroscopy. - Thermotropic properties of glycolipids show a number of unusual properties, most notably high (60-120) relative dielectric constants mainly proportional to the number of polar sugar heads. The relaxation of this dielectric mode is found to be governed by the hydrogen bonding between sugar heads. - Studies on novel bent-core liquid crystals reveal a new optically isotropic ferroelectric phase, molecular chirality-induced polarity, and transitions between molecular chirality and polarity driven phases. - Mixtures of several bent-core substances with nematic, polar SmA and SmC phases, and a simple amphiphilic sugar lipid with SmA mesophase found to obey the well known miscibility rules, i.e. the sugar lipid mixes best with the polar SmA bent-core material. In addition, the chiral sugar lipid was found to induce tilt to the non-tilted polar SmA phase, which represents a new direction among the chirality – polarity – tilt relations. - The effects of the surface properties and electric fields were studied on various colloid particles – liquid crystal systems. It is found that the surface properties (hydrophobicity, roughness, rubbing) of the substrates are important in determining the size and symmetry of colloidal crystals. The director field of the liquid crystal infiltrated in the colloid crystals can be rendered both random and uniform along one of the crystallographic axis. We present the first observations of DC electric-field-induced rotational and translational motion of finite particles in liquid crystals. The electrorotation is essentially identical to the well - known Quincke rotation, which in liquid crystals triggers an additional translational motion at higher fields. Analysis of the electro-rotation and translations provides new ways to probe local rheological properties of liquid crystals.
Advisors/Committee Members: Jákli, Antal.
Subjects: Physics, General
Keywords: Bent-core; Liquid Crystal; C12G1; Crystal; polarization; smectic; Liquid
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18.
Liu, Hui.
Optical Characterization of Lyotropic Chromonic Liquid Crystals.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2006, Kent State University
► Lyotropic chromonic liquid crystals (LCLCs) represent a special class of lyotropic mesophases…
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▼ Lyotropic chromonic liquid crystals (LCLCs) represent a special class of lyotropic mesophases markedly different from conventional amphiphilic mesogens. Materials forming LCLCs are composed of plank-like molecules with a polyaromatic central core and hydrophilic ionic groups at the periphery. The individual molecules tend to assemble into rodlike aggregates that form the N phase once the concentration exceeds about 0.1M. The LCLC materials show a tremendous potential for applications in optics as self-assembling polarizing and compensating films and in the area of real-time biological sensing. The emerging applications require an understanding of basic properties of LCLC. This work addresses these needs by providing the optical characterization of LCLC. We studied in detail the optical anisotropic properties of three different nematic LCLCs: disodium cromoglycate (DSCG), Blue 27, and Violet 20. We determined the birefringence of these three materials as the function of the temperature T and wavelength λ and the corresponding dependencies of the absorption coefficients for Blue 27 and Violet 20. The birefringence is negative and significantly lower in the absolute value as compared to the birefringence of typical thermotropic N materials. We determined the scalar order parameter of the nematic phase of Blue 27 and its temperature dependence. The scalar order parameter is close to the one predicted by the classic Onsager theory for solutions of rigid rods. However, this similarity is not complete, as the measured scalar order parameter depends on temperature. The I-N pretransitional fluctuations in an aqueous solution of DSCG were studied by light scattering. We obtained the correlation length of the orientational order-parameter fluctuations of isotropic DSCG solution. The pretransitional behavior of light scattering does not completely follow the classic Landau-de Gennes model. This feature is explained by the variable length of DSCG aggregates as a function of temperature.
Advisors/Committee Members: Lavrentovich, Oleg D.
Keywords: Lyotropic Chromonic; Liquid Crystal; Birefringence; Order Parameter; Aggregate; Light Scattering
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19.
Lopatina, Lena M.
Statistical Mechanics of Nanoparticle Suspensions and Granular Materials.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2011, Kent State University
► In this dissertation I present three projects united by the idea of…
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▼ In this dissertation I present three projects united by the idea of using statistical mechanics to study systems in which the main component is an ensemble of particles. Each project studies a distribution of particles either in an interacting ensemble by itself or in a host medium. I analyze connections between the internal properties of individual particles and the resulting macroscopic properties of the material with many particles. The first project is research on thermal conductivity in suspensions of nanoparticles in base fluids. We develop a model for heat conduction through a fluid containing nanoparticles and agglomerates of various geometries. The calculations show that elongated and dendritic structures are more efficient in enhancing the thermal conductivity than compact spherical structures of the same volume fraction. Results prove that the geometry, agglomeration state, and surface resistance of nanoparticles are the main variables controlling thermal conductivity enhancement in nanofluids. In the second project, the system of interest is a liquid crystal with a suspension of ferroelectric particles. Experiments on these suspensions have reported enhancement of the isotropic-nematic transition temperature, and enhanced sensitivity to applied electric fields. We present two theories to explain these enhancements: first a Landau-like theory, which provides a simple approach to the statistical mechanics of the suspension, and then a Maier-Saupe-type model, which gives more detailed predictions. These predictions apply even when electrostatic interactions are partially screened by moderate concentrations of ions, and are in good agreement with experiments. In the third project we examine the jamming transition in a two-dimensional granular polymer system. The jamming density decreases with increasing length of the granular chain due to the formation of loop structures, in excellent agreement with recent experiments. The jamming density can be further reduced in mixtures of granular chains and granular rings, also as observed in experiment. We show that the nature of the jamming in granular polymer systems has pronounced differences from the jamming behavior observed for polydisperse two-dimensional disk systems. This result indicates that there is more than one type of jamming transition.
Advisors/Committee Members: Selinger, Jonathan V.
Subjects: Physics
Keywords: liquid crystal; nanofluid; nanoparticle; ferroelectric; enhancement; agglomerate; thermal conductivity; nematic; Landau-like theory; Maier-Saupe-type theory; ion; granular material; jamming; grain; chain; bidisperse; granular polymer; point J
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20.
Lu, Lu.
CONTROLLABLE LIQUID CRYSTAL ALIGNMENT WITH THE ASSISTANCE OF REACTIVE MONOMERS.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2012, Kent State University
► Controllable liquid crystal alignment with the assistance of reactive monomers is realized…
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▼ Controllable liquid crystal alignment with the assistance of reactive monomers is realized by the Surface Localized Polymer Alignment (SLPA) system. The SLPA system allows fully controllable liquid crystal pretilt angles, which could open new types of liquid crystal device fabrication techniques. The SLPA method not only allows a fully controllable pretilt angle, but also allows different pretilt angles to be set in different areas. More specifically, the concept of this type of alignment is to polymerize a low concentration of reactive mesogens in a liquid crystal cell while a voltage is applied. In the SLPA system, the polymer layer is designed to be only present on the cell surface. Since most of monomers are polymerized on the cell surface and form a rigid polymer layer which is preferably less than a wavelength of light in thickness, the light scattering issue can be solved.
Advisors/Committee Members: Bos, Philip.
Subjects: Information Technology; Materials Science; Optics; Physics
Keywords: liquid crystal alignment, electro-optical devices, reactive monomers
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21.
Lu, Shin-Ying.
Electrically-tunable Colors of Chiral Liquid Crystals for Photonic and Display Applications.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2010, Kent State University
► Electrically tunable colors with chiral liquid crystal materials have been discussed in…
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▼ Electrically tunable colors with chiral liquid crystal materials have been discussed in this thesis. The switchable color device can be used as a color filter and may be an important step toward eco-friendly color reflective displays. The first approach in achieving electrically tunable color is a polymer-stabilized cholesteric liquid crystal. The selected Bragg reflective wavelength can be electrically switched to reflect blue or green from a cell initially reflecting a red color. The color change is a result of the compression of the helical pitches in planar layers in response to the applied voltage. The unique feature of the polymer stabilization is in the fact that the electrically switched colors preserve good reflectance and the colors are reversible with voltage ramping. The polymer-stabilized cholesteric display exhibits anisotropic reflection and has a reflected color which is independent of temperature. A second method is a polymer-stabilized blue phase liquid crystal, in which the Bragg reflected color of the blue phase can be switched to reflect a second color through the electrostriction effect. The phase-separated three-dimensional polymer network transcribes the cubic structure of a BP liquid crystal and restrains the deformation from cubic lattice by the external electric field.
Advisors/Committee Members: Chien, Liang-Chy.
Subjects: Physics
Keywords: polymer stabilized; cholesteric liquid crystals; blue phase; carbon nanotubes
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22.
Mbanga, Badel L.
HYBRID PARTICLE-FINITE ELEMENT ELASTODYNAMICS SIMULATIONS OF NEMATIC LIQUID CRYSTAL ELASTOMERS.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2012, Kent State University
► Liquid crystal elastomers are cross-linked polymer networks covalently bonded with liquid crystal…
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▼ Liquid crystal elastomers are cross-linked polymer networks covalently bonded with liquid crystal mesogens. In the nematic phase, due to strong coupling between mechanical strain and orientational order, these materials display strain-induced instabilities associated with formation and evolution of orientational domains. In building a simulation model of these materials, we consider the limit in which the orientational order equilibrates rapidly compared to the strain, so that the local order tensor remains in continuously evolving quasi- static equilibrium as the strain relaxes. Our method allows us to study the onset of stripe formation in a monodomain film stretched along an axis perpendicular to the nematic director, the transition from polydomain to monodomain states, and the interaction of nematic liquid crystal elastomers with external stimuli such as an electric field. We intend through this model to further our understanding of the basic physics governing the dynamic mechanical response of nematic elastomers and also provide a useful computational tool for design and testing of potential engineering device applications.
Advisors/Committee Members: Selinger, Robin.
Subjects: Physics
Keywords: Liquid crystal, nematic, elastomer, finite element method
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23.
Morvan, Jason.
HIGHLY PIEZOELECTRIC SOFT COMPOSITE FIBERS.
Degree: MS, College of Arts and Sciences / Department of Chemical Physics, 2012, Kent State University
► This work reports on the fabrication and studies of highly piezoelectric composite…
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▼ This work reports on the fabrication and studies of highly piezoelectric composite fibers. The polylactic acid fibers embedded with barium titanate were created using the standard electrospinning method and utilized the high electric fields required for the electrospinning process. Fibers with micron and sub-micron diameters were spun together to produce fibrous mats. These fibrous mats were analyzed for both their direct and converse piezoelectric properties. The converse piezoelectric effect was measured by applying both DC and AC fields to see the electromechanical response of the mats by looking at the movements of the mats. Along with this, the direct piezoelectric effect was measured by applying mechanical forces to the mats, and measured its electrical response. It was found that the piezoelectric coupling constant of the fibrous mat is larger, per weight, than that of single crystal barium titanate films. These materials, unlike the crystalline films, are flexible and can be beneficial in such things as bio-sensors, artificial muscles, and energy harvesting devices. Along with being flexible, the electrospinning process can be used to make similar composite fibers with materials tailored to a desired need, allowing for a multitude of applications to be considered for this method of creating piezoelectric composite fibers.
Advisors/Committee Members: Jakli, Antal.
Subjects: Physics
Keywords: barium titanate, electrospinning, ferroelectric ceramics, polymer fibers, polylactic acid, piezoelectricity
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24.
Neal, Jeremy R.
Orientationally Ordered Particles: Characterization and Applications.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2010, Kent State University
► The theme of this work is orientational order of anisometric inclusions in…
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▼ The theme of this work is orientational order of anisometric inclusions in elastomers. These inclusions can refer to rod-like molecules, which are chemical constituents of a liquid crystalline elastomer (LCE), metallic nanoparticles, which are embedded in an elastomer host, or macroscopic particles placed in the bulk or on the surface of rubber blocks or sheets. The salient feature of all these systems is that deformations of the elastomer network couple to anisometric shape of the inclusions, which results in a systematic reorientation of the inclusions. The initial chapters of this dissertation concern the synthesis and characterization of LCEs. We use novel experimental methods to characterize the anisotropic nature of the molecular ordering as a function of strain. We then show ways to exploit this coupling to do mechanical work. Metallic nanoparticles are expected to have novel optical properties due to plasmon resonance. We use the simulation scheme Discrete Dipole Approximation and the package Microwave Studio to determine the optical response of such particles. These results are used to interpret experimental measurements of the properties of metallic nanoparticles embedded in elastomers and oriented by the network. These simulations also provide insights about LCEs doped with nanoparticles which show orientational order in the absence of strain. Finally, we study the order of macroscopic particles on rubber sheets, which interact through viscous or static friction when strained. We develop a theoretical framework for this coupling and then carry out experiments to compare observations with theoretical predictions. In the Conclusion section, we discuss the common features of the materials and methods used to examine the nature of interactions between particles and elastomeric hosts and the connection which results in orientational order. We discuss practical implications and indicate directions for future work.
Advisors/Committee Members: Palffy-Muhoray, Peter.
Subjects: Physics
Keywords: orientational order; strain alignment; rubber; stretching; elastomer synthesis; liquid crystal elastomer, gold nanoparticle
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25.
Park, Heung-Shik.
Self-assembly of lyotropic chromonic liquid crystals: Effects of additives and applications.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2010, Kent State University
► Lyotropic chromonic liquid crystals (LCLCs) are composed of plank-like molecules with rigid…
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▼ Lyotropic chromonic liquid crystals (LCLCs) are composed of plank-like molecules with rigid polyaromatic cores and two or more ionic groups at the periphery. These molecules typically stack on top of each other leaving the ionic solubilizing groups at the aggregate-water interface. As the concentration of LCLC increases, the aggregates multiply, elongate, and align parallel to each other and then form mesophases. The two most commonly met phases in LCLCs are the uniaxial nematic phase and the columnar phase with aggregates forming a hexagonal lattice in the plane perpendicular to the average orientation of aggregates. This thesis explores how the aggregate structure and the phase diagrams of LCLCs in water depend on their concentration, temperature, pH of the solution, and the presence of various additives, such as salts and neutral polymers. The two main mechanisms associated with the role of additives are (a) electrostatic interactions within and between the aggregates and (b) excluded volume effects induced by the neutral additives. Mono- or divalent salts enhance the stability of the N phase when the concentration of LCLCs and salts is small, while they suppress the mesophases when the concentration of LCLCs and salts is large. The addition of non-ionic additives such as PEG to the SSY solution leads to phase-separation into a condensed liquid crystalline (LC) region with a high concentration of SSY and a PEG-rich isotropic region. In the condensed LC region, the distance between the SSY aggregates decreases and the average length of the aggregates increases when the concentration of PEG increases. This dissertation also describes a potential application of LCLCs as a functional material for nanofabrication, namely, a controlled and reversible assembly of gold nanorods. The anisotropic electrostatic interaction between the metallic NRs and chromonic stacks allows one to achieve either side-by-side or end-to-end assembly, depending on the surface charge of the NRs. The assembly of NRs can be controlled by a number of factors influencing the self-assembly of chromonic materials, such as the concentration and pH of the solution. We hope that these studies provide a basic understanding of phase behavior and the physical properties of the reversible self-assembled chromonic materials and expand the opportunities for practical applications of LCLCs.
Advisors/Committee Members: Lavrentovich, Oleg.
Subjects: Physics
Keywords: lyotropic chromonic liquid crystals; phase diagram; disodium cromoglycate; Sunset Yellow; gold nanorod; anisotropic interaction; side-by-side assembly; end-to-end assembly; surface plasmon peak; reversible assembly
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26.
Pishnyak, Oleg.
New electro-optical applications of liquid crystals: from beam steering devices and tunable lenses to negative refraction and field-induced dynamics of colloids.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2009, Kent State University
► Modern electro-optical applications require low cost, fast operation speed and low weight.…
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▼ Modern electro-optical applications require low cost, fast operation speed and low weight. Liquid crystals (LCs) are providing all these features, but there are problems still unsolved and there are phenomena still unexplored. In the Dissertation we explored new approaches for electro-optical applications of LCs. We described new designs of LC-based devices as well as shed some light on relatively new phenomena, such as negative refraction and electrically-controlled dynamics of colloidal particles in LCs. The main results obtained in this work are as follows: 1) Electrically tunable amphoteric (negative and positive) refraction was observed in nematic LC. The very existence of negative refraction in LC has been proven. Applications of the demonstrated phenomenon may include, e.g., beam steering devices. 2) Applicability of SmA materials for birefringent prisms useful in digital beam steerers and dual-frequency nematic for fast achromatic polarization rotator based on 45-degree twisted nematic cells has been demonstrated. 3) Applicability of dual-frequency nematic for LC-based lens with a hole-patterned electrode structure, which focal length can be tuned by the electric field from negative to positive values, has been demonstrated. Such a design can be used to achieve fast optical communication between multiple channels, for example, in microlens arrays, in beam steering or scanning devices or for fast non-mechanical zooming in miniature cameras. 4) Behavior of colloidal particles dispersed in a nematic LC and surrounded by hyperbolic-type defects has been explored. We observed a number of interesting effects, such as levitation of particles in the bulk, selective movement of particles toward the opposite substrates according to the orientation of satellite defects and bidirectional motion controlled by electrically-induced backflow. The observed phenomena open the possibility for electrically-driven particles manipulation in LCs. This may result in many practical applications, including particles sorting, electrically-driven LC microfluidic devices, development of LC-based electrophoretic displays, etc. The studied LC-based colloidal dispersions also offer an excellent model system to study viscoelastic properties of complex fluids, Brownian motion, interparticle interactions, hydrodynamics, pedestrian and traffic behavior, aggregation and jamming phenomena, etc. These studies are currently in progress.
Advisors/Committee Members: Lavrentovich, Oleg.
Subjects: Materials science; Optics; Physics
Keywords: Electro-optical applications of liquid crystals; beam steering devices; polarization rotator; negative refraction; electrically tunable lens; colloidal dynamics; bidirectional motion of colloidal particles in liquid crystals controlled by backflow
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27.
Reznikov, Dmytro Yu.
Effect of surface alignment layer on electro-optical properties of ferroelectric liquid crystal displays.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2008, Kent State University
► SiOx alignment layers have been shown to allow defect-free SmC* devices with…
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▼ SiOx alignment layers have been shown to allow defect-free SmC* devices with near optimum bistable orientation of the director. In the first part of the dissertation I investigate the effect of the thickness of this type of alignment layer on the required amplitude of an applied voltage pulse to cause bistable switching and present the results of an experimental investigation and numerical modeling. I find that for thicker layers, the amplitude is controlled by the voltage drop across the alignment layers and by the effect of polar interactions between the LC and the alignment layers. For thin alignment layers the amplitude is weakly dependent on the details of the alignment layer, being more strongly influenced by the properties of the liquid crystal material. In the second part of the dissertation, the effect of surface alignment on the achievement of analog (“V”- shaped ) electric field control of director rotation in SmC* liquid crystal devices is investigated experimentally and through numerical modeling. The study of the alignment layers is enhanced through the use of I-A-C materials that allow the alignment of smectic layers in a bookshelf structure for any type of alignment layer, including those promoting vertical alignment of the director. I show that one dimensional modeling is not valid for many types of alignment layers, and that the effect of the alignment layer has a stronger effect than predicted by the model. However, I demonstrate that for the case of vertical alignment layers, a one-dimensional model is valid and excellent analog response can be achieved. I further model and discuss the effect of a thin layer of nematic at the surface and the possibility of gliding of the easy axis during switching.
Advisors/Committee Members: Bos, Philip J.
Subjects: Physics
Keywords: liquid crystal, smectic, display, ferroelectric
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28.
Semyonov, Alexander N.
Design, Synthesis and Characterization of Fluorescent Dyes and Liquid Crystal Semiconductors.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2006, Kent State University
► A series of new 3,6-diphenyl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) fluorescent dyes has been synthesized and…
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▼ A series of new 3,6-diphenyl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) fluorescent dyes has been synthesized and their fluorescent properties studied. Some of the prepared DPP compounds have been utilized in single-molecule fluorescent spectroscopy experiments. The dyes have been shown to posses high quantum yields and useful (up to 84 nm) Stokefs shifts. Maleimide, hydroxy, halogen and some other functional groups have been incorporated into the DPP bicyclic structure. N,NŒ-Diarylated DPPs have been prepared by two new methods. Cysteine-specific fluorescent probes based on Nile Red and DCDHF fluorophore cores have been synthesized and applied to testing of the local polarity and conformational changes in a single-cysteine mutant of GroEL chaperonin of E. coli. A series of iodinated aromatic compounds has been prepared and purified to study charge transport in organic molecular crystals. Several polyalkyl- and polyalkoxy- acenes (anthracenes, tetracenes) and some important key intermediates en route to polyalkoxypentacenes have been prepared. None of the synthesized compounds exhibited mesogenic properties. A reliable, highly effective purification protocol has been elaborated for purification of hexapentyloxytriphenylene and its analogs. New trinitro-HAT-5 has been synthesized and characterized as a discotic liquid crystal in the temperature range 34c140 ‹C.
Advisors/Committee Members: Twieg, Robert J.
Subjects: Chemistry, Organic
Keywords: dyes; discotic liquid crystals; organic semiconductors; single-molecule spectroscopy; 3,6-diphenyl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione; DPP; pentacene; tetracene; anthracene; iodoarenes; Nile Red; fluorophores; fluorescence
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29.
Senyuk, Bohdan.
Dielectric Response of Liquid Crystals Formed by Bent-Core and Chiral Molecules.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2010, Kent State University
► The various homogeneous and inhomogeneous structures in liquid crystals (LCs) are often…
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▼ The various homogeneous and inhomogeneous structures in liquid crystals (LCs) are often behind the interesting physical phenomena and practically useful properties. The ability to induce or control them with relatively weak electric fields makes LCs a desirable material for displays, optics, photonics and microfluidics applications. In the Dissertation, we studied the spontaneous, confinement or electric-field induced LC structures and their response to an applied electric field in nematic, cholesteric and smectic phases formed by rod-like (either achiral or chiral) and bent-core molecules. The main conclusions from this work are the following: First, we studied experimentally surface alignment, dielectric and optical properties and topological defects in thermotropic bent-core materials A131, C7 and C12, and found that observed features of these materials, such as anchoring transitions, dependence of splitting of isogyres in conoscopic images on applied electric fields or design of samples, stable isolated point defects (hedgehogs and boojums), are consistent with those of a uniaxial nematic in the entire temperature range of nematic phase. Secondly, using the fluorescence confocal polarizing microscopy for imaging the director field, we performed the first experimental studies of the scenario of layer undulations in a full three-dimensional lamellar system, represented by a short pitch cholesteric LC. We demonstrated that both qualitative and quantitative features of undulations strongly depend on the surface anchoring at the cell boundaries. We showed that the shape of undulating layers changes from sinusoidal at the onset of instabilities to zig-zag at moderate field and to the lattice of parabolic walls at high electric fields. Additionally, we demonstrated that the spatial modulation of an average refractive index resulted from layer undulations in cholesterics can be used for electric field controlled two-dimensional diffraction gratings. Finally, we described an orbiting motion of spherical colloidal particles in a smectic LC under a dc electric field. The effect is due to the field induced Quincke rotation that triggers translation of spheres through the hydrodynamic interaction with bounding walls. In the smectic A phase, the spheres can be trapped in regions with strong director distortions and forced to follow a pre-determined pathway.
Advisors/Committee Members: Lavrentovich, Oleg.
Subjects: Optics; Physics
Keywords: liquid crystals, nematics, cholesterics, smectics, uniaxial, biaxial, dielectric response, layer undulations, defects, conoscopy, fluorescense confocal polarizing microscopy
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30.
Shi, Lei.
Tunable liquid crystal polarization gratings.
Degree: PhD, College of Arts and Sciences / Department of Chemical Physics, 2009, Kent State University
► The dissertation will first review the background and previous work of liquid…
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▼ The dissertation will first review the background and previous work of liquid crystal beam steering technology in chapter one. In chapter two, we will investigate the causes and solutions of low efficiency for optical beam steering devices based on liquid crystal Decentered micro-Lens Arrays (DLAs). In the following chapter three, we will focused on basic design and concepts of tunable liquid crystal polarization gratings with a nematic liquid crystal (LC) optical phase plate, with a large continuous in-plane gradient that is variable, and its application to a beam steering device with high efficiency. High steering efficiency of over 95% is demonstrated by modeling of the liquid crystal director field, and its effect on transmitted light. The period of the V-COPA grating can be varied by adjusting an applied voltage profile, which allows for continuous angular control of the diffraction angle. This continuous steering combined with such high efficiency is unprecedented. To improve the important issue of tuning speed, in chapter four, we consider the dynamics of the LC polarization gratings. In this chapter we study the dynamics of discrete changes in the phase profile, and also continuous changes in the phase profile through acquired data and numerical modeling. We show that a design based on liquid crystals whose dielectric anisotropy can change sign (as a function of frequency) allows continuous tuning with reasonable response times.
Advisors/Committee Members: Bos, Philip.
Subjects: Optics
Keywords: liquid crystal; beam steering; polarization gratings; diffraction efficiency; decentered microlens array
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