Doctor of Philosophy, University of Akron, 2010, Polymer Engineering

A phenomenological model for elucidating phase diagrams of hexagonal columnar/nematic liquid crystal mixtures has been developed on the basis of the combination of the Flory-Huggins (FH) free energy of isotropic mixing, Maier-Saupe (MS) free energy for nematic ordering, and Chandrasekhar-Clark free energy for hexagonal ordering. Self-consistent calculations show the theory is capable of predicting the various phase diagrams, covering nematic, hexagonal columnar, and isotropic phases. The model has been tested with the eutectic phase diagram of hexagonal columnar liquid crystal, 2, 3, 6, 7, 10, 11-hexapentyloxy triphenylene (HPTP)/reactive nematic mesogens, 4-(3-acryloyloxypropyloxy)-benzoic acid 2-methyl-1, 4-phenylene ester (RM257) mixtures determined by using DSC, polarized optical microscope (POM), and wide-angle X-ray diffraction (WAXD). The self-consistent calculation displays isotropic (I), nematic (N), hexagonal columnar (Colh), N + I, and Colh + I, and Colh + N coexistence regions. These phase regions has been confirmed by thermal quenching various compositions from the isotropic melt to different phase regions. Guided by the established phase diagram of HPTP/RM257 mixtures, photo-polymerization of the mixture has been carried out in different phase regions. The as-cured HPTP/p-(RM257) composites fabricated at isotropic phase (130 °C) remains single isotropic phase under optical microscope, whereas the SEM and TEM results show the bead-like microstructure with sub-micrometer scale. Polymerization-induced mesophase transition experiments have been carried out at isotropic temperatures slightly above the clearing point of the mixtures. Of particular interest is the development of liquid crystalline spherulites. Moreover, the fixation of the morphology is observed when the photopolymerization is carried out in the N, N + I, and N + Colh region. A generalized thermodynamic model for describing smectic A and smectic B ordering has been developed based on (open full item for complete abstract)

Committee: Thein Kyu Dr. (Advisor) Subjects: Polymers

PHD, Kent State University, 2023, College of Arts and Sciences / Materials Science Graduate Program

This dissertation describes stimuli-responsive liquid crystals and elastomers including thermal/electro-active ionic liquid crystal elastomers, UV responsive twist bend nematic liquid crystal dimmers and fast-switching chiral ferroelectric nematic liquid crystals with detailed studies on its nanoscale structures, electrical and optical properties for possible electric, optical and electro-optical applications. In this dissertation, the first preparation, physical properties, and electric bending actuation of a new class of active materials - ionic liquid crystal elastomers (iLCEs) are described. iLCEs can be actuated by low frequency AC or DC voltages of less than 1 V. The bending strains of the not optimized first iLCEs are already comparable to the well-developed ionic electroactive polymers (iEAPs). Additionally, iLCEs exhibit several novel and superior features. For example, pre-programmed actuation can be achived by patterning the substrates with different alignment domains at the level of cross-linking process. Since liquid crystal elastomers are also sensitive to magnetic fields, and can also be light sensitive, in addition to dual (thermal and electric) actuations in hybrid samples, iLCEs have far-reaching potentials toward multi-responsive actuations that may have so far unmatched properties in soft robotics, sensing and biomedical applications. The following two works are the understanding of the structure of the twist-bend nematic (NTB) phase. The first work presents hard and tender resonant X-ray scattering studies of two novel sulfur containing dimer materials for which we simultaneously measure the temperature dependences of the helical pitch and the correlation length of both the helical and positional order. In addition to an unexpected strong variation of the pitch with the length of the spacer connecting the monomer units, we find that at the transition to the NTB phase the positional correlation length drops. In the second work we use tender (open full item for complete abstract)

Committee: Antal Jakli (Committee Chair); Robin Selinger (Committee Member); Robert Clements (Committee Member); Robert Twieg (Committee Member); Deng-Ke Yang (Committee Member) Subjects: Materials Science; Nanotechnology; Optics; Physical Chemistry; Physics

PHD, Kent State University, 2019, College of Arts and Sciences / Department of Physics

Nematic liquid crystals are widely used for light management applications due to their anisotropic properties. The guest-host system is formed by combining a nematic liquid crystal “host” with a dichroic “guest” molecule. This mixture exhibits new and useful properties. We can either control the transmission of the light or characterize a new configuration for electrically controlled random laser action. The first project is focused on the dynamic scattering mode in nematic liquid crystals which is considered for various applications to control the transmission of light: notably, their use in therapeutic eyewear. This technology would be useful in the treatment of certain ophthalmic conditions, for example amblyopia and strabismus. Specifically, using this technology to controllably and measurably degrade vision in the stronger eye can lead to improved muscle strength and control in the weaker eye. Using a guest-host nematic mixture we have constructed devices that can be switched between a transparent, “off” state to a scattering state having adjustable cloudiness and/or opacity. The second project is related to the observation of electric field induced random lasing in a dye doped liquid crystal system. There is a continued interest in lasing in dye doped liquid crystals (DDLCs) based on prospects for building compact, all organic, tunable mirrorless lasers, with potential applications ranging from miniature spectroscopic and medical tools to large area holographic laser displays. We investigated a configuration that can provide a “reverse mode” operation of guest host based RLs.

Committee: James T Gleeson Dr (Advisor); Bahman Taheri Dr (Committee Member); Micheal Tubergen Dr (Committee Member); Samuel Sprunt Dr (Committee Member); Edgar Kooijman Dr (Committee Member) Subjects: Materials Science; Physical Chemistry; Physics

PHD, Kent State University, 2017, College of Arts and Sciences / Department of Physics

This research described in this dissertation comprises three experimental topics and includes the development of an appropriate theoretical framework to understand the various observations in each. In the first part, we present results from angle-resolved second-harmonic light scattering measurements on three different classes of thermotropic nematic liquid crystals: polar and non-polar rodlike compounds, and a bent-core compound. We analyze the data in terms of the “flexoelectric” polarization induced by distortions of the nematic director field around topological defects known as inversion walls, which are analogous to Neel walls in magnetic spin systems and which often exhibit a closed loop morphology in nematic systems. The second part of this dissertation explores the possible existence of a helical polarization field in the nematic twist-bend (NTB) phase of dimeric liquid crystals, utilizing a similar nonlinear light scattering approach. The NTB phase is characterized by a heliconical winding of the local molecular long axis (director) with a remarkably short, nanoscale pitch. According to theoretical conjecture, a helical electric polarization field accompanies this director modulation, but, due to the short pitch, presents a significant challenge for experimental detection. Our study focuses on topological defects, classified as parabolic focal conics, in two achiral, NTB-forming liquid crystals. These defects generate distortions of the polarization field on sufficiently long (micron) lengths to enable a confirmation of the existence of polar structure. We analyze our results with a coarse-grained free energy density that combines a Landau-deGennes expansion of the polarization field, the elastic energy of a nematic, and a bilinear coupling between the two. The last part of the dissertation focuses on the layer dynamics of thin, free-standing membranes of a smectic-A liquid crystal, with a particular consideration of the surface (interfacial) paramete (open full item for complete abstract)

Committee: Samuel Sprunt (Advisor); James Gleeson (Committee Member); Elizabeth Mann (Committee Member); Antal Jakli (Committee Member); Hiroshi Yokoyama (Committee Member) Subjects: Materials Science; Optics; Physics

Master of Science, University of Akron, 2016, Physics

Nanoparticles can affect some physical properties of liquid crystals. We investigate the influence that nanoparticles have on the Kerr coefficient by measuring the electric field Kerr effect (induced birefringence proportional to the square of the applied electric field), above the nematic-isotropic transition temperature by using various concentrations of cerium dioxide nanoparticles in 4-octyl-4'-cyanobiphenyl (8CB) liquid crystal. On cooling toward the nematic-isotropic transition temperature the nematic correlation length increases (but does not diverge because the transition is first order). When the correlation length is of the same order as the characteristic distance between the nanoparticles, the nematic fluctuations are disrupted and the Kerr coefficient will no longer increase as rapidly with decreasing temperature as predicted by Landau-DeGennes theory.

Committee: Sasa V. Dordevic Dr. (Advisor); Charles Rosenblatt Dr. (Advisor); Jutta Luettmer-Strathmann Dr. (Committee Member) Subjects: Physics

PHD, Kent State University, 2015, College of Arts and Sciences / Chemical Physics

Disclination lines play a decisive role in determining the equilibrium structures of topologically constrained liquid crystal systems including cholesteric blue phases, twist grain boundary phases and liquid crystal colloids. The extra energy associated with disclinations is key to stabilizing one particular director structure over another, yet our knowledge of disclination energetics is limited as are characterization methods. In this work, we detail our approach which has focused onbuilding versatile one-of-a-kind instruments for studying liquid crystal systems. This work details the development and use of two novel instruments: an automated maskless photoalignment pattern generator (maskless system) and ad ynamic-cell system that allows for the automated mechanical adjustment of the liquid crystal cell thickness, twist angle and temperature. Both instruments were extensively refined and characterized for maximum performance. In addition, both instruments were designed as versatile platformsfor new research. In this work, we used the maskless system to create novel surface alignments and Pancharatnam-phase devices, and we employed the dynamic-cell system for the generation and characterization of reverse-twist-domain defect loops.

Committee: Hiroshi Yokoyama (Advisor); Philip Bos (Committee Member); Antal Jakli (Committee Member); Elizabeth Mann (Committee Member); Michael Tubergen (Committee Member) Subjects: Chemistry; Physical Chemistry; Physics

PHD, Kent State University, 0, College of Arts and Sciences / Chemical Physics

Long range orientational order of nematic liquid crystals has been an inspiration for both theoretical works and practical applications. Using a combination of analytic calculations and numerical simulations, we investigate the interplay between orientational order and geometric constraints. In the first part of this dissertation, we concentrate on nematic order in liquid membranes, where the curvature induces non-uniformity and vice versa. Variations in nematic order, especially near topological defects, play an essential role in the interaction with curvature. In the second part, we consider the combination of the anisotropy of nematic liquid crystals with the large reversible deformations of elastomers as a mechanism for programmable deformations in soft materials.

Committee: Jonathan Selinger (Advisor); Robin Selinger (Committee Member); Antal Jakli (Committee Member); John Portman (Committee Member); Arden Ruttan (Committee Member) Subjects: Physics

Doctor of Philosophy, University of Akron, 2013, Chemistry

Semiflexible polymers of sufficient stiffness exhibit liquid crystalline order at low temperature and high polymer concentration. Blends of liquid crystalline and flexible polymers have interesting physical properties and important applications in organic electronics. We investigate melts and blends of flexible and semiflexible polymers with the aid of Monte Carlo simulations of an extension of Shaffer's bond-fluctuation model. To control chain stiffness we include a bending term in the Hamiltonian and investigate two models for semiflexibility that differ in the range of penalized bond angles. A study of structural, dynamic and thermodynamic properties of the first model shows that it describes melts of semiflexible chains that do not undergo a transition to a liquid crystalline state. Simulations of the second model reveal orientational order without positional order at high density and low temperature. The transition from the isotropic high-temperature phase to the nematic low-temperature phase, the IN transition, is accompanied by discontinuous changes in structural and thermodynamic properties. This agrees with mean-field theories and experimental observation that show that the IN transition is a discontinuous transition. To characterize our system fully, we determine the phase diagram and find that the IN transition temperature increases with increasing filling fraction, which agrees qualitatively with predictions by Onsager and Flory. Since pair distribution functions give insight into structure and morphology of polymers, we construct same-chain and different-chain distributions that we further differentiate by flexible and rod-like chain conformations. A study of same-chain pair distributions shows that the rod-like chains in our model align with a face diagonal in the nematic phase. Results for different-chain pair distribution functions show that a melt phase separates into a dense ordered region and a low-density disordered region when undergoing t (open full item for complete abstract)

Committee: Jutta Luettmer-Strathmann Dr. (Advisor); David Perry Dr. (Committee Member); Alper Buldum Dr. (Committee Member); David Modarelli Dr. (Committee Member); Kevin Cavicchi Dr. (Committee Member) Subjects: Chemistry; Physics; Polymers

PHD, Kent State University, 2009, College of Arts and Sciences / Department of Physics

The discovery of thermotropic biaxial nematic phase in bent-core mesogens, have engendered interest in these systems. Also, it undergoes optical switching about 100 times faster than conventional uniaxial nematic liquid crystal. Azo-substituted bent-core compounds, A131 and A103, were investigated as both offer an opportunity to observe their structures and phase transitions from the uniaxial nematic (Nu) to biaxial nematic (Nb) phase and from Nb to the underlying smectic-C (SmC) phase. Plank-like molecular systems are also expected to form Nb phase. Chromonic liquid crystals formed by aqueous solutions of plank-like dye molecules are interesting for their unique self-assembly and structural evolution. They have applications in optical element, coloring in food and textiles, and etc. Both systems were investigated with synchrotron x-ray scattering, polarizing optical microscopy, and differential scanning calorimetry. Temperature dependence of d-spacing and positional order correlations along the director clearly mark the phase boundaries where Nu-Nb transition was approximately 27o below the clearing point. Positional order correlation length of A131 increased from 1.5 in Nu to 3.3 molecular lengths in Nb phase, before it jumps by a factor of at least 5 in SmC phase. The lack of large discontinuous changes in the structural parameters and the subtle signatures in heat capacity establish the second order nature of Nu-Nb and Nb-SmC phase transitions. The chromonic system investigation results provide quantitative information of structural properties in nematic and columnar mesophases. We studied water solutions of (achiral) sunset yellow dye and (chiral and achiral) dihydrochloride salts of perylenebis-dicarboxydiimide. Positional order correlation lengths' measurements, parallel and perpendicular to the aggregate axis, revealed that they increase with concentration and decrease with temperature. Temperature dependence of correlation lengths yielded the scission en (open full item for complete abstract)

Committee: Satyendra Kumar Prof. (Advisor); David Allender Prof. (Committee Member); Elizabeth Mann Prof. (Committee Member); Arne Gericke Prof. (Committee Member); Oleg Lavrentovich Prof. (Committee Member) Subjects: Physics

PHD, Kent State University, 2009, College of Arts and Sciences / Department of Physics

In part because of its anticipated application for faster and lower power-consuming electro-optic devices, the biaxial phase of nematic liquid crystals, characterized by two optic axes, has long been sought after. We have investigated the existence of thermotropic nematic biaxiality in a relatively new system, liquid crystalline tetrapodes, through dynamic light scattering. Liquid crystalline tetrapodes are formed by the attachment of four mesogenic molecules to a single silicon or germanium atom through flexible siloxane chains. Our results, obtained in various scattering geometries and tested against available theory, strongly support the existence of a biaxial nematic phase. The temperature dependent slowing down of the biaxial order parameter fluctuations indicates that the uni – biaxial transition is weakly first order in a 4-ring tetrapode and second order in a 3-ring homolog. The temperature dependence of the relaxation rates of the biaxial order parameter mode and of the scattered intensity associated with biaxial optic axis fluctuations is explained by a Landau-deGennes model of the free energy. In particular, we have confirmed that the intensity exhibits the expected scaling with the uniaxial and biaxial order parameter magnitudes, for several distinct geometries, in the biaxial phase.Another relatively new class of nematics is the lyotropic chromonic liquid crystals (LCLCs) formed from concentrated mixtures of disc-like dye molecules in water. Because of their potential applications in dye based polarizing and compensating films and in biological sensing, and because of recent analogies drawn between LCLCs and the liquid crystalline phases of DNA solutions, LCLCs are interesting systems to study. Using dynamic light scattering on well aligned samples, we have explored particularly the temperature dependence of the elastic constants and orientational viscosities of nematic Disodium cromoglycate LCLCs. These parameters show a significant anisotropy. In part (open full item for complete abstract)

Committee: Samuel Sprunt PhD (Advisor) Subjects: Physics

PHD, Kent State University, 2005, College of Arts and Sciences / Department of Physics

Dynamical properties of the nematic and isotropic phase of several relatively new bent-core liquid crystals, as contrasted to conventional straight-core (“calamitic”) liquid crystals, have been systematically studied by dynamic light scattering. Objective: Comparison of properties of bent-core nematics with straight-core nematics and search for a biaxial nematic phase due to the bent-core shape Observations: Nematic phases are rather uncommon in bent-core compounds because nematic structure occurs only if the molecules can rotate relatively freely around their long axis, the condition apparently met in only a fraction of bent-core materials synthesized so far. The obtained results show that the elasticity to viscosity ratio for uniaxial bent-core nematics is typically two orders of magnitude lower than for straight-core nematics, due evidently to the large viscosity associated with optic axis distortions in the bent-core case. This result is independent of the normal mode of the director fluctuations probed. In one compound polarized light scattering data reveal fluctuations associated with the biaxial order parameter, occurring as a pretransitional effect on approach to the biaxial smectic-C phase. However, results on a homeotropically-aligned sample of another compound provide preliminary light scattering evidence for nematic biaxiality. Objective: To determine nature of isotropic to nematic phase transition and to measure dielectric fluctuations in the isotropic phase near the transition. Observations: The orientational order parameter fluctuations in the isotropic phase have been studied for the first time in bent-core compounds. Analogous to classic light scattering experiments performed on calamitic liquid crystals, fluctuations in nematic order in bent-core compounds exhibit a mean-field-like critical slowing down on approach to the isotropic-nematic transition from above. The fluctuations are intrinsically several orders of magnitude slower than for typical (open full item for complete abstract)

Committee: Samuel Sprunt (Advisor) Subjects:

PHD, Kent State University, 2024, College of Arts and Sciences / Materials Science Graduate Program

Stimuli-responsive functional soft materials have been the focus of attention and been widely applied in advanced devices. Chiral nematic liquid crystals (also called cholesteric liquid crystals, CLCs), which possess intrinsic self-organized helical superstructures, are good candidates to create diffraction gratings (DGs) for optical devices, due to the characteristics of adjustable pitch under external stimuli like light, temperature, electric field, and so forth. Here, we develop two novel photoresponsive CLCs, which are enabled by adding two novel axially chiral molecular switches into the nematic LC host, respectively. Those chiral molecular switches exhibit superior compatibility, high helical twisting power (HTP), and a big HTP change under photoisomerization. Accordingly, electro- and photo-driven orthogonal switching of CLC diffraction gratings, and visible light, temperature, and electric field-driven in-plane rotation of CLC diffraction gratings are demonstrated, which exhibit great potential application in beam steering, spectrum scanning, and beyond. Like CLCs, twist-bend nematic liquid crystals (NTB LCs) also possess an intrinsic heliconical structure although the constituent molecules are achiral, but the molecular director is tilted with a cone angle around the conic helical axis and the heliconical pitch is very small. We study the structure and optical properties of NTB LCs when the chiral dopant is introduced. We show that adding chiral dopant does not induce a twisting of the heliconical axis, but increases the cone angle. Then, based on this fundamental study, we develop a novel thermally switchable smart window enabled by phase transition from NTB phase to chiral nematic phase. Such smart window is energy-saving and exhibits great potential in applications for buildings, vehicles, and beyond. Moreover, we develop a novel switching mechanism between planar and focal conic states in bistable reflective display. The CLC is switched from the foca (open full item for complete abstract)

Committee: Deng-Ke Yang (Advisor); Quan Li (Advisor); Barry Dunietz (Committee Member); Xiaoyu Zheng (Committee Member); Philip J. Bos (Committee Member) Subjects: Chemistry; Materials Science; Physical Chemistry; Physics

MS, Kent State University, 2023, College of Arts and Sciences / Department of Physics

LIQUID CRYSTALSLIQUID CRYSTALSThe present work studied the physical properties of a nematic LC material compound RT11165. The recently discovered ferroelectric nematic phase that carries a non-zero dipole moment exhibits in this material. Experimentally we used the Freedericksz transition method in the nematic phase by measuring the light intensity through a planar nematic layer as a function of the amplitude and frequency of an applied voltage. The threshold voltage shows dependence on the frequency. In addition, the dielectric anisotropy and the splay elastic constant K11 are measured. The dielectric anisotropy indicates critical behavior at high frequencies relatively low as cooling toward the NF phase, while at low frequencies, it shows strong positive dielectric permittivity. Also, the dielectric spectroscopy shows two molecular relaxation modes detected at low frequency and temperature in the perpendicular component of the dielectric permittivity, and the strength of the relaxation frequency was found to be decreased. In contrast, it increases rapidly and smoothly in the parallel component.

Committee: James Gleeson Dr (Advisor) Subjects: Physics

PHD, Kent State University, 2023, College of Arts and Sciences / Department of Physics

Liquid crystal is a state of matter where constituents show orientational order, despite lack of translational order. For regular nematic liquid crystals, the ground state of orientational distribution of mesogens is described by a single axis, known as the director. Due to effects such as surface anchoring or chiral nature of added liquid crystal molecules, the uniformity in an orientational order field can be broken. The short-range spatial correlation persisting in the orientational order field, as well as topological defects enabled by the uniaxial symmetry manifested from the local orientational order of a nematic liquid crystal, often gives rise to abundant intriguing and sophisticated pattern formation in nematic liquid crystals. Studying the pattern formation and the topological defects in those orientational order fields is essential for understanding rheological and optical properties of nematic liquid crystals. Employing analytical and numerical tools, this dissertation explores the implications of elasticity theory which is commonly used to characterize the deformation of a uniform orientational order field, and the motion of different topological defects in nematic liquid crystals. In the conventional Oseen-Frank elasticity theory, a uniform ground state is protected by the elastic constants satisfying Ericksen inequalities. To examine the scope of the elasticity theory beyond the Ericksen inequalities, we revisit the Oseen-Frank elasticity theory for nematic liquid crystals from the perspective of a reformulated form and find a new set of necessary inequalities for Frank elastic constants to ensure the existence of stable solutions, which is weaker than the Ericksen inequalities. We therefore identify a regime where the Ericksen inequalities are violated but the system is still stable. Remarkably, lyotropic chromonic liquid crystals are in that regime. We investigate the nonuniform structure of the director field in that regime, show tha (open full item for complete abstract)

Committee: Jonathan Selinger (Advisor); Jonathan Selinger (Committee Chair); Xiaoyu Zheng (Committee Member); Robin Selinger (Committee Member); Hiroshi Yokoyama (Committee Member); Edgar Kooijman (Committee Member) Subjects: Physics

PHD, Kent State University, 2023, College of Arts and Sciences / Materials Science Graduate Program

Nematic Liquid Crystal Sessile Droplets in Electric and Magnetic Fields Abstract of Dissertation Zoltan Karaszi Materials Science Graduate Program, Kent State University Sessile droplets with uniform director structure could be used as tunable optical lenses where the focal length could be controlled by light polarization, viewing angle, and magnetic or electric fields. To achieve that, one must understand the liquid crystal director structure in various external fields. In this dissertation, I presented detailed experimental studies. I summarized the theoretical description of the director structure of uniaxial nematic liquid crystals, such as the formation and dynamics of Neel wall type metastable inversion walls, either in magnetic or electric fields or magnetic and electric fields combined. Sessile nematic droplets allow for studying the combined effect of anchoring at solid and gas interfaces. The combination of various alignments at the two surfaces and external fields results in various director distribution schemes, ranging from a defect-free, almost homogeneous state to configurations with point-, line- and wall defects. We designed a polarizing optical microscope made of non-magnetic materials that could be placed between an electromagnet's poles. The design allowed us to study the effect of various combinations of electric and magnetic fields on nematic liquid crystal sessile droplets. Additionally, a long-range microscope was used to observe the side view of the LC drop. We also built another experimental setup that enabled us to measure the focal length in response to electric fields while rotating the sample between crossed polarizers. (1) Our main experimental findings can be summarized as follows. We showed that under low magnetic fields applied along the base plane of a sessile droplet with homeotropic alignment, the director structure becomes distorted and gradually leads to a defect wall that moves toward the periphery. We explained the dir (open full item for complete abstract)

Committee: Antal Jákli Dr. (Advisor); Antal Jákli Dr. (Committee Chair); Hamza Balci Dr. (Committee Member); Liang-Chy Chien Dr. (Committee Member); James Gleeson Dr. (Committee Member); Philip Bos Dr. (Committee Member) Subjects: Chemistry; Materials Science; Physics

PHD, Kent State University, 2022, College of Arts and Sciences / Department of Chemistry and Biochemistry

Liquid crystals (LCs) show unique optical and physical properties, and so these compounds are attractive for many applications, including displays, sensors, reconfigurable mirrors, photonic devices, etc. [1-5]. Due to the cost and performance limitations of devices based on existing LC materials, it is essential to develop novel systems that are cost-effective and with enhanced performance. Also, it is crucial to explore the properties of these soft matter materials with respect to the molecular modifications to better understand their broad applicability. This is especially the case if the phase types involved are recently discovered. Many chiral dopants required for cholesteric preparations are either obtained from conventional chemical synthesis or from plant extracts. So, in search for novel dopants which are of biological origin and are relatively cheaper to produce in large volume, a study has been carried out to develop chiral compounds made from the fermentation derived bio-precursors. Likewise, the recently discovered nematic ferroelectric (NF) LCs [6] have not yet gained practical utility though they have potential to be applied in next generation displays and optoelectronic devices. Although these NF LCs show response towards ultra-small driving voltage ~1V/cm [7], the realistic applications are limited due to their current shortcomings such as monotropic LC phase, high temperature phase transition, thermochemical instability, etc. Thus, to develop novel improved systems and to seek their pragmatic applications, a variety of unique NF LCs have been synthesized and studied. Similarly, to explore the synthetic challenges and the properties related to their structural features, several classic compounds such as cyanobiphenyls (CBs), CB dimers, and triphenylene discotics have been prepared and examined. Reference: [1] Geelhaar, T. Liquid Crystals for Display Applications. Liq. Cryst. 1998, 24 (1), 91–98. https://doi.org/10.1080/026782998207613. (open full item for complete abstract)

Committee: Robert J. Twieg (Advisor); Christopher J. Fenk (Committee Member); Brett Ellman (Committee Member); Antal Jákli (Committee Member); Scott Bunge (Committee Member) Subjects: Materials Science; Organic Chemistry

MS, Kent State University, 2021, College of Arts and Sciences / Materials Science Graduate Program

The classical X-Y rotor model is used for demonstrating the temperature dependence of pitch of Cholesteric Liquid Crystals. Polymer Stabilized Cholesteric Liquid Crystals prepared from Nematic Liquid Crystals with a large birefringence (∆n) and an absolute value of dielectric anisotropy(∆ε) are used to achieve the whole-visible-spectrum bandgap symmetric broadening by 54V DC voltage (cell gap: 18.2 μm). The samples with DC voltage in the curing process gain a permanent wider bandgap. The bandgap of Polymer Stabilized Cholesteric Liquid Crystal doped with Direct Yellow 4 dye is found to have a bandgap of 230nm under only 27V DC voltage. POM images are used for demonstrating the focal-conic state in samples destroyed by overloading.

Committee: Liang -Chy Chien (Committee Chair); Robin Selinger (Committee Member); Antal Jákli (Committee Member) Subjects: Materials Science; Optics; Physics

PHD, Kent State University, 2019, College of Arts and Sciences / Chemical Physics

In this dissertation, we explore how the molecular structure of nematic liquid crystals influences their elastic behavior. In the first part, we study the structure-property relationship of flexible low-molecular weight liquid crystal dimers. These dimers were recently demonstrated to form a new liquid crystalline phase, the so-called twist-bend nematic. We report temperature dependencies of material properties such as dielectric anisotropy, birefringence, splay, K1, twist, K2, and bend, K3, elastic constants in the uniaxial nematic phase of these materials and compare these properties to the properties of conventional rod-like nematics. Our studies demonstrate striking differences between flexible dimers and rod-like mesogens. In the case of dimers, the temperature dependent birefringence and bend elastic constant show a non-monotonous behavior on approaching the nematic-to-twist-bend nematic phase transition. Additionally, the conventional relationship of rod-like mesogens follows the trend K3>K1>K2, whereas, in all the studied dimeric compounds we observe a very different trend with K1>K2> K3. The second part of the dissertation addresses stimuli-responsive nematic elastomer coatings formed by polymerized mesogens. The molecular orientation of the liquid crystal elastomers is coupled to rubber-like elasticity. The orientational order defines their mechanical response to external stimuli such as temperature or light. We demonstrate a dynamic thermal control of surface topography of the elastomers prepared as coating with patterned in-plane molecular orientation. Upon heating, the inscribed director pattern determines whether the initially flat coating develops elevations, depressions or in-plane deformations. We explain this deterministic relationship between the in-plane orientations and out-of-plane variations of coatings' profile by the activation forces concept. We employ the light-activated elastomer coatings with 2D inscribed orientational order as a tool f (open full item for complete abstract)

Committee: Oleg Lavrentovich (Advisor); Antal Jákli (Committee Member); Samuel Sprunt (Committee Member); Min-Ho Kim (Committee Member) Subjects: Chemistry; Materials Science; Physics

PHD, Kent State University, 2018, College of Arts and Sciences / Department of Physics

Cholesteric liquid crystals experience geometric frustration when they are confined between surfaces with anchoring conditions that are incompatible with the cholesteric twist. Because of this frustration, they develop complex topological defect structures, which may be helicoids or skyrmions. We develop a theory for these structures, which extends previous theoretical research by deriving exact solutions for helicoids with the assumption of constant azimuth, calculating numerical solutions for helicoids and skyrmions with varying azimuth, and interpreting the results in terms of competition between terms in the free energy. We have also performed numerical simulations based on director tensor relaxation and used a finite difference method to demonstrate different defect structures in confined liquid crystals. Using our model, we have studied the formation of Skyrmions and helicoids (stripes) in cholesterics confined in rectangular micron-channels. Depending on the ratio, (cholesteric pitch)/(channel depth), Skyrmions or stripes can form. These results were in agreement with experiments done by QiHuo Wei's group. In another study we demonstrated forming complex defect textures such as disclination loops and arches in Nematics confined between patterned substrates. These results were also compared with experiments done by QiHuo Wei's group. In our final project we have modeled the microstructural evolution in cholesterics under voltage pulses and studied different Cholesteric phases.

Committee: Robin Selinger Professor (Committee Member); Jonathan Selinger Professor (Committee Member); Deng-Ke Yang Professor (Committee Member); John Portman Professor (Committee Member); Elda Hemann Assistant Professor (Committee Member); BARRY DUNIETZ Associate Professor (Committee Member) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy, Case Western Reserve University, 2018, Physics

I present a body of work regarding topological defects (TDs) in nematic liquid crystals. Defects having specific strengths were created in specified locations using atomic force microscope (AFM) lithography and the means by which the defects relieve the diverging strain energy near their cores was characterized as a function of cell depth and by probing with an electric field. I also work towards nanoparticle trapping in the scribed cores by doping a host liquid crystal with fluorescent nano-emitters. The technique of scribing an easy axis by AFM lithography was extended by writing Python scripts that produce densely-packed paths for the AFM tip to follow. I create several arrays of defects using this method in thin cells. I then probe the structure of the nematic director near each scribed core by applying a perpendicular electric field to a positive anisotropy liquid crystal. Of interest is the means by which the TDs relax the diverging energy at the defect cores. I show qualitatively that smaller cell depths promote defect splitting, whereas thicker cells promote defect escape, i.e., the director rotates out of the plane. The voltage profile of the transmitted intensity under crossed polarizers was examined, and shows that the liquid crystal can have a Freedericksz threshold voltage near a split defect, but not near an escaped defect. I then assemble thick cells, such that the disclination lines caused by the scribing run near the master surface and terminate on nearest-neighbors. I apply an in-plane field and show that the disclinations can deflect, interact, and can swap termination partners to effect a change in orientation of 90$^\circ$. The local electric field required to make the disclinations interact was measured. Then I consider a technique whereby the diverging energy of the defect cores can be relaxed by suspending nano-emitters, specifically quantum and carbon dots, into the liquid crystal. We make progress towards trapping these nanoparticles in scr (open full item for complete abstract)

Committee: Charles Rosenblatt (Advisor) Subjects: Physics