Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session D17: Liquid Crystalline Order in Polymer and Complex Fluids |
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Sponsoring Units: DPOLY Chair: Aniket Bhattacharya, University of Central Florida Room: B116 |
Monday, March 15, 2010 2:30PM - 2:42PM |
D17.00001: Highly Viscous Cholesteric Mixtures: New Domain of Responsive Materials Petr Shibaev, Robert Uhrlas, Sean Woodward, Cristina Schlesier Recently, considerable efforts were directed towards design and study of cholesteric elastomers that could respond reversibly to different types of deformation by changing helical pitch and keeping these changes in the deformed state. The alternative approach to design mechanically responsive cholesteric materials would be the use of highly viscous cholesteric mixtures that display a viscoelastic behavior.\footnote{Petr V. Shibaev \textit{et. al}., Opt. Express \textbf{16}, 2965-2970 (2008)} In this presentation we review our recent results in designing novel viscoelastic cholesteric materials and show that some of their properties are superior to those of cholesteric elastomers. Gigantic color changes and tunable lasing displayed by highly viscous cholesteric materials are discussed in detail and the model describing mechanically induced shift of the selective reflection band is presented. The ways of increasing hydrophilicity of these novel materials that make them attractive for designing environmentally sensitive sensors are also considered and the design of novel pH sensors is presented. At the end, the properties of highly viscous cholesteric mixtures that bring them to the domain of novel responsive materials are discussed. [Preview Abstract] |
Monday, March 15, 2010 2:42PM - 2:54PM |
D17.00002: Structural origin of circularly polarized iridescence in jeweled beetles Matija Crne, Vivek Sharma, Jung O. Park, Mohan Srinivasarao The iridescent metallic green beetle, Chrysina gloriosa, selectively reflects left circularly polarized light. The exoskeleton is decorated by hexagonal cells ($\sim $10 micron) that coexist with pentagons and heptagons. We find that the fraction of hexagons decreases with an increase in curvature. In bright field microscopy, each cell contains a bright yellow core, placed in a greenish cell with yellowish border, but the core disappears in the dark field. Using confocal microscopy, we observe that these cells consist of nearly concentric, nested arcs that lie on surface of a shallow cone. We infer that the patterns are structurally and optically analogous to the focal conic domains formed spontaneously on the free surface of a cholesteric liquid crystal. The microstructure provides the bases for the morphogenesis as well as key insights for emulating the intricate optical response the exoskeleton of scarab beetles. [Preview Abstract] |
Monday, March 15, 2010 2:54PM - 3:06PM |
D17.00003: Rotation in magnetic fields yielding near-single crystal quality lamellar mesophases Pawel Majewski, Chinedum Osuji We present a novel method to obtain spatially ordered lamellar surfactant mesophases with the use of magnetic fields. The alignment is obtained by continuous rotation of the sample in the presence of the field while cooling through the disorder-order transition of the system. This facile approach allows one to overcome the degeneracy that is inherent in the alignment of these materials by a uniaxial field due to the lack of interfacial curvature combined with the negative diamagnetic anisotropy of the mesophase. We report on the influence of the field intensity, sample cooling rate and the rotation speed on the degree of alignment of a model non-ionic lyotropic system. Further, we demonstrate the use of the aligned mesophase as a template for the synthesis of nanomaterials with long-range order. [Preview Abstract] |
Monday, March 15, 2010 3:06PM - 3:18PM |
D17.00004: Bent Core Liquid Crystal Polymers and Elastomers Rafael Verduzco, Seung Ho Hong, John Harden, Antal Jakli, Sam Sprunt, Jim Gleeson Bent-core liquid crystals (LCs) have a kinked, or bent, molecular shape in contrast to the more common rod-like LCs. Due to their bent molecular shape, bent-core LCs form locally polar clusters, which result in novel LC phases and potentially useful properties such as ferroelectricity. Polymeric bent-core LCs are of particular interest because they can lead to new nanostructured soft materials with confined bent-core LCs. In this work, we investigate the synthesis, nanoscale structure, and physical properties of a variety of bent-core LCs and polymeric bent-core LCs. SAXS reveals the presence of polar clusters over a wide temperature range in the nematic phase for all materials studied, including bent-core side-group LC polymers and bent-core LC elastomers. The presence of locally polar clusters can account for the unexpected physical properties in nematic bent-core LCs, such as enhanced flexoelectricity. Direct flexoelectric measurements on pure bent-core LCs and swollen LCEs show that nematic bent-core materials have a flexoelectric coupling three orders orders of magnitude larger than calamitic LCs. Nematic clusters in bent-core LCs represent an unexpected and potentially useful phenomenon for building responsive LC devices. [Preview Abstract] |
Monday, March 15, 2010 3:18PM - 3:30PM |
D17.00005: Anisotropic conformation of a side group liquid crystalline polymer in an isotropic small molecule LC solvent Paul Pirogovsky, Zuleikha Kurji, Rohan Hule, Julia Kornfield While it is well known that SGLCPs adopt an anisotropic conformation when in a nematic solvent, we were intrigued to observe that anisotropy of the coil conformation can persist above T$_{NI }$-- despite the isotropic state of the solvent and the flexible nature of the backbone. Small angle neutron scattering (SANS) was performed on dilute (1 wt{\%}) solutions of SGLCPs dissolved in deuterated 5CB. The isotropic-nematic transition of the d-5CB was not shifted by this low concentration of polymer. In the absence of a magnetic field, the SANS patterns are isotropic. A magnetic field that is sufficiently weak that it does not perturb the order parameter at T $<$ T$_{NI}$, nor the value of T$_{NI}$, serves to break symmetry. The anisotropic SANS pattern observed at T$_{NI}$ + 2\r{ }C indicates to us that the polymer exerts a local nematic field upon the solvent causing it to take on a nonzero order parameter within the pervaded volume of the coil. The temperature dependence as well as the role of mesogenic side group geometry and molecular weight will also be presented. [Preview Abstract] |
Monday, March 15, 2010 3:30PM - 3:42PM |
D17.00006: Self-Assembly of Side Group Liquid Crystalline Block Copolymers in a Nematic Solvent Rohan Hule, Zuleikha Kurji, Paul Pirogovsky, Julia Kornfield The local chain conformation and self-assembled morphology of diblock copolymers consisting of Side-Group Liquid Crystalline Polymers (SGLCPs) and random coil blocks are investigated using SANS, USANS and TEM. The innate orientational order of the mesogen couples with the flexible backbone in the SGLCP block. Two distinct types of mesogen chemistry and orientational coupling are examined that produce either a prolate or oblate spontaneous anisotropy of the SGLCP conformation in a small molecule nematic solvent. The conformational preference of the SGLCP block induces formation of unusually large, anisotropic self-assembled microstructures that display a remarkable restructuring across the nematic-to-isotropic transition temperature of the thermotropic solvent driven by favorable solvation of the coil. Dimensions extracted via core-shell form factor fits and Guinier analyses have been substantiated by TEM observations. Morphological and conformational comprehension of these coil-SGLCP diblocks opens new opportunities in responsive LC interfaces with in-built functionality of the coil domain. [Preview Abstract] |
Monday, March 15, 2010 3:42PM - 3:54PM |
D17.00007: Mechanisms of Photo-Induced Deformations of Liquid Crystal Elastomers Nathan Dawson, Mark Kuzyk, Jeremy Neal, Paul Luchette, Peter Palffy-Muhoray Over a century ago, Alexander Graham Bell invented the photophone, which he used to transmit mechanical information on a beam of light. We report on the use of an active Fabry-Perot interferometer to encode and detect mechanical information using the photomechanical effect of a liquid crystal elastomer (LCE) that is placed at a critical point between the reflectors. These are the first steps in the creation of ultra smart materials which require a large photomechanical response. Thus, understanding the underlying mechanisms is critical. Only limited studies of the mechanisms of the photomechanical effect, such as photo-isomerization, photo-reorientation and thermal effects have been studied in azo-dye-doped LCEs and in azo-dye-doped polymer fibers have been reported. The focus of our present work is to use the Fabry-Perot transducer geometry to study the underlying mechanisms and to determine the relevant material parameters that are used to develop theoretical models of the response. We use various intensity-modulated optical wave forms to determine the frequency response of the material, which are used to predict the material response. [Preview Abstract] |
Monday, March 15, 2010 3:54PM - 4:06PM |
D17.00008: Generalized Hydrodynamic Boundary Conditions Applied to Dynamic Switching of Nematic Liquid Crystal Cells Angbo Fang As the size of a liquid crysta display (LCD) cell becomes smaller, boundary effects become more important. The dynamic coupling between flow and orientation order in surface layers can play a significant role when nematic molecules are weakly anchored to the solid boundaries and transitions between bistable configurations involve anchoring breaking. We use Onsager's variational approach along with a heuristic scaling analysis for surface layers to obtain hydrodynamic equations and generalized boundary conditions for nematic liquid crystal cells. As an application, we demonstrate how surface translation-rotation coupling can be utilized to optimize performance of LCD cells. We also discuss how surface dynamics affects the flow-induced transitions in hybrid aligned nematic cells. [Preview Abstract] |
Monday, March 15, 2010 4:06PM - 4:18PM |
D17.00009: Spatiotemporal chaos with shear banding in a driven nematogenic fluid Debarshini Chakraborty, Chandan Dasgupta, Sriram Ramaswamy, Ajay Sood We present the results of a numerical study of a model of the hydrodynamics of a sheared nematogenic fluid in which spatial variations are allowed only in the gradient direction and the effects of order parameter stresses on the velocity profile are taken into account. When the value of a dimensionless viscosity parameter is so chosen that the order parameter stress is comparable to the bare viscous stress, the system exhibits steady states with the characteristics of shear banding, In addition, a non-zero choice of a parameter that governs the effect of the velocity field on stretching the nematic order parameter leads to the appearance of a new steady state in which the features of both spatiotemporal chaos and shear banding are present. [Preview Abstract] |
Monday, March 15, 2010 4:18PM - 4:30PM |
D17.00010: Transient Molecular Orientation Distribution during Injection Molding of Liquid Crystalline Polymers: Experiment and Simulation Jun Fang, Wesley Burghardt, Robert Bubeck We report a coordinated experimental and computational study of molecular orientation development during injection molding of commercial thermotropic liquid crystalline polymers. Three ``short shots'' were purposely produced to represent intermediate stages ($\sim$40\%, 60\%, 100\% filled) during the mold filling. Two-dimensional wide-angle x-ray scattering (2D-WAXS) in transmission mode was used to map out the detailed transient molecular orientation distribution along the centerline and three representative cross-sectional regions. Three distinct zones of orientation states were observed due to the complex spatial variations of the extensional flow effects (dominant near the flow front region) and the shear flow effects (dominant close to the centerline region). The experiments are used to test process simulations performed using commercial molding processing software. An analogy between the Folgar- Tucker fiber orientation model and the Larson-Doi polydomain model for textured liquid crystalline polymers is exploited to allow for the first tests of Larson-Doi model predictions against the transient molecular orientation development during injection molding processing. The numerical predictions successfully capture many features in the orientation distribution across the sample plaque. [Preview Abstract] |
Monday, March 15, 2010 4:30PM - 4:42PM |
D17.00011: Hairy-rod Complexes of Conjugated Polymers for Solar Cells S. Zhang, E. Beach, P. Anastas, L. Pfefferle, C. Osuji The development of new high efficiency organic solar cells in many cases is largely predicated on the formation of ordered nanostructures in the active donor-acceptor layer. Hairy-rod conjugated polymers, consisting of a stiff conjugated backbone and flexible long side chains, self-assemble into ordered nanostructures via phase separation. In particular, they can form nematic, smectic and columnar phases depending on the volume fraction of side chains.[1] In combination with suitable second-phase semiconducting nanomaterials, they offer a potential route to the realization of ordered heterojunction devices. In this work, we report phase behaviors of novel hairy-rod complexes consisting of a polythiophene backbone with surfactant side chains. Two kinds of the complexes were prepared via ionic bonding: one contains the flexible hydrocarbon sequence in side chains whereas the other utilizes the rigid mesogenic moiety. The interactions between polythiophene and different surfactants were investigated. Phase transitions of the complexes were identified by a combination of various techniques including x-ray scattering and calorimetry. Both thermotropic and lyotropic liquid crystalline behaviors were observed. We discuss a simple model for the formation of the liquid crystalline mesophases in these supramolecular systems. Reference [1] R. Stepanyan, A. Subbotin, M. Knaapila, O. Ikkala, G. ten Brinke, Self-organization of hairy-rod polymers, Macromolecules, 2003, 36, 3758 [Preview Abstract] |
Monday, March 15, 2010 4:42PM - 4:54PM |
D17.00012: Liquid Crystalline Properties of Amyloid Protein Fibers in Water Raffaele Mezzenga, Jin-Mi Jung We have studied the liquid crystalline features of two colloidal systems consisting of food protein amyloid fibrils in water, obtained by heat-denaturation and aggregation of $\beta $-lactoglobulin, a globular dairy protein. The resulting fibrils, have a monodisperse cross section of about 4 nm and two groups of polydisperse contour lengths: (i) fibrils 1-10 $\mu $m long, showing semiflexible polyeletrolyte-like behaviour and (ii) rigid rods 100-200 nm long. In both systems, the fibers are highly charged (+5 e/nm) and stable in water at low ionic strength (0.01 M) and low pH (pH 2). The physical properties of these systems are studied using a polymer physics approach and phase diagrams of these two systems are obtained by changing concentration and pH. Both systems exhibit rich phase behaviours. Interestingly, the experimentally measured isotropic-nematic phase transition was found to occur at concentrations more than one order of magnitude lower than what expected based on Onsager theory. Experimental results are revisited in terms of the Flory theory developed for rigid polymers in solvent of varying conditions. [Preview Abstract] |
Monday, March 15, 2010 4:54PM - 5:06PM |
D17.00013: Coacervation in Symmetric Mixtures of Oppositely Charged Rodlike Polyelectrolytes Rajeev Kumar, Glenn Fredrickson Phase separation in the salt-free symmetric mixtures of oppositely charged rodlike polyelectrolytes is studied using quasi-analytical calculations. Stability analyses for the isotropic-isotropic and the isotropic-nematic phase transitions in the symmetric mixtures are carried out. It is shown that electrostatics favor nematic ordering. Also, the coexistence curves for the symmetric mixtures are computed, and the effects of the linear charge density and the electrostatic interaction strength on the phase boundaries are studied. It is found that the counterions are uniformly distributed in the coexisting phases for low electrostatic interaction strengths characterized by the linear charge density of the polyelectrolytes and the Bjerrum's length. However, the counterions also phase separate along with the rodlike polyelectrolytes with an increase in the electrostatic interaction strength. It is shown that the number density of the counterions is higher in the concentrated (or coacervate) phase than in the dilute (or supernatant) phase. In contrast to the rodlike mixtures, flexible polyelectrolyte mixtures can undergo only isotropic-isotropic phase separation. A comparison of the coexistence curves for the weakly charged rodlike with the flexible polyelectrolyte mixtures reveals that the electrostatic driving force for the isotropic-isotropic phase separation is stronger in the flexible mixtures. [Preview Abstract] |
Monday, March 15, 2010 5:06PM - 5:18PM |
D17.00014: Nematic Ordering in Confined Geometry Applied to DNA Packaging in Viral Capsids Nikolay Oskolkov, Per Linse, Alexei Khokhlov We propose a density functional theory of conformation of a double stranded DNA inside a spherical viral capsid. For this purpose we apply the mathematical apparatus elaborated in the classic theories on nematic ordering to describe the arrangement principles of a long persistent polymeric chain in confined geometry. In this way, the local self-assembly of different segments of the chain can be considered as a nematic phase with a non-constant (distorted) director. Taking into account the only steric interactions in the second virial approximation is shown to be enough to demonstrate the principle possibility of the nematic ordering. As a result, the polymer density and orientational order distributions throughout the volume of the capsid were obtained. It is observed that a short and flexible polymeric chain is concentrated at the center of capsid being orientationally disordered, and behaving similar to a simple polymeric coil in the spherical cavity. In contrast, upon the increasing of the length and rigidity of the chain, it shifts towards the surface of the capsid locating predominantly at the equator, and undergoes the nematic transition. [Preview Abstract] |
Monday, March 15, 2010 5:18PM - 5:30PM |
D17.00015: Dissolution of carbon nanotubes in superacids: Modeling and Applications M. Green, A. Parra-Vasquez, N. Behabtu, C. Pint, R. Hauge, M. Pasquali, E. Kesselman, J. Schmidt, Y. Cohen, Y. Talmon Controlling the phase behavior of single-walled carbon nanotubes (SWNTs) in fluids is critical for the production of SWNT-based macroscopic materials. Yet, dissolution has been a major hurdle for researchers over the past decade. We have previously shown that SWNTs can be dissolved in superacids at high concentration and form liquid crystalline (LC) phases, which are readily processed into fibers and sheets. Here we model the phase behavior of polydisperse SWNTs in superacids using the Onsager excluded volume ansatz a range- dependent rod-rod potential to capture the effects of the solvent. The model indicates that in chlorosulfonic acid (ClSO3H) the attractive van der Waals forces between SWNTs are fully counterbalanced by charge-induced repulsion; therefore, ClSO3H behaves as an athermal solvent for SWNTs, yielding true molecular solutions. This startling result is confirmed by cryo-TEM images of SWNT/acid solutions where the SWNTs are dissolved as individuals. We also show that this acid can even be used to dissolve long (hundreds of microns) nanotubes; these solutions can also form LC phases. Dissolution of such large nanotubes had previously been thought impossible and holds great promise for processing of high-performance SWNT materials. [Preview Abstract] |
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