Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H30: Liquid Crystalline Polymers |
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Sponsoring Units: DPOLY Chair: Patrick Mather, Case Western Reserve University Room: LACC 505 |
Tuesday, March 22, 2005 8:00AM - 8:36AM |
H30.00001: BREAK - H30
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Tuesday, March 22, 2005 8:36AM - 8:48AM |
H30.00002: Polydomain Liquid Crystalline Networks as Actuators Patrick Mather, Haihu Qin, Ingrid Rousseau We have separately designed and synthesized rigid nematic networks and compliant smectic-C networks, each existing with polydomain textures at equilibrium and accessible isotropization temperatures. The materials share molecular similarity by use of identical mesogens, but in the nematic case these mesogens are linked directly together by ADMET polymerization, leaving residual unsaturation for crosslinking, while in the smectic-C case the mesogens are bridged by short siloxane spacers that afford the macroscopic compliance. In this presentation we show that despite dramatically different stiffnesses and phase symmetries for these materials, they share in common reversible elongation/contraction on cooling and heating through liquid crystallization/isotropization, respectively. It is argued that this common feature derives from a polydomain-monodomain transition possible in both types of material due to their existence as highly textured materials. We further show that large strain ``fixing'' is possible in both types of materials, such fixing being possible by vitrification of the entire material in the nematic case or of the mesogen-rich layers only in the smectic case. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H30.00003: Molecular orientation of commercial thermotropic liquid crystalline polymers in transient shear flow Stanley Rendon, Wesley Burghardt, Robert Bubeck There is still limited fundamental understanding of the rheology of commercial main-chain thermotropic LCPs. Limited physical and chemical stability at the high melt temperatures of commercial LCPs have to date rendered fundamental 'monodomain' studies impossible, and complicate conventional shear rheometry. It is not established whether commercial thermotropes are of the shear-tumbling or shear-aligning classification. In studies on idealized materials, direct measurements of molecular orientation in transient shear flows (reversals, step-changes and flow cessation) have often shed light on the underlying director dynamics. Here we report attempts to apply such methods to two commercial thermotropic LCPs (Vectra A950 and B950). Synchrotron x-ray scattering in conjunction with a high speed detector provides sufficient time resolution to reduce the total time spent in the melt during testing, while enhancements to an x-ray capable shear cell provide a robust experimental platform for working with LCP melts at high temperatures. The transient orientation response to changes in flow condition do not yield definitive signatures of either tumbling or alignment. However, Vectra A shows clear responses to step-increase or step-decrease in shear rate, which contrasts with expectations and experience with shear-aligning nematics. The two polymers show opposite trends in orientation following flow cessation, which are correlated with evolution of dynamic modulus. [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H30.00004: Confined Discotic Liquid Crystalline Self-Assembly in a Novel Coil-Coil-Disk Triblock Oligomer Li Cui, Jianjun Miao, Lei Zhu, Igors Sics, Benjamin Hsiao An asymmetric ABC coil-coil-disk triblock oligomer, based on polyethylene-\textit{block}-poly(ethylene oxide)-\textit{block}-pentakis(pentyloxy)triphenylene (PE-$b$-PEO-$b$-P5T or EEO-P5T), was successfully synthesized by coupling a hydroxyl group terminated PE-$b$-PEO diblock oligomer and 2-hydroxy-3,6,7,10,11-pentakis(pentyloxy)triphenylene using oxalyl chloride. The structure and morphology of supramolecular self-assembly in bulk EEO-P5T was studied by small- and wide-angle X-ray scatterings and transmission electron microscopy. The PE block was found to crystallize into interdigitated, extended chain crystals with a chain-tilting angle of 23$^{\circ}$ from the lamellar normal, where their melting temperature ($T_{m})$ was at 93.8$^{\circ}$ C. The PEO block remained in the amorphous state because both its ends were tethered to other two blocks. Bilayer P5Ts, sandwiched between amorphous PEO layers, exhibited a nematic columnar (N$_{col})$ to nematic discotic (N$_{D})$ transition at \textit{ca.} 23$^{\circ}$ C. Intriguingly, the N$_{D}$ to isotropic (or \textit{vise versa}) transition was observed to coincide with the melting (or crystallization) of the PE block. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H30.00005: From vulcanization to isotropic and nematic rubber elasticity Xiangjun Xing, Swagatam Mukhopadhyay, Paul Goldbart, Annette Zippelius A Landau theory is constructed for the vulcanization transition in cross-linked polymer systems with spontaneous nematic ordering [1]. The neo-classical theory of the elasticity of nematic elastomers is derived via the minimization of this Landau free energy; this neo-classical theory contains the classical theory of rubber elasticity as its isotropic limit. Our work not only reveals the statistical-mechanical roots of these elasticity theories, but also demonstrates that they are applicable to a wide class of random solids. It also constitutes a starting-point for the investigation of sample-to-sample fluctuations in various forms of vulcanized matter. [1] X. Xing et al., cond-mat/0411660 [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H30.00006: Slow dynamics and the glass transition in anisotropic polymer liquids Folusho Oyerokun, Kenneth Schweizer A microscopic theory has been developed at the coarse-grained segment level for the onset or crossover temperature ($T_{c})$ to highly activated dynamics in deeply supercooled anisotropic polymer liquids. A generalization of a simplified mode coupling theory is employed which utilizes structural and thermodynamic information from anisotropic PRISM theory. Conformational alignment or /and deformation modifies equilibrium properties thereby inducing anisotropic segmental dynamics. For liquid crystalline polymers a suppression of $T_{c }$with increasing nematic or discotic orientational order is predicted. The underlying mechanism is reduction of the degree of coil interpenetration and intermolecular repulsive contacts due to chain alignment. For supported thin films on neutral substrates a significantly larger suppression of $T_{c}$ is found which emerges due to the presence of both segmental alignment and deformation. Reasonable agreement with experiment has been demonstrated. The theory can also be applied to brush-like systems and rubber networks where chain deformation results in more intermolecular contacts, an enhanced bulk modulus and an elevation of $T_{c}$. Extension to treat directionally-dependent collective barrier formation and hopping is also possible. [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H30.00007: The Origin of Helical Suprastructure from Achiral 4-Biphenyl Carboxylic Acid Molecules Kwang-Un Jeong, Jason J. Ge, Shi Jin, Matthew J. Graham, Brian S. Knapp, Frank W. Harris, Stephen Z. D. Cheng A novel series of achiral 4-biphenyl carboxylic acid molecules (BPCA-Cn-PmOH) connected with phenol at meta-position by alkoxyl chains with various carbon numbers (n = 6-10) formed spontaneous helical suprastructures in the low ordered SmC phase. In order to understand the origin of phase chirality without configurational chirality in BPCA-Cn-PmOH, four other achiral 4-biphenyl carbonyl model compounds with an alkoxyl chain (n = 6) were studied using a combination of techniques. It is concluded that both the twisted conformation of the self-assembled head-to-head dimers and the phenyl groups at the end of the dimers are essential to form a stable helical suprastructure, but the -OH or -OCH3 functions at the meta- or para-position of phenyl ring is not necessary. [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H30.00008: Free Energy Functional for Bend-core Liquid-Crystal Molecules Rui Zhang, An-Chang Shi Bend-core liquid-crystal molecules, such as polymers containing banana-shaped mesogens, exhibit very rich phase behaviour. Theoretical studies of this fascinating system include Landau theory and simulations. Starting from a molecular model and using a self-consistent field theory approach, we develop a field-theoretical description of the thermodynamics for bend- core liquid-crystal systems. We incorporate the order parameters, which characterize the particular geometry and potential structures of bent-core molecules, into the free energy functional. The resulting free energy functional is used to study phases and phase transitions of the system, including a group of important phases under diverse physical conditions. The results will be compared with several recent experiments. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H30.00009: Electric-field-induced motion of colloid particles in smectic liquid crystals Antal Jakli, Guangxun Liao, Ivan Smalyukh, Jack Kelly, Oleg Lavrentovich We present the first observations of DC electric-field-induced rotational and translational motion of finite particles in liquid crystals. The electro-rotation is basically identical to the well known Quincke rotation, which triggers the translational motion at higher fields. From the electric field dependence of the angular velocity of the rotation we obtain the viscosity of the liquid crystals. The analysis of the translational motion in smectic liquid crystals indicates elastic responses near the threshold for translation. At increasing fields the speed of the particles is increasing and at sufficiently high speeds the flow of the smectic A and smectic C liquid crystal around the beads become purely viscous. Colloid particles in smectic materials maybe considered as model systems for understanding motion of proteins in cell membranes. [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H30.00010: Crystallization of Polyelectrolyte-Surfactant Complexes at the Air-Water Interface Alex Travesset, David Vaknin, Gilat Nizri, Shlomo Magdassi A system consisting of a strong polyelectrolyte (poly-allyldimethylchloride) and surfactant (sodium-dodecyl-phosphate) is studied by surface synchrotron x-ray techniques combined with surface-tension measurements. It is found that distinct phases are formed at the gas-water interface in regimes where no order is present in bulk. In particular, we find that the addition of salt (NaCl) to a very dilute surfactant in polyelectrolyte solution induces a crystallization of cylindrical micelles with distorted-hexagonal symmetry. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H30.00011: Mesophase Behavior of Polyion-Complexed Azobenzene Chromophores in the Bulk C. Geraldine Bazuin, Carmen M. Tibirna, Qian Zhang We have complexed ammonium-functionalized 4-nitro-4'-alkoxy-azobenzene mesogens to various polyelectrolytes (carboxymethylcellulose, cellulose sulfate, polyacrylate, polystyrene sulfonate, etc.) via ion-exchange procedures. The bromine-neutralized mesogens melt directly into the isotropic phase, whereas the complexes are thermotropic liquid crystals with high mesophase stability. Stoichiometric complexes show well-defined glass transitions in the vicinity of ambient temperature, and often a second (weak) Tg-like transition at much higher temperatures. The clearing temperatures are generally reached in the 130-180 degree C range, above the mesogen melting points. They are organized as disordered monolayer lamellar (SmA) mesophases. The variations in transition temperatures (glass transitions, clearing temperatures) are generally moderate, and can be correlated with specific molecular parameters such as spacer length and polymer backbone. The transitions are particularly sensitive to the stoichiometry of the components. Some preliminary results regarding film-making and optical responses of these materials will also be presented. [Preview Abstract] |
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