Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Z43: Liquid Crystalline Order in Polymer and Complex Fluids |
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Sponsoring Units: DPOLY Chair: Chinedum Osuji, Yale University Room: A306/307 |
Friday, March 25, 2011 11:15AM - 11:27AM |
Z43.00001: Nonlinear Dielectric Response of the Liquid Crystal 8CB near the phase transitions Hannah Buchanan The nonlinear dielectric response of the liquid crystal (LC) 8CB (4'-octyl-4-cyanobiphenyl) was measured near the smectic-nematic and nematic-isotropic phase transitions. The sample was filled in a commercially available LC capacitor cell of dimensions (1 cm $\times $ 1 cm $\times $ 9 $\mu $m). The cell was mounted in a temperature-controlled environment with a stability and resolution of 1 mK. The capacitance of the cell was measured at different temperatures in the range 25 -- 45\r{ }C covering both phase transitions, and over a range of frequencies up to 100 kHz, and a signal level in the range of 0 -- 5V, using a lock-in amplifier (SRS830) and LCR meter (Fluke PM6304). Nonlinear effects were observed in the capacitance even at a 200 mV signal level, and very large changes in the capacitance, both linear and nonlinear, were observed in the nematic phase, near each phase transition. *Undergraduate physics students **Recent physics graduates [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z43.00002: A Novel Liquid Crystal Elastomer with Large Spontaneous Length Changes Patricia Cladis, Simon Krause, Yusril Yusuf, Shohei Hashimoto, Leonid Fel, Shoichi Kai, Heino Finkelmann An order parameter describes an elastomer where a liquid crystal is cross linked at a temperature T$_{L}$ far from its clearing temperature. At T$_{L}$, there is a first order constant density phase transition where the order parameter vanishes and the elastomer has cubic symmetry. Below T$_{L}$ it is an icosahedral nematic and above a biaxial nematic. This theory quantitatively accounts for spontaneous shape change, nonlinear elasticity and gelation dynamics in a novel liquid crystal elastomer. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z43.00003: Magnetic field directed self-assembly of liquid crystalline block copolymers for membrane applications Manesh Gopinadhan, Pawel W. Majewski, Chinedum O. Osuji The use of magnetic fields is presented as a facile approach to the control of long range order and alignment of block copolymers. Using SAXS we demonstrate the alignment of lamellar and hexagonally packed cylinder morphologies of a poly(ethylene oxide)-based LC diblock copolymer by slow cooling in the presence of the field through the order-disorder transition. Non-degenerate alignment of the lamellar system is enabled by sample rotation and alignment in the system is shown to be driven by the diamagnetic anisotropy of the LC mesogen, and not anisotropy resulting from crystallization of the PEO block. We consider the effects of lithium doping and field strength on the order-disorder transition of the system, and the effect of lithium content on the critical field required for attaining well aligned films. The controlled alignment of PEO channels over large areas offers a route to selective ion transport in solid state batteries. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z43.00004: Temperature Dependent Self-Assembly of Side-Group Liquid Crystalline Block Copolymers in LC Solvent Paul Pirogovsky, Zuleikha Kurji, Rohan Hule, Julia Kornfield Side Group Liquid Crystalline Polymers (SGLCPs) take on anisotropic conformations in a small molecule LC solvent. This conformation is affected by both the temperature and the sense of attachment of the mesogenic side group (either parallel or perpendicular to the backbone). Covalently linking the SGLCP with a random coil (PS) block leads to a block copolymer that self assembles into anisotropic micelles. Transmission Electron Microscopy was used to view these micelles in real space and determine their size and structure. Small Angle Neutron Scattering (SANS) was performed on dilute (1 wt{\%}) solutions of a series of SGLCPs (homopolymers and block copolymers with a range of polystyrene block lengths) dissolved in deuterated 5CB. A rubbed alignment layer and a magnetic field were used to break symmetry and allow the ordered structure to be seen. PS-b-SGLCP block copolymers were seen to form self-assembled structures that changed qualitatively with the changing PS block length and the temperature. Most intriguingly, several polymers exhibit mutually orthogonal anisotropies at different length scales. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z43.00005: In-Situ X-Ray Diffraction Study of the Elongation Behavior of a Main Chain Liquid Crystal Elastomer Sonal Dey, D.M. Agra-Kooijman, Leela Joshi, Satyendra Kumar, Wanting Ren, Whitney M. Kline, Anselm C. Grrifin We studied the structural evolution of a main chain smectic elastomer under uniaxial stress and during strain recovery. At low strains, both the alkyl chains and the smectic layers are oriented on average parallel to the strain direction. At higher strains, the development of layer undulations is observed at $\sim $50{\%} which gradually evolve into chevron-like smectic structures as revealed by the four diffuse spots in small angle x-ray diffraction patterns. This is accompanied with an enhancement of the smectic order as inferred from higher intensity of small angle peaks and larger orientational order parameter, $S$ [for example, $S $(20{\%}) = 0.33; $S $(110{\%}) = 0.86]. During strain recovery, two relaxation rates of 6.5 min and 38.5 min are observed which are associated with relaxations of the mesogenic part and the elastomer network, respectively. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z43.00006: Evaporative Self-Assembly and Formation of the Lyotropic Liquid Crystalline Phase of Poly(3-hexyl thiophene) Min Sang Park, Avishek Aiyar, Jung Ok Park, Elsa Reichmanis, Mohan Srinivasarao In this study, we electrically and optically interrogated the evolution of the thin film structure in conjugated systems using poly(3-hexylthiphene) (P3HT) as a model semiconducting polymer. In an effort to understand the electrical properties of the conducting channel in terms of polymer chain orientation and relaxation in solution, we performed in-situ micro-Raman measurements using polarized incident light. We measured the extent of molecular chain alignment during the process of film formation and showed the existence of a lyotropic liquid crystal phase at the three-phase contact line. The variation of frequency dispersion and the shift of position for Raman active mode, combined with the structural anisotropy of P3HT films, suggest a phase transition to the lyotropic liquid crystalline phase. The orientational order of P3HT chains in the liquid crystalline phase was quantified as a function of evaporation time using solidified solvent, 1,3,5-triclorobenzene (1,3,5-TCB). [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z43.00007: Lyotropic nematic droplets of single-walled carbon nanotubes in super acids: equilibrium shape and dynamics Natnael Behabtu, Budhadipta Dan, Lee Taewoo, Ivan Smalyukh, Matteo Pasquali The balance between surface tension, elastic energy and surface anchoring creates nematic droplets with elongated shape, called tactoids. Here we report equilibrium shape and coalescence dynamics of single-walled carbon nanotube (SWNT) nematic droplets in superacids. Surprisingly, we observe bipolar tactoids with uniform aspect ratio. The uniform aspect ratio is not expected based on theoretical arguments that indicated that aspect ratio should drop with domain volume. Bipolar order contrasts recent findings of uniform order in liquid crystalline tactoid droplets of aqueous SWNTs stabilized with bile salts. The coalescence dynamics is highly influenced by the drop elasticity. Coalescence pathways that lead to defect creation are not observed, unlike spherical nematic droplets. Instead coalescence proceeds by tangentially matching the nematic director of two drops. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z43.00008: The unique behavior of chiral templated polymers swollen with liquid crystals Michael McConney, Jennifer Hurtubise, Vincent Tondiglia, Timothy White, Timothy Bunning Liquid crystals (LC) have the capability of impart a degree of order onto the molecular structure of polymers, essentially acting as ``smart solvents.'' Furthermore, structured materials have the ability to force liquid crystals into structures that are far from the bulk LC structure. Our studies are aimed at exploring the interplay between LC templated polymers and polymer templated LCs. Specifically we investigated a unique swelling-deswelling phase transition involving an ordered liquid crystal solvent and a structured polymer. The polymer is formed through anisotropic photopolymerization of LC monomers in a chiral LC fluid, is only attached from one surface of a single cell, and only spans a fraction of the cell thickness. The details of the transition were studied with various techniques including DSC, confocal microscopy, polarized microscopy, and visible/near-IR spectrometry. The unusual thermally induced structural changes result in large changes to the reflection wavelength of the cholesteric LC cells. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z43.00009: The Shape of Structured Polymer Vesicles Mark Bowick, Min-Hui Li, Homin Shin, Xiangjun Xing, Zhenwei Yao Polymer vesicles are stable robust vesicles made from block copolymer amphiphiles. Recent progress in the chemical design of block copolymers has lead to the creation of a variety of polymer vesicles with varying internal structure, functionality and shape. By choosing suitable liquid-crystalline polymers for one of the copolymer components one can create vesicles with internal nematic or smectic order. This talk will address the possible shapes of these \textit{structured }vesicles as well as their inherent topological defects. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z43.00010: Curvature-induced transitions in two-dimensional nematics Badel L. Mbanga, C.D. Santangelo, G.M. Grason Anisotropic particles absorbed to a fluid interface are known to significantly alter the structure and mechanics of these surfaces, possibly even stabilizing surfaces of complex, bicontinuous topology, as in ``bijels'' formed from arrested spinodally- decomposing fluid mixtures. We study the intricate interplay between the morphology of these interfaces and the alignment of anisotropic elongated particles with nematic order. Here we present results from computer simulation of nematic order on hyperbolic surfaces using a model that accounts for the contribution of both the intrinsic and extrinsic curvatures to the energetics of topological defects defects. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z43.00011: Effect of chain stiffness on structural and thermodynamic properties of polymers Kiran Khanal, Jutta Luettmer-Strathmann The stiffness of the chains affects many properties of polymers. We investigate structural and thermodynamic properties of a bond- fluctuation lattice model for semiflexible polymer chains. Monte Carlo simulations for polymer melts for a range of values of the bending penalty, density, and temperature show elongation of the polymer conformations with increasing chain stiffness but no transition to a nematic phase. Results for average bead-bead interaction energy and bending energy were studied separately and showed that the bending energy is almost independent of the filling fraction, suggesting that the stiffness of the chains can be treated as a single chain property. We calculated the bending energy from the partition function of a pair of neighboring bonds and find excellent agreement between theory and simulation. Equation of state properties of the lattice model were determined from insertion methods and thermodynamic integration. We develop a theoretical description of these properties and use it to determine model parameters for real polymer melts from a comparison with experimental data. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z43.00012: Symmetric reflection band broadening of weakly polymer stabilized cholesteric thin films using low DC electric fields Madeline Dunning, Christopher Bailey, Anastasia Voevodin, Vincent Tondiglia, Lalgudi Natarajan, Timothy White, Timothy Bunning We report on a new, low field electro-optical effect in weakly polymer stabilized cholesteric liquid crystals with negative dielectric anisotropy. By applying low DC electric fields ($<$3V/$\mu $m), a symmetric broadening of the cholesteric reflection band can be seen, resulting in band width increases by factors of two or more. An intensive study of the various experimental parameters combined with numerical calculations of the transmission spectra, indicate that the polymer interacts with the electric fields resulting in an approximately constant pitch gradient across the cell thickness. Our results show that the maximum pitch distortions reach values of approximately 15{\%} the zero voltage value for notches in the visible range (pitches of 300-400nm), but increase along with the pitch. Possible physical mechanisms will be explored and discussed that might explain this interesting electro-optical effect. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z43.00013: Structure of Rigid Hard-Ring Fluids Mariam Nouri, Marc Robert Structure of fluids of molecules consisting of rigid rings of hard spheres is studied in two, quasi two, and three dimensions, using Monte Carlo computer simulations in the canonical ensemble. For rings of various size and for a wide range of densities, results are reported for the pair distribution function of the ring centers and for the pair distribution of the ring orientations. For dense fluids in two dimensions, a shoulder, precursor of the freezing transition, is observed in the second peak of the pair distribution function of the ring centers, as previously seen in the simple hard-sphere fluid. In quasi two dimensions, where the centers of the rings are confined to a plane but the rings themselves can wobble out of plane, a liquid crystalline nematic phase is observed at sufficiently high densities. Results are also presented for three dimensions. [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z43.00014: Fast Off-Lattice Monte Carlo Simulations with a Novel Soft-Core Spherocylinder Model Jing Zong, Xinghua Zhang, Qiang (David) Wang Fast off-lattice Monte Carlo simulations with soft-core repulsive potentials that allow particle overlapping give orders of magnitude faster/better sampling of the configurational space than conventional molecular simulations with hard-core repulsions (such as in the Lennard-Jones potential).\footnote{\textit{Q. Wang and Y. Yin}, \textbf{J. Chem. Phys., 130}, 104903 (2009).} Here we present our fast off-lattice Monte Carlo simulations on the structures and phase transitions of liquid crystals and rod-coil diblock copolymers based on a novel and computationally efficient anisotropic soft-core potential that gives exact treatment of the excluded-volume interactions between two spherocylinders (thus the orientational interaction between them favoring their parallel alignment). Our model further takes into account the degree of overlap of two spherocylinders, thus superior to other soft-core models that depend only on their shortest distance. It has great potential applications in the study of liquid crystals, block copolymers containing rod blocks, and liquid crystalline polymers. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z43.00015: Solvent involved self-crystallization of C$_{70}$ molecules into high definition cube microstructure Chibeom Park, Hee Cheul Choi C$_{70}$ molecules dissolved in mesitylene (good solvent) are self-crystallized into cube shape microstructure by the addition of isopropyl alcohol (poor solvent). Through control experiments attempted with different types of alcohols as well as in the replacement of mesitylene with other similar solvents, such as toluene, m-xylene, and m-dichlorobenzene, it is confirmed that mesitylene plays a critical role to guide C$_ {70}$ molecules to form cube microcrystal with high definition edges and surfaces. Thermal gravimetric and crystallographic analyses show that the crystal structure is simple cubic whose unit cell is composed of one C$_{70}$ and two mesitylene molecules. The photoluminescence intensity from C$_{70}$ cube crystals are enormously increased compared to C$_{70}$ powder. Such abnormal photoluminescence increase is mainly attributed to the high crystallinity of C$_{70}$ cubes as confirmed by time-resolved photoluminescence lifetime measurements. [Preview Abstract] |
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