Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B31: Focus Session: Nano to Meso-Scale Structure in Ordered Soft Matter: Liquid Crystal Structure, Dynamics and Function I |
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Sponsoring Units: DPOLY Chair: Alberto Fernandez de las Nieves, Georgia Institute of Technology Room: 339 |
Monday, March 18, 2013 11:15AM - 11:27AM |
B31.00001: Self-Assembly of Polyhedral Oligomeric Silsesquioxane-Based Giant Molecular Shape Amphiphiles Yiwen Li, Stephen Cheng A series of giant molecular shape amphiphiles based on functional polyhedral oligomeric silsesquioxane (POSS) particles was designed and synthesized. The supramolecualr structures of these assemblies along with the resulting ordered structures are fully investigated to determine their structure-property relationships. For example, functional POSS cages with different surface chemistry and sizes were employed to construct dumbbell- and snowman-like molecular Janus particles with various symmetry breakings. These particles could self-organize into hierarchically ordered supramolecular structures in the bulk. Another illustrating example is a series of novel giant surfactants, lipids and gemini surfactants possessing a hydrophilic POSS head and polymer or alkyl chain tails. Diverse architectures of this class of materials have been constructed and their self-assembly processes in solution and bulk state have been discussed. This set of research results not only has general implications in the basic physical principles underlying their self-assembly behaviors, but also create unique materials for developing advanced technologies by combining the properties of hybrid materials [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B31.00002: Nanoparticle Solubility in Liquid Crystalline Defects Jonathan K. Whitmer, Julio C. Armas-Perez, Abhijeet A. Joshi, Tyler F. Roberts, Juan J. de Pablo Liquid crystalline materials often incorporate regions (defects) where the orientational ordering present in the bulk phase is disrupted. These include point hedgehogs, line disclinations, and domain boundaries. Recently, it has been shown that defects will accumulate impurities such as small molecules, monomer subunits or nanoparticles. Such an effect is thought to be due to the alleviation of elastic stresses within the bulk phase, or to a solubility gap between a nematic phase and the isotropic defect core. This presents opportunities for encapsulation and sequestration of molecular species, in addition to the formation of novel structures within a nematic phase through polymerization and nanoparticle self-assembly. Here, we examine the solubility of nanoparticles within a coarse-grained liquid crystalline phase and demonstrate the effects of nanoparticle size and surface interactions in determining sequestration into defect regions. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B31.00003: Liquid Crystal Phase Transition driven three-dimensional Quantum Dot Organization Andrea L. Rodarte, R.J. Pandolfi, S. Ghosh, L.S. Hirst We use a nematic liquid crystal (LC) to create organized assemblies of CdSe/ZnS core/shell quantum dots (QDs). At the isotropic-nematic LC phase transition, ordered domains of nematic LC expel the majority of dispersed QDs into the isotropic domains. The final LC phase produces a series of three dimensional columnar QD assemblies that are situated at defect points in the LC volume. Within each assembly the QD emission is spectrally-red-shifted due to resonant energy transfer. We use this spectral shift as a measure of the inter-dot separation and find that the QDs are packed uniformly in these assemblies over distances of microns between the glass plates of a standard LC cell. In addition, because the QD clusters form at defects, we can deterministically control the location of the assemblies by seeding the LC cell with defect nucleation points. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B31.00004: Ordering of Lyotropic Chromonic Liquid Crystal Films In Cylindrical Micropost Arrays Marcello Cavallaro, Matthew Lohr, Daniel Beller, Laura Laderman, Kathleen Stebe, Randall Kamien, Peter Collings, Arjun Yodh The use of micropost arrays is explored as a means for controlling self-assembly and director alignment in nematic chromonic liquid crystal (CLC) films. Experiment and numerical solutions reveal that the micropost arrays induce bistable director alignment in the film, along either diagonal of a square micropost lattice. We demonstrate stabilization of large domains of a single director orientation by rubbing the substrate surface along a single diagonal, a procedure which biases planar CLC director alignment in the film. Additionally, by varying the rubbing angle we investigate the competition between alignment via micropost patterns versus substrate rubbing, and we find the resulting assemblies to be largely controlled by micropost geometry. Variation of micropost layout, spacing and dimensions leads to further interesting self-assembled patterns and defect geometries. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:39PM |
B31.00005: Modelling liquid crystal elastomers and potential application as a reversibly switchable adhesive Invited Speaker: James Adams Liquid crystal elastomers (LCEs) are rubbery materials that composed of liquid crystalline polymers (LCPs) crosslinked into a network. The rod-like mesogens incorporated into the LCPs are have random orientations in the high temperature isotropic phase, but can adopt the canonical liquid crystalline phases as the temperature is lowered. Smectic liquid crystal elastomers have highly anisotropic mechanical behaviour. This arises in side chain smectic-A systems because the smectic layers behave as if they are embedded in the rubber matrix [1]. The macroscopic mechanical behaviour of these solids is sensitive to the buckling of the layers, so is a multiscale problem. A coarse grained free energy that includes the fine-scale buckling of the layers has been developed [2], which enables continuum modelling of these systems. In the first part of this talk I present a model of the mechanical behaviour of side chain smectic elastomers. The properties of nematic LCEs, such as their high loss tangent, and mechanical strain hardening, might enable them to be used as reversibly switchable pressure sensitive adhesive (PSA). PSAs are typically made from viscoelastic polymers. The quality of their adhesion can be measured by the \emph{tack energy}, which is the work required to separate two bodies. To obtain a high tack energy a PSA should be capable of a large strain. It should strain soften at low strain to produce crack blunting, and then strain harden at high strain to stiffen the fibrils formed late in the debonding process. I will present a model of the tack energy of weakly crosslinked nematic polymers. To describe the constitutive properties of this system the nematic dumbbell model of Maffettone \emph{et al.} was used [3]. This constutitive model was then combined with the block model of Yamaguchi \emph{et al.} describing PSAs [4]. It was found that the parallel orientation of the nematic has a higher tack energy than both the isotropic and the perpendicular director orientation [5].\\[4pt] [1] C. M. Spillmann et al, Phys. Rev. E 82, 031705, (2010).\\[0pt] [2] J. Adams, S. Conti and A. DeSimone, Mathematical Models and methods in Applied Sciences, 18, 1 (2008).\\[0pt] [3] P. L. Maffettone and G. Marrucci, Journal of Rheology 36 (8) 1547 (1992).\\[0pt] [4] T. Yamaguchi, H. Morita, and M. Doi, Eur. Phys. J. E 20, 7 (2006).\\[0pt] [5] D. R. Corbett and J. M. Adams, Soft Matter, DOI:10.1039/C2SM26868J (2012). [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B31.00006: Compliant random fields in gels formed from side-chain liquid crystalline polymers Paul Goldbart, Fangfu Ye, Bing Lu, Xiangjun Xing Localized polymer-chain backbones in gels formed from side-chain liquid crystalline polymers serve to create random fields that induce local orientational order of the nematogenic pendants of the side chains. ~These random fields differ, however, from conventional ones, in that they are compliant, and thus themselves undergo thermal fluctuations. ~We develop a free energy that describes local nematic ordering in presence of such compliant random fields. ~In particular, we show that, as a result of this compliance, the free energy has a qualitatively new structure, unattainable via truly static random fields. ~We discuss the physical implications this free energy, focusing on the consequences of the compliant nature of the random fields. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B31.00007: Phase Behavior of Semi-flexible-Coil Block Copolymers Studied by Monte Carlo Simulations Tao Wei, Robert Riggleman Semi-flexible/coil and rod/coil polymers have attracted increasing interest in the applications of organic electronics and biomaterials due to their novel supramolecular structures with nanoscale architecture and tunable domain size. The coupling of microphase separation and liquid-crystalline ordering, stemming from chain rigidity, yields complex phase behaviors. In this work, phase morphologies and phase diagram of semi-flexible/coil block copolymers were identified with efficient Theoretical informed coarse-grained Monte Carlo (TIMC) simulations, which tracks the local density of each grid, rather than computationally demanding pair-wise interactions. Besides the common Flory-Huggins interactions between dissimilar components, we incorporate anisotropic interactions through a Maier-Saup\'e potential. Due to the increased complexity of semi-flexible polymer, parameter number is significantly larger compared to fully flexible polymers. We will illustrate the TIMC method for semi-flexible/coil polymers and examine fluctuation effect on various phase diagrams. We demonstrate the influence of the relative strength of Maier-Saup\'e parameter to Flory-Huggins parameter, as well as the geometric factors that characterize the size of the semi-flexible block relative to the coil block. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B31.00008: Competition of Elasticity and Flexoelectricity for bistable alignment of nematics on patterned substrates Timothy Atherton, James Adler We show that patterned surfaces can promote bistable configurations of nematics for reasons other than the symmetry of the surface. Numerical and analytical calculations reveal that a nematic liquid crystal in contact with a striped surface is subject to the competing aligning influences of elastic anisotropy, differing energy cost of various types of deformation, and flexoelectricity, curvature-induced spontaneous polarization. These effects favor opposing ground states where the azimuthal alignment is, respectively, parallel or perpendicular to the stripes. Material parameters for which the effect might be observed lie within the range measured for bent-core nematogens. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B31.00009: Effect of ionic additives on elasticity of lyotropic chromonic liquid crystal Shuang Zhou, Adam J. Cervenka, Yogesh Singh, Luana T. Tortora, Carmen C. Almasan, Oleg D. Lavrentovich Using a magnetic Frederiks transition technique, we determine how the splay $K_{1}$ and bend $K_{3}$ elastic constants of lyotropic chromonic liquid crystal Sunset Yellow (SSY) depend on concentration of ionic additives, sodium chloride (NaCl) and magnesium sulfate (MgSO$_{4}$). Both salts increase the ratio $K_{1}/K_{3}$, by mainly increasing $K_{1}$ (MgSO$_{4}$) or mainly decreasing $K_{3}$ (NaCl). The effects are attributed to the screening of electrostatic repulsions of chromonic molecules, which is expected to increase the contour length (thus increasing $K_{1}$) and to decrease the persistence length (thus decreasing $K_{3}$) of the chromonic aggregates in which the molecules are stacked face-to-face. As in salt-free SSY, the ratio $K_{1}/K_{3}$ increases when the temperature decreases. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B31.00010: Tactoids and Defects in Nematic-Isotropic Phase Transition in Lyotropic Chromonic Liquid Crystal Young-Ki Kim, Oleg D. Lavrentovich We explore the structure of nuclei (tactoids) and topological defects (disclinations) in the first-order nematic-isotropic phase transition in self-assembled lyotropic chromonic liquid crystals. The shape of tactoids is determined by orientational elasticity of the liquid crystal, surface tension, and surface anchoring of the director. The positive tactoids (nuclei of the nematic phase) show two pointed ends (cusps). The negative tactoids (nuclei of the isotropic phase) show a variety of shapes, with one, two, or three cusps, depending on whether they nucleate at the core of disclinations of strength 1/2, in a homogeneous nematic, or at the core of a (-1/2) disclination, respectively. Zero-cusp and four-cusp formations are also possible at the core of stabilized disclinations of strength 1 and -1, respectively. The results demonstrate a profound role of surface tension and its anisotropy in the morphological dynamics of phase transitions in liquid crystals. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B31.00011: Liquid Crystal Switching Response by Localized Surface Plasmon Induced Electric Fields Zachary Nuno, Linda Hirst, Sayantani Ghosh We investigate the effect of electric fields induced by localized surface plasmons (LSPs) from gold nanoparticles (AuNPs) on the director of a nematic liquid crystal (LC). We deposit LC thin films on a self-assembled AuNP layer and excite the LSPs in the AuNPs using 530 nm excitation light. Using polarized optical microscopy we follow the birefringence of the LC film as the excitation is turned on and off and observe the homeotropic alignment of the LC change to planar. This realignment response is observed to be dependent on the excitation wavelength, excitation power, and temperature; occurring only within 1 degree Celsius of the LC phase transition from nematic to isotropic. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B31.00012: Tunable lithography masks using chiral nematic fluids Hyeon Su Jeong, Mohan Srinivasarao, Hee-Tae Jung We present a facile route for pattern formation using chiral nematic fluids as tunable masks in lithography process. The chiral nematic phase prepared by adding a chiral dopant (CB15) to 5CB acted as a set of parallel cylindrical lenses and as a polarization selective photomask for the preparation of periodic line patterns. The pitch of the helical twist was easily controlled by the concentration of chiral agent and the feature size of the resulting pattern was easily tuned. Because of the high mobility of the small liquid crystalline compound, the preparation of chiral nematic fluids based lithography masks requires only a few seconds. This approach has significant advantages including facility, range of surface ordering, and rate of forming periodic arrays. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B31.00013: Light sensitive liquid crystals: Focusing on surface and bulk transitions Petr Shibaev, Seth Bourg, Shannon Rosario, Daniel Bateman, Andrey Iljin The study of transitions in liquid crystalline matrix resulting from light-induced conformational changes in newly synthesized light sensitive molecules are studied and discussed. Light sensitive molecules (novel chiral and non-chiral azo dyes, spiropyrans) were either embedded in a polymer film serving as a container for liquid crystals or dissolved in a bulk of liquid crystals. In both cases light-induced re-orientation of director was observed in nematic liquid crystals. In chiral liquid crystals the family of regular domains with a different helical pitch was formed. One of the most ordered representatives of this family was observed earlier in [1]. Interestingly, the shape and structural characteristics of the domains were different in liquid crystalline droplets and in the films contained between glass plates. It is possible to freeze the regular structure of the domains in highly viscoelastic liquid crystals and obtain free standing films. The role of viscosity is discussed and a simple theoretical model of observed effects is presented. The studied films can be used in photonic devices and also as detectors of environmental changes. \\[4pt] [1] P. Shibaev. R. Sanford, D. Chiappetta, A. Genack and A. Bobrovsky Optics Express, Vol. 13, Issue 7, pp. 2358-2363 (2005) [Preview Abstract] |
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