P. Pasini and S. Žumer- Liquid Crystal Phases and Nano-Structures

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<p>International School of Liquid Crystals 15th Workshop</p> <p>Liquid Crystal Phases and Nano-StructuresA Workshop to celebrate ten years of the International Liquid Crystal School and Claudio Zannoni 60th birthday E. Majorana Centre for Scientific Culture, Erice 27 October 1 November 2008</p> <p>TALKS</p> <p>Directors of the Workshop: P. Pasini and S. umer</p> <p>Director of the School: C.Zannoni</p> <p>Sponsors: Italian Liquid Crystal Society, INSTM-CRIMSON, E4 Computer Engineering</p> <p>Thermotropic Biaxial Nematics: An Enduring Challenge? Geoffrey Luckhurst School of Chemistry, University of Southampton, Southampton SO17 1BJ, United Kingdom1</p> <p>Superstructures in nematic colloidsSlobodan umer1,2, Miha Ravnik1, and Brina rnko1 Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia; 2 Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia</p> <p>The discovery of a new liquid crystal phase is an exciting event especially if the phase has been predicted theoretically. In principle, one example of such a phase is the biaxial nematic first predicted by Marvin Freiser1 in 1970. Following his prediction there was much theoretical work but apparently no experimental attempts to find the phase until 1980. Then, in a seminal study, Yu and Saupe discovered the biaxial nematic but for a lyotropic liquid crystal2. Since then although simulations3 of model systems have certainly revealed the existence of the thermotropic biaxial nematic the discovery of a real system has remained elusive. However, many compounds have been claimed to form a biaxial nematic but the evidence was not always strong and when studied with deuterium NMR it was invariably found that the nematic is uniaxial4. In recent years more detailed evidence in support of claims has been presented and the existence of the biaxial nematic seems more convincing. One example of such a claim is for a tetrapode in which four mesogenic groups are tethered by flexible spacers to a central silicon atom5. In addition, it seems that as well as finding the elusive biaxial phase the FTIR studies also determined the four orientational order parameters characterising both the uniaxial and the biaxial nematics5. The identification of the phases was then supported by NMR using a deuteriated calamitic nematogen as a spin probe6. Given the reassuring nature of these investigations we had decided to undertake our own studies. In the first of these the four order parameters were used to test a rather general molecular field theory of the biaxial nematic in which the molecular biaxiality is characterised by two parameters. Attempts to fit the theory to the experimental results proved to be interesting which prompted us to compare the order parameters found for the uniaxial and biaxial nematic phases of a biaxial Gay-Berne mesogen3(b) again with interesting results7. Independently of these theoretical studies we had also determined the phase symmetry with the aid of NMR and a selection of deuteriated spin probes over the entire nematic range of the tetrapode. To help appreciate these NMR results the orientational ordering of the probes in a phase of known biaxiality was determined. Further studies of the tetrapode at low temperatures were inadvertently undertaken and these suggest how some of our puzzling results, both experimental and theoretical, can be understood. The thermotropic biaxial nematic phase would appear to present an enduring challenge at least as far this tetrapode is concerned.M J Freiser, Phys Rev Lett 24, 1041 (1970). L L Yu and A Saupe, Phys Rev Lett 45, 1000 (1980). (a) G R Luckhurst and S Romano, Mol Phys 40, 129 (1980); (b) R Berardi and C Zannoni, J Chem Phys 113, 5971 (2000) 4 G R Luckhurst, Thin Solid Films 393, 40 (2001) 5 K Merkel, A Kocot, J K Vij, R Korlacki, G H Mehl and T Meyer, Phys Rev Lett 93, 237801 (2004). 6 J L Figueirinhas, C Cruz, D Filip, G Feio, A C Ribeiro, Y Frere, T Meyer and G Mehl, Phys Rev Lett 94, 107802 (2005). 7 F Bisi, G R Luckhurst and E G Virga, Phys Rev E 78, 021710 (2008).2 3 1</p> <p>Our recent theoretical predictions are contrasted with the latest experimental studies of colloidal structures in spatially confined nematic liquid crystals. Modeling is based on phenomenological Landau- de Gennes type free energy where also effects of confinement and external fields are taken into account. The complexity of effective inter-colloidal couplings in a nematic solvent leads to numerous colloidal structures not present in simple liquids. Here we focus on thin nematic films where colloidal particles can share nematic defects. In such a case the coupling is string-like and is therefore much more robust as compared to an interaction based on an array of localized disclinations. These structures can be realized either via -1/2 disclination lines that entangle two or more colloidal particles [1] or via -1 disclination loops with nonsingular cores where topological constrains result in sharing of the corresponding areas of deformed nematic by neighboring particles. The resulting colloidal dimmers, chains, lattices, and braids are the first steps toward assembling of robust nematic colloidal crystals and hierarchical structures [2] that are of potential interest for photonic applications. [1] M. Ravnik, M. karabot, S. umer, U. Tkalec, I. Poberaj, D. Babi, N. Osterman, and I. Muevi, Entangled Nematic Colloidal Dimers and Wires, Phys. Rev. Lett. 99, 247801 (2007) [2] M. karabot, M. Ravnik, S. umer, U. Tkalec, I. Poberaj, D. Babi, I. Muevi, Hierarchical self-assembly of nematic colloidal superstructures, Phys. rev. E 77, 061706 (2008),</p> <p>Evolutionary strategies for solving extremely complicated NMR spectra of solutes in nematic solventsC.A. de Lange Laser Centre, Vrije Universiteit De Boelelaan 1081, 1081 HV Amsterdam The Netherlands E.E. Burnell Chemistry Department, University of Britisch Columbia 2036 Main Mall, Vancouver (B.C.) V6T 1Z1 Canada W.L. Meerts Molecular- and Biophysics Group, Institute for Molecules and Materials Radboud Universiteit Nijmegen, P.O. Box 9010, 6500 GL Nijmegen The Netherlands The complexity of NMR spectra of solutes in partially ordered solvents such as liquid crystals increases rapidly with the number of spins. Spectra of simple solutes with sufficient symmetry and containing not too many spins (typically ~ 6) are readily analysed. The analysis of larger spin systems is more difficult, and often impossible. In this contribution the application of a general automated Evolutionary Algorithm to solving the highly complex proton NMR spectrum of the twelve-spin system pentane will be presented. This molecule interconverts rapidly among several symmetry-unrelated conformations on the NMR time scale. The interpretation of the spectral parameters that are obtained from the analysis requires the use of a model to connect relative orientational orders in symmetry-unrelated conformers.</p> <p>RECENT ADVANCES IN THE DYNAMICS OF CONFINED NEMATIC LIQUID CRYSTALS WITH DEFECTS AND BOUNDARY NON-HOMOGENEITIES A. F. Martins* and A. Vron Depart. Materials Science, The New University of Lisbon, 2829-516, Caparica, Portugal. (* e-mail: asfm@fct.unl.pt)</p> <p>AbstractIn spite of intense research and significant progress in the last thirty years, some aspects of the dynamics of real nematic liquid crystals confined between parallel plates (LCD geometry) are still not clear. In particular, local non-homogeneities in the boundary alignment of the molecules may have a significant effect on the electric field driven orientational dynamics that has not been recognized until recently [1]. This effect will be discussed in the present communication. We present 2-D computer simulations of the electric field driven director dynamics in thin nematic samples confined between parallel plates, in the presence of topological defects or boundary or volume non-homogeneities in the sample. Deuterium NMR spectra directly computed from the numerical results are good agreement with the experimental data [2]. We propose that in addition to the well known collective modes of reorientation (observed simultaneously over the whole sample), a qualitatively different mode of reorientation, that we may call progressive mode, exists and may dominate the director reorientation. This mode has now been observed in thin samples of low-molecular mass liquid crystals [2] as well as in polymeric systems [3]. The progressive mode originates in the local non-homogeneities and then propagates through the whole sample building up a periodic pattern. Its deuterium NMR signature a doublet with time-dependent intensity and constant splitting [1,2] - is clearly distinct from that of collective modes - a doublet with time-dependent splitting and constant intensity [4-6]. [1] A.F. Martins, A. Vron, Thin Solid Films (2008), doi: 10.1016/j.tsf.2008.09.052. [2] G.R. Luckhurst, A. Sugimura, B.A. Timimi and H. Zimmermann, Liquid Crystals 32, 1389 (2005). [3] A.F. Martins et al., to be published. [4] A.F. Martins, P.Esnault and F. Volino, Phys. Rev. Lett. 57, 1745 (1986). [5] A. Vron, A.E. Gomes, C.R. Leal, J. vd Klink, A.F. Martins, Mol. Cryst. Liquid Cryst. 331, 499 (1999). [6] A.F. Martins, A.E. Gomes, A. Polimeno, L. Orian, Phys. Rev. E 62, 2301 (2000).</p> <p>International workshop, Liquid Crystal Phases and Nanostructures, Erice, Italy, 27 Oct.-1 Nov. 2008</p> <p>Nematic fluctuations and semi-soft elasticity in liquid crystal elastomersMartin opi1,2 and Andrej Petelin21</p> <p>An atomistic model for the elastic constants of nematicsMirko Cestari, Alessandro Bosco and Alberta Ferrarini Dipartimento di Scienze Chimiche, Universit di Padova, via Marzolo 1, 35131, Padova, Italy present address: SISSA, via Beirut 2-4, Trieste, Italy e-mail: alberta.ferrarini@unipd.it Several molecular models for the elastic constants of nematics have been presented, since the early work of Nehring and Saupe,1 and have contributed to shed light on the connection between the intermolecular interactions and the elastic response of liquid crystals to director distortions [for a review see ref. 2]. However, the prediction of elastic coefficients on the basis of the molecular structure remains a challenge. This is a difficult task even for Molecular Dynamics techniques, since atomistic simulations of liquid crystals are very demanding and suitable theoretical models are needed to link long-scale deformations to observables on the length-scale of the simulation box.3 Modeling approaches, allowing for a realistic account of the molecular structure, can be a useful complement to simulations. We have derived molecular expressions for the elastic constants of nematics on the basis of the so called 'Surface Interaction' (SI) model, which was developed some years ago and since then successfully used to predict order and thermodynamic properties of nematics,4 as well as cholesteric induction5 and flexoelectric coefficients.6 Within this framework, the anisotropy of the molecular field experienced by molecules in the nematic environment is described in terms of the coupling between the molecular surface and the field of the nematic director. The chemical structure of mesogens can be introduced through a suitable representation of the molecular surface. In this way, molecular-scale features and long-scale elastic deformations are simultaneously taken into account. The elastic free energy is derived from the orientational distribution function in the presence of director distortions. The molecular flexibility can be easily handled, which is important for mesogenic systems. The method has been applied to typical mesogens; elastic constants have been calculated by averaging over conformers with geometries calculated at the quantum mechanical level7 and the Sanner representation of the molecular surface.8 In this communication, the derivation of molecular expressions for the elastic constants will be presented and the results obtained for PAA and 5CB will be discussed, highlighting the role of features like molecular shape and rigidity.1. 2. 3. 4. 5. 6. 7. 8.</p> <p>Faculty of Mathematics and Physics, University of Ljubljana Jadranska 19, SI-1000 Ljubljana, Slovenia2 J. Stefan Institute Jamova 39, SI-1000, Ljubljana, Slovenia</p> <p>In liquid crystal elastomers the nematic director is coupled to the strain of the polymer network. This leads to some interesting properites of the LC elastomers like very large length change with temperature, potentially useful as artificial muscles, and soft elasticity. With light scattering we measured the relaxation rate of the orientational fluctuations which is finite at zero wave vector due to the internal stress, frozen in the sample at the time of preparation. There is a weak fast signal also above the nematic-isotropic transition showing that there is a residual nematic order also in the high temperature phase, in aggreement with other experiments showing that the system is above critical point. When the strain is applied perpendicular to the director, so that the elastic response is semisoft, the relaxation rate of the director flucutations decreases to a very low value. The data are in good agreement with the theory of semisoft elasticity.</p> <p>J. Nehring, A. Saupe, J. Chem. Phys. 54, 337 (1971); 56, 5527 (1972). M.P. Allen, A.J. Masters, J. Mat. Chem. 11, 2678 (2001); M.R. Wilson, Int. Rev. Phys. Chem. 24, 421 (2005). S. Singh, Phys. Rep. 277, 283 (1996). A. Ferrarini, G.J. Moro, P.L. Nordio, G.R. Luckhurst, Mol. Phys. 77, 1 (1992). A. Ferrarini, G.J. Moro, P.L. Nordio, Phys. Rev. E 53, 681 (1996). A. Ferrarini, Phys. Rev. E 64, 021710 (2000). M.J. Frisch et al , Gaussian 03, Revision C.02 (Gaussian Inc., Wallingford CT, 2004). M.F. Sanner, J.C. Spehner, A.J. Olson, Biopolymers 38, 305 (1996).</p> <p>Simulation studies of structure and organisation in chromonic phases and macromolecular liquid crystals.M. R. Wilson, J. S. Lintuvuori and F. Chami Department of Chemistry, Durham University, Science Laboratories, South Road, Durham, DH1 3LE, UK 1) Molecular order within a chromonic phase is not well understood. From system to system, little is known about how molecules order. Moreover, we know little about the strength of interactions between molecules in stacks or the ordering of water molecules around stacks. We presents new results from an atomistic simulation study designed to answer some of these fundamental questions. Results are presented from atomistic simulations of a chromonic stack for the molecule edicol (sunset yellow) in water, looking at the ordering of edicol molecules in the columns, the structure of the bulk phases and the free energy change on binding to a chromonic stack. Results are also presented for quantum predictions of NMR shielding. Simulation results are linked to recent experimental measurements in solution, which aim to providing a fundamental understanding of how self-assembly occurs in chromonics. 2) Macromolecular liquid crystals composed of polyphilic segmen...</p>