On the structure of the Nx phase of symmetric dimers: inferences from NMR

Anke Hoffmann,   Alexandros G. Vanakaras,   Alexandra Kohlemeir,   Georg H Mehl and   D J Photinos
Soft Matter, 2014, Accepted Manuscript; DOI: 10.1039/C4SM02480J
Abstract: NMR measurements on a selectively deuterated liquid crystal dimer CB-C9-CB, exhibiting two nematic phases, show that the molecules in the lower temperature nematic phase, NX, experience a chiral environment and are ordered about a uniformly oriented director throughout the macroscopic sample. The results are contrasted with previous interpretations that suggested a twist-bend spatial variation of the director. A structural picture is proposed wherein the molecules are packed into highly correlated chiral assemblies.
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Surface-Induced Ordering on Model Liquid Crystalline Dendrimers

Zerihun G. Workineh and Alexandros G. Vanakaras, Polymers 2014, 6(8), 2082-2099; doi:10.3390/polym6082082

Abstract: The surface alignment of liquid crystalline dendrimers (LCDrs) is a key factor for many of their potential applications. Here, we present results from Monte Carlo simulations of LCDrs adsorbed on flat, impenetrable aligning substrates. A tractable coarse-grained force field for the inter-dendritic and the dendrimer-substrate interactions is introduced. We investigate the conformational and ordering properties of single, end-functionalized LCDrs under homeotropic, random (or degenerate) planar and unidirectional planar aligning substrates. Depending on the anchoring constrains to the mesogenic units of the LCDr and on temperature, a variety of stable ordered LCDr states, differing in their topology, are observed and analyzed. The influence of the dendritic generation and core functionality on the surface-induced ordering of the LCDrs are examined.

Keywords: liquid crystal dendrimers; surface alignment; Monte Carlo simulations; surface confinement of dendrimer

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On the structure of the Nx phase of symmetric dimers

Anke Hoffmann, Alexandros G. Vanakaras, Alexandra Kohlmeier, Georg H. Mehl, Demetri J. Photinos, arXiv:1401.5445 [cond-Mat]. (2014). http://arxiv.org/abs/1401.5445.

Abstract: NMR measurements on a selectively deuteriated liquid crystal dimer CB-C9-CB exhibiting two nematic phases show that the molecules in the lower temperature nematic phase, Nx, experience a chiral environment and are ordered about a unique direction. The results are contrasted with previous reports that proposed a twist-bend spatial variation of the director. A structural model is proposed wherein the molecules show organization into highly correlated assemblies of opposite chirality.

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Phase diagram of hard board like colloids from computer simulations

Stavros D. Peroukidis, Alexandros G. Vanakaras, S. D. Peroukidis, A G. Vanakaras, Soft Mater, 9, 7419-7423 (2013).  DOI: 10.1039/C3SM51165K.

The rich mesophase polymorhism and the phase sequence of board-like colloids depends critically on their shape anisometry. Implementing extensive Monte Carlo simulations, we calculated the full phase diagram of sterically interacting board-like particles, for a range of experimentally accessible molecular dimensions/anisometries of colloids of this shape. A variety of self organized mesophases including uniaxial and biaxial nematics, smectic, cubatic and columnar phases have been identified. Our results demonstrate clearly that the molecular anisometry influences critically not only the structure and the symmetry of the mesophases but also, and perhaps more interestingly, the phase sequence among them. New classes of phase sequences such as nematic-nematic and, for the first time, a direct transition from a discotic and a biaxial nematic to an orthogonal smectic A phase have been identified. The molecular geometry requirements for such a phase behavior have been located.

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European Conference on Liquid Crystals, ECLC-2013

The purpose of this biennial conference is to bring together the research community working in the interdisciplinary field of Liquid Crystals and soft anisotropic materials. The conference will be held from September 22-27, 2013 in the Paradise Mare Hotel, a five star resort, in Rhodes, Greece.

Please visit the web page of the conference: http://eclc2013.upatras.gr

The organizing committee:

• George Floudas, University of Ioannina
• Panayota K. Karahaliou, University of Patras
• Emmanouil Kriezis, University of Thessaloniki
• Polykarpos Pissis, NTUA
• Dimitris Tsiourvas, N.C.S.R “Demokritos”
• Stavros Peroukidis, University of Patras

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Light scattering by a metallic nanoparticle coated with a nematic liquid crystal

V. Yannopapas, N. Fyttas, V. Kyrimi, E. Kallos, A.G. Vanakaras, D.J. Photinos, PSSA, DOI: 10.1002/pssa.201228489.

We study the optical properties of gold nanoparticles (NPs) coated with a nematic liquid crystal (NLC) whose director field is distributed around the NP according to the anchoring conditions at the surface of the NP. The distribution of the NLC is obtained by minimization of the corresponding Frank free-energy functional whilst the optical response is calculated by the discrete-dipole approximation (DDA). We find, in particular, that the anisotropy of the NLC coating does not affect much the (isotropic) optical response of the NP. However, for strong anchoring of the NLC molecules on the surface of NP, the inhomogeneity of the coating which is manifested by a ring-type singularity (disclination or Saturn ring), produces an enhancement of the extinction cross spectrum over the entire visible spectrum.

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Modeling and Computer Simulation Methods for Dendrimers

An international school on “Modeling and Computer Simulation Methods for Dendrimers” will be held in the University of Patras, Department of Materials Science from 12 to 15 December 2011

Updated information and program here

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Layer-multiple-scattering theory for metamaterials made from clusters of nanoparticles

V. Yannopapas and A. G. Vanakaras, Phys. Rev. B, 84, 085119 (2011).

We present a layer-multiple-scattering method of electromagnetic waves for the study of periodic metamaterials formed as a lattice of cavities which are filled by clusters of spherical nanoparticles. Our approach is a three-stage process where we take fully into account all the multiple-scattering events involved: (a) among the spheres of the cluster inside the cavity, (b) between the cluster and the cavity, and (c) among the cavities (containing the clusters) within the metamaterial. As an example, we study the transmission, reflectance, and absorbance spectra of light incident on a finite slab of a $SiO_2$-inverted opal whose voids contain clusters of gold nanoparticles. We find, in particular, that finite slabs of this metamaterial act as highly efficient absorbers over a wide frequency range, from 2–4.5 eV. Also, around the local maxima of the absorbance spectrum, the metamaterial exhibits anomalous dispersion, wherein the real part of the group velocity is opposite to (the real part of) the phase velocity. ©2011 American Physical Society

DOI: 10.1103/PhysRevB.84.085119
PACS: 42.70.Qs, 42.25.Bs, 42.50.Gy, 78.67.Pt
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Dirac point in the photon dispersion relation of a negative/zero/positive-index plasmonic metamaterial

V. Yannopapas and A. Vanakaras, Phys. Rev. B., 84, 045128, (2011).

We report on the emergence of a Dirac point in the dispersion relation of a plasmonic metamaterial. It is realized as a three-dimensional crystal (cubic or orthorhombic) whose lattice sites are decorated by aggregates of gold nanoparticles embedded in a high-index dielectric material. The Dirac-type dispersion lines of the photon modes are not a result of diffraction as in photonic crystals but due to subwavelength features and emerge from the gapless transition from a negative to a positive index band. The Dirac point is manifested as a dip in the spectrum of light transmittance through a finite slab of the metamaterial; however, transmittance does not decrease diffusively but exponentially due to the inherent losses of gold in the given spectral regime. ©2011 American Physical Society

DOI: 10.1103/PhysRevB.84.045128
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