Expected and Unexpected Behavior of the Orientational Order and Dynamics Induced by Azobenzene Solutes in a Nematic

We have explored the changes in the phase stability, orientational order, and dynamics of the nematic 4-cyano-4'-n-pentylbiphenyl (5CB) doped with either the trans or the cis form of different p-azobenzene derivatives using the ESR spin-probe technique. In particular, we have studied the effects induced by each of the seven nonmesogenic 4-R-phenylazobenzenes (R = H, F, Br, CH3, CF3, On-Bu, Ot-Bu) at 1% and 7% mole fraction on the order parameter <P2> and on the shift of the nematic-isotropic transition temperature (TNI), as reported by a nitroxide spin probe, and we have tried to relate them to the solute shape and charge distribution. In all the cases the presence of the azo-derivative causes a depression of T(NI), more pronounced for the cis isomers. The dependence of <P2> on the reduced temperature T* = T/T(NI) remains the same as that of pure 5CB in all trans-doped samples at 1% and 7% and decreases only slightly in the cis at 1%. However, we observe different and in some cases large variations (up to 25%) in <P2> for the cis at 7%, showing solute effects that go beyond the shift in T(NI). Surprisingly enough, even at the highest concentration, the probe dynamics appears to be essentially independent of the nature, the configuration, and the concentration of the different solutes and very similar to that observed in the pure 5CB.

A computer simulation study of the formation of liquid crystal nanodroplets from a homogeneous solution

The aggregation of liquid crystal nanodroplets from a homogeneous solution is an important but not well understood step in the preparation of various advanced photonic materials. Here, the authors performed molecular dynamics computer simulations of the formation of liquid crystalline nanodroplets, starting from an isotropic and uniform binary solution of spherical Lennard-Jones (solvent) and elongated ellipsoidal Gay-Berne (solute) rigid particles in low (<10%) concentration. They studied the dynamics of demixing and the mesogen ordering process and characterized the resulting nanodroplets assessing the effect of temperature, composition, and specific solute-solvent interaction on the morphology, structure, and anisotropy. They find that the specific solute-solvent interaction, composition, and temperature can be adjusted to tune the nanodroplet growth and size.

Order parameters of alpha,omega-diphenylpolyenes in a nematic liquid crystal from an integrated computational and 13C NMR spectroscopic approach

The orientational order parameters and conformational behavior of five relatively large rodlike molecules, biphenyl, trans-stilbene, 1,3-diphenyl-butadiene, 1,3,5-diphenyl-hexatriene, and 1,3,5,7-diphenyl-octatetraene, dissolved in the thermotropic liquid crystal ZLI-1167, have been studied using an integrated approach combining (13)C NMR measurements and quantum mechanical computations of carbon chemical shift tensors. Besides biphenyl, the phenyl moiety of all structures has been found to have a high rotational mobility in the temperature range of the present experiments. The rank-two order parameter <P(2)> in the nematic phase is found to increase steadily from the shortest to the longest term of the series at any temperature within the nematic range. The molecular biaxiality order parameter <D(02) (2)> is found to be small and essentially constant with temperature, giving further support to the common assumption of effective uniaxiality for these probes.

Molecular diffusion in liquid crystals and chiral discrimination. II. Model calculations

We perform model calculations of the macroscopic diffusion coefficient for a solute moving in a chiral nematic (cholesteric) liquid crystal (LC) phase applying the methodology developed by Frezzato et al. [J. Chem. Phys. 122, 164904 (2005)]. Three types of solutes with different features are studied: ellipsoid [roto-translational coupling (RTC) absent], bent rod (RTC present), and two-blade propeller (with RTC and chiral shape). For each prototype molecule we estimate the effect of cholesteric helix pitch and local order on the diffusion along the helix axis. For the ellipsoidal particle we find that translational diffusion is slowed down by rotation around the short axis. For the chiral solute we show that the enantiomer with shape chirality opposite to that of the LC phase is slowed down more than the other. This provides a proof of principle of the possibility of separating the two enantiomers via transport in a suitable chiral medium.

Memory effects in nematics with quenched disorder

We present a combined experimental and Monte Carlo study of a nematic phase in the presence of quenched disorder. The turbidity of a nematic liquid crystal embedded in a porous polymer membrane is measured under different applied field conditions for field-cooled and zero-field-cooled samples. We find that a significant permanent alignment of the nematic can be induced by fields as low as 0.1 V/microm applied during the isotropic to nematic transition. An analogous effect and dependence on sample history is found by studying the order parameter of a sprinkled disorder Lebwohl-Lasher spin model, indicating that dilute quenched randomness is sufficient to produce memory effects in nematics. The large memory induced by field cooling appears to be written in the system during the transition as a result of the field action on freely oriented nematic nuclei. At lower temperature the nuclei consolidate into permanent nematic textures developed from the interaction with quenched disorder.

External field-induced switching in nematic elastomers: a Monte Carlo study

We present a Monte Carlo study of external field-induced switching in nematic elastomers, employing a coarse-grained shearable lattice model. In large enough systems a full-wavelength Fréedericksz effect is observed --as opposed to the half-wavelength effect seen in ordinary nematics-- that clearly reflects in simulated polarized light textures, as well as in deuterium magnetic resonance spectra. The reorientation of mesogenic units is accompanied by pronounced shear deformations.

An empowerment-based educational program improves psychological well-being and health-related quality of life in Type 1 diabetes

Educational programs are reported to improve metabolic control and well-being in Type 1 diabetes mellitus (DM), but the effects of newly- structured interventions, aimed at promoting empowerment in educated patients in active selfcare, have received little attention. Ninety patients with Type 1 DM in intensive insulin treatment were invited to an empowerment-based educational intervention. Changes in quality of life and psychological well-being in the 54 patients participating in the program (median age, 44 yr) were compared with those measured in patients who refused. The following questionnaires were administered at baseline and 12 months later: Psychological General Well-Being (PGWB), Medical Outcome Survey Short-Form 36 (SF-36), and Well-Being Enquiry for Diabetics (WED). Baseline values were indicative of moderate, but significant, psychological distress in the whole cohort. At follow-up, the experimental group had a better metabolic control {glycosylated hemoglobin, -0.4% [time x treatment analysis of variance (ANOVA), p = 0.005 vs controls]}, and a general improvement in comprehensive indices and most scales of PGWB and SF-36. Vitality (p = 0.042) and Social Functioning (p = 0.039) were no longer different from population norm. Similarly, the Symptoms (p = 0.005), Discomfort (p = 0.043) and Impact scales (p = 0.032) of WED, reflecting physical functioning, diabetes-related worries and familial relationships, role functioning and social network, improved significantly in treated patients. An educational empowerment-based intervention significantly improves the psychosocial aspects of diabetes and quality of life also in patients in active and effective self-care. Repeated educational interventions are the way towards a normal life with Type 1 DM.

A Monte Carlo study of the mesophases formed by polar bent-shaped molecules

Liquid crystal phases formed by bent-shaped (or "banana") molecules are currently of great interest. Here we investigate by Monte Carlo computer simulations the phases formed by rigid banana molecules modeled combining three Gay-Berne sites and containing either one central or two lateral and transversal dipoles. We show that changing the dipole position and orientation has a profound effect on the mesophase stability and molecular organization. In particular, we find a uniaxial nematic phase only for off-center dipolar models and tilted phases only for the one with terminal dipoles.

Effect of nanoconfinement on liquid-crystal polymer chains

We apply a Monte Carlo polymerization model for Gay-Berne [J. Chem. Phys. 74, 3316 (1981)] monomers that we have recently introduced [J. Chem. Phys. 121, 9123 (2004)] to investigate with computer simulations the effects of nanoconfinement and anchoring type on the structure of the main-chain liquid-crystal polymers formed in thin films, in the presence of several types of surface alignment: parallel to the interface (random and uniform) or perpendicular to it (homeotropic). We perform first a study of the confined monomers and then we examine the features of the polymer chains obtained from an isotropic or nematic sample. We find a significant effect of the anchoring conditions on the characteristics of the chains and particularly striking differences between planar and homeotropic boundaries. Furthermore, our results indicate that the choice of different anchorings could be used to tune the linearity and degree of polymerization of the chains.