Fitting a free-form scattering length density profile to reflectivity data using temperature-proportional quenching

A technique for fitting a free-form scattering length density profile to reflectivity data via least-squares minimization is presented. The approach combines aspects of simulated annealing with a parametrized representation of the scattering length density profile. The ability of the algorithm to accurately recover the scattering length density profile from arbitrary initial parameter values is demonstrated for simulated and experimental data.     

Multilinear gradient elution optimisation in reversed-phase liquid chromatography using genetic algorithms

The treatment presented in a recent paper [P. Nikitas, A. Pappa-Louisi, J. Chromatogr. A, 1068 (2005) 279] is extended to multilinear gradients, i.e. continuous gradients consisting of a certain number of linear portions. Thus, the experimental lnk versus phi curve, where k is the retention factor of a sample solute under isocratic conditions and phi is the volume fraction of the organic modifier in the water-organic mobile phase, is subdivided into a finite number of linear portions resulting in simple analytical expressions for the solute gradient retention time. These expressions of the retention time are directly used in an optimisation technique based on genetic algorithms. This technique involves first the determination of the theoretical dependence of k upon phi by means of gradient measurements, which in turn is used by the genetic algorithm for the prediction of the best gradient profile. The validity of the analytical expressions and the effectiveness of the optimisation technique were tested using fifteen underivatized amino acids and related compounds with mobile phases modified by acetonitrile. It was found that the adopted methodology exhibits significant advantages and it can lead to high quality predictions of the gradient retention times and optimisation results.     

An investigation of the porosity and surface roughness of liquid crystal alignment layers using neutron reflectivity

Neutron reflectivity has been used to investigate the porosity and surface roughness of three different liquid crystal alignment layers to elucidate how they orient the director. The absorption of hexane into these alignment layers was measured by neutron reflection and the volume fraction profile of the hexane within the layers was determined using the contrast variation method. Measurements were made on rubbed polyimide and silicon monoxide (SiO) evaporated at 5 and 30 to the substrate. The porosity and surface roughness of the alignment layers were found to be correlated with the induced pre-tilt of the director. The low pre-tilt rubbed polyimide and SiO 30 alignment layers were found to be smooth, uniform and impervious to hexane, whilst the high pre-tilt SiO 5 was porous and extremely rough. These observations suggest that both polyimide and SiO 30 rely on an anisotropic interaction with the surface rather than one induced by the surface topography. In contrast, the alignment of liquid crystals by SiO 5 probably originates from an interaction between the mesogens and the rough, porous surface.     

Simultaneous calorimetric and optical reflectivity measurements of extremely thin liquid crystal films

Abstract

An extremely sensitive measuring system has been developed to obtain simultaneously heat capacity and optical reflectivity data from thin free-standing liquid crystal films. Results from 3(10)OBC near the smectic A to hexatic B transition will be reported and discussed in the context of recent theory regarding phase transitions in two dimensions.     

Probing the protic ionic liquid surface using X-ray reflectivity

The structure of the free liquid surface of three protic ionic liquids, ethylammonium nitrate (EAN), propylammonium nitrate (PAN), and ethylammonium formate (EAF), has been elucidated using X-ray reflectivity. The results show all three liquids have an extended interfacial region, spanning at least five ion pairs, which can be divided into two parts. Adjacent to the gas phase are aggregates consisting of multiple cations and anions. Below this are layers oriented parallel to the macroscopic surface that are alternately enriched and depleted in cation alkyl chains and polar domains of cation ammonium groups and their anions, gradually decaying to the isotropic sponge-like bulk structure. The most pronounced layering is observed for PAN, driven by strong solvophobic interactions, while reduced hydrogen bonding in EAF results in the least structured and least extensive interfacial region.     

Fitting adsorption isotherms to the distribution data determined using packed micro-columns for high-performance liquid chromatography

Knowing the adsorption isotherms of the components of a mixture on the chromatographic system used to separate them is necessary for a better understanding of the separation process and for the optimization of the production rate and costs in preparative high-performance liquid chromatography (HPLC). Currently, adsorption isotherms are usually measured by frontal analysis, using conventional analytical columns. Unfortunately, this approach requires relatively large quantities of pure compounds, and hence is expensive, especially in the case of pure enantiomers. In this work, we investigated the possible use of packed micro-bore and capillary HPLC columns for the determination of adsorption isotherms of benzophenone, o-cresol and phenol in reversed-phase systems and of the enantiomers of mandelic acid on a Teicoplanin chiral stationary phase. We found a reasonable agreement between the isotherm coefficients of the model compounds determined on micro-columns and on conventional analytical columns packed with the same material. Both frontal analysis and perturbation techniques could be used for this determination. The consumption of pure compounds needed to determine the isotherms decreases proportionally to the second power of the decrease in the column inner diameter, i.e. 10 times for a micro-bore column (1 mm I.D.) and 100 times for capillary columns (0.32 mm I.D.) with respect to 3.3 mm I.D. conventional columns.     

Optimizing doped libraries by using genetic algorithms

The insertion of random sequences into protein-encoding genes in combination with biologicalselection techniques has become a valuable tool in the design of molecules that have usefuland possibly novel properties. By employing highly effective screening protocols, a functionaland unique structure that had not been anticipated can be distinguished among a hugecollection of inactive molecules that together represent all possible amino acid combinations.This technique is severely limited by its restriction to a library of manageable size. Oneapproach for limiting the size of a mutant library relies on doping schemes, where subsetsof amino acids are generated that reveal only certain combinations of amino acids in a proteinsequence. Three mononucleotide mixtures for each codon concerned must be designed, suchthat the resulting codons that are assembled during chemical gene synthesis represent thedesired amino acid mixture on the level of the translated protein. In this paper we present adoping algorithm that reverse translates a desired mixture of certain amino acids into threemixtures of mononucleotides. The algorithm is designed to optimally bias these mixturestowards the codons of choice. This approach combines a genetic algorithm with localoptimization strategies based on the downhill simplex method. Disparate relativerepresentations of all amino acids (and stop codons) within a target set can be generated.Optional weighing factors are employed to emphasize the frequencies of certain amino acidsand their codon usage, and to compensate for reaction rates of different mononucleotidebuilding blocks (synthons) during chemical DNA synthesis. The effect of statistical errors thataccompany an experimental realization of calculated nucleotide mixtures on the generatedmixtures of amino acids is simulated. These simulations show that the robustness of differentoptima with respect to small deviations from calculated values depends on their concomitantfitness. Furthermore, the calculations probe the fitness landscape locally and allow apreliminary assessment of its structure.     

Energy minimization of peptide analogues using genetic algorithms

A genetic algorithm is used to minimize the energy of peptide analogues in the dihedral angle space. It is interfaced to MOPAC, which computes the energy employing the AM1 Hamiltonian. The genetic algorithm identified the global energy minimum of glycine dipeptide analogue, alanine dipeptide analogue, diglycine, and dialanine. It identified three low-energy conformations of tetraalanine, including the reported global minimum, all of which contained three hydrogen bonds. A structure with a lower energy than the reported global minimum has been generated in which one hydrogen bond is replaced by another one. © 1995 John Wiley & Sons, Inc.     

Ionic relaxation in nematic liquid crystal cells

We investigate the dependence of the relaxation time of the current flowing in a nematic cell submitted to an external dc voltage on the physical properties of the substrate. We show that previously presented analyses of the same problem are not very useful for practical applications. We compare our theoretical predictions with experimental data, and show that the agreement is rather good. The influence of the adsorption-desorption phenomenon on the relaxation time is also discussed.     

Crosslinked liquid crystal polymers from liquid crystal monomers: Synthesis and mechanical properties

Difunctional acrylates and methacrylate monomers have been made which are high order smectic liquid crystal (or crystalline) at room temperature. This report discusses materials with the following structure: F–S–M–S–F, where F is a functional group, acrylate or methacrylate (A or M); S is a spacer (CH₂)_n(n), and M is a mesogen—in this case 4,4′-dioxybiphenyl (B). They are codified as BnA or BnM where n is the number of methylenes in the spacer. High conversion with high T_g can be obtained when polymerizing in the smectic state because the reactive end groups are concentrated in a small volume and can react well with little or no diffusion. B2A, B3A, B6A, B11A, and B3M were polymerized in the smectic state and compared to polymers made at temperatures where the monomers were isotropic. High conversion was obtained below final T_g—even then, probably because the polymers were ordered. All the polymers were studied by WAXD and dynamic mechanical spectroscopy. Solid-state NMR on B3A showed that there was very high conversion of the double bonds at all temperatures. B3A photopolymerized in the smectic state (60–76°C) produced a crystalline polymer with T_g = 185°C (1 Hz). When photopolymerized at 85°C, above the isotropization temperature (T_i), a poorly organized polymer was obtained with a T_g of 155°C (1 Hz). Monomers with an odd number of methylene groups as spacers were crystalline after polymerization. With an even number of methylene groups, they lost most of their crystallinity on polymerization below T_i, but retained a low order smectic structure. Similar structures were obtained with all the monomers when they were polymerized above T_i. There was little effect of polymerization temperature on T_g when the spacers had an even number of methylene groups. © 1993 John Wiley & Sons, Inc.     

Dye-sensitized TiO2 solar cells using imidazolium-type ionic liquid crystal systems as effective electrolytes

A novel ionic liquid crystal (ILC) system (C(12)MImI/I(2)) with a smectic A phase used as an electrolyte for a dye-sensitized solar cell (DSSC) showed the higher short-circuit current density (J(SC)) and the higher light-to-electricity conversion efficiency than the system using the non-liquid crystalline ionic liquid (C(11)MImI/I(2)), due to the higher conductivity of ILC. To investigate charge transport properties of the electrolytes in detail, the exchange reaction-based diffusion coefficients (D(ex)) were evaluated. The larger D(ex) value of ILC supported that the higher conductivity of ILC is attributed to the enhancement of the exchange reaction between iodide species. As a result of formation of the two-dimensional electron conductive pathways organized by the localized I(3)- and I- at S(A) layers, the concentration of polyiodide species exemplified by I(m)- (m = 5, 7, ...) was higher in C(12)MImI/I(2). However, as the increment of the concentration of polyiodide species is less than that of D(ex), the contribution of a two-dimensional structure of the conductive pathway through the increase of collision frequency between iodide species was proposed. Furthermore, a quasi-solid-state ionic liquid crystal DSSC was successfully fabricated by employing a low molecular gelator. Addition of the 5.0 g/L gelator to ILC improved light-to-electricity conversion efficiency through the increase of J(SC) due to the enhancement of the conductivity in C(12)MImI/I(2)-gel.     

Stamping oriented molecular monolayers using liquid crystal inks

Thermotropic liquid crystal (LC)-based inks are combined with patterned anchoring stamps to deposit organic monolayer films with simultaneous control over positional and molecular orientational order in a single step.     

High-transmittance liquid crystal cell fabricated using nanoparticle-doped polyimide

We found that we can increase the transmittance of a liquid crystal (LC) cell by doping the alignment material with nanoparticles. We fabricated an LC cell using indium–tin–oxide glass substrates coated with SiO₂-doped polyimide. We confirmed that the transmittance of LC cells fabricated using SiO₂-doped polyimide was higher than that of LC cells fabricated using pure polyimide. To account for the increase of transmittance, we measured the order parameter of the fabricated LC cells and investigated the surface morphology of the alignment layer using atomic force microscopy.     

Passive broadband reflector using photocrosslinkable liquid crystal molecules

This work consists in creating passive optical filter reflecting light in a broadband wavelength with photo‐polymerizable mesogenic liquid crystal (LC) molecules from a reproducable method. The influence of several parameters was studied and it appears that reflective wavelength range can be modified by chiral agent concentration and/or temperature. Band broadening is influenced by several process parameters such as layer thickness, polymerization and dissolution times. A semi‐quantitative analysis allows the variation of conversion ratio as a function of polymerization and dissolution times to be determined. Copyright © 2002 John Wiley & Sons, Ltd.     

The characteristics of SnPc-doped ferroelectric liquid crystal cells

Tin phthalocyanine (SnPc)-doped polymer films were used to align ferroelectric liquid crystals (FLC). A stripe-shaped domain structure was obtained spontaneously in the initial alignment of the surface-stabilized cells. Atomic force microscopy (AFM) was introduced to investigate the aligning films. It was discovered that the modified aligning films show a polymer aggregate chain structure oriented along the rubbing direction that may play an important role in the FLC alignment. The stability, memory capability and response time of the cell were improved and enhanced through the doping process.     

Dynamic processes in ferroelectric liquid crystal filled cells

The switching process in a ferroelectric liquid crystal cell is of great interest. The precise way in which the optic tensor structure reorientates during switching between states is, however, difficult to determine. Here we consider the use of guided modes and surface plasmon-polaritons as techniques for the investigation of this. It is seen that because of the nature of the dynamic processes the guided mode data is inconclusive, but surface plasmon-polariton data show the surface reorientation mechanism.     

Dynamic processes in ferroelectric liquid crystal filled cells

Abstract

The switching process in a ferroelectric liquid crystal cell is of great interest. The precise way in which the optic tensor structure reorientates during switching between states is, however, difficult to determine. Here we consider the use of guided modes and surface plasmon-polaritons as techniques for the investigation of this. It is seen that because of the nature of the dynamic processes the guided mode data is inconclusive, but surface plasmon-polariton data show the surface reorientation mechanism.     

Oscillating Poiseuille flow in photo-aligned liquid crystal cells

We present an experimental study of thin liquid crystal (LC) layers under the action of a harmonically varied pressure gradient. Optical measurements were performed to register the linear oscillations of a nematic director related to homeotropic and homeoplanar (hybrid) initial states. In the latter case one of the inner surfaces of the rectangular channels was treated by ultraviolet light to provide a relatively weak planar anchoring. The optical response of hybrid and homeotropic LC cells under an oscillating pressure gradient was investigated in relation to on the amplitude and frequency of the pressure gradient. A hydrodynamic model is developed taking into account the LC polar anchoring strength and the surface viscosity responsible for a fast LC surface dynamics. Our estimates show that the thickness of the boundary layer corresponding to the surface viscosity does not exceed 10^-6 m, and further experiments are needed with thinner LC cells and higher frequency oscillations to achieve a more precise value. An oscillating Poiseuille flow in the hybrid cell was found to be useful for characterizing elastic and viscous properties of a weakly anchoring LC surface layer in a fast surface dynamic process.