Liquid Crystalline Samples Research Articles

Rheology of schizophyllan solutions in isotropic and anisotropic phase regions

In this paper, we discuss the rheological properties of aqueous solutions of a rigid triple-helical polysaccharide, schizophyllan (SPG), in isotropic, biphasic, and fully anisotropic phases. Both steady shear and dynamic rheological behaviors reveal remarkable changes when SPG solutions pass through the three phases. The steady shear flow exhibits shear thickening at low shear rates for anisotropic SPG liquid crystalline samples, which is attributed to the shear-induced cholesteric to nematic transformation. The first normal stress difference and transient rheological experiments demonstrate that director tumbling is absent or negligible in SPG liquid crystals in the range of examined shear rates. Additionally, the stress relaxation of SPG liquid crystals after flow cessation shows an inverse relation between the relaxation time and preshear rate, as expected by Larson and Mead’s theory [Larson and Mead (1989)]. Small amplitude oscillation measurements following flow cessation show decreasing complex modulus with time for SPG liquid crystals, which is probably related to an increase of molecular orientation after flow cessation. The evolutions of complex modulus after flow cessation are discussed in terms of chain persistence length.

Performance and application of a new planar array infrared spectrograph operating in the mid-infrared (2000-975 cm(-1)) fingerprint region.

A no-moving-part planar array infrared spectrograph (PA-IR) equipped with a 256 x 256 mercury cadmium telluride (MCT) focal plane array has been designed and constructed. The performance of the instrument, whose frequency range extends from 2000-975 cm(-1), has been assessed in terms of resolution, bandwidth, and signal-to-noise ratio. The PA-IR spectrograph is able to record spectra with an 8.7 ms time resolution and has peak-to-peak noise levels as low as 2.4 x 10(-4) A.U. As a demonstration of the potential of PA-IR, the dynamics of reorientation of a liquid crystalline sample exposed to a single electric field pulse has been studied. It was shown that PA-IR can be used for the simultaneous acquisition of two orthogonally polarized spectra. The advantages and limitations of PA-IR, step-scan Fourier transform infrared (FT-IR), and ultrarapid-scanning FT-IR for real-time studies of reversible and irreversible phenomena are thoroughly discussed.

Heteronuclear dipolar recoupling in liquid crystals and solids by PISEMA-type pulse sequences

A pulse sequence is described for the recoupling of heteronuclear dipolar interactions under MAS. The method is similar to the PISEMA experiment, but employs a well-defined amplitude modulation of one of the two radio-frequency fields. The technique is used for measurements of 1H– 13C dipolar couplings in unoriented solid and liquid-crystalline samples.

Using Switched Angle Spinning to Simplify NMR Spectra of Strongly Oriented Samples

This contribution describes a method that manipulates the alignment director of a liquid crystalline sample to obtain anisotropic magnetic interaction parameters, such as dipolar coupling, in an oriented liquid crystalline sample. By changing the axis of rotation with respect to the applied magnetic field in a spinning liquid crystalline sample, the dipolar couplings present in a normally complex strong coupling spectrum are scaled to a simple weak coupling spectrum. This simplified weak coupling spectrum is then correlated with the isotropic chemical shift in a switched angle spinning (SAS) two-dimensional (2D) experiment. This dipolar-isotropic 2D correlation was also observed for the case where the couplings are scaled to a degree where the spectrum approaches strong coupling. The SAS 2D correlation of C(6)F(5)Cl in the nematic liquid crystal I52 was obtained by first evolving at an angle close to the magic angle (54.7 degrees ) and then directly detecting at the magic angle. The SAS method provides a 2D correlation where the weak coupling pairs are revealed as cross-peaks in the indirect dimension separated by the isotropic chemical shifts in the direct dimension. Additionally, by using a more complex SAS method which involves three changes of the spinning axis, the solidlike spinning sideband patterns were correlated with the isotropic chemical shifts in a 2D experiment. These techniques are expected to enhance the interpretation and assignment of anisotropic magnetic interactions including dipolar couplings for molecules dissolved in oriented liquid crystalline phases.

Two-Dimensional NMR Correlations of Liquid Crystals Using Switched Angle Spinning

We present the first application of switched angle spinning (SAS) to correlate the first-order dipolar spectrum of a liquid crystalline sample with the isotropic magic angle spinning (MAS) spectrum in a two-dimensional experiment. In this experiment we are able to select the degree of dipolar couplings introduced via mechanical manipulations of the liquid crystal director in a single oriented sample. The 19F SAS-COSY correlation of iodotrifluoroethylene, an AMX spin system, dissolved in the nematic liquid crystal 4-octylphenyl-2-chloro-4-(4-heptylbenzoyloxy)-benzoate provides assignment of both the J and dipolar couplings in a single experiment. This work demonstrates the use of oriented samples and sample spinning to resolve homonuclear dipolar couplings using isotropic chemical shifts.

Light scattering of thin azobenzene side-chain polyester layers

Light scattering properties of liquid crystalline and amorphous azobenzene side-chain polyester layers used for optical data storage were examined by means of transmissive scatterometry. Comparative experiments show that the amorphous polyester has significantly lower light scattering characteristics than the liquid crystalline polyester. The amorphous samples have negligible polarization part orthogonal to the incident beam, the liquid crystalline samples have relative high orthogonal polarization part in light scattering. The light scattering results can be used to give a lower limit for the domain size in thin liquid crystalline polyester layers being responsible for the dominant light scattering. The characteristic domain sizes obtained from the Fourier transformation of polarization microscopic pictures confirm these values.

Selective detection of the proton NMR spectra of molecules containing rare spins at natural abundance in liquid crystalline samples.

It is shown that the proton NMR spectra of molecules containing rare spins at natural abundance dissolved in a liquid crystalline solvent can be obtained free from the strong lines from the spectrum of the abundant isotopomer by the 2D HSQC NMR experiment. The technique can also give the individual chemical shifts of the rare spins, and, for a molecule containing another abundant nucleus, such as fluorine, the rare spin--(19)F total anisotropic couplings are also obtained. The usefulness of the technique is demonstrated for molecules containing (13)C as the rare spins.

Detection of sodium ions in anisotropic environments through spin-lock NMR.

A new method for selectively detecting sodium ions in anisotropic environments is presented. A spin-lock (SL) sequence, followed by a coherence transfer pulse, generates rank-two zero-quantum coherences, and converts them into observable transverse magnetization. The quadrupolar polarization is only generated when there are residual quadrupolar couplings in the sample, and provided the SL field strength is comparable to these couplings. This filter has proved to be more efficient than a double-quantum magic-angle (DQ-MA) filter in generating observable signal from ions in anisotropic media in both a nasal bovine cartilage sample and a liquid crystalline DNA sample. Finally, the SL filtering technique does not rely on a flip angle effect for the selection of the desired signal component, as does a DQ-MA filter, and may therefore prove desirable in an imaging experiment, due to its better tolerance to phase and flip angle imperfections.

Production of ordered collagen matrices for three-dimensional cell culture

The aim of this study was to produce collagen gels with controlled fibrillar order as matrices for cell culture. Their structural characterization and colonization by human dermal fibroblasts are presently reported. Ordered matrices are obtained by using the property of type I collagen monomers to self-assemble in liquid crystalline arrays by slow evaporation of acidic solutions at high concentrations. Induction of fibrillogenesis concomittent with the stabilization of the supramolecular order is then obtained, within petri dishes, by gelation of the viscous preparations under ammoniac vapours. For comparison, dermal equivalents, in which collagen compaction depends on fibroblasts contraction, are made according to the method of Bell et al. (Proc. Natl. Acad. Sci. 76(3) (1979) 1274). The fibrillar arrangement of the collagen network in the samples is determined by polarizing optical microscopy and by transmission electron microscopy. Whereas dermal equivalents exhibit heterogeneous distributions of fibrils, two differents types of order are obtained in the stabilized liquid crystalline collagen samples, namely aligned, i.e. nematic, at 20 mg/ml, or crimped, i.e. precholesteric, at 40 mg/ml. The morphology and behaviour of fibroblasts seeded on the surface of the matrices are analysed from day 1 to day 21. The cells are viable, proliferate at the surface of ordered matrices and migrate up to 400 μm in depth. Production of concentrated and ordered collagen matrices provides new perspectives to study the behaviour of cells in a valorized three-dimensional context where the fibrillar organization becomes close to in vivo situations.

Polarization hysteresis loops induced by surface phenomena

In this work we present a model that describes the influence of surface interactions on polarization hysteresis loops in thin liquid crystalline samples. Experimental results that illustrate this contribution are referred.

Solvent Effects on the Absorption and Emission Spectra of Some Nematic Liquid Crystals: Determination of Dipole Moment and First-Order Hyperpolarizability

The effect of solvent on the absorption and fluorescence spectra of two liquidcrystalline molecules LC1 and LC2 were studied. These data were used to evaluatethe values of the change in the dipole moment between the ground and excitedstate and the first-order hyperpolarizability “β” in each solvent. The values of βgives an indication of how these liquid crystalline samples can find applicationsas nonlinear materials. The β values are in the range 1–1.9 × 1029 esu for LC1and LC2. The values of β indicate that these compounds can be used as goodnonlinear materials. Here solvatochromism has been used as an effective tool toestimate the hyperpolarizability in solution.

Hierarchy of Order in Liquid Crystalline Polycaps.

When outfitted with long alkyl chains, polycaps, capsules along a polymer chain, spontaneously organize themselves into a two-dimensional liquid crystalline phase. Further organization results from shearing or pulling the liquid crystalline samples, producing three-dimensional assemblies of micrometer-wide, infinitely long fibers (see schematic representation).

Characteristics of Solvation Dynamics of Rhodamine 700 in Cyanobiphenyls

The results of solvation dynamics measurements of rhodamine 700 in the isotropic phase of two different liquid crystalline substances, 8CB and 4CB, are presented. It is shown that solvation takes place on a length scale shorter than the orientational correlation length of the pseudonematic domains. Two processes contribute to the solvation. The slower process can be described by dielectric relaxation. The faster component is interpreted in terms of intermo-lecular interactions specific to liquid crystalline samples.

Solvent-induced ordering phenomena in a segmented-chain mesogenic polymer with a flexible substituent at the rigid moiety

Solvent-induced ordering phenomena of a liquid-crystalline polymer, characterized by having moderate flexibility in the main chain and a highly flexible lateral substituent, was investigated in different liquids. Diffusion and sorption were measured at room temperature. The solvent-uptake kinetics were found either Fickian or anomalous, depending on the solubility parameter of the liquids. The curve of weight uptake of the different solvents, reported as a function of the solubility parameters, shows a maximum correspondent to chloroform. Liquids with low solubility parameter, as pentane and n-hexane, do not induce any change in the structural organization of the starting sample, whereas ordering phenomena are evident for all the solvents that strongly interact with the liquid crystalline sample.

Symmetry and Phase-Selected NMR Spectra of Liquid Crystalline Samples

It is demonstrated that the NMR spectra of liquid crystalline samples can be simplified by using multiple quantum filtering. In a system of N spin-12nuclei, the N or (N-1)-multiple quantum filtered spectra (NQF or (N-1)QF) contain lines which originate only from transitions among the eigenstates belonging to the highest symmetry class of the spin permutation group. In addition the NQF spectra are divided further into two sets of lines which differ in phase by 180°. A method for simulating and analysing multiple quantum filtered spectra is described, with examples from molecules with up to eight interacting spins.

Rheological behaviour of a lamellar liquid crystalline surfactant–water system

The rheological behaviour of a lamellar liquid crystalline surfactant–water system was investigated using a Haake RS-100 viscometer. The repeat distance of lamellae as a function of surfactant concentration and temperature was measured by small-angle X-ray scattering. The rheological characteristics were measured at different temperatures with systems containing surfactant in different concentrations. The frequency-dependent storage and loss modulus were found to be characteristic to the lamellar phase in the linear viscoelastic region.The results are analysed on the basis of Jones–McLeish slip-plane theory. Trends of the fitted constants are discussed based on the general knowledge on the interactions in dispersions stabilized by non-ionic surfactants, and the structure of lamellar liquid crystalline samples. Time-dependent compliance was also measured. The instantaneous elasticity measured in the creep tests was compared with that predicted from oscillatory tests. Burger's model was used to describe the time-dependent compliance.Viscosity measurements in the non-linear region were also done. A modified Carreau equation is used to describe the viscosity versus shear rate curves. The changes in sample under shear is described briefly.

Lyotropic System Potassium Laurate/1-Decanol/Water as a Carrier Medium for a Ferronematic Liquid Crystal: Phase Diagram Study

Synthesis of lyotropic mixtures of potassium laurate, 1-decanol, and water is presented, and the phase diagram of this ternary system is investigated in the vicinity of the nematic region. Several phases are identified and the corresponding concentration and temperature-induced phase transitions are studied. To prepare ferronematic liquid-crystalline samples, positively charged magnetic nanoparticles are introduced into the lyotropic matrix. Modifications of the phase behavior of the system induced by the particle incorporation are described.

Interactions between fenoprofen sodium and poly (ethylene oxide)

Interactions of the amphiphilic drug fenoprofen sodium (FNa) in solution below and above its critical micelle concentration with poly (ethylene oxide) (PEO) of different chain length (PEO 400 to PEO 20 000) and between a liquid crystalline formulation of fenoprofen (FLC; containing FNa, fenoprofen acid and water) and PEO were investigated, using surface tension measurements, viscometry, cloud point temperature measurements, [ 1H]NMR, polarised light microscopy, transmission electron microscopy, and differential scanning calorimetry. Interaction between FNa solutions and PEO: the investigations suggest that an interaction starts below the critical micelle concentration (CMC) of FNa. This can be concluded from [ 1H]NMR experiments (an upfield shift of the PEO proton signal was found at FNa concentrations below the CMC of FNa), surface tension measurements (absence of a critical association concentration) and cloud point temperature determinations. The surfactant does not seem to bind quantitatively on the PEO molecules (a higher FNa concentration was needed to cause the same upfield shift of the PEO proton signal than for more lipophilic surfactants, and no plateau phase in the surface tension reduction isotherm could be determined).Interactions were found to be independent from the chain length of the PEO. Interactions between FLC and PEO/pluronics: partial or complete dissolution of the fenoprofen mesophase (detected by [ 1H]NMR, polarised light microscopy and transmission electron microscopy) occurred after addition of PEO at concentrations between 2 and 10% (w/w), independent from the molecular weight of the PEO. A comparable amount of water added to the liquid crystalline samples does not change the mesophase into a liquid crystalline dispersion or a micellar solution. The liquid crystalline particles in the dispersion formed by the addition of PEO, had a higher transition temperature into an isotropic phase (between 54°C and 57°C), than in liquid crystalline dispersions without polymer (40°C). The interactions between FNa and PEO can be interpreted in terms of a hydrophobic interaction with an association of the drug molecules on the polymer, i.e. the interaction between FNa and PEO occurs at a molecular rather than a micellar level. The interaction leads to a dissolution of the fenoprofen liquid crystal and the formation of an isotropic phase. No phase separation of the oily, amorphous, practically water insoluble fenoprofen acid could be found. Addition of PEO also seems to affect the composition of the remaining mesophase.

Simplification of19F NMR Spectra of Liquid Crystalline Samples by Multiple-Pulse COSY Experiments

A general method is presented of converting second-order fluorine-19 spectra of liquid crystalline samples into first-order spectra. This is achieved by recording a19F COSY spectrum with a multiple pulse dipolar reducing sequence (MPS) operating in thet1period, leading to aF1projection which is first order. The method is illustrated by recording spectra on a sample of 1,3-dichloro-4-fluorobenzene dissolved in the nematic solvent ZLI 1167. Experiments have been done in which the MPS is either the MREV-8 or Flip-Flop-16 sequence. The first-orderF1projections are analyzed to give reduced total1H–19F couplings,KCSTHFijand the reduction factors,KCS, are obtained by comparison with theTHFijavailable from an analysis of the ordinary 1D spectrum. The reduction factors are compared with values predicted by average Hamiltonian theory (AHT). The reasons for the differences found between observed and predicted values ofKCShave been investigated by performing numerical simulations of the 2D MPS COSY experiments.

NMR excitation of quadrupolar order using adiabatic demagnetization in the rotating frame

Adiabatic demagnetization and remagnetization in the rotating frame (ADRF and ARRF) are shown to be practical and efficient techniques for exciting and observing quadrupolar order, T2,0, in NMR of quadrupolar nuclei such as H2 or Na23. A detailed theoretical description of ADRF and ARRF of spin I=1 and 3/2 nuclei, based on the well-known vector model of NMR, is presented and used to derive analytically a variety of pulse shapes for ADRF and ARRF. This theory is also used to calculate the state of the spin I=1 and 3/2 density operators following an ADRF or ARRF pulse and it is shown that the desired coherence transfer processes have the maximum amplitudes allowed by the well-known “universal bound” theorem. In principle, therefore, ADRF is shown to be superior as a method of exciting quadrupolar order to the Jeener-Broekaert experiment since the latter fails to excite the maximum T2,0 amplitude for spin I=3/2 nuclei. The performance of ADRF is investigated using computer calculations and simulations and the conditions under which it yields broadband (i.e., non-oscillatory) excitation of quadrupolar order are derived. Using both H2 (I=1) and Na23 (I=3/2) NMR of liquid crystalline and biological samples, ADRF and ARRF are demonstrated experimentally. The predicted broadband excitation behavior is observed for both ADRF and ARRF, whereas both the Jeener-Broekaert and double-quantum filtration experiments show excitation profiles that oscillate sinusoidally as a function of either the quadrupolar splitting parameter, ωQ, or the duration of the pulse sequence, τ. Finally, a more general discussion of ADRF and ARRF of quadrupolar nuclei is presented and it is shown that the maximum coherence transfer amplitudes are achieved for nuclei of any spin quantum number I.