Some N.M.R. experiments using lyotropic nematic liquid crystals

Abstract

A series of lyotropic nematic liquid crystals based on aniso-dimensional micelles can be subdivided according to micelle shape and diamagnetic susceptibility anisotropy. The proton magnetic resonance spectra of benzene dissolved in dilute solution in several of these phases have been analysed. The signs of the partially averaged dipole–dipole coupling give information about the relative alignment of the six-fold symmetry axis of benzene, the director and the magnetic field. The system caesium perfluorooctanoate/water is unusual in two respects. The system forms a nematic phase with only a binary mixture and the disk micelles align in a perpendicular plane to the magnetic field. As a result the binary system has been studied previously in some detail (see N. Boden, S. A. Come and K. W. Jolley, 1987, J. phys. Chem. 91, 4092). New measurements on the interesting phase diagram have been accomplished using caesium-133 and deuterium magnetic resonance measurements at various temperatures. Some dilatometric measurements have been made to complement the N.M.R. studies.     

Phase behaviour of n-alkyl- and bi-alkylbenzenesulphonates. Nematic lyotropics from double chain surfactants

The lyotropic mesophases in binary systems of surfactants in water: n-alkylbenzenesulphonates (C₈-C₁₂), two chain C₁₂-surfactants, and dodecyl-benzenesulphonic acid, were investigated. The micellar properties were examined by conductometry and viscosimetry. The phase diagrams were determined using crossed polarizers, ²H NMR spectroscopy and polarization microscopy. Besides lamellar and inverse cubic phases, new nematic lyotropic phases have been found, presenting precursors for the lamellar phases, and exhibiting very fast alignment in a magnetic field.     

N.M.R. investigations and the molecular field theory of nematic mixtures

N.M.R. measurements have shown that the mixture E5 can be described by a single order parameter. Based on molecular field theory the nematic-isotropic transition and the temperature dependence of the order parameter in the nematic phase are considered for a binary mixture of nematogens. Guided by the results of the N.M.R. measurements the binary mixture is treated as an effective medium characterized by a single order parameter. Soft attractive forces are taken into account as well as the excluded volume. The coexistence of nematic and isotropic phases in the phase transition region is discussed in detail.     

Orientational order and refractive indices in binary nematic mixtures

Mean field theory is used to calculate the temperature-composition phase diagram and component order parameters of binary nematic mixtures. Experimental values for the mixture order parameter of a binary nematic mixture close to the nematic/isotropic transition have been obtained from refractive index measurements. The experimental results qualitatively confirm the predictions of the theory.     

Order parameters of dye molecules in lyotropic nematic mesophases

The alignment in a magnetic field of two similar azo dyes in binary nematic lyotropic phases (C₇F₁₅CO₂NH₄ and C₈F₁₇CO₂NH₄ in D₂O) has been studied. The order 5 parameter for the dyes was determined by measuring quadrupolar splittings (deuterium N.M.R.), the anisotropy of the electric conductivity and dichroism. The order parameter of the dye fixed in the disc-like perfluoro micelle with positive diamagnetic anisotropy was determined as a function of magnetic field strength, surfactant concentration, temperature and chain length. In addition the time constants for orientation and disorientation were determined.     

Ultra-slow director rotation in nematic side-group polymers detected by N.M.R.

A novel N.M.R. method is described in which very high values of the rotational viscosity in nematic liquid crystals are measured with an improved accuracy of several orders of magnitude as compared to conventional N.M.R. methods. It consists of monitoring the deuteron line splitting while rotating the sample in a magnetic field. The rotation speed of the sample container is chosen such that the director orientation follows the container orientation, albeit with a certain phase lag. By synchronizing the data acquisition with the container orientation the phase lag is monitored for several hours. A simple relation, derived from the Ericksen-Leslie-Parodi equations, holds between the rotational viscosity and the phase lag. Frequencies as low as 10^-6 Hz can be measured precisely.     

Observation of textures of nematic polymers and estimation of the elastic constants

The texture of a copolyesteramide, Vectra® B950, has been studied by optical microscopy. At rest, the defects are typical of a biaxial nematic phase, but under the influence of a magnetic field or under shear, the nematic transforms into a uniaxial phase. The Freedericksz geometry yields a K₁ elastic constant of the order of 10^-8 N. The diffusivities K_III/_n and K_II/_n are obtained by measuring the light intensity transmitted through a sample during shear start-up and relaxation, and from this we estimate K_III ≅ 10 ^-9N and K_II ≅ 10 ^-10N. Since the biaxiality is small, these uniaxial constants adequately describe the phase at rest.     

Field-cycling NMR detection of magnetoacoustically manipulated nematic ordered states: Memory effects

The proton spin–lattice relaxation time (T₁) dispersion was studied under simultaneous sonication in the nematic phase of 5CB. It appears that metastable ordered states subject to a memory effect can be induced by the combined action of an amplitude-modulated ultrasonication and a pulsed magnetic field. We argue that the acoustic amplitude modulation adds instability to the nematic phase through director order fluctuation enhancement. Different manipulated states of the director were unambiguously identified by the Larmor frequency dispersion of T₁. The field-cycling NMR technique was used for T₁ measurements.     

Novel phase behaviour in two (smectic/cholesteric) liquid crystal mixtures

The phase behaviours of mixed liquid crystal systems having either Sm/N or Sm/Ch properties have been studied. The (smectic/nematic) binary system formed smectic phases over a wide and much enhanced range of temperature (42 C) and a broad concentration range (0-90 wt %). The ternary smectic/cholesteric system, in appropriate concentration ranges, exhibited the smectic A phase, a TGBA-like twist grain boundary A phase, the cholesteric phase and blue phases. The TGBA-like phase appeared in the cholesteric-smectic phase transition range. Three textures (chiral pitch, fan-shaped and scale-like) for the cholesteric phase of the ternary smectic/cholesteric mixtures were observed in the ranges 0-7, 7-43 and 43 wt % respectively, of cholesteric CB15, in a binary Sm/N mixture.     

Phase diagrams of binary mixtures of biaxial nematogens

A mean field theory is used to describe nematic phases of binary mixtures of biaxial molecules. Using a general pseudopotential consistent with the D_2h symmetry of the constituent particles, the theory is used to calculate the elements of the order tensors necessary to describe the orientational order in binary mixtures in both uniaxial and biaxial nematic phases. For a single component, the model only requires one parameter, r₂, a ratio of anisotropic interaction strengths, to predict the temperature dependence of the four order parameters. The temperature dependence of the orientational distribution functions is illustrated for both rod-like and plate-like molecules. For binary mixtures, three anisotropic interaction strengths, r₁, r₂, and r₃, are needed to calculate the order parameters of both components as a function of concentration and temperature. The free energy is evaluated to predict the phase stability of the mixture. By systematically varying the anisotropic interaction strengths, temperature-concentration phase diagrams for a variety of molecular shapes are presented. The theoretical predictions suggest that binary mixtures of molecules with highly asymmetric shapes will display stable biaxial nematic phases.     

A lyotropic nematic phase of lamellar micelles (N L) obtained by a non-ionic surfactant in aqueous solution

A non ionic surfactant with a rigid rod-like hydrophobic group has been synthesized. Owing to the molecular geometry of the surfactant only lamellar micelles are formed in aqueous solution.This system exhibits a lyotropic nematic phase (N _L), which for the first time has been found for a binary non ionic surfactant/water system.Herrn Professor Dr. H.-G. Kilian mit herzlichen Glückwünschen zum 60. Geburtstag gewidmet.     

Magnetic resonance study on lamellar phases of ammonium perfluorooctanoate

The aggregation properties of ammonium perfluorooctanoate (NH₄-PFO) in concentrated aqueous phases have been investigated by magnetic resonance techniques and have been compared with the aggregation properties in dilute solutions. Magnetic resonance methods indicated that NH₄-PFO—water systems with surfactant concentrations below 45% (w/w) behaved as isotropic purely micellar solutions in the temperature range 285–340 K. For higher concentrations the system exhibited a rather complex structure, having both isotropic and anisotropic components. The nematic nature of the anisotropic fraction was demonstrated by ¹⁹F NMR studies. The ¹⁹F NMR and EPR of nitroxides (TempTMA⁺, 5- and 16-DXSA) inserted as paramagnetic probes into the concentrated NH₄-PFO—water systems allowed us to establish that the lamellar phase could be mechanically oriented between quartz slides. The EPR investigation also gave details concerning the dynamics of both the oriented and non-oriented structures.     

The phase diagram of the lyotropic nematic mesophase in the TTAB/NaBr/water system

The phase diagram of the nematic mesophase present in the tetradecyltrimethylammonium bromide/sodium bromide/water ternary system was determined. A calamitic nematic mesophase (N_C) was observed which extends to very high concentrations of electrolyte. The order parameters of the surfactant head group in the mesophases were studied by the NMR quadrupolar splitting of the deuterated surfactant. On increasing the temperature of nematic mesophases with low electrolyte concentrations, a phase separation occurs with the formation of a more highly ordered hexagonal phase and an isotropic phase. Diffusion measurements of the isotropic micellar solution by the NMR PFG method were used to estimate hydrodynamic radii at low surfactant concentrations and to study micelle diffusion as the concentration of the surfactant was increased to the liquid crystalline region. At higher surfactant concentrations, the diffusion coefficient reached a limiting value. The calamitic nematic mesophase in this surfactant/electrolyte/water system appears to be formed by long wormlike micelles.     

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar-Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal-smectic, smectic-nematic, and nematic-isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.     

Synthesis and characterization of new liquid crystalline materials containing a non-activated arylazoindolinobenzospiropyranyl group as a chiral unit. Part II

Two series of new liquid crystalline compounds containing a non-activated arylazoindolinobenzospiropyran, ABP-SPAB 1a-1e (series 1) and SPAP-ABPC 2a-2e (series 2), have been synthesized. These LC dyes were characterized by a differential scanning calorimetry polarizing optical microscopy, X-ray diffraction and electro-optical measurements. All but one of the series 1 compounds examined exhibit monotropic second and/or third transition liquid crystal phases on cooling from the isotropic liquid. In particular, ABP-SPAB 1b shows a monotropic SmC phase, in addition to a SmA phase. In series 2, most of the compounds exhibit a monotropic nematic phase on cooling. SPAP-ABPC 2c forms an enantiotropic nematic phase and a monotropic SmA phase; 2e shows enantiotropic nematic and SmA phases.     

Behavior of liquid crystals confined to mesoporous materials as studied by 13C NMR spectroscopy of methyl iodide and methane as probe molecules

The behavior of thermotropic nematic liquid crystals (LCs) Merck Phase 4 and ZLI 1115 confined to mesoporous controlled pore glass materials was investigated using 13C nuclear magnetic resonance spectroscopy of probe molecules methyl iodide and methane. The average pore diameters of the materials varied from 81 to 375 A, and the temperature series measurements were performed on solid, nematic, and isotropic phases of bulk LCs. Chemical shift, intensity, and line shape of the resonance signals in the spectra contain lots of information about the effect of confinement on the state of the LCs. The line shape of the 13C resonances of the CH3I molecules in LCs confined into the pores was observed to be even more sensitive to the LC orientation distribution than, for example, that of 2H spectra of deuterated LCs or 129Xe spectra of dissolved xenon gas. The effect of the magnetic field on the orientation of LC molecules inside the pores was examined in four different magnetic fields varying from 4.70 to 11.74 T. The magnetic field was found to have significant effect on the orientation of LC molecules in the largest pores and close to the nematic-isotropic phase transition temperature. The theoretical model of shielding of noble gases dissolved in LCs based on pairwise additivity approximation was utilized in the analysis of CH4 spectra. For the first time, a first-order nematic-isotropic phase transition was detected to take place inside such restrictive hosts. In the larger pores a few degrees below the nematic-isotropic phase transition of bulk LC the 13C quartet of CH3I changes as a powder pattern. Results are compared to those derived from 129Xe NMR measurements of xenon gas in similar environments.     

Overlapping spheres Xα calculatons of the electron affinities and ionization potentials of ethylene,parabenzoquinone and their perfluoro derivatives

Microscopy and deuterium NMR spectroscopy are employed to demonstrate the existence of a thermodynamically independent nematic amphiphilic mesophase in mixtures of perfluoro-octanoate and water between 37.0 to 87.0% by weight of ²H₂O and 284.0 to 348.3 K at 1 atmosphere pressure. This phase is in equilibrium with an isotropic micellar solution and a smeetic lamellars mesophase to, respectively, high and low temperatures. The lamellar to nematic transition line changes from first to second order at a tricritical point corresponding to 42.5% of ²H₂O and 328.0 K, whilst the nematic to isotropic micellar solution transition is first order at all compositions. The measurements suggest the nematic phase is a solution of disc shaped micelles which orient with their unique axis parallel to the direction of an applied magnetic field.     

Vibrational spectroscopy of liquid crystals : 2. Normal coordinate analysis of benzaniline: basis structure for some liquid crystals

The vibrational spectrum of benzalaniline, some derivatives of which have been found to exhibit crystalline properties in the liquid phase, has been interpreted in the light of new experimental data and from an accurate normal coordinate calculation. Kinematic and dynamic couplings are of great importance even for the modes corresponding to the central group. A modified Urey-Bradley force field refined on three isotopic species of benzene has been determined.

In the course of studies on the vibrational spectra of nematogenic Schiff bases, it has been found that the complete vibrational assignment of the parent molecule, benzaniline or benzylidene-aniline (B.A.) has never been reported. In a previous report [1], one of us (G.V.) showed the similarity in the Raman spectra of B.A. and (methoxy)-benzylidene-(butyl)-aniline (M.B.B.A.) which is a nematic type liquid crystal. For structural studies of the nematic phase itself and for the use of liquid crystals as solvents in vibrational spectroscopie studies of solute molecules, one needs to know the full assignment. The purpose of the present paper is to discuss the interpretation of the vibrational spectrum of B.A. in the light of new experimental data and from an accurate normal coordinate calculation.     

Virtual isotropic‐nematic transitions in alkyl cyanobiphenyls

Pretransitional fluctuations in the isotropic phase of liquid crystalline and non‐liquid crystalline alkyl cyanobiphenyls have been investigated using light scattering and magnetic birefringence measurements. We find evidence for a virtual isotropic‐nematic phase transition in short‐chain alkyl cyanobiphenyls with no observable nematic phase. The measured temperature dependence of fluctuations is well‐described by mean‐field theory. Virtual phase transition temperatures extrapolated from separate light scattering and magnetic birefringence experiments are in good agreement. Landau–de Gennes model parameters for the compounds investigated are calculated from the experimental results.     

Pulsed gradient NMR study of anisotropic surfactant diffusion in the caesium perfluoro octanoate/D2O system

We use pulsed field gradient ¹⁹F NMR to measure the diffusion coefficients of surfactant molecules in the isotropic and lamellar phases of the caesium perfluoro octanoate (CsPFO)/D₂O system. An aligned lamellar sample is created by cooling through the nematic phase in the presence of a 1·4 T magnetic field. The director in the lamellar phase does not respond to ordinary field strengths, thus the aligned sample can be rotated clockwise or counterclockwise to place the director at a magic angle, where measurement of diffusion coefficients becomes possible. From a pair of so-obtained coefficients, we derive the principal values of the diffusion tensor corresponding to the directions parallel and perpendicular to the director (D_⊥ and D_⊥). We found D_∥ to be at least 20 times D_∥ a much larger anisotropy than is seen in electrical conductivity and water diffusion in similar systems. These results are compared to electrical conductivity, water and dye diffusion measurements.