Proton NMR investigation of a hydrogen-bonded liquid crystal gel

Proton NMR is employed to determine director distributions in a hydrogen-bonded liquid crystal gel in the presence of a magnetic field. The samples consist of the mesogen 8CB mixed with small percentages of gelator, which forms a hydrogen-bonded network when the sample is cooled below the gelation point in the isotropic liquid. Since the gelation occurs above the clearing point, a non-oriented random director configuration is frozen in. The configuration of the hydrogen-bonded network is found to be isotropic. It fixes a preferential random local orientation of the nematic director, even in the presence of an external magnetic field of a few Tesla. The NMR spectra of the samples give information on orientation and order in such systems. A simple model for the director field is provided.     

Evaluation of translational friction coefficients of micro-sized spherical probes in nematic liquid crystals

We study the translational friction coefficients of a spherical micrometric probe moving in nematic liquid crystalline fluids, by solving numerically the constitutive hydrodynamic equations of uncompressible isothermal nematic fluids (Leslie–Ericksen equations). The nematic medium is described by a vector field, which specifies the director orientation at each point and by the velocity vector field. Simulations of director dynamics surrounding the moving probe are presented, and the dependence of translational diffusion upon liquid crystal viscoelastic parameters is discussed. The time evolution of director field is studied in the presence of an orienting magnetic field in two characteristic situations, i.e. direction of motion parallel and perpendicular to field. In particular, a detailed analysis is given for the case of a spherical probe in rectilinear motion in nematic MBBA (4-methoxibenzylidene-4′-n-butylaniline), together with a comparison with other nematogens.     

Streamline Image Analysis: a new tool for investigating defects in nematic liquid crystals

A bidimensional investigation of the director field near defects normal to the symmetry plane in a nematic liquid crystal is here presented, based upon a new 'streamline' image processing of the data obtained by polarized light microscopy. With a visualization of vector field streamlines related to the actual director configuration, the disclination topological strength is revealed. The streamline visualization is also suitable for detecting smooth local variations of the director of field as functions of both space and time. Moreover, the orientation of the director field projection in the cell plane is determined with high sensitivity ( 2%) and the possible presence of the out-of-plane escape of the director field is evaluated.     

Anisotropy of non-mesogenic polymer networks dispersed in a liquid crystal matrix

We report here original results characterizing in situ the interactions between a smectic liquid crystal phase and a polymer network dispersed in it. These results have been obtained by neutron scattering on smectic liquid crystal (8CB) samples containing a physical network of 1.5wt% polymer. The samples were polymerized in the isotropic, or in the smectic A phases. For the first time it is experimentally proved that the polymerization of non-mesogenic monomers in an aligned smectic A matrix induces anisotropy in the resulting network. The network becomes elongated along the liquid crystal director. When the polymerization is carried out in the isotropic phase the polymer network has an isotropic distribution even if a magnetic field, which orients the liquid crystal director, is later applied. On the other hand, studies show that after several thermal cycles, the liquid crystal orientational order still remained. Without other external constraints, the polymer network freezes the alignment of the liquid crystal. It is probably imposed by pendant reticulates on the diffuse liquid crystal-polymer interfaces.     

Electric-field driven director oscillations in a nematic liquid crystal: a NMR investigation

We have investigated the oscillatory behavior of the nematic director for 4-pentyl-4'-cyanobiphenyl (5CB) when it is subjected to a static magnetic field and a sinusoidal electric field. In these experiments the two fields were inclined at about 50 degrees and the frequency of the electric field was varied from several hertz to approximately 1000 Hz. The director orientation was measured using time-resolved deuterium NMR spectroscopy since this has the advantage of being able to determine the state of director alignment in the sample. In fact, for all of the frequencies studied the director is found to remain uniformly aligned. Since the diamagnetic and dielectric anisotropies are both positive the director oscillates in the plane formed by the two fields. These oscillations were observed to continue for many cycles, indicating that the coherence in the director orientation was not lost during this motion. The maximum and minimum angles made by the director with the magnetic field were determined, as a function of frequency, from the NMR spectrum averaged over many thousand cycles of the oscillations. At low frequencies (several hertz) these limiting angles are essentially independent of frequency but as the frequency increases the two angles approach each other and become equal at high frequencies, typically 1000 Hz. Our results are well explained by a hydrodynamic theory in which the sinusoidal time dependence of the electric field is included in the torque-balance equation. This analysis also shows that, for a range of frequencies between the high and low limits, these NMR experiments can give dynamic as well as static information concerning the nematic phase.     

Liquid single crystal elastomers (lsce) - mechanical optical and electric properties

Liquid crystal elastomers can be macroscopically ordered with respect to the director by applying a mechanical field similar to electric/magnetic field effects of low molar mass liquid crystals. Introducing network anisotropy a priori by the synthesis, uniformly aligned nematic or smectic elastomers are available without external mechanical field. These LSCE's combine optical properties of single crystals with entropy elasticity of elastomers. Due to uniform director orientation without defects, the LSCE's exhibit excellent transparency which makes them applicable for optical elements. Non-linear optical properties can be easily optimized by attaching suitable I.c.-moieties to the polymer network. On the other hand, due to the rubber elasticity of the LSCE's, electromechanical effects can be observed, e. g. piezoelectricity of chiral smectic C-LSCE's.     

Experimental study of the director pattern dynamics in the vicinity of the Fréedericksz twist geometry

We present an experimental study of the transient periodic structures appearing in the nematic director field in the magnetically induced reorientation of the director in the vicinity of the twist Fréedericksz geometry. Thin nematic samples (50?µm thick) were exposed to magnetic fields of variable intensity and orientation relative to the surface aligning direction of the sample. The director reorientation was induced by a rapid rotation of the sample in the static magnetic field producing a misalignment between the director and the magnetic field. The director field was optically monitored during the reorientation process and the transient periodic structures were characterized. Two types of periodic structures could be identified, namely bands and walls. Walls grow from bands close to the twist Fréedericksz geometry. The time dependence of the wave length and inclination of the periodic structures was obtained as a function of the magnetic field intensity and orientation relative to the surface aligning direction of the sample. The results for the bands are compared with the predictions of a model that we specifically developed to account for the non-orthogonal field orientations. It is seen that our model can account rather well for the experimental results considering that it uses only the field rotation time as adjustable parameter. All other model parameters are known.     

Liquid crystal alignment on isotropic submicro-textured surfaces of homeotropic-planar states

The equilibrium configuration of liquid crystal molecules in contact with a periodicly patterned substrate has been theoretically analysed considering the variation of the order parameter at the interface, and a first order transition was predicted for a critical periodicity, or induced by temperature or an electric field. In this work, we experimentally investigated the equilibrium configuration of the director for a submicron patterned substrate, alternating homeotropic and random planar regions in the substrate. Experimentally, it was possible to access a starting point, theoretically close to the first order transition, however no transition was observed either on varying the temperature or on applying an electric field. The possible reasons for the non-observation of the phase transition are discussed.     

Liquid crystal alignment on isotropic submicro‐textured surfaces of homeotropic‐planar states

The equilibrium configuration of liquid crystal molecules in contact with a periodicly patterned substrate has been theoretically analysed considering the variation of the order parameter at the interface, and a first order transition was predicted for a critical periodicity, or induced by temperature or an electric field. In this work, we experimentally investigated the equilibrium configuration of the director for a submicron patterned substrate, alternating homeotropic and random planar regions in the substrate. Experimentally, it was possible to access a starting point, theoretically close to the first order transition, however no transition was observed either on varying the temperature or on applying an electric field. The possible reasons for the non‐observation of the phase transition are discussed.     

Magnetic field controlled optical phase retardation in a hybrid nematic cell

We have studied the phase retardation of linearly polarized light in a hybrid nematic liquid crystal cell. For a certain range of directions of the applied magnetic field the phase retardation is found to change non-monotonically with the magnetic induction. The observed behaviour is described rather well by the standard Frank elastic theory. The corrections resulting from subsurface deformations, which are characteristic both for second order elasticity approach and for surface field theory, are also considered. The analysis of the experimental data suggests that the presence of distortions in the zero-field director configuration is the necessary condition for the non-monotonic phase retardation, which implies that such an experiment could be used for the detection of misalignment of the effective pretilts in a nematic cell.     

The 1H NMR spectrum of pyrazole in a nematic phase

The experimental ¹H nuclear magnetic resonance (NMR) spectrum of 1H‐pyrazole was recorded in thermotropic nematic liquid crystal N‐(p‐ethoxybenzylidene)‐p‐butylaniline (EBBA) within the temperature range of 299–308 K. Two of three observable dipolar D_HH‐couplings appeared to be equal at each temperature because of fast prototropic tautomerism. Analysis of the Saupe orientational order parameters using fixed geometry determined by computations and experimental dipolar couplings results in a situation in which the molecular orientation relative to the magnetic field (and the liquid crystal director) can be described exceptionally by a single parameter. Copyright © 2016 John Wiley & Sons, Ltd.     

Analysis of transient periodic textures in nematic polymers

A numerical solution of the Leslie-Ericksen equations for nematic liquid crystals is obtained for in-plane rotation of a strong magnetic field. A transient periodic orientation develops as a result of in-plane director motion and the induced shear flow. At long times the in-plane director orientation results in steady splay-bend inversion walls. A linear stability analysis shows that the inversion walls are unstable to perturbations out of the plane for elastic coefficients characteristic of nematic polymers. Calculations of transmitted light intensity through crossed polarizers for the computed orientation development predict the evolution of a banded texture, as observed experimentally.     

The influence of electric field-induced orientation on the retraction of a liquid crystal droplet immersed in a flexible polymer matrix

We measured the apparent interfacial tension between a liquid crystal and a flexible polymer by deformed droplet retraction method. An external electric field is applied to change the director orientation in liquid crystal droplet. The deformation and recovery of a single liquid crystal droplet dispersed in a polydimethylsiloxane (PDMS) matrix were realized by a transient shear flow and observed by polarized optical microscope. In order to control the director orientation in LC droplet, the electric field is applied perpendicular and parallel to the flow field, respectively. The different orientation induced by electric field in liquid crystal droplet has different behavior during droplet retraction and affect the apparent interfacial tension between liquid crystal and flexible polymer.     

Inertial effects in rotating lyotropic liquid crystals

The orientation of the director of a lyotropic liquid crystal with negative anisotropy of the diamagnetic susceptibility is studied when the sample is spun at various speeds and angles from the magnetic field. Different behaviours is observed at low and high spinning speeds where the orientation is governed by the magnetic and the inertial torques, respectively. At intermediate frequencies it is possible to reach an equilibrium between these two torques. The evolution of this equilibrium with the average magnetic torque allows one to form a conclusion on the inertial nature of this effect.     

Sensitive through-space dipolar correlations between nuclei of small organic molecules by partial alignment in a deuterated liquid solvent

Through-space residual dipolar correlations in NMR spectra can be measured between nuclei of small organic molecules by partially aligning them with respect to the magnetic field in a pure deuterated liquid solvent, 4-pentyl-4'-cyanobiphenyl. A simple temperature change of this liquid phase enables spectra to be compared between samples under isotropic tumbling conditions and weakly oriented anisotropic states. This should provide access of a number of small nonpolar molecules to more sensitive through-space nuclear correlations than possible through NOE experiments, depending on the net orientation of specific nuclear pairs with respect to the magnetic field and the specific coherence transfers employed.     

Inertial effects in rotating lyotropic liquid crystals

Abstract

The orientation of the director of a lyotropic liquid crystal with negative anisotropy of the diamagnetic susceptibility is studied when the sample is spun at various speeds and angles from the magnetic field. Different behaviours is observed at low and high spinning speeds where the orientation is governed by the magnetic and the inertial torques, respectively. At intermediate frequencies it is possible to reach an equilibrium between these two torques. The evolution of this equilibrium with the average magnetic torque allows one to form a conclusion on the inertial nature of this effect.     

ESR studies of the orientational and reorientational properties of mesomorphic polymers

Abstract

The orientational properties of two mesomorphic azobenzene-containing polyacrylates are investigated by ESR spectroscopy using the spin probe technique. Procedures to achieve a uniform alignment of the director in the presence of a static magnetic field are described in detail. The recovery of the alignment after the creation of controlled non-equilibrium states in the presence of a magnetic field is studied. It is found that the recovery time is proportional to the reciprocal square of the static magnetic field, analogous to that which occurs in low molar mass liquid crystals.     

Field Effects in Nematic Liquid Crystals in Terms of Catastrophe Theory

The field effects in nematic liquid crystal layers are reanalysed using catastrophe theory. The layer with pretilted director orientation and obliquely applied magnetic field, the hybrid aligned nematic cell and twisted nematic structures are considered. The stable solutions are identified and transitions between them are specified. The results are in essential agreement with previous work. Some details concerning the behaviour near the threshold are revealed.     

Magnetic orientation of diamagnetic amorphous polymers

Molecular order in an amorphous polymer with anisotropic magnetic susceptibility is altered by applying external magnetic fields. Disks of atactic polystyrene (a‐PS) are prepared by solvent casting outside or inside a magnet. The effect of the magnetic field on the polymer samples is investigated by magnetic levitation and solid state NMR spectroscopy. Magnetic levitation of the a‐PS disks shows that the orientation of disk symmetry axis with respect to the magnetic field gradient depends on the magnitude and direction of the applied field during casting. Similarly, carbon‐13 two‐dimensional cross‐polarization/magic angle spinning rotor‐synchronized NMR measurements in these samples show modulation patterns of the spinning side bands only on disks prepared in the presence of a magnetic field. These findings suggest that macromolecular order could be induced in a fluid or fluid–solid phase transition with cooperative segmental motions reorienting the diamagnetic susceptibility tensor to minimize the magnetic contribution to free energy of the sample. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1009–1015, 2010