Ralf Stannarius

Dielectric properties of the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl in porous membranes

Abstract Broadband dielectric spectroscopy (up to 109 Hz) is employed to study the molecular dynamics of the liquid crystal 4-n-pentyl-4′-cyanobiphenyl (5CB) in the free bulk phase and confined in cylindrical channels of Anopore membranes having a diameter of 0.2 μm and length of about 60 μm. The bulk samples of 5CB orient almost homeotropically between the untreated metal electrodes of the measurement set-up, and two relaxation processes are observed: the slower δ-relaxation is assigned to hindered rotation (180° flips) of the molecules around their molecular short axis, and a faster second process is attributed to the tumbling of the molecules about this axis. In the confined 5CB samples, the membrane pores align the nematic director axially or radially depending upon their surface preparation. Planar (axial) alignment is always found in untreated membranes, whereas radial alignment was achieved by treatment with decanoic acid. Consequently the director field is fixed perpendicular or parallel to the el...

Measurement of orientational order and mobility of a nematic liquid crystal in random nanometer confinement

Confinement of a nematogen in submicrometer cavities can produce substantial effects on the nematic-isotropic phase transition and on order and orientation of the nematic phase. We combine the methods of 13C- and 1H-NMR and broadband dielectric spectroscopy to probe liquid crystalline order and molecular mobility. We find that the dynamics of mesogenic molecules (5CB) in the free volume of nanoporous sol-gel glass (5 nm pore diameter) is as fast as in the free bulk phase. In addition, a boundary layer of molecules with reduced mobility covers the pore walls. Isotropic-nematic transition as well as crystallization of the confined nematogen are considerably suppressed. In order to extract the temperature dependence of the nematic order parameter from NMR data, a mathematical model is applied which describes the combined influences of orientational order and diffusional averaging in randomly oriented pores on NMR spectra of the nematogens.Confinement of a nematogen in submicrometer cavities can produce substantial effects on the nematic-isotropic phase transition and on order and orientation of the nematic phase. We combine the methods of 13C- and 1H-NMR and broadband dielectric spectroscopy to probe liquid crystalline order and molecular mobility. We find that the dynamics of mesogenic molecules (5CB) in the free volume of nanoporous sol-gel glass (5 nm pore diameter) is as fast as in the free bulk phase. In addition, a boundary layer of molecules with reduced mobility covers the pore walls. Isotropic-nematic transition as well as crystallization of the confined nematogen are considerably suppressed. In order to extract the temperature dependence of the nematic order parameter from NMR data, a mathematical model is applied which describes the combined influences of orientational order and diffusional averaging in randomly oriented pores on NMR spectra of the nematogens.

Unexpected liquid crystalline behaviour of three-ring bent-core mesogens: bis(4-subst.-phenyl) 2-methyl-iso-phthalates

Three-ring bent-core bis(4-subst.-phenyl) 2-methyl-iso-phthalates exhibiting nematic, SmA and SmC phases are reported. The occurring mesophases have been identified by their optical textures and X-ray diffraction measurements which give also geometrical structural parameters like layer spacing and molecular tilt. Quantum chemical calculations on single molecules and X-ray structure analysis in the crystalline state indicate wide opening angles (about 155°) of the molecular legs due to the lateral methyl group in position 2 of the central phenyl ring. However solid state NMR spectroscopy in the liquid crystalline phases finds stronger molecular bending (bending angle to be about 138° in the SmA and about 146° in the nematic phase). Dielectric and SHG measurements give evidence that in the SmA phase a polar structure can be induced by application of an electric field which disappears in the isotropic liquid phase. The electric field not only leads to a slight textural change even in the SmA phase but also polar-type electric current response (PS about 200 nC cm−2) is observed. This unusual electro-optical behaviour is discussed on the basis of the orientation of polar clusters formed by the bent molecules. In the paper we not only attempt to characterize the mesophases and to describe their physical properties, but we also show that these types of molecules represent the borderline between bent-shaped and calamitic liquid crystals.

Granular materials composed of shape-anisotropic grains

Granular physics has made considerable progress during the past decades in the understanding of static and dynamic properties of large ensembles of interacting macroscopic particles, including the modeling of phenomena like jamming, segregation and pattern formation, the development of related industrial applications or traffic flow control. The specific properties of systems composed of shape-anisotropic (elongated or flattened) particles have attracted increasing interest in recent years. Orientational order and self-organization are among the characteristic phenomena that add to the special features of granular matter of spherical or irregularly shaped particles. An overview of this research field is given.

Inclusions in free standing smectic liquid crystal films

Colloidal inclusions in thin free standing liquid crystal films are ideal model systems for 2D anisotropic dispersions. Different types of self-organization in chain and lattice structures have been observed. The orientational elasticity of the anisotropic matrix and capillary forces are the dominating interaction mechanisms between solid or liquid inclusions, the director field, and dislocations of the films. We give an overview of the progress in this field, focussing on different inclusion types and their interactions in thermotropic smectic films.

At the boundary to banana-shaped liquid crystals: polar properties of phases formed by new asymmetric achiral four-ring bent-core mesogens

New asymmetric four-ring bent-core compounds derived from N-benzoylpiperazine are presented. The mesophases were characterized by polarizing microscopy, DSC, X-ray diffraction, dielectric, electro-optical and SHG measurements. In some homologues the SmA as well as SmC phase can be transformed into a polar structure by applying an electric field. It was found for the first time that the polar SmI phase shows a ferroelectric switching which is obviously based on the collective rotation of the bent molecules around their long axes. It is remarkable that the ferroelectric structure is preserved on cooling into the solid state.

Comment on “Dynamics of electro-optical switching processes in surface stabilized biaxial nematic phase found in bent-core liquid crystal” [J. Appl. Phys. 101, 034105 (2007)]

Lee et al. [J. Appl. Phys 101, 034105 (2007)] studied the Freedericksz transition of a nematic bent-core mesogen in a planar cell. They claim that they have distinguished two different nematic phases in optical experiments, a uniaxial phase at a high temperature and a biaxial phase at a lower temperature. The commented paper contains several problematic passages and the substantial parts of the conclusions are invalid. Some experimental data appear to be inconsistent or incorrectly interpreted. We review the data and their interpretation, and analyze which conclusions are actually justified.

Surface tensions of smectic liquid crystals

We determine surface tensions σ of smectic liquid crystals from the curvature pressure of smectic films. A new technique is introduced for the comparison of surface tensions of different smectic materials. The method is based on the relation of curvatures of smectic films drawn on communicating vessels. The measurement of the temperature dependence of σ reveals anomalies in the vicinity of phase transitions to low temperature smectic modifications. This anomalous slope dσ/dT can be related to the surface excess entropy of the material in the corresponding temperature range. The surface tension values determined for a number of mesogens fit well into the classification proposed by Mach et al.

Preparation of actuating fibres of oriented main-chain liquid crystalline elastomers by a wetspinning process

We present a versatile method to prepare oriented fibres with a defined thickness from main-chain liquid crystalline elastomers. A microfluidic setup is utilized to inject a solution of a photocrosslinkable smectic A main-chain polymer into a co-flowing stream of silicone oil. Diffusion of the solvent into the oil yields solid polymer filaments that are crosslinked in a continuous way by UV-irradiation. The obtained fibres are highly oriented and show a reversible and significant contraction during the liquid crystals phase transition.

A fibre forming smectic twist–bent liquid crystalline phase

We demonstrate the nanostructure and filament formation of a novel liquid crystal phase of a dimeric mesogen below the twist–bend nematic phase. The new fibre-forming phase is distinguished by a short-correlated smectic order combined with an additional nanoscale periodicity that is not associated with density modulation.