PubMed Health⌕ Search

Biomedical subjects

W Weissflog

Publications and source records attributed to W Weissflog.

11 recordsLinked to original sources
PubMed2006

Electric-field-induced chirality flipping in smectic liquid crystals: the role of anisotropic viscosity.

We demonstrate the homogeneous and permanent reversal of the chirality of a condensed phase by an applied electric field. Tilted chiral smectic layers exhibit a coupled polarization density and molecular orientation fields which reorient about the layer normal as couple of fixed handedness in response to small applied electric fields. Experiments on some bent-core smectics show that above a threshold field the induced rotation can occur instead about the molecular long axis and that, as a result, the handedness of the phase can be flipped. The effect is quantitatively described by a nonequilibrium dissipative model of chiral smectic dynamics with anisotropic rotational viscosities.

Journal Article↗
PubMed2005

Optically isotropic liquid-crystal phase of bent-core molecules with polar nanostructure.

We found that the optically isotropic (I(M)) mesophase observed recently below the nematic phase of the bent-core liquid crystal 4-chlororesorcinol bis[4-(4-n-dodecyloxybenzoyloxy)benzoate] shows ferroelectric-type switching. Polarizing microscopic, electric current, dielectric, and dynamic light scattering studies lead us to propose that the I(M) phase is composed of interconnected orthoconic racemic smectic (Sm-Ca P(F)) nanodomains with random layer orientations. Near the nematic phase, where the polarization can be saturated by electric fields, the system responds in a fashion analogous to the granular structure of a magnetic spin glass--in particular, we observed that the relaxation back to the nonpoled structure follows a similar, inverse logarithmic rule.

Journal Article↗
PubMed2003

Optical studies on free-standing films of an achiral smectic liquid crystal.

Employing null transmission ellipsometry and depolarized reflected light microscopy, we have studied two smectic phases, Sm-C1 and Sm-C2, of one achiral mesogen. Our results show that Sm-C1 and Sm-C2 are the synclinic Sm-C and anticlinic Sm-C(A) phases, respectively. We find no evidence to support recent claims that the Sm-C1 is chiral and ferroelectric [R. Stannarius et al., Phys. Rev. Lett. 90, 025502 (2003)].

Journal Article↗
PubMed2003

Polarization-modulated smectic liquid crystal phases.

Any polar-ordered material with a spatially uniform polarization field is internally frustrated: The symmetry-required local preference for polarization is to be nonuniform, i.e., to be locally bouquet-like or "splayed." However, it is impossible to achieve splay of a preferred sign everywhere in space unless appropriate defects are introduced into the field. Typically, in materials like ferroelectric crystals or liquid crystals, such defects are not thermally stable, so that the local preference is globally frustrated and the polarization field remains uniform. Here, we report a class of fluid polar smectic liquid crystals in which local splay prevails in the form of periodic supermolecular-scale polarization modulation stripes coupled to layer undulation waves. The polar domains are locally chiral, and organized into patterns of alternating handedness and polarity. The fluid-layer undulations enable an extraordinary menagerie of filament and planar structures that identify such phases.

Journal Article↗
PubMed2003

Experimental evidence for Sm-CG-->Sm-CP polymorphism in fluorinated bent-shaped mesogens.

We report the experimental results on pure fluorinated bent-shaped mesogens showing very unusual textures and electro-optical behavior. In the view of recent publications such behavior can be explained by triclinic symmetry of the mesophase (Sm-C(G)) formed by these compounds. Based on the results of the x-ray diffraction and electro-optical investigations, we give evidence for the Sm-C(G) -->Sm-CP polymorphism.

Journal Article↗
PubMed2003

Field-induced switching between states of opposite chirality in a liquid-crystalline phase.

A tilted smectic phase of a new achiral banana-shaped mesogen is presented. It possesses liquidlike order within the layers and appears with a fan-shaped texture. At sufficiently high electric fields, this texture can be transformed into a texture that displays a complete extinction between crossed polarizers and it forms randomly distributed chiral domains. Above a threshold these domains can be reversibly switched into a state of opposite handedness.

Journal Article↗
PubMed2002

Observation of unusual surface ordering in a uniaxial Sm-A phase formed by a highly biaxial compound.

We have studied one banana-shaped compound using null-transmission ellipsometry. By studying free-standing films of various thicknesses we confirm that this compound exhibits the Sm-A phase. This is the first banana-shaped compound with a relatively small bend angle (approximately 140 degrees) in which a uniaxial phase has been observed. At the lower temperature end of the Sm-A phase we observe unusual ordering at the surface of the film.

Journal Article↗
PubMed2001

Detailed optical studies of several banana-shaped compounds in liquid crystal B2 phase.

Null-transmission ellipsometry has been performed on free-standing films of one compound in the liquid crystal B2 phase. We have studied films of thickness from 1 to 121 layers. One of the compounds used has an unusually wide 59 K window for the B2 phase. The tilt angle was investigated as a function of temperature and found to be constant over this temperature range to within our resolution of 1 degrees. The one-layer films studied exhibit the same structure as the thicker films, and have helped us to refine the optical model for the B2 phase. For thin films we find that modeling a single smectic layer as two uniaxial layers is a better description of the data than a single biaxial layer, but that for thick films the model used does not effect the simulated result appreciably. Preliminary results also find that the surface layers are less tilted than the interior layers, in contrast to rodlike liquid crystals, which show an enhanced surface tilt.

Journal Article↗
PubMed1985

[Synthesis and biologic activity of some stable structure analogs of platelet activating factor].

Physiologically stable structure analogues of the platelet-activating factor (PAF-acether), the acetyl group of which had been substituted by short-chain alkyl rests, had been synthesized and been tested with regard to their biological activity against thrombocytes. Five differently substituted 2-alkylmalonic acid esters with methyl rest up to n-pentyl rest had been applied the basic products. The n-propyl compound exhibits the most intense biological activity, which is, however, essentially reduced against the PAF-acether. With conditions of the examination, all the other derivatives possess a more decreased activity.

Chemical Phenomena↗
PubMed1984

[Synthesis of 2-substituted alkanolphosphoric acid esters with lytic effects].

Lysolipid analogues were synthetized and tested for haemolytic activity. Initial substances were long-chain 2-methylalkanols, as well as 2-hexadecylpropane-1,3-diol. The latter compound was prepared starting with hexadecylmalonic acid diester. Among the compounds synthetized, there were also 1-acetoxy-2-hexadecylpropan-3-ol-phosphocholine and acidic-reacting lysolipid analogues.

Chemical Phenomena↗