Biomedical subjects

Jozef Mieczkowski

Publications and source records attributed to Jozef Mieczkowski.

8 recordsLinked to original sources

PubMed2007

Molecular factors responsible for the formation of the axially polar columnar mesophase Col(h)P(A).

The structure of hexacatenar bent-shape molecules has been systematically modified in order to determine the main molecular factors responsible for the appearance of the axially polar columnar mesophase. It was found that the stability of the polar phase is very sensitive to the subtle modifications of the molecular shape: the phase is solely preserved if the modification is made at the terminal parts of the mesogenic core, whilst any other modifications destabilize the phase. It can be concluded that the main factor driving the transition between the phase made of flat supramolecular discs and the axially polar phase made of the cone-like units is the ability to fulfill close packing conditions in order to eliminate voids between neighboring molecular rigid cores.

Liquid Crystals↗

PubMed2006

Electric-field-induced polar biaxial order in a nontilted smectic phase of an asymmetric bent-core liquid crystal.

A polarization reorientation process has been studied by means of optical second-harmonic generation in an optically uniaxial smectic phase of an asymmetric bent-core liquid crystal. A nontilted polar smectic order with a biaxial order is induced by applying an electric field to the uniaxial nonpolar smectic phase. This phenomenon was well simulated by the two-dimensional Langevin process, i.e., electric-field-induced continuous molecular reorientation against thermal agitation. The simulation suggests that about 200s of molecules form a polar domain and cooperatively respond to the applied field. The existence of the polar domains and their reorientation are consistent to the dielectric measurement; a high dielectric constant of about 60 at the kHz range is markedly suppressed by applying a bias field.

Journal Article↗

PubMed2006

Polar order in columnar phase made of polycatenar bent-core molecules.

Columnar phases made of polycatenar molecules with bent-shaped mesogenic cores are studied. The polar order in this system is associated with the change of the column building blocks from flat disks (Colh phase) into cones (Colh PA phase), which allows for axial polarization of the columns. The nature of the Colh and Colh PA phase transition changes from first order for short homologues to continuous for the longest one. This can be attributed to decreasing intercolumnar interactions due to broadening of the columnar scaffold made of partially melted terminal alkyl chains. Decrease of intercolumnar interactions is also responsible for a strong increase of the pretransitional fluctuations in the Colh phase. The mesophase observed for the longest homologues is reminiscent of the relaxor phase observed for solid crystals.

Journal Article↗

PubMed2005

Paraelectric-antiferroelectric phase transition in achiral liquid crystals.

Critical freezing of molecular rotation in an achiral smectic phase, which leads to polar ordering through the second order paraelectric-antiferroelectric (Sm-A-->Sm-APA) phase transition is studied theoretically and experimentally. Strong softening of the polar mode in the Sm-A phase and highly intensive dielectric mode in the Sm-APA phase are observed due to weak antiferroelectric interactions in the system. In the Sm-APA phase the dielectric response behaves critically upon biasing by a dc electric field. Such a behavior is found general for the antiferroelectric smectic phase with significant quadrupolar interlayer coupling.

Journal Article↗

PubMed2005

Spontaneous breaking of minimal surface condition: labyrinths in free standing smectic films.

We present a study of thin free standing films made of intercalated smectic-C liquid crystal in which, upon lowering the temperature, the minimal surface area condition is broken. A periodic modulation of the film thickness is obtained and a labyrinth structure of crests and valleys is formed. Thickness variation is coupled to the spatial variation of the molecular orientation. The transition to the labyrinth structure is explained to be driven by the mass density difference between the surface and the bulk layers.

Journal Article↗

PubMed2005

Switching mechanism in polar columnar mesophases made of bent-core molecules.

The behaviour of polar, broken-layer-type columnar phases made of bent molecules (B(1Rev) and B(1RevTilted)) was studied under an applied electric field. There are two competing mechanisms of ferroelectric switching in the polar B(1RevTilted) columnar phase: collective rotation around the long molecular axis and collective rotation around the tilt cone. The proposed model shows that the main factor discriminating the type of switching is the width of the column cross-section.

Journal Article↗

PubMed2004

Axially polar columnar phase made of polycatenar bent-shaped molecules.

The polycatenar bent-shaped molecules are able to form columnar phases with column stratum built of few molecules, arranged in coplanar or conelike geometry. In the latter case, the phase becomes axially polar, with electric spontaneous polarization reorientable in the electric field by flipping the cone axis. The phase is antiferroelectric; in the plane perpendicular to columns, the ferroelectric hexagonal order exists, but the columns are broken along the z direction and the polarization direction alternates between the blocks.

Journal Article↗

PubMed2003

Ferroelectric mesophase with randomized interlayer structure.

We present the experimental and theoretical evidence that a new Sm X phase made of asymmetric bent-shaped molecules has a layered, nontilted, optically uniaxial and polarly ordered structure with random direction of the layer polarization. The randomness results from the sign degeneracy of the difference in polarization directions in neighboring layers, although the magnitude of the phase difference is constant. Lifting the degeneracy by external fields two additional structures of the Sm X phase are possible.

Journal Article↗