Cornelia Motoc

Behaviour of ferrocholesterics under external magnetic fields

Abstract The influence of an external magnetic field on the orientational behaviour of a ferrocholesteric with a positive magnetic anisotropy is investigated. Both the phenomena arising when the field was switched on or switched off are considered. It is found that the field needed for a ferrocholesteric–ferronematic transition B FC ↑ is higher when compared to that obtained for the pure cholesteric ( B C ↑). A similar result was obtained when estimating the critical field for the homeotropic ferronematic–ferrocholesteric (focal conic) transition, occurring when the magnetic field was decreased or switched off. We found that B FC ↓> B C ↓. These results are explained when considering that the magnetic moments of the magnetic powder are not oriented parallel to the liquid crystal molecular directors, therefore hindering their orientation under a magnetic field.

Mesomorphic Properties of Some Sitosteryl Esters

Abstract The mesomorphic properties of some esters formed by aromatic acids with β-sitosterol are reported. It is shown that para-substituted benzoates are compounds exhibiting enantiotropic cholesteric and some smectic mesophases. Connections between the clearing points of sitosteryl esters and their homologous cholesteryl derivatives are discussed.

MAGNETO-OPTIC EFFECTS IN NEMATIC LIQUID CRYSTAL DOPED WITH AZO-DYES

Nematic liquid crystal cells were subjected to different DC magnetic fields and the optical transmission of a He-Ne laser beam was recorded for different field values. It was found that a small addition of Methyl Orange (MO) induced optical activity into the nematic sample. When the cells were subjected to increasing magnetic field, the rotatory power (rotation angle) varied periodically; the same occurred for the light transmission and ellipticity. We found that the light emerging from the liquid crystal cell is elliptically polarized and in some cases circularly polarized. After UV light irradiation (365 nm) the rotatory power of the sample increases. The position of rotation angle maxima and minima is slightly affected by the irradiation.

PHASE TRANSITION INVESTIGATIONS IN POLYMER/LIQUID CRYSTAL COMPOSITE MATERIALS

We prepared polymer dispersed liquid crystal (PDLC) composite films using polymethyl methacrylate (PMMA) and the ferroelectric liquid crystal (LC) Felix 015/000 (Hoechst) by the solvent-induced phase separation method. We studied the phase transitions by the thermally stimulated depolarization currents (TSDC) method and by differential scanning calorimetry (DSC), for the composite film and the corresponding liquid crystal. Polarized microscopy was also used to characterize the phase transitions. When the LC is mixed with the PMMA, its characteristic transition temperatures are shifted down a few degrees and the current peaks revealed by the TSDC method are broadened due to the dispersion of microdroplets and the consequential presence of a large interface between the LC and the polymer matrix.

Static and Dynamic Behavior of Ferronematics Under Magnetic Fields

The static and dynamic behavior under magnetic elds of ferronematics obtained by adding an organomagnetic material to the nematic E3 is investigated. First, by using both optical and electrical measurements, an increase of the threshold field for magnetic Freedericksz transition was noticed. Second, we examined both theoretically and experimentally the relaxation phenomena occurring in these ferronematics when the magnetic field was suddenly varied. We found that for ferronematics the relaxation processes are delayed when compared to those occurring in pure nematics. The threshold fields for magnetic Freedericksz transition were determined by means of the relaxation time and the results are in good agreement with the ones obtained when using the optical and electrical methods reported before.

Anisotropy of Electric Properties in Binary Cholesteric Mixtures

Abstract Dielectric permittivity and electric conductivity anisotropies were determined for cholesterol laurate, cholesterol caprilate as well as for their mixtures 25:75%, 50:50% and 75:25% (by weight). Connection between anisotropies of pure substances and their mixtures were found. The mixtures 75:25% and 25:75% have positive conductivities anisotropies within the cholesteric range as for the initial esters; the sign of the dielectric anisotropy is the same as for the prevailing ester. Within the smectic range the dielectric anisotropy is negative for both mixtures and pure compounds, in accordance to Carrs hypothesis with reference to the conduction mechanism in smectics. The phase transitions are revealed in most cases by maxima or minima of the dielectric anisotropies.

Laser-Induced Optical Nonlinearities In Ferronematics

Nematic and ferronematic samples were subjected to high power laser irradiation. Optical nonlinearities such as diffraction rings and self focussing effects were examined as function of light intensity. It is shown that the self-focusing effect appearing in a ferronematic(obtained by dispersing 0.1%yttrium iron garnet into nematic 4799 Merck)is smaller when compared to that encountered in pure nematic. A theoretical approach was elaborated to explain these optical nonlinearities in ferronematics and good agreement with the experimental findings was found.

Electrical Properties of Some Fatty Acids‐Cholesterol Mixtures

Abstract Some electric properties of some fatty acids(arachidic, arachidonic, elaidic and lauric) and fatty acid-cholesterol mixtures were determined. The I = I(U) plots revealed a hysteresis behaviour, the hysteresis area being dependent on the choleaterol content. From the I = I(t) plots, the relaxation time τ and the space charge Qsp were determined and it was shown that in case of arachidic acid-cholesterol mixtures 1+1 molar, τ and Qsp reached maxima. The results are explained by considering a hopping mechanism for conduction.

Electro-optical and conductive properties of polymer/liquid crystal composite film

Polymer-dispersed liquid crystal films, using polymethyl methacrylate and nematic liquid crystal E3 were obtained. The electro-optical effects were investigate using He-Ne lasers of different powers and a.c. electric fields of different frequencies applied across the composite film. The measurements of the transmitted light intensity through the composite film were carried out without any polarizers. At low frequencies,the emergent optical signal is modulated in amplitude, doubling the frequency of the a.c. driving field. In the kHz frequency range, the decrease of the fluctuations of the transmitted light with the increase of the frequency was noticed. The conductive properties of the polymer dispersed liquid crystal composite films and the contribution of the non-linear effects to the electric conduction were determined.

Nonlinear Optical Properties of Some Fatty Acids‐Cholesterol Mixtures

Abstract It is shown that fatty acids such as the arrachidic and the lauric acids and their mixtures with cholesterol exhibit nonlinear optical properties. The effects of a 20 mW He-Ne laser (λ= 6328 A) on such mixtures exhibiting smectogenic mesomorphism were examined and the optical activity dependence on the cholesterol percentage was determined.