Rodica Bena
Politehnica University of Bucharest
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Featured researches published by Rodica Bena.
Modern Physics Letters B | 1997
Doina Manaila-Maximean; Rodica Bena; Ana-Maria Albu
We obtained polymer-nematic liquid crystal composite films by the solvent induced phase separation (SIPS) method. We used a novel polymer obtained by the synthesis of a macromolecular structure with a matrix basis of methyl methachrylate and a chromophore group with extended conjugation (MMA + I) and the nematic liquid crystal E3 (BDH). We studied the electro-optical effects using He–Ne lasers of two different intensities, for a.c. driving fields of different frequencies and voltages. At low frequencies, the emergent optical signal is modulated in amplitude, doubling the frequency of the a.c. driving field. At higher intensities of the incident laser beam, the variation of the transmission with the a.c. driving electric field is much smaller. In the KHz frequency range, the decrease of the fluctuations of the transmitted light intensity with the increase of the frequency was observed. We also plotted the volt-ampere (V-I) characteristics of the polymer/LC composite films in different coordinates systems i...
Czechoslovak Journal of Physics | 2003
Constantin Rosu; Doina Mănăila-Maximean; Rodica Bena; Emil Petrescu; Stanislav Klosowicz; Krzysztof L. Czuprynski
Polymer-dispersed-liquid-crystal films were obtained using the photopolymerization-induced-phase-separation method. The thermally stimulated depolarization current measurements indicate a space-charge-limited current. Conduction of electronic type was proven and the activation energy, specific to the conduction process, was calculated. Optical measurements have been performed simultaneously with the measurements of thermally stimulated currents and the nematic-isotropic phase transition was detected by the switch of the optical transmission. The change of the optical transmission versus applied a.c. voltage was the indication for the on-off switch behavior, useful in electro-optic applications.
Molecular Crystals and Liquid Crystals | 2001
Doina Manaila-Maximean; Rodica Bena; Constantin Rosu; Maurizio Furlani
Abstract We obtained polymer dispersed nematic liquid crystal films E7/PMMA. Using the thermally stimulated depolarization currents method and registering the optical signal simultaneously we studied the dependence of the phase transitions temperatures on the polarizing electric field. Thermal switch behavior and its dependence on polarizing electric field is obtained.
Modern Physics Letters B | 1999
Doina Manaila-Maximean; Maurizio Furlani; Rodica Bena; Bengt-Erik Mellander; Constantin Rosu; Tatiana Pop; Cornelia Motoc
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.
ROMOPTO 2000: Sixth Conference on Optics | 2001
Doina Manaila-Maximean; Ana Maria Albu; Boris A. Umanski; Constantin Rosu; Rodica Bena
We prepared a copolymer of the maleic anhydride with styrene, to which a colored structure designated for the functionalization was added. The new polymer was used to prepare reverse-mode PDLC films. Reverse-mode PDLC are designed for applications where it is important that the failure of the device is a transparent state, rather than a scattering one. We obtained reverse-mode PDLC using a nematic liquid crystal with negative anisotropy (Delta) (epsilon) <0, LCM 3141 (NIOPIK), and we measured the optical transmission of the film. The system was switched from OFF-transparent state to the ON-scattering one, by a sufficiently small voltage, but with a rather low contrast ratio.
SIOEL '99: Sixth Symposium on Optoelectronics | 2000
Doina Manaila-Maximean; Maurizio Furlani; Rodica Bena; Victor Stoian; Constantin Rosu
We obtained polymer dispersed liquid crystal (PDLC) films using a nematic liquid crystal E7 and polymethyl methacrylate by the Solvent Induced Phase Separation method. Using the Thermally Stimulated Depolarization Current method we determined the clearing point of the pure liquid crystal and the PDLC films. The glass transition temperature of the PMMA matrix has also been determined. The result have been compared with those obtained by Differential Scanning Calorimetry. During the TSDC measurements we registered the optical transmission as a function of temperature, thus the thermal switch operation being demonstrated.
ROMOPTO '97: Fifth Conference on Optics | 1998
Doina Manaila-Maximean; Constantin Rosu; Rodica Bena; Alexandru I. Albu; Ion M. Popescu
We present the experimental results concerning hybrid optical bistable devices without resonant cavity using as electro-optic modulators a nematic liquid crystal cell and a polymer dispersed liquid crystal film. Bistable operation was studied for different control voltages and multistable behavior was studied for the polymer dispersed liquid crystal film as a function of the control voltage frequency.
Liquid Crystals | 1998
Doina Manaila-Maximean; Rodica Bena; Cornelia Motoc
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.
Journal of The Optical Society of America B-optical Physics | 1998
Doina Manaila-Maximean; Constantin Rosu; Rodica Bena
We have obtained polymer-dispersed liquid-crystal (PDLC) films by using poly(methyl methacrylate) and the nematic liquid crystal E7 in a solvent-induced phase separation method. The optical transmission of the PDLC sample is modulated by application to the sample of a control voltage from a reaction block, which electronically sums a feedback signal, an ac electric signal from an ac generator, and a dc bias voltage; the resultant signal is proportional to the output optical power detected by a photomultiplier. Because the PDLC film also exhibits a quadratic electro-optic effect (Kerr effect), which depends on the control voltage, multistability is experimentally obtained. The conditions for obtaining a differential gain and hysteresis are also theoretically analyzed; good agreement with the experimental results is observed.
Liquid Crystals | 1989
Iuliana Cuculescu; Rodica Bena; Gabriela Smeianu
Abstract The optical transmission and the electric power was simultaneously recorded for the binary compensated cholesteric mixture cholesteryl chloride + cholesteryl nonanoate (70:30 molar). The values of the critical voltages U 1 and U 2 (at which the planar texture changes into a conic focal one, and the cholesteric-nematic transition take place) at different temperatures were obtained. The temperature dependence of the anchoring constant is determined.