Otilia Catanescu

Synthesis and characterization of some aliphatic–aromatic poly(Schiff base)s

Abstract Polymers with azomethine structure containing 1,5-naphthyl or 1,4-phenyl moieties were synthesised through polycondensation of some dialdehydes with diamines. Both monomers and polymers were characterised by IR and 1 H-NMR techniques. Thermogravimetric analyses were made for all the synthesised polymers in order to study their thermal behaviour.

High birefringence difluoroisothiocyanate biphenyl tolane liquid crystals

Several difluoroisothiocyanato‐based biphenyl tolanes exhibiting high optical birefringence are presented and their representative molecular structure is shown below. The optical birefringence of these liquid crystals lies between 0.44 to 0.48. Several eutectic mixtures of the biphenyltolanes with other compounds have been formulated and their properties investigated. The molecular simulations of polarizability and dipole moment of biphenyl tolanes are also presented.

High Figure-of-Merit Laterally Fluorinated Biphenyltolane-Isothiocyanates

We have designed, synthesized, and evaluated the physical properties of some high birefringence (Δn) isothiocyanato biphenyl-tolane liquid crystals. These compounds exhibit Δn ∼ 0.48–0.52 at room temperature and wavelength λ = 633 nm. Laterally substituted short alkyl chains and fluorine atom eliminate smectic phase and lower the melting temperature. The moderate melting temperature and very high clearing temperature make those compounds attractive for eutectic mixture formulation. A eutectic mixture based on those compounds was formulated and its physical properties evaluated.

Phase transfer catalysis in polycondensation processes XXIV. On the thermal stability of some aromatic polyethers containing a propylenic spacer

Abstract We have studied the correlation between thermal stability and molecular structure, for some copolyethers. Thermal stability was evaluated from thermogravimetric analyses and molecular modeling. The oligomers were synthesized by a phase transfer catalysis technique, starting from 1,3-dibromopropane and various bisphenols, 4,4′-dihydroxyazobenzene, 4,4′-dihydroxydiphenyl, bisphenol A and 4,4′-dihydroxybenzophenone. Use of bisphenols with different geometry or polarity permitted us to modify chain conformation and polarity and to correlate these factors with thermal stability. The study demonstrated that the presence of azobenzene groups in the chain induces lower stability: the weight loss starts around 260–300°C. The most stable product was that based on biphenyl and bezophenone. Chain polarity is less important than chain periodicity, the most stable products being those with an equimolecular ratio between the bisphenols. The presence of bent bisphenol in the chain decreases thermal stability, but this effect can be compensated by an increase of chain polarity. Two heating rates were used, the compensation effect calculated demonstrating that the degradation mechanism is similar for both rates.

Liquid crystalline polymers, 9. Synthesis and thermal behavior of some oligomers containing a diethyletheric spacer

Polyethers based on bis(2-chloroethyl) ether and various bisphenols were obtained using a phase transfer catalysis technique in a liquid/liquid system. 4,4′-Dihydroxyazobenzene, 4,4′-dihydroxybiphenyl, bisphenol A, 2,7-dihydroxynaphthalene and 4,4′-dihydroxydiphenyl) sulfide were used. Only the polymers with azobenzene and biphenyl units exhibited mesophases, since the others have semi-crystalline or amorphous structures. The polymers were characterized by 1H NMR, DSC and optical microscopy in polarized light. The molecular weights of the samples were low, situated in the oligomeric domain. The various transfer rates of the bisphenols from the aqueous to the organic phase, in some cases, led to very different copolymerization ratios as compared to the feed ratios. The LC polymers exhibited monotropic mesophases, probably for conformational reasons. A comparison between similar polyether structures containing the diethyletheric spacer and an oxetanic one was effectuated. Polyether aus Bis(2-chlorethyl)ether und verschiedenen Diphenolen wurden durch Phasentransferkatalyse in einem zweiphasigen flussigen System hergestellt. Als Diphenol-Verbindungen wurden 4,4′-Dihydroxyazobenzol, 4,4′-Dihydroxybiphenyl, Bisphenol A, 2,7-Dihydroxynaphthalin und 4,4′-Dihydroxydiphenylsulfid eingesetzt. Lediglich die Polymeren mit Azobenzol- und Diphenyleinheiten bildeten Mesophasen, wohingegen die anderen Produkte semikristalline oder amorphe Strukturen ausbildeten. Die Polymeren wurden durch 1H NMR, DSC und Polarisationsmikroskopie charakterisiert. Die Proben besasen niedrige Molekulargewichte im Bereich von Oligomeren. Die unterschiedlichen Transfergeschwindigeiten der Diphenole von der wasrigen in die organische Phase fuhrten in einigen Fallen zu vom Ausgangsmonomerverhaltnis sehr verschiedenen Copolymerzusammensetzungen. Die flussigkristallinen Polymeren bildeten, vermutlich aufgrund der Molekulkonformation, monotrope Mesophasen. Die Ergebnisse wurden mit denen von Polyethern ahnlicher Struktur mit Diethylether- und Oxetan-Spacern verglichen.

High frequency performance extending to millimeter-waves in inverted-microstrip-line-type LC phase shifter

Various liquid crystal (LC) phase shifters that operate in the super-high-frequency electromagnetic-wave regions have been investigated using planar-type excellent waveguides such as the microstrip line (MSL) and coplanar waveguide (CPW). First planar-type LC phase shifters were constructed using MSL, which was developed as an excellent planar waveguide for super-high-frequency electromagnetic waves. CPW-type LC phase shifters have attracted continued attention, because when they are used, all the signal and ground electrodes are at the same surface, which leads to ease in integration for constructing various functional devices. However, they suffer from an essential drawback of degradation in the phase shift magnitude, which is because the propagating electromagnetic waves encounter the permittivity of both the substrate and the LC materials, which reduces the modulation effect of the LC materials to less than half. In this work, a novel MSL-type LC phase shifter is investigated to achieve excellent phase shifting performance while maintaining ease in integration, as offered by the CPW-type phase shifter. Several device structural parameters are investigated to improve the transmission and phase shifting properties. Some LC materials are also tested for further improvement in the high-frequency operation extended to the millimeter-wave region.

Molecular engineering of high-birefringence liquid crystals

Several linearly conjugated isothiocyanato (NCS) tolanes, biphenyl-tolanes and bistolanes were synthesized and their properties evaluated. Eutectic mixtures compromising these compounds were formulated. The temperature dependent birefringence, visco-elastic coefficient and figure-of-merit were measured. These isothiocyanato tolane mixtures exhibit a favorably high figure of merit at elevated temperatures. Potential applications of these mixtures for laser beam steering, tunable-focus lens and telecom variable optical attenuator are emphasized.

Improved High-Frequency Performance of Microstrip-Line-Type Liquid Crystal Phase Shifter

A novel microstrip-line (MSL)-type liquid crystal (LC) phase shifter is designed to retain both the ease of fabrication with a coplanar waveguide (CPW)-type LC phase shifter and the excellent phase-shifting properties of the MSL-type one. The converting circuit, which is a key component of the device, is optimized so as to improve the operating frequency. Then, the phase-shifting properties and loss phenomena are investigated by using various LC materials. The operational frequency is extremely improved by employing a novel large birefringence LC material and the optimized design.

Thermal and kinetic characteristics in non-isothermal conditions of some aromatic copolyethers containing an octomethylenic spacer

The paper presents a study of the thermal properties of some aromatic polyethers obtained by phase trasfer catalysis technique. The polymes were synthesized starting from 1,8-dichloro-octane and various bisphenols: 4,4′-dihydroxyazobenzene, 4,4′-dihydroxydiphenyl, bisphenol A and 2,7-dihydroxynaphthalene. Molecular simulations were performed in order to obtain supplementary information concerning the chain conformation and inter-chain interactions. The presence of azobenzenic moieties induces high phase transition values, situated near the thermal stability limit. Therefore, the thermogravimetrical study of these polymers is very important for establishing the temperature limits for DSC and optical microscopy studies. All the polymers present a good thermal stability situated above the isotropization values. In these circumstances no special precautions are necessary for the characterization methods of the liquid crystalline behavior. The kinetic characteristics suggest a similar degradation mechanism by successive reactions. The inter-chain interactions do not influence the thermostability of the samples if the polar surface of the polymer is situated below a certain value.