Beata Mossety-Leszczak

Photochromic and nonlinear optical properties of azo-functionalized POSS nanoparticles dispersed in nematic liquid crystals

It is well known that addition of a small amount of nanoparticles to liquid crystals may enhance their nonlinear optical properties. To date, in most reports, semiconductor- or metallic-core nanoparticles in LCs have been described. Here, we report on the synthesis of two types of novel nanoparticles based on a cubic siloxane cage (RSiO3/2)8, known as polyhedral oligosilsesquioxane (POSS) with the Si–O skeleton surrounded by covalently attached eight groups (R) of azo-benzene mesogenes differing in the length of their aliphatic chains. These dielectric-core nanoparticles substituted with mesogenic groups are well adapted to the calamitic liquid crystal matrix. Azo-benzene units add an important light-driven functionality to the system via their conformational photoisomerisation. We report on the photochromic properties of these new compounds in chloroform. By embedding the nanoparticles in a nematic liquid crystalline matrix, a functional hybrid nanocomposite material has been obtained. Our research was focused on the characterization of the third order nonlinear optical properties in this system linked to photochromism. The experimental results of all-optical switching in an optical Kerr effect configuration are discussed in the framework of light-induced modification of the aliphatic chains containing azo-benzene photoisomerizing moieties and following local director reorientation in nematic liquid crystals. The described properties are useful in the field of real-time holography.

Synthesis of glycidyl esters

A novel method of synthesis of glycidyl carboxylates (2,3-epoxypropyl carboxylates, RCOOCH2CH-CH2O) has been developed by carrying out the reaction of epichlorhydrin with a carboxylic acid in the presence of chromium acetate catalyst and subsequent dehydrochlorination of the resulting mixture of chlorohydrin esters followed by their separation. © 1999 Society of Chemical Industry

Synthesis and phase transitions of mesogenic compounds with functional groups in the tail

The properties of two pairs of recently synthesised compounds with mesogenic unit were compared. The four compounds consist of a central segment (based on biphenyl or phenylbenzoate) with symmetric carbon chains ended with vinyl or epoxy groups. Typical measurement techniques (including microscopic observations, differential scanning calorimetry and wide-angle X-ray scattering and dielectric studies) were used to examine phase transitions in the studied materials. The phase transition sequences were determined for all materials. A liquid crystalline phase appeared in some cases. It was observed that replacing the functional group in the tails visibly changes the temperatures and sequences of phase transitions.

Comparing Liquid Crystalline Properties of Two Epoxy Compounds Based on the Same Azoxy Group

Two recently synthesised diepoxy compounds based on the same rigid central segment with carbon chains of different lengths were compared. Microscopic observations, differential scanning calorimetry (DSC) and dielectric measurements confirmed the existence of a mesophase in both compounds and enabled determination of the temperatures of phase transitions. Wide-angle X-ray scattering (WAXS) measurements and optical observations enabled identification of mesophase types. Nematic phase appeared in both compounds; in the material with longer chains smectic A phase was also detected. Lengths of the molecules and average nearest neighbor distances were estimated using the wide-angle X-ray scattering results. Lengths of the molecules were also computed by quantum mechanical methods resulting in very good agreement with the experimental data.

Liquid crystal epoxy resins based on biphenyl group cured with aromatic amines - studied by dielectric spectroscopy

In this paper we investigate the dielectric response of cured epoxy resins based on biphenyl group. We chose two monomers, which were earlier synthesised, containing the same mesogen but with long and short symmetric tails. They were cured with standard amines (DDM, DMAP). Firstly we optimised conditions of the curing process by dielectric and optical observation in situ. Small differences in the dynamics of the curing process were observed in different mixtures. For the four selected products we made dielectric measurements in a wide range of frequency and temperature. Although, generally, dielectric response is similar in all the cured materials, differences can be seen in the details. At lower temperatures, quite strong relaxation process is seen in all products, its low activation energy points towards a β-process, which appears in many polymers. The Havriliak-Negami formula was fitted to the data and characteristic parameters of the observed relaxation processes were compared in detail. In some cases the α-process also appears. We conclude how the longer spacer and the type of curing process influence the electrical properties of the resin.

Liquid crystalline polymer networks based on a nematic epoxy resin with azoxy group

The paper presents research results on curing two recently synthesized liquid crystalline epoxy materials with selected amines. The process of cross-linking, the final product of curing, and the pure monomers were examined using polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and dielectric spectroscopy (DS). Chemical structure of the products was confirmed using spectroscopic methods. The authors attempted to demonstrate how selection of curing conditions (such as the amine used as curing agent, the curing temperature or preparation of the surface in contact with the sample) influences optical properties of the cured product.

Dielectric relaxations in new liquid crystalline diepoxy monomer

A series of new liquid crystalline divinyl and diepoxy monomers of different length of stiff segments, were synthesized. Relaxation phenomena in one of these compounds were investigated at different temperatures using Solartron 1260 Impedance Analyzer with Chelsea Dielectric Interface in the frequency range 10-2 Hz - 1 MHz. The liquid crystalline material was introduced between two parallel glass plates covered with conducting ITO layer and polyimide thin film with anti-parallel rubbing. The microscope observations have revealed a planar texture in this kind of sample. In order to find the influence of the polyimide thin film on the texture the investigations of a sample with out the polyimide film were also carried out. In this case the homeotropic structure was detected at zero external electric field. Three loss peaks were detected in the investigated frequency range. The dielectric response is sensitive to the phase transitions observed in this material. The temperatures of the phase transitions determined from the dielectric study are in a reasonable agreement with those obtained from thermo-optical analysis.

Quantitative analysis of the polymeric blends

A method of quantitative analysis of polypropylene/linear low density polyethylene blends was determined by using differential scanning calorimetry. The samples were prepared by means of extrusion process. The method of quantitative analysis was based on the principle that the melting enthalpies of the components in the mixture are proportional to their amount, but it has been found that the presence of two polymers in the blend has influence on the crystallization of its components. Previous studies seemed to neglect this phenomenon, so a linear correlation allowing to eliminate discrepancies between calculated and actual quantitative blend composition has been developed. This approach was proven to be more accurate than earlier directly proportional enthalpy—quantity dependence.

Optical Kerr effect in nematic doped with azo-benzene functionalized POSS nanoparticles

In this work, two novel azo-benzene substituted polyhedral oligomeric silsesquioxanes (POSS) compounds having m =4 and m = 10 in order to check their nonlinear optical properties when dissolved in common nematic mixture E-7 were synthesized. The method of optically induced birefringence of LC or so called Optical Kerr Effect (OKE) was used. The light-induced birefringence in nematic liquid crystals can be treated as a third order nonlinear optical effect because of changes of refractive index of the form: n = no + n2I where nonlinear optical coefficient n2 is proportional to the cubic optical susceptibility. Excitation within absorption n-π* band of azo-group was able to induce a cycle of trans-cis photoisomerisations leading to inducement of rearrangement of nematic molecules forming a layer in a homogenic LC cell. The results of this work prove that nematic LC doped with POSS based azo-functionalized nanoparticles is a reliable system for dynamic all-optical switching as well as can found application in a real-time holography.