Henryk Galina

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.

Mean-Field Kinetic Modeling of Polymerization: The Smoluchowski Coagulation Equation

The Smoluchowski coagulation equation derived back in 1916 is usually linked with the diffusivity and size of aggregating particles. It can also be used as a versatile tool for mean-field kinetic analyses. In this paper it is shown to be an efficient tool for studying the relationships between the reactivity of functional groups in monomers and the size distribution of polymer species in step and chain growth polymerizations. The Smoluchowski coagulation equation and its modifications are applied as models of kinetically controlled growth reactions (with irreversible elementary reaction steps). Advantages and limitations of this method of modeling polymerization processes are discussed.

Kinetic Models of the Polymerization of an AB2 Monomer

Two mean-field kinetic models of the polymerization of an AB 2 monomer that lead to hyperbranched polymers are presented. The first model deals with oranched molecules only and constitutes a new yersion of the Flory theory. Its modification, which takes into account ring formation, is the second model. Preliminary results of the application of the models are shown and discussed.

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.

Methods of narrowing the molecular size distribution in hyperbranched polymerization involving AB2 and B2 monomers

A kinetics model was developed describing the time evolution of the molecular size distribution in hyperbranched condensation polymerization of monomers AB2 and B2 (A and B are functional groups). The rate equations derived from the model were used to calculate moments of the distribution of polymer species. The calculation procedure was adjusted to simulate introduction of fresh AB2 monomer into the polymerization reactor in equal portions, with each portion introduced after the previous one reached a predetermined conversion degree. The core monomer B2 was introduced with the first portion. The calculation procedure also made it possible to subtract unreacted monomer from the system. This was yet another method of narrowing the molecular size distribution of polymers in addition to the feed scheme. Dedicated to Professor John L. Stanford on the occasion of his 60th birthday.

A three parameter kinetic model of formation of linear polyurethane from 2,4-toluenediisocyanate and butane-1,4-diol

A model of linear isothermal polymerization of two bi-functional monomers, one of which has alike functional groups of different reactivities, is presented. The model has been applied to the polymerization of 2,4-toluenediisocyanate (TDI) a and butane-1,4-diol carried out in solution at 86 or 101°C. The rate constant K of the reaction between an isocyanate group in position 4 of TDI and a hydroxy group, the ratio κ of reactivities of groups in position 4 relative to that in position 2, and the ratio k Φ of reactivities of an isocyanate group in the monomer relative to that at the end of an oligomer chain, have been evaluated from experimental data to be 6.51.10 -4 dm 3 .mol -1 .s -1 , 1.47, and 1.55 at 86°C, and 17.2.10 -4 dm 3 .mol -1 .s -1 , 1.55, and 1.62 at 101°C, respectively.

1H NMR STUDY OF (HYDROXYMETHYL)MELAMINE REARRANGEMENT IN DMSO SOLUTIONS

(Hydroxymethyl)melamines (HMMs) are readily obtained by dissolving melamine in formalin at pH 8-9. They are widely used as precursors of melamine-form-aldehyde resins belonging to an important group of plastics. There are nine HMM isomers possible, with six hydrogen atoms in amino groups of melamine available for substitution. In aqueous solution they form an equilibrium mixture the composition of which was the subject of both theoretical and experimental studies. Each bis-, tris-, and tetrakis(hydroxymethyl)melamine consists of two constituent isomers. The isomers differ in the way the hydroxy-methyl groups are distributed among amino groups.

Phase transitions and molecular properties of new divinyl and diepoxy compounds

The aim of this paper is to compare some physical properties of six new divinyl and diepoxy compounds. The compounds differ in the length of the rigid central segments. Optical observations and differential scanning calorimetry confirmed the existence of a mesophase in five compounds and enabled the determination of the phase transition temperatures. The wide angle X-ray scattering (WAXS) measurements of samples aligned in a magnetic field and the optical observations allowed the identification of the mesophase types. In four compounds a nematic mesophase was detected during both heating and cooling. In one case two higher ordered phases were observed during cooling. The analysis of WAXS images and the positions and shapes of the scattering intensity peaks suggests a smectic B–crystal E transition. Using the data obtained in WAXS measurements carried out in the nematic phase, the lengths of the molecules and the average nearest neighbour distances were estimated. The average intermolecular distances and the layer spacings were also estimated for the SmB and E phases. The lengths of the molecules were also calculated by means of theoretical ab initio methods. The results obtained are in very good agreement with the experimental data.

An algorithm for Monte Carlo modeling of degradation of polymer networks

Abstract An algorithm for Monte Carlo simulation of polymer network degradation has been devised. The object was a random nearly regular molecular graph with almost all vertices of degree three. The vertices represented units of the polymer and edges were bonds or subchains of the network. The algorithm was particularly designed to study the effect of the substitution degree of units on the degradation process. The abrupt change of molecular parameters observed at a certain extent of degradation was referred to as the gel–sol transition. The position of the transition depended on the rate constants modeling the effect of units substitution.

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.