Dimitris Tsiourvas

Liquid crystalline behaviour of hydrogen bonded complexes of a non-mesogenic anil with p-n-alkoxybenzoic acids

Phase diagrams of binary mixtures of the non-mesogenic N -(p -methoxy- o -hydroxybenzylidbe ene)- p -aminopyridine with a series of p - n -alkoxybenzoic acids ranging from methoxy to hexadecyloxy were established using differential scanning calorimetry and polarising optical microscopy. The key results obtained are: (1) the formation of 1 1 hydrogen bonded complexes between the pyridine derivative and the alkoxybenzoic acids, (2) the stability of the alkoxybenzoic acid mesophases over a wide range of compositions (up to slightly over 50 mol% of the pyridine derivative), (3) the absence of additional mesophases corresponding specifically to the 1 1 complexes, and (4) the complete miscibility of the acids with the complexes in the mesomorphic state. With alkoxy chains from methoxy to heptyloxy, mixtures produce only nematic phases; they produce both nematic and smectic phases with chains from octyloxy to dodecyloxy, and only smectic phases with chains from tetradecyloxy to hexadecyloxy. The formation of hyd...

Thermotropic liquid crystalline behaviour of piperazinium and homopiperazinium alkylsulphates

A series of 1,4-piperazinium di-n-alkyl sulphates was synthesized and compared with an analogous series of 1,5-homopiperazinium di-n-alkyl sulphates. Their thermotropic liquid crystalline behaviour was studied by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. For the piperazinium salts, two ordered smectic phases were established, in which the lateral packing of the molecules within the layers is rectangular or tetragonal. Both phases are characterized by an alternate periodic packing of the positive and negative ionic groups in the polar sublayer, while the lipophilic sublayers of the alkyl chains are in a disordered conformation. Substitution of the piperazinium by the less symmetrical homopiperazinium group disfavours organization within the smectic layer and as a result smectic A phases were obtained.

Cubic and columnar thermotropic mesophases of potassium dialkylphosphate salts

Long chain potassium dialkylphosphate salts, from dioctyl to dioctadecyl, were synthesized and their thermal stability was checked by thermogravimetry. Their thermotropic liquid crystalline behaviour was investigated using differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. Crystalline at room temperature, they were all found to exhibit upon heating a Ia3d body-centred cubic phase followed by a columnar mesophase of hexagonal symmetry. Structures and structural parameters are discussed.

Thermotropic liquid crystals from alkali metal dihexadecylphosphates

Lithium, sodium, potassium, rubidium, and caesium dihexadecylphosphates were synthesized. Their thermal stability was checked by thermogravimetry. Their ability to show thermotropic mesomorphic behaviour was investigated using differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. All these compounds were found to exhibit a lamellar structure in the crystalline state at room temperature and a columnar structure of hexagonal symmetry in the mesomorphic state at high temperature. The potassium, rubidium, and caesium derivatives were found to exhibit an additional Ia 3 d body-centred cubic structure in the temperature range between the crystal and the columnar phase. Structures and structural parameters are briefly discussed.

SMECTIC LIQUID CRYSTALLINE CHARACTER OF N-ALKYLAMMONIUM PYROGLUTAMATES

A series of n-alkylammonium and n-dialkylammonium pyroglutamates were synthesized and characterized. Their thermotropic liquid crystal behaviour was investigated using differential scanning calorimetry and optical microscopy. The structures of their crystal and smectic A phases were studied by X-ray diffraction.

Vibrational Spectroscopy of Certain Polymaleic- and Polyacrylic-Based Mesomorphic Polymers

The FT-IR/ATR technique has been employed in the investigation of the forces leading to the organization of a number of polymeric materials exhibiting thermotropic liquid crystalline character. Two types of compounds have been studied, i.e., one bearing the carboxylic polar group near the main chain and the other at the end of the side chain. Intermolecular and intramolecular hydrogen bonding between the carboxylic groups was found to lead to the formation of “facing” and “sideways” structures. Enhanced stability of these supramolecular structures was obtained by the presence of intramolecular hydrogen bonds between the amide groups, resulting in the formation of smectic phases. Furthermore, it has been found that the strengthening of hydrogen bonding between carboxylic groups is associated with a weakening of hydrogen bonds between the amide groups.

Comparative Experimental and Computational Study of Monoalkyl Chain Phosphatidylcholine-Containing Thermoresponsive Liposomes.

Liposomes containing lysophospholipids are intensively studied as drug delivery systems that are stable at normal body temperature but exhibit fast release of their drug load at slightly elevated temperatures. In this study, the stability and release properties of dipalmitoylglycerophosphocholine (DPPC)-based liposomes incorporating the commonly used lysophosphatidylocholine (lyso-PC), and a series of monoalkyl chain ether-linked phosphatidylcholine, i.e., the biologically relevant monoalkyl chain platelet activating factor (PAF) and its derivatives lyso-PAF and methyl-PAF, were investigated. To this end a series of PEGylated small unilamellar liposomes with DPPC:monoalkyl lipid compositions of 5% and 10% molar ratio were prepared and compared with regard to stability (37 °C) and release properties at elevated temperatures (38-43 °C). All systems were characterized with respect to size distribution, ζ-potential, and phase transition characteristics. The presence of ether-lipids endows liposomes with superior (∼10% increase) release properties at 5% incorporation compared to lyso-PC, while at 10% molar ratio the formulations do not differ significantly, the release being close to 90%. The findings are supported by atomistic molecular dynamics simulations that suggest a correlation between the enhanced permeability and increased penetration of water molecules within the bilayers with density fluctuations resulting from the increased area-per-lipid and the disorder of the lysolipids alkyl chains.

Dilatometric studies of liquid crystalline sodium and rubidium dihexadecylphosphate

Abstract The thermotropic liquid crystal polymorphism of sodium and rubidium dihexadecylphosphates, previously investigated by polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction, was further investigated using dilatometry. Their molar volumes and thermal expansion coefficients in the cubic and columnar mesomorphic state were measured.