High strength disclinations in a rigid rod nematic polytolane

Transient disclinations of strength ± 3/2 were observed in the nematic phase of a single-component, thermotropic, rigid rod polytolane. These disclinations were characterized by 6 extinction brushes in textures observed by hot stage transmitted polarized light microscopy. The small number and short lifetime of these disclinations is discussed in terms of their energy per unit line length, and in terms of the force that causes them to be attracted to and annihilated at other disclinations.     

High strength disclinations in the nematic and smectic states of 2,5-bis(4-alkoxybenzoyloxy)- p -benzoquinones

High strength disclinations were observed in both nematic and smectic phases exhibited by members of the 2,5-bis(4-alkoxybenzoyloxy)- p -benzoquinone series, beyond 4- n -pentoxy. The shorter members of the series show only a nematic mesophase, the highest members only a smectic mesophase, while intermediate members exhibit both types of mesophase. These high strength disclinations seen in a single component system are very stable due to energy dispersion and the high viscosity of the smectic state. All the high strength disclinations are negative in sign.     

Recent advances in the observation of disclinations of main-chain nematic polymers

Experimental observation of disclinations of main-chain nematic polymers is reviewed. The geometry of disclination structure is discussed both from observations of schlieren texture and the theory of curvature elasticity for a two-dimensional case with equal splay and bend moduli. The effects of non-equal elastic moduli, the nature of the disclination core and the observation of inversion wails are briefly mentioned. Examples of experimental observation of disclinations of high topological strengths up to S=4 are given. The main experimental methods to map the director field around disclinations are 1) lamellar decoration, 2) band-texture decoration and 3) surface microcrack decoration. The methods are discussed and compared, and examples illustrated by electron micrographs or photomicrographs are presented.     

Supramolecular assemblies of gold nanoparticles induced by hydrogen bond interactions

The formation of two-dimensional self-assemblies of ordered, packed gold nanoparticles is described. The nanoparticles were coated by a diamide thiol ligand (DASH) that ensured the connection of the nanoparticles through hydrogen bonds. The stability of the colloidal solutions is discussed with respect to the stability of the normal dodecanethiol-coated particles.     

Electroconvective dendrites in nematic phases having short range smectic order exhibited by the 4,n-alkoxybenzoic acids

Electroconvective (EC) dendrites (parabolic type) in nematic (N) liquid crystals with short range smectic order shown by 4,n-octyloxybenzoic acid (8-OBA) were observed. The driving of the dendrites by lateral d.c. and a.c. electric field parameters in the nematic temperature range was carried out. The dynamics of dendrite growth were studied and analysed by the Ivantsov two-dimensional solution for the diffusion equation. Using various combinations of blocked and unblocked electrodes, we studied dependences between the dimensionless electric field parameter and product of the dendrite parabolic radius and the velocity of the dendrite growth and found that the type of electrode significantly modifies these basic dependences. We attribute this effect possibly to the opposing ion propagation, i.e. gradient of electric field in direction opposite to that of the charge injection. A possible mechanism of EC dendrite growth is suggested. A comparison with thermal dendrites in liquid crystals is presented.     

Calorimetric study of the dissociation of the 3-carboxy pyridinium ion. The effect of the hydrogen bond on the base strength

A calorimetric study of the protonation of 3-carboxy pyridine is presented.The solute—solvent interactions (hydrogen bonds and dipoles), more so than the effects intrinsic to the molecule (the greater degree of s character in the hybrid molecular orbital containing the lone pair), seems to have the main influence on the protonation of pyridine in the liquid phase.     

Influence of intramolecular hydrogen bond strength on OH-stretching overtones

Vapor-phase OH-stretching overtone spectra of 1,3-propanediol and 1,4-butanediol were recorded and compared to the spectra of ethylene glycol to investigate the effect of increased intramolecular hydrogen bond strength on OH-stretching overtone transitions. The spectra were recorded with laser photoacoustic spectroscopy in the second and third OH-stretching overtone regions. The room-temperature spectra of each molecule are dominated by two conformers that show intramolecular hydrogen bonding. Anharmonic oscillator local-mode calculations of the OH-stretching transitions have been performed to aid assignment of the different conformers in the spectra and to illustrate the effect of the intramolecular hydrogen bonding. The hydrogen bond strength increases in the order ethylene glycol, 1,3-propanediol, and 1,4-butanediol. The overtone transitions of the hydrogen-bonded hydroxyl groups are more difficult to observe with increasing intramolecular hydrogen bond strength. We suggest that the bandwidth of these transitions increases with increasing hydrogen bond strength and with increasing overtone and furthermore that these changes are in part responsible for the lack of observed overtone spectra for complexes.     

Estimation of O-H bond dissociation energies in alcohols and acids from kinetic data

The O-H bond dissociation energies (D _O-H) in five alcohols and six acids have been determined from experimental data (rate constants of radical reactions). The ratio of the rate constants of the reactions R¹O˙+RH→R¹OH+R˙ and R ⁱ O˙+RH→R ⁱ OH+R˙ and the intersecting parabolas method are used in the estimation procedure. The D _O-H values are used to calculate the activation energies and rate constants for hydrogen abstraction from 2-methylbutane, butene-1, and cumene by alkoxyl and carboxyl radicals. The geometric parameters of the transition state are calculated for these reactions.     

Direct decoration of disclinations by solidification-induced band texture for a nematic side chain liquid crystalline polymer

For a nematic polymethacrylate side chain liquid crystalline polymer, g 154 N 298 I (°C), the solidification-induced band texture has been observed aligned along the disclination under a polarizing optical microscope, when the specimen was quenched from 280°C to room temperature. The decoration technique of solidification-induced band texture, which is usually reported for main chain liquid crystalline polymers, was then introduced to reveal the director field pattern along a disclination for this side chain liquid crystalline polymer. It was found by infra-red dichroism measurements that the director orientation is parallel with the direction of the band. On this basis, disclinations with strength s=±1/2 and s=±1 were mapped according to the corresponding pattern of solidification-induced band texture. In addition, two types of inversion wall, loop-like and splay-type walls, were also found to be decorated by the solidification-induced band texture.     

Relative strength of hydrogen bond interaction in alcohol–water complexes

Hydrogen binding energies are calculated for the different isomers of 1:1 complexes of methanol, ethanol and water using ab initio methods from MP2 to CCSD(T). Zero-point energy vibration and counterpoise corrections are considered and electron correlation effects are analyzed. In methanol–water and ethanol–water the most stable heterodimer is the one where the water plays the role of proton donor. In methanol–ethanol the two isomers have essentially the same energy and no favorite heterodimer could be discerned. The interplay between the relative binding energy is briefly discussed in conjunction with the incomplete mixing of alcohol–water systems.     

Creation of nematic and reentrant nematic phases in bi- and multi-component smectic mixtures

Abstract

Experimental results referring to the transformation of smectic phases, mainly smectic A, into nematic and reentrant nematic phases are reviewed. A new explanation of some experimental results is proposed. Factors which are responsible for the depression of smectic phases in mixtures of polar mesogens are discussed and the possibility of forming mixtures with a broad temperature range of nematic phase from smectic compounds, which can be useful for liquid crystal displays (LCDs), is shown. A nematic gap observed in some cases between monolayer (S_A1) or monolayer and partially bilayer (S_Ad) smectics results from the differences in the organization of the molecules in the smectic layers. It is concluded that polar phase from smectic A₁ phases can be divided into two groups: (a) the first one is characteristic for compounds with the -NCS, -F, -CI, -I or ?COC _m H_{2m + 1} terminal group. The spacing of the smectic layer slowly expands with the increase in alkyl chain length and the structure of the smectic A₁ phase slowly changes to be more like the smectic A_d phase (d/1 > 1). It is proposed that such a smectic is called an enhanced monolayer smectic (S_A1e (b) the second one is typical for compounds with the -CN terminal group. This kind of smectic A₁ phase is rapidly transformed into the smectic A_d phase with increasing alkyl chain length. These latter monolayer mesogens easily form the reentrant nematic phase when they are mixed with other polar smectic mesogens.     

Direct observation of director fields of disclinations in the nematic mesophase of a main chain thermotropic aromatic polyester by surface microcrack decoration

For a main chain thermotropic aromatic polyester with a flexible spacer, Cr 194 N 245 I (°C), the director fields around disclinations in the nematic mesophase can be decorated by both the solidification-induced band texture and surface microcracks. Director fields of various types of disclinations, including inversion walls, in the nematic mesophase of this semi-rigid polyester have been observed directly by polarizing optical microscopy. It was found that when the polymer was pre-sheared in the nematic state and then quenched to room temperature, a shear-induced band texture was observed, which relaxed slowly during annealing at 200°C, and then on quenching the solidification-induced band texture and surface microcracks appeared, displaying the pattern of the disclination fields. Pre-shearing is a necessary condition for the appearance of microcracks. On annealing, disclinations of various types were generated, quite often connected by inversion walls in the direction of pre-shearing. In some areas of the specimen where the shear-induced bands had not been completely relaxed during annealing, the shear-induced band texture was shown by the birefringence and surface microcracks to have a supermolecular structure of sinusoidal chain fibrils.     

Direct observation of director fields of disclinations in the nematic mesophase of a main chain thermotropic aromatic polyester by surface microcrack decoration

For a main chain thermotropic aromatic polyester with a flexible spacer, Cr 194 N 245 I (°C), the director fields around disclinations in the nematic mesophase can be decorated by both the solidification-induced band texture and surface microcracks. Director fields of various types of disclinations, including inversion walls, in the nematic mesophase of this semi-rigid polyester have been observed directly by polarizing optical microscopy. It was found that when the polymer was pre-sheared in the nematic state and then quenched to room temperature, a shear-induced band texture was observed, which relaxed slowly during annealing at 200°C, and then on quenching the solidification-induced band texture and surface microcracks appeared, displaying the pattern of the disclination fields. Pre-shearing is a necessary condition for the appearance of microcracks. On annealing, disclinations of various types were generated, quite often connected by inversion walls in the direction of pre-shearing. In some areas of the specimen where the shear-induced bands had not been completely relaxed during annealing, the shear-induced band texture was shown by the birefringence and surface microcracks to have a supermolecular structure of sinusoidal chain fibrils.     

Conformational studies by nuclear magnetic resonance in nematic phases: Benzyl chloride

Spectra of benzyl chloride were analysed in two nematic phases (PCH and ZLI). The existence of the molecule as conformer with the C? Cl bond perpendicular or parallel to the benzene plane can be safely excluded. The data are consistent with a conformer with the C? Cl bond forming an angle of 60° with the benzene plane. A full discussion of the conformational problem was not possible owing to the small number of independent D_ij couplings related to the internal motion. In addition, when dealing with low potential barriers, the LCNMR technique does not give rigorous answers.     

Intermolecular hydrogen bond complexes by in situ charge transfer complexation of o-tolidine with picric and chloranilic acids

A two new charge transfer complexes formed from the interactions between o-tolidine (o-TOL) and picric (PA) or chloranilic (CA) acids, with the compositions, [(o-TOL)(PA)(2)] and [(o-TOL)(CA)(2)] have been prepared. The (13)C NMR, (1)H NMR, (1)H-Cosy, and IR show that the charge-transfer chelation occurs via the formation of chain structures O-H?N intermolecular hydrogen bond between 2NH(2) groups of o-TOL molecule and OH group in each PA or CA units. Photometric titration measurements concerning the two reactions in methanol were performed and the measurements show that the donor-acceptor molar ratio was found to be 1:2 using the modified Benesi-Hildebrand equation. The spectroscopic data were discussed in terms of formation constant, molar extinction coefficient, oscillator strength, dipole moment, standard free energy, and ionization potential. Thermal behavior of both charge transfer complexes showed that the complexes were more stable than their parents. The thermodynamic parameters were estimated from the differential thermogravimetric curves. The results indicated that the formation of molecular charge transfer complexes is spontaneous and endothermic.