G. A. Oweimreen

The effect of quasispherical solutes on the nematic to isotropic transition in liquid crystals

When nonmesomorphic solutes are added to nematic liquid crystals, the nematic–isotropic transition temperature is depressed and a two‐phase region is observed. The slopes, βn and βi, of the nematic and isotropic phase boundary lines in the reduced nematic–isotropic transition temperature (T*) vs solute mole fraction (x) diagrams are reported for binary mixtures of quasispherical solutes with nematic liquid crystals. The systems studied are Me4Sn, Et4C, Et4Sn, Pr4Sn, Bu4Pb, and Bu4Sn (Me = methyl, Et = ethyl, Pr = propyl, and Bu = butyl), mixed with p‐methoxybenzylidene‐p′‐n‐butylaniline (MBBA), and CCl4 mixed with p‐p′‐di‐n‐hexyloxyazoxybenzene (DHAB). Also reported are infinite dilution solute activity coefficients for several mixtures involving Et4C and CCl4. The experimental results are compared with predicted values from statistical–mechanical theories based on (a) a lattice model of hard rectangular parallelepipeds and cubes, (b) a virial expansion treatment of hard spherocylinders and spheres, and (c) a proposed modification of a mean field treatment (Maier–Saupe type) of axially symmetric and spherical molecules. All three models correctly predict the observed trend of increasing βn and βi with increasing solute size and yield predicted slopes which are at least within a factor of 2 of the experimental slopes. The modified mean field approach is found to give the best agreement with the activity coefficient data. A simple experimental method is proposed for estimating the effect of potential impurities (x?0.01) on the nematic–isotropic transition, and possible directions for future research in this area are suggested.When nonmesomorphic solutes are added to nematic liquid crystals, the nematic–isotropic transition temperature is depressed and a two‐phase region is observed. The slopes, βn and βi, of the nematic and isotropic phase boundary lines in the reduced nematic–isotropic transition temperature (T*) vs solute mole fraction (x) diagrams are reported for binary mixtures of quasispherical solutes with nematic liquid crystals. The systems studied are Me4Sn, Et4C, Et4Sn, Pr4Sn, Bu4Pb, and Bu4Sn (Me = methyl, Et = ethyl, Pr = propyl, and Bu = butyl), mixed with p‐methoxybenzylidene‐p′‐n‐butylaniline (MBBA), and CCl4 mixed with p‐p′‐di‐n‐hexyloxyazoxybenzene (DHAB). Also reported are infinite dilution solute activity coefficients for several mixtures involving Et4C and CCl4. The experimental results are compared with predicted values from statistical–mechanical theories based on (a) a lattice model of hard rectangular parallelepipeds and cubes, (b) a virial expansion treatment of hard spherocylinders and spheres, and (...

DSC studies on p-(n-alkyl)-p′-cyanobiphenyl (RCB’s) and p-(n-alkoxy)-p′-cyanobiphenyl (ROCB’s) liquid crystals

Abstract Solidification (supercooled nematic-to-solid or supercooled smectic- A -to-solid), melting (solid-to-nematic or solid-to-smectic- A ), smectic- A -to-nematic, and clearing (nematic-to-isotropic or smectic- A -to-isotropic) transition temperatures ϑ s , ϑ m , ϑ AN , and ϑ c , respectively, and enthalpies Δ H s , Δ H m , Δ H AN , and Δ H c , respectively, have been determined by differential scanning calorimetry (DSC) for members of the p -( n -alkyl)- p ′-cyanobiphenyl (RCB) and the p -( n -alkoxy)- p ′-cyanobiphenyl (ROCB) homologous series of liquid crystals, where ‘ R ’ is the number of carbon atoms in the n -alkyl or n -alkyloxy tails. The study shows that the transition of the less ordered nematic phase to the more ordered smectic- A phase, in liquid crystals that exhibit both phases (those with R =8 or 9), involves a transient phase of intermediate order. Broad or overlapping solidification exotherms indicate that two mesogenic aggregations supercool simultaneously. Finally, there is evidence for a transition between two solid forms that occurs at temperatures preceding ϑ m or concurrent with it depending on ‘ R ’ and the heating rate. This refutes an earlier view that during melting the solid form with the higher melting point solidifies.

The effect of quasispherical and chainlike solutes on the nematic to isotropic phase transition in liquid crystals

The effects of solute molecular structure (size, shape and flexibility) and solvent molecular structure (length‐to‐breadth ratio and end‐chain flexibility) on nematic phase stability in dilute binary mixtures of nonmesomorphic solutes and nematogenic solvents are examined through experiment and theory. Addition of the perturbing solute to the liquid‐crystalline solvent leads to depression of the nematic–isotropic (NI) transition temperature and formation of a two‐phase region. Directly determined moduli of the slopes, βn and βi, of the nematic and isotropic phase boundary lines in the reduced NI transition temperature (T*) vs solute mole fraction (x2) diagrams are reported for quasispherical and chainlike solutes with two nematogenic solvents. The systems studied are the quasispheres Et4C (tetraethylmethane) and R4Sn (R = CH3, C2H5, n‐C3H7 and n‐C4H9) and the chains n‐C8H18 through n‐C14H30, mixed with p‐methoxybenzylidine‐p′‐n‐butylaniline (MBBA) and p‐n‐pentyl‐p′‐cyanobiphenyl (5CB). Also reported are i...

DSC studies on p-cyanophenyl p-(n-alkyl)benzoate liquid crystals : evidence for polymorphism and conformational change

Temperatures and enthalpy changes are reported for the melting, clearing, and nematic-to-solid transition of the p-cyanophenyl p-(n-alkyl)benzoate (RCBz) liquid crystals. The results reveal that several solid structures exist for the RCBzs. Evidence that the transitions between the various solid forms involve conformational changes is provided by exotherms and endotherms appearing in heating and cooling cycles, respectively. The study also demonstrates that as long as melted RCBzs are not allowed to solidify they undergo repeated, reversible nematic-to-isotropic and isotropic-to-nematic transitions making them suitable for use in liquid crystal displays (LCD).

The depletion of dibenzyl disulfide from a mineral transformer insulating oil

This study examines the kinetics of the net depletion of dibenzyl disulfide (DBDS) from its solution in mineral insulating oil due to copper corrosion at elevated temperatures in power transformers. The concentration of DBDS is determined using liquid-liquid extraction followed by GC detection. Measurements, at 24 h intervals, up to 168 h, show that the depletion of DBDS from its solutions in mineral insulating oil at 150 °C and initial concentrations of 3.00, 6.00, 9.00, 12.00 and 15.00 mg L-1 is very slight compared to identical measurements in the presence of copper strips in the mineral oil. The analysis of the kinetic data reflects the complexity of the corrosion process and its occurrence at a concentration as low as 1 mg L-1 which is much lower than what has been previously believed. Measurements on solutions at initial DBDS concentrations of 150 mg L-1 indicate that even when the insulation oil matrix is in contact with copper strips the corrosion does not occur to any measurable extent below 100 °C and starts to speed up appreciably only beyond 125 °C. From data at 100, 125 and 150 °C an energy of activation equal to 123 kJ mol-1 is estimated.

Thermodynamics of solution of non-mesomorphic solutes at infinite dilution in the smectic-A, nematic and isotropic phases of p-n-octyl-p'-cyanobiphenyl : a gas-liquid chromatographic study

Abstract From specific retention volumes measured at four temperatures in each of the three fluid phases of p-n-octyl-p′-cyanobiphenyl (8CB), infinite-dilution solute activity coefficients, partial molar excess enthalpies and entropies, and enthalpies and entropies of solution were determined for 21 solutes varying in molecular size, shape and flexibility. The reported results and their trends are discussed in terms of a previously proposed solution model. The results for 8CB are compared with those for the lower alkyl homologs in this liquid-crystalline solvent series, 5CB, 6CB and 7CB. The thermodynamic behavior within each of five solute groups is interpreted.

Assessment of dibenzyl disulfide and other oxidation inhibitors in transformer mineral oils

In recent years some unexpected operational failures of power transformers were attributed to the presence of the undeclared dibenzyl disulfide (DBDS) antioxidant in mineral insulating oils. This study examines the characteristics of DBDS and other antioxidants dissolved in a mineral oil. Acidities as neutralization numbers, interfacial tension values and depletion rates were obtained at 100°C for solutions of the antioxidants 2,6-di-tert-butyl-p-cresol (DBPC), 2,6-di-tert-butyl-phenol (DBP), dibenzyl disulfide (DBDS), 2-tert-butyl-p-cresol (2-t-BPC), Nphenyl- 1-naphthylamine, 1,2,3-Benzotriazol (BTA) and methylated-BTA in transformer mineral oil that have been exposed to an accelerated aging process involving flow of oxygen through them at a rate of 1.0 L h-1 over intervals of 24, 48, 72, 96 and 120 hours. The results show that the excellent antioxidant characteristics obtained for DBDS are lost when a copper plate is placed in the mineral oil sample. This is attributable to sulfur corrosion. The results also suggest that DBPC is most suited for use as an antioxidant in transformer mineral oil with DBP a close second. The higher depletion rates encountered for 2-t-BPC and N-phenyl-1- naphthylamine make them less suitable antioxidants. The lower effectiveness of 2-t-BPC as an antioxidant relative to DBPC and DBP may be attributed to the lesser steric hindrance of its - OH group. The results for BTA and methylated-BTA and their higher depletion rates indicate that they would not be suitable antioxidants.

Evidence of tilt angle in an 8CB-10CB mixture

Abstract EPR spectra using perdeuterated 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxy as spin probe on a mixture of p - n -octyl- p ′ -cyanobiphenyl and p - n -decyl- p ′ -cyanobiphenyl at a 0.30106 mole fraction for p - n -decyl- p ′ -cyanobiphenyl revealed the existence of a tilt angle that has been attributed to the longer p - n -decyl- p ′ -cyanobiphenyl molecules. Temperature control to within ±0.01°C revealed the existence of two-phase regions across the smectic A-to-nematic and the nematic-to-isotropic transitions which point to first-order transitions.

Thermodynamic data for nonmesomorphic solutes at infinite dilution in the nematic and isotropic phases of hexylcyanobiphenyl

AbstractInfinite dilution solute activity coefficients γo2, partial molar excess enthalpies