Edward M. Barrall

Some Flow Characteristics of Mesophase Types

Characteristic flow properties for each of the three nominal mesophase types have been empirically established. Results are given for the nematic, smectic, and cholesteric types of mesophase. Discussion is centered on mesophases or liquid crystals exhibited by pure compounds within defined temperature intervals between the regular solid and the isotropic liquid. For nematic mesophases, the prominent flow orientation, consequent low viscosity, and the unusual flow behavior near the nematic—isotropic transition are discussed. The remarkably high and shear‐dependent viscosities of cholesteric mesophases are contrasted to the simple flow properties of these cholesterol derivatives at temperatures for their respective isotropic liquids. Descriptions of flow properties for smectic mesophases are based essentially upon data on a single compound, ethyl‐p‐azoxybenzoate. Viscosity data on this compound from several sources are intercompared for the first time. Some of the new empirical conclusions given here are at variance with those reported previously in several reviews.

Evaluation of Thermal Transitions in Some Cholesteryl Esters of Saturated Aliphatic Acids

Abstract The heats and temperatures of transition for fourteen cholesteryl esters of saturated aliphatic acids have been evaluated using a Perkin-Elmer Differential Scanning Calorimeter, DSC-IB. Most esters tested were first recrystallized from n-pentyl alchol. Among several recrystallization solvents tested, n-pentyl alchol was most effective in removing impurities and eliminating ester transitions caused by these impurities. Ester purity was determined by a recently reported technique which is based on an analysis of the complete DSC recorder scan for transitions. The transition data in this study indicate that the lowest molecular weight saturated aliphatic ester of cholesterol to show mesophase behavior is the propionate. For the formate or acetate less pure samples may be supercooled in the melt so that mesophases may be observed. New transition data for the odd esters of cholesterol from undecanoate to nonadecanoate have also been obtained. Cholesteryl eicosanoate was examined and found to have only...

Gas chromatography using cholesteryl ester liquid phases.

Abstract The acetate, n -valerate, and n -nonanoate esters of cholesteroal, when distributed in the form of pure compounds as liquid phases on a gas chromatographic packing, exhibit anomalous log elution time−temperature relationships. Sharp changes in the elution relationship for benzene, toluene, ethylbenzene, n -octane, and n −decane are noted at or near previously reported liquid crystal or mesophase transition temperatures. The elution times increrase sharply, as much as 3.5 times, in the transition from the cholesteric mesophase to the isotropic liquid. This represents an anomalous elution time increase with increasing temperature. Near mesophase transitions, the relationship between elution time and eluant boiling point also exhibits an anomaly. Aromatic compounds exhibit shorter elution times than those for corresponding aliphatic materials of equal carbon number. When a given mesophase is supercooled, the solute elution characteristics lie on a curve which extrapolates from the data derived within the normal mesophase range. The gas chromatographic method appears to be an excellent and general determination for liquid crystal transition temperatures. The unique temperature behavior of the cholesteryl esters also gives them some importance as liquid phases in analytical gas chromatography.

Interpretation of Mesophase Transitions

New data have provided a means for quantitatively testing the statistical theory of Frenkel for explaining the pretransitional effects adjacent to first‐order transitions in mesophase systems. This theory has been frequently suggested as a qualitative explanation for pretransitional changes in physical properties that occur within 10°C of mesophase–isotropic transitions. The one previous quantitative test used compressibility and volume expansion data on p‐azoxyanisole, which forms a single mesophase of the nematic type. It was concluded that the Frankel theory was of limited applicability; that (a) the equations did not apply closely, within about l°C of the transition, and (b) did not apply to pretransition effects below the transition. Calculations made here using new data on p‐azoxyanisole show, in contrast, that the Frenkel theory (a) applies well near the nematic–isotropic transition, (b) the theory may be applied equally well on both sides of the transition, (c) that asymmetrical pretransitional ef...

The Polymorphism of Cholesteryl Esters: Differential Thermal and Microscopic Measurements on Cholesteryl Myristate

Abstract The techniques of differential thermal analysis and photomicroscopy have been combined to eludicate the complex phase relationships and crystallography of cholesteryl myristate. Also, the amount of polarized light rotated by the samples as a function of temperature has been measured at a fixed sample thickness. Cholesteryl myristate exhibits a complex liquid crystal mesophase system on heating. In addition to the phase transformations obtainable on heating, photomicrographs were obtained of the phases which result on subjecting the smectic and cholesteric mesophases to a very low shear. Excellent correlation was obtained between the present work and earlier studies of the myristate ester. The combined instrumental and microscopic method appears to be a valuable tool in the study of subtle phase changes in complex systems.

The effect of purity on the thermodynamic properties of cholesteryl heptadecanoate

Abstract The thermodynamic properties of two samples of cholesteryl heptadecanoate have been measured and contrasted. As little as 2% impurity (presumably oxidized cholesterol and free acid) profoundly effects all of the thermodynamic properties. The mesophases are the most sensitive to alteration and distortion by impurities. A transition diagram has been developed for this ester.

Composition analysis of polybutenes

Abstract The structure and composition of polyolefins made from pure isobutylene and from isobutylene diluted with various amouts of other light olefins have been investigated. The techniques used were nuclear magnetic resonance and combined anaerobic pyrolysis and gas chromatography. Nuclear magnetic resonance gave a measure of molecular weight and of isobutylene polymerized into the polymers. Pyrolysis chromatograms of pure polyisobutylene indicated a broad range of volatile hydrocarbon products, principal of which were propylene and isobutylene. For three mixed olefin polymers, the yields of propylene and isobutylene were directly proportional to the amount of isobutylene polymerized into the copolymer. Each polymer composition exhibited a pyrolysis temperature for maximum isobutylene yield. This temperature increased with the isobutylene content of the polymer. Pyrolysis chromatograms were found to be virtually identical for a molecular weight series of pure polyisobutylenes. Fractionated olefin copolymers did show a distinct increase in isobutylene production from pyrolysis with increasing molecular weight. This is because the molecular weight of butene copolymers generally increases with the isobutylene content of the copolymerized mixture.

Depolarized Light Intensity and Optical Microscopy of some Mesophase-forming Materials

Abstract Eight mesophase-forming compounds were studied by the technique of depolarized light analysis. The temperatures at which anisotropy changes occurred were compared with results from differential thermal analysis. Although anisotropy change and uptake of heat are usually synchronous, in some cases thermal uptake is almost complete prior to a significant change in the anisotropic behavior of the compound. The observations made explain many of the discrepancies between mesophase transition temperatures observed by differential thermal analysis and optical methods. Photomicrographs of the various mesophase types examined are presented.

A depolarized light intensity apparatus for use with differential thermal analysis

Abstract The technique of depolarized light intensity (DLI) measurement yields useful information on polymer melting points, crystallinity, and rate of crystallization. Most of the DLI instruments described to date have been separate, complete units. This paper describes an apparatus to function as an auxiliary module with a differential thermal analyzer. Thus, the differential thermal analyzer provides the necessary temperature programmer, amplifier, and temperature and signal recorder. The necessary auxiliary equipment to obtain DLI measurements is relatively simple and convenient to operate.

The effect of temperature gradient on the chromatographic fractionation of polyisobutene

Abstract The advantage as proposed by Baker and Williams of the application of a temperature gradient in addition to a solvent gradient in the column chromatography of polymers has been questioned. In the present work, fractionations were carried out (a) using a 30–60°C temperature gradient, (b) isothermally at 30°C, and (c) isothermally at 60°C on identical polyisobutylene samples. The molecular weight distributions of the fractions collected from a , b , and c were evaluated by gel permeation chromatography. The temperature gradient produced a system which was no more efficient than the isothermal 30°C condition. A small improvement in efficiency was noted with the temperature gradient at low molecular weights. The isothermal fractionation at 60°C was the least efficient of the three conditions studied. For the specific polymer solvent system used in the present study, the temperature gradient appears to have little effect on column resolution.