Julian F. Johnson

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.

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.

Polymer degradation VI—Distribution changes on polyisobutene degradation in laminar flow

Abstract Precisely established polymer molecular weight distribution changes with variations in shear degradation history are extensively interpreted. The systems consist of solutions of polyisobutenes degraded in homogeneous, laminar-flow, shear fields. The polymer solutions studied were 9·6 volume per cent in n-hexadecane and 9·7 volume per cent in 1,2,4-trichlorobenzene. The procedure for preparing equilibrium shear degraded systems is described. A new computer programme is used to derive integral and differential distributions as well as molecular weight averages from number to Z +1. The programme also computed distribution inhomogeneities and standard deviations from the different molecular weight averages. It is found that the standard deviation, derived from number and weight average, changes linearly with weight average molecular weight. A predominantly random mechanism of degradation in laminar flow shear is postulated to explain the results. The efficiency of bond rupture due to storage of shear energy in polymer bonds is shown to be low. Equivalent results were obtained in the two solvents.

Gel Permeation Chromatography with Organic Solvents

Abstract Fractionation of polymers either as a means of determining molecular-weight distributions for the characterization of polymers or the preparation of fractions with defined molecular-weight distributions, for subsequent physical and chemical testing, is a valuable and widely used technique. There are many ways of performing fractionations and of measuring molecular-weight distributions; see, for example, the forthcoming book Polymer Fractionation [1]. Gel permeation chromatography is the newest of the fractionation methods and has already found widespread applications; see, for example, the forthcoming review by Altgelt and Moore [2]. Because of its speed and high resolving power and the possibility of a high degree of automation gel permeation chromatography is the most promising current technique for the fractionation of polymers on an analytical scale and, probably, on a preparative scale.

Recording High Shear Viscometer for Measurement at Shear Rates Near 106 Sec−1

An improved recording rotational high shear viscometer has been developed. The instrument employs concentric steel cylinders with unusually narrow and precisely defined shear clearances. Both shear rate and stress are continuously and independently recorded. Improved cylinders and other additional features have led to accurate viscosity measurements at shear rates from 5×102 to 2×106 sec−1. The temperature range is −20 to +150°C with a viscosity range which varies inversely with the desired shear rate for measurement. The shear rate range and the upper limit, 2×106 sec−1, exceed all previous reports of measurement at defined conditions in a homogeneous, isothermal shear field.

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.

Gel Permeation Chromatography Using a Bio-Glas Substrate Having a Broad Pore Size Distribution

Abstract Gel permeation chromatography has been performed using a porous glass of broad pore size distribution, which was subjected to hexamethyl-disilazane treatment. The elution volumes and peak widths of narrow molecular weight distribution polystyrenes using toluene as solvent have been determined. The physical characteristics of the porous glass have been studied by the methods of mercury porosimetry. nitrogen ad-sorption-desorption isotherms, and electron microscopy. The characteristics of this column packing material are compared with other packing materials in popular use.

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.