Julian F. Johnson

Mechanical Properties of Polymers: The Influence of Molecular Weight and Molecular Weight Distribution

Abstract The molecular weight and molecular weight distribution of a polymer can markedly affect its mechanical properties. While this has been recognized for a long time, quantitative studies have been relatively difficult to perform for a number of reasons. A major problem has been the characterization of the complete molecular weight distribution curve by fractionation methods because fraction collection and the determination of the various molecular weight averages of polymers is time consuming. Additionally, measurement of the number-average molecular weight, [Mbar]n, by classical osmometry procedure is often unreliable.

Order and Flow of Liquid Crystals: The Nematic Mesophase

Liquid crystals offer a unique opportunity for physical studies. The nematic mesophase is an aggregate, yet fluid state, which consists of anisometric, rodlike structures. The flow of three nematic‐forming compounds, p‐azoxyanisole, anisaldazine, and p‐anisal‐p‐aminoazobenzene, was studied in both their isotropic and anisotropic states. Studies were made with capillary viscometers and with a high‐shear concentric cylinder viscometer. New density, coefficient of expansion, and transition temperature data are also given. The instrumentation used is described. The results add definition to the viscosity and density anomalies at nematic‐isotropic transitions. The data for p‐anisal‐p‐aminoazobenzene are entirely new. Values for anisaldazine establish previous viscosity and density results on an absolute basis. New results on p‐azoxyanisole contribute a temperature and shear rate extension to previous studies.

Order and Flow of Liquid Crystals: The Cholesteric Mesophase

The flow properties of three cholesteryl esters have been studied in both their liquid crystal and isotropic ranges. Tests were made with low‐shear capillary viscometers and with a high‐shear rotational concentric cylinder instrument. Measurements on cholesteryl acetate are compared with relative viscosities given by Ostwald. Ostwalds viscosity data on cholesteryl propionate and butyrate are reevaluated. New flow data are also given for cholesteryl palmitate and stearate. Viscosity and flow activation energy are reported as a function of temperature, shear, and ester molecular weight. Results indicate that all cholesteryl esters show Newtonian flow in their isotropic states and non‐Newtonian flow in their respective cholesteric liquid crystal phases. New transition temperature and density data are also given for the three esters. These values and earlier density data for the benzoate ester are discussed in terms of the aggregate size of liquid crystals. Comparisons are made between the major types of mes...

TECHNIQUES FOR THE ANALYSIS OF CROSSLINKED POLYMERS

Abstract This review describes the current status of methods for characterizing the crosslink structure in network polymers. It is not intended to be an exhaustive summary of the literature itself, but rather a critical survey of key papers in the field. It is hoped that this information will provide researchers with an up-to-date background of presently available techniques and suggest alternatives to more traditional methods of analysis.

CHEMICAL AND PHYSICAL FACTORS AFFECTING PERFORMANCE OF PRESSURE-SENSITIVE ADHESIVES

Abstract Considerable attention has been given to the general area of adhesion and adhesives [1–3], but a need now exists to review more specific topics in these areas. The nature of pressure-sensitive adhesives (PSAs) and the factors contributing to their performance are reviewed here. A description for measuring performance of pressure-sensitive adhesives is followed by a treatment of physical and chemical factors that influence adhesive behavior. These include surface energetics, rheological behavior, molecular weight, and functional-group contributions.

The Viscosity of Mesophases Formed by Cholesteryl Myristate

Abstract This investigation provides the first reported viscosity measurements on cholesteryl myristate. Viscosities were measured as a function of both temperature and shear over temperatures corresponding to the isotropic liquid, the cholesteric mesophase, and the smectic mesophase. The myristate ester was chosen for study because its multiple phase transitions have been previously shown to be reproducible. The phases also persist over relatively long temperature ranges. The viscosity measurements were made at lower shear rates than previously reported for esters of cholesterol by using a Weissenberg Rheogoniometer. These are the first cone-and-plate viscosity measurements reported for any esters of cholesterol. The shear rates in this cone-and-plate viscometer are homogeneous and variable. The results show large breaks in viscosity behavior at the independently-measured thermodynamic transitions between the crystal, smectic, cholesteric, and isotropic states. Viscosities for the isotropic state are New...

Non‐Newtonian Viscosity of Polymers

The rheology of concentrated and bulk polymer systems has been investigated. Studies have been made on a low molecular weight series of several polymers using capillaries and a high shear concentric cylinder viscometer. The compositional and shear conditions for non‐Newtonian flow have been closely defined. Limiting high and low shear viscosities and the path of non‐Newtonian flow may be consistently correlated for concentrated systems of linear, flexible polymers. The minimum molecular weight for non‐Newtonian flow is related to the transition at low shear in log viscosity vs log molecular weight correlations. For non‐Newtonian systems, changes in apparent activation heat for viscous flow are reported as a function of shear. The conditions for laminar flow shear degradation of polymers are also given.

Prediction of Eutectic Temperatures, Compositions and Phase Diagrams for Binary Mesophase Systemst

Abstract Examination of a number of binary mesophase systems shows that the Schroder-van Laar equations for predicting the liquidus temperature and the eutectic temperature and composition are widely applicable. Additionally emperical correlations between the melting point and eutectic composition and temperature for binary mixtures having one common component are shown to be applicable to a considerable number of systems.

Polyisobutene Degradation in Laminar Flow: Composition and Shear Variables

The mechanical degradation of polyisobutene solutions in laminar flow under known shear stress conditions was performed in a thin‐film, high‐shear, rotational viscometer. Polyisobutenes with molecular weights from 40 000 to over a million were studied at several temperatures and concentrations in n‐hexadecane. Reduced variables, of a form similar to those used in studies of temporary viscosity losses, can be used successfully to correlate permanent viscosity losses due to degradation. The variables which superimpose are stress, temperature, and concentration. The range in which reduced variables apply corresponds to concentrations and molecular weights where degradation is commonly observed and to the region where polymer entanglements exist.

Sonic Degradation of Polyisobutylene in Solution

Narrow, well‐defined molecular weight fractions of polyisobutylene were irradiated in n‐hexadecane as 3% concentrations by a 10‐kc/sec sonic oscillator. The irradiated polymers were recovered by precipitation and evaporation; the molecular weight distributions were determined by column chromatographic fractionation. Narrow fractions for study had viscosity molecular weights of 15 100, 40 000, 74 400, and 137 000. The changes in distribution are compared with those from a broad distribution polymer with a molecular weight of 40 000. The limiting degree of polymerization below which degradation will not take place is at least as low as 200, and probably 100, for polyisobutylene in solution, considerably lower than the 1000–2000 reported for several polymers.