High Molecular Weight Portion Research Articles

Viscosity/molecular weight relation in bulk polymers—II. Onset of non-Newtonian Flow

The strong dependence of polymer melt viscosity on molecular weight can be attributed to the ability of polymer molecules to participate in entanglements. When unidirectional shear stress is applied, the melt viscosity reduces because of molecular motion which results in a reduced statistical requirement for segmental cooperation. Qualitative examination of polymer motion suggests that the shear stress required to reduce melt viscosity from its Newtonian flow varies inversely with molecular weight and depends on the degree of polymer polydispersity. Experiments confirm the proposed dependence of the initial shear stress for non-Newtonian flow on molecular weight, and indicate the potential use of this shear stress as a characteristic of the high molecular weight portion of the distribution in a given polymer.

A comparison of the thermal gradient and elution methods of polymer fractionation

The elution method is essentially a single-stage process, whereas the thermal gradient method is a multistage process which depends upon a thermal gradient to bring about reprecipitation of polymer in the fractions. As a test of the effectiveness of the thermal gradient, comparisons have been made of fractionation by these two column methods on high molecular weight polystyrene samples. It was found that the thermal gradient method definitely provides superior resolution and reproducibility, as expected. However, the degree of fractionation obtained by the elution method was surprising, accounting for at least 80% of the sample under the usual conditions and giving complete fractionation with certain modifications of conditions. These results indicate the difference in performance of the two methods is less than expected from an elementary consideration of the operation of the columns, and fractionation by the elution method, as conducted here, exceeds that expected for a single-stage extraction process. Although the reasons for the observed behavior are not clear, the following conclusions have been reached about certain factors which influence fractionation. Alternative methods of controlling the concentration of polymer in the fractions give almost equivalent results but enhanced resolution of the high molecular weight portion of the sample is obtained with extended solvent gradients. The inhibitor, tert-butyl catechol, which it was necessary to add to the solvents to limit degradation of the very high molecular weight sample, plays a specific role in the fractionation due to a reaction with the polystyrene which alters the fractionation behavior without affecting the molecular weight. Also, trace amounts of chemical heterogeneity in the polymers, presumably hydroxyl groups, have a marked adverse effect on fractionation by the elution method and probably account for molecular weight reversals observed in some fractionations by the thermal gradient method. It is suggested that adsorption on the surface of the beads is responsiblp for the adverse effect of chemical heterogeneity on the fractionation and that possibly an adsorption which increases with molecular weight contributes to fractionation by the elution and thermal gradient methods.

Some Osmotic Measurements of the Molecular Weight of a High Molecular-Weight Fraction of Rubber Hydrocarbon

Abstract Fractionation of the rubber hydrocarbon in temperate climates has usually resulted in high molecular-weight fractions, with a molecular weight of the order of one million. Bloomfield has shown that fresh latex contains a considerable proportion of hydrocarbon having an intrinsic viscosity (η) of 10 or over and, therefore, a molecular weight of well over 106. The fractionation technique used by Bloomfield in Malaya has now been applied by the writer to smoked sheet and to F rubber, working in the United Kingdom. No very high molecular-weight fractions were found in the smoked sheet, but the F rubber yielded a fraction of (η)=7.3 and a number average molecular weight 6×106, determined osmometrically. The average molecular weight of natural rubber when freshly prepared is probably well over a million, and includes a substantial portion having a molecular weight of several millions. By the time smoked sheet has reached temperate climates, the high molecular-weight portion has probably been converted to gel. F rubber, presumably because of its different method of preparation, retains the major part of its high molecular-weight material during prolonged storage.

Biophysical Studies of Blood Plasma Proteins

Summary Marked differences in antibody-activity are found in human γ1-and γ2-globulin antibody preparations. The hemagglutinins and typhoid “O” agglutinin appear to be γ1-globulins. The β-hemagglutinin of plasma represents a small amount of the high molecular weight portion of the γ1-globulin. The antibody responses of rabbits to human γ1- and γ2-globulins, fibrinogen and albumin as studied by the antigen-dilution technic showed that these essentially electrophoretically pure proteins did not represent distinct immunological entities.