OSMOTIC PRESSURE MEASUREMENT AND THE INSTABILITY OF HIGH POLYMERS

Abstract

In attempting to obtain accurate osmotic data in the low concentration range, it was found that the results were influenced by the diffusion of solvent from the cell through the membrane gasket. This behaviour was prevented by auxiliary sealing in a cell of new design, and results obtained with the improved cell are compared with those obtained with previous types of osmometer. A partial re-examination of the influence of preparation of the membrane was made in order to check results reported by Robertson, McIntosh, and Grummitt, and the different behaviour of caustic swollen and water swollen cellophane membranes has been confirmed, although quantitative agreement for water swollen membranes between the two investigations was not obtained. Using caustic treated membranes the molecular weight of a polyvinyl acetate sample was found to be the same within experimental error in three solvents.Further results on the changes in intrinsic viscosity and molecular weight brought about by ageing at elevated temperatures in the presence and absence of oxygen are reported for polyvinyl acetate and polymethyl methacrylate. It appears that oxygen is necessary to bring about molecular weight changes, and the degradation is due to scission of the chains at weak points. The evidence also suggests that changes in molecular shape may occur, since intrinsic viscosity values altered in some cases without evidence of molecular weight variation. This last observation is apparently outside the limits of error in reproducing osmotic pressure values.