Rheological properties of concentrated polymer solutions I. Growth of pressure fluctuations during prolonged shear flow

Abstract A new apparatus, similar to the Weissenberg Rheogoniometer, has been developed for the determination of the differences of normal stress components in steady shear flow. The liquid is sheared between a rotating cone and a fixed plate, the pressure distribution on the latter being measured by means of a diaphragm-capacity gauge having a response time (about 0.1 sec) considerably shorter than those (about 1 h) of the capillary gauges which have been used hitherto. Preliminary experiments reported here have revealed the unexpected result that with certain systems [poly(methyl methacrylate) in dimethylphthalate, polystyrene in dimethylphthalate] the observed pressure during prolonged shear flow (20 min at 20 sec −1 ) develops a random fluctuation with ‘periods’ of up to 0.4 sec and amplitudes up to about one third of the shear stress. A liquid mixture of polydimethylsiloxanes showed no fluctuations under similar conditions. The growth of pressure fluctuations was accompanied by the development, in the free liquid surface near the rotating cone, of irregularities or ruffling visible to the naked eye. The observations may possibly be explained in terms of structural changes in the solutions leading to the growth of inhomogeneities or gel particles of dimensions (about 0.4 mm) comparable to the gap between cone and plate.