Shear-induced demixing in a polystyrene/poly(vinyl methyl ether) blend: in-situ fluorescence and rheometry

Abstract

In miscible blends of polystyrene with poly(viny1 methyl ether), a shear-induced demixing occurs at temperatures as much as 40 K below the lower critical solution temperature. Demixing occurs at relatively low shear rates ranging from 1.5 to 4 s-l depending on the temperature and composition; the corresponding shear stresses and first normal stress differences range from 20 to 35 kPa and 30 to 120 kPa, respectively. There appears to be no threshold value of the shear rate or the shear stress for the onset, but a certain value of rate of work done on the sample roughly describes the steady-state demixing at a fixed temperature. When shear does induce demixing, some of the concentration differences are stable under shear for as long as 24 h. However, in other cases, a demixing appears for a shorter time, along with relatively high stresses, shortly after the inception of shear but disappears as the stresses drop at longer times. The time scale for the development of demixing ranges from hundreds to thousands of seconds and remixing after cessation of the flow typically occurs in thousands of seconds. Particularly at temperatures near the lower critical solution temperature, a substantial time is required for the stress development, even when there is no measurable demixing as indicated by fluorescence. Transmission electron microscopy on sheared samples confirms the presence of a shear-induced structure.