Demixing of unsheared and sheared solutions of polystyrene in tert-butylacetate and the pressure influence on their flow behaviour

Abstract

For the system tert-butyl acetate/polystyrene (M= 670000) the phase separation behaviour (upper critical solution temperatures, upper critical solution pressures) is investigated by means of visual and turbidimetric cloud-point measurements and by viscometry. For near-critical concentrations the visually determined cloud-points are found at higher, for low concentrations at lower temperatures as compared with the turbidimetrically determined binodal temperature,Tbin Plots of the invers of the turbidity measured atTbin as a function of the polymer concentration,w2, yield a minimum from which the critical composition can be determined. The viscometrically obtained demixing points (break-down of the viscosity at normal and at elevated pressures) show that the polymer solutions are stabilized at moderate concentrations. Irrespective of the preselected temperatures and of the present shear-rates (90-640 s−1, the demixing pressures are reduced by almost 20 bar. At sufficiently dilute or concentrated solutions the viscometric demixing points approach the binodal points. The above influences of the shear-field are explained by the rupture of intermolecular segment/segment contacts, which will be most efficient in the neighbourhood of the critical concentration for the formation of entanglements. In the case of the homogeneous solutions, the viscosity coefficientn increases in a fairly exponential manner by a factor of 3-4 per 1000 bar within the range ofp,T,w2 and shear-rate under investigation (1–1000 bar, -25 to f20 °C, 4–10 wt-% and 25–640 s−1). In plots of logn or ofV* (volume of activation) vs. concentration, extra effects show up when the critical conditions are approached; they result in a “hump” in logn vs.wn2 and in a minimum in V* vs.w2.