Effects of shear on a miscible polymer blend: In situ fluorescence and rheometry

The effects of shear flow on the phase behavior of a miscible blend of polystyrene with poly(vinyl methyl ether) have been investigated by a combination of fluorescence and rheometry. For this purpose, a Rheometrics RMS-800 rheometer was modified for stress and in-situ fluorescence measurements under well-controlled temperature and deformation conditions. Following equilibration at approximately 25 K below the coexistence temperature, slow heating to temperatures in the two-phase region shows that shearing at a constant rate increases the apparent coexistence temperature. Within the experimental error, the elevation above the quiescent spinodal temperature is related to the shear rate as $\Delta T/T\sb{s}$ = (0.015 $\pm$ 0.002)$\sbsp{\gamma}{\cdot 0.59 \pm 0.04}$; this correlation is independent of the composition in the range of 20-60% (w/w) polystyrene. A shear-induced demixing also occurs in the same blend at temperatures as much as 40 K below the LCST. There appears to be no threshold value of the shear rate or the shear stress for the onset of demixing, but a certain value of rate of work done on the sample roughly describes the onset of demixing at a fixed temperature. When shear does induce demixing, the fluorescence intensity remains much higher than that of the quiescent blend for as long as 24 hours, showing that the segregated phase structures can be stable under shear. 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. Transmission electron microscopy on sheared samples confirms the presence of a shear-induced structure. The influence of phase separation on the linear viscoelastic behavior was also studied by measuring the complex moduli $G\sp\prime$ and $G\sp{\prime\prime}$ of the blend at temperatures between 25 and 155$\sp\circ$C. For the 20/80 and 40/60 PS/PVME blends, the terminal zone was in the accessible frequency window and phase separation was accompanied with a large increase in G$\sp\prime$ and $G\sp{\prime\prime}$. In contrast, the complex modulus of the 60/40 PS/PVME blend could not be measured near the terminal zone and the G$\sp\prime$ and G$\sp{\prime\prime}$ did not exhibit any significant changes near the phase transition temperature