Rheological investigation of shear induced-mixing and shear induced-demixing for polystyrene/poly(vinyl methyl ether) blend

The phase behavior of polystyrene (PS) and poly(vinyl methyl ether) (PVME) blend has been investigated rheologically as a function of temperature composition and oscillating shear rate as well as different heating rates. An LCST (lower critical solution temperature)-type phase diagram was detected rheologically from the sudden changes in the slopes of the dynamic temperature ramps of G' at given heating and shear rate values. The rheological cloud points were dependent on the heating rate, q, and oscillating shear rate, gamma. The cloud points shifted a few degrees to higher temperatures with increasing q and reached an equilibrium value (heating rate independent) at q greater than or equal to 2 degreesC/min. The phase diagrams of the blends detected at gamma = 0.1 and 1 rad/s were located in lower temperature ranges than the quiescent phase diagram, i.e., oscillating shear rate induced-demixing at these two values for the shear rate. On the other hand, at gamma = 10 rad/s, the phase diagram shifted to higher temperatures, higher than the corresponding values found under quiescent conditions, i.e., shear induced-mixing took place. Based on these two observations, shear induced-demixing and shear induced-mixing can be detected rheologically within a single composition at low and high shear rate values, respectively, and this is in good agreement with the previous investigation using simple shear flow techniques. In addition, the William, Landel and Ferry (WLF)-superposition principle was found to be applicable only in the single-phase regime; however, the principle broke-down at a temperature higher than or equal to the cloud point. Furthermore, different spinodal phase diagrams were estimated at different oscillating shear rates based on the theoretical approach of Ajji and Choplin.