Influence of phase separation on the linear viscoelastic behavior of a miscible polymer blend

Abstract

The influence of phase separation on the linear viscoelastic response has been studied in miscible blends of polystyrene and poly(vinyl methyl ether) with a lower critical solution temperature near 110 °C. At temperatures between 25 and 155 °C, and for compositions in the range 20% to 60% polystyrene, the complex moduli G′ and G″ were measured at frequencies in the range of 0.01 to 100 rad/s. Time–temperature superposition was applied in the single phase region to obtain the complex modulus over eight decades of frequency. Increasing the polystyrene content resulted in an increase in the zero-shear viscosity and a shift of the terminal behavior to lower frequencies or longer times. The phase separation above the lower critical solution temperature was measured as a sudden increase in the fluorescence intensity of an anthracene-labeled polystyrene (approximately 1 wt % in the blend), using an optical probe in the rheometer fixture. For the 20/80 and 40/60 PS/PVME samples, the terminal zone was in the accessible frequency window and phase separation was accompanied with a large increase in G′ and G″. In contrast, the complex modulus of the 60/40 PS/PVME blend could not be measured near the terminal zone and the G′ and G″ did not exhibit any significant changes near the phase transition temperature.