Binary miscible blends of poly(methyl methacrylate)/poly(α-methyl styrene-co-acrylonitrile): I. Rheological behavior

Viscoelastic properties of poly(α-methylstyrene-co-acrylonitrile) and poly-(methyl methacrylate) blends have been systematically investigated below and above the lower critical solution phase-separation temperatures. In the one-phase regime, the viscoelastic characteristic parameters of the blends, such as zero shear viscosity, η₀, and entanglement plateau modulus, G_N⁰, were found to be composition dependent. A negative deviation of the composition dependence of G_N⁰ from the linear-mixing rule was detected indicating that the total number of entanglement points per unit volume in the blends is smaller than the sum of those in the two constituent pure components. Similar behavior was also seen in the composition dependence of η₀. The linear-viscoelastic properties of the blends were found to be greatly changed by phase separation in the two-phase regime. The change in the viscoelastic properties was clearly observed in the failing of the Williams-Landel-Ferry (WLF) superposition principle and sudden changes in the slopes of the temperature ramps of some of viscoelastic material functions (G′, G″, η₀) as well as an appearance of a second plateau at a small value of frequency in the classical frequency dependence of the complex dynamic viscosity (η*). This large change in the linear-viscoelastic properties might be attributed to an additional contribution of concentration fluctuations to the material functions at the phase-separation temperatures. The phase diagram of the blends was also estimated rheologically and the result was in good agreement with the cloud point measurements obtained visually under quiescent condition at a heating rate of 1°C/min.