Effect of viscosity ratio on structure evolution in miscible polymer blends

Structure development through the softening/melting compounding regime is investigated for very-low-viscosity-ratio model miscible blends consisting of poly(styrene-co-acrylonitrile) (SAN)/poly(methyl methacrylate) (PMMA) and polyethylene (PE)/polyethylene. Blends with viscosity ratios of 1.12, 0.11, 0.03, 0.003, and 0.00015 are pigmented, compounded, and visually interrogated. These experiments reemphasize both the importance of the softening/melting regime to the development of morphology in low-viscosity-ratio, miscible blends, and the dynamic similarity between the behavior of these miscible blends and their immiscible counterparts at short mixing times. Additionally, these experiments reveal the presence of a phase-inversion-like process in the lowest viscosity-ratio blends, as evidenced by both local and global maxima in the mixer torque traces. This is the first such observation in a miscible polymer blend. We postulate a mechanism of morphology development that incorporates the Scott/Macosko lacing/sheeting mechanism, as well as the observed phase-inversion-like process. Finally, we confirm that the presence of a low-viscosity additive significantly delays the onset of mixing, even when thermodynamically miscible with the major component.