Effect of viscosity ratio on structure evolution in miscible polymer blends

Structure development through the softening/melting compounding regime is i nvestigated for very-low-viscosity-ratio model miscible blends consisting o f poly(styrene-co-acrylonitrile) (S AN)/poly(methyl methacrylate) (PMMA) an d polyethylene (PE)/polyethylene. Blends with viscosity ratios of 1.12,0.11 , 0.03, 0.003, and 0.00015 are pigmented, compounded, and visually interrog ated. These experiments reemphasize both the importance of the softening/me lting regime to the development of morphology in low-viscosity-ratio, misci ble blends, and the dynamic similarity between the behavior of these miscib le blends and their immiscible counterparts at short mixing times. Addition ally, these experiments reveal the presence of a phase-inversion-like proce ss in the lowest viscosity-ratio blends, as evidenced by both local and glo bal maxima in the mixer torque traces. This is the first such observation i n a miscible polymer blend. We postulate a mechanism of morphology developm ent 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 mixi ng, even when thermodynamically miscible with the major component. (C) 2001 Elsevier Science Ltd. All rights reserved.