Phase inversion detection in immiscible binary polymer blends via zero-shear viscosity measurements

In the present work, we demonstrate that zero-shear viscosity is a sensitive rheological function to detect phase inversion in immiscible binary polymer blends characterized by a viscosity ratio larger than one. The phase inversion of poly(propylene) (PP)/low-density poly(ethylene) (LDPE) and poly(styrene) (PS)/LDPE, at various compositions, was assessed via our novel approach. For both blends, three distinctive regions could be determined through zero-shear viscosity measurements; the LDPE matrix, the co-continuous phase, and the PS or PP matrix. For PP/LDPE blends, the co-continuous structure was between 50 and 75 wt.% PP, and for PS/LDPE blends the co-continuous structure was between 45 and 75 wt.% PS, in agreement with scanning electron microscopy analysis, empirical model predictions, and literature data. Highlights: Phase inversion revealed via viscosity measurements. Limitations of linear viscoelastic models for immiscible blends assessed.