Interplay of rheology and morphology in melt elongation and subsequent recovery of polystyrene/poly(methyl methacrylate) blends

In this study, blends of polystyrene (PS) and poly(methyl methacrylate) (PMMA) are investigated in melt elongation and subsequent recovery using the uniaxial elongational rheometer RME. The volume concentration Phi of the PS phase ranges from 15% to 85% so that the full range of morphology from PS drops to cocontinuous structures to PMMA drops is spanned. A small and a large Hencky strain rate are chosen which cover the cases of small and large capillary numbers and lead to small and large drop extensions. In simple elongation, the elongational viscosity of the blends is mainly determined by the volume fraction of the PS phase. In recovery, our experimental data clearly reveal that the molecular recovery portions of the blend components superpose to the total recovery of the blend at short recovery times. At large recovery times, the interfacial tension contributes to the recovered stretch for blends with droplet as well as cocontinuous morphologies. The time scale of the interfacial tension driven recovery attains a maximum for the blend with a cocontinuous structure (Phi = 50%). We compare our experimental data of the recovered stretch with the results of a model that is based on an effective medium approximation. Our model reproduces the experimental results for the transient recovered stretch and agrees well with the experimental data for large capillary numbers. (C) 2004 The Society of Rheology.