Phase morphology development in immiscible PP/(PS/PPE) blends influence ofthe melt-viscosity ratio and blend composition

Immiscible blends of isotactic polypropylene (PP) with a miscible amorphous phase containing varying concentrations of polystyrene (PS) and poly (2,6- dimethyl-1,4-phenylene ether) (PPE) were prepared in the melt, to study the influence of the blend composition and the melt-viscosity ratio, p, on the phase morphology. This model blend system offers the unique opportunity to vary the composition of the miscible amorphous PS/PPE phase, without affec ting the global interfacial tension, a crucial parameter with respect to ph ase morphology development. All immiscible PP/(PS/PPE) blends were prepared in a co-rotating twin-screw mini-extruder under constant processing condit ions. The location of the phase inversion region was strongly related to th e viscosity ratio. A composite-like morphology was observed in this region. To be able to separate the effects of droplet break-up and coalescence wit h respect to particle size, blends containing only 1 wt.% dispersed phase w ere investigated over a viscosity ratio range from 0.05 to 20. The results showed a clear dependence of the blend phase morphology on the viscosity ra tio; highly viscous matrices (p much less than 1) enhance droplet break-up due to their efficient shear stress transfer towards the dispersed phase an d the higher dispersive forces acting on it; low viscous matrices (p > 1) o ften act as a lubricant for the dispersed phase reducing droplet break-up. The influence of the viscosity ratio on droplet break-up is reflected in th e particle diameter in blends with a concentration of the dispersed phase u p to 20 wt.%. In the latter case, blends with a low viscosity ratio (p < 1) offer the best approach towards a fine and stable phase morphology, unlike suggestions in the literature. Blends containing higher concentrations of the minor phase (>20 wt.%) exhibit strong coalescence during melt-mixing; t he influence of the viscosity ratio on the final blend phase morphology bec omes less obvious, and the finest dispersion was observed at p = 1. Only bl ends of a lower viscous matrix in which a highly viscous phase has to be di spersed, do not follow the previous tendency as a result of the strong impa ct of a changing overall melt-viscosity. A quiescent thermal treatment of t he blends showed that the concentration of the dispersed phase is the most important factor determining phase coarsening in blends having nearly equal melt-viscosities. Blending a highly viscous component with a low viscous c omponent seems to counteract quiescent phase coarsening. (C) 1999 Elsevier Science Ltd. All rights reserved.