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

Abstract

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 affecting the global interfacial tension, a crucial parameter with respect to phase morphology development. All immiscible PP/(PS/PPE) blends were prepared in a co-rotating twin-screw mini-extruder under constant processing conditions. The location of the phase inversion region was strongly related to the viscosity ratio. A composite-like morphology was observed in this region. To be able to separate the effects of droplet break-up and coalescence with respect to particle size, blends containing only 1 wt.% dispersed phase were 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 ratio; highly viscous matrices ( p≪1) enhance droplet break-up due to their efficient shear stress transfer towards the dispersed phase and the higher dispersive forces acting on it; low viscous matrices ( p>1) often 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 the particle diameter in blends with a concentration of the dispersed phase up 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; the influence of the viscosity ratio on the final blend phase morphology becomes less obvious, and the finest dispersion was observed at p=1. Only blends of a lower viscous matrix in which a highly viscous phase has to be dispersed, do not follow the previous tendency as a result of the strong impact of a changing overall melt-viscosity. A quiescent thermal treatment of the 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 component seems to counteract quiescent phase coarsening.