SHEET FORMATION IN IMMISCIBLE POLYMER BLENDS - MODEL EXPERIMENTS ON INITIAL BLEND MORPHOLOGY

Model experiments were performed to determine the controlling paramete rs in the evolution of the phase morphology of immiscible polymer blen ds from pellets to micrometre-sized particles. It has been established that during melting in both twin-screw extruders and batch mixers, th e dispersed phase is stretched into sheets. These sheets develop into cylinders and the cylinders ultimately break into spherical droplets v ia Rayleigh-type instabilities. Here, we show that micrometre thick sh eets can be created from millimetre-sized pellets by shearing in the p arallel discs geometry. Extensional flow is not required to generate t he sheets. Pellets are seen to break up in three ways: (1) by stretchi ng into cylinders with drops streaming off the end; (2) by extending s heets that form fingers at the edges; and (3) by stretching into thin sheets that break up by forming holes. A map of the different regions of breakup is given using the Deborah number and the ratio of the firs t normal stress difference of the matrix to the restoring stress of th e pellet (drop). The drop-restoring stress is the sum of the surface s tress resulting from interfacial tension and the first normal stress d ifference of the drop. The masterplot explains why sheets can be easil y formed from large drops and gives a window where the sheets are stab le and do not form holes.