Effect of supercritical carbon dioxide on morphology development during polymer blending

Supercritical carbon dioxide (scCO(2)) was added during compounding of polystyrene and poly(methyl methacrylate) (PMMA) and the resulting morphology development was observed. The compounding took place in a twin screw extruder and a high-pressure batch mixer. Viscosity reduction of PMMA and polystyrene were measured using a slit die rheometer attached to the twin screw extruder. Carbon dioxide was added at 0.5. 1.0, 2.0 and 3.0 wt% based on polymer melt flow rates. A viscosity reduction of up to 80% was seen with PMMA and up to 70% with polystyrene. A sharp decrease in the size of the minor (dispersed) phase was observed near the injection point of CO2 in the twin screw extruder for blends with a viscosity ratio, eta PMMA/eta polystyrene, of 7.3, at a shear rate of 100 s(-1). However, further compounding led to coalescence of the dispersed phase. Adding scCO(2) did not change the path of morphology development; however, the final domain size was smaller. In both batch and continuous blending, de-mixing occurred upon CO2 venting. The reduction in size of the PMMA phase was lost after CO2 venting. The resulting morphology was similar to that without the addition of CO2. Adding small amounts of fillers (e.g, carbon black, calcium carbonate, or nano-clay particles) tended to prevent the de-mixing of the polymer blend system when the CO2 was released. For blends with a viscosity ratio of 1.3, at a shear rate of 100 s(-1), the addition of scCO(2) only slightly reduced the domain size of the minor phase.