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 CO 2 in the twin screw extruder for blends with a viscosity ratio, ηPMMA/ηpolystyrene, 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 CO 2 venting. The reduction in size of the PMMA phase was lost after CO 2 venting. The resulting morphology was similar to that without the addition of CO 2 . 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 CO 2 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.