Effect of supercritical carbon dioxide on morphology development during polymer blending

Supercritical carbon dioxide (scCO(2)) was added during compounding of poly styrene and poly(methyl methacrylate) (PMMA) and the resulting morphology d evelopment was observed. The compounding took place in a twin screw extrude r and a high-pressure batch mixer. Viscosity reduction of PMMA and polystyr ene were measured using a slit die rheometer attached to the twin screw ext ruder. Carbon dioxide was added at 0.5. 1.0, 2.0 and 3.0 wt% based on polym er melt flow rates. A viscosity reduction of up to 80% was seen with PMMA a nd up to 70% with polystyrene. A sharp decrease in the size of the minor (d ispersed) phase was observed near the injection point of CO2 in the twin sc rew extruder for blends with a viscosity ratio, eta PMMA/eta polystyrene, o f 7.3, at a shear rate of 100 s(-1). However, further compounding led to co alescence of the dispersed phase. Adding scCO(2) did not change the path of morphology development; however, the final domain size was smaller. In bot h batch and continuous blending, de-mixing occurred upon CO2 venting. The r eduction in size of the PMMA phase was lost after CO2 venting. The resultin g morphology was similar to that without the addition of CO2. Adding small amounts of fillers (e.g, carbon black, calcium carbonate, or nano-clay part icles) 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 rat e of 100 s(-1), the addition of scCO(2) only slightly reduced the domain si ze of the minor phase.