METASTABLE POLYMER BLENDS BY PRECIPITATION WITH A COMPRESSED FLUID ANTISOLVENT

Metastable polymer blends of polycarbonate (PC) and poly(styrene-co-ac rylonitrile) (SAN) are formed by spraying polymer solutions into liqui d and supercritical fluid CO2. Because of the rapid mass transfer betw een the CO2-phase and the solution phase, the blends are trapped in a metastable state before they can phase separate. A transition from a m etastable blend with a single glass transition temperature, T-g in the form of particles, to a phase-separated 100 mu m fibre is observed wi th an increase in polymer concentration from 3.0 to 9.0wt%. This trans ition is related to the calculated concentration for the dilute to sem i-dilute transition, C, of PC/SAN in THF and the characteristic times for polymer-polymer and polymer-solvent phase separation. PC and SAN do not form a blend at 35 degrees C because of plasticization by CO2. They also do not form a metastable blend when 1.0mm droplets are preci pitated into a liquid antisolvent, heptane. (C) 1997 Elsevier Science Ltd.