FORMATION OF MICROPOROUS POLYMER FIBERS AND ORIENTED FIBRILS BY PRECIPITATION WITH A COMPRESSED FLUID ANTISOLVENT

Polymer morphology is controlled over a continuum from microspheres to interconnected bicontinuous networks to fibers with a versatile new p rocess: precipitation with a compressed fluid antisolvent. The results are explained qualitatively as a function of phase behavior, mass-tra nsfer pathways, and the formation rates of skin on the flowing jet. By spraying dilute polystyrene in toluene solutions into liquid carbon d ioxide, extremely small 100 nm microspheres are formed. For concentrat ions above the critical composition, fibers are produced that are not only microcellular, but, in some instances, even hollow. Mass-transfer pathways that cross the binodal near the critical composition produce interconnected networks, likely due to spinodal decomposition. In thi s region, fibers composed of highly oriented microfibrils are produced at high shear rates. Preaddition of CO2 influences the morphology bec ause of dilution, in a similar manner as a liquid antisolvent, except that the viscosity reduction is larger due to added free volume. Becau se CO2 diffuses through the glassy polystyrene skin faster than does a conventional liquid antisolvent such as methanol, it produces more po rous fibers, which are also more cylindrical. (C) 1993 John Wiley & So ns, Inc.