The formation of polymer fibers from the rapid expansion of supercritical fluid solutions

Fibers of organic polymers (polystyrene, cellulose acetate, and polypropylene) were formed by the rapid expansion of supercritical fluid solutions through a small diameter nozzle to ambient temperature and pressure. Solutions were prepared either by dissolving the polymer directly in room temperature pentane, or in an autoclave at elevated temperatures and pressures for less soluble polymers. The fibers were collected on substrates mounted in the expansion jet. The diameters of the fibers formed (typically 1‐5 μm) were much smaller than the opening of the nozzle, although fiber diameter was observed to generally increase with nozzle diameter. The aspect ratios of the fibers, produced by this process were on the order of 10 3 or larger. Optimum conditions for fiber formation occurred at fluid expansion temperatures near the melting point of the polymer, with particle formation mechanisms favored at higher and lower temperatures.