Effect of concentration and degree of saturation on RESS product morphology and size

The goal of this work is to explain the formation of structure and size (i.e., fibers vs. particles and submicron- vs. micron-sized structures) for the rapid expansion of supercritical solutions (RESS) containing a polymer and solvent. RESS experiments were performed on (1) the fluorinated acrylate polymer poly(HDFDA) (Mw = 254,000, Mw/Mn = 2.95) in solution with CO2 and (2) cellulose triacetate (Mw = 145,700, Mw/Mn = 2.07) in solution with ethyl acetate, with both concentration and degree of saturation being varied in a systematic manner. Phase behavior measurements were carried out beforehand in order to locate the liquid-liquid equilibrium (LLE) phase boundaries, and thus establish the degree of saturation for a given concentration and pre-expansion temperature and pressure. RESS results for both polymer-solvent systems indicate that the phase state of the pre-expansion mixture controls product size, whereas the overall concentration of the pre-expansion mixture controls product morphology, specifically fiber vs. particle formation. Furthermore, to produce well-formed fibers and particles, the rapid expansion path must include penetration into a region of LLE, which must exist over a sufficient pressure range so that a separate, polymer-rich liquid phase has time to develop before the onset of vapor-liquid equilibrium (VLE). If rapid expansions are carried out at temperatures near or below the lower critical end point (LCEP), the expansion path leads directly into a region of VLE, and hollow particles, hollow structures, and even foams are produced.; The usefulness of intrinsic viscosity as a predictor of product morphology for the RESS process was explored. An oscillating-piston viscometer was calibrated with pure CO2, and then used to measure viscosities for dilute solutions of poly(HDFDA) in CO2 at temperatures and pressures identical to those applied in our earlier RESS work. These measurements were then used to calculate intrinsic viscosities and, from overlap concentration theory considerations, overlap concentrations c*. The results are consistent with theory, as fibers are produced by RESS from homogeneous solutions with polymer concentrations greater than c*, and particles are formed from solutions with concentrations less than c*.