Supercritical solutions rapid expansion polymers

Journal of Applied Polymer Science 11, No.7, 15th Aug.2000, p. 1478-87 MORPHOLOGIES OF BLENDS OF ISOTACTIC POLYPROPYLENE AND ETHYLENE COPOLYMER BY RAPID EXPANSION OF SUPERCRITICAL SOLUTION AND ISOBARIC CRYSTALLIZATION FROM SUPERCRITICAL SOLUTION... 

By utilizing the rapid expansion of supercritical solutions, small-size particles can be produced from materials which are soluble in supercritical solvents. In this process, a solid is dissolved in a pressurized supercritical fluid and the solution is rapidly expanded to some lower pressure level which causes the solid to precipitate. This concept has been demonstrated for a wide variety of materials including polymers, dyes, pharmaceuticals and inorganic substances. 

In the latest literature, the production by supercritical techniques of pharmaceuticals-loaded bio-polymer micro-particles is widely considered [34], All of these applications take advantage of the solvent or anti-solvent power of CO2. Various techniques have been proposed so far, such as the rapid expansion from supercritical solution (RESS) [35], the gas... 

The rapid expansion of supercritical solutions (RESS) was explored by several authors as a novel route to the formation of microparticles. Ohgaki [1] produces fine stigmasterin particles by the rapid expansion of a supercritical C02 solution. Amorphus fine particle and whisker-like crystals (0,05 - 3 pm) were obtained with different preexpansion pressures. Johnston [2] obtained submicron particles from different polymers. Loth [3] described the mirconisation of phenacetin with supercritical fluids. 

The supercritical fluid mefhod is a relafively new method, which can minimize the use of organic solvents and harsh manufacturing conditions taking advantage of two distinctive properties of supercritical fluids (i.e., high compressibility and liquid-like density). This method can be broadly divided into two parts rapid expansion of supercritical solutions (RESS), which utilizes the supercritical fluid (e.g., carbon dioxide) as a solvent for the polymer, " and supercritical antisolvent crystallization (SAS), using the fluid as an antisolvent that causes polymer precipitation. Recent reviews of the supercritical technology for particle production are available in the literature. ... 

The magnetic metals were also prepared by a method [25] based on the rapid expansion of supercritical fluid solutions (RESS) coupled with chemical reduction to produce nickel, cobalt, iron, and iron oxide nanopartides of reasonably narrow size distribution. Under the protection of a polymer stabilization agent, the largely amorphous metal nanopartides form stable suspensions in room-temperature solvents. 

Matsuyama, K., Mishima, K., Hayashi, K., and Matsuyama, H. (2003). Microencapsulation of TiC>2 nanoparticles with polymer by rapid expansion of supercritical solution. J. Nanoparticle Res. 5, 87-95. 

Meziani, M. J. and Sun, Y. P. (2003). Protein-conjugated nanoparticles from rapid expansion of supercritical fluid solution into aqueous solution. J. Am. Chem. Soc. 125, 8015-8018. Meziani, M. J., Pathak, P., Hurezeanu, R., Thies, M. C., Enick, R. M., and Sun, Y. P. (2004). Supercritical fluid processing technique for nanoscale polymer particles. Angew. Chem. Int. Ed. Engl. 43, 704—707. 

SCF technologies deserve a special mention as they have been less commonly applied on a laboratory scale in the preparation of nanoparticles (73). Scheme 7 and Figures 8 and 9 summarize the approach used. A drug and polymer mixture is dissolved in an organic solvent or carbon dioxide. Under certain conditions of pressure and temperature, the liquid phase is transformed into the supercritical state as seen in Figure 8. Here the supercritical state is found at pressures >74 bar (atmospheric pressure = 1.013 bar) and at temperatures >31°C. The rapid expansion of this supercritical solution on exposure to atmospheric pressure causes the formation of microspheres or nanospheres Figure 9 on the other hand illustrates the use of a solvent, which can be formed into a... 

Petersen RC, Matson DW, Smith RD. The formation of polymer fibers from the rapid expansion of supercritical fluid solutions. Polym Eng Sci 1987 27 1693-1697. 

Micro-encapsulation of drug-polymer systems using flic RESS (Rapid Expansion of Supercritical Fluid Solutions) techniques have been initiated with limited success due to poor understanding of the complex phenomena involved in co-nucleation of components. Not only do the particles have to be nucleated with the desired particle size and shape but also encapsulate the material simultaneously in an uniform fashion. 

When a supercritical solution that contains a dissolved solute is expanded across a micro-oriflce, the solvent density decreases dramatically and the solute is rejected from solution. Petersen et al. (1) were the first to call this process the rapid expansion of supercritical solutions, or RESS for short. Because the characteristic speed of the expansion is the speed of sound, the process is quite rapid, with residence times in the orifice on the order of 1 rs. The rapid pressure reduction across the expansion nozzle leads to both uniform conditions and very high supersaturation ratios in the postexpansion environment. These two characteristics are a key feature of RESS and favor the formation of small particles with narrow size distributions (2). When materials such as polymers are used, other product morphologies are possible. For example, RESS solutions can be sprayed to form thin films (3). In other cases, the very high extensional rates in the expansion nozzle can be used to make microfibers (4-6). 

RESS has been particularly popular for the processing of polymeric materials. According to the original report by Krukonis (55), rapid expansion of a polypropylene solution in supercritical propylene resulted in the formation of fiber-like particles. The report marked the beginning of an extensive discussion on the issue of particles vs. fibers involving the use of RESS with polymers. 

PbS is a semiconductor that exhibits extreme quantum confinement effects, with the absorption spectral properties being highly sensitive to the physical parameters of the nanoparticles. Thus, the effects of the experimental parameters in RESOLV on the PbS nanoparticles produced can be evaluated in a systematic fashion. For example, PbS nanoparticles were prepared by the rapid expansion of a supercritical ammonia/Pb(N03)2 solution at various temperatures. In the experiments all parameters [Pb(N03)2 concentration, expansion nozzle size, and concentration of Na2S and PVP polymer in the room-temperature ethanol solution] were held constant, except the temperature of the ammonia/Pb(N03)2 solution for rapid expansion. The absorption spectra of the two PbS-nanoparticle samples obtained by rapid expansions at 160°C and 130 C were quite similar. The similarity in absorption properties probably reflected the fact that the PbS nanoparticles obtained at the two different preexpansion temperatures had similar particle sizes, which was supported by the similar x-ray powder diffraction patterns of the nanoparticle samples. 

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