Dioxide microspherical form

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... 

Typically, this process allows forming particles from a great variety of substances that need not be soluble in supercritical carbon dioxide, especially from some polymers that absorb a large concentration (10-40 wt.%) of CO2 [8]. This process can also be performed with suspensions of active substrates in a polymer or other carrier substance leading to composite microspheres. 

A parallel study has reported the synthesis of crosslinked polymer microspheres in supercritical carbon dioxide [54]. Heterogeneous free-radical polymerization of divinyl benzene and ethyl benzene were carried out at 65 C and 310 bar using AIBN initiator to form the crosslinked polymer. It is shown that in the absence of surfactants as stabilizers, polymerization of the mixture containing 80 % divinyl benzene + 20 % ethyl benzene leads to poly(divinylbenzene) microspheres of about 2.4 micron diameter [Figure 14]. In the presence of a carbon dioxide-soluble diblock copolymer as a stabilizer, polymerization of the mixture with the same monomer ratio proceeds as an emulsion and lead to smaller crosslinked particles (ca. 0.3 micron). Thermal analysis shows that the crosslinked polymer that is formed from these polymerizations is stable up to 400 C. 

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