The reaction parameters and progress of the dispersion polymerization of methyl methacrylate (MMA) utilizing poly(l,l-dihydroperfluorooctyl acrylate) [poly(FOA)] as a polymeric stabilizer in supercritical CO2 were investigated. Spherical and relatively uniform polymer particles were formed. Poly(methyl methacrylate) (PMMA) latex particles with diameters ranging from 1.55 to 2.86 um were obtained with poly(FOA) stabilizer [A/w = 1.0 (± 0.4) x 106 g/mol] concentration from 16 to 0.24 wt %.Investigations of the particle size and conversion as a function of reaction time indicate that a gel effect occurs between 1 and 2 h of reaction time. In addition, the particle diameter increases with an increase in monomer concentration, presumably due to an increase in the solvency of the reaction medium. Dispersion polymerizations of MMA carried out under different pressures (145-331 bar) produced latex particles with similar diameters, molecular weights, and yields, suggesting that the results are insensitive to the pressure under the reaction conditions investigated. In addition, the phase behaviors of poly-(FOA) were thoroughly investigated. Cloud point experiments indicate lower critical solution temperature (LCST) phase behavior for the poly(FOA)/C02 system with much higher polymer solubilities than for hydrocarbon polymers.
Rapid expansion from supercritical solution (RESS) of a crystalline fluoropolymer, polyol,2,2-tetrahydroperfluorodecyl acrylate) or poly(TA-N), in carbon dioxide produces submicron to several micron sized particles and fibers. The understanding of the RESS mechanism has been clarified by careful design of experimental variables and procedures. The concentration of the poly(TA-N)/CC>2 solution was held constant (at 0.5 and 2.0 wt %), the solution cloud point curves were obtained, the pre-expansion temperature was varied above and below the cloud point, and the length to diameter {LID) ratio of the nozzle was varied from 8.5 to 508. The morphology is explained in terms of the location of phase separation within the expansion nozzle. The LID is the most influential variable for achieving a transition from particles or fibers. In most cases, manipulation of the solution concentration and the pre-expansion temperature did not produce this transition but did have a large effect on the sizes of the particles and fibers. These results are an important step in demonstrating C02-based spray processes which do not require any volatile organic solvents.
Stable suspensions of submicron particles of cyclosporine, a water-insoluble drug, have been produced by rapid expansion from supercritical to aqueous solution (RESAS). To minimize growth of the cyclosporine particles, which would otherwise occur in the free jet expansion, the solution was sprayed into an aqueous Tween-80 (Polysorbate-80) solution. Steric stabilization by the surfactant impedes particle growth and agglomeration. The particles were an order of magnitude smaller than those produced by RESS into air without the surfactant solution. Concentrations as high as 38 mg/mL for 400-700 nm particles were achieved in a 5.0% (w/w) Tween-80 solution.
A coaxial nozzle was developed to achieve further control over the morphology of microparticles precipitated from solution by carbon dioxide as a compressed fluid antisolvent. The polymer solution was sprayed through the core of the nozzle and CO 2 through the annulus. For the coaxial nozzle versus a standard nozzle, polystyrene and poly(L-lactic acid) particles can be larger by a factor of 3-8 with less flocculation. A reduction in the Weber number reduces atomization and larger droplets are formed in the jet, delaying precipitation. However, because of the much higher Reynolds number for the high velocity CO 2 , the mass transfer in the suspension outside of the jet is faster leading to less flocculation and agglomeration. For polyacrylonitrile, the delayed precipitation produces a transition from highly oriented microfibrils to microparticles.
Poly(1,1-dihydroperfluorooctyl acrylate) (poly(FOA)) based stabilizers greatly reduce, and in some cases eliminate, flocculation of amorphous poly(methyl methacrylate) (PMMA) and polystyrene (PS) microparticles formed by precipitation into liquid CO2 at 23 °C. The microparticle stabilization mechanism is explained in terms of the stabilizer-CO2 phase behavior, the spray characteristics, and the interfacial activity of the stabilizer. Compared with the homopolymer poly(FOA), the diblock copolymer PS-b-poly(FOA) produces smaller and more spherical primary particles (0.1-0.3 µm) and also prevents flocculation at lower stabilizer concentrations. These differences are due to the greater interfacial activity of PS-b-poly(FOA). Steric stabilization commences in the jet on the order of several tenths of milliseconds and continues for seconds throughout the precipitator. With the use of a coaxial nozzle, precipitation is delayed and the stabilizers become even more effective at preventing flocculation.
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