Abstract
The preparation of polymer composites using sub/supercritical fluids and the measurement of diffusion coefficients in CO2-expanded liquids are included in this dissertation. There are four chapters, which are organized as follows: In Chapter 1, the physical properties of supercritical fluids, and the applications of supercritical fluids in polymer processing as well as the characteristic properties of gas-expanded liquids were described. In Chapter 2, the preparation of TiO2/PC and TiO2/PET composites using supercritical impregnation was investigated. The extent of polymer expansion could be controlled by adjusting the pressure and temperature of supercritical fluids and the impregnation of TiO2 nanoparticles into the polymer matrix with supercritical fluids was therefore achived. Three different mechanisms of impregnation were studied in this chapter, and the optimum way of impregnation was found when the polymer film was immersed in the suspension solution of TiO2 prior to the introduction of compressed CO2 into the solution. However, the impregnation of TiO2 in this study was observed to occur only on the polymer surface. In Chapter 3, the preparation of polymer composites including TiO2/PS nanocomposites, PMMA particles, and PS/PMMA blending polymers by using continuous PCA method was studied. When the polymer solution with suspended TiO2 was sprayed into an antisolvent environment through a nozzle, the supersaturation of PS was achieved and the precipitation of PS on TiO2 surface occurred. The morphology of TiO2/PS core/shell structure was obtained for a temperature of 298 K, a pressure of 6.41 MPa, a liquid CO2 level in the precipitator of 1/4, a PS concentration of 0.72 wt%, and a PS/TiO2 ratio of 8. Due to the facile adsorption of CO2 in PMMA, spherical PMMA particles were not easily formed in compressed CO2 environment. In this chapter, the preparation of PMMA particles without adding any stabilizers was conducted by adjusting several operating variables, and the spherical PMMA submicron-sized particles were obtained for a temperature of 298 K, a pressure of 6.41 MPa, a polymer solution of 5 mL/min, a CO2 flow rate of 2000 mL/min, a liquid CO2 level in the precipitator of 1/8, and a PS concentration equal to or less than 1 wt%. In the preparation of PS/PMMA blends, the obtained blends were mostly in submicron-sized particles for a temperature of 298 K, a pressure of 6.41 MPa, a polymer solution of 5 mL/min, a CO2 flow rate of 2000 mL/min, a liquid CO2 level in the precipitator of above 1/4, and a blending polymer concentration equal to or less than 1 wt%. When the blending polymer concentrations ranged between 2 and 8 wt%, the obtained blends were shown in submicron and micron-sized particles. By the way, a spherical blending polymer of PS/PMMA core/shell morphology could be observed for a PS/PMMA ratio of 9/1, a liquid CO2 level in the precipitator of 1/2, a PS molecular weight of 144,000, and a PMMA molecular weight of 36,000. In Chapter 4, the measurement of diffusion coefficients of solutes in CO2-expanded liquids was described. Due to the low solubility of hydrogen in organic solvents, and the existence of interfacial resistance between gas and liquid phases, as well as the resistance of diffusion within the liquid, the reaction rate of hydrogenation reaction would be limited. When compressed CO2 was dissolved into the organic liquids, CO2-expanded liquids (CXLs) were formed. In CXLs, the solubility of hydrogen in organic liquids increased, and the density and viscosity of organic liquids decreased and thus the resistance of diffusion within liquids decreased. The diffusion coefficients of p-chloronitrobenzene in CO2-expanded methanol and the diffusion coefficients of benzonitrile in CO2-expanded ethanol were investigated in this chapter. Several variables including temperature, pressure, and the CO2 concentration in organic liquids were studied. It was found that the diffusion coefficients of solutes in CXLs were higher than those in pure liquids, indicating the benefit of the dissolution of CO2 in liquids for diffusion. For a fixed pressure, the diffusion coefficients of solutes were observed to increase with temperature. The diffusion coefficients of solutes were also found to decrease with increasing pressure at a fixed temperature. The corresponding equations of diffusion coefficients were established according to different systems of diffusion.