Phase behavior and phase separation kinetics in polymer solutions under high pressure

The phase behavior and phase separation kinetics in polymer solutions in binary mixtures of supercritical carbon dioxide (CO2) and organic solvents were studied for two systems. Solutions of polyethylene (PE) in CO 2 + n-pentane were selected as one model system to study both the solid-fluid (S-F) and liquid-liquid (L-L) phase transitions as well as the interplay of these two types of phase separations on the final morphological and thermal properties of PE crystals. Solutions of polysulfone (PSF) in CO2 + tetrahydrofuran (THF) were selected as another model system because of the technological importance of this membrane forming polymer and because of the broad interest in developing new solvent/non-solvent systems for forming microporous materials. These phase boundaries were determined using a high-pressure view-cell and optical techniques over a temperature range of 90--165°C and pressures up to 55 MPa for PE/n-pentane/CO2 system, and over a temperature range of 25 to 155°C and pressures up to 70 MPa for PSF/THF/CO 2 system.; For PE solutions, it has been found that the addition of CO2 to the PE/n-pentane system shifts the L-L phase boundary to significantly higher pressures, but moves the S-F phase boundary only slightly to higher temperatures. The S-F phase boundary which represents the crystallization/melting process in the polymer solution was about 10°C lower than the crystallization/melting temperatures of the neat polyethylene samples determined by differential scanning calorimetry (DSC). It was further found that the S-F phase boundary in n-pentane displays a unique sensitivity to the pressure-temperature conditions and moves to lower temperatures in the pressure range from 38 to 42 MPa. This effect even though not as augmented remains also for the S-F boundary in the solutions in CO2 + n-pentane mixtures.; The miscibility of PSF in THF + CO2 was investigated at CO 2 levels up to 14 wt%. This system shows lower critical solution temperature (LCST)-type phase behavior at low CO2 content, which is shifted to upper critical solution temperature (UCST)-type at higher CO2 levels along with an increase in the miscibility pressures. In contrast to the PE system, this system was found to display multiple miscibility windows. A 'U'-shaped phase boundary in 92% THF + 8% CO2 mixture was observed to transfer to a "W"-shaped phase boundary at 10 wt% CO2, which was further separated into a double 'U'-shaped phase boundary at 13 wt% CO2. The specific volume of the polysulfone solutions were found to display a variation parallel to this changing pattern in the phase boundaries, with reduced miscibility being accompanied with an increase in the specific volume.; The phase separation kinetics in these two polymer solutions were investigated using time- and angle-resolved light scattering techniques. With the PE solutions, the focus was on the kinetics of S-F phase separation (crystallization) and miscibility and (melting) in n-pentane. Experiments were conducted with relatively dilute solutions at concentrations up to 2.3 wt%. The results show that the crystallization which was induced by cooling at constant pressure is dominated by a nucleation and growth process. In the majority of the experiments the particle growth process was observed to last for about 1 minute with a slight dependence on the crystallization pressure. (Abstract shortened by UMI.)...