| Precipitation polymerization of acrylic acid in supercritical carbon dioxide (ScCO2) has received considerable attention during the last decade. By replacing water and organic solvents with ScCO2, this technique eliminates the costly polymer drying and purification processes required by conventional poly(acrylic acid) (PAA) production.; The present thesis provides detailed study on continuous precipitation polymerization of acrylic acid in ScCO2 in a continuous stirred tank reactor (CSTR) using 2,2'-azobis (2, 4-dimethyl-valeronitrile) (V-65B) as the free-radical initiator. The reaction temperature was between 50 and 90°C, the pressure was 207 bar, and the average residence time was between 12 and 40 minutes. The product polymer was a white, dry, fine powder that dissolved in water. A wide range of polymer molecular weights (5 to 200 kg/mol) was obtained.; The effect of the operating variables on the polymerization rate and on the polymer molecular weight was evaluated. The polymerization showed distinct characteristics of precipitation polymerization. Three simple kinetic models have been developed to describe this polymerization, and the models have been compared with the experimental results for rate of polymerization and viscosity average molecular weight. The first model, the "solution polymerization model", is based on the assumption that all of the polymerization reactions take place in the fluid phase, and that no reaction takes place in the polymer phase. In the second model, the "surface polymerization model", chain initiation is assumed to occur in the fluid phase, but chain propagation and chain termination occur in a thin zone on the surface of the polymer particles. The third model, the "particle polymerization model", is similar to the "surface polymerization model", except that chain propagation and chain termination take place uniformly throughout the polymer particles. The surface polymerization model and the particle polymerization model both provided a much better fit of the data from the CSTR than the solution polymerization model. However, the data from a batch reaction calorimeter showed the particle polymerization model described the polymerization behavior better.; Scanning electron micrographs showed that three types of polymer particles were obtained: coagulum of primary particles of about 100 nanometers in size, irregular particles of 5-20 micrometers, and spherical particles of 10-100 micrometers. (Abstract shortened by UMI.)... |
