| The thesis describes a method to prepare thermally expandable polymer microspheres encapsulating volatile hydrocarbon blowing agent by a suspension polymerization process. When the surrounding temperature was high enough to vaporize the blowing agent and soften the polymer shell, the microspheres expanded due to the pressure difference across the shell. A diffusion-viscoelasticity model was developed to describe the expansion process of the microspheres, and to investigate their thermal expansion properties. The expansion performance of the microspheres were found to be dependent on many polymer latex properties, such as particle size, ratio of polymer to blowing agent, gas permeability and viscoelastic properties of the shell and particle morphologies. Statistical design of experiments and contour mapping were employed to investigate the critical parameters to control the expansion properties and the size of the polymer microspheres. It was found that the use of short chain alcohols, such as ethanol and ethylene glycol, could increase the particle size, and dramatically affect both the emulsion stability and the particle morphology. The stabilization mechanism of the O/W emulsion stabilized by silica colloids and polyelectrolyte was further investigated. The effects of aqueous pH and alcohol concentration on the emulsion stability revealed that a moderate flocculation of the silica colloids by the polyelectrolyte promoted the stability. The polymer microspheres were reported to have different morphologies, such as poor-encapsulation, symmetric single-cellular and asymmetric single-cellular structures, and multi-cellular structure. The use of increasing amounts of ethanol and ethylene glycol changed the morphology from poor-encapsulation to single-cellular, and then to multi-cellular structure. However, the use of glycidyl methacrylate or hydroxypropyl methacrylate was found to have an opposite effect. The particle morphology was found to depend on the early phase separation in the oil phase. When the phase separation was promoted by high concentration of alcohols or initiators, single-cellular and multi-cellular structured microspheres were produced. The findings suggested that the particle morphology was controlled by the combination of the kinetics and thermodynamics of the phase separation process. |
