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Abstract
Miniemulsion polymerization has received significant attention because polymer particles are generated directly from monomer droplets. However, the problem with current methods of synthesizing miniemulsions is the requirement for high energy mixing approaches. The ability to synthesize miniemulsions using low energy methods would be of very significant academic and practical importance. In the present thesis, radical polymerization in heterogeneous systems under compressed gases has been investigated with a view to the development of low energy miniemulsion polymerization.
The mechanistic aspects of radical polymerization of styrene in aqueous emulsions pressurized by CO2 have been investigated. It has been demonstrated that CO2 pressurization to the so-called transparency pressure (PT) of an emulsion comprising styrene/hexadecane/Dowfax 8390/water leads to a decrease in droplet size as well as an increase in emulsion stability. However, the radical polymerization under CO2 pressure reveals that these polymerizations do not proceed as true miniemulsion polymerization systems (i.e., via exclusive monomer droplet nucleation) but are also characterized by a significant contribution of particle formation via secondary nucleation as in an ab initio emulsion polymerization.
Photopolymerization of miniemulsions under compressed CO2 has been performed under UV and visible-light irradiation. For the UV-light system, the rate of polymerization of a transparent miniemulsion under compressed CO2 was found not to be affected by the transparency. However, the transparency affected the polymerization rate under visible-light irradiation. This is due to the transparency of refractive index matched miniemulsions favours less light scattering than turbid miniemulsions at the at the appropriate wavelength (transparency is associated with high visible light transmission).
Pressurization of an emulsion comprising styrene/hexadecane/Dowfax 8390/water to PT has been performed under compressed ethane. Under the same condition, the value of PT for emulsions under compressed ethane is lower than that of the compressed CO2 due to the solubility difference between ethane and CO2 in water. Polymerization of the emulsion under compressed ethane has been conducted with the initiator VA-044. The polymerization has been found to proceed via conventional emulsion polymerization mechanism as revealed by the mechanistic investigation.