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Authors: PAUL TAN
Issue Date: 1992
Abstract: The polymerisations of methyl methacrylate (MMA) and/or styrene in oil-in-water microemulsions stabilised with dodecyltrimethylammonium bromide (DTAB) were initiated by potassium persulphate at 60°C. The growth of the particle size during polymerisation of MMA seems to follow an exponentially decay law. The initial growth, during the nucleation period, was dominated by the diffusion of monomers into the growing particles. There was an increase in the number of the polymerised microemulsion particles, nd, in the nucleation stage until it reached the maximum rate of polymerisation (Rp). Subsequently, the merger of the polymerised microemulsion particles became significant resulted in the increase of the particle sizes and decrease the number of microemulsion particles. The weighted average molecular weight, Mw, of the polymers in the polymerised microemulsions at 60°C was high (106D) and the number of polymer per particle, np, was about 1-5. The particle sizes of the microemulsion after polymerisation at 60°C for 18 hours depended on the types and concentrations of monomers. For polymerised microemulsions containing poly (methymethacrylate), the particle sizes ranged from 30 - 110 nm in diameter, whereas they were smaller (30-60 nm) for those containing Polystyrene (PS) or the copolymer PS/PMMA. Polymerised microemulsion containing PS was much more stable than those containing PMMA. The particle size (35.7 nm) of styrene-polymerised microemulsion determined by TEM was in agreement with that (36.2 nm) from light scattering measurements. However, they were quite different for those microemulsions containing PMMA. It is believed that PMMA is unstable under the electron beam resulting in dimensional changes. Most Microemulsions polymerised at 60°C for 18 hours and kept at 20°C remained stable even up to one year. The polymerised microemulsions containing PMMA that were kept at 60°C for an extended period of time became phase separated when the particles grew to about 100nm in diameter. But microemulsions containing PS remained stable with no much changes in particle sizes even after one year on standing at 60°C. Thermodynamic treatments of the microemulsion system were carried out using Safran's model with a slight modification. The determination of the thermodynamic parameters showed that the total free energy of the system increased as polymerisation proceeded. This is consistent with the observation that the polymerised microemulsions became increasingly unstable. The total free energy for microemulsions containing PMMA became positive on long standing at 60°C. That is the onset of phase separation.
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