POLYMERIZATION OF MONOMERS IN MICROEMULSIONS FOR FORMING POROUS POLYMERIC MATERIALS
CHIENG TIONG HAN
CHIENG TIONG HAN
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Abstract
This thesis presents the studies of polymerization of methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) in both water-in-oil (w/o) and bicontinuous microemulsions for forming porous polymeric materials using nonpolymerizable and polymerizable surfactants. The following three objectives have been accomplished from this study. 1. Formation of porous polymeric materials by polymerization of monomers in w/o and bicontinuous microemulsions using non-polymerizable anionic, cationic surfactants and a polymerizable anionic surfactant. 2. Development of novel methods for microemulsion polymerization to control the microstructures and pore sizes of the porous polymeric materials. 3. Understanding of possible polymerization mechanisms in the formation of porous polymeric materials and the factors affecting their microstructures. The transparent (isotropic) and thermodynamically stable microemulsion regions of systems consisting of monomer (MMA), copolymerizable cosurfactant (HEMA), a non-polymerizable anionic surfactant (SOS) or cationic surfactant of n-alkyltrimethylammonium bromide (CnTAX, where n=12 -16 and X= Br- or Cl-) or a polymerizable anionic surfactant (Na11-EAAU) and water were determined. The microstructure of each microemulsion system was established before proceeding to polymerization. Polymeric materials with coagulated spherical aggregates were obtained from the polymerization of microemulsion systems using non-polymerizable anionic surfactant (SDS). Opaque polymeric materials with open-cell structures were generally formed from precursor transparent microemulsions containing water contents exceeding 20 wt%. But transparent polymeric materials with closed-cell structures were obtained at water content less than 20 wt%. Pore sizes of the polymeric materials in the range of 1-10 µm were obtained depending on the water content of the precursor microemulsions used. The initial polymerization of bicontinuous microemulsions lead to the generation of spherical polymer particles as revealed by TEM. The particles grew in sizes and coagulated with neighbouring particles through strong hydrophobic interactions. The pores/voids are believed to be water-filled spaces generated in between incompletely coalesced aggregates. The formation of large spherical aggregates (micron sizes) indicate the inefficiency of SDS molecules in stabilizing the growing polymer particles in the reaction medium. As a result, the phase separation occurred through the extensive coalescence between the polymer particles. For porous polymeric materials prepared from precursor microemulsions formulated with CnTAX, they were transparent when the water contents used were less than 35 wt%. The opacity of the polymeric materials diminished on increasing the surfactant-water ratios. This is in contrast to the system using SDS where an increase in surfactant concentration did not seem to have any appreciable change in appearance except causing the system less stable. Various factors affecting the morphology of the porous polymeric materials, which include water content, surfactant concentration, alkyl chain length of CnTAX mixed ratio of long and short alkyl chain length of CnTAX and MMA to HEMA ratio were investigated. It was found that the higher the water content, the lower the surfactant concentration, the longer the alkyl chain length of CnTAX and the higher the weight ratio of long to short chain length of CnTAX, the larger was the pore dimension of the polymeric materials. The incorporation of polymerizable anionic surfactant Na11-EAAU into the microemulsion systems ensured no apparent phase separation during the polymerization for obtaining transparent and microporous polymeric materials. All the organic components in this microemulsion system could easily be cross-polymerized to form polymeric solids. Their microporous structures may be related to the water domains in the precursor microemulsions. By adjusting the water content in the precursor microemulsions, transparent solid polymers with a closed-cell or open-cell structure were produced readily. Based on the understanding of the possible polymerization mechanisms and the judicial choice of the above-mentioned three types of surfactants, new polymerization methods have been developed to control the microstructures and pore sizes of polymeric materials.
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Date
1996
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Thesis