Comprehensive study of semicontinuous emulsion polymerisation processes for producing nanoparticles

• Main Author: Chen, Yan
• Published: King's College London (University of London) 2012
• Subjects: 547
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.628125

Semicontinuous emulsion polymerisation offers flexibility and versatility for manufacturing colloids for a wide range of applications. The technique has been studied since 1950s, however, its importance has only recently realized as evidenced by increasing attentions it has received in recent years. Despite all attentions, the potentials of this process to produce novel materials have not been well recognized yet by polymer industry. The main aim of the thesis is to investigate the underlying mechanisms of semicontinuous emulsion polymerisation processes in order to improve and/or control the properties of resulting polymer latexes, and design new colloids. Polymerisations have been carried out in a jacketed glass reaction vessel. The conversions of monomers were measured by gravimetric method. Particle sizes and size distributions, their zeta potentials and molecular weights were measured by dynamic and static light scattering techniques. Turbidity as well as phase transition behaviour of latexes was measured by UV-visible spectroscopy. Glass transition temperature of polymers was measured by differential scanning calorimetry. Morphologies of particles were investigated by transmission electron microscopy and scanning electron microscopy. Semcontinuous microemulsion polymerisation of styrene has first been carried out in order to study the true features of nucleation stage. A major step has been taken forward to reconcile experimental data with theory. The average diameter of particles (D) decreased and the particle size distribution (PSD) narrowed with decreasing rate of monomer addition (Ra). Nanolatexes with extremely sharp particle size distribution with average particle size as small as 10 nm could be easily produced by the end of feeding. Consistent with recent theoretical developments, D at the end of nucleation was found to be independent of surfactant concentration ([5]). Furthermore, the average diameter of particles was successfully reduced by almost 25%. Thermosensitive and water-soluble poly(n-isopropylacrylamide) (polyNIPAM) colloids have recently attracted increasing attentions for many applications. A set of kinetic data has been reported for the batch polymerisation of neat NIPAM using a wide range of monomer and surfactant concentrations for the first time. A novel method has been applied to produce fast dissolving polyNIPAM latexes with high solids content, based on layered structures produced via semicontinuous process. The application of above method has been extended to fabrication of ultrafme thermal-sensitive nanocapsules. The core material produced via semicontinuous process could be easily and completely removed via self-removing process in water, in comparison with the conventionally made core material. Furthermore, the thickness of crosslinked polyNIPAM shells could also be precisely controlled via semicontinuous process. In recent years, considerable effort has been expended on research in polymer/silica core-shell hybrid systems due to their promising applications in surface coatings and biotechnology. Comparing with conventional batch process, hybrid particles obtained by semicontinuous surfactant-free emulsion polymerisation can incorporate more silica nanoparticles into the shell. The incorporation of silica nanoparticles into the shell can be further improved by gradually feeding silica dispersion during polymerisation. A mechanistic scheme for particle formation of hybrid particles under different pH has been developed. Monomers with intermediate water solubility like methyl methacrylate (MMA) can only produce stable composite particles at acidic conditions, at which silica nanoparticles are less stable. By contrast, more water-soluble monomers like vinyl acetate (VA) can easily yield composite particles at both basic and acidic conditions. Polymerisation on the surface of silica nanoparticles is believed to occur in the course of polymerisation for water-soluble monomers, which can suppress the electrostatic repulsion between both negatively charged silica and polymer particles and thus promote the adhesion of silica to polymer particles. For highly water-soluble monomers such as NIP AM, uniform silica armoured polyNIPAM particles could only be obtained under basic conditions. Interestingly, under acidic conditions, agglomeration of growing polymer particles with silica particles was extensive, resulting insoluble gels (even in the absence of crosslinker). TEM images revealed that under basic conditions, polymer-core/silica-shell structures were produced by batch process, but homogenous hybrids of polyNIPAM/silica nanoparticles by semicontinuous process.