MICROEMULSION POLYMERIZATION AND CHARACTERIZATION OF COLLOIDAL POLYMER PARTICLES

Abstract

This thesis presents the studies of polymerizations of styrene and methyl methacrylate (MMA) and copolymerizations of styrene-MMA, styrenetetrahydrofurfuryl methacrylate (THFMA) and styrene-acrylonitrile (AN) in ternary oil-in-water (o/w) microemulsions. Th~ following objectives were achieved: 1) Proposal of a possible polymerization mechanism which includes both particle nucleation and growth for microemulsion polymerisations of styrene and MMA based on kinetic results obtained. 2) Investigation of stability of resulting PMMA and polystyrene (PS) particles stabilized by cationic surfactant 3) Determination of copolymerization loci based on the monomer reactivity ratios obtained from microemulsion copolymerization. 4) Determination of the parameters affecting the size of nanoparticles obtained and the possibility of synthesizing monodispersed nanoparticles via o/w microemulsion polymerization. The stable microemulsion regions of ternary systems consisting of monomer (MMA or styrene), an alkyltrimethylammonium halide either tetradecyltrimethylammonium bromide (TTAB), tetradecyltrimethylammonium chloride (TT AC), cetyltrimethylammonium bromide (CTAB ), cetyltrimethylammonium chloride (CTAC), stearyltrimethylammonium bromide (STAB) or stearyltrimethylammonium chloride (STAC) and water before and after polymerization were investigated. PS nanoparticles (Rh = 10 - 35 nm) were very stable even being stored at 60°C for more than 200 days. In contrast, the stability of PMMA strongly depended on the chain-length and concentration of surfactant used. The TTAB- or TTAC-MMA system produced the least stable PMMA latex, while the stability of STAC-MMA polymerized system was comparable to the styrene-polymerized system. The polymerization rates for both styrene and MMA systems usually show only two rate-intervals, i.e., the rate increased to a maximum and then decreased. However, an additional constant rate period could be observed when low concentration of initiator was used or it was polymerized at low temperatures (e.g. 50°C). It is established that the main source of effective free radicals for polymerization using either water-soluble KPS or oil-soluble AIBN are from the aqueous phase. The dependencies of rate of polymerization, number of resulting particles and molecular weights of polymers on the concentration of initiator, monomer and surfactant were also established. The dependencies of polymerization rate on the concentrations of initiator and surfactant for microemulsion polymerizations of MMA and styrene are similar to the predicted values (0.4 and 0.6 respectively) based on the Smith-Ewart theory (Case 2). This indicates that the principal polymerization loci for both styrene and MMA were in microemulsion droplets via the micellar nucleation mechanism. For microemulsion polymerization of styrene, the size of monomer-swollen particles increased rapidly to a maximum at about 4-7% conversion and then decreased, whereas a continuous growth in sizes of monomer-swollen particles was observed in MMA systems. The number of particles continuously increased for both microemulsion polymerizations of styrene and MMA due to the continuous nucleation mechanism. The results of 1 to 3 polymer chains per particle and the average of free radicals per particles (less than 0.5) also support the continuous nucleation mechanism. Based on the results of kinetics studies, the nucleation and growth mechanisms are proposed for o/w microemulsion polymerization. Copolymerizations of styrene-MMA, styrene-THFMA and styrene-AN have been carried out in ternary o/w microemulsions. The monomer reactivity ratios of styrene-MMA and styrene-AN for microemulsion copolymerization are different from those obtained from bulk or solution polymerization. The influence of microenvironment of polymerization loci on the compositions of the copolymers are discussed on the basis of the monomers distribution between polymerization loci and the aqueous phase. It is found that the copolymerization proceeds only in the microemulsion droplets. Various parameters affecting the resulting nanoparticles were also investigated. The size is affected by the concentrations of initiator, monomer, surfactant and crosslinker as well as polymerization temperature and seeding method. The conclusion is that the preparation of monodispersed nanoparticles of polymers or copolymers via usual o/w microemulsion polymerization cannot be achieved due to the continuous particle nucleation.