Charge-Stabilized Dispersion Polymerization To Prepare Polymer Particles

Dispersion polymerization is a unique method to prepare monodisperse polymer particles with diameters in the 1-15μm size range in a single polymerization step. While the stability of the particles or latex, both during polymerization and as end-products, is normally achieved via a steric/electrosteric stabilization mechanism from chemically grafted or physically adsorbed polymers with an applied amount of at least 5 wt%, and the commonly used electrostatic stabilization mechanism was studied less. In this thesis, the polymerization characters and mechanism of charge-stabilized dispersion polymerization system was studied in detail, which enriched the dispersion polymerization, and developed new methods to fabricate clean polymer particles. The main contents were listed as followed:1. A simple and highly efficient in-situ self-stabilized dispersion polymerization system by copolymerization of cation-charged monomer 2-(methacryloyloxy)ethyltrimethylammonium chloride (DMC) with styrene (St) in a methanol/water (MeOH/H2O) mixture was studied. Monodisperse cation-charged polystyrene (PS) particles with average diameters of approximately 200-1600 nm could be directly obtained. The polymerization rate was very fast and a much lower amount of DMC (0.025 wt% based on styrene) was required to prepare monodisperse and stable PS particles. The reaction parameters:solvent composition, stabilizer content, monomer and initiator concentration had similar effect on particle size and size distribution with the conventional dispersion polymerization. By using X-ray photoelectron spectrometry (XPS), NMR and ion-exchange/conductometric titration to characterize the composition of the particles and the surface charge density, and by synthesizing model monomer to replace DMC to proceed the polymerization, it was found that DMC was copolymerized on the particle surface to stabilize the particles, and the true stabilizer was the PS-PDMC copolymer formed in-situ.2. In St/DMC/MeOH/H2O polymerization system, by following the polymerization process, the metastable state of the nucleation stage, their aggregates and the aggregating process, was first observed experimentally; the molecular weight of the deposited polymer played a very important role on the formation of spherical particles; and water content had great effect on the particle nucleation/growth process. Three polymerization modes were obtained:1) A water/methanol (20/80) system, corresponding to a typical dispersion polymerization mode where the particle nucleation occurred in the solution phase and growth in the particle phase; 2) a pure methanol system, including a first nucleation in the solution phase with growth by absorption of the small particles formed in this phase, and a secondary nucleation when high molecular weight copolymers appeared in the solution phase with growth in the particle phase; and 3) a water/methanol (5/95) system, similar to the conventional dispersion polymerization mode during the first 90 min, with subsequent epitaxial growth. It was very important to enrich the dispersion polymerization process and understand the particle nucleation/growth mechanism.3. Using a polymerizable sodium styrene sulfonate (NaSS) as the stabilizer, methanol/water mixture as the reaction medium to produce clean PS particles was investigated. Surface-charged and monodisperse PS particles with average diameters of approximately 470-1600 nm could be obtained. NaSS was quite efficient as the stabilizer, and as little as 0.05 wt% was enough to prepare stable latex with monodisperse particles. The reaction parameters:solvent composition, stabilizer content, monomer and initiator concentration had similar effect on particle size and size distribution with the DMC stabilized polymerization system. XPS result indicated that NaSS had copolymerized on the particle surface.4. It was difficult to produce highly cross-linked PS particles in the conventional dispersion polymerization. In St/DMC/MeOH/H2O polymerization system, using one-step polymerization method, as the increasing of the divinylbenzene (DVB) content, the gel content:and the glass transition temperature (Tg) of the produced particles both increased. By following the polymerization process of the cross-linked particles, it was found that the polymerization mechanism was similar with that of uncorss-linked particles:including the nucleation in the solution phase and growth in the particle phase, and phase separation occurred and polymer chains formed in the solution attached on the particle surface at the later polymerization time because the interior of the particles were cross-linked, and thus particles with coarse surface were obtained. By tuning DMC concentration, cross-linked PS particles with 85% gel content were produced by one-step polymerization method. Using two-step method, by adding cross-linker during the fastest polymerization stage, highly cross-linked PS particles which could maintain their shape in THF were obtained; changing the adding amount of DVB at the second step, unsymmetrical cross-linked PS particles could be produced; PS/cross-linked PS core-shell particles were produced by adding DVB 2.5h after the reaction began.5. In St/NaSS/MeOH/H2O polymerization system, using two-step polymerization method, PS particles with epoxy groups, carboxylic groups, temperature response polymer and photoreactive groups on the surface were produced by adding glycidyl methacrylate (GMA), acrylic acid (AA), N-isopropylacrylamide (NIPAm) and 4-allyloxy-2-hydroxybenzophenone (BP-OH) into the reaction system at the later stage of the polymerization. According to the results, it was found that hydrophobic monomers were likely to copolymerize on the particle surface by the two-step polymerization method, while hydrophilic monomers were not. 6. In DMC/MeOH/H2O polymerization system, when the ratio of MeOH/H2O was less than 70/30, stable polymethylmethacrylate (PMMA) particles were produced; when the ratio of butyl acrylate (BA)/St was greater or equal to 1/1, the Tg of the obtained PS/PBA particles were less than room temperature, and the Tg of the particles decreased with the increasing of BA content. Using two-step polymerization method, by adding St into the polymerization system at the later stage, PBA/PS core-shell polymer particles could be obtained.