SYNTHESIS AND CONTROL OF MORPHOLOGY OF POLY METHYL METHACRYLATE AND POLY ACRYLIC ACID MICRO-PARTICLES BY THE MODIFIED SUSPENSION POLYMERIZATION TECHNIQUES
Kim, Yunju Jung
Choi, Kyu Yong
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This thesis studied the synthesis and control of morphology of two kinds of polymers by modified suspension polymerization techniques. The first polymer, poly (methyl methacrylate), is a transparent thermoplastic polymer, which is typically used in diffusing film in the backlight unit of an LCD. Also, the synthesis of micron-sized polymer particles with complex internal morphologies such as hollows, multihollows, and multiporous structures is of growing interest in many technological applications such as microelectronic displays and microencapsulation. The direct synthesis of such materials is carried out in heterogeneous processes with controlled phase separation mechanisms. In such systems, detailed knowledge of heterogeneous polymerization kinetics and phase separation phenomena is essential for investigating the process characteristics. An in situ polymerization and phase separation technique has been used to construct a ternary phase diagram for the free radical precipitation polymerization of methyl methacrylate (MMA), n-hexane, and poly(methyl methacrylate) (PMMA) system. The onset of the phase separation point during polymerization is directly monitored in real time by laser light scattering (LLS) technique for a broad range of polymer concentrations. The presented method overcomes the difficulty of determining the cloud points by titrating unreactive blends of polymer and solvent at high initial monomer concentrations that lead to high polymer concentration and high viscosity of the mixture fluid at the system phase separation point. We present the micro dispersive suspension polymerization (MDSP) process to produce complex particle morphologies in a single-stage process. MDSP is a hybrid of suspension and dispersion polymerizations. The micron-sized polymer particles are polymerized by suspension polymerization, and the internal morphology of particle is polymerized by dispersion polymerization inside the polymer particles. Varying the initial conditions for the phase separation in precipitation and dispersion polymerizations, final particles’ morphology may change from solid polymer particles to complex porous polymeric structures. In this heterogeneous process, the system evolution depends on the composition and molecular characteristics of the coexisting phases and on the characteristics of the interface. Using MDSP, we were able to develop a phase diagram to show the regions of multi-hollow/porous and core-shell/pomegranate-like poly (methyl methacrylate) (PMMA) particles. We also show that controlling morphology of polymer particles by thermodynamic and kinetic variables is technically feasible. The second polymer, poly (acrylic acid), is an absorbing polymer. Superabsorbent polymers (SAP) can absorb and retain extremely large amounts of water or aqueous solutions relative to their own mass. Partially neutralized sodium polyacrylate is industrially a very important polymer for many applications. However, in industry sodium polyacrylate is mostly manufactured by bulk polymerization, and the resultant bulk polymer is pulverized using a kneader to obtain small discrete polymer particles. It is environment-unfriendly process and the produced granules from bulk have irregular shapes, rather than a spherical shape. This study is aimed at investigating the inverse suspension polymerization of acrylic acid to make spherical polymer particles. In particular, the study is focused on how the resulting polymer morphology and characteristics are affected by the polymerization conditions. A feasible and simple technique to obtain Na-polyacrylate microparticles with sizes below 10 µm was investigated using a high shear mixing device. To maintain the stability of submicron size of aqueous droplets in the oil medium, a co-surfactant system containing Span 80 and Tween 80 was used. Neutralization of acrylic acid was proved from EDX analysis. Na-polyacrylate submicron particles were characterized by size, surface morphology, swelling capacity, and conversion. When the speed of homogenization was lowered from 3000 rpm to 1000 rpm, particles over 10 µm were obtained, but more nano-sized particles were present outside. We also developed the technique above to increase polymer particle size to tens of microns. In this process, a wrinkled and cracked surface of Na-polyacrylate particles was observed in the special environment of post treatment. Surface area, swelling capacity, and swelling speed of different morphologies and sizes were characterized and analyzed. In order to synthesize spherical Na-polyacrylate particles with smooth surface regardless of post treatment, polymerization time was progressed longer than 20 hr. Na-polyacrylate particles had a solid structure at high conversion over 0.996 after longer than 15 hrs of polymerization, which made particles maintain their shapes regardless of post treatment. When a high monomer concentration was used in this polymerization, perfectly smooth and spherical polymer particles were obtained after 9 hr, which was faster than when a lower monomer concentration was used.