| Summary: | 博士 === 國立清華大學 === 化學工程研究所 === 75 === The kinetics and mechanism of emulsifier-free emulsion polymerizations of styrene in the presence of hydrophilic comonomer of ionic type (sodium methallyl sulfonate, NaMS), or of nonionic type (2-hydroxyl ethyl methacrylate, HEMA), and of the initiator, potassium persulfate, are studied. For NaMS comonomer system, the number density of polymer particles (Np) is found to depend on 0.5-power of the initiator concentration and shows a minimum in the comonomer concentration plot under constant ionic strength. Under constant concentrations of the monomer, comonomer and initiator, Np is found to depend on-1. 1-power of the ionic strength. In the earlier period, the presence of styrene oligomer having MW about 1000 and water soluble poly(NaMS) or copolymer with high NaMS content suggest a micellar nucleation mechanism. For HEMA comonomer system, Np is found to depend on 0.97-power of the initiator concentration and 0.17-power of the comonomer concentration under constant ionic strength; and under constant concentrations of monomer, comonomer and initiator, Np is found to depend on -0.78-power of the ionic strength. In the earlier period, the generation of water-insoluble but hydrophilic copolymer and the absence of oligomer suggest a homogeneous nucleation mechanism.
In the particle growth period,m because of the larger particle size, the monomer diffusion rate is not fast enough to keep a homogeneity and saturation of monomer within the polymer particle, and the motion of polymer radical end within the particle will be restricted by the other hydrophilic chain end anchored on the particle surface, so the mechanism of particle growth transfers from homogeneous polymerization via a gradient polymerization to shell part polymerization. In order to elucidate the role of particle size in the occurrence of core-shell structure mechanism (shell part polymetization),and the shell polymerization during the growth period, seeded emulsion polymerization with different sizes of seed are carried out. It is found that, as the diameter of particle is larger than 2000 A, the particle growth follows the core-shell thickness is about 100 to 160 A. But for the system with HEMA as comonomer, as the chemical composition of the particle surface changes, the shell thickness also changes, owing to that the monomer swelling property and monomer diffusion property within the particle will change with the chemical nature of the particle surface.
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