迷你乳化聚合製備奈米複合乳膠顆粒PS/Fe3O4，P(AA-SA)/ZnO及PMMA-b-PBA — 成核機制，形態控制及動力分析

Abstract

Composite materials were made from two or more constituent materials with different chemical or physical properties and have been demonstrated an enhanced performance. It has been widely applied in many fields, such as military hardware, aviation, architecture and fitness equipment. In this work, composite latex particles and block copolymers were synthesized via miniemulsion polymerization. The reaction mechanisms and related applications were investigated. This research was divided into three parts. The objective of the first part was to prepare PS/Fe3O4 composite latex particles by oil in water (O/W) miniemulsion polymerization. The objective of the second part was to prepare P(AA-SA)/ZnO composite latex particles by water in oil (W/O) miniemulsion polymerization. The objective of the third part was to synthesize PMMA-b-PBA block copolymers via controlled/living radical polymerization. In the first part, there were two synthesis pathways to prepare PS/Fe3O4 nano composite latex particles. The first pathway contained a two-step process. First, the Fe3O4 nanoparticles were prepared by a coprecipitation method followed by the surface treatment with lauric acid. Hence, the surface modified Fe3O4 was hydrophobic in nature. In the second step, the monomer droplets containing Fe3O4 and costabilizer were dispersed in water with surfactant by ultrasonication. As the miniemulsion polymerization was initiated by water-soluble initiator, potassium persulfate (KPS), or oil-soluble initiator, 2,2'-azobisisobutyronitrile (AIBN), magnetic composite latex particles could be obtained. The influences of initial formulation on the monomer conversion, size distributions of monomer droplets and latex particles, nucleation mechanism, and morphology of composite particles were investigated in depth. Another synthesis pathway contained a three-step process. On the first step, the Fe3O4 dispersion was prepared by a coprecipitation method followed by the surface treatment with lauric acid and sodium dodecyl sulfate (SDS). On the Second step, the one-to-one copy of monomer droplets to latex particles could be synthesized via polymerization of an equilibrium stabilized miniemulsion prepared from a less stringent preparation process. The size distribution of obtained latex particles was relatively narrow. On the third step, by mixing the Fe3O4 dispersion with latex particles, the magnetic composite latex particles could be fabricated from heterocoagulation. Moreover, an all-atom molecular dynamics simulations were employed to explore the influences of sizes and surface polarity of polymer and inorganic particles on the morphology of composite latex particles. The simulation results were in agreement with our experimental results. In the second part, P(AA-SA) latex particles were synthesized via W/O miniemulsion polymerization. In order to minimize the monomer dissolving in continuous phase, cyclohexane, the polymerization was carried out in the presence of ammonium persulfate/sodium metabisulfite (APS/SMBS) redox initiators at 0-5oC. The influences of costabilizer on the stability of miniemulsion, its morphology and nucleation mechanism were studied. The pKa and pH regulation capacity of P(AA-SA) latex particles synthesized in this work were investigated in depth. Furthermore, the ZnO nanoparticles were fabricated by a hydrothermal synthesis method in ethanol followed by oleic acid surface modification for dispersing the nanoparticles in cyclohexane. Based on our previous experimental procedure of synthesizing P(AA-SA) latex particles, P(AA-SA)/ZnO composite particles could be fabricated by introducing modified ZnO nanoparticles into continuous phase. ZnO was not only amphoteric substance but also owned an excellent performance in photocatalyst and UV shielding. The applications of P(AA-SA)/ZnO composite particles were various besides pH regulation. The reaction mechanism, morphology, pH regulation capacity and UV/Vis absorbance properties of composite latex particles were examined. In the third part of this work, 1,1-diphenylethene (DPE) was employed to control the living free radical polymerization of MMA. When the reaction temperature was low (less than 95oC), the molecular weight of synthesized polymer remained almost a constant throughout the reaction time regardless of changing the amounts of DPE and initiator. A living polymerization kinetic model was established and compared with our experimental results. The kinetic rate constants involved in the DPE mechanism were estimated. The rate constant k2, corresponding to the reactivation reaction of the DPE- capped-dormant chains, was found to be very small, that accounted for the result of a constant molecular weight of polymer synthesized throughout the polymerization. In order to increase k2, the polymerization temperature was increased to 135oC, and the molecular weight of polymers increases with conversion, demonstrating the living nature of DPE mechanism. In addition, by using a preheating treatment, all the initiators dissociated into radicals at the very beginning of the polymerization. Then the synthesized polymers with narrow molecular weight distribution could be prepared successfully. From the trace of GPC diagram, a unimodal rather than bimodal molecular weight distribution was observed throughout the polymerization. Finally, PMMA-b-PBA block copolymers were synthesized by two polymerization methods (homogeneous polymerization and miniemulsion polymerization) using DPE-containing PMMA as a macroinitiator. The influences of solvent and polymerization methods on the polymerization rate, controlled living character, molecular weight (Mn) and molecular weight distribution (PDI) throughout the polymerization were studied and discussed.