Investigation On Organic-inorganic Hybrid Nanoparticles And Nanocapsules By (Mini) Emulsion Polymerization

Nanoparticles and nanocapluses with organic-inoragnic hybrid structure can exhibit the excellent properties of both inorganic and polymer materials. They can be used in some pioneer fields, such as controlled release, biotechnology, medicine etc. In this thesis, a special functional comonomer, 3-trimethoxysilyl propyl methacrylate (MPS), is copolymerized with styrene in the seeded emulsion (or miniemulsion) polymerization process to prepare the organic-inoragnic hybrid nanoparticles with core-shell morphology or nanocapluses.It is found that this process is determined by the coupling reaction of free radical polymerization and hydrolysis-condensation. Therefore, partition and hydrolysis/condensation rates of MPS are the key factors determining the reaction process. In the thesis, MPS partition behavior is characterized by gas chromatography (GC). It is found that MPS concentration both in water and particle phase increase linearly up to the saturated concentration when MPS addition amount is about 11%wt of polymer seeds. Then the concentrations in these two phases keep constant at the saturated concentration, which are 0.37 and 0.014 mol/L in particle and water phases, respectively. The MPS partitions in each phase have constant coefficients, which can be expressedas: Meanwhile, the hydrolysis andcondensation rate constants are measured at different pH value with ²⁹Si liquid-state NMR. It is found that the hydrolysis and condensation rate constants-pH plots show inverted bell-type with a lowest value at pH=7 and 4, respectively. A formula is presented to correlate the hydrolysis and condensation rate constants at different pH value as:Based on the reaction mechanism and the thermodynamic criterion governing the final morphology of the particles, organic-inorganic hybrid core-shell nanoparticles are designed and synthesized by seeded emulsion polymerization. Through morphology design, polystyrene, as seeded cores, are synthesized firstly;then MPS is added to form MPS-styrene copolymer shell on the surface of polystyrene core;and finally the organic-inorganic hybrid shell is formed through sol-gel process via the hydrolysis-condensation of organic alkoxysilane groups in MPS. The operation conditions, such as pH value, saturation degree of MPS, etc, have great influence on the latexmorphology and microstructures: the hydrolysis-condensation rates are quite slow at pH=7, free radical polymerization is the dominated reaction in the system and MPS enter into particles very quickly, only a little parts of SiOR groups are hydrolyzed to form slightly hydrogen bonded crosslink;in the case of pH=8.5 and unsaturation, high hydrolysis-condensation rate constants will lead the copolymer shell to have C-C main polymer chain with side chains of Si-O-Si oligomers or cyclic molecules;and in the case of pH=8.5 and saturation with MPS, higher MPS concentration and higher hydrolysis-condensation rates in water phase will promote the growing up and agglomeration of the Si-O-Si oligomers or cyclic molecules in the water and make the emulsion unstable finally;at pH=2, the extreme high hydrolysis rate (much higher than condensation rate) leads to the formation of multi-SiOH resultants, which act as crosslinker and cause the gelation of the system.A monomer partition model was developed firstly using constant partition coefficients;then a free radical emulsion polymerization model with the monomer partition was presented. On the base of these two models and the features of hydrolysis-condensation reaction for organosilane at various phases in the heterogeneous system, a complete kinetic model of emulsion polymerization with the participation of hydrolysis-condensation reaction was derived. This model is used in the emulsion copolymerization system of MPS and styrene, and are compared with the experimental data obtained by GC and solid-state ²⁹Si NMR. It is found that the model predicted results are in good agreement with the experimental data. It is concluded that the characteristic parameter f of the heterogeneous system is determined by the surface area and the difficulty of functional groups diffusingonto the particle surface, and can be estimated byBy using the synthetic hybrid core-shell nanoparticles, nanocapsules can be obtained after removing the polystyrene core through centrifuge-redisperse or dialysis methods. However, morphology of the nanocapsules obtained by centrifuge-redisperse method is greatly destroyed because of the strong mechanic force during the produced process. The dialysis process can produce perfect nanocapsules, but it consumes quite a long time and the core cannot be removed completely. Therefore, miniemulsion process is used to synthesize hybrid nanocapsules by only one step with using low molecular weight droplets as template. In this process, styrene and MPS are copolymerized on the surface of the well-dispersed isooctane droplets to form capsules. It is found that the micelles and homogenous nucleation methods in the synthetic process must be avoided, so that theaddition amount of surfactant must be less than its critical micelle concentration (3.3g/L) and hexadecane used as costabilizer. It is also found that the initial volume fraction of the monomer in the recipe must be lower than 0.36 to obtain capsules, because less monomer fraction in droplets is in favor of the phase separation after polymer is produced, and higher MPS fraction is favor in the formation of capsules, but too higher MPS content in capsules will lead to collapse.The kinetics of the penetrative process is researched by ultraviolet-visible spectrum (UV), it is found that the diffuse behavior is consisted with diffuse model in themicroporous media: cw - coe B r ', DB =Daï¿¡/r2 . The diffusion coefficients andtortuosities are calculated and suggest that the diffusion velocity is increased with higher MPS content in capsules due to the larger porosity;the tortuosity is less in cresol red-methanol system than that in anthracene-THF system, and nearly didn't change with the MPS content in capsules. The loading of the anthracene is proved by FTIR, and the release velocity is much slower than the loading velocity because the special microstructure of the hybrid nanocapsules.