Control Over Formation Efficiency And Particle Properties Of Nanocomposite Particles In SiO₂-containing Miniemulsion Systems

Polymeric/inorganic nanocomposite particles（NCPs）have excellent overall properties,as they combine polymers and inorganic particles.The enhanced mechanical property,thermal stability,and processability of the polymeric/inorganic NCPs can be ascribed to the synergistic effect between polymers and inorganic particles.Moreover,the polymeric/inorganic NCPs may display excellent catalytic,electromagnetic,and optical properties through incorporation of functional inorganic particles.Therefore,they are widely used in high-performance coatings,biology,medicine,catalysis,and textiles.Miniemulsion polymerization is one of the most widely used techniques for preparation of polymeric/inorganic NCPs.However,the formation efficiency of polymeric/inorganic NCPs is relatively low in most cases.Therefore,it is necessary to study the particle formation mechanism during polymerization in the miniemulsion systems containing inorganic nanoparticles（NPs）,to know the main reason for the formation of pure polymer NPs,and furthermore,to propose specific strategies for efficiently improving the formation efficiency of polymeric/inorganic NCPs.These studies will definitely promote the application of miniemulsion polymerization in the preparation of polymeric/inorganic NCPs.In this thesis,miniemulsion systems were built through using SiO₂ NPs modified with 3-（methacryloxy）propyltrimethoxysilane（MPS）as representative inorganic NPs,methyl methacrylate（MMA）and styrene（St）as comonomers,and sodium dodecyl sulfate（SDS）,Tween 20,or their combination as surfactants.The particle formation mechanism,the strategy to enhance the formation efficiency of the polymer/SiO₂ NCPs,and the control over the particle characteristics of the polymer/SiO₂ NCPs were studied.In the SiO₂-containing miniemulsion stabilized with SDS,the influence of the SiO₂ and SDS amounts on the particle morphology and particle size,and the formation efficiency of the polymer/SiO₂ NCPs were studied.The results showed that IX pure polymer NPs dominated in the final emulsion regardless of the SiO₂ and SDS amounts when the sonication power was 95 W,and the number fraction of polymer/SiO₂ NCPs(f_NCP)was lower than 4%.Centrifugation experiments indicated that the final emulsion contained three types of particles,namely,a large number of pure polymer NPs,a small amount of the polymer/SiO₂ NCPs.and a very small amount of the sub-micrometer polymer/Si O₂ aggregates.The mechanistic investigation showed that these three types of particles evolved from the small plain monomer droplets,medium monomer droplets containing several SiO₂ NPs,and micrometer monomer droplets containing many SiO₂ NPs,respectively.It should be pointed out that although the number of the micrometer monomer droplets was extremely low,each of them contained many Si O₂ NPs,leading to the inhomogeneity in the number distribution of SiO₂ NPs among monomer droplets and the low formation efficiency of polymer/SiO₂ NCPs.On the basis of the mechanistic understanding,enhanced sonication powers were adopted to improve the homogeneity in the number distribution of SiO₂ NPs among monomer droplets.The f_NCP markedly increased to ³9% when the sonication power was increased to 190 W.However,the f_NCP could not be improved with the further increase of the sonication power.In order to further improve the formation efficiency of polymer/SiO₂ NCPs,Tween 20 was selected as the surfactant to build miniemulsion systems,as Tween 20 has good emulsifying ability but relatively weak protection ability.The f_NCP value of latex particles was ⁹4% when 0.075 g of Tween 20 was used.High f_NCP values of latex particles（above 80%）could be achieved in the range of 0.075–0.15 g of Tween-20.Cryo-transimission electron microscopy indicated that the distribution of SiO₂ NPs among monomer droplets was relatively more homogeneous when Tween 20 was used as the surfactant,compared with SDS.The evolution of the particle morphology and particle size during polymerization was monitored by conventional transmission electron microscopy and dynamic light scattering.It was found that the mergence among the dispersed objects occurred during polymerization.The formation of pure polymer NPs was avoided because of the disappearance of the plain monomer droplets through mergence.A formation mechanism of particles was proposed on the X basis of the effects of Tween 20 amount on the droplet size distribution and particle morphology,and the evolution of the particle size and morphology during polymerization.The main reasons for the efficient preparation of the polymer/SiO₂ NCPs are as follows: 1)The distribution of SiO₂ among monomer droplets is relative homogeneous;2)the mergence between the dispersed objects inhibits the formation of pure polymer NPs through the droplet nucleation of the plain droplets.Finally,a special use strategy of surfactants was employed to efficiently synthesize narrowly size-distributed polymer/SiO₂ NCPs in SiO₂-containiing miniemulsion systems.First,0.15 g Tween 20 was used to prepare a monomer miniemulsion,and then,a trace amount of SDS was added to the prepared monomer miniemulsion.The post-addition of SDS could increase the colloidal stability of the dispersed phase during polymerization,partially suppress the mergence of the dispersed objects,and thus obtain the polymer/SiO₂ NCPs with a relatively small size.In comparison with the miniemulsion systems stabilized with Tween 20,the high formation efficiency of polymer/SiO₂ NCPs was maintained,indicative of a high f_NCP value of 97% in the emulsion stabilized with 0.15 g of Tween 20 and 2.5 mg of post-added SDS.More promisingly,the particle size of the polymer/SiO₂ NCPs decreased from 341 nm to 217 nm,and the particle size distribution was remarkably narrowed through post-addition of SDS.The effects of the post-added SDS amount,the modification degree and amount of the SiO₂ NPs on the particle size and its distribution,and particle morphology of polymer/SiO₂ NCPs,and the f_NCP value were systematically studied.The particle size and f_NCP value decreased with the increase of SDS amount.The SiO₂ modification degree had a significant influence on the f NCP value.To obtain high formation efficiency of polymer/SiO₂ NCPs,the SiO₂ modification degree should be higher than 10%.The SiO₂ amount only had a minor effect on the particle size and f_NCP value.The particle size distribution of polymer/SiO₂ NCPs was always narrow regardless of the post-added SDS amount,the modification degree and amount of the SiO₂ NPs.In conclusion,we discovered the particle formation mechanism of SiO₂-containing miniemulsion polymerization systems,provide an effective strategy to enhance the formation efficiency of polymer/SiO₂ NCPs,and designed a special use strategy of surfactants to control over the particle properties of polymer/SiO₂ NCPs.The conclusions obtained in this thesis may guide the efficient preparation of versatile polymeric/inorganic NCPs with controlled particle properties through the miniemulsion polymerization technique.