Control and evaluation of dispersion structure of tetrapod nanoparticles

Abstract

Inorganic nanoparticles have the unique electric and optical properties which is determined by their size and shapes, thus various researches about the synthesis of well-defined nanoparticles as well as the analysis of their functionality for the diverse applications. Especially tetrapod nanocrystals are greatly interested because of their large surface area for junction with neighboring materials and formability of interconnected structures from their unique structural feature. Meanwhile, dispersion of inorganic nanoparticles within polymer matrix is most important for full development of their unique properties. Surface modification of inorganic nanoparticles with organic polymers is one way to make well-dispersed inorganic nanoparticle/polymer system. However, even though various techniques for control the dispersion structure have been studied, these are hardly certified as the reliable method without accurate evaluation of dispersion structure. Typically, microscopic method has been used for characterization of dispersion structure. However, recently, scattering methods is not only used for the complement but also the alternative to characterize the structural properties, due to the detectability about ensemble averages of dispersion structure. Therefore we characterize the dispersion structure of tetrapod using scattering method, and control the dispersion structure of tetrapod nanoparticles by hybridization. The features of tetrapod nanocrystals, organic/inorganic hybridization and characterization of dispersion structure using scattering method are introduced in chapter 1. In chapter 2, scattering model for tetrapod is presented by theoretical calculation for characterization of dispersion structure of tetrapod nanocrystals. Geometry of tetrapod is simplified using regularity of identical four cylindrical arms with tetrahedral include angles. Because anisotropic tetrapod needs three rotations for considering the random orientations, additional rotation about bisector of tetrapod is considered. Scattering intensity and their pair distance distribution functions of presented model are calculated numerically. Verification about scattering model is proved by numerical calculation and simulation as well as small angle x-ray scattering experiment of tetrapod dispersed in solvent. In chapter 3, dispersion structure of tetrapod nanoparticles dispersed within polymer matrix is characterized using small angle x-ray scattering. Because dispersion of inorganic nanocrystals within polymer matrices is regarded as the most important for full realization of unique properties of inorganic materials, the dispersion structure has to be characterized preferentially. Scattering intensity from colloidal structure is consist with form factor, which informs the size and shape of particles and structure factor, which contains the correlational information between particles. Form factor of tetrapod is obtained from dilute solution of tetrapod dispersed within good solvent to exclude the correlational effects. Dispersion structure of correlation between particles, structure factor, is deduced by dividing the total scattering intensity by form factor. In order to compare the differences between well-dispersed structure and aggregated structure, SAXS is performed for hybrid, which is the composite of surface modified tetrapod and matrix polymer, and simple blends without surface modification. Comparing the structure factors of hybrid and blends, it is found that dispersion structure is not only analyzable but also quantitatively comparable by scattering method. In chapter 4, method for control of the dispersion structure of tetrapod nanoparticles within polymer matrix as well as polymer particles is presented. polystyrene-b-poly(cysteamine methyl disulfide) diblock copolymer, which are synthesized for surface modification of tetrapod nanocrystals by RAFT polymerization, facilitate the amphipathic properties due to polystyrene main chain and amide functional groups. Therefore, they forms the globular polymer particles in polar solvent, while polystyrene homopolymers are precipitated quickly. When the block copolymer is particulated in company with tetrapod nanoparticles, tetrapods are incorporated to polymer particles. It is found that size of the polymer particles are controlled by concentration of block copolymers and solvent polarity and the number of tetrapod per polymer particle is also controlled by weight ratio of polymer and tetrapod nanoparticle. These series of studies about the control and evaluation of the dispersion structures are present the approaching way to apply the tetrapod nanocrystals for various fields, which need the controllability of dispersion structures, like the solar cells as well as the transistors.