Synthesize And Synchrotron Small Angle X-ray Scattering Study Latex Particles

Latices are dispersions of polymeric particles in a suspending medium, usually water. They are wildly used in emulsion paints, paper coatings, adhesives and so on. It is well known that colloidal crystals are formed when the latex dispersions are dried. And they are attracting significant attention because of their potential applications in photonic crystals, photonic band gap materials, optical filters and so forth. The properties of materials are determined by their microscopic structure. Therefore, it is very important to study the underlying physical mechanism in latices and colloidal crystals. It can establish the relations between microscopic structure and macroscopic properties, which would provide theoretical guidance for improving material properties.Mono-dispersed polystyrene latex was synthesized by emulsion polymerization. The latex dispersion was coated on silicon wafer and an iridescent color film was obtained after it was dried in vacuum oven. The brilliant color could be varied depending on the viewing angle and the particle size. The color of the film comes from the diffraction of natural light by the resulting crystalline packing of the particles. Difference in refractive index between the polystyrene particles and air provides necessary contrast for the diffraction to occur. The films exhibit beautiful angle-dependent color effects, which can be explained by Bragg equation. Scan electron microscopy (SEM), atomic force microscopy (AFM) were used to characterize particle diameter and surface structure of the films. And the statistical average particle size was gained by synchrotron small angle X-ray scattering (SAXS) method. And the face-centered cubic (FCC) close packing of the colloidal crystal was confirmed by SAXS for our samples. The changes of surface structure of the film annealed at different temperature were observed by SEM and AFM. And the variations were also tracked by in-situ grazing incident small-angle X-ray scattering (GISAXS) using a self-designed heating stage. The GISAXS data indicate that surface structure not always accord with inner structure.The films formed from mono-dispersed hard polymer particles are too fragile for any applications. However, the color of the films yielded from drying the dispersions of mono-dispersed soft polymer particles normally vanish, due to the coalescence of latex particles, and the mechanical strength of the film is also too weak. Core-shell structured polymeric particles with hard core and soft shell can simultaneously hold good film forming ability and high mechanical strength. We synthesized mono-dispersed core-shell polymer particles with hard core and soft shell. SEM and SAXS were used to track the growth of particles; Differential Scanning Calorimetry (DSC) can prove that relevant polymeric material exist in core-shell structure particles. The scattering curves of core-shell particles were obtained from SAXS experiment and they were well fitted by theory. The thickness of each layer was obtained from the theoretical fitting. It demonstrated that SAXS experiment is every suitable to characterize the structure of core-shell latex particles. And it provides a new characterization method for core-shell structure particles.