A. van Blaaderen

Monodisperse Colloidal Silica Spheres from Tetraalkoxysilanes: Particle Formation and Growth Mechanism

The mechanisms behind the formation and growth of silica particles prepared from tetraalkoxysilanes in alcoholic solutions of water and ammonia were investigated. By analyzing the competitive growth of a dispersion of silica spheres with a bimodal size distribution, it was established that the growth proceeds through a surface reaction-limited condensation of hydrolyzed monomers or small oligomers. By following the hydrolysis of tetraethoxysilane with 13C liquid NMR and the particle growth with time-resolved static light scattering, it was found that both processes were described by the same first-order rate constants. Therefore, despite the fact that the incorporation of hydrolyzed monomers proceeds through a reactionlimited process, the overall rate of the particle growth is limited by the first-order hydrolysis rate of the alkoxide. It was concluded that the particle formation (or particle nucleation) proceeds through an aggregation process of siloxane substructures that is influenced strongly by the surface potential of the silica particles and the ionic strength of the reaction medium. These conclusions were based on the dependence of the particle stability and final particle size on additions of LiN03 to the reaction and dispersion medium and the independence of the growth rate on the same additions. 0 1992

Rare-earth doped polymers for planar optical amplifiers

Optical waveguide amplifiers based on polymer materials offer a low-cost alternative for inorganic waveguide amplifiers. Due to the fact that their refractive index is similar to that of standard optical fibers, they can be easily coupled to existing fibers with low coupling losses. Doping the polymer with rare-earth ions that yield optical gain is not straightforward, as the rare-earth salts are poorly soluble in the polymer matrix. This review article focuses on two different approaches to dope a polymer waveguide with rare-earth ions. The first approach is based on organic cage-like complexes that encapsulate the rare-earth ion and are designed to provide coordination sites to bind the rare-earth ion and to shield it from the surrounding matrix. These complexes also offer the possibility of attaching a highly absorbing antenna group, which increases the pump efficiency significantly. The second approach to fabricate rare-earth doped polymer waveguides is obtained by combining the excellent properties of SiO2 as a host for rare-earth ions with the easy processing of polymers. This is done by doping polymers with Er-doped silica colloidal spheres.

Particle Morphology and Chemical Microstructure of Colloidal Silica Spheres Made from Alkoxysilanes

Abstract Monodisperse colloidal silica spheres with radii in the range 10–500 nm were prepared by hydrolysis and condensation of tetraethoxysilane (TES) in a mixture of water, ammonia and a lower alcohol at several temperatures. It was attempted to establish a relation between the morphology and the chemical microstructure of the particle. Particle morphologies were examined with transmission electron microscopy and static and dynamic light scattering. The microstructure of the spheres was studied with quantitative direct excitation 29Si nuclear magnetic resonance (NMR) spectroscopy and a combination of qualitative cross-polarization 13C NMR and elemental analysis. A comparison was made between these particles and particles prepared from TES in an ammonia/water in cyclohexane microemulsion and also with Ludox® silica particles. The siloxane microstructure was found to show almost no variation as a function of concentration of reagents, catalyst, co-solvent and temperature. Around 65% of the silicon nuclei was bonded through siloxane bonds with four other silicons, approximately 30% was bonded with three other silicons and a few percent with only two. It was shown that under most experimental conditions several percent of the ethoxy groups never leave the TES molecule and end up inside the silica. The Ludox particles were found to consist of a more condensed silicon structure as compared with particles synthesized in alcohol, ammonia, water mixtures, whereas the spheres prepared in the microemulsion were less condensed and contained more alkoxy groups. The differences in particle morphologies — ranging from irregularly shaped rough particles to perfect, smooth spheres — are not caused by differences in siloxane and ethoxy microstructure. It is proposed instead, that a smooth and spherical particle shape is the result of the growth by monomers or small oligomers, and that a rough, irregular shape is the result of growth by larger silicon structures.

Hollow Silica Spheres: Synthesis and Mechanical Properties

Core-shell polystyrene-silica spheres with diameters of 800 nm and 1.9 microm were synthesized by soap-free emulsion and dispersion polymerization of the polystyrene core, respectively. The polystyrene spheres were used as templates for the synthesis of silica shells of tunable thickness employing the Stöber method [Graf et al. Langmuir 2003, 19, 6693]. The polystyrene template was removed by thermal decomposition at 500 degrees C, resulting in smooth silica shells of well-defined thickness (15-70 nm). The elastic response of these hollow spheres was probed by atomic force microscopy (AFM). A point load was applied to the particle surface through a sharp AFM tip, and successively increased until the shell broke. In agreement with the predictions of shell theory, for small deformations the deformation increased linearly with applied force. The Youngs modulus (18 +/- 6 GPa) was about 4 times smaller than that of fused silica [Adachi and Sakka J. Mater. Sci. 1990, 25, 4732] but identical to that of bulk silica spheres (800 nm) synthesized by the Stöber method, indicating that it yields silica of lower density. The minimum force needed to irreversibly deform (buckle) the shell increased quadratically with shell thickness.

Self-assembly of colloids with liquid protrusions

A facile and flexible synthesis for colloidal molecules with well-controlled shape and tunable patchiness is presented. Cross-linked polystyrene spheres with a liquid protrusion were found to assemble into colloidal molecules by coalescence of the liquid protrusions. Similarly, cross-linked poly(methyl methacrylate) particles carrying a wetting layer assembled into colloidal molecules by coalescence of the wetting layer. Driven by surface energy, a liquid droplet on which the solid spheres are attached is formed. Subsequent polymerization of the liquid yields a wide variety of colloidal molecules as well as colloidosomes with tunable patchiness. Precise control over the topology of the particles has been achieved by changing the amount and nature of the swelling monomer as well as the wetting angle between the liquid and the seed particles. The overall cluster size can be controlled by the seed size as well as the swelling ratio. Use of different swelling monomers and/or particles allows for chemical diversity of the patches and the center. For low swelling ratios assemblies of small numbers of seeds resemble clusters that minimize the second moment of the mass distribution. Assemblies comprised of a large number of colloids are similar to colloidosomes exhibiting elastic strain relief by scar formation.

Structure of electrorheological fluids

Specially synthesized silica colloidal spheres with fluorescent cores were used as model electrorheological fluids to experimentally explore structure formation and evolution under conditions of no shear. Using confocal scanning laser microscopy we measured the location of each colloid in three dimensions. We observed an equilibrium body-centered tetragonal phase and several nonequilibrium structures such as sheet-like labyrinths and isolated chains of colloids. The formation of nonequilibrium structures was studied as a function of the volume fraction, electric field strength, and starting configuration of the colloid. We compare our observations to previous experiments, simulations, and calculations.

Long‐time self‐diffusion of spherical colloidal particles measured with fluorescence recovery after photobleaching

Long‐time self‐diffusion coefficients in concentrated colloidal dispersions of silica spheres, with various interaction potentials, were measured with the fluorescence recovery after photobleaching technique. Charge stabilized spheres were measured in solutions of LiCl in dimethylformamide with varying ionic strength. Sterically stabilized hard‐sphere‐like stearyl silica dispersions were studied in cyclohexane. The fluorophore used, fluorescein‐isothiocyanate, was covalently attached to the surface of the spheres (for the charged particles) or buried inside the silica core (for the hard spheres). The particles were characterized by electrophoresis, static and dynamic light scattering, and transmission electron microscopy. The experimental results are discussed and compared with existing theories on long‐time self‐diffusion.

A new colloidal model system to study long-range interactions quantitatively in real space

We present quantitative three-dimensional real space measurements by confocal microscopy on fluorescently labelled and sterically stabilized dispersions of polymethylmethacrylate spheres dispersed in index and density-matched solvent mixtures with a relative dielectric constant 5 <e r < 10. In this new model system Debye screening lengths (κ −1 ) comparable to the particle size (diameter σ ) can be realized even for particles with sizes of several micrometres. Moreover, by addition of salt (tetrabutylammonium chloride) κ −1 can be varied and the surface charge of the particles can be set roughly in between the values +100 and −100 mV, as determined by electrophoresis. By a comparison of radial distribution functions and displacements from lattice positions with Monte Carlo computer simulations we found that both the structure in the liquid and the crystallization volume fraction could be described with a Yukawa potential characterized by one set of parameters, a surface potential of 36 mV and κσ = 5, where the particle diameter σ = 2 µm. Anomalous (‘phase’) behaviour such as extreme long-range repulsions, ‘coexistence’ of high-density and low-density colloidal crystals and void formation, previously observed for deionized dispersions in water, was observed as well, and can now be studied in ad ifferent system without ion exchange resin. These anomalous effects are seen relatively soon after preparing the systems and are absent or short-lived in systems with grounding and at higher salt concentrations.

Colloidal Ellipsoids with Continuously Variable Shape

tBuMA block was 84 800 in PMMA-b-PtBuMA with polydispersity of 1.18. All the homopolymers used in this study were obtained from Aldrich Chemical Company, Inc., and (except for HEMA) were precipitated twice from methylene chloride into methanol for purification. For the purification of HEMA, precipitation from methanol with hexane/methylene chloride (1:1) was carried out twice. Film Preparation: 0.2 g of polymer was dissolved in 10 mL toluene and poured into an aluminum petri dish with a diameter of 5 cm. The solution was dried in ambient conditions overnight, then dried on a hotplate at 50 C for 1 day, and finally dried completely in a vacuum oven at 100 C for 2 days. The thickness of the films was about 100 lm. TEM: The films were embedded in visible-light curable resin, D-800 (JEOL DATUM Co. Ltd.), and were cured by irradiating with the light from a xenon lamp source through a UV cut filter at 100 mW/cm 2 for 5 min. The films were microtomed to give a thickness of 50 nm at room temperature. The Carl Zeiss EM 902 was operated at an accelerating voltage of 80 kV, and the images were recorded using the Imaging Plates system, FDL5000 (Fuji Photo Films Co. Ltd.). The number-average diameter of the clusters was calculated with image processing software: Ultimage Pro 2.6 (Graftek France). More than 200 particles were counted and individual diameters were assigned to those circles with equivalent area. UV-vis Absorption Spectroscopy: Measurements were carried out on a Shimadzu UV-2500PC. The 50 lm films prepared by solvent casting as mentioned above were used.

Patterning surfaces with colloidal particles using optical tweezers

A method for positioning colloidal particles on surfaces in any designed pattern is described. Optical tweezers are used to bring particles from a reservoir to the substrate where opposite surface charges are used to immobilize particles on the surface. Both chemical surface modification and polyelectrolyte coating of either substrate or colloids make the method generally applicable. We show that using this technique large, two-dimensional patterns can be created that can be dried without distortions by critical point drying. As an example we show the positioning of 79 nm radius metallodielectric particles and we show how two-dimensional patterns can be used to direct three-dimensional epitaxial crystal growth. The method is inexpensive, relatively fast, and can be fully automated.