Characterization of the nanomorphology of polymer-silica colloidal nanocomposites using electron spectroscopy imaging

Zinc Oxide-From Synthesis to Application: A Review

Zinc oxide can be called a multifunctional material thanks to its unique physical and chemical properties. The first part of this paper presents the most important methods of preparation of ZnO divided into metallurgical and chemical methods. The mechanochemical process, controlled precipitation, sol-gel method, solvothermal and hydrothermal method, method using emulsion and microemulsion enviroment and other methods of obtaining zinc oxide were classified as chemical methods. In the next part of this review, the modification methods of ZnO were characterized. The modification with organic (carboxylic acid, silanes) and inroganic (metal oxides) compounds, and polymer matrices were mainly described. Finally, we present possible applications in various branches of industry: rubber, pharmaceutical, cosmetics, textile, electronic and electrotechnology, photocatalysis were introduced. This review provides useful information for specialist dealings with zinc oxide.

Correcting for a density distribution: particle size analysis of core-shell nanocomposite particles using disk centrifuge photosedimentometry

Many types of colloidal particles possess a core-shell morphology. In this Article, we show that, if the core and shell densities differ, this morphology leads to an inherent density distribution for particles of finite polydispersity. If the shell is denser than the core, this density distribution implies an artificial narrowing of the particle size distribution as determined by disk centrifuge photosedimentometry (DCP). In the specific case of polystyrene/silica nanocomposite particles, which consist of a polystyrene core coated with a monolayer shell of silica nanoparticles, we demonstrate that the particle density distribution can be determined by analytical ultracentrifugation and introduce a mathematical method to account for this density distribution by reanalyzing the raw DCP data. Using the mean silica packing density calculated from small-angle X-ray scattering, the real particle density can be calculated for each data point. The corrected DCP particle size distribution is both broader and more consistent with particle size distributions reported for the same polystyrene/silica nanocomposite sample using other sizing techniques, such as electron microscopy, laser light diffraction, and dynamic light scattering. Artifactual narrowing of the size distribution is also likely to occur for many other polymer/inorganic nanocomposite particles comprising a low-density core of variable dimensions coated with a high-density shell of constant thickness, or for core-shell latexes where the shell is continuous rather than particulate in nature.

All-acrylic film-forming colloidal polymer/silica nanocomposite particles prepared by aqueous emulsion polymerization

The efficient synthesis of all-acrylic, film-forming, core-shell colloidal nanocomposite particles via in situ aqueous emulsion copolymerization of methyl methacrylate with n-butyl acrylate in the presence of a glycerol-functionalized ultrafine silica sol using a cationic azo initiator at 60 °C is reported. It is shown that relatively monodisperse nanocomposite particles can be produced with typical mean weight-average diameters of 140-330 nm and silica contents of up to 39 wt %. The importance of surface functionalization of the silica sol is highlighted, and it is demonstrated that systematic variation of parameters such as the initial silica sol concentration and initiator concentration affect both the mean particle diameter and the silica aggregation efficiency. The nanocomposite morphology comprises a copolymer core and a particulate silica shell, as determined by aqueous electrophoresis, X-ray photoelectron spectroscopy, and electron microscopy. Moreover, it is shown that films cast from n-butyl acrylate-rich copolymer/silica nanocomposite dispersions are significantly more transparent than those prepared from the poly(styrene-co-n-butyl acrylate)/silica nanocomposite particles reported previously. In the case of the aqueous emulsion homopolymerization of methyl methacrylate in the presence of ultrafine silica, a particle formation mechanism is proposed to account for the various experimental observations made when periodically sampling such nanocomposite syntheses at intermediate comonomer conversions.

Detection of charge distributions in insulator surfaces

Charge distribution in insulators has received considerable attention but still poses great scientific challenges, largely due to a current lack of firm knowledge about the nature and speciation of charges. Recent studies using analytical microscopies have shown that insulators contain domains with excess fixed ions forming various kinds of potential distribution patterns, which are also imaged by potential mapping using scanning electric probe microscopy. Results from the authors' laboratory show that solid insulators are seldom electroneutral, as opposed to a widespread current assumption. Excess charges can derive from a host of charging mechanisms: excess local ion concentration, radiochemical and tribochemical reactions added to the partition of hydroxonium and hydronium ions derived from atmospheric water. The last factor has been largely overlooked in the literature, but recent experimental evidence suggests that it plays a decisive role in insulator charging. Progress along this line is expected to help solve problems related to unwanted electrostatic discharges, while creating new possibilities for energy storage and handling as well as new electrostatic devices.

Efficient preparation of polystyrene/silica colloidal nanocomposite particles by emulsion polymerization using a glycerol-functionalized silica sol

Colloidally stable polystyrene/silica nanocomposite particles of around 200-400 nm diameter and containing 22-28 wt % silica can be readily prepared by aqueous emulsion polymerization at 60 degrees C using a cationic azo initiator in combination with a commercially available glycerol-functionalized ultrafine aqueous silica sol in the absence of any surfactant, auxiliary comonomer, or nonaqueous cosolvent. Optimization of the initial silica sol concentration allows relatively high silica aggregation efficiencies (up to 95%) to be achieved. Control experiments confirm the importance of selecting a cationic initiator, since nanocomposite particles were not formed when using an anionic persulfate initiator. Similarly, the glycerol groups on the silica sol surface were also shown to be essential for successful nanocomposite particle formation: use of an unfunctionalized ultrafine silica sol in control experiments invariably led to polystyrene latex coexisting with the silica nanoparticles, rather than efficient nanocomposite formation. Electron spectroscopy imaging transmission electron microscopy studies of ultramicrotomed polystyrene/silica nanocomposite particles indicate well-defined "core-shell" particle morphologies, which is consistent with both X-ray photoelectron spectroscopy and aqueous electrophoresis studies.

Efficient synthesis of poly(2-vinylpyridine)-silica colloidal nanocomposite particles using a cationic azo initiator

Colloidal poly(2-vinylpyridine)-silica nanocomposite particles can be efficiently prepared by emulsion polymerization at 60 degrees C using a commercial 20 nm aqueous silica sol as the sole stabilizing agent. Unlike previously reported colloidal nanocomposite syntheses, transmission electron microscopy studies indicate very high silica aggregation efficiencies (88-99%). The key to success is simply the selection of a suitable cationic azo initiator. In contrast, the use of an anionic persulfate initiator leads to substantial contamination of the nanocomposite particles with excess silica sol. The cationic azo initiator is electrostatically adsorbed onto the anionic silica sol at submonolayer coverage, which suggests that surface polymerization may be important for successful nanocomposite formation. Moreover, the 2-vinylpyridine can be partially replaced with either styrene or methacrylic comonomers to produce a range of copolymer-silica nanocomposite particles. The poly(2-vinylpyridine)-silica nanocomposite particles have a well-defined core-shell morphology, with poly(2-vinylpyridine) cores and silica shells; mean diameters typically vary from 180 to 220 nm, and mean silica contents range from 27 to 35% by mass.

Low-energy-loss EFTEM imaging of thick particles and aggregates

Electron spectroscopy imaging is a powerful tool for the elucidation of colloidal particle morphology and microchemistry, but it normally requires the use of very thin samples, typically less than 50 nm, to avoid the effects of multiple scattering. This work shows that many aspects of the internal morphology of thick particles and aggregates and the chemical component distribution are revealed using low-energy-loss electron imaging in the transmission electron microscope, benefiting from multiple scattering as well as small but significant differences in the low-energy-loss spectra of aggregate constituents. Low-loss images reveal morphological details of thick aggregates made out of colloidal polymers (natural rubber and styrene-acrylic latex) and inorganic particles (silica, montmorillonite, and aluminum phosphate) at a spatial resolution close to that achieved in the bright-field images and much better than in the elemental maps, showing the advantages of the simultaneous use of low-loss images and standard thin-cut elemental maps.

Nanoscale indentation of polymer and composite polymer-silica core-shell submicrometer particles by atomic force microscopy

Atomic force microscopy was employed to probe the mechanical properties of surface-charged polymethylmethacrylate (PMMA)-based terpolymer and composite terpolymer core-silica shell particles in air and water media. The composite particles were achieved with two different approaches: using a silane coupling agent (composite A) or attractive electrostatic interactions (composite B) between the core and the shell. Young's moduli (E) of 4.3+/-0.7, 11.1+/-1.7, and 8.4+/-1.7 GPa were measured in air for the PMMA-based terpolymer, composite A, and composite B, respectively. In water, E decreases to 1.6+/-0.2 GPa for the terpolymer; it shows a slight decrease to 8.0+/-1.2 GPa for composite A, while it decreases to 2.9+/-0.6 GPa for composite B. This trend is explained by considering a 50% swelling of the polymer in water confirmed by dynamic light scattering. Close agreement is found between the absolute values of elastic moduli determined by nanoindentation and known values for the corresponding bulk materials. The thickness of the silica coating affects the mechanical properties of composite A. In the case of composite B, because the silica shell consists of separate particles free to move in the longitudinal direction that do not individually deform when the entire composite deforms, the elastic properties of the composites are determined exclusively by the properties of the polymer core. These results provide a basis for tailoring the mechanical properties of polymer and composite particles in air and in solution, essential in the design of next-generation abrasive schemes for several technological applications.

Synthesis of biocompatible poly[2-(methacryloyloxy)ethyl phosphorylcholine]-coated magnetite nanoparticles

A well-defined, double-hydrophilic diblock copolymer comprising poly[2-(methacryloyloxy)ethyl phosphorylcholine]-block-(glycerol monomethacrylate) (PMPC30-PGMA30, where the numbers represent the average degrees of polymerization for each block) was evaluated for the synthesis of colloidally stable ultrafine magnetite sols. Sterically stabilized paramagnetic sols were prepared in aqueous solution by chemical coprecipitation of ferric and ferrous salts in the presence of this block copolymer. The PMPC30-PGMA30-stabilized magnetite sol had a mean transmission electron microscopy (TEM) diameter of 9.4 +/- 1.7 nm and a mean hydrodynamic diameter of 34 nm. This sol exhibited improved colloidal stability with respect to long-term storage and pH variation compared with magnetite sols prepared in the presence of alternative water-soluble homopolymers and diblock copolymers. Fourier transform infrared (FT-IR) spectroscopy, thermogravimetry, electron spectroscopy imaging (ESI), and zeta potential studies indicate that the PMPC30-PGMA30 diblock copolymer was adsorbed onto the surface of the sol via the PGMA30 block, with the PMPC30 chains acting as the stabilizing block. Such sterically stabilized sols are expected to be improved contrast agents for magnetic resonance imaging (MRI) applications.

ESI-TEM imaging of surfactants and ions sorbed in Stöber silica nanoparticles

The sorption of surfactants and NaCl in silica nanosized particles creates unexpected spatial distributions of solutes that were evidenced by electron spectroscopy imaging in the transmission electron microscope (ESI/TEM). The spectral images show that simple ions (Na(+), Cl(-), Br(-)) are actually absorbed within the particles irrespective of their charges, while surfactant chains are adsorbed at the particle surfaces. The expected effect of the surfactants on particle aggregation is also observed in the micrographs. In the case of salt, close-packed silica particle arrays are formed at low ionic strength, but only coarse aggregates form at higher salt concentrations. The particles absorb both Na(+) and Cl(-) ions in similar amounts, from 0.5 mol L(-)(1) NaCl, but Na(+) ions are depleted from the particles' immediate outer vicinity, where Cl(-) ions are in turn accumulated. These results confirm that Stöber silica nanoparticles are highly porous and reveal their potential usefulness as carriers of small molecules and ions, due to the small particle size, exceptional colloidal stability, and this newly found sorption behavior.

Effects of pH and salt concentration on oil-in-water emulsions stabilized solely by nanocomposite microgel particles

Aqueous dispersions of lightly cross-linked poly(4-vinylpyridine)/silica nanocomposite microgel particles are used as a sole emulsifier of methyl myristate and water (1:1 by volume) at various pH values and salt concentrations at 20 degrees C. These particles become swollen at low pH with the hydrodynamic diameter increasing from 250 nm at pH 8.8 to 630 nm at pH 2.7. For batch emulsions prepared at pH 3.4, oil-in-water (o/w) emulsions are formed that are stable to coalescence but exhibit creaming. Below pH 3.3, however, these emulsions are very unstable to coalescence and rapid phase separation occurs just after homogenization (pH-dependent). The pH for 50% ionization of the pyridine groups in the particles in the bulk (pK(a)) was determined to be 3.4 by acid titration measurements of the aqueous dispersion. Thus, the charged swollen particles no longer adsorb at the oil-water interface. For continuous emulsions (prepared at high pH with the pH then decreased abruptly or progressively), demulsification takes place rapidly below pH 3.3, implying that particles adsorbed at the oil-water interface can become charged (protonated) and detached from the interface in situ (pH-responsive). Furthermore, at a fixed pH of 4.0, addition of sodium chloride to the aqueous dispersion increases the degree of ionization of the particles and batch emulsions are significantly unstable to coalescence at a salt concentration of 0.24 mol kg(-1). The degree of ionization of such microgel particles is a critical factor in controlling the coalescence stability of o/w emulsions stabilized by them.