Latex Process

The latex process utilizes two different solvents that are not soluble each other to form a uniform and homogeneous microemulsion with the help of surfactants and dispersions.

Microemulsion is usually a transparent and isotropic ther-modynamic system composed of surfactants, dispersions (polymers, alcoholic, etc.), oil (hydrocarbons), and water (or electrolytical water solutions). Polymer dispersions made of a variety of monomers, including styrene, butyl acrylate, and methyl methacrylate, surround the latex droplet. The solid phase could be from the emulsion, so that the processes of nucleation, growth, assembly, aggregation, etc. could be limited inside a micro/nanoemulsion droplet to form the micro/nanoparticles in the shape of spheres. The formation of micro/nanospheres avoids further aggregation of the nanoparticles. The key to this process is to form a system that every water solution droplet with precursors must be surrounded by a continuous oil phase and the precursors are not soluble in this oil phase. Namely, it is necessary to form a w/o-type latex so that the water phase cannot be formed, but instead a pseduophase forms. In the latex, the latex beads serve as micro/nanoreactors, because each bead of several to hundreds of nanometers in size is separated by the layer of monomers. In some cases, the latex process using polymer dispersions can be combined with the polymerization process. The shape and the structure of the nanoparticles can be controlled by processes of layer-by-layer deposition, covering, coating, assembly, etc. The spherical latex can also serve as a template for the preparation of the nanomaterials, like porous materials, networks, etc. Thus the latex process can be used for synthesizing different types of nano-materials, including nanoparticles, nanocapsules, nanoshells, etc. The creation of core-shell particles by the latex process has been attracting a great deal of interest because of the diverse applicability of these colloidal particles (e.g., as building blocks for photonic crystals, in magnetic storage, in multienzyme biocatalysis, and in drug delivery). The advantages of the latex process are narrow size distribution, easy control of size and shape, various synthesizable nanostruc-tures, etc.

The processing, microstructure, and optical properties of CdS semiconductor nanoparticles sequestered in spin-coated polymer films were investigated [344]. A simple processing protocol was developed to form thin film structures consisting of CdS nanoparticles dispersed in the interstices created by a close-packed stacking of polystyrene spheres.

Poloxamer 407 and poloxamine 908 were used to modify the surface of both model latex and biodegradable nanospheres showing reduced protein adsorption in vitro and extended circulation times in vivo [345].

Model polymeric nanoparticles [aqueous colloidal polymer dispersions: Eudragit(R) RL 30D, L 30D, NE 30D, or Aquacoat(R)] with different physicochemical properties were incorporated into various solid dosage forms (granules, tablets, pellets, or films) [346]. The compatibility of the nanoparticles with commonly used tabletting excipients and the redispersibility of the nanoparticles after contact of the solid dosage forms with aqueous media were investigated.

The core-shell latex morphology was used to produce three-dimensionally ordered arrays of fluorescent nano-particles in polymer composite materials [347]. With their soft, inert shells, the particles were easily ordered into an array, after which heat treatment melted the soft shells to give an array of nanoparticles in a transparent polymer matrix.

Colloidal particles with magnetic properties are attractive due to their tunable anisotropic interactions. A class of polystyrene-core magnetite-shell particles was produced by the sequential adsorption of nanoparticles and poly-electrolyte, a process that allowed the shell thickness and composition to be controlled with nanometer precision [348-350]. The nanocomposite particles were aligned in the presence of a magnetic field. Mesoscale hollow spheres of ceramic materials were prepared by templating an appropriate sol-gel precursor against a crystalline array of monodisperse polystyrene beads [351]. An array of TiO2 hollow spheres of well-controlled, uniform size, and homogeneous wall thickness resulted.

Functional core-shell particles were prepared by assembling a composite multilayer shell of charged polyelec-trolytes and luminescent CdTe(S) nanocrystals (cadmium telluride with a certain content of sulfide) via their consecutive electrostatic adsorption from solution onto micrometer-sized latex particles [352]. Variation of the size and surface chemistry of the semiconductor nanoparticles, the size and shape of the colloid templates, and the nature of the poly-electrolyte opened avenues for the production of a variety of core-shell materials.

Submicrometer core-shell polymer particles, with molec-ularly imprinted shells, were prepared by a two-stage polymerization process [353]. Particles, prepared with a cholesterol-imprinted ethyleneglycol dimethacrylate shell and in the absence of porogen, were found to be 76 nm in diameter with a surface area of 82 m2 g-1. Imprinted shells were also prepared over superparamagnetic polymer cores and over magnetite ferrocolloid alone. The cholesterol binding to magnetic particles was very similar to that of equivalent nonmagnetic materials. Magnetic particles could be sedimented in as little as 30 s in a magnetic field.

Hierarchical assembly of zeolite nanoparticles into ordered macroporous monoliths was studied using core-shell building blocks [354]. Latex beads were used as a combined template and porogen in the fabrication of monolithic silica containing a hierarchy of pores. Prefabricated core/shell particles, prepared by the layer-by-layer assembly of zeolite (silicalite) nanoparticles onto spherical latex templates, were assembled into macroscopic close-packed structures. Calcination removed all organic components and caused densification of the inorganic structure producing a macroporous zeolite in which both the pore size and wall thickness can be varied.

Spherical polystyrene latex beads of about 2.0 /m diameter were coated with islands of silver or gold metal, about 5-200 nm in diameter, by reduction of aqueous silver or gold ions in the presence of sugar-coated polystyrene latex beads [355]. The metal islands were held on the bead surface by a polymeric sugar derivative, aminodextran, covalently bound to the polystyrene aldehyde/sulfate bead.

A review presented the state-of-the-art strategies for engineering particle surfaces, such as the layer-by-layer deposition process, which allows fine control over shell thickness and composition [356]. Nanocoating, the covering of materials with a layer on the nanometer scale, or covering of a nanoscale entity, to form nanocomposites and structured materials using the sol-gel process was reviewed [357]. Templates from spherical nanoparticles to complex bicontinuous networks were discussed where either the coated material or the structured inorganic hollow frame resulting from removal of the template were of interest in fields of application ranging from information storage to catalysis.

The ultrasonic induced encapsulating emulsion polymerization technique was used to prepare polymer/inorganic nanoparticle composites [358]. The main factors affecting ultrasonic induced encapsulating emulsion polymerization were studied. The pH value, the type of monomers, the type, content, and surface properties of nanoparticles, and the type and concentration of surfactant had great influence on the ultrasonic induced encapsulating emulsion polymerization and the obtained latex stability. The mechanism of ultrasonic induced encapsulating emulsion polymerization and the composite latex stabilization were proposed.

Mesoscopic hollow spheres of ceramic materials with functionalized interior surfaces were synthesized [359]. Amphiphilic colloidal particles with hydrophobic cores and hydrophilic shells were prepared via a two-step method [360]. First, polystyrene cores were obtained through the concentrated emulsion polymerization. In the second step, the polystyrene particles were dispersed in water, after which acrylamide, N, N'-methylenebisacrylamide, and ferrous sulfate were added.

A procedure was developed to coat colloidal polystyrene spheres with a smooth and well-defined layer of amorphous titanium dioxide [361]. The thickness of the coating was easily varied from a few nanometers upward and increased further by seeded growth. The resulting composite particles were very monodisperse. The core-shell particles were turned into spherical hollow titania shells by dissolution of the polystyrene cores in suspension or by calcination of the dried particles in a furnace.

Polymer dispersions made of a variety of monomers, including styrene, butyl acrylate, and methyl methacrylate, were generated by the miniemulsion process in the presence of a coupling comonomer, a hydrophobe, and silica nanoparticles [362]. Depending on the reaction conditions and the surfactants employed, different hybrid morphologies were obtained, comprising a "hedgehog" structure where the silica surrounded the latex droplet and provided stabilization even without any low molecular weight surfactant.

The preparation of biocolloids with organized enzyme-containing multilayer shells for exploitation as colloidal enzymatic nanoreactors was described [363]. Urease multilayers were assembled onto submicrometer-sized polystyrene spheres by the sequential adsorption of urease and polyelectrolyte, in a predetermined order, utilizing electrostatic interactions for layer growth.

Fluorescently labeled core-shell latex particles composed mainly of the thermoresponsive polymer poly-N-isopropylacrylamide, were synthesized such that an energy transfer donor (phenanthrene) and an energy transfer acceptor (anthracene) were covalently localized in the core and shell, respectively [364]. Core-shell particles with different shell thicknesses displayed identical phase transition temperatures, with a clear increase in collapse temperature as the shell thickness was increased.

Encapsulation of silica nanoparticles was performed by dispersion polymerization of styrene, butyl acrylate, and butyl methacrylate in aqueous alcoholic media [365]. The silica beads were modified by reacting on their surface the 3-trimethoxysilyl propyl methacrylate coupling agent.

Core-shell composite materials, consisting of a silica core and a polystyrene shell, were prepared by colloidal assembly of polystyrene nanospheres onto silica microspheres [366]. The assembly process was controlled by specific chemical (amine-aldehyde) or biochemical (avidin-biotin) interactions between the nanospheres and microspheres. Colloidal assembly was performed using polymer nanoparticles and silica particles (3-10 /m diameter). Heating the assembled materials to temperatures above the glass transition (Tg) of the polymer nanoparticles allowed the polymer to flow over the microsphere surfaces, resulting in uniform core-shell materials. Nanosphere packing density on the microsphere surfaces influenced the uniformity of the resulting polymer shell.

A type of micrometer-sized entity containing silver was introduced [367]. Three different approaches were employed to modify the colloidal particles with silver. According to the first technique, the particles were first coated with layers of poly(styrenesulfonate) and silver ions followed by the reduction of silver by photoirradiation. The second technique involved coating the colloidal particles with a shell capable of reduction, and sequential addition of silver salt resulted in the reduction of metal in the shell matrix. The last approach utilizied the reaction of a silver mirror to silver particles.

Acrylic/nanosilica composite latexes were prepared by blending via high shear stirring or ball milling and in-situ polymerization [368]. For comparison, composites filled with microsilica were also prepared. The mechanical and optical properties of the polymers formed by the composite latex filled with nano- or microsilica were investigated.

Raspberrylike hybrid organic-inorganic materials consisting of spherical silica beads supporting smaller polystyrene particles were prepared through a heterophase polymerization process [369]. In a first step, micrometer-sized silica particles were synthesized according to procedures inspired from the literature. In a second step, a poly(ethylene gly-col) macromonomer was adsorbed on the surface of the silica beads. Finally, polymerization of styrene was achieved in water with a nonionic surfactant as an emulsifying agent and sodium persulfate as an initiator.

Nanoparticles of polystyrene (Mw = 1.0-3.0 x 106 g/mol) latexes were prepared from their respective dilute polystyrene (commercial) solutions in cyclohexane, toluene/ methanol, or cyclohexane/toluene at each temperature [370]. The cationic surfactant cetyltrimethylammonium bromide was used to stabilize the formed PS latex particles. By varying different concentrations of cetyltrimethylammonium bromide and polystyrene solution of various Mw, stable bluish-transparent latex particles ranging from about 10 to 30 nm in diameter were produced.

Magnetically controllable photonic crystals were formed through the self-assembly of highly charged, monodisperse superparamagnetic colloidal spheres [371]. These super-paramagnetic monodisperse charged polystyrene particles containing nanoscale iron oxide nanoparticles were synthesized through emulsion polymerization.

Polystyrene latex particles containing silanol groups were synthesized in emulsion polymerization using 3-(trimethoxysilyl)propyl methacrylate as a functional comonomer [372]. The surface properties of the functional-ized polymer latexes were investigated using electrophoretic measurements and the soap titration method. This work illustrated the determining role of interfaces in the structuration of organic-inorganic colloids.

Figure 6 shows a scanning electron microscope (SEM) photograph of the conducting polymer deposited as an ultra-thin overlayer onto preformed polystyrene latex particles [373]. A series of sterically stabilized polystyrene latex particles in the size range 0.1-5.0 /m was coated with ultra-thin (<50 nm) overlayers of either polypyrrole, polyaniline, or poly(3,4-ethylenedioxythiophene) [373]. In each case the conducting polymer overlayer allowed the latex particles to acquire surface charge and hence be accelerated up to hypervelocities (> 1 km s-1) using a Van de Graaff accelerator. These coated latexes had two main advantages. First, a wider particle size range can be accessed. Second, the particle size distributions of the coated latexes are much narrower than those of the pure polypyrrole particles.

Polyaddition reactions in miniemulsions were performed by miniemulsification of mixtures of di-, tri-, and tetrae-poxides with varying diamines, dithiols, or bisphenols and subsequent heating to 60 °C [374]. Figure 7 shows a TEM micrograph of polyaddition latex comprised of Epikote E828 and 4,4'-diaminobibenzyl. Depending on the chemical nature of the monomer, the amount of surfactant, and the pH of the reaction mixture, latex particles with diameters between 30 and 600 nm and narrow size distributions were obtained [374].

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