Core-shell particle/morphology

A series of SBR copolymers were prepared by emulsion polymerisation and an SAN copolymer was polymerised by a semicontinuous process in the presence of SBR to form a core/shell morphology. The effects of initiator concentration, monomer feeding rate, core/shell ratio, and gel fraction of the core on the core/shell particles morphology were studied. Morphology and Tg were characterised by TEM, DSC, and dynamic mechanical spectroscopy. 28 refs. 

Table 21. Selection of Core Shell Particle Morphologies for Particular Applications... 

Devon et al. [33] prepared a series of acrylic latexes with core-shell particle morphologies. The minimum film formation temperatures (MFFT) of the latexes are expected to change with the core-shell characteristics in the following order ... 

The advantage of the low Cex RCTAs is that latexes with controlled particle size distributions using seeded polymerisations can be made. These latex particles in a second stage polymerisation can be further reacted with other monomers to make block copolymers with core-shell particle morphology or even latex particles in which the second block has functional groups, that is, reactive latexes (Monteiro de Barbeyrac, 2002). 

When monomers of drastically different solubiUty (39) or hydrophobicity are used or when staged polymerizations (40,41) are carried out, core—shell morphologies are possible. A wide variety of core—shell latices have found appHcation ia paints, impact modifiers, and as carriers for biomolecules. In staged polymerizations, spherical core—shell particles are made when polymer made from the first monomer is more hydrophobic than polymer made from the second monomer (42). When the first polymer made is less hydrophobic then the second, complex morphologies are possible including voids and half-moons (43), although spherical particles stiU occur (44). 

The benefit of the LbL technique is that the properties of the assemblies, such as thickness, composition, and function, can be tuned by varying the layer number, the species deposited, and the assembly conditions. Further, this technique can be readily transferred from planar substrates (e.g., silicon and quartz slides) [53,54] to three-dimensional substrates with various morphologies and structures, such as colloids [55] and biological cells [56]. Application of the LbL technique to colloids provides a simple and effective method to prepare core-shell particles, and hollow capsules, after removal of the sacrificial core template particles. The properties of the capsules prepared by the LbL procedure, such as diameter, shell thickness and permeability, can be readily adjusted through selection of the core size, the layer number, and the nature of the species deposited [57]. Such capsules are ideal candidates for applications in the areas of drug delivery, sensing, and catalysis [48-51,57]. 

The concept of spinodal decomposition can be extended to multicomponent systems and allow the prediction of various morphologies, e.g., core-shell particles, dual-continuous phases, dual-dispersed phase, and more complex structures. Spinodal decomposition simulations, in contrast to cellular automata, do... 

Core-shell nanoparticles can also be fabricated using microemulsions. This was performed using a two-stage microemulsion polymerization beginning with a polystyrene seed [62]. Butyl acrylate was then added in a second step to yield a core-shell PS/PBA morphology. The small microlatex led to better mechanical properties than those of similar products produced by emulsion polymerization. Hollow polystyrene particles have also been produced by microemulsion polymerization of MMA in the core with crosslinking of styrene on the shell. After the synthesis of core-shell particles with crosslinked PS shells, the PMMA core was dissolved with methylene chloride [63]. The direct cross-... 

As for the linear properties, numerous approaches have been proposed to predict and explain the nonlinear optical response of nanocomposite materials beyond the hypothesis leading to the simple model presented above ( 3.2.2). Especially, Eq. (27) does not hold as soon as metal concentration is large and, a fortiori, reaches the percolation threshold. Several EMT or topological methods have then been developed to account for such regimes and for different types of material morphology, using different calculation methods [38, 81, 83, 88, 96-116]. Let us mention works devoted to ellipsoidal [99, 100, 109] or cylindrical [97] inclusions, effect of a shape distribution [110, 115], core-shell particles [114, 116], layered composites [103], nonlinear inclusions in a nonlinear host medium [88], linear inclusions in a nonlinear host medium [108], percolated media and fractals [101, 104-106, 108]. Attempts to simulate in a nonlinear EMT the influence of temperature have also been reported [107, 113]. 

The core and shell type of particulates are similar to one of the deposit morphologies formed on an Fe-Ni alloy from CO at temperatures above 500°C where the core consisted of a metal particle in the size range 0.09 to 0.2 pm, with a shell thickness typically of 0.04 jjm(23). The structure of the particles, i.e. a carbon layer on metal, is comparable to the laminar film on the metal, suggesting that the carbon in the shell has been precipitated. Free metal particles have not been observed on the iron foils that could serve as active centres for growth directly from the gas phase. Therefore, it must be concluded that a solution-precipitation process plays a part in determining the final morphology of the core / shell particles, but further details of the mechanism of growth cannot be established at present. 

The successful production of kinetically stable core-shell particles will rely entirely on prevoiting radicals generated in the aqueous phase from diffusing into the seed particles, i.e. as long as the mraiomer feed rate keeps pace with the rate of polymerization. Jonsson s conclusions also underline the importance of the Tg of the seed polymer swollen by the corresponding amount of monomer accumulated in the system. He found that the deviations from the designed core-shell morphology were correlated to variations in the tonperature difference between the polymerization temperature and the Tg of the seed polymer. 

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