Composite latex spheres

Single-particle optical analyzers are especially useful for continuous measurement of particles of uniform physical properties. However, as discussed earlier, uncertainties develop in the measurement of particle clouds that are heterogeneous in composition because the refractive index may vary from particle to particle. Thus, in making atmospheric aerosol measurements, workers have assumed an average refractive index characteristic of the mixture to estimate a calibration curve or have reported data in terms of the equivalent particle diameter for a standard aerosol, such as suspended polystyrene latex spheres. 

Latex spheres with the same composition can be synthesized in a wide range of sizes by a similar procedure. Under the conditions described, the size of the spheres is dependent on the concentration of total monomer. As shown in Figure 1, the diameter of the spheres can be decreased from 3 0 nm to 35 nm by reducing the total monomer concentration from 35 to 3 A linear relation between these parameters is observed. 

Three cases can be considered for TEM image contrast of the finished latexes (see Figure 6.11). In the first case. Figure 6.11a, the latex spheres are taken to be homogeneous in composition and to stain uniformly with OSO4. In TEM, the particles would be darkest in the interior, as expected for the image of any sphere viewed in transmission. 

Sihcate composites can provide an efficient size-exclnsion membrane. The pore size distribntion can be controlled, at least to some extent, by incorporation of water-solnble polymers such as poly(ethylene glycol) and their leaching after sol-gel formation or by pH control. On the low-pore-size side, low-pH silicate processing may yield silicate membranes that exhibit Knudsen flow and can separate nitrogen from air [245]. The npper limit for porous silicates probably lies in living cell templated materials [246,247] or latex sphere templates [248], and in sol-gel-derived ruthenium and vanadinm oxide aerogels for supercapacitor applications [249,250]. 

The types of surface moieties stabilizing the latex also are important. The binders used in waterborne coatings are not the hard-sphere, model polymer colloids used in adsorption studies. They are soft (low glass transition temperature), deformable moieties that are stabilized by grafted polymer fragments [e.g., (hydroxyethyl)cellulose (16) or poly(vinyl alcohol)] or by terpolymerized acid monomers extended from the surface of the colloid (IT). Such stabilizers produce a far less hydrophobic surface than is generally depicted in colloid texts. This situation is particularly true if the composition of the latex is predominately methacrylate or vinyl acetate, as they are in most U.S. commercial products. 

The materials which have been mentioned here so far are predominantly shaped in planar films of hierarchical order. However, the synthesis of hierarchically structured particles is also highly desirable, as they might be further processed and used for the preparation of composite porous materials. Wu et al. showed the synthesis of raspberry-like hollow silica spheres with a hierarchically structured, porous shell, using individual PS particles as sacrificial template [134]. In another intriguing approach by Li et al. [135], mesoporous cubes and near-spherical particles (Fig. 10) were formed by controlled disassembly of a hierarchically structured colloidal crystal, which itself was fabricated via PMMA latex and nonionic surfactant templating. The two different particle types concurrently generated by this method derive from the shape of the octahedral and tetrahedral voids, which are present in the template crystal with fee lattice symmetry. 

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