Hollow organic spheres

Typical product applications for hollow organic spheres are shoe soles of TPU, SBS or PVC, synthetic wine stoppers, cables, extruded profiles, hoses, and interior auto-... 

Figure 21.4, a comparison of the data on a weight basis (i.e., the moduli divided by the corresponding densities) results in approximately a 20% increase in the foamed materials [18]. Within this context, the hollow organic spheres do exhibit a reinforcing effect. The presence of closed cells is assumed to be the reason, in combination with the adhesion of the walls to the matrix. This has also been observed for phenolic resin microspheres [19]. 

Thermosets also benefit from the foam structure, as evidenced by improved thermal insulation, sound dampening and mechanical stress absorption responses to temperature changes or impact. Hollow spheres with an already set volume are normally used, that is, pre-expanded microspheres. The reason is that the curing reactions often interfere with any expansion before a sufficient volume increase has been obtained. Hollow organic spheres are found in products such as sealants, adhesives, putties, pipes, cultured marble, body fillers, model-making materials, and pastes [2, 3, 19]. Common suitable matrix materials are epoxies, PUR, and polyesters. 

As an alternative to crashing an alloy into small particles, Ostgard et al. [43] first proposed the manufacture of hollow skeletal catalyst spheres. Precursor alloy is deposited on an organic polymer sphere that is later oxidized completely by heating in air. The hollow alloy spheres that remain are then leached as usual to give the catalyst. 

Preformed metal oxide nanoparticles have been successfully coated on polymer spheres by the use of the layer-by-layer method. This involves the coating of the template spheres with polyelectrolyte layers, which are oppositely charged to the metal oxide nanoparticles to be deposited. Alternating the polyelectrolyte and nanoparticle deposition has led to the successful formation of silica [67,68] and titania [69] coated PS spheres. Using this approach preformed crystalline nanoparticles can be deposited on the organic spheres and crystalline hollow spheres can be obtained without the need of calcination. On removal of the template and the polymer interlayers by heating, hollow spheres of the inorganic material can be obtained [68-70]. This procedure is described in detail in the chapter by Dr Frank Caruso. 

Following a related approach, Castelvetro et al. reported the formation and properties of hybrid latex films resulting from the coalescence of low 7 poly(BA-co-MMA-co-MPTMS) terpolymer latex particles coated by a silica shell [78], The latex was synthesized at neutral pH by semi-continuous emulsion polymerization under starved-feed conditions in order to protect the MPTMS monomer from premature hydrolysis and condensation reactions. A substantial amount of free silanols were therefore available for further reaction with the silica precursor. In order to avoid the formation of a densely crosslinked silica network around the latex core, which may significantly alter film formation, the pH was kept at around 2 (at this pH, hydrolysis is promoted and condensation is significantly retarded). TEM and AFM studies of the nanocomposite film indicated that the silica shell formed a continuous percolating network throughout the polymer matrix. A porous film of interconnected hollow silica spheres was next elaborated by thermo-oxidative decomposition of the organic phase. 

Organic spheres are predominantly polymeric, consisting of synthetic or natural polymers. The field of polymeric nano- and microparticles is vast, comprising, for instance, latex particles for coatings, hollow particles for syntactic foams, and microcapsules for foaming and additive release. In addition, there are core-shell microbeads and coated polymeric particles, where the particles can exhibit multiple functionalities, thanks to the individual features of their different layers 1]. As fillers in thermosets and thermoplastics, hollow microspheres and expandable microcapsules are among the most frequently used in commercial applications. 

Solid polymeric colorants may also be prepared by using solid absorbent materials such as metal oxides, metal salts of aluminosilicate, clays, diatomites, hydro-talcite, silica, zeolite, hollow glass spheres, organic-inorganic mesoporous hybrid... 

The preparation of porous hollow silica nanoparticles for a drug delivery applications was described by Chen et al. The authors used the time release profile of cefradine as a test example. Caruso et al. " prepared hollow silica spheres and silica-polymer layered composite spheres. A controlled pore structure remained after decomposition of the organic material and possible applications in medical and pharmaceutical applications are described in their publication. Another drug release example was described by Chamay et al., ° who incorporated ibuprofen, a well... 

Excess iron is incorporated into ferritin and stored in this form in the liver and other organs. The ferritin molecule consists of 24 subunits and has the shape of a hollow sphere (bottom left). It takes up Fe ions, which in the process are oxidized to Fe " and then deposited in the interior of the sphere as fer-rihydrate. Each ferritin molecule is capable of storing several thousand iron ions in this way. In addition to ferritin, there is another storage form, hemosiderin, the function of which is not yet clear. 

The process utilizing supramolecular organization involves pore expansion in silicas. A schematic view of such micelles built from the pure surfactant and those involving in addition n-alkane is shown in Figure 4.9. Another example of pore creation provides a cross-linking polymerization of monomers within the surfactant bilayer [30]. As a result vesicle-templated hollow spheres are created. Dendrimers like that shown in Figure 4.10 exhibit some similarity to micellar structures and can host smaller molecules inside themselves [2c]. Divers functionalized dendrimers that are thought to present numerous prospective applications will be presented in Section 7.6. 

A broad variety of sacrificial colloidal cores have been used for hollow capsule fabrication. They are inorganic or organic particles from tens of nanometers and up to tens of micrometerss, like melamine formaldehyde (MF), polysterene spheres, CaCC>3 and MgCQ3 particles, protein and DNA aggregates, small dye... 

Figure 3.30 Synthesis route of PMO magnetic hollow spheres (a) hydrophobic, stearic acid-capped Fes04 nanoparticles in an organic phase are treated with CTAB to produce (b) water-dispersible nanoparticles, (c) Formation of FC4 vesicles leads to encapsulation of the CTAB-stabilized Fe304 nanoparticles, (d) Addition of BTME/CTAB forms the outer ethane-bridged PMO shell surrounding the vesicles. (See color insert.)...

To what extent can the example of a solid exoskeleton be replicated in the laboratory Going against most contemporary examples of flexible artificial cells, Muller and Rehder published an example of a complex molybdenum oxide that spontaneously forms discrete nanospheres [23], The hollow spheres were porous and allowed lithium cations to pass through the exoskeleton. While this a perhaps an extreme example of what may be considered an artificial cell, the authors assert that the presence of ion selective channels through the encapsulating oxide is directly analogous to natural ion channels in organic cells. 

F. Caruso uses monodisperse polymer spheres and their colloidal crystals only as templates to create hollow capsules or extended opal arrays with the layer-by-layer technique. Again this is a typical colloid chemistry tool which is unparalleled in low molecular weight organic chemistry, and hollow mesostruc-tures systems with astonishingly high complexity and chemical function can be generated. 

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