Shell Polymers

Scheme 4. Synthetic routes for a silica particle with hyperbranched polymer shell (a) and branched polyelectrolyte shell (b)...

Figure 2. TCS-D strategy. A resin layer is produced outside each support particle. Metallation-reduction of the polymer shell will lead to size-controlled metal nanoclusters. Gentle thermal degradation of the organic shell will lead to size-controlled metal nanoclusters, expected to be evenly dispersed on the support particles surface.

These two seemingly dissimilar applications have a common basis—both are examples of the pressure-sensitive release of a chemical. How are these products designed Tiny spherical capsules (microcapsules or microspheres) with a glass or polymer shell are filled with a liquid core and glued onto paper. For a scratch-and-sniff ad, the core of the microcapsules contains a liquid with the desired scent for carbonless paper, a liquid ink or dye is encapsulated within the... 

Figure 7.1. (a) Transmission electron microscopy image of a collection of 200-nm magnetic emulsion droplets obtained from emulsifying an octane-based ferrofluid. (b) One droplet is shown after polymerization. A polymer shell is visible that encapsulates the iron oxide nanoparticles. (With permission of Ademtech). 

Donath E, Sukhorukov GB, Caruso F, Davis SA, Mohwald H. Novel hollow polymer shells by colloid-templated assembly of polyelectrol3fes. Angew Chem Int Ed 1998 37 2202-2205. 

Pellegrino T, Manna L, Kudera S, Liedl T, Koktysh D, Rogach AL, Keller S, Radler J, Natile G, Parak WJ (2004) Hydrophobic nanocrystals coated with an amphiphilic polymer shell A general route to water soluble nanocrystals. Nano Lett 4 703-707... 

Core carboxylated polymer, Shell hydrophobic polymer... 

The thickness of the imprinted polymer shell can be also tuned in the range 10—40 nm by changing the relative amounts of functionalised silica nanoparticles and polymer shell precursors. The resulting core-shell particles exhibit enhanced capacity of rebinding the TNT template over 2,4-dinitrotoluene in comparison to particles prepared by precipitation polymerisation. Nevertheless, this strategy, although leading to impressive results, cannot be easily applied to other templates and monomers. 

These facts preclude the surface treatment with monomers to form a wood-polymer shell around the wood. Ray cells in the radial direction are often connected to the longitudinal cell structure making shell loading improbable. 

A recent example of how silica nanoparticles prepared by the Stober method can be used as supports was the work of Gao et al., who derivatized a silica nanoparticle first with APTMS and then with acryloyl chloride to form reactive vinyl groups.74 A polymer shell with sites imprinted with the template, TNT, was then formed around the silica nanoparticle using conventional acrylic organic polymerization procedures. The capacity and binding kinetics were shown to be significantly better than traditional imprinted particles.74... 

Peptides can be used to direct the nanoscale assembly of amphiphilic synthetic polymers. A common feature is that the self-assembly of the peptides proceeds as it would do in the absence of the polymer conjugates, with the peptide suprastructure forming a core, surrounded by the polymer random coil. The polymer shell acts to limit aggregation of the peptides beyond a certain size limit. A particularly striking example of this is the self-assembly of cyclopeptide-polymer composites, which form hollow... 

Nanoparticle 7) contrast agents are being studied as well. Gd-chelates can be surrounded by a polymer with a high affinity for the metal and again by a porous hydrophobic polymer shell that modulates access to the core [157]. The coatings increase the circulation time, which has been the primary limitation of Gd chelates. When administered intravenously, the nanoparticles remain in the intravascular space, and thus provide excellent visualization of the vasculature. 

More recently, this method has been successfully extended by us18 to form the inverse systems, i.e. water core/polymer shell particles dispersed, initially in oil, but then transferred to an aqueous continuous phase. Clearly, whether one needs an oil or a water core depends on the nature of the active material to be released. Now one starts with a water/oil emulsion, rather than an oil/water emulsion, but the basic principles are very similar. A variety of shell polymer systems were prepared, including PMMA and poly(tetrahydrofuran) [PTHF]. The high vapor pressure liquid used in this case was in general, acetone. It turned out, however, that these water core systems are intrinsically more difficult to make than the equivalent oil core systems, because large amounts of acetone were required to dissolve the polymers initially in the water-acetone mixtures. An oil was then required which did not mix too well with acetone. In general, mineral oil worked reasonably well. In order to transfer the water core capsules into an aqueous continuous phase, the particles were centrifuged in... 

---