Hollow particle

The preparation of hollow particles through emulsion polymerisation is very interesting, for instance, in the hght of using these particles as drug carriers in controlled drug release. Other applications are in surface coatings and as opacifiers. 

Core-shell emulsion polymerisation. VanderholF et al. (1991) prepared particles consisting of a core of a copolymer of methacryUc acid and methyl methacrylate and a shell of crosslinked material. After neutralisation with NH4OH, the core material collapses and the particles contain voids of between 130 and 760 run. A similar approach was applied by Okubo and Ichikawa (1994) where the particles were produced by an emulsion-free terpolymerisation of styrene, butyl acrylate and methacrylic acid. The effect of pH, temperature and time of acid treatment on the multi-hoUow structure formed were investigated. 

A somewhat different approach is where an organic solvent is used to extract to core material. In one example (Okubo etal, 1991 Okubo Nakagawa, 1994) large polystyrene seed particles, produced by dispersion polymerisation, are used as a seed in a second stage polymerisation where a shell of polystyrene-divinylbenzene was polymerised around the core. The core material was then extracted with toluene under reflux. Depending on the 

Vesicle polymerisation. There are several options to achieve polymerisation in/or of vesicles (Paleos, 1990)  

The most flexible route is the one where the bilayer is swollen with the monomers of choice. In that case the glass transition temperature, permeability, layer thickness and degradabihty of the polymer layer can be varied more easily than in the other approaches. 

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All 12 active sites of HslV are located on the inner walls of the hollow particle. In the E. coli particle, each active site has neighboring active sites 28 A away on the same ring and 22 A and 26 A away on the opposite ring. The environment of the nucleophilic Thrl looks similar to that in proteasomes, and the presence of a (putatively protonated) lysine residue near the active site probably helps to lower the pKa of the N-terminal a-amino group so that it is present in the unprotonated form, which can act as the general base to accept a proton from Thrl. 

Sanji T, Nakatsuka Y, Ohnishi S, Sakurai H. Preparation of nanometer-sized hollow particles by photochemical degradation of polysilane shell cross-linked micelles and reversible encapsulation of guest molecules. Macromolecules 2000 33 8524-8526. 

Song LY, Ge XW, Zhang ZC (2005) Interfacial fabrication of silica hollow particles in a reverse emulsion system. Chem Lett 34 1314-1315... 

Since this process depends on the stoichiometry of the reactants, sufficient amounts of EDA must be present to produce fully solidified polymer particles. Incomplete reactions yielded a polyurethane shell, which on the removal of unreacted liquid in the core by evaporation resulted in hollow particles (68). It would appear that the solid encapsulating polymer inhibits the diffusion of EDA into the rest of the original droplet. 

Powder C had particles with more and deeper folds or shrinkages and had frequently, as shown in Figure 5, broken hollow particles,possibly due to ballooning effect, as compared to powders A and B. The observed change is further magnified (1800 x) in Figure 4. 

The core may be a solid or a liquid, or indeed a gas but this is unusual in the context of release applications. Hollow particles have found applications in other areas such as surface coatings, where they offer a high refractive index contrast with film itself, and therefore good light scattering properties. The shell... 

Fig. S TEM images of (a) a multi-walled nanotube of WS2 and (b) hollow particles (inorganic fullerenes) of WS2. (Reproduced with permission from refs. 5a and ft, respectively).

Particles may also occur as gas-filled hollow drops or as particle-filled hollow particles. Fly ash is an example of this latter type of material. Thus particle density can be significantly different from the density of the parent material. 

Fig. 25 Hollow particles prepared from SCMs of poly(l,l-dimethyl-2,2-dihexyldisilene)-f -poly(methacrylic acid), (a) AFM image on Pyrex glass plate with operating in the contact mode, (b) vertical profile of the hollow particle shown in part (a), and (c) in the tapping mode under THF wet conditions. Reproduced with permission from [50] Sanji et al. (2000) Macromolecules 33 8524. American Chemical Society...

Fig. 15 Illustration of the preparation of hollow particles using PS-6-P2VP diblock copolymer together with PAE. Reprinted with permission from [127]. 2005 American Chemical Society...

The anionic polymerization of masked disilenes proceeds via living anions, and therefore block copolymerization with a conventional vinyl monomer is possible. Recently, interesting hydrophobic block copolymer of PMHS with poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(methacrylic acid) (PMMA) have been prepared (Scheme 11). These polymers can be self-assembled and are transformed into polysilane micelles, shell cross-linked micelles (SCM), and nanometer-sized hollow particles. ... 

In this process, porous particles are easily formed. The porosity is controlled by changing the precursor concentration in the droplets or by adjusting temperature profile in the furnace. Hollow particles can also be prepared when the solute concentration gradient is created during evaporation of solvent. Easy scaling up is a major advantage of this method, too. 

Hyuk Im S, Jeong U, Xia Y (2005) Polymer hollow particles with controllable holes in their surfaces. Nat Mater 4 671-675. 

Significantly different seemed intiaUy the crystal morphology of polyethylene, polybutene-1, polypropylene, polystyrene, poly(4-methyl pen-tene-1), and polyisoprene polymerized with varying solvents and at varying temperatures (114, 123). Discrete hollow particles with a fibrous texture could be observed. The fibrils had an appearance similar to polyethylene crystallized from solution sheared by rapid stirring (118). A closer analysis of this similarity was carried out by Wikjord and Manley (124), Keller and Willmouth (117), and Ingram and Schindler (125) for polyethylene. 

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