Polymers capsule shells

A unique feature of in situ encapsulation technology is that polymerization occurs ia the aqueous phase thereby produciag a condensation product that deposits on the surface of the dispersed core material where polymerization continues. This ultimately produces a water-iasoluble, highly cross-linked polymer capsule shell. The polymerization chemistry occurs entirely on the aqueous phase side of the iaterface, so reactive agents do not have to be dissolved ia the core material. The process has been commercialized and produces a range of commercial capsules. 

Asymmetrical membrane capsules were prepared, for example, by dip coating mandrels with solutions containing 15 wt% CA and 33 wt% ethanol in acetone. After the mandrels were withdrawn from the coating solution they were immersed in water to precipitate the polymer and create the asymmetrical membrane capsule shell. This process was used to create both the body and the cap for the capsules. The capsules were sealed at the juncture between the cap and body by banding with a solution of CA in acetone. 

Coating materials with degradable bonds including capsule shells Hydrogels and matrices consisting of cross-linked, degradable polymers... 

Polymer capsules Core-shell Flavour composition entrapped in capsule consisting of solvent core covered by protein shell Dry product or liquid dispersion Limited (oxidative) stability of encapsulated flavour Flavour can be loaded in empty particles Flexible particle size Relatively slow release/burst-like release... 

Furthermore, porous CPs (e.g., polypyrrole, polyanUine) films have been used as host matrices for polyelectrolyte capsules developed from composite material, which can combine electric conductivity of the polymer with controlled permeability of polyelectrolyte shell to form controllable micro- and nanocontainers. A recent example was reported by D.G. Schchukin and his co-workers [21]. They introduced a novel application of polyelectrolyte microcapsules as microcontainers with a electrochemically reversible flux of redox-active materials into and out of the capsule volume. Incorporation of the capsules inside a polypyrrole (PPy) film resulted in a new composite electrode. This electrode combined the electrocatalytic and conducting properties of the PPy with the storage and release properties of the capsules, and if loaded with electrochemical fuels, this film possessed electrochemically controlled switching between open and closed states of the capsule shell. This approach could also be of practical interest for chemically rechargeable batteries or fuel cells operating on an absolutely new concept. However, in this case, PPy was just utilized as support for the polyelectrolyte microcapsules. 

Fig. 18 Formation of polymer capsules by polymer nanoprecipitation on preformed miniemulsion droplets. Left An aqueous solution containing a lipophobe (can also be a functional molecule, e.g., chlorohexidine digluconate [87, 88]) is dispersed in a solution of a polymer in a solvent/nonsolvent mixture. Middle After homogenization, solvent is evaporated in a controlled manner. Right The polymer precipitates on the aqueous droplets and eventually forms a polymeric shell...

Hao, L., Zhu, C., Chen, C., Kang, E, Hu,Y., Fan, W., Chen, Z., 2003. Fabrication of silica core-conductive polymer polypyrrole shell composite particles and polypyrrole capsule on monodispersed silica templates. Synth. Met. 139,391-396. 

With regard to the preparation of multicompartment polymer capsules based on this SIP method, a successful approach was demonstrated by Kang et al. [17] who used a two-step distillation/precipitation polymerization of methacrylic acid (MAA) and A-isopropylacrylamide (NIPAM), respectively, onto silica nanoparticles as templates. More interestingly, due to the PMAA inner shell and PNIPAM outer shell, the hollow structure can respond independently to changes in pH and temperature. After loading DOX into the capsules, temperature- and pH-controlled release of anticancer drug behavior was demonstrated. 

Fluorescent dyes as markers can also be used to follow particle-cell interactions, via LSM and FACS measurements. Hence, polyure thane/urea capsules were created in inverse miniemulsion that could encapsulate a fluorescent dye with 90% efficiency [129]. In this case, carboxymethylation was carried out on the particle surface, followed by the physical adsorption of poly(2-aminoethylmethacrylate) or polyethylene imine polycations. As expected, the rate of uptake of capsules modified by the polycation was higher than for non-modified capsules. Rosenbauer et al. applied the same synthetic procedure, but in the presence of a surfactant that crosslinked the shell [130]. The commercially available surfactant containing several amine groups reacted with the diioscyanate monomer subsequently, the capsule shell wall was found to be less permeable than capsules synthesized with a non-crosslinkable surfactant. Baier el al. used the above-described synthesis to perform a polymerase chain reaction (PCR) in crosslinked starch nanocapsules [131]. The permeability of the shell was also evaluated using fluorescence spectroscopy. The combination of a cleavable polyurethane [132] with the interfacial polyaddition described above [126] afforded polymer shells that could be opened by ultraviolet (UV) irradiation, or by modifying the temperature or pH [133], In order to determine the release of encapsulated sulforhodamine dye, polyurethanes with... 

Simple Coacervation. Aqueous solutions of water-soluble polymers are phase-separated in aqueous media when sufficient salt is added to such solutions. This phenomenon is called simple coacervation. As long as phase separation produces a liquid polymer-rich phase, simple coacervation can be used to produce microcapsules (5). Microcapsules with a gelatin or poly(vinyl alcohol) shell have been formed in this manner. The use of poly(vinyl alcohol) as a capsule shell material is of great interest in various applications because it is a widely available synthetic polymer with excellent oxygen and oil barrier properties (see Barrier Polymers Vinyl Alcohol Polymers). 

Figure 4c also describes the spontaneous polymerization of para-xylylene diradicals on the surface of solid particles dispersed in a gas phase that contains this reactive monomer (12) (see Xylylene Polymers). The poly(p-xylylene) polymer produced forms a continuous capsule shell that is highly impermeable to the transport of many penetrants, including water. This is an expensive encapsulation process, but it has produced capsules with impressive barrier properties. It is a type B encapsulation process, but is included here for the sake of completeness. 

Figure 4d represents in situ encapsulation processes (13,14), an example of which is presented in more detail in Figure 6 (14). The first step is to disperse a water-immiscible liquid or solid core material in an aqueous phase that contains urea, melamine, water-soluble urea-formaldehyde condensate, or water-soluble urea-melamine condensate. In many cases, the aqueous phase also contains a system modifier that enhances deposition of the aminoplast capsule shell (14). This is an anionic polymer or copoljuner (Fig. 6). Shell formation occurs once formaldehyde is added and the aqueous phase acidified, eg, pH 2-4.5. The system is heated for several hours at 40-60°C.

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