Capsule Formation by Phase Separation

Capsule morphologies could also be observed in a system applying the biodegradable surfactant lecithin and the eco-friendly hydrophobe Neobee M5 (triglyceride) [171] and, after controlled radical copolymerization of styrene and divinyl benzene (DVB), stabilized by poly(vinylalcohol) (PVA) [172]. 

The acid sensitive photoinitiator Lucirin TPO could effectively be shielded from an acidic environment by encapsulating it into PMMA or PBA-co-PMMA particles [173]. The hybrid particles show a core-shell morphology (Fig. 16c) which is generated by phase separation during the polymerization process. While the photoinitiator is readily soluble in the monomer(s), it becomes insoluble in the polymer. Therefore, Lucirin TPO precipitates during the polymerization and forms an amorphous core surrounded by a polymeric shell. The encapsulation efficiency was determined as about 90%. 

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Fig. 16 Capsule formation by phase separation, (a) Scheme a solution of monomer and hydrophobic oil (ie/t) is dispersed in an aqueous surfactant solution middle). Phase separation between the growing polymer and the oil occurs, leading to core shell morphology with encapsulated liquid, (b) Nanocapsules with hexadecane by phase separation [35]. (c) Encapsulation of Lucirin TPO [173] and (d) the fragrance 1,2-dimethyl-1-phenyl-butyramide [174]...

It was found that the nanocapsules are formed in a miniemulsion process by a variety of monomers in the presence of larger amounts of a hydrophobic oil. Hydrophobic oil and monomer form a common miniemulsion before polymerization, whereas the polymer is immiscible with the oil and phase-separates throughout polymerization to form particles with a morphology consisting of a hollow polymer structure surrounding the oil. The differences in the hydro-philicity of the oil and the polymer turned out to be the driving force for the formation of nano capsules. 

Miniemulsion is a special class of emulsion that is stabilized against coalescence by a surfactant and Ostwald ripening by an osmotic pressure agent, or costabilizer. Compared with conventional emulsion polymerization process, the miniemulsion polymerization process allows all types of monomers to be used in the formation of nanoparticles or nanocapsules, including those not miscible with the continuous phase. Each miniemulsion droplet can indeed be treated as a nanoreactor, and the colloidal stability of the miniemulsion ensures a perfect copy from the droplets to the final product. The versatility of polymerization process makes it possible to prepare nanocapsules with various types of core materials, such as hydrophilic or hydrophobic, liquid or solid, organic or inorganic materials. Different techniques can be used to initiate the capsule wall formation, such as radical, ionic polymerization, polyaddition, polycondensation, or phase separation from preformed polymers. 

Coacervation n. The separation of a polymer solution into two or more liquid phases, one of which is a polymer-rich liquid. The term was introduced to distinguish this phenomenon from the precipitation of a polymer solute in solid form. The process is used in microencapsulation by emulsifying or dispersing the material to be encapsulated with a solution of the polymer. By changing the temperature or concentration of the mixture, or by adding another polymer or solvent, a phase separation may be induced and the polymeric portion forms a thin coating on the external surfaces of the particles. After further treatment to solidify the polymeric wall, the capsules can be isolated in powder form by filtration. It is an intermediate stage between sol and gel formation. 

The formation of these DNA gel particles constitutes an example of strong associative phase separation. An indication of the strength of this interaction is the formation of a stronger film (or skin) constituted by the polyelectrolyte-surfactant complex. Preliminary results of S AXS measurements have supported the existence of an ordered structure formed on the hydrated skin of the obtained particles. SEM images of the cross-section of the particles have given evidence for the existence of a shell structure, its formation being more pronounced in the case of ss-DNA. The capsule shells obtained may be considered as physical networks in which surfactant micelles form polycationic-multianionic electrostatic complexes as crosslink points. 

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