Microcapsule morphology

FIGURE 32.2 Schematic diagram of microcapsules morphology (a) reservoir system (simple wall) (b) matrix system (c) simple wall (liquid core) (d) multicore (e) simple wall (solid and irregular core) and (f) matrix (solid core dispersed into the polymeric matrix). 

A key feature of encapsulation processes (Figs. 4a and 5) is that the reagents for the interfacial polymerisation reaction responsible for shell formation are present in two mutually immiscible Hquids. They must diffuse to the interface in order to react. Once reaction is initiated, the capsule shell that forms becomes a barrier to diffusion and ultimately begins to limit the rate of the interfacial polymerisation reaction. This, in turn, influences morphology and uniformity of thickness of the capsule shell. Kinetic analyses of the process have been pubHshed (12). A drawback to the technology for some apphcations is that aggressive or highly reactive molecules must be dissolved in the core material in order to produce microcapsules. Such molecules can react with sensitive core materials. 

The term microcapsule is defined, as a spherical particle with the size varying between 50 nm and 2 mm containing a core substance. Microspheres are, in a strict sense, spherically empty particles. However, the terms microcapsules and microspheres are often used synonymously. In addition, some related terms are used as well. For example, microbeads and beads are used alternatively. Spheres and spherical particles are also employed for a large size and rigid morphology. Due to attractive properties and wider applications of microcapsules and microspheres, a survey of their applications in controlled drug release formulations is appropriate. 

Sah, H.K. Toddywala, R. Chien, Y.W. Biodegradable microcapsules prepared by a w/o/w technique effects of shear force to make a primary w/o emulsion on their morphology and protein release. J. Microencapsul. 1995, 12 (1), 59-69. 

The surface of microcapsules has a granular structure (Fig. 2b). The average size of grains is 84.0 13.0 nm. Such morphology is probably related to... 

Rgure 8.6 Representation of the variety of polymer morphologies in solution and in the gel (or microgel) or solid states. In solution the conformation of the polymer depends on the nature of polymer-solvent interactions and whether or not the polymer chains associate to form micellar aggregates. Crystals of polymer and microcrystals can be prepared, and gels can be formed from covalently crosslinked or polymer chains associated by hydrogen bonding or hydrophobic interactions. Listed are the forms in which most polymers can be fabricated membranes, fibres, composites, matrices microspheres and microcapsules can also feature, as discussed later in this chapter. 

The authors also reported on the supramolecular self-assembly from rod—coil—rod triblock copolymers prepared by copolymerization of 5-acetyl-2-aminob-ezophenone with diacetyl functionalized polystyrene with low polydispersity (Scheme 12).110 In contrast to the rod—coil diblock copolymers which exhibit multiple morphologies, the triblock copolymers were found to spontaneously form only microcapsules or spherical vesicles in solution as evidenced by optical polarized, fluorescence optical, and scanning electron microscopies (Figure 33). 

Polymea mictocapsules containing 0,0-3,5,6-trichloro-2-pyridyl phosphorothioate (Dursban) were prepared by reacting a polyisocyanate and a poly amine and the factors affecting the formation of the microcapsule wall examined. The formation of polyurea was confirmed by FTIR spectroscopy, the thermal properties of the microcapsules were investigated by DSC and the morphology of the microcapsules detenninedby scanning electron microscopy. Optimum conditions for the formation of a thin surface l er and a porous matrix were established. 9 lefs. 

Journal of Microencapsulation 18, No.6, Nov./Dec. 2001, p.801-9 MORPHOLOGY AND STRUCTURE OF MICROCAPSULES PREPARED BY INTERFACIAL POLYCONDENSATION OF METHYLENE BIS(PHENYL ISOCYANATE) WITH HEXAMETHYLENE DIAMINE Jabbari E... 

Polyurethane-urea microcapsules were prepared by the interfacial polycondensation of methylene bis(phenyl isocyanate), hexamethylene diamine and anionic sodium hgnin sulphonate, as the emulsifying agent, and 2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide, as a herbicide. The morphology amd microstractuie of these microcapsules were investigated by TEM and scanning electron microscopy and the effect of caldrun chloride on the stabihty of the microcapsule stractuie examined. 22 refs. 

The morphology of the resulting solid material depends both on the material structure (crystalline or amorphous, composite or pure, etc.) and on the RESS parameters (temperature, pressure drop, distance of impact of the jet against the surface, dimensions of the atomization vessel, nozzle geometry, etc.)[ l It is to be noticed that the initial investigations consisted of pure substrate atomization in order to obtain very line particles (typically of 0.5-20 m diameter) with narrow diameter distribution however, the most recent publications are related to mixture processing in order to obtain microcapsules or microspheres of an active ingredient inside a carrier. 

The morphology of microcapsules depends mainly on the core material, how it is distributed within the system, and the deposition process of the shell. Similarly, the morphology of the internal structure of a microparticle depends largely on the selected shell materials and the microencapsnla-tion methods that are employed. The microcapsules may be categorized into several arbitrary and overlapping classifications such as... 

In addition to these three basic morphologies, microcapsules can also be mononuclear with multiple shells (such as layering of shells), or they may form clusters of microcapsules (as noted in Figure 1.1). 

Encapsulation is a formulation technique in which an active compound can be enclosed inside a second material, such as polymeric or nonpolymeric carrier. The product obtained by this process is referred to as microcapsules. These microcapsules are obtainable in two types of morphology— either matrix type or core-shell (Figure 11.8). Matrix-type microcapsules are in which the API is homogeneously distributed within the carrier system. In contrast, core-shell microcapsules consist of a solid shell made up of polymeric materials surrounding a core-forming space, which entraps the API. 

FIGURE 11.8 Schematic morphologies of the two types of microcapsules, (a) Core-shell microcapsule or reservoir and (b) matrix type microcapsule. (From Lembo, D. and Cavalli, R., Antivir. Chem. Chemother., 21, 53,2010.)... 

The formation of a microcapsule wall through interfacial polycondensation/addition takes place in two steps. First step is the deposit of the oligomer (initial wall) at the oil droplet, and the second step is the wall thickness builds up. As described earlier, the polymerization occurs in oil phase, and the formed initial wall can limit the diffusion of the reactants. This reduces the polymerization rate that has great impact on the surface morphology and thickness of the microcapsule wall. - - Polycondensation by which polyamide, polyester, and polycarbonate microcapsules are prepared can generate acid byproduct during the process therefore, a base is needed to neutralize the acid and drive the reaction to complete. ... 

Zhuo, L. Chen, S. Effects of catalyst and core materials on the morphology and particle size of microcapsules. International Journal of Polymeric Materials (2004), 53(5), 385—393. 

Salts like KCl, Na2S04, KH2P04, etc., can help to reduce the processing viscosity and achieve high solid content microcapsule slurry with increased wall impermeability. NaCl can increase the encapsulation efficiency " and has positive impact on the microcapsules surface morphology. ... 

Dietrich, K. Herma, H. Nastke, R. Bonatz, E. Teige, W. Amino resin microcapsules, II Preparation and morphology. Acta Polymerica (1989), 40(5), 325-331. 

Zhao, D. Liu, F. Mu, W. Han, Z.-R. Factors affecting morphology and encapsulation ratio of chlorpy-rifos microcapsules with UF-resin during preparation. Yingyong Huaxue (2007), 24(5), 589-592. 

The speed in which this exchange occurs determines the final morphology of the membrane or microcapsule. 

Panisello C, Pena B, Gumi T, Garcia Vails R. Polysulfone microcapsules with different wall morphology. JAppl Polym Sci. 2013 129 1625-1636. 

Torras C, Pitol L, Garcia Vails R. Two methods for morphological characterization of internal microcapsule structures. J Membr Sci. 2007 305(l-2) l-4. 

Microcapsules can be classified in three basic categories according to their morphology as mono-cored (mononuclear), poly-cored (poly-nuclear), and matrix types. Mono-cored (mononuclear) microcapsules contain the shell around the core. 

Rocha-Selmi, G. A. Favaro-Trindade, C. S. Grosso, C. R. F., Morphology, stability, and application of lycopene microcapsules produced by complex coacervation. Journal of Chemistry (2013). 

Leimann, F. V., Gonsalves, O. H., Machado, R. A. E, and Bolzan, A. (2009). Antimicrobial activity of microencapsulated lemongrass essential oil and the effect of experimental parameters on microcapsules size and morphology. Materials Science and Engineering C, 29,430-436. 

The lowest particle size of microparticles is 1 pm and the largest size is 1 mm. Commercial microparticles have a diameter range of 3 to 800 pm. Microparticles may be formulated as microcapsules or as microspheres that differ in morphology and internal structure. In addition other terms such as beads, microbes are also being used alternatively. 

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