Capsules formation, interfacial polymerization

The amine-based Henry reaction catalyst was encapsulated via the interfacial polymerization of oil-in-oil emulsions. PEI was encapsulated by dispersing a methanolic PEI solution into a continuous cyclohexane phase. Upon emulsification, 2,4-tolylene diisocyanate (TDI) was added to initiate crosslinking at the emulsion interface, forming polyurea shells that contain free chains of PEI. The microcapsules crenate when dry and swell when placed in solvents such as methanol and dimethylformamide, suggesting a hollow capsule rather than a solid sphere formation. The catalyst loading was determined to be 1.6 mmol g . ... 

The first step in all interfacial polymerization processes for encapsulation is to form an emulsion. This is followed by initiation of a polymerization process to form the capsule wall. Most commercial products based on interfacial or in situ polymerization employ water-immiscible liquids. For encapsulation of a water-immiscible oil, an oil-in-water emulsion is first formed. Four processes are schematically illustrated in Figure 5.82. In Figure 5.82(a), reactants in two immiscible phases react at the interface forming the polymer capsule wall. For example, to encapsulate a water-immiscible solvent, multifunctional acid chlorides or isocyanates are dissolved in the solvent and the solution is dispersed in water with the aid of a polymeric emulsifier, e.g., poly(vinyl alcohol). When a polyfunctional water-soluble amine is then added with stirring to the aqueous phase, it diffuses to the solvent-water interfece where it reacts with acid chlorides or isocyanates forming the insoluble polymer capsule wall. Normally some reactants with more than two functional groups are used to minimize a regation due to the formation of sticky walls. 

FIGURE 5.86 Schematic description of the process of capsule formation by interfacial polymerization of DdexMA (see text). (After Jiang, B., Hu, L., Gao, C., and Shen, J. 2006. Acta Biomaterialia, 2, 9. With permission.)... 

Hydrophilic materials can be encapsulated with the inverse minianulsions by using interfacial polymerization such as polyaddition and polycondensation, radical, or anionic polymerization. Crespy et al. reported that silver nitrate was encapsulated and subsequently reduced to give silver nanoparticles inside the nanocapsules. The miniemulsions were prepared by anulsilying a solution of amines or alcohols in a polar solvent with cyclohexane as the nonpolar continuous phase. The addition of suitable hydrophobic diisocyanate or diisothiocyanate monomers to the continuous phase allows the polycondensation or the cross-linking reactions to occur at the interface of the droplets. By using different monomers, polyurea, polythiourea, or polyurethane nanocapsules can be formed. The waU thickness of the capsules can be directly tuned by the quantity of the reactants. The nature of the monomers and the continuous phase are the critical factors for the formation of the hollow capsules, which is explained by the interfacial properties of the systan. The resulting polymer nanocapsules could be subsequently dispersed in water. 

Other encapsulations utilize more or less similar methods for the formation of the capsule wall. Complex coacervation utilizes the reaction of an anionic water-soluble polymer with a cationic material to form the shell wall that separates from the solution. As the coacervate separates from the solution, it will tend to coat suspended particles with a protective shell. The shell wall is then hardened with a cross-linking agent. In situ polymerization is used to form urea formaldehyde or melamine formaldehyde shells by using heat to cross-link the monomers forming the shell waU. Interfacial polymerization with isocyanates via hydrolysis is another method to form a shell wall at an organic-water interface. In this case, water acts to hydrolyze some of the polyisocyanate to an amine, which cross-links to form the polyurea microcapsule waU. 

Formation of microcapsules by in situ interfacial polymerization (where the monomers are entirely in the oil phase of the capsule core) yields microcapsules with a high core-to-wall ratio and a bilayer wall with an outer layer (about 0.05 urn) and an inner reinforcing spongy layer (0.5 fim). This method has been used to encapsulate a range of insecticides, pheromones, and herbicides, many of which have been available commercially (37). The capsule size may be varied from submicrometer to 100 /um diameter and the permeability selected for rapid or slow release of... 

Figure 4a represents interfacial polymerization encapsulation processes in which shell formation occurs at the core material-continuous phase interface due to reactants in each phase diflfiising and rapidly reacting there to produce a capsule shell (1,8). The continuous phase normally contains a dispersing agent in order... 

Figure 5 illustrates the type of encapsulation process, shown in Figure 4a, when the core material is a water-immiscible liquid. Reactant X, a multifunctional acid chloride, isocyanate, or a combination of these reactants, is dissolved in the core material. The resulting mixture is emulsified in an aqueous phase that contains an emulsifier such as partially hydrolyzed poly(vinyl alcohol) or a hgno-suMbnate. Reactant Y, a multifunctional amine or a combination of amines such as ethylenediamine, hexamethylenediamine, or triethylenetetramine, is added to the aqueous phase, thereby initiating interfacial polymerization and formation of a capsule shell. If reactant X is an acid chloride, a base is added to the aqueous phase in order to act as an acid scavenger. 

A key feature of encapsulation processes (Figs. 4a and 5) is that the reagents for the interfacial polymerization reaction responsible for shell formation are present in two mutually immiscible liquids. They must diffuse to the interface in order to react. Once reaction is initiated, the capsule shell that forms becomes... 

In interfacial polymerization, the monomers A and B are polylunctional monomers capable of causing polycondensation or polyaddition reaction at the interlace [126, 127]. Examples of oil soluble monomers are polybasic acid chloride, bishalo-formate and polyisocyantates, whereas water soluble monomers can be polyamine or polyols. Thus, a capsule wall of polyamide, polyurethane or polyurea may be formed. Some trifunctional monomers are present to allow crosslinking reactions. If water is the second reactant with polyisocyanates in the organic phase, polyurea walls are formed. The latter modification has been termed in situ interfacial polymerization [128]. 

The suspension polymerization process allowed the formation of capsules of l-30 rm consisting of migrin oil as core and polyurea as wall material. The latter was formed by interfacial polycondensation reactions between different diisocyanates and emulsified ethylenediamine [106],... 

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