Interfacial and in Situ Polymerization

FIGURE 5.82 Microcapsule formation by interfacial and in situ polymerization. A and B are reactants, while— (A-B) —and—(A) —are polymeric products. See text for explanation. (After Thies, C. 1989. Biomaterials and Medical Applications, Encyclopedia Reprint Series, J. I. Kroschwitz, ed., pp. 346-367. John Wiley, New York.)... 

Two types of microencapsulation are known in the art based upon the shellwall forming chemistry. These are interfacial polymerization and in-situ polymerization. Encapsulating plastic shellwalls are synthesized at the 0/W (Oil-in-Water) interface of a pesticide emulsion by reacting oil-soluble monomers dissolved in the pesticide with water-soluble monomers added to the emulsion. This process is referred to as interfacial polymerization. 

Other microcapsules are produced by surface and in situ polymerization methods or, generally, interfacial reactionsThe capsule walls may be soluble or insoluble and impermeable or permeable, whereby the rate of release of the encapsulated material or the exchange of material through the membrane walls can be adjusted and controlled. Originally, drugs, chemicals, toner materials, pigments, and the like were encapsulated to improve handling and... 

Microencapsulation techniques make use of sol-gel processes, coacervation, surface and in situ polymerization methods or, generally, interfacial reactions to produce soluble or insoluble and impermeable or permeable capsule walls. In addition to the coating and spray drying methods that were discussed previously, a growing number of other processes deposit particles onto cores or solid surfaces whereby the binding mechanisms of agglomeration are utilized [B.97] (Chapter 11). 

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. 

Polymerization plays a key role in chemical microencapsulation. The basic mechanism of this method is to put a polymer wall (can be multilayer) through polymerization on a core material, which is in a form of small liquid droplets, solid particles, or even gas bubbles or to embed the core material in a polymer matrix through polymerization. Interfacial polymerization is one of the most important methods that have been extensively developed and industrialized for microencapsulation. According to Thies and Salaun, interfacial polymerization includes live types of processes represented by the methods of emulsion polymerization, suspension polymerization, dispersion polymerization, interfacial polycondensation/polyaddition, and in situ polymerization. This chapter is only focnsed on interfacial polycondensation and polyaddition in a narrow sense of interfacial polymerization. 

There are different types of encapsulation processes [25], for example complex coacervation, polymer-polymer incompatibility, interfacial polymerization and in situ polymerization, and each certainly has its different process challenges which might need to be addressed. The following discussion focuses on the topic of interfacial polymerization. 

An important application in agrochemicals is that of controlled-release formulations. Several methods are used for controlled release, of which microcapsules (CS) are probably the most widely used. These are small particles with size range 1-1000 pm consisting of a core material and an outer wall. The latter isolates the core material from the environment and protects it from degradation and interaction with other materials. The core active ingredient is designed to be released in a controlled manner as required. Microencapsulation of agrochemicals is usually carried out by interfacial condensation, in situ polymerization or coacervation, all of which are determined by the interfacial properties. 

Microencapsulation of agrochemicals is mainly carried out by interfacial condensation, in situ polymerization and coacervation. Interfacial condensation [126,127] is perhaps the most widely used method for encapsulation in industry. The a.i., which... 

Recently, many synthetic polymers such as urea/formalin resin, melamine/formalin resin, polyester, and polyurethane have been widely used as the wall material for the microcapsule, though the gelatin microcapsule is still used. Microcapsules using a synthetic polymer wall have several advantages over those using a gelatin wall (1) the preparation process is simple, (2) the size of the microcapsules is well balanced, (3) the microcapsule concentration can be increased twofold or more and (4) the microcapsules have a high resistance to water and many chemicals. Synthetic microcapsules are prepared by interfacial polymerization or in situ polymerization. 

Y. Ou, F. Yang, J. Chen, Interfacial interaction and mechanical properties of nylon 6-potassium titanate composites prepared by In-situ polymerization, Journal of Applied Polymer Science, vol. 64, pp. 2317-2322,1998. 

The ultrathin dense barrier layer and the thick porous underlayer can be separately fabricated from various materials and laminated together to give an asymmetric membrane. Thus a thin barrier membrane can be formed on the porous matrix by casting from a polymer solution, in-situ polymerization, or in-situ-interfacial condensation polymerization. 

As shown above, aromatic rings are connected by an amide linkage, -CONH-. While the aromatic ring attached to -NH- is m a-substituted, the ring attached to -CO- is the mixture of meta- and para-substitutions, which gives more flexibility to the polymeric material. Aromatic polyamide remains one of the most important materials for RO membranes because the thin selective layer of composite membranes is aromatic polyamide synthesized by interfacial in situ polymerization. 

The next section describes measurements of interfacial tension and surfactant adsorption. The sections on w/c and o/c microemulsions discuss phase behavior, spectroscopic and scattering studies of polarity, pH, aggregation, droplet size, and protein solubilization. The formation of w/c microemulsions, which has been achieved only recently [19, 20], offers new opportunities in protein and polymer chemistry, separation science, reaction engineering, environmental science for waste minimization and treatment, and materials science. Recently, kinetically stable w/c emulsions have been formed for water volume percentages from 10 to 75, as described below. Stabilization and flocculation of w/c and o/c emulsions are characterized as a function of the surfactant adsorption and the solvation of the C02-philic group of the surfactant. The last two sections describe phase transfer reactions between lipophiles and hydrophiles in w/c microemulsions and emulsions and in situ mechanistic studies of dispersion polymerization. 

Chemical functionalization of CNT surfaces could improve their dispersion in the polymer matrix and enhance the nanotube-polymer interfacial interaction and the mechanical load transfer. The effects of nanotube functionalization on the properties of CNT-TPU composites have been investigated in details. Xia and Song have synthesized polycaprolactone polyurethane (PU)-grafted SWNTs (PU-g-SWNTs) and corresponding PU-g-SWNT-PU composites by in-situ polymerization. The results show that PU-g-SWNTs improve the dispersion of SWNTs in the PU matrix and strengthen the interfacial interaction between the PU and SWNTs. Compared with neat PU and pristine SWNT PU composites, PU-g-SWNT-PU composites demonstrate remarkable enhancement on Young s modulus. The Young s modulus of a 0.7 wt /o PU-g-SWNT-PU composite increases by 178% over the blank PU and 88% over the 0.7 wt% pristine SWNT-PU composite, respectively. 

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