In situ interfacial polycondensation

Figure 6. Formation of a composite reverse osmosis membrane via in-situ interfacial polycondensation.

Improvement of Water Permeability (UTC-70L) In our past experiments of various polyamide composite membranes, introduction of end acids is preferable to obtain better water permeability and decrease of end amines is preferable to obtain better tolerance to chloride. From the view point, we tried to improve water permeability of UTC-70. Our strategy for introduction of end acids and decrease of end amines is an improvement of acid chlorides reactivity by using catalyst for in-situ interfacial polycondensation. Thus, we found common catalysts for acylation worked effectively as we had expected, and water permeability of UTC-70 were increased without severe decrease of membrane selectivity. This type of membrane are commercialized as "UTC-70L", and membrane performance is shown Figure 7. 

Transmission electron microscopy (TEM) This technique is used when the MPCM is in nanometer size range. The specimen must have a low density, allowing the electrons to travel through the sample. There are different ways to prepare the material it can be cut in very thin slices either by fixing it in plastic or working with it as frozen material. Pan et al. studied nanostructures that were prepared through the methodology in-situ interfacial polycondensation. 

Tarameshlou et al. [28] reported a novel method for preparation of PA 6.6/OMMT nanocomposites, using in situ interfacial polycondensation of an aqueous hexamethylene-diamine and a nonaqueous adipoyl chloride in dichloromethane solutions containing varied amounts of OMMT. Exfoliated or highly intercalated PA 6.6/OMMT nanocomposites were obtained in their studies. Similar interfacial polycondensation was employed to prepare PA 6.6/Na-MMT nanocomposites [29]. 

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 next approach used the int acial polycondensation reaction to form a vay thin polymeric layer onto a substrate. Morgan first proposed this approach (Morgan, 1965), and then Scala et al. (1973) and Van Heuven (1976) actually applied this approach to obtain an RO membrane. But it was Cadotte who invented the high-performance membrane using the in situ interfacial condensation method (Cadotte, 1985). In his method, interfacial condensation reactions between polymeric polyamine and monomeric polyfimctional acid halides or isocyanates takes place on a substrate material to deposit a thin film barrier onto a substrate. Some of the composite membranes were succeeded in industrial fabrication by another method, which was designated as PA-300 or RC-100. 

This method, developed by Cadotte and coworkers of Film Tech in the 1970s, is currently most widely used to prepare high-performance reverse osmosis and nanofiltration membranes.A thin selective layer is deposited on top of a porous substrate membrane by interfacial in situ polycondensation. There are a number of modifications of this method primarily based on the choice of the monomers.However, for the sake of simplicity, the polycondensation procedure is described by a pair of diamine and diacid chloride monomers. 

A diamine solution in water and a diacid chloride solution in hexane are prepared. A porous substrate membrane is then dipped into the aqueous solution of diamine. The pores at the top of the porous substrate membrane are filled with the aqueous solution in this process. The membrane is then immersed in the diacid chloride solution in hexane. Because water and hexane are not miscible, an interface is formed at the boundary of the two phases. Poly condensation of diamine and diacid chloride will take place at the interface, resulting in a very thin layer of polyamide. The preparation of composite membranes by the interfacial in situ polycondensation is schematically presented in Fig. 3. 

The solution coating technique was used in the preparation of the cellulose triacetate membrane discussed above. A solution of cellulose triacetate in chloroform was deposited on the porous support and the solvent was then evaporated leaving a thin film on the porous support. Thin film polymerization was used to prepare a polyfuran membrane barrier layer on polysulfone. In this case, the monomer furfuryl alcohol is polymerized in situ by adjustment of pH and temperature. This membrane proved to be highly susceptible to oxidizing agents and is of limited value. By far the most valuable technique in the formation of membrane barrier layers is interfacial polycondensation. In this method, a polymer is formed on the porous support surface at the interface of organic and aqueous phases by reaction of specific molecules dissolved in each phase. It is by this method that a number of polyamides and polyurea membrane barrier layers have been formed on polysulfone. Elements containing these membranes are available commercially. 

A main process of membrane fabrication is an in-situ inter cial polycondensation, which is an interfacial reaction between aqueous solution of amines and organic solution of acid chlorides. In the process, many factors, eg. diffusion rate of amines, membrane thickness, surface charge related to end groups, affect membrane perfoimance (Figure 6). 

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. 

In-situ processes such as emulsion, suspension, precipitation or dispersion polymerization and interfacial polycondensations are the most important chemical techniques used for microencapsulation [85-90]. An image of microcapsules with an aqueous core and silicone shell prepared using in-situ polymerization is shown in Figure 1.10. 

An in-depth discussion on the major in-situ polymerization processes is provided in Chapter 4. In addition, encapsulation using the mini emulsion process is discussed in Chapter 2, and interfacial polycondensations processes are described in Chapter 5. 

Z. S. Kalkan and L. A. Goettier, In situ polymerization of polyamide 66 nanocomposites utilizing interfacial polycondensation. II. Sodium montmorillonite nanocomposites. Polymer Engineering and Science, 49 (2009), 1825-31. 

A thin polymer him can be formed in situ on the surface of a porous substrate membrane by an interfacial polycondensation process. A classical example of this method was described by Rozelle et al. in detail for the formation of the North Star NS-100 membrane 30]. According to their description, the polysulfone support films are placed, shiny surface upwards, into a 0.67% aqueous polyethyicnimine (PEI) solution in an aluminum tray. After 1 min, the PEI solution is poured off, and the tray held in a vertical position for 1 min to allow the excess solution to drain from the surface of the him. Then the wet surface is contacted with a 0.5% solution of toluene 2,4-diisocyanate (TDI) for 1 min at room temperature. After draining the excess TDI solution, the u y is placed horizontally at 11S C for 10 min. After the heat curing, the composite membrane is easily peeled off from the aluminum surface. 

---