Reverse interfacial polymerization

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

Membranes made by interfacial polymerization have a dense, highly cross-linked interfacial polymer layer formed on the surface of the support membrane at the interface of the two solutions. A less cross-linked, more permeable hydrogel layer forms under this surface layer and fills the pores of the support membrane. Because the dense cross-linked polymer layer can only form at the interface, it is extremely thin, on the order of 0.1 p.m or less, and the permeation flux is high. Because the polymer is highly cross-linked, its selectivity is also high. The first reverse osmosis membranes made this way were 5—10 times less salt-permeable than the best membranes with comparable water fluxes made by other techniques. 

Interfacial polymerization membranes are less appHcable to gas separation because of the water swollen hydrogel that fills the pores of the support membrane. In reverse osmosis, this layer is highly water swollen and offers Httle resistance to water flow, but when the membrane is dried and used in gas separations the gel becomes a rigid glass with very low gas permeabiUty. This glassy polymer fills the membrane pores and, as a result, defect-free interfacial composite membranes usually have low gas fluxes, although their selectivities can be good. 

UF Membranes as a Substrate for RO An important use of UF membranes is as a substrate for composite reverse-osmosis membranes. After the UF membrane (usually polysulfone) is prepared, it is coated with an aqueous solution of an amine, then dipped in an organic solution of an acid chloride to produce an interfacially polymerized membrane coating. 

Geong and coworkers reported a new concept for the formation of zeolite/ polymer mixed-matrix reverse osmosis (RO) membranes by interfacial polymerization of mixed-matrix thin films in situ on porous polysulfone (PSF) supports [83]. The mixed-matrix films comprise NaA zeoHte nanoparticles dispersed within 50-200 nm polyamide films. It was found that the surface of the mixed-matrix films was smoother, more hydrophilic and more negatively charged than the surface of the neat polyamide RO membranes. These NaA/polyamide mixed-matrix membranes were tested for a water desalination application. It was demonstrated that the pure water permeability of the mixed-matrix membranes at the highest nanoparticle loadings was nearly doubled over that of the polyamide membranes with equivalent solute rejections. The authors also proved that the micropores of the NaA zeolites played an active role in water permeation and solute rejection. 

Although these composite fibers were developed lor reverse osmosis lltcir acceptance in the desalinatiun industry has been limited due to insufficient selectivity and oxidative stability. The concept, however, is extremely viable composite membrane fiat films made from interfacial polymerization have gained wide industry approval. Hollow libers using Ibis technique to give equivalent properties and life, yet lo be developed, should be market tested during the 1990s. 

Figure 3.20 Schematic of the interfacial polymerization process. The microporous film is first impregnated with an aqueous amine solution. The film is then treated with a multivalent crosslinking agent dissolved in a water-immiscible organic fluid, such as hexane or Freon-113. An extremely thin polymer film forms at the interface of the two solutions [47]. Reprinted from L.T. Rozelle, J.E. Cadotte, K.E. Cobian, and C.V. Knopp, Jr, Nonpolysaccharide Membranes for Reverse Osmosis NS-100 Membranes, in Reverse Osmosis and Synthetic Membranes, S. Sourirajan (ed.), National Research Council Canada, Ottawa, Canada (1977) by permission from NRC Research Press...

Since the discovery by Cadotte and his co-workers that high-flux, high-rejection reverse osmosis membranes can be made by interfacial polymerization [7,9,10], this method has become the new industry standard. Interfacial composite membranes have significantly higher salt rejections and fluxes than cellulose acetate membranes. The first membranes made by Cadotte had salt rejections in tests with 3.5 % sodium chloride solutions (synthetic seawater) of greater than 99 % and fluxes of 18 gal/ft2 day at a pressure of 1500 psi. The membranes could also be operated at temperatures above 35 °C, the temperature ceiling for Loeb-Sourirajan cellulose acetate membranes. Today s interfacial composite membranes are significantly better. Typical membranes, tested with 3.5 % sodium chloride solutions,... 

Table 5.1 Characteristics of major interfacial polymerization reverse osmosis membranes...

Composite Nonporous barrier on microporous substrate Flat-sheet, hollow fiber Direct coating, interfacial polymerization, plasma polymerization Reverse osmosis, gas separation, perstraction... 

Jeong, Byeong-Heon, Eric M.V. Hoek, Yushan Yan, Arun Subramani, Xiaofei Huang, Gil Hurwitz, Asim K. Ghosh, and Anna Jawor, "Interfacial Polymerization of Thin Film Nanocomposites A New Concept for Reverse Osmosis Membranes," Journal of Membrane Science, 294,2007. 

L.E. Black, Interfacially polymerized membranes and reverse-osmosis of organic solvent solutions using them, EP 421676, 1991. 

Teijin s product, trademarked Teijinconex, is a 100/97/3 copolymer of MPD/IQ/TCl [83]. The polymer is prepared by interfacial polymerization, isolated and dissolved in NMP to form spin dopes of approximately 20% solids concentration [86]. The resulting isotropic solutions are stable at 100°C and are suitable for wet spinning. The solution has two solubility limits that include reversible and irreversible regions, as shown in Figure 13.1 [87]. If the irreversible limit is exceeded, the polymer becomes soluble only in sulfuric acid. 

Black, L. E. 1991. Interfacially polymerized membranes for reverse osmosis separation of organic solvent solutions. U.S. Patent No. 5173191. 

The fastest growing desalination process is a membrane separation process called reverse osmosis (RO). The most remarkable advantage of RO is that it consumes little energy since no phase change is involved in the process. RO employs hydraulic pressure to overcome the osmotic pressure of the salt solution, causing water-selective permeation from the saline side of a membrane to the freshwater side as the membrane barrier rejects salts [1-4], Polymeric membranes are usually fabricated from materials such as cellulose acetate (CA), cellulose triacetate (CTA), and polyamide (PA) by the dry-wet phase inversion technique or by coating aromatic PA via interfacial polymerization (IFP) [5]. 

Jeong BH, Hoek EMV, Yan Y, Subramani A, Huang X, Hurwitz G, Ghosh AK, and Jawor A, Interfacial polymerization of thin film nanocomposites A new concept for reverse osmosis membranes, Journal of Membrane Science 2007, 294, 1-7. 

State of the art composite membranes for reverse osmosis consist of three layers 1) the discriminating surface layer (commonly a polyamide produced by interfacial polymerization of m-phenylenediamine - trimesoyl chloride), 2) a supporting ultrafiltration layer (commonly polysulfone), and 3) a non-woven fabric that provides the majority of the mechanical strength [28, 30]. This trilayer composite reduces the resistance to permeation in the supporting layers without compromising mechanical integrity. 

Emulsions are involved when interfacial polymerization is used to microencapsulate a lipophilic drug. In this case the drug is placed in an oil phase together with a hydrophobic monomer, an added surfactant and suitable mechanical shear allows these to be incorporated into the dispersed oil droplets of an O/W emulsion. A hydrophilic monomer is then dissolved in the aqueous phase and the two monomers interact at the oil/aqueous interface to create polymer films that form the capsule walls. For a hydrophilic drug all of the above could simply be reversed. 

In recent years reverse osmosis membranes have become the most popular desalination technology because of their high energy efficiency. To improve the separation features of polyamide films, typically used as a selective layer, different fillers have been dispersed in the polymer phase. The in situ dispersion of the nanoparticles during the interfacial polymerization reaction of the polyamide layer is reported to be able to achieve a suitable dispersion. 

Xu, X. and Kirkpatrick, R.J. 2006. NaCl interaction with interfacially polymerized polyamide films of reverse osmosis membranes A solid-state Na NMR study. J. Membr. Sci. 280 226-233. 

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