Composite polyamide membranes interfacial polymerization

Kurihara and coworkers at Toray Industries prepared several aminated derivatives of polyepichlorohydrin, then formed composite polyamide membranes by interfacial reaction with isophthaloyl chloride.38 Polyepichlorohydrin was converted to polyepiiodohydrin, then reacted with either4-(aminomethyl)piperidine, 3-(methylamino)hexahydroazepine, or 3-(amino)hexahydroazepine. Also, poly-epiaminohydrin was prepared by reduction of the azide derivative of polyepiiodohydrin. Best salt rejections were obtained if the polymeric amine formulation contained a substantial proportion of the monomeric amines as coreactants in the interfacial reaction. In tests on 3.5% sodium chloride at 800 psi and 25 C, salt rejections of 99.5% at fluxes of 8 to 9 gfd were characteristic. A three-zone barrier layer was produced, consisting of a heat-crosslinked polyamine gel (as in NS-100), a polyamide layer incorporating both the polymeric... 

The initial studies by Cadotte on interfacially formed composite polyamide membranes indicated that monomeric amines behaved poorly in this membrane fabrication approach. This is illustrated in the data listed in Table 5.2, taken from the first public report on the NS-100 membrane.22 Only the polymeric amine polyethylenimine showed development of high rejection membranes at that time. For several years, it was thought that polymeric amine was required to achieve formation of a film that would span the pores in the surface of the microporous polysulfone sheet and resist blowout under pressure However, in 1976, Cadotte and coworkers reported that a monomeric amiri piperazine, could be interfacially reacted with isophthaloyl chloride to give a polyamide barrier layer with salt rejections of 90 to 98% in simulated seawater tests at 1,500 psi.4s This improved membrane formation was achieved through optimization of the interfacial reaction conditions (reactant concentrations, acid acceptors, surfactants). Improved technique after several years of experience in interfacial membrane formation was probably also a factor. 

In the patent by Kurihara, Uemura and Okada,38 combinations of a polymeric amine with a monomeric amine were used to produce composite polyamide membranes having high salt rejections. The membranes were described as having a bilayer polyamide barrier film a surface polyamide zone rich in monomeric amine, and a subsurface polyamide zone incorporating both monomeric and polymeric amine. This patent disclosure demonstrated an understanding of the mechanism of interfacial polyamide barrier layer formation. 

One of the more interesting fronts of development includes the search for improved membrane materials. While no new polymeric RO membranes have been introduced commercially over the last 20 to 30 years, there have been developments in performance (see Figure 1.5). These improvements in performance were achieved via modifications to the membrane itself (surface modifications made possible due to more advanced membrane characterization techniques) and closer tolerances in the interfacial polymerization reaction to make the membrane, and enhancements of the module design. Membranes with these improvements are commercially available today. While work is continuing with modifications to the current thin-film composite polyamide membranes, researchers are looking toward additional materials that might be suitable for use as RO membranes. 

Nanofiltration membranes usually have good rejections of organic compounds having molecular weights above 200—500 (114,115). NF provides the possibility of selective separation of certain organics from concentrated monovalent salt solutions such as NaCl. The most important nanofiltration membranes are composite membranes made by interfacial polymerization. Polyamides made from piperazine and aromatic acyl chlorides are examples of widely used nanofiltration membrane. Nanofiltration has been used in several commercial applications, among which are demineralization, oiganic removal, heavy-metal removal, and color removal (116). 

In composite RO membranes, the selective top layer and the porous support layer are usually made of different polymeric materials. The selective top layer is formed on the porous support in a second step, typically by an interfacial polymerization reaction. For example, a commercially available thin film composite RO membrane is made by coating a porous polysulfone support with a polyamide thin film formed by the interfacial reaction of m-phenylenediamine and 1,3,5-benzenetricarbonyl trichloride. Details regarding membrane structures can be found elsewhere in the... 

The functional thin coating in composite membranes for water desalination is often a polyamide formed in-situ on the porous substrate by interfacial polymerization. Interfacial polymerization of polyamides is a polymerization technique that was pioneered by Du Pont (20). 

Tbe next mejor comasereial success was the family of composite membranes. They feature a very thin RG membrane on a suitable substrate, usually a UF membrane. Most of the RO composite membranes are polyamide cnatings in which the separating layer is produced by interfacial polymerization of a diamine and a multibasic acid chloride. The most snecessfol recent memhrane is based on an inteifacial polymer of 1,3-diamino benzene and 1,3,5-benzene tricarboxylic acid chloride coated on a polysulfone membrane substrate. 

Polyamides clearly dominate the field of thin-film composites by interfacial polymerization. The composition and morphology of the membranes depend on different parameters, including the concentration of the reactants, their partition coefficients and reactivities, the kinetics and diffusion rates of the reactants, the presence of by-products, competitive side-reactions, cross-linking reactions and postreaction treatment... 

Poly(tetrafluoroethylene)/polyamide thin-film composite membranes via interfacial polymerization for pervaporation dehydration on an isopropanol aqueous solution. Journal of Membrane Science 315 106-115. 

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. 

Kwak and Ihm [7] used AFM and solid state NMR spectroscopy to characterize structure-property-performance correlations in high-flux RO membranes. The membranes were thin film composites, whose thin active layers were based on aromatic polyamide formed by the interfacial polymerization of MPD and trimesoyl chloride (TMC). These membranes, each coded as SH-I, SH-II, and SH-III, were provided by Saechan (Yongin-city, Korea). The variations among these commercial membranes are difficult to know. Most likely, they vary by the amount of catalyst or surfactant added to the aqueous MPD solution. Table 8.2 shows water flux, salt rejection, and the roughness parameter of those membranes, together with the data for another membrane, MPD/TMC, which was prepared at the laboratory of Kwak and Ihm [7]. 

Acid activated composite membranes were experimentally prepared in the same way than the experimental PAO polyamide/polysulfone composite membrane. Different concentrations of di-(2-ethyl hexyl)dithiophosphoric acid (DTPA) were added to interfacial-polymerization monomer solutions. Molecular structure of this organic acid is shown in Scheme 4. This activating agent is expected to be the carrier for heavy metallic ions, such as thallium, cadmium, zinc or uranium, between the media at both membrane sides [8-9, 63-65]. In this chapter, two activated membranes are studied DT50 and DT200 fabricated from 50 and 200 mM acid solutions, respectively. 

Chu et al. [70] demonstrated a simple and effective route for the hydrophilic surface modification of ceramic-supported PES membranes by synthesizing a polyfvinyl alcohol) (PVA)/polyamide (PA) composite thin sinface layer with an interfacial polymerization method (IP) method. The reaction of the interfadal polymerization is schematically shown in Figure 2.7. A prepared tubular ceramic-supported PES membrane (both ends sealed) was immersed in a terephthaloyl chloride solution in benzene and... 

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. 

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