Interfacial polyamidation

Interfacial reactions have been employed using the reactions of dicar-boxylic acid chlorides with diamines to form polyamides, with hydrazine and hydrazides to form polyhydrazides disulphonyl halides have been used to prepare polysulphonamides diisocyanates and glycols have been used to prepare polyurethanes interfacially (Section 7) and diisocyanates and diamines have been reacted interfacially to form polyureas [6] (Section 8). Polyesters prepared by this technique were discussed in Section 5.7. 

In interfacial polyamidation the reaction takes place close to the interface between a solution or suspension (usually aqueous) of the diamine and a solution of the diacyl or disulphonyl chloride in an immiscible organic solvent. The aqueous layer often contains an acid acceptor such as triethylamine, pyridine, sodium hydroxide or sodium carbonate, to neutralize the hydrochloric acid produced in the reaction. Often an emulsifier is used to increase interfacial area. The reaction is believed to occur just within the organic solvent layer. The rate of reaction is so fast that the process becomes diffusion-controlled. 

The nature of the organic solvent used to dissolve the acid chloride can affect the course of the reaction. Factors that may be involved include the rate of the principal reaction, rates of side reactions, the rate of polymer precipitation, the permeability of the polymer film to monomers, etc. All these factors doubtless influence the molecular weight of the polymer produced. In one example [106], the viscosity of poly(p-phenylene terephthalamide) made interfacially varied with the organic solvent as follows 

The solvent power of the organic phase can be varied by including in it an organic liquid that is a solvent for the polyamide. Sokolov and Turetskii [107] showed that for mixtures of tricresol (polymer solvent) and dibutyl ether (non-solvent) there was an optimum ratio of the two solvents for maximum polymer viscosity and therefore for maximum molecular weight (Fig. 7). The content of tricresol in those experiments also influenced the yield of polymer [107] (Fig. 8). 

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Fig. 9. Schematic diagram of concentrations in an interfacial polyamidation. Amine groups (A) are in the water phase. Acid chloride (C) and amine groups at partition equilibrium (B) are in the organic phase. P and the dotted lines show the relation of the reactive groups when the first incremental layer of polymer forms (from ref. 6).

Table 5.4 Effect of Cation and Anion Valence on Rejection of Various Salts by a Piperazine Trimesamide Interfacial Polyamide Membrane...

In the book, Condensation Polymers By Interfacial and Solution Methods, by P.W. Morgan,67 interfacial polyamide formation is stated to occur in the organic phase, that is, on the organic solvent side of the interface. Several proofs are presented in support of this statement. For instance, monofunctional acyl halides added to the difunctional acyl halide in the organic phase always lowered polymer molecular weights. However, monofunctional amines added to the difunctional amines in the aqueous layer did not always show this effect. In this latter case, partition coefficients became a factor, particularly when relative reactivities of the amines were comparable. Mass transfer rate of diamine across the interface into the organic phase was noted to be the rate-controlling step at all concentrations of diamine. 

A visually graphic display of this mechanism is illustrated in Figure 5.15. An insoluble colored powder, deposited on the face of an interfacial polyamide film, becomes incorporated into the polyamide film if the powder is introduced on the organic phase side. However, if the powder is deposited on the aqueous phase side of the growing interfacial film, it remains loose and unattached. 

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. 

A patent has also appeared by Fukuchi and coworkers on the combination of polyethylenimine and several monomeric amines (including piperazine, 2,5-dimethylpiperazine, and 4-aminomethylpiperidine) in interfacial polyamide membranes. This patent also claimed very high salt rejections. 

Interfacial polymerization will tolerate the presence of impurities in the reactants that simply dilute the material and thereby produce nonequivalence of reactants. These diluents might be water or inert contaminants in the acid chloride. Reactive monofunctional species are harmful in either phase. To maximize molecular weight, it is essential to use high purity monomers. Molecular weight control can be achieved, if desired, with appropriate use of monofunctional reagents. Examples of impurities interfering with the interfacial polyamidation of MPDI are half hydrolyzed acid chloride, monoamide, partially oxidized amines, and reactive surfactants. 

Asm, L., et al. 2001. Sequential poly(ester amide)s based on glyeine, diols, and diearboxylic acids thermal polyesterifieation versus interfacial polyamidation. Characterization of polymers containing stiff units. Journal of Polymer Science Part A Polymer Chemistry 39 (24) 4283 293. 

Typical chemical systems are fast reactions between two difimctional monomers, AXA + BYB. The first monomer (diamine, bisphenolate) is dissolved in a water solution (in alkaline media in both cases), and the other monomer, with low water solubility (acid chloride, phosgene), is usually dissolved in an organic solvent. Either the neutral form of AXA is in an appreciable amount (in the case of amines), or a phase transfer catalyst is needed (as in polycarbonate synthesis), since ionized forms will not dissolve in the organic phase. A decrease in the pH is often used to quench interfacial polyamidation. 

Asin L, Armelin E, Montane J, Rodriguez-Galan A, Puiggali J (2001), Sequential poly(ester amide)s based on glycine, diols and dicarboxylic acids Polyesterification versus interfacial polyamidation , J. Pol. Sci. Part A Chemistry, 39 (24), 4283-4293. 

As with polyesters, the amidation reaction of acid chlorides may be carried out in solution because of the enhanced reactivity of acid chlorides compared with carboxylic acids. A technique known as interfacial polymerization has been employed for the formation of polyamides and other step-growth polymers, including polyesters, polyurethanes, and polycarbonates. In this method the polymerization is carried out at the interface between two immiscible solutions, one of which contains one of the dissolved reactants, while the second monomer is dissolved in the other. Figure 5.7 shows a polyamide film forming at the interface between an aqueous solution of a diamine layered on a solution of a diacid chloride in an organic solvent. In this form interfacial polymerization is part of the standard repertoire of chemical demonstrations. It is sometimes called the nylon rope trick because of the filament of nylon produced by withdrawing the collapsed film. 

Polypropylene block and graft copolymers are efficient blend compatibilizers. These materials allow the formation of alloys, for example, isotactic polypropylene with styrene-acrylonitrile polymer or polyamides, by enhancing the dispersion of incompatible polymers and improving their interfacial adhesion. Polyolefinic materials of such types afford property synergisms such as improved stiffness combined with greater toughness. 

Els and McGill [48] reported the action of maleic anhydride on polypropylene-polyisoprene blends. A graft copolymer was found in situ through the modifier, which later enhanced the overall performance of the blend. Scott and Macosko [49] studied the reactive and nonreactive compatibilization of nylon-ethylene-propylene rubber blends. The nonreactive polyamide-ethylene propylene blends showed poor interfacial adhesion between the phases. The reactive polyamide-ethylene propylene-maleic anhydride modified blends showed excellent adhesion and much smaller dispersed phase domain size. 

Acid chlorides are very reactive and have as a condensation product hydrochloric acid.4,7 9 This hydrochloric acid can form an amine salt with unreacted amine groups, which should be avoided. To prevent this happening, acid binders, which are more reactive than the amines, are added. Polyamidation can be earned out using a solution and with an interfacial method. With the interfacial method one has the choice between a stirred and an unstirred process. In an unstirred process, the polymerization is at the interface and a rope can be drawn from the interface,... 

Acid anhydride-diol reaction, 65 Acid anhydride-epoxy reaction, 85 Acid binders, 155, 157 Acid catalysis, of PET, 548-549 Acid-catalyzed hydrolysis of nylon-6, 567-568 of nylon-6,6, 568 Acid chloride, poly(p-benzamide) synthesis from, 188-189 Acid chloride-alcohol reaction, 75-77 Acid chloride-alkali metal diphenol salt interfacial reactions, 77 Acid chloride polymerization, of polyamides, 155-157 Acid chloride-terminated polyesters, reaction with hydroxy-terminated polyethers, 89 Acid-etch tests, 245 Acid number, 94 Acidolysis, 74 of nylon-6,6, 568... 

In certain cases the organic dibasic acid is not sufficiently reactive for the purpose of polymerisation, and so it is replaced either with its anhydride or its acid chloride. For example polyamides (nylons) are often prepared by reaction of the acid chloride with the appropriate diamine. In the spectacular laboratory prepatation of nylon 6,6 this is done by interfacial polymerisation. Hexamethylenediamine is dissolved in water and adipyl chloride in a chlorinated solvent such as tetrachloromethane. The two liquids are added to the same beaker where they form two essentially immiscible layers. At the interface, however, there is limited miscibility and nylon 6,6 of good molar mass forms. It can then be continuously removed by pulling out the interface. 

Jha A., Bhowmick A.K., Eujitsuka R., and Inoue T. Interfacial interaction and peel adhesion between polyamide and acrylate rubber in thermoplastic elastomeric blends, J. Adhes. Sci. Technol., 13(6), 649, 1999. 

Van Duin, M. and Borggreve, R., Blends of polyamides and maleic anhydride containing polymers Interfacial chemistry and properties, in Reactive Modifiers for Polymers, Al-Malaika, S. (Ed.), Blackie Academic Professional, London, 1997. 

The predominant RO membranes used in water applications include cellulose polymers, thin film oomposites (TFCs) consisting of aromatic polyamides, and crosslinked polyetherurea. Cellulosic membranes are formed by immersion casting of 30 to 40 percent polymer lacquers on a web immersed in water. These lacquers include cellulose acetate, triacetate, and acetate-butyrate. TFCs are formed by interfacial polymerization that involves coating a microporous membrane substrate with an aqueous prepolymer solution and immersing in a water-immiscible solvent containing a reactant [Petersen, J. Memhr. Sol., 83, 81 (1993)]. The Dow FilmTec FT-30 membrane developed by Cadotte uses 1-3 diaminobenzene prepolymer crosslinked with 1-3 and 1-4 benzenedicarboxylic acid chlorides. These membranes have NaCl retention and water permeability claims. 

We now report a convenient method for the interfacial polycondensation of 1,1 -bis(3-aminoethyl)ferrocene (1) with a variety of diacid chlorides and diisocyanates, leading to ferrocene-containing polyamides and polyureas. In some instances, we have been able to observe film formation at the interface. Moreover, the polymerization reactions can be conveniently conducted at ambient temperatures in contrast to earlier high-temperature organometallic condensation... 

Interfacial or solution polycondensation, with or without stirring, was the general procedure utilized for the preparation of the polyamides and polyureas.l a Details are given in Table I. An important point to be noted is that, in the unstirred interfacial condensation polymerization of 1 with sebacoyl chloride or tere-phthaloyl chloride in the organic phase and triethylamine as the proton acceptor, immediate film formation took place at the interface. The polyamide films were removed after 1 h, dried, and utilized for taking electron micrographs. 

One previous synthesis of ferrocene-containing condensation polymers via interfacial methods at room temperature has been reported by Knobloch and Rauscher, who formed low molecular weight polyamides and polyesters by reacting l,l -bis(chloro-formyl)ferrocene with various diamines and diols. Further, Carraher and co-workers have utilized interfacial techniques in the formation of other types of organometallic polymers. 

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. 

Relationship Between Nodular and Rejecting Layers. Nodular formation was conceived by Maler and Scheuerman (14) and was shown to exist in the skin structure of anisotropic cellulose acetate membranes by Schultz and Asunmaa ( ), who ion etched the skin to discover an assembly of close-packed, 188 A in diameter spheres. Resting (15) has identified this kind of micellar structure in dry cellulose ester reverse osmosis membranes, and Panar, et al. (16) has identified their existence in the polyamide derivatives. Our work has shown that nodules exist in most polymeric membranes cast into a nonsolvent bath, where gelation at the interface is caused by initial depletion of solvent, as shown in Case B, which follows restricted Inward contraction of the interfacial zone. This leads to a dispersed phase of micelles within a continuous phase (designated as "polymer-poor phase") composed of a mixture of solvents, coagulant, and a dissolved fraction of the polymer. The formation of such a skin is delineated in the scheme shown in Figure 11. 

The condensation polymerization process, employed recently by Skourlis et al. (1993) and Duvis et al. (1993), involves immersion of carbon fibers in a solution containing hexamethylenediamine and sodium carbonate. Dried carbon fibers are then immersed in a dipolychloride solution in carbon tetrachloride where the interfacial polycondensation reaction takes place. The result is that a thin layer of polyamide (nylon 6,6) coating is deposited on the continuous carbon fiber, whose thickness is controlled though by varying the diamine concentration. 

Fully aromatic polyamides are synthesized by interfacial polycondensation of diamines and dicarboxylic acid dichlorides or by solution condensation at low temperature. For the synthesis of poly(p-benzamide)s the low-temperature polycondensation of 4-aminobenzoyl chloride hydrochloride is applicable in a mixture of N-methylpyrrolidone and calcium chloride as solvent. The rate of the reaction and molecular weight are influenced by many factors, like the purity of monomers and solvents, the mode of monomer addition, temperature, stirring velocity, and chain terminators. Also, the type and amount of the neutralization agents which react with the hydrochloric acid from the condensation reaction, play an important role. Suitable are, e.g., calcium hydroxide or calcium oxide. 

In principle, the attainment of chemical equilibrium can be accelerated by catalysts however, in contrast to polyester formation, catalysts are not absolutely essential in the above-mentioned polycondensations. The first two types of reactions are generally carried out in the melt solution polycondensations at higher temperature, e.g., in xylenol or 4-fert-butylphenol are of significance only in a few cases on account of the poor solubility of polyamides. On the other hand, polycondensation of diamines with dicarboxylic acid chlorides can be carried out either in solution at low temperature or as interfacial condensation (see Sect. 4.1.2.3). 

As in the preparation of polyesters, also in the preparation of polyamides, the reaction temperature can be considerably reduced by using derivatives of dicarbo-xylic acids instead of the free acids. Especially advantageous in this connection are the dicarboxylic acid chlorides which react with diamines at room temperature by the Schotten-Baumann reaction this polycondensation can be carried out in solution as well as by a special procedure known as interfacial polycondensation (see Examples 4-11 and 4-12). 

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