Pore-filled composite membranes

An interesting pore-filled composite membrane, made by photograft copolymerization onto a solvent-stable PAN UF membrane, has been established [47]. High flux and selectivity for PV separation of organic-organic mixtures were achieved by a very thin selective barrier and prevention of swelling of the selective polymer in the pores of the barrier. 

For pore-filling composite membranes (see Table 1), in single solute experiments, enantioselectivity, i.e., an enhanced flux for the template, was observed for the MIP membrane while the blank membrane showed the same flux for D and L enantiomers. However, no selectivity could be observed in experiments with mixtures, i.e., under real enantiomer separation conditions [100]. Presumably, this could be caused by nonselective transport pathways due to uneven/incomplete pore filling, a hypothesis also in agreement with the exceptionally high fluxes. 

In this process, the first network generally determines the morphology of the final material. In addition, the composition range is limited by the maximum swelling capacity of the first network by the second network of precursors. It is important to note that the pore-filling electrolyte membranes can be considered as IPNs depending on whether or not the polymer substrate is crosslinked. Indeed, the pores of a porous polymer substrate are filled with polyelectrolyte precursors and the linear or network polyelectrolyte is synthesized within the porous substrate [69]. Ihis is shown in Figure 15.4. 

An excellent example of such a composite membrane type is represented by the work recently published by Tanaka et al. (2008). The starting material used for preparing the novel pore-filled palladium membrane was the YSZ supported on macroporous a-alumina. At this step, palladium particles could be successively deposited as a top layer of the YSZ-7-AI2O3 membrane (Fig. 3.10) or as nano-size pores of ceramic y-Al203 (Fig. 3.lid). 

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. 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

FIGURE 16.8 Stmcture of a pore-filled membrane. A responsive hydrogel is incorporated into the pores of the substrate membrane to form a stable composite membrane. 

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. 

One of the first zeolite based membranes were composite membranes, obtained by dispersion of zeolite crystals in dense polymeric films in order to make zeolite filled polymeric membranes [59,60,61], These membranes have been developed at the end of the 80 s for both gas separation and pervaporation. The clogging of zeolite pores by the matrix and the quality of the interface between the zeolite crystals and the polymer matrix (non-selective diffusion pathways) were key points. 

An ultrathin-film composite membrane selective for theophylline has been reported [48]. The theophylline-imprinted polymer was prepared inside pores of a microporous alumina support membrane with a thickness of 500 nm and a pore size of 20 nm, in which pores of the membrane were filled by the polymerization solution containing the template theophylline, methacrylic acid, and ethylene glycol dimethacrylate, and the membrane was illuminated with UV light for 1 h, followed by immersion in methanol containing 10 %(v/v) acetic acid to remove the template and any excess monomer. Because the membrane is extremely thin, the flux rate is high, being at least two orders of mag-... 

K. Kuraoka, H. Zhao, T. Yazawa, Pore-filled palladium-glass composite membranes for hydrogen separation by novel electroless plating technique, J. Mater. Sci. 2004, 39, 1879-1881. 

When used in different kinds of electrochemical equipment the membranes are in contact with aqueous solutions of the low molecular weight electrolytes in which they swell. Moreover, a certain amount of the ambient solution penetrates the voids or pores in the membrane. So the swollen membrane is a multiphase system composed of an ion containing component appearing in a gel state, an inert partly crystalline polymer, and the electrolyte filling any voids or nonselec-tive domains, all of them in varying amounts. For such a system it is possible to calculate the approximate phase composition based on the conductivity and the multilayer electrochemical model. We presented such a model at the First Italian-Polish Seminar on Multi-component Polymeric Systems in 1979. 

Pore-filling MIP composite membranes had been first prepared by Dzgoev and Haupt [100]. They casted the reaction mixture into the pores of a symmetric microfiltration membrane from polypropylene (cutoff pore size 0.2 pm) and performed a cross -linking copolymerization of a functional polyacrylate for imprinting protected tyrosine. Hattori et al. [101] had used a commercial cellulosic dialysis membrane (Cuprophan) as matrix and applied a two-step grafting procedure by, (i) activation of the cellulose by reaction with 3-methacryloxypropyl trimethoxysilane from toluene in order to introduce polymerizable groups into the outer surface layer, (ii) UV-initiation of an in situ copolymerization of a typical reaction mixture (MAA/EDMA, AIBN) for imprinting theophylline. 

However, studies with self-supported and much thinner membranes by Sergeyeva et al. [90,91] (cf. Section III.D.l) provided very clear evidence for the gate effect a most remarkable template specificity of conductivity response could be observed (seeTable 1). Similar, supporting results hadbeen obtainedby another group [104]. One more convincing proof for the gate effect was results of Hattori et al. [101] with pore-filled MIP composite membranes (cf. Section III.E) The transport rate of another solute (creatinine) increased 1.23-fold in the presence of the template (Tho) while without any additive and with Caf the flux was the same (seeTable 1). 

Filling the pores of a track-etched membrane with a polyelectrolyte results in a new class of polymer composite membrane, where the filler can enhance or regulate transport in the desired direetion and the traek-etched membrane can provide meehanieal stability and durabihty. Compared to dense membranes, these eomposite membranes can be tailored by the type of filler, pore size and porosity of the membrane to provide tunable transport properties. 

The effect of polyelectrolyte domain size and voliune fraction on vapor transport properties is more clearly shown in Table 2. Due to a porous stracture, PETE membranes with selected pore sizes and porosities exhibit high permeability to both water and DMMP vapor. No obvious effect of the pore stractures was observed on transport properties. Filling the pores with linear PAMPS did not change the transport properties of PETE membrane with 500 nm pore sizes. A distinct reduction on DMMP vapor permeability was observed when the domain size of the linear PAMPS decreased to 50 nm, which increased the selectivity by nearly 5 times compared to the original membrane. Table 2 also shows that by increasing the volume fraction of linear PAMPS (in 100 nm pores), the composite was less permeable to DMMP vapor with a higher selectivity. 

Composite pore-filled membranes, which have a porous polymer structure filled with a polyelectrolyte as schematized in Figure 12.6, are receiving growing attention in the field of separation and purification and recently in fuel cell application (Yamaguchi et al. 2003). In these membranes, the porous polymer substrate acts as an inert host that constrains the swelling of an anchored polyelectrolyte and provides high mechanical strength for the obtained membranes. 

FIGURE 12.8 SEM images of cross sections of (a) pristine PVDF film, (b) grafted polystyrene pore-filled PVDF film (G = 30%), and (c) its corresponding sulfonated composite membrane. (From Nasef, M.M., Zubir, N.A., Ismail, A.F., Dahlan, K.Z.M., Saidi, H., and Khayet, M., J. Power Sour., 156, 200-210, 2006. With permission.)... 

Gas-diffusion membranes Hydrophobic porous polymer membranes with air filling the membrane pores have been used successfully in the online separation of volatile and semivolatile analytes between two miscible liquid streams in flow injection analysis (FIA) systems. The corresponding technique is frequently referred to as gas-diffusion EIA. The mass transfer of an analyte across a gas-diffusion membrane is controlled by the membrane pore size and the solubility of the analyte in the feed and receiver solutions. The latter can be manipulated by appropriately modifying the chemical composition of the two solutions. In this way it is possible to enhance both the evaporation of the analyte from the feed solution into the membrane pores and its subsequent absorption into the receiver solution. 

The work of Henis and Tripodi [29] made industrial gas separation economically feasible. They placed a very thin homogeneous layer of a polymer with high gas permeability on top of an asymmetric membrane, ensuring that the pores in the toplayer were filled and that a leak-free composite membrane suitable for gas separation was obtained. 

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