Asymmetric Polymeric Membranes

The more important factors from an industrial point of view are a high flux or productivity and a high selectivity or separation eflectiveness. It is here that asymmetric membranes find more application, due to their high flux. When the same material forms two layers differing in their structure, with a thin active dense skin layer associated with another layer that acts as a mechanical support and has no significant resistance to mass flux, the resulting membranes are called integral. 

In our case, these membranes were made of the same material and prepared by phase inversion process [72, 73]. The dense selective skin layer was possible because of the evaporation of solvent during the initial period and the macroporous layer sticking to the skin layer was formed due to the exchange between the solvent and nonsolvent systems inside the precipitation bath [74]. 

In order to avoid or to minimize the influence of micropores appearing on the surface of the dense layer, the membranes were recovered by a thin layer of 

The structural characterization of these materials is difficult. The main factor here should be the thickness of the several dense or porous layers. In effect, the width of the dense skin layer determines the flux. Because of this, it is interesting to obtain thin membranes [75-78], with good mechanical, thermal and chemical resistances [79-81]. Actually, the borders within the several layers of the membrane are not easy to detect as far as they correspond to the same material. 

14/10 iTieiTibrane evaporated during 40 seconds Cross sections 

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Hydraulic permeability, A 70 to 10,000 g/s-m. MPa. Pressure 0.3 to 0.5 MPa for ceramic. Capacity/unit 0.001 to 1 L/s. Liquid permeate flux 0.001 to 0.2 L/s-m with the permeate flux through ceramic membranes 2 to 3 times higher than through symmetric polymeric or sintered metal membranes and 5 to 10 times higher than through asymmetric polymeric membranes because ceramic operates at higher pressure. 

The evolution and expansion of the nse of ultrafiltration (UF) on an industrial scale became possible after the development of asymmetric polymeric membranes, especially of cellulose acetate and aromatic polysulfones (PSs). These membranes were initially developed for desalination of seawater by reverse osmosis and then were used in various applications from other polymeric materials. Until the development of asymmetric membranes, considered as second generation. 

A. Bottino, G. CapanneUi, and S. Munari. (1986). Factors affecting the stmctnre and properties of asymmetric polymeric membranes. In Membrane and Membrane Processes, E. DrioU and M. Nakagaki (Eds.), Plenum Press, New York. 

To prepare an asymmetric membrane, either the phase-inversion process (skin and support made of the same material) or a two-step process (barrier layer deposited on a porous substructure) is used. In the latter case, the barrier and support structures are usually made from different materials. Symmetric and asymmetric polymeric membranes can be prepared using the phase separation process. ° A precipitation/solidification process is used to transform a polymer solution into two phases (a polymer-rich solid and a polymer-lean liquid phase). The following techniques can be used to solidify the polymer ... 

Most ultrafiltration membranes are porous, asymmetric, polymeric stmctures produced by phase inversion, ie, the gelation or precipitation of a species from a soluble phase (see Membrane technology). 

In gas separation applications, polymeric hollow fibers (diameter X 100 fim) are used (e.g. PAN) with a dense skin. In the skin the micropores develop during pyrolyzation. This is also the case in the macroporous material but is not of great importance from gas permeability considerations. Depending on the pyrolysis temperature, the carbon membrane top layer (skin) may or may not be permeable for small molecules. Such a membrane system is activated by oxidation at temperatures of 400-450 C. The process parameters in this step determine the suitability of the asymmetric carbon membrane in a given application (Table 2.8). 

Most ultrafiltration membranes are porous, asymmetric, polymeric structures produced by phase inversion, i.e., the gelation or precipitation of a species from a soluble phase. See also Membrane Separations Technology. Membrane structure is a function of the materials used (polymer composition, molecular weight distribution, solvent system, etc) and the mode of preparation (solution viscosity, evaporation time, humidity, etc.). Commonly used polymers include cellulose acetates, polyamides, polysulfoncs, dyncls (vinyl chlondc-acrylonitrile copolymers) and puly(vinylidene fluoride). 

It is typically on the order of several hundred nanometers. In practice the minimum thickness for polymeric membranes is 50gm or greater, which is far more than one would expect from (6.53). This is apparendy due to the fact that these membranes hydrate in the bulk, thus increasing the dielectric constant. They also form a hydrated layer at the solution/membrane interface (Li et al 1996) which affects their overall electrochemical properties and selectivities. Macroscopic ISEs use relatively thick membranes ( 500jU.m). In contrast, it is desirable to use thin membranes in the construction of asymmetric solid-state potentiometric ion sensors, in order to make their preparation compatible with the thin-layer preparation techniques. 

The types of polymeric membranes that have attracted much interest for analytical applications and are nowadays in common use are characterized as nonporous membranes such as low-density polyethylene (LDPE), dense PP and PDMS silicone rubbers, and asymmetric composite membranes... 

Polymeric materials are still the most widely used membranes for gas separation, and for specific apphcations the separation technology is well established (see Section 4.6). Producing the membranes either as composites with a selective skin layer on flat sheets or as asymmetric hollow fibers are well-known techniques. Figure 4.5 shows an SEM picture of a typical composite polymeric membrane with a selective, thin skin layer of poly(dimethyl)siloxane (PDMS) on a support structure of polypropylene (PP). The polymeric membrane development today is clearly into more carefully tailored membranes for specific... 

Eckner and Zottola (cited in Ref. [124]) reported that sterilizing microfilters, such as the 0.2 p,m membranes commonly used in the pharmaceutical industry, would foul too rapidly when operated in the conventional crossflow manner, resulting in low flux and permeates with an undesirable solids profile, making them of little practical value in the dairy industry. Guerra et al. [102] employed the combined benefits of reverse asymmetric 0.87 p.m pore size polymeric membranes and backshock technique to control the adverse effects of concentration polarization and fouling in the removal of bacterial spores in skim milk... 

The other chapters then lead from the simple to the more complex molecular assemblies. Syntheses of simple synkinons are described at first. Micelles made of 10-100 molecules follow in chapter three. It is attempted to show how structurally ill-defined assemblies can be most useful to isolate single and pairs of molecules and that micelles may produce very dynamic reaction systems. A short introduction to covalent micelles, which actually are out of the scope of this book, as well as the discussion of rigid amphiphiles indicate where molecular assembly chemistry should aim at, namely the synkinesis of solid spherical assemblies. Chapter four dealing with vesicles concentrates on asymmetric monolayer membranes and the perforation of membranes with pores and transport systems. The regioselective dissolution of porphyrins and steroids, and some polymerization and photo reactions within vesicle membranes are also described in order to characterize dynamic assemblies. 

There are essentially four different types of membranes, or semipermeable barriers, which have either been commercialized for hydrogen separations or are being proposed for development and commercialization. They are polymeric membranes, porous (ceramic, carbon, metal) membranes, dense metal membranes, and ion-conductive membranes (see Table 8.1). Of these, only the polymeric membranes have seen significant commercialization, although dense metal membranes have been used for commercial applications in selected niche markets. Commercial polymeric membranes may be further classified as either asymmetric (a single polymer composition in which the thin, dense permselective layer covers a porous, but thick, layer) or composite (a thick, porous layer covered by a thin, dense permselective layer composed of a different polymer composition).2... 

PREPARATION OF POLYMERIC MEMBRANES Membranes Without Asymmetric Structures... 

Three different techniques are used for the preparation of state of the art synthetic polymeric membranes by phase inversion 1. thermogelation of, a two or more component mixture, 2. evaporation of a volatile solvent from a two or more component mixture and 3. addition of a nonsolvent to a homogeneous polymer solution. All three procedures may result in symmetric microporous structures or in asymmetric structures with a more or less dense skin at one or both surfaces suitable for reverse osmosis, ultrafiltration or microfiltration. The only thermodynamic presumption for all three preparation procedures is that the free energy of mixing of the polymer system under certain conditions of temperature and composition is negative that is, the system must have a miscibility gap over a defined concentration and temperature range (4). 

Interesting progress has been made recently in chemicaUy modifying the barrier-layer surface of asymmetric polymeric gas permeation membranes by reactive gaseous or liquid treatment (e.g., fluorination) to improve membrane permselectivity or stability [42]. Such surface treatments modify the ultrathin barrier layer almost exclusively and aUow conversion of that layer into a compositionaUy difierent structure. The result may be a more permselective membrane without significant permeabUity loss, a more fouling resistant membrane. 

Symmetric polymeric membranes possess a uniform pore structure over the entire thickness. These membranes can be porous or dense with a constant permeability from one surface to the other. Asymmetric (also sometimes referred to as anisotropic) membranes, on the other hand, typically show a dense (nonporous) structure with a thin (0.1-0.5 pm) surface layer supported on a porous substrate. The thin surface layer maximizes the flux and performs the separation. The microporous support structure provides the mechanical strengdi. 

Ultraflltraiion membranes are commonly asymmetric (skinned) polymeric membranes prepared by the phase inversion process. Materials commercially made into membranes include cellulose nitrate, cellulose acetate, polysulfone. aramids, polyvinylidene fluoride, and nctylonitrile polymers and copolymers. Inorganic meni-braues of hydrous zirconium oxide deposited on a tubular carbon backing are also commercially available. 

K. K. Sirkar, Separation of Gaseous Mixtures with Asymmetric Deose Polymeric Membranes,... 

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