Polymeric membranes asymmetric structure

Figure 4.3 shows a cross section of a CA membrane. The structure is asymmetric or "anisotropic," with a solute-rejecting layer on top of a microporous support, all made of the one polymeric material. 

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... 

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). 

For the investigations reported here polyvinyl alcohol (PVA) and its derivatives such as polyvinyl acetate, polyvinyl ether etc. were used as the basic polymeric materials. These compounds can easily be converted into polymeric analogues [1]. It was shown in an earlier work [2] that PVA-membranes with an asymmetrical structure can be obtained by phase-inverted precipitation similar to the method of Loeb and Sourirajan [3]. These membranes can also be rendered inso].uble in water by... 

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. 

Thus, nodular structures are always found at the surface of polymeric membranes. Based on the size of the structural units, Resting suggested the following four superimposed tiers of structure in asymmetric membranes prepared by the phase inversion technique [5] ... 

As discussed, the cross-sectional view of the membranes observed by AFM has similar characteristics to those observed by high-resolution FE-SEM, confirming that AFM can be used to study the cross-sectional structure of polymeric membranes, particularly in terms of their nodular structures. The void spaces between the nodules may form water channels in reverse osmosis and ultraflltration. They may also become defects when they appear at the densely packed monolayer of nodules or nodular aggregates. Information on the nodular structure will therefore help to eliminate the unwanted defects in the skin layer of the asymmetric membranes. 

Impedance plots (Nyquist and Bode plots) for the PS/PA-PEG5 and PS/PA-PEG25 membranes are shown in Figure 2.5b and 2.5c, where the effect of the asymmetric structure on the impedance (Nyquist) plot is indicated, but the differences depending on the PEG concentration are also evident. The equivalent circuit for the total membrane system, (R C ) - (RmQmX is also indicated in Figure 2.5b the depressed semicircle, attributed to the nonconstant phase circuit element (Q, is due to the porous structure of these membranes and the mixture of the relaxation times associated with their electrical response (polymeric matrix and solution). 

Porous membranes are used in microfiluation and ulu-afiltration processes. These membranes consist of a polymeric matrix in which pores within the range of 2 nm to 10 jLm are present. A large variety of pore geomeoies is possible and figure V 5 gives a schematic representation of some of the characteristic structures found. Such structures exist over the whole membraite thickness in microfUtration membranes and here the resistance is determined by the total membrane thickness. On the other band, ultiafiltration membranes generally have an asymmetric structure, where the porous top-layer mainly determines the resistance to transport Here, the transpon length is only of the order of 1 fim or less. 

Membranes can be fabricated to possess different morphologies. However, most membranes that have found practical use are mainly of asymmetric structure. Separation in membrane proeesses takes place as a result of differences in the transport rates of different species through the membrane structure, which is usually polymeric or ceramic [32]. 

Most polymeric OSN membranes have an asymmetric structure and are porous with a dense top layer. This asymmetry can be divided into two major types the integral type, where the whole membrane is composed of the same material, and the thin-film composite (TFC), where the membrane separating layer is made of a different material. 

As pointed out by Nunes and Peinemann [108], inorganic membranes are usually preferred because many processes at the industrial level are carried out at high temperature. However, polymeric membranes can be used for H2/hydrocarbon separation in the platformer off gases from refineries and for CO2 separation in coal plants. Polymeric manbranes for GS can be symmetric or asymmetric, but should have a dense selective layer. Three types of membrane structures can be employed (1) homogeneous dense manbranes (symmetric) (2) integrally skinned asymmetric membranes and (3) composite membranes. 

Synthetic membranes may be synunetric (with an identical structure over the cross-section of the membrane) or asymmetric, and may be flat, tubular or hollow fibre (see Fig. 1.1 for the main types of polymeric membrane structures). Table 1.1 shows the molecular structures of conunon polymers used in membrane preparation. 

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 ... 

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