The structure of a membrane is vital for the performance of the membrane. The membranes are either symmetrical, where the properties of the membrane do not change through the cross-section of the membrane or they are asymmetrical (also known as composite). In the latter case the membrane is composed of a thin selective layer and a strong support layer. With some techniques it is possible to create both membrane and support layers in one single preparation step.
Asymmetric membranes are advantageous here compared to symmetric membranes because the flux determining top layer is thin, while the sublayer is responsible for the mechanical strength. A schematic representation of symmetric and asymmetric membranes is given in Figure 9.
Asymmetric membranes offer great possibilities in optimizing the membrane separation properties by varying the preparation parameters of especially the thin top layer. Also the ongoing development of polymers throughout the decades, like polysulfone (1965), polyether-ether-ketone (1980), and polyetherimide (1982), offers product engineers more and more possibilities for new membrane types.
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