Membrane Modification Methods

The development of high-performance polymeric membranes involves the selection of a suitable membrane material and the formation of this material into a desired membrane structure. However, it is often necessary to modify the membrane material or the structure to enhance the overall performance of the membrane. Generally, the objectives for modification of pre-formed membranes are: (i) increasing flux and/or selectivity and (ii) increasing chemical resistance (solvent resistance, swelling, or fouling resistance).

The first reported membrane modification method involved annealing of porous membranes by heat-treatment. Zsigmondy and Bachmann demonstrated in the early 1920s that the pore size of pre-formed nitrocellulose membranes could be decreased with a hot water or steam treatment. Loeb and Sourirajan used the same method to improve the salt rejection of integrally-skinned asymmetric cellulose acetate reverse osmosis membranes.

During the development of integrally-skinned asymmetric cellulose acetate gas separation membranes it was found that water-wet membranes collapse and form an essentially dense film upon drying. This collapse occurs because of the strong capillary forces within the finely porous structure during the drying process. This phenomenon can be described by the well-known Young-Laplace relationship ((hp = 2y/r) in the case of perfect wetting of the liquid in the pores). Hence, the capillary pressure is directly proportional to the surface tension of a liquid, but inversely proportional to the pore radius. If the modulus of the membrane material (in the swollen state) is lower than the capillary force of the liquid in the pore space, the pores will collapse and form a dense polymer film. Because water has a very high surface tension, it is often difficult to dry water-wet membranes without collapsing the membrane structure. An exchange of water with liquids having lower surface tension, such as alcohols or aliphatic hydrocarbons, results in maintaining the original membrane structure upon drying. Typical solventexchange methods involve replacing water first with iso-propanol and then with w-hexane. Other methods of eliminating the collapse of finely porous membrane structures include freeze-drying and the addition of surfactants to the water prior to drying of the wet membranes.

In the 1970s, commercialization of gas separation membranes was severely limited by the very poor reproducibility of making ultrathin, defect-free membranes on a large scale. Methods for production of thin-film composite membranes as well as integrally-skinned asymmetric membranes with separating layer thicknesses of less than 0.2 |im were known. However, production of these membranes without defects was not possible. Defects as small as 20 A over an area fraction of less than 10~4% can severely reduce the selectivity of gas separation membranes. However, a thin coating of a highly permeable polymer, such as polydimethylsiloxane, can render defective membranes suitable for gas separations. Modification methods developed by Browall for thin-film composite membranes and, in particular, Henis and Tri-podi for integrally-skinned asymmetric membranes resulted in rapid commercialization of gas separation membranes. Surface coatings are also applicable in improving the fouling resistance of membranes for ultrafiltration or nanofiltration applications. Chemical surface modification methods of gas separation membranes include treatment with fluorine, chlorine, bromine, or ozone. Typically, these treatments result in an increase in membrane selectivity coupled with a decrease in flux. Cross-linking of polymers is often applied to improve the chemical stability (swelling resistance) and selectivity of membranes for elec-trodialysis, reverse osmosis, pervaporation, and gas separation applications.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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