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SYNTHESIS AND PROCESSING: MORPHOLOGICALLY SPECIFIC METHODS. 54 MEMBRANES Broadly, membranes are structural materials involved with the process of permeation. The membrane may be used for separations, to conduct electrical current, to conduct protons, to prevent permeation (packaging), or to chemically modify the permeating species (e.g., ion exchange membranes) (Lloyd, 1984). Structurally, membranes can be classified as porous (containing void space) or nonporous. The tremendous diversity in type and use precludes a systematic approach, so that here we shall only be concerned with semipermeable membranes as illustrative of the opportunities in ultrafine-scale structural design. A semipermeable membrane is defined as a material structure through which one or more substances in a mixture may pass, but not all. An ideal membrane would selectively pass a specific substance or substances at a high rate and at the same time be impervious to one or all of the other substances in the mixture. The structure of such a membrane would be one of intricate uniformity, consisting of identical pores (or channels of a second phase if the membrane is nonporous) in a geometrical array. For example, the simplest type of sieve is a two- dimensional woven mesh. The pores consist of the regions between the spanning weaves, the selectivity of separation being controlled only by the pore size. A more complex membrane could consist of three-dimensional periodically interconnected pore space with specific chemical groups lining the pores, providing both size and compositional selectivity. Current technological goals are toward ever-finer pore spaces (channels) with ever-higher overall porosity (channel volume fraction) for high throughputs. Such advances must also provide controlled pore geometry and interconnectivity (topology) as well as uniformity. Interconnectivity is important because the clogging of one- dimensional channels results in the stoppage of flow, whereas, if the pores are interconnected with a high coordination number, there are many routes for flow, and such materials can sustain considerable contamination before needing replacement. Pore uniformity is critical for high selectivity. Such membranes can be used in reverse osmosis, microfiltration, and ultrafiltration. There is a vast number of growing applications of semipermeable membranes (e.g., the removal of pollutants from all types of industrial and agricultural waste). In many cases, the availability of an improved membrane material could significantly affect the viability of a given industrial process. Some examples of submicron-scale membrane materials are sintered-particle membranes, solution-cast polymeric filters, and molecular sieves (dehydrated crystalline zeolites). The sintered particle membranes usually have a quite nonuniform pore space with a highly polydisperse pore size distribution and contorted three- dimensionally interconnected channels. At present, their size selectivity extends down to only
SYNTHESIS AND PROCESSING: MORPHOLOGICALLY SPECIFIC METHODS. 55 slightly below 1 µm. Solvent-cast cellulose acetate membranes were a breakthrough in membrane technology in the early 1960s as desalination membranes (Kesting, 1985). Zeolites feature extremely small and uniform pores (0.3 to 1 nm) with several types of periodic three-dimensional interconnectivity and reasonable porosity (50 percent). Because of their polar nature, they selectively absorb polar molecules. Advances in ultrafine-scale semipermeable membranes can arise from progress in nanoscale materials science. Membrane microstructures need to be combined with membrane properties, such as flexibility and environmental tolerance. Either man-made assemblies or self-assembled composite materials, in which one phase either possesses selective high transport or for which methods are available to selectively remove one of the phases, can provide suitable membranes. For example, spinoidally-decomposed borosilicate glass after acid leaching yields a high-porosity hyperfiltration material. Well-controlled ceramic precursor materials with nearly monodisperse particle size (Bowen, 1986) could, under appropriate sintering conditions, yield a highly uniform, three-dimensional interconnected fine-scale membrane with the inherent advantages afforded by ceramics-- namely, thermal and chemical resistance and strength. The exploitation of polymer gels is also an avenue toward new membrane materials. In particular, gels formed from rod-like macromolecules show promise as interesting structural materials because of their mechanical anisotropy and low density. In addition, these materials can be coagulated from an oriented precursor state, thus providing an anisotropic porespace (Cohen and Thomas, 1985). Indeed, the general field of phase separation of materials to provide fine-scale composites is a fertile area, since the length scale of phase separation can be readily controlled by synthesis of well-defined starting materials and time and temperature processing history. Recent work has utilized the microphase separation of block copolymers to form regularly arrayed and uniform-sized channels. For example, 10-nm diameter one-dimensional cylindrical channels oriented normal to the membrane surface were formed by temperature and solvent gradient phase separation of a polystyrene-isoprene diblock copolymer. This membrane material is a competitor for current nuclear track filters that consist of parallel cylindrical pores formed by the chemical etching of high-energy radiation tracks. Porosity is, however, low because of the need to avoid overlap of tracks. Elegant work utilizing a four-component pentablock copolymer to form a charge mosaic membrane builds on the rational manipulation of controlled synthesis and phase separation (Fujimoto et al., 1984). These membranes contain an alternating lamellar structure with the normal to the lamellae parallel to the external film surface. One of the polymeric phases (vinylbenzyl dimethyl diamine) can be quarternized, which then provides ionically charged layers passing completely across the film thickness separated by 20-nm dielectric insulating layers. Such films have demonstrated both high transport and
