Synthetic Membranes—Preparation, Structure, and Application

Abstract

tions n (negative). We prefer the abbreviations I and u, because P and p are reserved for Specification of axial, planar, and helical elements of chirality [28].[3 I] The descriptors re and si introduced by K. R. Hanson [J. Am. Chem. SOC. 88 (1966) 27311 for the specification of the faces of planar trigonal centers of chirality are differentiated by using Re and Si (beginning with capital letters) for specification of reflection-variant 2-dimensionally stereogenic centers and re and si for reflection-invariant stereogenic centers or heterotopic half-spaces, respectively; this is in analogy with R.S and r,s for three-dimensionally stereogenic centers [28, 291.[32] For molecules containing pairs of constitutionally identical elements of chirality, the terms mesoand (d.0or racemic form do not lead to ambiguity.After a long period of dormancy, membrane separation processes have begun to emerge as technically significant and commercially relevant unit operations. Prior to the mid-sixties, synthetic membranes were employed for those few specialized laboratory applications which could tolerate low permeability and poor selectivity or in electrochemical applications excluding, e. 9.. batteries, fuel cells, chloride-alkali electrolysis, where marginal chemical stability remained a severe limitation. Within the framework of a broad R & D program started in the US in the mid-fifties and devoted to the production of fresh water from brackish and seawater, developments of more suitable membranes arose out of the application of the principles of physical chemistry, modern polymer chemistry (especially surface or interfacial polymerization and polycondensation technology), and electron microscopy. In particular, it was learned that asymmetric membrane structures comprise a very thin consolidated barrier layer (5000 A or less for membranes with economically practical filtration rates) supported by an integral but less dense substrate which does not participate in the transport process. Later and after much effort, composite membranes were developed in which the salt-rejecting skin (still only 5000 A thick) was placed atop a supporting matrix formed from a more chemically and mechanically stable polymer.-The main desalination research effort led to several spin-off developments in related membrane fields, e. g. the successful preparation and commercialization of ultrafiltration technology in the automobile, food, and chemical industries. Also, ion-exchange membranes prepared from perfluorinated polymers offered the electrochemical industry much better chemical stability than the earlier phenolic-resin-based ion-exchange membranes. -Current efforts are aimed at the improved selectivity and stability required for very specific separation processes (e. g. separation of heavy metal salts from waste water or selective enrichment of gases). In the future, the mechanisms of biological processes will have to be exploited for successful development of synthetic membranes suitable for more sophisticated separations.

Síntese de polímeros condutores em matrizes sólidas hospedeiras

“…A seguir, o filme sofre um lixiviamento químico. O ataque ocorre preferencialmente nas regiões atravessadas pelas partículas e o diâmetro dos poros aumenta com o tempo de imersão no banho de lixiviamento 103,104 . As membranas de alumina porosa são preparadas eletroquimicamente a partir de alumínio metálico 105 .…”

Influence of coagulation temperature on pore size and properties of cellulose membranes prepared from NaOH–urea aqueous solution

“…Nowadays, various cellulose membrane materials such as regenerated cellulose (RC), cellulose acetate (CA), and cellulose acetate butyrate (CAB) have been developed for a series of industry applications for microfiltration, ultrafiltration, reverse osmosis, and gas separation. It is noted that each application demands specific requirements from the morphology and structure of the membranes (Pusch and Walch 1982;Mulder 1992;Nues and Peinemann 2001), and usually, they depend strongly on the coagulants and conditions during coagulation. In particular, the composition change that occurs in the course of coagulation, and the subsequent phase separation, affect morphology of the membranes and this, in turn, controls porosity, permeability, and selectivity (Laity et al 2001).…”

Carrier-Facilitated Coupled Transport Through Liquid Membranes

“…While many polymers have been examined for use as membrane materials, only a few are widely used. Detailed description of polymeric membranes can be found in reviews [100][101][102][103][104][105][106].…”