Permselective conducting polymer

Kowalski, D Ueda, M and Ohtsuka, T. (2010) Self-healing ion-permselective conducting polymer coating. J. Mater. Chem., 20, 7630-7633. 

Similar to metal electrode materials, conducting polymers stimulation requires reversible reactions driven under stimulation currents to ensure low voltage excursion and reduced production of by-products. Reports suggest that conducting polymers, especially PPy, are susceptible to irreversible oxidation [120]. Overoxidation of conducting polymers was reported to cause permselectivity changes, loss of conductivity, and de-doping [58,121-122]. 

This material was first synthesized in the middle 1960s by E.I. Du Pont de Nemours and Co., and was soon recognized as an outstanding ion conductor for laboratory as well as for industrial electrochemistry. The perfluorinated polymeric backbone is responsible for the good chemical and thermal stability of the polymer. Nation membrane swollen with an electrolyte solution shows high cation conductivity, whereas the transport of anions is almost entirely suppressed. This so-called permselectivity (cf. Section 6.2.1) is a characteristic advantage of Nation in comparison with classical ion-exchange polymers, in which the selective ion transport is usually not so pronounced. 

In conclusion, it can be claimed that a combination of kinetic and equilibrium conductance and membrane potential measurements provides a powerful method for investigating the permselective properties of membranes of low fixed charge density. Such methods should be applicable also to other polymers useful in hyperfiltration if they can be prepared in the form of homogeneous membranes. 

There are essentially four different types of membranes, or semipermeable barriers, which have either been commercialized for hydrogen separations or are being proposed for development and commercialization. They are polymeric membranes, porous (ceramic, carbon, metal) membranes, dense metal membranes, and ion-conductive membranes (see Table 8.1). Of these, only the polymeric membranes have seen significant commercialization, although dense metal membranes have been used for commercial applications in selected niche markets. Commercial polymeric membranes may be further classified as either asymmetric (a single polymer composition in which the thin, dense permselective layer covers a porous, but thick, layer) or composite (a thick, porous layer covered by a thin, dense permselective layer composed of a different polymer composition).2... 

Electroosmotic effects also influence current efficiency, not only in terms of coupling effects on the fluxes of various species but also in terms of their impact on steady-state membrane water levels and polymer structure. The effects of electroosmosis on membrane permselectivity have recently been treated through the classical Nernst-Planck flux equations, and water transport numbers in chlor-alkali cell environments have been reported by several workers.Even with classical approaches, the relationship between electroosmosis and permselectivity is seen to be quite complicated. Treatments which include molecular transport of water can also affect membrane permselectivity, as seen in Fig. 17. The different results for the two types of experiments here can be attributed largely to the effects of osmosis. A slight improvement in current efficiency results when osmosis occurs from anolyte to catholyte. Another frequently observed consequence of water transport is higher membrane conductance, " " which is an important factor in the overall energy efficiency of an operating cell. 

Polymeric ion exchange membranes are a particular class of these ion-containing polymers. We have been interested in understanding their microstructure in order to explain membrane characteristics such as permselectivity, ionic conductivity, and water diffusion. 

Flemion is quite different from prior membranes in that it is based on specific perfluorinated copolymers with pendant carboxylic acid as a functional group. The introduction of carboxylic functions in the polymer has realized high permselectivity in cation transport with high conductivity, which is indispensable to electrochemical application of ion exchange membranes. 

Ion-exchange membranes (lEM s) are polymeric materials that contain ionic functionalities chemically bound to the polymer backbone. The bound ionic functionalities can be anionic or cationic. In order to maintain electroneutrality within the membrane, each bound ionic site must be paired with an ion of the opposite charge. When the membrane Is swollen with an appropriate solvent, these ions become mobile and can be exchanged with other ions of like chau ge. The Internal structure of certain lEM s such as Naflon (23) results in a property known as permselectivity. Permselective lEM s reject Ions of the same charge as the bound ionic sites, l.e. in Nafion vltually all the ionic conductivity through the membrane is due to the mobile cations. 

Nonconducting pol5nners are not so frequently used in DNA biosensors as conducting ones. They have high resistivity, but their permselectivity is very useful in preventing interferences in electrochemical biosensors [114]. In this group of polymers, polyethyleneimine (PEI) and chitosan (CHIT) are very often used for the preparation of DNA biosensors. 

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