Poly electrolyte conductivities

While the condition of stoichiometric neutrality invariably must hold for a macroscopic system such as a space-network polyelectrolyte gel, its application to the poly electrolyte molecule in an infinitely dilute solution may justifiably be questioned. In a polyelectrolyte gel of macroscopic size the minute excess charge is considered to occur in the surface layer (the gel being conductive), which is consistent with the assumption that the potential changes abruptly at the surface. This change is never truly abrupt, for it must take place throughout a layer extending to a depth which is of the order of magnitude of the... 

Figure 4. Plot of poly-I conductivity as a function of potential. A series of potential step of 20mV were employed on a sandwich electrode. Each potential was held until Faradaic current ceased, where upon a DC conductivity measurement, AE = 60MV, was taken, before proceding with the next potential. The results are for 0.05 M II electrolyte in acetonitrile vs. Ag+/Ag.

From the experiments it is clear that poly electrolyte is adsorbed on the surface of the black lipid film. This applies both to the experiments with gelatin and bovine serum albumin, which gave no decrease of film resistance, and to the experiments with bovine erythrocyte ghost protein and polyphosphate. The adsorption of protein on the phospholipid-water interface may be controlled independently by investigating the electrophoretic behavior of emulsion droplets, stabilized by phospholipid, in a protein solution, as a function of pH. In this way Haydon (3) established protein adsorption on the phospholipid-water interface. If the high resistance (107 ohms per sq. cm.) of black lipid films is to be ascribed to the continuous layer of hydrocarbon chains in the interior of the film, as is generally done, an increase in film conductivity is not expected from adsorption without penetration. 

Further important detectors utilize electrochemical as well as conductivity and capacitance measurements. Electrochemical detectors are absolute while conductime-ters with extremely small measuring cells are very sensitive and rather specific detectors for gel chromatography of (poly)electrolytes. 

When an anionic polyelectrolyte is used as a supporting electrolyte, this poly electrolyte is incorporated as a dopant [6]. Such an anionic polyelectrolyte can also be incorporated by the chemical polymerization method [7]. Owing to the tight hybridization of the polyelectrolyte and the conducting polymer, the anionic polyelectrolyte dopant is not liberated even when the polymer is electrochemically reduced and a pseudo-cathodic doping takes place to conserve electroneutrality. Accordingly, a cationic functional molecule can be incorporated (Table 9.1 (2)) [6]. 

The Structural Analogue Poly electrolyte Multilayers and Their Conductivity... 

There are many subtleties inherent in treating solution conductivities involving polyelectrolytes, and Kreft and Reed [30] developed a simple model for interpreting the marked nonlinearities of a versus [VB]. Automatic Continuous Online Monitoring of Polymerization Reactions data can serve as grist for more refined and rigorous models. In Reference [30], t) is the fraction of noncondensed Na counterions, is the conductivity due to added salt in the reactor at the reaction temperature, which is constant during the reaction, is the specific conductivity of free VB (it includes both the conductivities of the anionic monomer and its free Na" counterions), is the specific conductivity of Na", and is the specific conductivity of VB in the poly electrolyte (i.e., the conductivity of a VB monomer incorporated into a copolymer chain). (0 was computed from o(0 by ... 

Electrochemistry provides methods for preparing conducting polymers in the convenient form of thin films on electrode (metal, possibly also carbon or semiconductor) surfaces. These preparative aspects are not discussed in this chapter. Instead, the resulting system consisting of the coated electrode immersed in an electrolyte, i.e, the arrangement metal (Me)/poly-mer(poly)/electrolyte (S), constitutes the basis for our discussions. Thus, the aim may be to oxidize or reduce the polymer film itself (which may be done in an inert electrolyte by electron exchange with the metal electrode), or the polymer film may act as a mediator for the oxidation or reduction of a depolarizer dissolved in the contacting electrolyte. The two approaches are illustrated in Fig. 20.1. 

Fang, J., Guo, X., Haroda, S., Watari, T., Tanaka, K., Kita, H., Okamoto, K., Novel sulfonated polyimides as poly electrolytes for fuel ceU apphcation. 1. Synthesis, proton conductivity, and water stability of polyimides from 4,4 -diaminodiphenyl ether-2,2 -disulfonic acid, Macromolecules, 2002, 35, 9022-9028. 

The ion conducting mechanism of polyelectrolyte was studied by Forsyth et al. (2003). The incomplete dissociation of poly electrolyte is considered to be the main obstacle to further enhancing the ion conductivity. However,... 

Yilmazturk, S., H. Deligoz, M. Yilmazoglu, H. Damyan, F. Oksuzomer, S. N. K09, A. Durmu , and M. A. Gurkaynak. 2009. A novel approach for highly proton conductive electrolyte membranes with improved methanol barrier properties Layer-by-layer assembly of salt containing poly electrolytes. J. Memb. Sci. 343 137-146. 

Poly(ethylene oxide) associates in solution with certain electrolytes (48—52). For example, high molecular weight species of poly(ethylene oxide) readily dissolve in methanol that contains 0.5 wt % KI, although the resin does not remain in methanol solution at room temperature. This salting-in effect has been attributed to ion binding, which prevents coagulation in the nonsolvent. Complexes with electrolytes, in particular lithium salts, have received widespread attention on account of the potential for using these materials in a polymeric battery. The performance of soHd electrolytes based on poly(ethylene oxide) in terms of ion transport and conductivity has been discussed (53—58). The use of complexes of poly(ethylene oxide) in analytical chemistry has also been reviewed (59). 

A second class of important electrolytes for rechargeable lithium batteries are soHd electrolytes. Of particular importance is the class known as soHd polymer electrolytes (SPEs). SPEs are polymers capable of forming complexes with lithium salts to yield ionic conductivity. The best known of the SPEs are the lithium salt complexes of poly(ethylene oxide) [25322-68-3] (PEO), —(CH2CH20) —, and poly(propylene oxide) [25322-69-4] (PPO) (11—13). Whereas a number of experimental battery systems have been constmcted using PEO and PPO electrolytes, these systems have not exhibited suitable conductivities at or near room temperature. Advances in the 1980s included a new class of SPE based on polyphosphazene complexes suggesting that room temperature SPE batteries may be achievable (14,15). 

Although polyacetylene has served as an excellent prototype for understanding the chemistry and physics of electrical conductivity in organic polymers, its instabiUty in both the neutral and doped forms precludes any useful appHcation. In contrast to poly acetylene, both polyaniline and polypyrrole are significantly more stable as electrical conductors. When addressing polymer stabiUty it is necessary to know the environmental conditions to which it will be exposed these conditions can vary quite widely. For example, many of the electrode appHcations require long-term chemical and electrochemical stabihty at room temperature while the polymer is immersed in electrolyte. Aerospace appHcations, on the other hand, can have quite severe stabiHty restrictions with testing carried out at elevated temperatures and humidities. 

The polymers which have stimulated the greatest interest are the polyacetylenes, poly-p-phenylene, poly(p-phenylene sulphide), polypyrrole and poly-1,6-heptadiyne. The mechanisms by which they function are not fully understood, and the materials available to date are still inferior, in terms of conductivity, to most metal conductors. If, however, the differences in density are taken into account, the polymers become comparable with some of the moderately conductive metals. Unfortunately, most of these polymers also have other disadvantages such as improcessability, poor mechanical strength, instability of the doped materials, sensitivity to oxygen, poor storage stability leading to a loss in conductivity, and poor stability in the presence of electrolytes. Whilst many industrial companies have been active in their development (including Allied, BSASF, IBM and Rohm and Haas,) they have to date remained as developmental products. For a further discussion see Chapter 31. 

Figure 11.9. Conductivity vs temperature plot for two ionically conducting crystals and for a polymer electrolyte, LiTf-aPtO40, which is based on amorphous poly(ethylene) oxide (after Ratner...

The first use of ionic liquids in free radical addition polymerization was as an extension to the doping of polymers with simple electrolytes for the preparation of ion-conducting polymers. Several groups have prepared polymers suitable for doping with ambient-temperature ionic liquids, with the aim of producing polymer electrolytes of high ionic conductance. Many of the prepared polymers are related to the ionic liquids employed for example, poly(l-butyl-4-vinylpyridinium bromide) and poly(l-ethyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide [38 1]. 

Figure 1. Temperature variation of the conductivity for a cross-section of polymer electrolytes. PESc, poly (ethylene succinate) PEO, polyethylene oxide) PPO, polypropylene oxide) PEI, poly(ethyleneimine) MEEP, poly(methoxyethoxy-ethoxyphosphazene) aPEO, amorphous methoxy-linked PEO PAN, polyacrylonitrile PC, propylene carbonate EC, ethylene carbonate.

---