Ion transport in polymers

More detailed theoretical approaches which have merit are the configurational entropy model of Gibbs et al. [65, 66] and dynamic bond percolation (DBP) theory [67], a microscopic model specifically adapted by Ratner and co-workers to describe long-range ion transport in polymer electrolytes. 

Another quasi-thermodynamical model of ion transport in polymers is based on the concept of minimum configurational entropy required for rearrangement of the polymer, giving practically identical o—T and D — T dependences as the preceding model. 

These materials are introduced in Chapter 5 and only brief mention of them is necessary here. It is important to appreciate that polymer electrolytes, which consist of salts, e.g. Nal, dissolved in solid cation coordinating polymers, e.g. (CH2CH20) , conduct by quite a different mechanism from crystalline or glass electrolytes. Ion transport in polymers relies on the dynamics of the framework (i.e. the polymer chains) in contrast to hopping within a rigid framework. Intense efforts are being made to make use of these materials as electrolytes in all solid state lithium batteries for both microelectronic medical and vehicle traction applications. 

The interaction of poly(ethylene oxide) and other polar polymers with metal salts has been known for many years (Bailey and Koleska, 1976). Fenton, Parker and Wright (1973) reported that alkali metal salts form crystalline complexes with poly(ethylene oxide) and a few years later, Wright (1975) reported that these materials exhibit significant ionic conductivity. Armand, Chabagno and Duclot (1978, 1979) recognised the potential of these materials in electro-chemical devices and this prompted them to perform more detailed electrical characterisation. These reports kindled research on the fundamentals of ion transport in polymers and detailed studies of the applications of polymer-salt complexes in a wide variety of devices. 

The electrolyte concentration is very important when it comes to discussing mechanisms of ion transport. Molar conductivity-concentration data show conductivity behaviour characteristic of ion association, even at very low salt concentrations (0.01 mol dm ). Vibrational spectra show that by increasing the salt concentration, there is a change in the environment of the ions due to coulomb interactions. In fact, many polymer electrolyte systems are studied at concentrations greatly in excess of 1.0 mol dm (corresponding to ether oxygen to cation ratios of less than 20 1) and charge transport in such systems may have more in common with that of molten salt hydrates or coulomb fluids. However, it is unlikely that any of the models discussed here will offer a unique description of ion transport in a dynamic polymer electrolyte host. Models which have been used or developed to describe ion transport in polymer electrolytes are outlined below. 

Because multilayer interconnecting networks are an important element of advanced chips and parallel processors, it is essential that an understanding of the corrosion processes that affect their reliability be developed. Needed are methods to quantify metal corrosion and ion transport in polymers and means to identify electrochemically reliable metal-polymer systems. 

The three-dimensional continuous nanostructured framework has a large specific surface area and favors rapid electron and ion transport in polymer-based electronics. The previously mentioned polymer composites with three kinds of structures can form three-dimensional porous structures when they were stacked. Besides, polymers can be incorporated into porous silica, carbon, and graphene as the retaining frameworks to form three-dimensional structured composites. The resultant composite materials exhibit surface properties of the polymers and high mechanical strength and high electric conductivity of the frameworks, which will provide new possibilities for advanced applications. The structures, such as the cormectivity, pore diameters, and shapes, are mainly decided by the frameworks. 

This method is widely used to simulate motions in liquids and solids and to study rapid diffusions in ionic lattices. Although the application of molecular dynamics (MD) to studying ion transport in polymer systems is still in a seminal state, it is included in this chapter because of its correlation with the expansion of computing power, which will probably result in its prominence as an investigative method in this field. 

The intensive studies on the ion transportation in polymer electrolytes have been carried out over the last two decades. To date, it has not been proven otherwise that both the cation and the anion are responsible as charge carriers in blend-based polymer electrolytes (Armand et al., 1979 Floganczyk et al., 2004 Cameron et al., 1989 Kim et al., 1996 Stoeva et al., 2003). 

Vargas, R.A.,Zapata, V.H., Delgado, M.I., Palacios, I. (2004) Effect of water vapor on the ion transport in polymer films of PV0H/LiH2P04/H20. Solid State Ionics, 175,729-732. 

It is of maximum importance to understand the ion transport in polymer electrolytes. It is the basic principle governing the interactions between ions... 

The electrochemistry of a polymer-modified electrode is determined by a combination of thermodynamics and the kinetics of charge-transfer and transport processes. Thermodynamic aspects are highlighted by cyclic voltammetry, while kinetic aspects are best studied by other methods. These methods will be introduced here, with the emphasis on how they are used to measure the rates of electron and ion transport in conducting polymer films. Charge transport in electroactive films in general has recently been reviewed elsewhere.9,11... 

A variety of other techniques have been used to investigate ion transport in conducting polymers. The concentrations of ions in the polymer or the solution phase have been monitored by a variety of in situ and ex situ techniques,8 such as radiotracer studies,188 X-ray photoelectron spectroscopy (XPS),189 potentiometry,154 and Rutherford backscatter-ing.190 The probe-beam deflection method, in which changes in the density of the solution close to the polymer surface are monitored, provides valuable data on transient ion transport.191 Rotating-disk voltammetry, using an electroactive probe ion, provides very direct and reliable data, but its utility is very limited.156,19 193 Scanning electrochemical microscopy has also been used.194... 

The Mechanism of Ion Transport in Solutions, Solids, Melts and Polymers... 

The mechanism of ion transport in such systems is not fully elucidated, but it is presumably dependent on the degree of crystallinity of the polymeric complex (which further depends on the temperature and the salt type). The ionic conductivity was initially attributed to cation hopping between fixed coordination sites in the depicted helical tunnel, i.e. in the crystalline part of the polymer. 

In polymer electrolytes (even prevailingly crystalline), most of ions are transported via the mobile amorphous regions. The ion conduction should therefore be related to viscoelastic properties of the polymeric host and described by models analogous to that for ion transport in liquids. These include either the free volume model or the configurational entropy model . The former is based on the assumption that thermal fluctuations of the polymer skeleton open occasionally free volumes into which the ionic (or other) species can migrate. For classical liquid electrolytes, the free volume per molecule, vf, is defined as ... 

In the example above, a short-chain poly(ethylene glycol) was added to a rigid polyelectrolyte to plasticise the material and thereby increase polymer-solvent motion in the vicinity of mobile ions. This strategy has been widely explored as a means of improving ion transport in electrolytes. 

The different techniques which have been applied to determine transport in polymer electrolytes are listed in Table 6.1. For a fully dissociated salt all the techniques yield the same values of t (small differences may arise due to second order effects such as long range ion interactions or solvent movement which may influence the different techniques in different ways). In the case of associated electrolytes, any of the techniques within one of the three groups will respond similarly, but the values obtained from different groups will, in general, be different. Space does not permit a detailed discussion of each technique, this is available elsewhere (see Bruce and Vincent (1989) and the references cited therein). However, we will consider one technique from each group to illustrate the differences. A solid polymer electrolyte containing an associated uni-univalent salt is assumed. 

Solvent-free polymer-electrolyte-based batteries are still developmental products. A great deal has been learned about the mechanisms of ion conductivity in polymers since the discovery of the phenomenon by Feuillade et al. in 1973 [41], and numerous books have been written on the subject. In most cases, mobility of the polymer backbone is required to facilitate cation transport. The polymer, acting as the solvent, is locally free to undergo thermal vibrational and translational motion. Associated cations are dependent on these backbone fluctuations to permit their diffusion down concentration and electrochemical gradients. The necessity of polymer backbone mobility implies that noncrystalline, i.e., amorphous, polymers will afford the most highly conductive media. Crystalline polymers studied to date cannot support ion fluxes adequate for commercial applications. Unfortunately, even the fluxes sustainable by amorphous polymers discovered to date are of marginal value at room temperature. Neat polymer electrolytes, such as those based on poly(ethyleneoxide) (PEO), are only capable of providing viable current densities at elevated temperatures, e.g., >60°C. 

The polymerized ionic liquid (IL) shows great promise for diverse applications. Some polymerization methods have already been oriented toward specific applications. Polymerized ILs are useful in polar environments or where there are ion species for transport in the matrix. Amphoteric polymers that contain no carrier ions are being considered for several porposes in polymer electrolytes. Zwitterionic liquids (ZBLs) were introduced in Chapter 20 as ILs in which component ions cannot move with the potential gradient. ZILs can provide ion conductive paths upon addition of salt to the matrix. It is therefore possible to realize selective ion transport in an IL matrix. If the resulting matrix can form solid film over a wide temperature range, many useful ionic devices can be realized. This chapter focuses on the preparation and characteristics of amphoteric IL polymers. 

---