Polymer salt/complexes

A chemical cross-hnking of MEEP was obtained by Shriver [606] by using polyethylene glycol (PEG) dialkoxide, which also forms polymer salt complexes. The cross-linked polymers were prepared by substituting a part (1 and 10 mole%) of the methoxyethoxyethoxy ethanol by PEG in the synthesis of MEEP. Contrary to the MEEP, the amorphous polymers obtained do not flow and are stable even at 140 °C. The maximum ionic conductivity at 30 °C, obtained after complexation with liSOjCFj, are 4.1x10" S cm for MEEP/PEG 1% complexed with 6.4 wt% salt and 3x10" S cm for MEEP/PEG 10% com-plexed with 8.9 wt% salt. These values are comparable with those obtained with the parent hnear polyphosphazenes. 

In conclusion, polymer electrolytes based on phosphazene backbone and containing ether side chains are, after complexation with alkali metal salts, among the highest ionically solvent-free polymer salt complexes, with conductivities in the order of 10" -10" S cm However, these conductivities are still below the value of 10 S cm" which is considered to be the minimum for practical applications. Therefore the design of new polyphosphazenes electrolytes with a higher conductivity and also a higher dimensional stability still remains a challenge for future researchers. 

Two general types of polymer electrolytes have been intensively investigated, polymer-salt complexes and polyelectrolytes. A typical polymer-salt complex consists of a coordinating polymer, usually a polyether, in which a salt, e.g. LiC104, is dissolved. Fig. 5.1(a). Both anions and cations can be mobile in these types of electrolytes. By contrast, polyelectrolytes contain charged groups, either cations or anions, covalently attached to the polymer. Fig. 5.1(h), so only the counterion is mobile. 

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. 

Many polymer-salt complexes based on PEO can be obtained as crystalline or amorphous phases depending on the composition, temperature and method of preparation. The crystalline polymer-salt complexes invariably exhibit inferior conductivity to the amorphous complexes above their glass transition temperatures, where segments of the polymer are in rapid motion. This indicates the importance of polymer segmental motion in ion transport. The high conductivity of the amorphous phase is vividly seen in the temperature-dependent conductivity of poly(ethylene oxide) complexes of metal salts. Fig. 5.3, for which a metastable amorphous phase can be prepared and compared with the corresponding crystalline material (Stainer, Hardy, Whitmore and Shriver, 1984). For systems where the amorphous and crystalline polymer-salt coexist, NMR also indicates that ion transport occurs predominantly in the amorphous phase. An early observation by Armand and later confirmed by others was that the... 

The rate of growth of polymer-salt complexes can provide fundamentally important information that is difficult to determine otherwise. The rate of crystal growth of (PEO)3 NaSCN from its undercooled liquid was measured and used to determine values for the diffusion coefficients of Na" " and SCN (Lee, Sudarsana and Crist, 1991). Also it was shown that the rate of the salt diffusion is independent of the molecular weight of the polymer for PEO molecular weights above 10. This result is fully consistent with the concept that ion motion is due to local segmental motion of the polymer. 

The structures of crystalline polymer-salt complexes provide insight into the structure of the more conducting amorphous materials. To date, large single crystals of polymer-salt complexes have not been prepared, but it has been possible to obtain structural information from single crystal X-ray diffraction applied to stretched oriented fibres in the PEO NaI and PEOiNaSCN systems (Chatani and Okamura, 1987 Chatani, Fujii, Takayanagi and Honma, 1990). One of the most detailed studies is of (PEO)3 NaI, Fig. 5.11(a). The sodium ion in this structure is coordinated to both the polymer and to the iodide ion and the polymer is coiled in the form of an extended helix. 

It is difficult to prepare stretched oriented fibres and such fibres may differ in their polymer chain conformation compared with the unstretched materials. Furthermore the quality of the single crystal X-ray data is poor and difficult to interpret. In contrast power X-ray data of relatively high quality may be obtained from polycrystalline polymer films. Lightfoot, Mehta and Bruce (1992) have obtained the first crystal structure of a polymer-salt complex, PEOjiNaClQ from powder X-ray data. Fig. 5.11(h). The structure is similar to the corresponding PEOjiNal structure, the PEO chains are wrapped around the Na ions with each Na ... 

An amorphous material sometimes referred to as amorphous poly(ethylene oxide), aPEO, consists of medium but randomly-variable length segments of poly(ethylene oxide) joined by methyleneoxide units. Fig. 5.13 (Wilson, Nicholas, Mobbs, Booth and Giles, 1990). These methyleneoxide units break up the regular helical pattern of poly(ethylene oxide) and in doing so suppress crystallisation. The aPEO host polymer and its salt complexes can crystallise below room temperature, but this is not detrimental to the properties of the polymer-salt complexes at or above room temperature. Similarly, dimethyl siloxy units have been introduced between medium length poly(ethylene oxide) units to produce an amorphous polymer. Fig. 5.14 (Nagoka, Naruse, Shinohara and Watanabe, 1984). 

Since the polyelectrolytes contain only one type of mobile ion, the interpretation of conductivity data is greatly simplified. Polyelectrolytes have significant advantages for applications in electrochemical devices such as batteries. Unlike polymer-salt complexes, polyelectrolytes are not susceptible to the build up of a potentially resistive layer of high or low salt concentration at electrolyte-electrolyte interfaces during charging and discharging. Unfortunately flexible polyelectrolyte films suitable for use in devices have not yet been prepared. 

Polymer-salt complexes involving the tripositive lanthanides have been investigated from the standpoint of conductivity, which is observed to be very low. In addition, the neutral complex Nd(DMP)3 (DMP = 2,2,6,6-tetraethyl,-3,5-heptane dionate) will dissolve in PEO although not electrically conductive this polymer may have utility as a laser material. To... 

Polymeric electrolytes, polymer-salt complexes, and gelled electrolytes, e.g., benzyl sulfonic acid siloxane, polyethylene oxide (imine, succinate)-LiC104, and PVDF gel in THF containing a mixture of Bu2Mg and AlEtCl2, respectively. 

Solid polymer electrolytes (SPE) represent the newest and one of the most important (i.e. as far as potential applications are concerned) class of FIC solids. The area of polymer/salt complexes became extremely active following the work of Wright, who first reported that PEO is an excellent polymer host for a number of salts and that the resnlting polymer/salt complexes have significant electrical conductivities near room temperature. Armand extended the investigation of the electrical properties of the polymer/salt electrolytes and... 

SPE can be prepared by dissolving the polymer and an alkali metal salt in a mutual solvent and then evaporating the solution on a teflon fluorocarbon resin plate in a dry atmosphere. SPE films are obtained by casting the solution and heating at relatively low temperatures ( 150°C) under vacuum. The existence of a polymer/salt complex, however, is no guarantee that the material will be a good ionic conductor. 

There has been a continuation of the study of the polymer-salt complex poly(ethylene oxide) (PE0)/LiCF3S03, in which the morphology and conduc-... 

Since the polymer and the metal salt involved are both solid materials, the preparation of a polymer salt complex is achieved by the dissolution of the two materials in a common solvent such as acetonitrile, methanol or THF followed by a slow removal of the solvent in vacuum. This results in either the bulk polymer-metal salt complex or a thin film depending upon the method of preparation. It is essential to ensure that no traces of moisture are present and hence the operations are carried out by using Schlenk techniques or glove box methods. The essential reaction that occurs in the formation of a polymer-metal salt complex can be written as... 

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