Polymer/salt complexes motion

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 classical example of a soUd organic polymer electrolyte and the first one found is the poly(ethylene oxide) (PEO)/salt system [593]. It has been studied extensively as an ionically conducting material and the PEO/hthium salt complexes are considered as reference polymer electrolytes. However, their ambient temperature ionic conductivity is poor, on the order of 10 S cm, due to the presence of crystalUne domains in the polymer which, by restricting polymer chain motions, inhibit the transport of ions. Consequently, they must be heated above about 80 °C to obtain isotropic molten polymers and a significant increase in ionic conductivity. 

Work on ion-conducting polymers started after it was reported during 1951 that salts can interact with PEO chains (Rg. 11.1) and the properties of polymer salt solutions were studied during the 1960s. Ionic conductivity in a PEO-alkaline metal ion complex was first reported by Wright in 1975. As the concentration of lithium salt was increased in the PEO, a general reduction in both the conductivity and number of lithium transfers was observed. The reduction was attributed to the motion of the polymer chains, responsible for ion mobility, being restricted and also formation of ion pairs. In turn, it lowered the number of free lithium ions available for conduction. Initial work was carried out by Armand as he realized that this... 

Although the motion of protons does not lead to electrical conduction in the case of benzoic acid, electronic and even ionic conductivity can be found in other molecular crystals. A well-studied example of ionic conduction is a film of polyethylene oxide (PEO) which forms complex structures if one adds alkaline halides (AX). Its ionic conductivity compares with that of normal inorganic ionic conductors (log [cr (Q cm)] -2.5). Other polymers with EO-units show a similar behavior when they are doped with salts. Lithium batteries have been built with this type of... 

Poly(oxyethylene) combinations with various other comonomers [46], are of interest as solid polymer electrolytes after complex formation with Li(I) (complexation with Na(I), K(I), Mg(II), Ba(II), etc. has also been studied) [1,5,46-48]. The synthesis is carried out by direct interaction of the ligand and metal ions in solution or, if cross-linked poly(oxyethylene) is employed, by immersing the polymer ligand into a solution of the metal salt. Poly(oxypropylene), modified polysiloxanes, cross-linked phosphate esters and ethers [46,49,50], and structurally different ligands such as 2,5-dimercapto-1,3,4-thiadiazol-polyaniline [51] have also been used as polymer ligands, The developments in this field are reviewed in [46], In this review the segmental motion of Li(I) in a poly(oxyethylene) is described as shown in Fig. 5-4. 

Above all, the work described in this review demonstrates that UV-visible spectroscopy is a very useful technique for learning about the coordination of certain transition metal ions in polymer electrolytes. The results demonstrate unequivocally that complex ions exist in polymer electrolytes of this sort, particularly at moderate to high salt concentrations and at elevated temperatures. Furthemore, it appears that free anions and anionic complex species are the predominant charge carriers in the electrolytes studied here. In fact, our studies indicate that the principal mechanism of cation mobility in Co(II) and Ni(II)-PEG electrolytes is by the motion of complex anions, not the diffusion of uncomplexed cations. 

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