Crystalline Polymer Salt Complexes

Investigation of ionic conductivity in crystalline soft solids has followed two paths crystalline polymer salt complexes and plastic crystals. Although the latter are not strictly polymer electrolytes they have similar mechanical properties and help to provide a more complete view of ion transport in non-amorphous soft solids. 

Crystalline polymer electrolytes have been incorporated into lithium ion cells which are being shown to sustain cychng thus demonstrating potential for apphcations.  

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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 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. 

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 relative advantages of drying at different temperatures have been discussed in detail elsewhere [73]. In summary, drying at high temperatures modifies the morphology and the amorphous/crystalline ratio and favours the formation of high temperature polymer-salt complexes in which the ions are too tightly bound to be mobile. 

In general, the composition of polymer-salt complexes is a result of rather complicated equilibrium and some non-equilibrium, kinetically limited phenomena (Fauteux and Robitaille 1985 Fauteux et al. 1985 Lee and Crist 1986 Minier et al. 1984 Munshi and Owens 1986 Robitaille and Fauteux 1986 Stainer et al. 1984). According to the Gibbs phase rule (Gibbs 1870 Mindel 1962), for all the compositions ranging from pure polymer up to that of the thinnest crystalline complex, two phases should be present pure, crystalline PEO and pure PEO-salt crystalline complex. Nevertheless, polymeric materials are intrinsically impure , for example due to their polydispersity. Additionally, their crystallisation is kinetically limited, therefore in all polymeric materials there are always amorphous domains. Thus PEO-salt complexes usually consist of three phases (Fig. 2.4) - pure crystalline PEO, crystalline PEO-salt complex and amorphous PEO-salt complex the latter is of undefined composition (Wieczorek et al 1989). 

The high-molecular-weight poly (propylene oxide) produced with hexacyanometalate salt complexes shows no crystallinity. Moreover, it was shown by Price et al. (18) and confirmed in our laboratory that these polymers have more than 953 head-to-tail enchainment. The amorphous fractions of partially crystalline polymers made with metal-alkyl and ferrio-chloride-based catalysts were shown by those authors to have considerable head-to-head enchainment. They postulated that this was the cause of the amorphous nature of these fractions. It seems clear, however, that the amorphous nature of the polymers prepared with hexacyanometalate salt complexes must be the result of their low degrees of tacticity. 

Most interesting are the effects of salt complexation on the mesomorphic behavior of liquid crystalline crown ethers and liquid crystalline crown ether polymers. Sodium triflate was added to poly(17) [34] and poly(25) (Scheme 14) [39]. The enantiotropic nematic and smectic phases of poly(17) were changed dramatically [40]. With increasing amounts of salt, the clearing temperatures are shifted to higher values while the melting transition increases only slightly. 

Although PEO is an excellent solvent for the solvation of alkali metal ions, polymer electrolytes derived from pure PEO-metal salt complexes do not show high ionic conductivities at ambient temperatures, due to the partial crystalline nature of PEO [27,29,37,59,79] (vide supra). 

Various methods have been employed to find out about the structure of polymer electrolytes. These include thermal methods such as differential scanning calorimetry (DSC), differential thermal analysis (DTA), X-ray methods such as X-ray diffraction and X-ray absorption fine structure (XAFS), solid state NMR methods particularlyusing7LiNMR,andvibrationalspectroscopicmethodssuch as infrared and Raman [27]. The objective of these various studies is to establish the structural identity of the polymer electrolyte at the macroscopic as well as the molecular levels. Thus the points of interest are the crystallinity or the amorphous nature of materials, the glass transition temperatures, the nature and extent of interaction between the added metal ion and the polymer, the formation of ion pairs etc. Ultimately the objective is to understand how the structure (macroscopic and molecular) of the polymer electrolyte is related to its behavior particularly in terms of ionic conductivity. Most of the studies have been carried out, quite understandably, on PEO-metal salt complexes. In comparison, there has been no attention on the structural aspects of the other polymers particularly at the molecular level. 

Other polymers such as MEEP, poly MEEM A and polysiloxanes are completely amorphous polymers and remain amorphous even in the polymer-metal salt complexes at least upto certain concentrations of the metal salt. The question of the amorphous nature of the polymer electrolytes is important in view of Berthi-er s demonstration in the PEO-polymer electrolyte system that the ionic conductivity occurs mostly in the amorphous phase [59]. Thus the room temperature ionic conductivities of the completely amorphous MEEP-LiX complexes is at least three orders of magnitude higher than the corresponding PEO-LiX complexes. This has led to the use of plasticizers and other modifications to suppress crystallinity and increase free volume as discussed above. 

The presence of still more complex species of general formula Al[Al3(OH)8] +3 has also been postulated10 to fit potentiometric data. Hydroxo-bridged polymers are known to be present in various crystalline basic salts. 

The crystallinity of a polymer can be reduced or eliminated by manipulating its structure. For example, Li salt complexes with low TgS and fully amorphous morphology have been obtained from PEO by interspersing them with oxymethylenes polymers called poly(oxymethylene-oligo-... 

PEO can coordinate alkali metal ions strongly and is used as a solid polymer electrolyte [20-22]. However, conventional PEO-Li salt complexes show conductivities of the order of 10 S/cm, which is not sufficient for battery, capacitor and fuel-cell applications. A high crystalline phase concentration limits the conductivity of PEO-based electrolytes. Apart from high crystallinity, PEO-based electrolytes suffer from low cation transport number (t ), ion-pair formation and inferior mechanical properties. Peter and co-workers [23] reported the modification of PEO with phenolic resin for improvement in mechanical properties and conductivity. 

XRD patterns of PEO/PVP blend films (Figure 19.1) showed peaks at around 19.2°, 22.5°, 23.6°, 25.5°, 26.2°, 27.1°, 29.7°, 32.6°, 36.6° and 39.6° and are attributed to the crystalline phase of PEO. These peaks appear to superimpose on a broad hump between 18 and 50 which could be due to the amorphous nature of PEO. A broad peak at around 13° is associated with the amorphous nature of PVP The intensity of all crystalline peaks decreases gradually in all XRD patterns with salt concentration suggesting a decrease in the degree of crystallinity of the complex. This could be due to the disruption of the semi crystalline structure of the films by salt. No sharp peaks correspond to NaF salt were observed in all PEO/PVP complexes, indicating the complete dissolution of salt in the polymer matrix. 

A specific feature of rigid-chain polyamides related to the possibility of dissolution of these polymers only due to the very energetic interaction of the elementary units of the polymer with the solvent molecules is manifested in these systems. At low temperatures, salt compounds crystallize out in the form of crystal solvates [42] with a constant polymer-sulfuric acid molar ratio. At high temperatures, the crystal solvate melts and equilibrium involving a liquid-crystalline phase is attained. The polymer-acid complex in this example is thermally unstable and decomposes at relatively low temperatures. Melting of the compound with decomposition— incongruent melting—results in the appear-... 

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