Polymer/salt complexes amorphous

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. 

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. 

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). 

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. 

The polymer [NP(0CH2CH20CH2CH20CH3)2]n is a eompletely amorphous polymer with a Tg of -84 °C. This polymer as well as other related polymers of the above type can be complexed with lithium salts such as LiCFsSOs, L1BF4, LiC104 etc., to afford polymer-salt complexes. Essentially the etheroxy side-chain-containing polymers act as solvents and the metal salt dissolves in the polymer to afford a solid solution. The ionic conductivities of such polymers are quite promising (lO -lO Scm" ) [21, 38]. Such polymer-salt complexes have been shown to be semiconducting. 

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. 

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. 

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 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-... 

Itaconates of structure II in which n = 1-5 are amorphous polymers. Their Li salt complexes, however, retain the amorphous morphology only when n 2. Even the amorphous electrolytes show low conductivity apparently because of large increases in the Tg upon complexation with Li salts. It is apparent that the amorphous morphology of a polymer electrolyte should be complemented by high polymer fluidity in order to have good conductivity at ambient temperatures. 

The results of this study lead to the conclusion that poly(acetylene) oxidized by iodine essentially consists of three different components unreacted bulk polymer, iodine covered surfaces and/or amorphous regions containing iodine and a metallic conducting polymer-iodine complex salt structure with a new lattice composed of rearranged chains together with I -chains. The conductivity data indicated a percolation process involving the metallic conductive parts of the sample as component of a system with a complex texture. 

Since the amorphous and crystalline fractions of a high polymer cannot be separated mechanically, e.g. by filtration as for the separation of liquid solutions from crystalline complexes, the phase behaviour of polymer-salt systems is more difficult to chart than that of liquid solvents. However, many techniques, e.g, polarized light microscopy, differential scanning calorimetry, " X-ray diffraction and NMR, have been used to study the partition of the salt and the temperature dependence of phase behaviour. 

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