Polymer/salt complexes structure

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 ... 

Recent work in the field of ceramic polymeric electrolytes is the so-called polymer-in-ceramic approach presented by Syzdek et aL (2009). It consists of introducing polymer-salt complexes into porous ceramic structures. In this manner we split the spatial and electronic separation and ionic conductivity into two phases again, but the difference is that ceramics are much more thermally stable, resistant to reactions with lithium and to puncturing with lithium dendrites that can appear in the cell. Also the wettability of ceramic materials is much better than that of polymers, therefore this approach can be considered as an alternative to the Bellcore process that... 

JThe effect of the substituent on the properties of the polyphosphazenes is not fully understood. For instance, [NP(OCH ) ]n and [NP C CH. homopolymers are elastomers (8,29). Synthesis using lithium, in contrast to sodium, salts is claimed to produce rubber-like fluoroalkoxyphosphazene polymers (30). The presence of unreacted chlorine or low molecular weight oligomers can affect the bulk properties (31,32). Studies with phosphazene copolymers both in solution and in the bulk state (29,33-38) indicate a rather complex structure, which points out the need for additional work on the chain structure and morphology of these polymers. 

Polymer electrolytes (e.g., poly (ethylene oxide), poly(propylene oxide)) have attracted considerable attention for batteries in recent years. These polymers form complexes with a variety of alkali metal salts to produce ionic conductors that serve as solid electrolytes. Their use in batteries is still limited due to poor electrode/electrolyte interface and poor room temperature ionic conductivity. Because of the rigid structure, they can also serve as the separator. Polymer electrolytes are discussed briefly in section 6.2. 

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... 

Figure 4. Backbone structures of salt-solvating polymers. The figure shows the similarity of backbone structure, with optimal spacing between electron-donating oxygens, of polymers that form ion-conducting salt complexes. PPL-poly-3-propiolac-tone PEO polyethylene oxide PPO 1,2- polypropylene oxide.18...

PAni has a very complex structure and doping behaviour, see Fig. 9.6, and the spectra are sensitive to the polymer morphology, the level of oxidation and degree of protonation. This accounts for the considerable variation in tire spectra that have appeared in the literature. The effects are illustrated in Fig. 9.33 for various forms of the protonated salt. These spectra refer to dried films, electrochemically prepared at different electrode potentials, and subject to oxidation by exposure to air. This variation in preparation conditions means that the degrees of oxidation and protonation are not well defined, as evidenced by the pronounced differences in the spectra of the emeraldine prepared at the... 

The recent example of the ab initio structure determination of the polymer electrolyte Poly (ethylene oxide)6 LiAsFe by Bruce et is a notable example of the complex structures that can be determined from powder diffraction on a pulsed neutron source. Polymer electrolytes consist of salts dissolved in solid high molecular weight polymers, and represent a unique class of solid coordination compounds. Their importance lies in their potential in the development of truly all-solid-state rechargeable batteries. The structure of the 6 1 complex is particularly important, as it is a region where the conductivity increases markedly. The structure of the complex is distinct from all known crystal structures of PEO salt complexes (see Figure 7). The Li-i- cations are arranged in rows, with each row located inside a cylindrical surface formed by two PEO chains, with the PEO chains adopting a previously unobserved conformation. Furthermore the anions are located outside the PEO cylinders and are not coordinated with the cations. 

Compared to block copolymers, there have been relatively fewer examples of using homopolymers for nanofabrication. Nevertheless, some polymers with amphiphilic properties were also used in the fabrication of nanostructures with various metal salts/complexes. For example, polyaniline (PANI) emeraldine base formed self-organized mesomorphic structures when mixed with Zn(DBS)2 by the coordination between Zn2+ and the imine nitrogen atoms on the polymer main chain.100 The resulting supramolecule PANI[Zn(DBS)2]0.5 had a comb-shaped... 

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. 

Infrared and Raman spectroscopic studies have been extensively carried out on PEO -metal salt complexes and have aided our understanding of the polymer structure, interaction of the ions with the polymer, as well as ion-ion interactions [67, 246]. 

Hydrated cellulose (viscous) fibers, unwoven materials (e.g. felt), with different fiber interweaving and chemical reagents of high purity were used. Hydrated cellulose was chosen as a polymer precursor. Its structure is a complex system composed of micro-fibrils and micro- and macropores and also of a branched network of microscopic capillaries. Cellulose has a large inner surface that plays a determining role in absorption of aqueous or organic liquids with polymer molecules. Under the impregnation of hydrated cellulose with aqueous solutions of salts, the liquid fills the space between fibers, pores on the fiber surface and interacts with cellulose macromolecules. 

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. 

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