Multivalent polymer electrolytes

The purpose of this chapter is to describe how ionic conductivity has been achieved in ways that retain the advantages of flexibility, processability, ease of handling and relatively low impact on the environment that polymers inherently possess. Electronically conducting polymers are addressed in the next chapter. The reader will also find particular aspects of ion conducting polymers discussed in detail in subsequent parts of this volume these include multivalent polymer electrolytes, key application areas such as lithium polymer batteries and smart windows, and the development of polymer hosts which permit greatly enhanced conductivity at room temperature. 

Key words multivalent polymer electrolytes, ionic conductivity, cationic... 

This chapter begins by highlighting the general theory behind the formation of stable multivalent polymer electrolytes, and by providing an overview of the kind of complexes formed between the polymer, PEO, and divalent or bivalent cation salts. Details of the associated studies are described briefly. 

Multivalent polymer electrolytes that use PEO as the polymer base are usually multiphase systems consisting of salt-rich crystalline phases, another crystalline phase of pure polymer, and amorphous phases with dissolved salts. Conductivity is thus often affected by factors such as slow crystallisation and salt redistribution processes between the phases, which result in values dependent on the thermal history, preparation methods, etc. The morphological and crystallographic structures of these polymeric systems are presented, and the influence of factors such as crystallisation of the pure polymer on ionic conduction, and the factors that influence them in turn, are highlighted. Discussion of conduction takes into account the two possible mechanisms of mobility of charged ionic clusters and exchange of... 

Finally, the thermal behaviour of several multivalent polymer electrolyte systems is reviewed. The importance of establishing equilibrium phase diagrams is discussed in some detail in the last section of this chapter, concluding that the thermodynamic interactions that exist in multivalent polymer electrolytes are, even qualitatively, very useful in understanding the mechanical properties, conduction and stability of these compounds. 

A great number of multivalent polymer electrolytes are semicrystalline in nature and belong to type II or III. One example of type II behaviour is shown by the PEO -Znl2 system, with the variation of ionic conductivity with temperature behaving differently below and above the transition temperature T. For T < Tt, the data gave a linear variation due to the existence of crystalline compounds below T, and the Arrhenius law was applied. For T > Tt, the data produced a convex variation, in accordance with the amorphous regime observed above T, and the VTF law was applied. 

This chapter was not intended to cover all the areas of science concerning polymer electrolytes, but rather to contribute towards a better understanding of some of the important aspects of systems containing multivalent cation salts and their behaviour. Examples of work carried out in the field of ionic transport have given a general overview of the influence of crystallinity and thermal behaviour of polymeric systems on the relevant electric properties of multivalent polymer electrolytes. 

The properties of polymer electrolyte membranes change with the cation associated with the sulfonic acid sites. The conductivity of the membrane decreases when larger cations replace the protons. Other cations are at least four times less mobile than protons. The mobility of multivalent cations is lower than monovalent cations at similar molecular weight, but this is somewhat offset by the fact that multivalent ions carry more than one charge per molecule. The cationic composition also affects other properties of the polymer electrolyte membrane. Nonproton forms of the membrane do not hold as much water as the protonic form. In addition, the electroosmotic drag of water by other cations is greater than that by protons. Parameters for membranes equilibrated with several cationic species and mixtures of species have been measured and reported in the literature [2-4]. 

Examples of type I PEO-MX (M = multivalent cation, X = anion) include complexes of PEO-based calcium and barium salts, PEO-based zinc chloride (in composition O/Zn = 4 and 8), PE0-Cu(C104)2 (certain compositions) and PE0-Tm(S03CF3)3. When the polymeric system is predominantly amorphous, conductivity-temperature behaviour is sometimes better described by the VTF law, for instance for the gel polymer electrolyte studied by Pandey et a/., where the addition of liquid electrolyte provokes substantial conformational changes in the crystalline texture of the host polymer due to immobilisation of the liquid electrolyte in the gel system. The polymer crystallinity almost disappears and the VTF equation applies to the a-T relationship. 

VIPLEX SR is a vinyl acetate homopolymer emulsion which is an excellent hand builder for textile water repellent finishes because it has a film of high water resistance. It also forms dilute dispersions in water which are stable to substantial concentrations of electrolytes such as borax, multivalent metallic ions and quaternary compounds which coagulate many types of vinyl acetate emulsion polymers. 

A polymer may modify this entropy contribution in a number of different ways. If it is ionic and has a similar charge, then we have a simple and relatively moderate electrolyte effect. If its charge is opposite, and it acts as a multivalent counterion, then the interaction becomes very strong since an association between polymer and micelle leads to a release of the counterions of both the micelles and the polymer molecules a very similar effect will be obtained in mixtures of two oppositely charged polymers. Indeed there is for such a case a lowering of the CMC by orders of magnitude. 

Aqueous solutions of the above polymers show widely different behavior under carefully controlled conditions of concentration pH, temperature, and mono- multivalent electrolyte concentration. 

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