Ionically conducting polyether composites

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Abstract
Ionic conductivity in polymer–salt electrolytes occurs in the amorphous regions of the complex. Poly(ethylene oxide) (PEO) is the best polyether for complexing salts. Unfortunately, it is partially crystalline at ambient temperatures. With inorganic (i.e., alumina) or organic (i.e., poly(acrylamide) (PAAM)) fillers the crystallization of PEO is inhibited and the room temperature conductivity is enhanced in these mixed phase systems by over two orders of magnitude (to ~ 10−4 S/cm) above the base PEO–salt system (− S/cm). Even adding PAAM to an initially amorphous system (oxymethylene-linked PEO–LiClO4) increases the room temperature conductivity by 2 to 3 times. Various alkali metal salts (Li, Na) and NH4SCN are used with α-Al2O3, θ-Al2O3, PAAM′ and poly(N,N′-dimethyl acrylamide) as fillers. The aluminas stiffen the complex and increase Tg. The addition of the organic fillers lowers Tg, as is to be preferred. It is suggested that changes in the conductivity with changes in salt and filler concentration are due to changes in the ultrastructure and morphology and are the result of an equilibrium between various Lewis acid – Lewis base reactions. Qualified success has been achieved in modelling ionic conductivity in these composite electrolyte systems using an effective medium approach. In this approach it has been assumed that the main conductivity enhancement takes place in thin amorphous layers of the polyether that coat the dispersed polyacrylamide particles separated in a microphase. In the best complexes this layer is identified by a second Tg. Key words: polyethers, composites, ionic conductivity, phase structure, Lewis acids and bases.