Polymer Ionics
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N/SEPTEMBER1989
particular, the Nafion materials a n d related fluorocarbons, when swollen by solvents such as water, exhibit very high ionic mobilities and some permselectivity. 4 Additionally, polyvinyl alcohol/ phosphoric acid swollen materials hâve been used as protonic electrolytes and sensors. 5 In the majority of thèse materials, however, the polymer host is largely présent for structural reasons, and the actual electroactive r e s p o n s e a r i s e s chiefly from pools of solvent. Our concentration h è r e is on p u r e p o l y m e r materials, in which such solvent pools are absent.
Table I: Conductivities for Characteristic Materials. Substance
a (S/cm)
p(EO)12LiCIO/» pfPOkLiCFaSOa»» (MEEP)4LiCF3S03 RbAgJs Ge NaLjA^CV,"» Cu 0.1 M NaCI(aqueous)
5.6 x 10"6 2.2 x 10* 1.0 x 10-" 6 x 10-' 5 x 10* 3 x 10-2 5x6-105 1 x 10 2
(a) (b) (c) (d)
At312K EO: -CH 2CH 20-, PO: -CHCH3CH2-0, MEEP: Structure 1 "Na-/8-alumina" Electronic conductivity
Polymer electrolytes represent the newest class of solid ionics. They stand in sharp contrast to the usual solid ionic materials based on ceramics or inorganic crystals. The most important différences involve, first, the value of the ionic conductivity (Table I)—polymer electrolytes conduct noticeably less well than the best ceramic or crystalline electrolytes — and the nature of the transport mechanism. Indeed, the polymer electrolytes transport charge well only above their
glass transition température, where on a local microscopic scale the material is, in fact, molten. In this sensé, ion transp o r t in t h è s e materials occurs in a molten micro-environment, and therefore might be expected (as is indeed observed) to be more similar to liquid electrolytes than ceramic electrolytes. A n u m b e r of experiments d e m o n strate that ions move by a local liquidlike process rather than by h o p p i n g from site to site in an ordered polymer host. For example, experiments using strong ion pair forming dopants demonstrated 6 that simple hopping in a helical tunnel could not be true. More detailed mechanistic studies (discussed later in this article) indicated clearly that ionic motion in thèse materials does not occur by hopping in a locally crystalline région, but rather by continuous motion in the amorphous région of the polymeric material. 7 In this sensé, the dynamics of the host material détermines the transport of the ionic guests. Polymer electrolytes are conveniently grouped into three major classes. The first class, corresponding to the earliest materials to be studied, are simply complexes of a sait with a polar polymer host material. Typical polymer hosts h â v e Lewis-base sites, a n d i n c l u d e PEO, poly(propylene oxide), poly(ethylene imine), poly(ethylene succinate), and a number of comb polymers, in which oligoether side chains are built on a flexible backbone. Characteristic comb polymers include the substituted phosphazene structure (Figure la), and substituted siloxane structure (Figure lb). The polymer/salt complex materials hâve been the most ex
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