(PAMPS) as a polyanion, at high humidity a dc conductivity of σ dc % 10
À5 S cm
À1
was achieved, which is in a realistic range for applications [28]. Different polyelectrolyte pairs tested for their conductivity even involved polymers known as ion
conductors in their bulk form, such as Nafion as an established proton conductor
[28], PEO as a classical polymer electrolyte [29] or polyphosphazene, an optimized
polymer electrolyte providing flexible main chains [30, 31].
However, despite these studies on conductivities of different types of PEM,
a fundamental understanding of ion transport properties in PEM is not yet in sight.
A major problem in the interpretation of conductivity data is the lack of knowledge
about the composition of the films. Because PEM are formed by self-assembly, the
compensation of surface charges upon chain adsorption controls the stoichiometry
of the films. The excess charges of an outermost polyelectrolyte layer might
become fully compensated by the oppositely charged segments of the subsequent
layer (“intrinsic charge compensation”), or, if this is sterically not favorable, small
counterions might incorporate into the film in order to compensate the polyion
charges (“extrinsic charge compensation”). Thus, PEM are a material of unknown
stoichiometry. Though in first approximation the gain of translational entropy of the
small counterions would always lead to intrinsic charge compensation being
favored, in a number of polyion combinations a deviation from a 1:1 stoichiometry
of the polyions has been found and a substantial degree of extrinsic charge
compensation by small counterions was concluded [32–34].
In hydrated multilayers, even protons can contribute to the conductivity.
Several authors employ Nafion in multilayer formation and discuss protons as the
dominating charge carriers in the dc conductivity [35–38]. The contribution of other
ions was found to be negligible in hydrated PEM [39]. Indeed, the hydration state of
the layer assembly has a strong influence on the conductivity, as shown when films
are compared in the dry and the completely hydrated state [27, 28, 35]. A review of
the activities up to 2007 is given by Lutkenhaus and Hammond [38]. It is generally
argued that protons carry the current; however, an analysis of the contributions of
other small counterions suffers from the lack of systematic knowledge about the
composition of PEM.
Therefore, PEC act as a model material with the same local molecular structure
of the complex, but have the advantage of a variable stoichiometry and known ion
content. In PEC, the content of small cations and anions is known because it
depends on the mixing ratio of the polyions. Furthermore, systems with mainly
one type of counterion can be prepared if excess salt is removed by dialysis. In this
way, conductivity data in dependence of the composition can be related to the
conductivity contribution of a single type of charge carrier [40, 41]. For this
purpose, solid PEC complexes have to be prepared from complexes formed in
aqueous solution. The broad composition range includes both water-soluble as well
as insoluble complexes, i.e. complex coacervates. Both can be treated by drying and
subsequently pressing the polymer material to form a dense solid [40].
In this article we review the knowledge gained from impedance spectroscopy on
such solid PEC materials. Dry and humidified complexes are investigated, the latter
in systematic dependence of the relative humidity (RH), both as a function of
composition and of temperature.
100
C. Cramer and M. Scho ¨nhoff
À5 S cm
À1
was achieved, which is in a realistic range for applications [28]. Different polyelectrolyte pairs tested for their conductivity even involved polymers known as ion
conductors in their bulk form, such as Nafion as an established proton conductor
[28], PEO as a classical polymer electrolyte [29] or polyphosphazene, an optimized
polymer electrolyte providing flexible main chains [30, 31].
However, despite these studies on conductivities of different types of PEM,
a fundamental understanding of ion transport properties in PEM is not yet in sight.
A major problem in the interpretation of conductivity data is the lack of knowledge
about the composition of the films. Because PEM are formed by self-assembly, the
compensation of surface charges upon chain adsorption controls the stoichiometry
of the films. The excess charges of an outermost polyelectrolyte layer might
become fully compensated by the oppositely charged segments of the subsequent
layer (“intrinsic charge compensation”), or, if this is sterically not favorable, small
counterions might incorporate into the film in order to compensate the polyion
charges (“extrinsic charge compensation”). Thus, PEM are a material of unknown
stoichiometry. Though in first approximation the gain of translational entropy of the
small counterions would always lead to intrinsic charge compensation being
favored, in a number of polyion combinations a deviation from a 1:1 stoichiometry
of the polyions has been found and a substantial degree of extrinsic charge
compensation by small counterions was concluded [32–34].
In hydrated multilayers, even protons can contribute to the conductivity.
Several authors employ Nafion in multilayer formation and discuss protons as the
dominating charge carriers in the dc conductivity [35–38]. The contribution of other
ions was found to be negligible in hydrated PEM [39]. Indeed, the hydration state of
the layer assembly has a strong influence on the conductivity, as shown when films
are compared in the dry and the completely hydrated state [27, 28, 35]. A review of
the activities up to 2007 is given by Lutkenhaus and Hammond [38]. It is generally
argued that protons carry the current; however, an analysis of the contributions of
other small counterions suffers from the lack of systematic knowledge about the
composition of PEM.
Therefore, PEC act as a model material with the same local molecular structure
of the complex, but have the advantage of a variable stoichiometry and known ion
content. In PEC, the content of small cations and anions is known because it
depends on the mixing ratio of the polyions. Furthermore, systems with mainly
one type of counterion can be prepared if excess salt is removed by dialysis. In this
way, conductivity data in dependence of the composition can be related to the
conductivity contribution of a single type of charge carrier [40, 41]. For this
purpose, solid PEC complexes have to be prepared from complexes formed in
aqueous solution. The broad composition range includes both water-soluble as well
as insoluble complexes, i.e. complex coacervates. Both can be treated by drying and
subsequently pressing the polymer material to form a dense solid [40].
In this article we review the knowledge gained from impedance spectroscopy on
such solid PEC materials. Dry and humidified complexes are investigated, the latter
in systematic dependence of the relative humidity (RH), both as a function of
composition and of temperature.
100
C. Cramer and M. Scho ¨nhoff