91
potential (other than water). More recently, however, incorporation of nanoparticulate ceramic particles has been shown to inhibit crystallization of the polymer and
enhance conductivity, while also increasing stability of the film [6]. The mechanism
for this enhancement is not yet fully understood and remains under investigation. In
one example, Chen et al. made cation exchange membranes with embedded silica
nanoparticles to observe the effects. The change in the microstructure of the membrane upon incorporation of nanoparticles was suggested as a reason for enhancement in this case [11]. Specifics of the nanoparticle surface are also of importance
for the ionic conductivity, which will be discussed more throughout this chapter and
depend on whether the nanoparticles are simply blended or functionalized.
It is necessary that the polymer provides a polar environment in which the ions
can be solvated to allow for dissociation and movement. Salts with a bulky counterion (i.e. lithium trifluoromethanesulfonate) are often used due to their lower lattice
energies and resulting higher solubility. PEO is the archetypal ion conducting polymer matrix, as it provides a coordinating environment via the ether’s oxygen, associating with the metal ions whereby the flexible segments of the ethylene oxide
provide a coil around the cation, similar to crown ether solvation of metal cations.
It is versatile in its solubility, allowing for anhydrous and hydrated composites [12,
13]. Solid or quasi-solid polymer electrolyte classes can be divided into three types:
(1) dry polymer-salt electrolytes, (2) solvent-incorporated polymer-salt electrolytes,
and (3) single-ion conducting polymer electrolytes. Nanocomposite mixtures of
these and related polymers will be discussed here. In some cases, composites of
polymers have been made with MOFs as fillers, to be discussed next.
2.2 Coordination Polymers and Metal Organic Frameworks
One of the newest forms of polymers to be developed are metal organic frameworks
(MOFs), which are a subtype of porous coordination polymers (PCPs). A coordination polymer consists of organic and inorganic groups bound together in a repeated
manner. They may maintain order in 1, 2, or 3 dimensions. The term MOF typically
implies 3-dimensional order, although 2-D MOFs are possible, as long as permanent porosity is maintained. Although the terms are not necessarily equivalent as
they have different origins and intentions, MOF and PCP are often used interchangeably. (See ref. [14] with a better description of IUPAC terminology). The inorganic
group might consist of a metal ion or metal cluster, and a diverse array of metals
have been used, from Be to transition metals to lanthanides. These groups are
referred to as the “nodes,” which are linked together via polydentate organic linkers,
which serve as the “struts.” The diversity in possible structures owes to the variations in metal sizes, coordination number, and possible geometries, as well as the
shape, size, and functionalities of the linkers [15]. Many of the original MOFs,
particularly cubic MOFs as part of the isoreticular MOF series (IRMOFs), were
formed from conjugated molecules having two carboxylate groups. In the archetypal MOF-5 (also known as IRMOF-1), [16] these carboxylates are 180° from one
Polymer Nanocomposites for Ion Transport
potential (other than water). More recently, however, incorporation of nanoparticulate ceramic particles has been shown to inhibit crystallization of the polymer and
enhance conductivity, while also increasing stability of the film [6]. The mechanism
for this enhancement is not yet fully understood and remains under investigation. In
one example, Chen et al. made cation exchange membranes with embedded silica
nanoparticles to observe the effects. The change in the microstructure of the membrane upon incorporation of nanoparticles was suggested as a reason for enhancement in this case [11]. Specifics of the nanoparticle surface are also of importance
for the ionic conductivity, which will be discussed more throughout this chapter and
depend on whether the nanoparticles are simply blended or functionalized.
It is necessary that the polymer provides a polar environment in which the ions
can be solvated to allow for dissociation and movement. Salts with a bulky counterion (i.e. lithium trifluoromethanesulfonate) are often used due to their lower lattice
energies and resulting higher solubility. PEO is the archetypal ion conducting polymer matrix, as it provides a coordinating environment via the ether’s oxygen, associating with the metal ions whereby the flexible segments of the ethylene oxide
provide a coil around the cation, similar to crown ether solvation of metal cations.
It is versatile in its solubility, allowing for anhydrous and hydrated composites [12,
13]. Solid or quasi-solid polymer electrolyte classes can be divided into three types:
(1) dry polymer-salt electrolytes, (2) solvent-incorporated polymer-salt electrolytes,
and (3) single-ion conducting polymer electrolytes. Nanocomposite mixtures of
these and related polymers will be discussed here. In some cases, composites of
polymers have been made with MOFs as fillers, to be discussed next.
2.2 Coordination Polymers and Metal Organic Frameworks
One of the newest forms of polymers to be developed are metal organic frameworks
(MOFs), which are a subtype of porous coordination polymers (PCPs). A coordination polymer consists of organic and inorganic groups bound together in a repeated
manner. They may maintain order in 1, 2, or 3 dimensions. The term MOF typically
implies 3-dimensional order, although 2-D MOFs are possible, as long as permanent porosity is maintained. Although the terms are not necessarily equivalent as
they have different origins and intentions, MOF and PCP are often used interchangeably. (See ref. [14] with a better description of IUPAC terminology). The inorganic
group might consist of a metal ion or metal cluster, and a diverse array of metals
have been used, from Be to transition metals to lanthanides. These groups are
referred to as the “nodes,” which are linked together via polydentate organic linkers,
which serve as the “struts.” The diversity in possible structures owes to the variations in metal sizes, coordination number, and possible geometries, as well as the
shape, size, and functionalities of the linkers [15]. Many of the original MOFs,
particularly cubic MOFs as part of the isoreticular MOF series (IRMOFs), were
formed from conjugated molecules having two carboxylate groups. In the archetypal MOF-5 (also known as IRMOF-1), [16] these carboxylates are 180° from one
Polymer Nanocomposites for Ion Transport