encountering any of the leakage problems typical for all low molecular weight ILs.
Today, four strategies lead to nanoPILs, mesoporous systems, and unconventional
nanostructured polyelectrolyte hybrid materials:
1. Self-assembly of amphiphilic and liquid crystalline PILs
2. Nanophase separation of IL blends with amphiphilic block copolymers
3. In-situ formation of nanoparticle dispersions in (P)ILs
4. Design of molecular nanoPIL architectures resembling micelles
The latter strategy is of particular interest for the development of robust
nanoPILs as polymer additives, which must sustain high shear forces and the
elevated temperatures typical for polymer melt processing.
2 Nanostructured Linear PILs
A wide variety of linear PILs have been prepared by free radical polymerization of
vinyl, (meth)acrylic, and styrenic monomers containing IL moieties [14, 16, 17]. In
PILs, the IL moieties are incorporated either in the PIL backbone or in the PIL side
chains. Selected members of the linear PIL family are displayed in Fig. 1. The
subsequent anion exchange of the counter anions enables facile tuning of the PIL
properties [18, 19]. Prominent representatives of PILs are polycations such as poly
(1-vinyl-3-alkylimidazolium), and polyanions such as poly(styrene sulfonate), which
contains IL moieties as counter cations. In addition to random copolymers, a variety
of block copolymers and zwitterionic PILs have been prepared [17, 20–24]. Among
them are film-forming PILs exhibiting very low glass transition temperatures (T g )
below À75
C, combined with high ion conductivity [21, 25–28].
In the first strategy, the formation of nanoPILs exploits nanophase separation
through self-assembly of liquid crystalline PILs [14, 29–36] and of amphiphilic PIL
and PIL block copolymers [23, 24, 37, 38]. For example, Kato and coworkers
polymerized an ion-conductive mesogenic monomer containing an IL moiety, thus
enabling the formation of self-standing liquid crystalline polymer films. Upon
macroscopic orientation, layered nanostructures are formed. Such oriented films
exhibit high ion conductivity of 10
À2 S cm
À1 , as measured at 150
C in the smectic
A phase [29]. In a recent advance, Binder and coworkers reported on the synthesis
of nanostructured thermally stable nanoPILs (see PIB-IL in Fig. 1). Using click
chemistry, they attached hydrophobic polyisobutylene (PIB) to methylimidazolium, pyrrolidinium, and triethylammonium IL moieties, which govern
nanostructure formation, rheology, and relaxation times at constant PIB chain
length [38].
According to the second strategy, polymer electrolytes as well as nonionic block
copolymers are dissolved in ILs in order to achieve nanostructure formation of
polymer-based ionic systems by nanophase separation. A great variety of polymer
electrolytes have been integrated into ILs with the aim of improving the ion
transport properties of electrochemical devices such as lithium ion batteries, fuel
Nanostructured Polymeric Ionic Liquids
433
Today, four strategies lead to nanoPILs, mesoporous systems, and unconventional
nanostructured polyelectrolyte hybrid materials:
1. Self-assembly of amphiphilic and liquid crystalline PILs
2. Nanophase separation of IL blends with amphiphilic block copolymers
3. In-situ formation of nanoparticle dispersions in (P)ILs
4. Design of molecular nanoPIL architectures resembling micelles
The latter strategy is of particular interest for the development of robust
nanoPILs as polymer additives, which must sustain high shear forces and the
elevated temperatures typical for polymer melt processing.
2 Nanostructured Linear PILs
A wide variety of linear PILs have been prepared by free radical polymerization of
vinyl, (meth)acrylic, and styrenic monomers containing IL moieties [14, 16, 17]. In
PILs, the IL moieties are incorporated either in the PIL backbone or in the PIL side
chains. Selected members of the linear PIL family are displayed in Fig. 1. The
subsequent anion exchange of the counter anions enables facile tuning of the PIL
properties [18, 19]. Prominent representatives of PILs are polycations such as poly
(1-vinyl-3-alkylimidazolium), and polyanions such as poly(styrene sulfonate), which
contains IL moieties as counter cations. In addition to random copolymers, a variety
of block copolymers and zwitterionic PILs have been prepared [17, 20–24]. Among
them are film-forming PILs exhibiting very low glass transition temperatures (T g )
below À75
C, combined with high ion conductivity [21, 25–28].
In the first strategy, the formation of nanoPILs exploits nanophase separation
through self-assembly of liquid crystalline PILs [14, 29–36] and of amphiphilic PIL
and PIL block copolymers [23, 24, 37, 38]. For example, Kato and coworkers
polymerized an ion-conductive mesogenic monomer containing an IL moiety, thus
enabling the formation of self-standing liquid crystalline polymer films. Upon
macroscopic orientation, layered nanostructures are formed. Such oriented films
exhibit high ion conductivity of 10
À2 S cm
À1 , as measured at 150
C in the smectic
A phase [29]. In a recent advance, Binder and coworkers reported on the synthesis
of nanostructured thermally stable nanoPILs (see PIB-IL in Fig. 1). Using click
chemistry, they attached hydrophobic polyisobutylene (PIB) to methylimidazolium, pyrrolidinium, and triethylammonium IL moieties, which govern
nanostructure formation, rheology, and relaxation times at constant PIB chain
length [38].
According to the second strategy, polymer electrolytes as well as nonionic block
copolymers are dissolved in ILs in order to achieve nanostructure formation of
polymer-based ionic systems by nanophase separation. A great variety of polymer
electrolytes have been integrated into ILs with the aim of improving the ion
transport properties of electrochemical devices such as lithium ion batteries, fuel
Nanostructured Polymeric Ionic Liquids
433