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Design and properties of supramolecular polymer gels

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Atsushi Noro,* Mikihiro Hayashi and Yushu Matsushita*
Received 19th January 2012, Accepted 20th March 2012
DOI: 10.1039/c2sm25144b
Supramolecular polymer gels are precisely designed physical gels brought together by reversible
secondary interactions to form three dimensional networks of melt macromolecules. Generally, theydiffer from supramolecular gels because they are comprised of polymers instead of low molecularweight compounds. Recently, much effort has focused on designing supramolecular polymer gels andrelated materials with excellent properties; indeed, improvements have been made in theirsupramolecular interactions, complementarity in the non-covalent bonding units, the nature of themacromolecular building blocks, and strand elasticity of supramolecular polymer networks. Owing to
the precise molecular design, they represent nanophase separation and characteristic viscoelasticity.
Here, we review supramolecular polymer gels in terms of molecular design, morphology, and rheology.
We also discuss future directions in practical application of supramolecular polymer gels.

1. Introduction‘‘Polymer gels’’1 are among the most attractive and versatile soft
materials. They are used in our daily lives for contact lenses,
superabsorbent polymers, etc., and more high-tech applications
are in development.2–11 Polymer gels typically contain a large

Department of Applied Chemistry, Graduate School of Engineering,
Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan.
E-mail: yushu@apchem.nagoya-u.ac.jp; noro@nagoya-u.jp; Fax: +81-52789-3210; Tel: +81-52-789-4604

Atsushi Noro is an AssistantProfessor in the Department ofApplied Chemistry at Nagoya
University. He earned his PhD
degree in Applied Chemistry at
Nagoya University in 2006
under the direction of Prof.
Yushu Matsushita. After that,
he joined Prof. Timothy P.
Lodge’s Group at the Department of Chemistry, Universityof Minnesota, as a JSPS postdoctoral fellow. He rejoined
Atsushi Noro
Nagoya University in 2007. In
2011, he was a Visiting Scientist
in Prof. Edwin L. Thomas’sGroup, Department of Materials Science and Engineering,
Massachusetts Institute of Technology. The main focus of his
research is the design and properties of supramolecular and nanohybrid materials.
6416 | Soft Matter, 2012, 8, 6416–6429

amount of solvent,12 which gives them a characteristic softness
originated from a liquid nature. On the other hand, they canmaintain shape like solid materials, unless excessive stress isapplied. Consequently, the combination of softness and shaperetention ability provides unique properties, particularly
mechanical properties.13–16
To understand the interesting mechanical properties of polymer gels, they should be considered at the molecular level.
Polymer gels are regarded as three dimensional networks ofcross-linked, melt macromolecules, typically surrounded bya large number of solvent molecules. This polymer network

Mikihiro Hayashi earned his B.
Eng in Applied Chemistry in2010 at Nagoya University. He
is currently a graduate student in
the Department of Applied
Chemistry at Nagoya University
under the direction of Prof.
Yushu Matsushita. His PhD
study focuses on the basic properties and applications ofsupramolecular polymer gels.

Mikihiro Hayashi

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consists of several elements, including bridging strands, crosslinks (or junctions), dangling ends, and loops. The latter twoelements can be regarded as defects in the network; the former
two are indispensable for maintaining a network. Here,a bridging strand is defined as a polymer chain that connects one
cross-link to another. Three or more strands1,17–20 must be connected to one cross-link to form a network structure.
On the other hand, a ‘‘supramolecular polymer gel’’21 (alsoknown as a supramacromolecular gel22,23), the subject of thisreview, can be regarded as three dimensional networks of
non-covalently bound, melt macromolecules. Here, the term‘‘supramolecular’’24,25 is defined as a state of self-assembling via
non-covalent bonding or interactions. These precisely designed
physical gels are different from polymer gels in that their bonding
is reversible; thus they are semi-solid at room temperature andliquefy at high temperatures. Like polymer gels, a supramolecular polymer gel is composed of polymer components termed
macromolecular building blocks (macro-building blocks);
however, these macro-building blocks are generally designed toparticipate in attractive interactions to form the non-covalentcross-links of a supramolecular polymer network. At the sametime, the macro-building blocks also include parts that are not
involved in non-covalent bonding, which typically serve as
bridging strands for network formation of supramolecularpolymer gels. Note that a similar term ‘‘supramolecular gel’’26–28
typically refers to physical gels built of low molecular weightgelators; another similar term ‘‘supramolecular polymers’’29–31
refers to a polymeric array built of monomeric units that arebrought together through reversible, highly directional
secondary interactions. Thus, the term ‘‘supramolecular polymer
gels’’ refers to a type of physical gel with a network structure, butbuilt of ‘‘polymer’’ chains.
As for molecular design for supramolecular polymer gels,there are two main types of design. One design uses single
polymer chains that include randomly arrayed, non-covalentbonding units.32–35 The other design is more precise: the noncovalent bonding units are attached at specific locations on the

Yushu Matsushita is currently
a Professor of Applied Chemistry at Nagoya University. He
has also served as a Vice-President of the University since2007. He received his Dr Eng in
Synthetic Chemistry, School ofEngineering at Nagoya University under the supervision ofProf. Mitsuru Nagasawa. Dr
Matsushita joined Nagoya
University in 1982; in 1994, hemoved to the Institute for Solid
Yushu Matsushita
State Physics, University ofTokyo. He rejoined Nagoya
University in 1999. He received
the Wiley Polymer Science Award in 1999 and the Award ofPolymer Science in 2007, from the Society of Polymer Science,
Japan (SPSJ). His research focus is the molecular design ofcomplex polymeric systems and control of their morphology.
This journal is ª The Royal Society of Chemistry 2012chains, particularly on both ends.36,37 In either case, when selfcomplementary non-covalent bonding units are used, a single
polymer chain can serve as both a bridging strand and a crosslink to produce a supramolecular polymer gel.
When we look at the gels in terms of chemical nature, strands
and cross-links are very different from each other. Thus they are
apt to repel each other at the nanoscopic level. The distance
between cross-links is easily defined in supramolecular polymer
gels, because they are built according to precise molecular designs.
Based on design, nanophase separation,38 also conventionallycalled ‘‘microphase separation’’,39–46 can be engineered intosupramolecular polymer gels. In fact, recent studies havedescribed supramolecular polymer gels that represented nanophase separation. However, because attractive interactions are
considered a key to form supramolecular polymer networks, mostresearchers have focused on gel preparation and physical properties, particularly viscoelasticity. In contrast, few studies havefocused on nanophase separation in supramolecular polymer gels.
In this review, we summarize the basic properties of supramolecular polymer gels. We focus on the molecular design,morphology, and viscoelasticity. We consider the molecular scale(preparation), nanoscopic scale (nanophase separation), andmacroscopic scale (viscoelasticity) of supramolecular polymer gels.
We also suggest some future directions in applications of the gels.

2. Molecular design and preparation
There are many reports and reviews on supramolecular polymer
gels and related materials.47–50 However, they mainly focused on
non-covalent bonding units themselves that connect macrobuilding blocks. Here, we first summarize the classifications of
non-covalent bonding (or supramolecular interactions) for
preparation. We also discuss the important features of themolecular design, including complementarity, macro-building
blocks, and strand elasticity.
2.1 Supramolecular interactions between non-covalent bondingunits
2.1.1 Hydrogen bonding. Hydrogen bonding is the mostversatile interaction that can be used for preparing supramolecular materials.34,36,51–56 Many functional groups can formhydrogen bonds. Stadler et al. used urazole and benzoic acid units
for hydrogen bonding.57–59 However, urazole and benzoic acid are
not a well-matched complementary pair; therefore, hydrogenbonding was sometimes formed between two urazole units or
between two benzoic acid units. Sijbesma and Meijer et al.
developed a novel, self-complementary, hydrogen bonding arrayknown as ureidopyrimidone; this molecule formed supramolecular polymers and gels with more precise designs than thoseformed by Stadler et al. (Fig. 1).60,61 Although ureidopyrimidone
is an excellent functional unit, it was only useful for selfcomplexation. Later, hetero-complementary pairs (e.g., cyanuric
acid and diaminopyrimidine-substituted isophthalamide) weresynthesized for preparing supramolecular assemblies of small
molecules.62,63 Inspired by those works, Binder et al. incorporated
heterocomplementary hydrogen bonding units into macrobuilding blocks for building supramolecular assemblies ofpolymers.64–67 All the units described above were completely
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Fig. 1 Dimerization of 2-ureido-4[1H]-pyrimidone via self-complementary hydrogen bonding. Reprinted with permission from ref. 60. Copyright 1998 American Chemical Society.

synthetic; nevertheless, there are naturally occurring, unique,heterocomplementary hydrogen bonding motifs, for example, thenucleotides that form DNA. In fact, some studies have preparedsupramolecular polymer materials based on nucleotide units.68–72
2.1.2 Metal-to-ligand coordination. Metal-to-ligand coordination is another popular interaction to bind building blocks.73–75
Thanks to the development and improvement of polymer
synthesis in recent years, this interaction has been used preferably to build up supramolecular assemblies of polymers.76–81
Although the assemblies were more like linear polymers, it ismeaningful to note the work of Velten and Rehahn et al. They
studied complexes of phenanthroline units and metal ions (Ag+ orCu+).82–84 Schubert et al. described supramolecular polymer
assemblies made from terpyridine units and metal ions (e.g., Ru2+
or Fe2+).85–94 Terpyridine units and these metal ions readily formed
a 2 : 1 complex (Fig. 2); therefore, they described their polymer
assemblies as linear polymers rather than a network. Rowan et al.
showed that bis(benzimidazolyl) pyridine units and trivalent
metal ions like La3+ and Eu3+ formed supramolecular polymer gelsin solvents.37,95–101 In addition, Craig et al. prepared supramolecular polymer gels from a complex of organopalladium and pyridine. They have been intensely studying viscoelasticity.102–105

this concept was applied to macromolecular systems.110–123
Supramolecular complexation with an acid and a base in solution
produces separated cation–anion pairs; in bulk, this complexation occurs within confined nanospaces and ion pairing occurs
through hydrogen bonding. This is termed ionic hydrogenbonding.124–128 Ionic interactions are not so directional, but they
can be used to generate branches at cross-links, which facilitates
use of the interactions within macromolecular systems. Therefore, ionic interactions hold promise for building up supramolecular polymer gels. Our recent study described simple
preparation for these types of gels; a carboxy-terminated telechelic polymer was mixed with a polymer that had many aminogroups in a bulk state (Fig. 3).21
Organic cation and anion pairs are also formed by mixing
anion salts (cf. sulfonate and carboxylate) and cation salts (cf.
pyridinium and ammonium).129,130 In fact, well-defined supramolecular polymer gels were formed recently in water by mixing
ionic ABA triblock copolymers (Fig. 4).131 Due to the ioniccharges, very clear nanophase separated structures wereobserved. In another study, hybrid supramolecular polymer gelswere successfully prepared by mixing sodium polyacrylatecoated clay nanosheets and dendritic molecules with multiple
guanidium ions in water.132 This fascinating interaction will be
studied in more detail in the future.
2.1.4 Inclusion complexation. Because non-covalent interactions are sometimes (or often) essential for inclusion complexation,
we consider inclusion complexation in the class of supramolecular

2.1.3 Ionic interactions. The term ‘‘ionic interactions’’ typically refers to electrostatic or Coulombic interactions; here, we
use this term specifically to describe interactions between organic
cations and organic anions.106–109 Organic cations and anions
readily form pairs by mixing an acid and a base, a well-known
neutralization reaction in classical and basic chemistry. Recently,

Fig. 2 Complex formation of terpyridine units and metal ions via metalto-ligand coordination. Reprinted with permission from ref. 85. Copyright 2002 John Wiley & Sons, Inc.

6418 | Soft Matter, 2012, 8, 6416–6429

Fig. 3 Schematic illustrations show macro-building blocks and thechemical structures of supramolecular polymer gels. Carboxylic acids on
one macro-building block (a) and many amines on the other building
block (b) form acid–base complexes (c). These complexes serve as crosslinks in polymer networks (c, inset). The blend of macro-building blocks
in bulk forms a gel at room temperature (photograph). Reprinted withpermission from ref. 21. Copyright 2011 RSC Publishing.

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complementary units contain both key and keyhole motifs, andthey connect by aligning appropriately.166–175 Heterocomplementary units are used in more definite molecular designs,
as shown by Lehn et al. In this design (Fig. 6), one unit serves asa key and the other as a keyhole for molecular recognition.63
Metal-to-ligand coordination and inclusion complexation
always represent heterocomplementary designs;176 they require
two different functional units to build supramolecular assemblies. Hydrogen bonding and ionic interactions are sometimes
self-complementary; thus, they require only one type of macrobuilding block for non-covalent bonding formation. In any case,heterocomplementary systems are more reliable for definedmolecular designs.
2.3 Macro-building blocks

Fig. 4 Formation of supramolecular polymer gels in water by mixing
ionic ABA triblock copolymers. (a) Chemical structure of a cationic ABA
triblock copolymer with guanidinium units. (b) Chemical structure of ananionic ABA triblock copolymer with sulfonate units. (c) Schematic
illustration of gelation in water formed via ionic interactions. Reprinted
with permission from ref. 131. Copyright 2011 John Wiley & Sons, Inc.

interactions. In fact, inclusion complexation is a powerful tool for
preparation of supramolecular polymer materials.133–139 Takata
et al. described the preparation of supramolecular polymer gels via
inclusion complexation between crown ether units and ammonium
salts (Fig. 5).140–144 More recently, Huang and Liu et al. produced
more defined gels with the same pair.145 Although this molecular
design is chemically elegant,146–160 it is complicated, because
functional units must be attached to the polymer side chains orends to synthesize macro-building blocks for supramoleculargelation. The simplified synthetic procedures will make thiscomplexation useful for the preparation.

Supramolecular polymer gels as well as covalent-bonded polymer gels comprise three dimensional networks, where each crosslink has more than two degrees of functionality. Therefore, thesimplest molecular design is to synthesize a macro-building blockwith three or more branches with self-complementary noncovalent bonding units attached to the branch ends. On the other
hand, a heterocomplementary molecular design typicallyrequires two kinds of building blocks. One building block musthave more than two non-covalent bonding units attached thatcan form branches when it is mixed with the other building block.
This requirement can be satisfied with a mixture of a bifunctional
small molecule and a multifunctional linear polymer (Fig. 7).177
A supramolecular polymer gel can also be formed from a mixtureof a linear polymer functionalized at both ends and a multifunctional polymer (Fig. 3).21
2.4 Elasticity of bridging strands
Supramolecular polymer gels require some molecular flexibility
in the network, i.e., chain elasticity. To achieve this, solvents can

2.2 Complementarity
There are two types of complementarity: self-complementarity(homocomplementarity) and heterocomplementarity.161–165 Self-

Fig. 5 Supramolecular polymer networks via inclusion complexation
between crown ether units and ammonium salts. Reprinted withpermission from ref. 140. Copyright 2004 John Wiley & Sons, Inc.

This journal is ª The Royal Society of Chemistry 2012

Fig. 6 Two heterocomplementary hydrogen bonding units (A ¼ keyhole
unit, and B ¼ key unit) form a functional pair. Reprinted with permission
from ref. 63. Copyright 2002 John Wiley & Sons, Inc.

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be similar. Therefore, we first review phase separation behaviourin supramolecular assemblies without networks.

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3.1 Morphologies of supramolecular assemblies of polymers
without network structures

Fig. 7 Schematic illustration of the formation of a supramolecularpolymer network. (Left) Multifunctional linear polymers with acceptor
units are mixed with (1) bifunctional small molecules with donor units.
Reprinted with permission from ref. 177. Copyright 2006 John Wiley &Sons, Inc.

be added to make the network strands elastic. Solvents can lower
the glass transition temperature of network strands; the quantitative effect can be approximated with the Fox equation or otherequations.178 Adding solvents is the simplest way to renderstrands elastic, but the drawback of this approach is that solvents
can evaporate, and this can cause supramolecular polymer gels tolose viscoelastic reversibility.
Recently, we developed use of an ionic liquid179–185 as a nonvolatile solvent for preparing supramolecular polymer gels.22,23
This molecular design (Fig. 8) enabled an exploration of thetemperature dependence of viscoelasticity over a wide range oftemperatures to nearly 200  C. Another novel molecular designthat precludes solvent evaporation is to use melt macro-building
blocks186–188 that have low glass transition temperatures below
room temperature (Fig. 3).21 The simplicity of this molecular
design will facilitate future investigations. It should be noted thatsupramolecular polymer gels do not always require solvents.

3. Nanophase separation and density fluctuation
As mentioned in Section 1 (Introduction), few reports havefocused on nanophase separation in supramolecular polymer
gels. One potential reason is that the phenomenon is difficult toobserve directly. However, knowledge of phase separation
provides better understanding of their viscoelastic properties.
There are more reports on phase separation in supramolecularassemblies without networks than on phase separation insupramolecular polymer gels, but the phenomena are expected toFig. 8 Schematic representation of a supramolecular polymer network
connected via hydrogen bonding in an ionic liquid. One macro-building
block is a telechelic-type polymer (pink) with ends of hydrogen bonding
units (green), and the other is a homopolymer (yellow) with manyhydrogen bonding units. Because the hydrogen bonded cross-links(magnified in the circle) have a finite lifetime, the gel state (right) and theliquid state (left) are reversible.

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3.1.1 Nanophase separation in block supramacromolecules.
One of the most studied supramolecular assemblies of polymers
representing nanophase separation is a ‘‘block-type supramacromolecule’’189–191 (or block supramacromolecule; Fig. 9b).
This term actually refers to a ‘‘block-type supramolecularassembly of polymers’’; in fact, some researchers favour the term‘‘supramolecular block copolymer.’’192,193 The latter is easy toremember and use, but it does not clearly express how supramolecular it is.194 Furthermore, the latter term can be misleading,because block copolymers themselves are sometimes used as
macro-building blocks for preparing195 ‘‘supramolecular block
copolymers.’’ To avoid confusion, we used the term ‘‘blocksupramacromolecule’’ throughout this paper.
From the 1960’s, it has been well established that block
copolymers represent nanophase separated structures when theconstituent blocks are immiscible.196–201 Here, nanophase separated structures are defined as ordered structures of several tensof nanometres (sometimes smaller, sometimes larger), that are
expected to become highly functional materials. In recent years,
many researchers in this field have investigated the morphologyof block supramacromolecules.202–207 In a pioneering work
performed by Russell et al., block supramacromolecules
were prepared with ionic interactions.208 The design included
macro-building blocks of two different kinds of polymers with
non-covalent bonding end-groups. The nanophase separated
structures were detected by small angle X-ray scattering (SAXS).
Similar molecular designs were employed in other studies, where
two different kinds of polymers with non-covalent bonding endgroup(s) were mixed to build block supramacromolecules withionic interactions.209–216
Recently, more precise molecular designs for supramacromolecules were employed with metal-to-ligand coordination,217–224 inclusion complexation,225–230 and hydrogenbonding.231–236 Some studies, including ours,189,190 have reported
the formation of nanophase separated structures from blocksupramacromolecules. SAXS and transmission electron microscopy (TEM) are the preferred methods for detecting periodic
structures.

Fig. 9 Schematic of different types of block-type assemblies. (a) A block
copolymer molecule has a junction with a covalent bond. (b) A blocksupramacromolecule has a junction with a non-covalent interaction; theblock supramacromolecule will dissociate into a polymer blend of macrobuilding blocks in response to external stimuli, like heat or applied stress.
The polymer blend will reform into a block supramacromolecule whenthe external stimuli are removed.

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3.1.2 Thermoresponsive morphology of block supramacromolecules. For the past decade, a central interest for
researchers in this field has been whether block supramacromolecules represent nanophase separated structures. This
question was addressed in several studies mentioned above. The
next issue was the thermo-responsiveness of block supramacromolecule morphology. The morphology of block supramacromolecules was expected to be thermally responsive,
because they are built of non-covalent bonding units, which
provide a temporary connection between macro-building blocks;
thus, high temperatures can weaken the interaction strength.
Conversely, at low temperatures, block supramacromolecules via
non-covalent bonding should behave as block copolymers representing nanophase separated structures. On the other hand,block supramacromolecules may not maintain an orderedstructure at high temperatures; they may behave as an immiscible
polymer blend, known as a macrophase separated mixture ofmacro-building blocks (Fig. 10). This thermo-responsiveness is
not typical in inorganic materials; therefore, this functionality ofblock supramacromolecules is expected to be developed for
creating novel thermoresponsive nanostructured materials.
Several studies have described the temperature dependence of
nanophase
separatedstructuresinblock
supramacromolecules.231,232,237 A pioneering study by Russell et al.
used SAXS to follow structural changes in block supramacromolecules. Increasing the temperature to around 300  Ctypically caused the polymer samples to degrade.208 Therefore,
they could not clearly determine the mechanism underlying
morphological transitions in block supramacromolecules. To
elucidate this issue, several recent studies have used different
molecular designs to investigate the temperature dependence of
nanophase separated morphology. For example, Noro et al.
added an ionic liquid as a non-volatile solvent to remove theeffects of glass transition temperatures of macro-building blocks.
The solvents also serve as a filler for the dissolved phase.
Then, the temperature was changed from 30 to 110  C and thestructural changes were followed by SAXS to determine theFig. 10 Schematic illustration of thermoreversible morphology. The
block supramacromolecule structure is represented on the molecularscale (upper section) and macromolecular scale (lower section). Changes
in temperature induce the transition between a nanophase separated
structure (left) and a macrophase separated mixture (right). Reprinted
with permission from ref. 191. Copyright 2009 American Chemical
Society.

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thermo-responsiveness of the morphology.191 At the lower
temperatures (e.g., 30  C), there were several sharp peaks withrelative scattering vector values of 1, 2, 3, and 4; this clearly
indicated a lamellar nanophase separated structure. In contrast,
no peaks appeared at temperatures above 90  C; this might haveindicated macrophase separation due to weakened interactions
between macro-building blocks (Fig. 11). This system wasinteresting because the morphological behaviour was completely
thermoreversible. The design should be simplified to facilitate its
application to preparation of thermoresponsive nanostructured
materials.
3.1.3 Phase of non-covalent bonding units. To approach the‘‘ideal’’ block supramacromolecule, which, in the extreme case,includes only ‘‘one’’ non-covalent bonding unit, a small numberof non-covalent bonding units is desirable. On the other hand,a large number of non-covalent bonding units provides greaterinteraction strength, which favours stable macro-building blockconnections. For instance, in the case of hydrogen bonding,multiple hydrogen bonds are typically utilized to ensure tightly
bound building blocks; in turn, these bonds occupy relatively
large volume fractions, and therefore, they may form a newphase.238–241
In fact, some reports on the nanophase separation of blocksupramacromolecules have demonstrated the formation of a newphase. Jiang et al. described nanophase separation in blocksupramacromolecules formed by mixing a polystyrene-bpoly(1,2-butadiene)-b-poly(methacrylic acid) triblock copolymer

Fig. 11 SAXS profiles of a block supramacromolecule in an ionic liquid
at various temperatures. The temperature was increased from 30  C to110  C, and then, decreased again to 30  C. Reprinted with permission
from ref. 191. Copyright 2009 American Chemical Society.

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with a polystyrene-b-poly(2-vinylpyridine) diblock copolymer.
There, the poly(methacrylic acid) and poly(2-vinylpyridine)
formed a hydrogen-bonded complex phase.242 Similar, but more
precise molecular designs have also resulted in block supramacromolecules with a clear nanophase separated structure anda complex new phase. Moreover, three-phase nanophase separated structures have been clearly observed in diblock–diblock243–245 and even diblock–triblock246 copolymer mixtures,
when the parent block copolymers carried hydrogen bonding
moieties (Fig. 12). Our recent work also demonstrated thata mixture of end-functionalized homopolymers and multifunctional homopolymers resulted in nanophase separated structures
made of grafted supramacromolecules via ionic hydrogenbonding (Fig. 13).247

3.2 Morphologies of supramolecular polymer gels
3.2.1 Nanophase separation in supramolecular polymer gels.
As described above, supramolecular assemblies represent nanophase separated structures that often have a phase of noncovalent bonding units, when the volume fraction of non-covalent bonding units is large. Accordingly, supramolecular polymer
gels should also exhibit nanophase separation with phases ofcross-links and strands. Although some works have implied that
nanophase separation plays an important role in gelation,
definitive experimental results were not available until recently.
In fact, our recent work is one of the most detailed reports todemonstrate clear experimental results on the occurrence of
nanophase separation. We prepared supramolecular polymer
gels by mixing a poly(2-vinylpyridine)-b-poly(ethyl acrylate)-bpoly(2-vinylpyridine) (VEAV) triblock copolymer and apoly(4-hydroxystyrene) (H) in an ionic liquid (Fig. 8). Thepoly(2-vinylpyridine) (V) and H formed a hydrogen-bonded
cross-link phase, which was excluded from the ionic liquid. On

Fig. 12 TEM image of a block supramacromolecule with hydrogenbonding created by mixing polystyrene-b-poly(4-hydroxystyrene) (Mn ¼
86k and fpolystyrene ¼ 0.86) and polyisoprene-b-poly(2-vinylpyridine)(Mn ¼ 80k and fpolyisoprene ¼ 0.91). The sample was stained with osmium
tetroxide. The lightest phase is polystyrene, the next darkest phase ispolyisoprene, and the darkest phase (spherical shapes) is a hydrogenbonded complex of poly(4-hydroxystyrene)–poly(2-vinylpyridine).
Reprinted with permission from ref. 243. Copyright 2005 American
Chemical Society.

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Fig. 13 Schematics of the chemical structures and macro-building
blocks that formed nanophase-separated supramolecular assemblies. (a)Carboxy-terminated poly(dimethylsiloxane). (b) Polyethyleneimine. (c)Supramolecular assemblies of these macro-building blocks. Reprinted
with permission from ref. 247. Copyright 2011 American Chemical
Society.

the other hand, a strand fraction of poly(ethyl acrylate) was
soluble in the ionic liquid. Therefore, with SAXS, we detected
nanophase separation between the matrix of poly(ethyl acrylate)
and the spherical (or cylindrical) cores of a hydrogen-bonded,cross-linked phase (Fig. 14).23 It should be noted that, in the ionicliquid, VEAV itself did not show a scattering peak. However,when VEAV was mixed even with a small amount of H in anionic liquid, it showed very broad peaks; this suggested that theaddition of H definitely induced nanophase separation.
A hydrogen-bonded, cross-link phase can be disassembled into
individual polymers at high temperatures or with high energy
stress. Therefore, we investigated the thermoresponsive nature of
the morphology of the supramolecular polymer gel in an ionicliquid (also called supramacromolecular ion gels or supramolecular ion gels).22,23 An ionic liquid is a non-volatile solvent;thus, we could investigate morphologies at comparatively hightemperatures (e.g., 180  C). At lower temperatures (e.g., 30  C),
the SAXS profile showed a peak at around 0.2 nm1 and a bumpat 0.35 nm1, which indicated the presence of an ordered nanophase separated structure. In contrast, a single and comparatively broad peak appeared at higher temperatures (e.g., 180  C),
which suggested a disordered state. The analysed data are shownin Fig. 15. As the temperature increased, the intensity of the firstpeak decreased and the peak width became broader. Thisphenomenon indicated an order-to-disorder transition, but thetransition occurred over a wide range of temperatures. As
described below, this order-to-disorder transition corresponds to
the gel-to-liquid transition that could be detected with oscillatory
shear measurements; it indicated the transition between assembled and disassembled supramolecular polymer networks.
3.2.2 Density fluctuation in supramolecular polymer gels.
Although the nanophase separation phenomenon appears to beimportant for gelation,248–254 Feldman et al. reported that noThis journal is ª The Royal Society of Chemistry 2012

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Fig. 16 SAXS profiles at various temperatures of supramolecularpolymer gels formed from telechelic-type triblock copolymers connectedby self-complementary hydrogen bonding. Reprinted with permission
from ref. 35. Copyright 2009 American Chemical Society.

a fairly reliable indicator for this gelation, because the product istypically large in supramolecular polymer gel systems.
Fig. 14 Comparison of SAXS profiles for supramacromolecular ion gels
prepared with different concentrations of a triblock copolymer (VEAV),
poly(4-hydroxystyrene) (H), and an ionic liquid (IL). The weight ratio of
VEAV : H : IL in the gels was systematically varied from 10 : 0 : 90 to
10 : 4 : 90 (bottom to top traces, respectively). See also ref. 23.

4. Viscoelasticity
A recent review on viscoelasticity of supramolecular polymer
networks was presented by Seiffert and Sprakel that focused on
dynamics.256 Here, we summarize viscoelastic properties of
stiffness, temperature dependence, damping performance, androbustness that are required to attain better supramolecularpolymer gels.

4.1 Stiffness

Fig. 15 The reciprocal of the maximum of scattering intensity (I1) andthe square of half-width of half-maximum (hwhm)2 as a function of
inverse absolute temperature. The sample is a supramacromolecular ion
gel with a weight ratio of VEAV : H : IL of 10 : 4 : 90 (see Fig. 14). The
vertical line represents the temperature that induces a gel-to-liquidtransition (Tgel; 141  C). See also ref. 23.

nanophase separation occurred in their supramolecular polymer
gels, despite the gelation detected with a rheometer.35 Theyobserved correlation hole peaks255 on SAXS profiles of theirmaterials (Fig. 16), which suggested an electron density fluctuation between the non-covalent bonding units and the strands.
Therefore, it should be noted that nanophase separation is notalways induced at the time of gelation; its occurrence depends on
the product of the effective interaction parameter (c) and thedegree of polymerization (N) between cross-links and strands,
similar to the interaction observed in block copolymers.
However, the nanophase separation phenomenon remains
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It can be difficult to carry out tensile measurements on supramolecular polymer gels, because they tend to have little stiffness
or toughness and they may not maintain their shape for a longtime. The viscoelastic properties of these soft materials are best
evaluated with shear measurements.
Several groups have carried out dynamic shear measurements
in linear viscoelastic regimes.257–263 In fact, we have performed
studies on supramolecular polymer gels composed of welldefined macro-building blocks in an ionic liquid to determine therelationship between stiffness and molecular weight.22,23 As
mentioned in Section 3.2.1, we mixed macro-building blocks of
VEAV triblock copolymers and H homopolymers in an ionicliquid, and networks were formed via hydrogen bonding. SAXS
results strongly suggested that these networks comprised crosslinks of a hydrogen-bonded V–H complex phase and strands ofpolyethyl acrylate (EA) that were soluble in the ionic liquid. We
could roughly estimate stiffness (Gx), based on the assumption of
an ideal (or Gaussian) network formation from the blend of H
and bridging VEAV in an ionic liquid. In this ideal network, Gx is
expressed as
Gx ¼ nkBT
here, n denotes the number of elastically effective strands per unit
volume, kB is the Boltzmann constant, and T is the absolutetemperature. At the appropriate stoichiometric ratio of V blocks
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and H homopolymers, a clear plateau was evident on curves ofthe storage modulus (G0 ) and the loss modulus (G00 ).
The number of elastically effective strands per unit volume, n,
was derived as follows:

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n ¼ cNA/Mx
where c is the concentration of elastically effective strands (i.e.,the concentration of bridging VEAV),264,265 NA is Avogadro’s
number, and Mx is the molecular weight of the molecules
between cross-links. Note that experimental value of the plateau
modulus (Gx) was around 3000 Pa (Fig. 17) and the calculated
value is almost the same, approximately 2500 Pa. The applicability of this expression indicated that the supramolecular polymer gel was analogous to an ideal network at the molecular level.
This is because the formation of supramolecular polymer
networks proceeds slowly and homogeneously at the time of
preparation. Note that the preparation procedure of theconventional covalently bonded polymer network produced withchemical reactions is quite different from that of supramolecularpolymer networks. In any case, when the macro-building blocks
are designed and synthesized precisely, then the desired stiffnessof the gels can be available.
4.2 Temperature dependence
Supramolecular polymer networks can be formed homogeneously during the preparation procedure. The key to homogeneous network formation is the cross-linking, which depends on
temporary and weak non-covalent bonding. In other words, bycontrolling the non-covalent bonding units, we can design thetemperature dependence of the gel.
4.2.1 Gel–liquid transition temperature. The interaction
strength of non-covalent bonding, particularly hydrogenbonding, is relatively high at low temperatures, and network
cross-links are able to maintain shape. However, as the temperature increases, the non-covalent bonding strength weakens to
the point that supramolecular cross-links will begin to fall apart;thus, the networks can collapse, and the macroscopic appearance
will become a liquid. However, the gel–liquid transition

temperature (Tgel) is difficult to determine from appearance
alone. A rheometer is needed for a quantitative measure of the
Tgel. Here, we define the Tgel of a supramolecular polymer gel as
the temperature where the storage modulus G0 (elasticity) andloss modulus G00 (viscosity) have the same value on dynamic
temperature ramp tests. The moduli values depend on theamplitude of angular frequency; thus, the Tgels from different
samples should be acquired at the same angular frequency forcomparison among samples. Although a Tgel does not represent
all the properties related to the temperature dependency, it iseasily understandable, and hence, it is a useful indicator for
preparing gels.
To date, few studies have systematically run dynamic
temperature ramp tests, because the tests are typically thought toprovide only qualitative assessments by experts in the field of
rheology. However, recent studies on supramolecular polymer
gels composed of VEAV and H in an ionic liquid clearly showed
the usefulness of the Tgel indicator. The Tgels of supramolecularpolymer gels could be increased systematically with increasing
amounts of H cross-linker molecules; this indicated that, withlarger amounts of H, higher energies were required to weaken the
cross-link bonds. This suggested that the addition of more H
induced a larger fraction of hydrogen bonding between V blocks
and H homopolymers. Recently, it was shown that the Tgel also
increased with the degree of hydrogen bonding units per crosslinker molecule (H), although an equal mass of H was added forgel preparation.266,267 Those results indicated that the Tgel can be
adjusted by designing the gel preparation conditions to achieve
an appropriate degree of non-covalent bonding units per crosslinker molecule.
4.2.2 Time–temperature superposition (TTS). Compared to
the few studies that used dynamic temperature ramp tests, morestudies have used systematic dynamic frequency sweeps. The
dynamic frequency sweep determines whether the time–temperature superposition (TTS) principle holds. The TTS master curveshows the change in a sample state at a given temperature overa given time. TTS master curves of supramolecular polymer gelswere analysed to investigate their molecular motion, or gel
dynamics. The results showed that supramolecular polymer gel
systems exhibited TTS over a wide temperature range and therelaxation time varied over a wide time range. This result
assumed that the networks were formed with cross-links of finite
lifetimes. Clearly, this property arose from the non-covalentbonding features based on the number of non-covalent bonding
units per cross-link.
4.3 Damping performanceFig. 17 Dynamic moduli of shear storage, G0 (B), and loss, G00 (,), ofa supramacromolecular ion gel as a function of temperature. The
supramacromolecular ion gel was prepared by mixing a triblock copolymer (VEAV), poly(4-hydroxystyrene) (H), and an ionic liquid (IL). The
weight ratio of VEAV : H : IL in the sample was 10 : 4 : 90. See also ref.
22 and 23.

6424 | Soft Matter, 2012, 8, 6416–6429

Gel materials with high damping performance are attractive,
because most other materials do not exhibit damping. Here,
damping is defined as the suppression of vibration and noise by
dissipating deformation energy. Damping performance can be
estimated rheologically with the loss tangent, tan d (¼ G00 /G0 ). A
nice study by Urayama et al.268 showed improvement in damping
performance with the use of a silicon elastomer; i.e., a chemical
gel in bulk solution. They prepared chemically bonded polymer
networks by simultaneously reacting mono- and bifunctional
precursors with trifunctional cross-linking agents. When
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View Article Onlinemono-functional precursors reacted with the cross-linking
agents, they could not extend the growth of the polymer
networks; instead, they became dangling ends in the networks.
This feature contributed to improving the damping performanceof the material.
This concept was successfully applied to supramolecularpolymer gels. In Fig. 18, the loss tangent changed as a function oftemperature for gels composed of VEAV triblocks (or VEA
diblocks) and H in an ionic liquid.23 Here, the loss tangent
depended on the stoichiometry of non-covalent bonding units
and also on the amount of non-telechelic VEA. An appropriate
stoichiometric balance could attain the most elastic state at
temperatures lower than the Tgel, but inappropriate stoichiometric ratios produced dangling ends in the networks, leading to
improved damping performance at T < Tgel. Homogeneouspolymer networks are readily prepared with the preparation
procedure described above; thus, supramolecular polymer gels
are good candidates for materials with tuned damping
performance.
4.4 Robustness
For practical applications, it is also important to note therobustness of supramolecular polymer gels. Estimation of thelinear viscoelastic regime is one of the best ways to evaluate
robustness. Unfortunately, in the gel state, supramolecularpolymer gels have a very narrow linear viscoelastic regime,
typically below approximately 10% strain. Fig. 19 shows
dynamic amplitude sweeps of the gels in an ionic liquid (a)23 andin water (b).132 The drops in the G0 modulus above a 10% strain
may be due to loss of cross-linking or dissociation of noncovalent bonding induced by a large strain. In recent studies, therobustness of the gel state (at G0 > G00 ) of supramolecular polymer gels was associated with non-Newtonian behaviour in theliquid state (at G00 > G0 ): shear-thickening and shear thinningphenomena.269,270 These properties will be investigated in moredetail with future studies.

Fig. 18 Loss tangent (tan d) of supramacromolecular ion gels as
a function of normalized absolute temperature (T/Tgel). The supramacromolecular ion gels were prepared by mixing a triblock copolymer(VEAV), poly(4-hydroxystyrene) (H), and an ionic liquid (IL). Red,
yellow, blue, and green curves represent supramacromolecular ion gelswith VEAV : H : IL weight ratios of 10 : 1 : 90, 10 : 2 : 90, 10 : 4 : 90,
and 10 : 8 : 90, respectively. The black curve represents a supramacromolecular ion gel made of VEA diblocks and H, where theconcentration of VEA : H : IL was 10 : 4 : 90. Reprinted with permission
from ref. 23. Copyright 2009 American Chemical Society.

This journal is ª The Royal Society of Chemistry 2012

Fig. 19 Strain amplitude sweeps for testing gel robustness. Supramacromolecular ion gels were prepared by mixing triblock or diblock
copolymers (VEAV or VEA, respectively) and poly(4-hydroxystyrene)(H) in an ionic liquid (IL). (a) The complex modulus of supramacromolecular ion gels of VEAV and H (,), and VEA and H (-) in an
IL was tested; (b) storage (red) and loss (blue) moduli of supramolecularpolymer gels in water were tested. Reprinted with permission from ref. 23
and 132. Copyright 2009 American Chemical Society and Copyright 2010
Nature Publishing Group.

5. Conclusions and future outlook
In this review, we discussed supramolecular polymer gels in termsof design, morphology, and rheology. The studies onmorphology and rheology depended on the preparation; therefore, much effort has been devoted to determining simple, efficient procedures for preparing homogeneous supramolecularpolymer gels. Thanks to that effort, many groups have produced
supramolecular polymer gels and applied many different
measurements. In particular, many basic rheological properties
have been revealed. Although this review also summarized thedata available on the basic features of gel viscoelasticity, many
issues remain to be addressed for practical applications. Further
fundamental studies are needed in the future to reveal more
about these basic properties.
Although supramolecular polymer gels are interesting andshould be useful materials for some applications, in themselves,
they cannot be considered perfect materials. As mentioned in
Section 4, these gels do not have sufficient robustness in the gelstate. Furthermore, their physical properties lack outstanding
optics, magnetism, electrical properties, etc., because they are
typically composed solely of organic materials, except in the caseof supramolecular polymer gels with metal-to-ligand coordination. On the other hand, they do have novel properties, including
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View Article Onlinea characteristic temperature dependence (self-healing and thermoresponsive properties), high damping performance, andcontrollable stiffness; these properties are lacking in othermaterials. Therefore, we propose three goals for improving theproperties of supramolecular polymer gels. One is to incorporate
covalently bonded cross-links into the supramolecular polymer
networks.34,271–276 The second is to hybridize the materials withinorganic components,277–279 particularly metal or semiconductor
nanoparticles.280–284 The third is to use supramolecular polymer
gels as thin films. Several groups are currently working on some
improvements. We expect that changes which compensate for the
deficits in supramolecular polymer gels will produce unprecedented materials with novel properties and functions in the
future.

Acknowledgements
The authors thank Prof. Timothy P. Lodge at the University of
Minnesota for collaboration on producing and measuring
supramolecular polymer gels in an ionic liquid. The authors also
express their special thanks to Prof. Hiroshi Watanabe at Kyoto
University for his kind suggestions on the analysis of rheology
data. Furthermore, the authors thank Akihisa Ohshika, He
Xudong, Yutaka Nagata, Akinori Tamura, Hajime Yamagishi,
Yoshio Sageshima, Koji Ishihara, Kota Higuchi, Akiko Minami
and Satoshi Matsushima, current and past graduate students at
Nagoya University, for their assistance in our studies in supramolecular polymer materials. The authors are also grateful for
financial support through the Grant-in-Aid projects (no.
21750217 (A.N.), no. 23655123 (A.N.), no. 24685035 (A.N.), no.
18068008 (Y.M.), and no. 22245038 (Y.M.)) from JSPS and
MEXT, Japan, the Global COE program in Chemistry at
Nagoya University, and the Program for Leading Graduate
Schools at Nagoya University.

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