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Current Opinion in Solid State and Materials Science 11 (2007) 47–54
Contents lists available at ScienceDirect
Current Opinion in Solid State and Materials Science
journal homepage: www.elsevier.com/locate/cossms
Nanocomposite hydrogels
Kazutoshi Haraguchi *
Material Chemistry Laboratory, Kamamura Institute of Chemical Research, 631 Sakado, Sakura, Chiba 285-0078, Japan
a r t i c l e
i n f o
Article history:
Received 21 May 2008
Accepted 22 May 2008
Keywords:
Nanocomposite
Hydrogel
Clay
Network structure
Stimulus-sensitivity
a b s t r a c t
Hydrogels, which consist of three-dimensional polymer networks and large amounts of water, have long
been believed to be interesting but mechanically fragile materials limited to specific uses. Recently, important breakthroughs have been made as a result of the creation of nanocomposite hydrogels (NC gels), and
most of the traditional limitations of hydrogels have been overcome. NC gels are prepared by in situ freeradical polymerization at high yield under mild conditions (near ambient temperature, without stirring),
and various shapes and surface forms are readily obtained. Because of their unique organic (polymer)/
inorganic (clay) network structure, high toughness and excellent optical properties and stimulus-sensitivity are simultaneously realized in NC gels. Furthermore, NC gels exhibit a number of interesting new characteristics. In this paper, the fundamental and recent developments related to NC gels are reviewed.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Organic/inorganic nanocomposites (NCs) are functional materials consisting of immiscible organic and inorganic components,
and complex nanometer-scale structures can be fabricated therefrom. As a typical example, polymer/clay NCs have been extensively studied and successfully developed for many applications
[1]. In 2002, Haraguchi reported the creation of a novel ‘‘nanocomposite hydrogel” with a unique organic–inorganic network structure by extending the concept of NC to the field of soft hydrogel
materials [2]. The nanocomposite hydrogel (abbreviation: NC gel)
exhibited extraordinary mechanical, optical, swelling/deswelling
properties which could simultaneously overcome the limitations
of conventional chemically crosslinked hydrogels (abbreviation:
OR gels). The construction of NC gels was achieved, not by the mere
incorporation of clay nano-particles into a chemically crosslinked
network, but by allowing the clay platelets to act as multifunctional crosslinkers in the formation of polymer/clay networks.
Due to their superior properties, NC gels have attracted much
attention and are believed to be a revolutionary type of hydrogel
[1]. This article reviews the fundamental and recent developments
in the field of NC gels.
2. Synthesis and network structure
2.1. Synthesis procedure and composition
The synthetic procedure for NC gel formation consists of the in
situ free-radical polymerization of a monomer (e.g., N-substituted
* Tel.: +81 43 498 2111; fax: +81 43 498 2182.
E-mail address: hara@kicr.or.jp
1359-0286/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.cossms.2008.05.001
acrylamides) in the presence of inorganic clay platelets uniformly
dispersed in an aqueous medium [2]. Here, the use of exfoliated
clay platelets instead of an organic crosslinker such as N,N-methylenebisacrylamide (BIS) is important. The use of both clay and BIS
(i.e., the use of clay as a reinforcing agent) results in hydrogels with
poor mechanical properties, similar to those of conventional OR
gels [3–5]. In order to achieve the superior characteristics of NC
gels, it is necessary for inorganic nano-particles (clay platelets) to
act as multifunctional crosslinkers [1,6,7]. Attempts to prepare
NC gels by other procedures, such as mixing clay and polymer solutions, have been unsuccessful [8].
As the inorganic component, the clays in the smectite group
(e.g., hectorite, montmorillonite), their modifications (e.g., by fluorination or addition of pyloric acid), synthetic mica, and so on, can
be used, providing that they can be swollen and exfoliated in
water. Several kinds of clay and their effects on the tensile
mechanical properties of NC gel are shown in Table 1 and Fig. 1
[9]. The non-water-swellable clay minerals (e.g., sepiolite) cannot
produce NC gels, although modified hectorite or montmorillonite
can be used to produce NC gels [9–13]. Concerning the size of
the clay particles, a diameter of about 30 nm (for synthetic hectorite) is enough for the construction of NC gels and clay particles of
larger sizes (e.g., >300 nm for natural montmorillonite) are not
necessary; sometimes large clay particles are not even effective
(Fig. 1), probably due to the insufficient exfoliation and moderate
aggregation in the aqueous solution. As for the other minerals,
polyhedral oligomeric silsesquioxane [14,15], rigid polysiloxane
[16,17], fibrillar attapulgite [18] and hydrotalcite [19] have been
used with selected polymers. Silica and titania nano-particles have
been found ineffective as multifunctional crosslinkers [8].
For the polymer, water-soluble monomers containing amide
groups, such as N-isopropylacryamide (NIPA), N,N-dimethylacryla-
48
K. Haraguchi / Current Opinion in Solid State and Materials Science 11 (2007) 47–54
patterned NC gel films have been successfully prepared by direct
replica molding [26].
Table 1
Compositions of various inorganic clays
Clay
Type
Composition (wt%)
SiO2
MgO
Li2O
Na2O
F
P2O5
2.3. Network structure and the gel formation mechanism
Water swelling
XLGa
Synthetic hectorite
SWNb Synthetic hectorite
a
XLS
XLG + dispersing agent 54.5
SWFb
Fluorinated hectorite
Ba
fluorinated hectorite
Sa
B + dispersing agent
59.5
54.3
26.0
54.5
55.0
51.0
27.5
27.9
0.8
27.2
27.0
25.0
0.8
1.6
5.6
1.6
1.4
1.3
2.8
2.7
–
5.8
3.8
6.0
–
–
4.1
2.9
5.6
5.0
–
–
Partially swelling in water
Fc
Natural montmorillonite
61.3
–
4.1
Al2O3 = 22,
Fe2O3 = 1.9
Non-swelling in water
Sepiolite
IGSd
Magnesium silicate
The extraordinary properties and new functions of NC gels are
attributed to their unique network structure. The organic (polymer)/inorganic (clay) network structure (Fig. 3) was first proposed
on the basis of analytical data from TEM, XRD, TGA, DSC and FTIR
measurements [2], and it has been confirmed to be consistent with
the observed mechanical, optical and swelling properties [6,7,27].
Based on classic rubber-elasticity theory, the number of crosslinking points per clay platelet for NC gels has been calculated [6,7],
and it is several tens to more than 100, indicating that each clay
particle acts as a multifunctional crosslinker. This conclusion has
also been confirmed by rheological measurements [28]. Also, the
effective crosslink density (me) has been calculated using the degree
of equilibrium swelling according to the Flory–Rehner theory
(Table 2) [24,29]. In general, the me value of NC gels is much smaller
than that of OR gels, which is consistent with the observed
mechanical properties.
The process of forming the unique organic/inorganic network
structure of NC gels has been studied based on the changes in viscosity, optical transparency (Fig. 4a), XRD data and mechanical
properties, and the formation mechanisms of the solution structure and the clay-brush particle have been proposed (Fig. 4b) [8].
The network structure and gelation mechanism have also been
investigated by dynamic light scattering (DLS) and small-angle
neutron scattering (SANS) measurements. It has been confirmed
that clay platelets disperse homogeneously in the polymer matrix
[30] and act as multiple crosslinkers [31,32]. Also, the thermal fluctuations of the clay platelets are largely suppressed upon network
formation [33]. The gelation of NC gels is classified as an ergode–
nonergode transition, as in OR gels, except that huge clusters of
NC microgels (corresponding to clay-brush particles) are formed
before the gelation threshold [34]. Further, by contrast-variation
SANS, it has been found that there is a polymer layer surrounding
the clay platelets with a thickness of about 1 nm, irrespective of
Cclay [34].
3.43
–
–
3.3
From Ref. [9].
a
Rockwood Additive Ltd.
b
Coop Chemical Ltd.
c
Kunimine Ind. Co.
d
Tomoe Ind. Co.
Fig. 1. Tensile stress–strain curves for (PDMAA-)NC gels prepared using various
kinds of clay shown in Table 1. Cclay = 3.9 102 mol/L H2O. From Ref. [9].
mide (DMAA) and acrylamide (AAm), are the most effective, probably due to their adequate interaction with the clay surface. Other
monomers (e.g., those containing carboxyl, sulfonyl or hydroxyl
groups) can also be used alone [20] or as co-monomers [21,22].
By using a similar procedure and a specific acrylate monomer
(2-methoxyethylacrylate), Haraguchi et al. succeeded in preparing
an interesting novel, flexible and transparent NC (M-NC: solid)
[23]. In M-NCs and NC gels, it should be noted that the content
of clay (Cclay) can be varied to a large extent because they are prepared from aqueous solutions, which is in stark contrast to the procedure for fabricating conventional polymer/clay NCs.
2.2. Various shapes and surface patterns
Since the synthetic procedure is quite simple and versatile, NC
gels can be prepared in many shapes, such as huge blocks, sheets,
thin films, rods, hollow tubes, spheres, bellows and uneven sheets,
by means of different molds (Fig. 2) [24]. This is due to the easy
injection of the reaction solution and the very high toughness of
the resulting gel. Also, microgels with dimensions of several hundred nm [25] and porous NC gels with a density of 0.2 g/cm3 [24]
have been prepared. Furthermore, micrometer-scale, surface-
3. Mechanical properties
For a long time, the mechanical properties were not the main
theme in the study of hydrogels such as PNIPA hydrogels, which
are typical smart gels. However, after the creation of NC gels by
Haraguchi et al. [2], it became possible to conduct all the conventional mechanical tests, and hydrogels can now be treated as rubbery materials. In fact, NC gels can withstand high levels of
deformation, not only in the form of elongation and compression,
but also bending, tearing, twisting and even knotting (Fig. 5) [6].
3.1. Tensile properties
In general, an NC gel can be elongated to more than 1000% of its
original length, and the extensibility depends on the kind of clay
and polymer used. In a specific clay/polymer system, the elongation at break (eb) is almost constant regardless of Cclay and the polymer content (Cp), e.g., 1000% for PNIPA-NC gel [6,27] and 1600% for
PDMAA-NC gel [7]. Thus, fragile OR gels are dramatically changed
to stretchable NC gels by simply changing the crosslinker from BIS
to clay. When the network is not effectively established, eb becomes much larger (>2000%), as shown in Fig. 1 [9]. Also, NC gels
exhibit high time-dependent recovery from large strains [27],
and this ability changes according to the clay/polymer system
[11] and composition [27].
K. Haraguchi / Current Opinion in Solid State and Materials Science 11 (2007) 47–54
49
Fig. 2. NC gels with various shapes: (a) thin film, (b) sheet, (c) uneven sheet, (d) hollow tube and (e) bellows.
modulus is increased (e.g., 500 fold) without sacrificing eb, which
is totally different behavior from that of conventional polymeric
materials. Consequently, the fracture energy increases to 3300
times that of OR gels [27,35]. Also, in NC gels, properties such as
time-dependent recovery, the tensile properties on the second cycle and the disappearance of the glass transition change at a critical
Cclay ðC clay 10 102 mol=L H2 OÞ [27]. The structural change during uniaxial stretching of NC gels has also been investigated by
SANS [31,36] and optical anisotropy [37].
3.2. Compression and other properties
Fig. 3. Schematic representation of the organic (polymer)/inorganic (clay) network
structure of NC gel. Dic is the inter-particle distance of the exfoliated clay platelets.
v, g1 and g2 represent the crosslinked chains, grafted chains and looped chains,
respectively. In the model, only a small number of polymer chains are depicted for
simplicity. From Ref. [7].
The modulus and strength can be improved greatly by increasing values of Cclay and Cp [6,7]. Here, it should be noted that the
NC gels generally withstand 90% compression, in contrast to
OR gels, which are readily broken into pieces under small strains.
The compression modulus and strength increase almost proportionally to Cclay [27]. Also, it has been found that, due to the formation of co-crosslinked network (NC–OR gel), the compression
strength improves considerably at low BIS contents (CBIS) [5]. This
is attributed to the formation of a microcomplex structure consisting of clay platelets with enhanced densities of chemical crosslinks.
In viscoelastic measurements, very stable G0 (storage modulus)
and G00 (loss modulus) (G0 > G00 ) values in the frequency range of
101 102 rad1 and relatively high tan d (loss factor) (0.1) values have been observed for PDMAA-NC [24] and PAAm-NC gels
[28], respectively, which indicates that the NC gels are much more
viscous than conventional OR gels.
4. Swelling and stimulus-sensitivity
4.1. Swelling
NC gels generally exhibit a high degree of equilibrium swelling
(DES) in water compared with OR gels [2,29]. This is due to the
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K. Haraguchi / Current Opinion in Solid State and Materials Science 11 (2007) 47–54
Table 2
Degree of equilibrium swelling (DES) and effective network density (me) for NC and OR gels, calculated according to the Flory–Rehner theory
Hydrogel
NC1
NC5
NC10
NC15
NC20
OR1
OR5
DES(Ws/Wdry)
me (mol/L)
50.94
0.0048
28.94
0.0099
22.57
0.0127
17.61
0.0170
12.94
0.0256
16.93
0.0431
8.55
0.1582
From Ref. [29].
Fig. 4. (a) Changes in optical transparency during the polymerization of NC gel and OR gel. (b) Model structures of (1–3) the reaction solutions, (4) the clay-brush particles
and (5) NC gel. From Ref. [8].
relatively low value of me for the polymer/clay network. The value
of DES changes depending on the kind of clay and polymer as well
as me. In a fixed clay/polymer system, the DES decreases with
increasing Cclay and Cp [7,29]. For the purpose of constructing a
superabsorbent (high-swelling) hydrogel, a more hydrophilic polymer (e.g., PAAm and PAAc) [11,38] or co-polymerization with an
ionic monomer [21,39] has been applied.
In the swelling of NC gels, unusual behaviors, such as the
appearance of a maximum in the swelling curve [40] and a remarkable increase in DES due to post-treatment in a specific NC gel with
the retention of high mechanical properties [41], have been observed. Both phenomena are explained by the rearrangement of
the entangled polymer chains and clay particles during the course
of swelling or post-treatment.
4.2. Stimulus-sensitivity
Thermo-sensitivity and its control in hydrogels have attracted
much attention because of their many potential applications [42].
However, for example, conventional PNIPA-OR gels have several
important limitations, such as low volume change (VC) and slow
deswelling rate (DSR), as well as low mechanical properties. It
has been revealed that these limitations are simultaneously solved
in (PNIPA-)NC gels, which exhibit large VC and high DSR as well as
excellent mechanical properties [2]. Interestingly, the effect of me
on the DSR is opposite in NC and OR gels [6]. The thermo-sensitivity of NC gels can be varied widely in a controlled manner by altering the gel composition [35], and even non-thermo-sensitive NC
gels can be obtained by increasing Cclay and thereby restricting
K. Haraguchi / Current Opinion in Solid State and Materials Science 11 (2007) 47–54
51
Fig. 5. NC gels exhibit extraordinary mechanical toughness: (a) stretching, (b) bending, (c) knotting and (d) compression.
Fig. 6. Semi-interpenetrated NC gel (sI-NCgel), consisting of a PNIPA/clay network including linear poly(acrylic acid) chains, exhibited outstanding temperature- and pHsensitive swelling/deswelling behaviors as well as superior tensile mechanical properties. From Ref. [14].
the thermal molecular motions of the PNIPA chains attached to the
hydrophilic clay surfaces or in their proximity [29]. The transition
temperature (lower critical solution temperature: LCST) of NC gels
is hardly changed by altering Cclay [6,29], but shifts toward a lower
or higher temperature on adding an inorganic salt or cationic surfactant to the surrounding aqueous solution, respectively [43].
NC gels with a semi-interpenetrating (semi-IPN) organic/inorganic network structure have been prepared using linear poly
(acrylic acid) [44] or linear carboxymethylchitosan [45]. The resulting semi-IPN NC gels manifest outstanding swelling/deswelling
behavior in response to both temperature and pH (Fig. 6) while
retaining their remarkable tensile mechanical properties. The response to electrostimulation has been studied in chitosan/clay
gel [46]. Haraguchi et al. have reported the first observation of
the reversible generation of retractive tensile forces in NC gels as
a result of the coil-to-globule transition of PNIPA chains, in response to the alternation of the temperature across the LCST [47].
5. New functions in NC gels
5.1. Transparency and its changes
Transparent OR gels generally turn opaque with increasing CBIS
(/me) due to the inhomogeneous distribution of crosslinking
points, and the critical value of CBIS at which the loss of transmittance changes depending on the polymer [6,7]. In contrast, NC gels
are generally transparent, almost regardless of the crosslink density (/Cclay and Cp) and the kind of polymer, except for a slight decrease at low Cclay (ca. 2 102 mol/L H2O) [7] and very high Cclay
(25 102 mol/L H2O) [27]. These findings indicate that the
52
K. Haraguchi / Current Opinion in Solid State and Materials Science 11 (2007) 47–54
organic/inorganic network structure forms uniformly throughout
the NC gel regardless of Cclay and Cp. The homogeneity of the network structure has been investigated in detail by SANS and DLS
[30,34].
load. The distinct change in frictional force with the surroundings
is particularly observed in NC gels with low Cclay.
5.2. Optical anisotropy and its changes
Hydrogels, which consist of a hydrophilic polymer with water
filling the interstitial space of the network, are naturally hydrophilic, and their surfaces generally show very low contact angles
for water (hw). Haraguchi et al. [49] have found that the surface
of an NC gel consisting of a PNIPA/clay network can exhibit
extraordinarily high hydrophobicity, although all the constituents
of the NC gel were hydrophilic under the testing conditions (Fig.
8). hw was generally greater than 100° over broad ranges of Cclay
and Cwater and showed a maximum contact angle of 151° at a specific composition. The high hydrophobicity of NC gels is primarily
attributed to the amphiphilicity of PNIPA and, more specifically,
to the spontaneous alignment of N-isopropyl groups at the gel–
air interface, and is enhanced by other factors, such as the network
structure and gel composition. Also, the surfaces of NC gels exhibit
reversible hydrophobic-to-hydrophilic changes on changing the
surroundings from dry (in air) to wet (in water) conditions, and
vice versa.
Since hydrogels consist of polymer networks swollen with large
amounts of water, hydrogels are normally amorphous (optically
isotropic). It has been found that an NC gel exhibits optical anisotropy when its Cclay exceeds a critical value ðC clay 10
102 mol=L H2 OÞ [27]. This C clay is consistent with the critical value
calculated for spontaneous clay aggregation (layer stacking) in NC
gels [29]. Furthermore, the optical anisotropy of an NC gel changes
uniquely on uniaxial deformation, regardless of the optical characteristics in the original (as-prepared) state [37]. That is, upon uniaxial stretching, all NC gels exhibit remarkable optical anisotropy,
and the birefringence shows distinct maxima and sign inversions
(Fig. 7). The contributions of clay and PNIPA to the birefringence
of stretched NC gels have been evaluated.
5.3. Sliding frictional behaviors
Sliding frictional measurements have been conducted on the
surfaces of NC gels under different environmental conditions and
loads [48]. In air, NC gels exhibit a characteristic force profile with
a maximum static force and a subsequent constant dynamic frictional force, both of which vary markedly depending on the gel
composition and the load on the sliding plate. In contrast, under
wet conditions, NC gels generally exhibit very low frictional forces
and the dynamic frictional coefficient decreases with increasing
5.4. High water contact angles
5.5. Cell cultivation and biocompatibility
It has been found that cells such as human dermal fibroblasts
and human umbilical vein endothelial cells (HUVEC) can be cultured to confluence on the surface of an NC gel consisting of a PNIPA/clay network (Fig. 9a and b); in contrast, cell cultures hardly
develop on OR gels. Also, the cultured cells can be detached as cell
sheets without trypsin treatment, simply by decreasing the temperature to 20 °C (below the LCST of PNIPA) (Fig. 9(1–3)) [50].
The effectiveness of NC gels in biomedical applications is being
investigated in terms of extraction tests for contact lenses, absorption of saline, sterilization by autoclaving, blood compatibility and
intramuscular implantation [51].
5.6. Porous NCs with layered morphology
Porous solid NCs with characteristic layered morphologies, such
as a three-layer morphology with controlled porosity, have been
Fig. 7. Birefringence, DNC, of (PNIPA-)NC gels with different Cclay (NC2–NC10)
values as a function of strain. The closed and open symbols represent measurements in the through- and edge-directions, respectively. The inserted photo images
(a–d) show polarized-light micrographs of stretched NC2 gels under crossed
polarizers in conjunction with a 530 nm retardation plate. From Ref. [37].
Fig. 8. Water droplets on PNIPA-NC gel film. (a) Value of the water contact angle for
a sessile drop on the surface of PNIPA-NC gel with a water content of 210 wt%
(¼ W H2 O =W dry-gel 100). From Ref. [49].
K. Haraguchi / Current Opinion in Solid State and Materials Science 11 (2007) 47–54
53
Fig. 9. Phase-contrast photomicrographs of (a) fibroblast and (b) HUVEC proliferated on (PNIPA-)NC gel. (1–3): Cell sheet detachment of fibroblast by decreasing the
temperature to 10–20 °C. From Ref. [50].
obtained by freeze-drying NC gels without the use of an added
porogen [52].
6. Conclusion
Hydrogels are transparent, soft materials made mainly of water,
and can possess characteristics of both a solid and a liquid. By creating NC gels, the limitations in properties and applications, which
had previously been thought impossible to overcome, have been
eliminated. Furthermore, a number of ‘‘super-functions” (the internal consistency of pairs of mutually conflicting properties), such as
softness and toughness, extensibility and rigidity, swelling and
deswelling, elongation and recovery, optical isotropy and anisotropy, cell cultivation and detachment, inorganic inclusion and
transparency, etc., can now be induced. Since the primary component is water, NC gels can be utilized as environmentally friendly
rubbery materials in the management of resources and waste,
and may open new doors in various areas of advanced research
and technology.
Acknowledgement
This work was partially supported by the Ministry of Education,
Science, Sports and Culture of Japan (Grant-in-Aid 20350109).
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