13
Click here to load reader
Click here to load reader
PAPER www.rsc.org/softmatter | Soft Matter
Publ
ishe
d on
07
June
201
0. D
ownl
oade
d by
Por
tland
Sta
te U
nive
rsity
on
06/1
0/20
13 1
8:46
:08.
View Article Online / Journal Homepage / Table of Contents for this issue
Nano-hybrid self-crosslinked PDMA/silica hydrogels
Linn Carlsson, S�everine Rose, Dominique Hourdet* and Alba Marcellan*
Received 19th February 2010, Accepted 8th April 2010
DOI: 10.1039/c0sm00009d
We discovered that the free radical polymerization of N,N-dimethylacrylamide in water can lead, above
a certain concentration, to gels without any added difunctional crosslinker. These so called ‘‘self-
crosslinked’’ hydrogels were prepared and their weak mechanical properties were improved by
introducing silica nanoparticles.
From swelling experiments performed at equilibrium in aqueous media, it was shown that silica
particles behave as adhesive fillers and strongly interact with PDMA chains. These interactions are
responsible for the reinforcement of mechanical properties. From initial elastic moduli, determined in
the preparation state, we show that the elastic behaviour of these hydrogels mainly originates from
entanglements and from physical crosslinks that can be controlled by the polymer concentration and
the ratio between silica particles and polymer chains, respectively. The mechanical behaviour was
characterized using: monotonic tensile tests, loading-unloading cycles at large strains and stress
relaxation experiments in order to investigate long time behaviour. The introduction of silica highly
increases the stiffness of the network without greatly reducing its extensibility, implying that strong
interactions take place between PDMA chains and silica surfaces.
Non-linear behavior was pointed out: softening at small deformations and hardening at high
deformations which is related to finite chain extensibility. All these effects have been shown to strongly
depend on the silica content. The analysis of hysteresis and residual strains induced by cycles, clearly
indicate that contrary to chemical crosslinkers, hybrid interactions increase the dissipative process.
1. Introduction
Hydrogels are crosslinked polymers forming a three-dimensional
network and are highly swollen in aqueous environments. From
a macroscopic point of view, if the lifetime of polymer crosslinks is
long enough compared to the experimental time, hydrogels behave
like solids; they have a defined geometry and they do not flow like
liquids in the corresponding timescale. Simultaneously, these
macromolecular reservoirs are similar to real solutions or dialysis
bags wherein the soluble molecules can diffuse and be outsourced
with diffusion constants depending on the mesh size of the network
and on the level of interactions. Since the beginning of the 80’s
hydrogels have known a continuous growth with important appli-
cations like molecular sieves, super absorbent polymers or contact
lenses.1–4 More recently, with the development of stimuli-responsive
hydrogels, they continue to attract a very important research activity
with potential applications in biomedical areas as biotechnological
devices, tissue engineering, and drug delivery systems.5–10
We can consider today that the chemist is able to design very
sophisticated macromolecular architectures capable of swelling or
collapsing under various stimuli like temperature, pH, ionic
strength, light, electric field, magnetic field, etc.,11–19 but one of the
main issues remaining is the mechanical integrity of these soft
materials. The mechanical behaviour of soft hydrogels is actually
an important research area and different groups are intensively
working in this field trying to solve the paradox: how to get tough
Physico-chimie des Polym�eres et des Milieux Dispers�es, UMR 7615,UPMC-CNRS-ESPCI, 10 rue Vauquelin, 75005 Paris, France. E-mail:[email protected]; [email protected]; [email protected]; [email protected]; Fax: +33 (0)1 40 79 46 86; Tel: +33 (0)1 40 79 46 82
This journal is ª The Royal Society of Chemistry 2010
hydrogels? During this decade, several groups, mainly in Japan
have proposed some new strategies to improve the mechanical
properties of soft networks. Two main paths have been investi-
gated. The first strategy consists in reducing as much as possible
the heterogeneities in the network, implying that defect free
materials will not easily nucleate cracks. We can mention the
pioneering works of Okumura et al. on slide-ring polyrotaxane
hydrogels20,21 and more recently the Tetra-PEG gels of Shibayama
and coworkers.22 These gels are highly extensible and elastic but
are not very resistant to the propagation of an existing crack. A
second path has consisted of toughening the material by enhancing
the energy dissipation during crack propagation. Gong et al.,23–25
with so-called double networks have designed partially inter-
connected interpenetrated networks (IPN), that greatly enhance
the fracture toughness by increasing the dissipative volume ahead
of the crack tip. These gels are however permanently damaged
upon deformation and are softer when loaded a second time.
Both of these strategies involve a rather sophisticated chemistry
which is not off the shelf. A third and much easier strategy has been
pioneered by Haraguchi et al. with nanocomposite hydrogels
(NC).26–31 Most NC gels were synthesized by free radical polymeri-
zation of N-isopropylacrylamide (NIPA) or N,N-dimethylacryla-
mide (DMA) in the presence of exfoliated clays (Laponite: diameter
20–30 nm and thickness 1 nm). Since no crosslinking agent was
added, they were classified as physical gels. The structure of the gels is
still not clear, but the authors assume that the polymerization starts
from the clay platelet surface, leading to a percolating network where
clay particles act as multi-functional crosslinks.30 Accordingly, the
elasticity of the hybrid network would originate from polymer
chains making bridges between the clay platelets. Haraguchi et al.31
attribute the high deformability of NC gels (3max ¼ 800–1000%),
Soft Matter, 2010, 6, 3619–3631 | 3619
Publ
ishe
d on
07
June
201
0. D
ownl
oade
d by
Por
tland
Sta
te U
nive
rsity
on
06/1
0/20
13 1
8:46
:08.
View Article Online
compared to chemically cross-linked gels (3max ¼ 20–50%), by the
larger average distance between cross-links. In that case the average
distance between particles (physical crosslinks) is directly controlled
by the clay/monomer ratio. NC gels combine a high modulus with
a high extensibility, suggesting a large density of elastic chains
(contributing to the modulus) but a large distance between cross-
link points (high extensibility). The key would then be in the mul-
tifunctionality of the crosslinks. The weakness of such nanoclay
gels is however their highly viscoelastic character and residual
deformation due to the lack of chemical crosslinks. The concept of
nanocomposite gels with reinforced mechanical properties has been
successfully extended to other polymers, like poly(acrylamide)
using modified clays32 but conversely the replacement of clay by
other inorganic particles like silica or titanium oxide has not
provided comparable properties.33
Quite recently a similar concept of hybrid networks applied to
semi-dilute solutions was developed in our group34,35 by using
specific interactions between silica nanoparticles and polymer
side-chains (PNIPA or PDMA) grafted onto a non-adsorbing
poly(acrylamide) backbone. In a semi-dilute solution, physical
gels having a long lifetime, can be easily formed by simply mixing
silica nanoparticles with the copolymer. The main problem with
this procedure is that homogeneous hybrid gels become rather
difficult to prepare at polymer concentration above 2 wt% due to
the strong interactions taking place between the graft copolymers
and the silica particles.
This limitation can be overcome by polymerizing the monomer
in the presence of the inorganic filler. Following this idea, the
simplest way was to replace graft copolymers by homopolymers
using only DMA as monomer and basing our approach on the
strong interaction that will be developed during polymerization
between PDMA chains and silica nanoparticles. This idea has
been applied successfully using some additional chemical cross-
linker N,N0-methylenebisacrylamide (MBA) in the preparation
of the hybrid network.36 From a detailed comparison of the
hybrids with unmodified PDMA gels, we show in a first
approach that the incorporation of only 7 vol% silica nano-
particles in the hydrogel increases the initial stiffness by a factor
of 6 and the fracture toughness up to an order of magnitude.36
With the idea of increasing the extensibility of these silica-filled
gels, it was clearly interesting to reduce or completely eliminate
the chemical crosslinker (MBA) from the formulation.
In this paper, we report the facile synthesis of new hybrid
hydrogels, what we have called ‘‘self-crosslinked’’ hydrogels,
prepared only from water, monomer, initiator and silica nano-
particles. Then we describe the swelling behaviour and the
mechanical properties of these hybrid networks as a function of
DMA and silica concentration, comparing also our results with
those obtained in the presence of some MBA crosslinker. Finally,
from mechanical analyses performed at large strains we will
discuss the role of physical crosslinks and dissipation processes
on the reinforcement of organic networks.
Experimental part
Materials
N,N-Dimethylacrylamide (DMA, 99%, Aldrich), potassium persul-
fate (KPS, Acros Organics), N,N,N0,N0-tetramethylethylenediamine
3620 | Soft Matter, 2010, 6, 3619–3631
(TEMED, 99,5%+ redistilled, Sigma Aldrich) and N,N0-methyl-
enebisacrylamide (MBA, Fluka) were used as received without
further purification.
The silica particles (Ludox TM-50 from Dupont) were kindly
supplied by IMCD France SAS. The silica suspension was
characterized by dynamic light scattering using a ALV/CGS-3
compact goniometer (ALV, Langen, Germany). The measure-
ments performed at different scattering angles (30�, 90� and 150�)
in dilute conditions give an average radius R ¼ 17 nm for the
silica particles and a specific area Sspe ¼ (3/rSiR) ¼ 77 m2 g�1,
considering rSi ¼ 2.3.106 g m�3 for the density of pure silica. The
silica suspension (52 wt%; pH¼ 8.5–9.5) was used as received for
the synthesis of hybrid hydrogels.
Preparation of hydrogels
Hydrogels were prepared at 25 �C by free-radical polymerisation
of DMA in an aqueous suspension of silica nanoparticles using
KPS and TEMED as a redox initiator. For all the syntheses, the
molar ratio (DMA)/(KPS)/(TEMED) was set equal to 100/1/1
while the relative amounts of DMA, silica and water were varied
keeping constant either the weight ratio polymer/water (PW) or
silica/polymer (SP). In some cases, a very small amount of
chemical crosslinker (MBA) was introduced (R ¼ (MBA)/
(DMA) ¼ 0.1–0.5 mol%) in order to investigate the impact of
covalent crosslinks on the mechanical properties.
The nomenclature of hydrogels is SPX_PWY-RZ, with S for
silica, P for polymer, W for water, R for crosslinker (when used).
X is the weight ratio between silica and DMA, Y is the weight
ratio between DMA and water and Z is the molar percentage
between MBA and DMA. The composition of the hydrogels we
tested is summarized in Table 1.
A typical procedure for the synthesis of hydrogels can be
described as follows in the case of the sample SP2_PW0.14.
First, 1.485 g of DMA and 0.0405 g of KPS were dissolved at
25 �C in an aqueous suspension of silica particles initially
prepared by mixing of 7.699 g of water and 5.712 g of Ludox
TM-50. The homogeneous suspension was purged with nitrogen
during 30 min under magnetic stirring and then 22.5 mL of
TEMED was rapidly added under vigorous stirring. The mixture
was then transferred under nitrogen atmosphere into a mold,
made with two hydrophobized glass plates (chemically treated
with octadecyltrichlorosilane) separated by a spacer. The poly-
merization was left to proceed during 1 night. In these condi-
tions, it was shown from size exclusion chromatography
experiments performed on hydrogel samples swollen in a large
excess of solvent that the conversion of monomer is quantitative
and that the amount of extractable (polymer chains) was less
than 5 wt%.
At the end of the reaction, the gels were cut with a die-cutter of
rectangular shape and the samples were stored into paraffin oil
until mechanical testing in order to avoid any change in hydrogel
composition induced by swelling or drying.
Unfilled DMA networks were also synthesized in plastic
containers of cylindrical shape (diameter ¼ 33 mm) in order to
produce sample shapes suitable for rheological experiments.
For each gel (see Table 1), the initial swelling in the prepara-
tion state (Q0) was calculated from their initial composition,
This journal is ª The Royal Society of Chemistry 2010
Table 1 Composition and nomenclature of hydrogels classified by increasing silica/polymer ratio and then by polymer/water ratio
Sample Name SPX_PWY-RZ DMA/g H2O/g Silica/g KPS/g MBA/g TEMED/g SwellingaQ0
SP0_PW0.041 0.594 14.38 0.016 0.007 26.5SP0_PW0.047 0.668 14.31 0.018 0.008 23.5SP0_PW 0.052 0.7425 14.23 0.02 0.009 21.2SP0_PW0.11 1.485 13.50 0.041 0.017 10.6SP0_PW0.14 1.485 10.44 0.041 0.017 8.4SP0.5_PW0.14 1.485 10.44 0.74 0.041 0.017 8.4SP0.5_PW0.12 1.485 12.67 0.74 0.041 0.017 10.0SP1_PW0.12 1.485 11.92 1.49 0.041 0.017 9.5SP1_PW0.14 1.485 10.44 1.49 0.041 0.017 8.4SP2_PW0.14 1.485 10.44 2.97 0.041 0.017 8.4SP2_PW0.24 2.093 8.6 4.19 0.057 0.025 5.1SP2_PW0.44 2.79 6.31 5.58 0.076 0.033 3.3SP2.2_PW0.47 2.79 5.89 6.18 0.076 0.033 3.2SP3.1_PW0.34 2.093 6.21 6.57 0.057 0.025 4.1SP3.5_PW0.14 1.485 10.44 5.20 0.041 0.017 8.4SP5_PW0.1 1.395 14.14 6.97 0.038 0.016 11.7SP5_PW0.14 1.395 9.81 6.97 0.038 0.016 8.4SP5_PW0.21 1.395 6.53 6.97 0.038 0.016 5.9SP2_PW0.14_R0.1 1.485 10.44 2.97 0.041 0.002 0.017 8.4SP2_PW0.14_R0.2 1.485 10.44 2.97 0.041 0.005 0.017 8.4SP2_PW0.14_R0.5 1.485 10.44 2.97 0.041 0.012 0.017 8.4
a Q0, swelling at the preparation state.
Publ
ishe
d on
07
June
201
0. D
ownl
oade
d by
Por
tland
Sta
te U
nive
rsity
on
06/1
0/20
13 1
8:46
:08.
View Article Online
assuming full conversion of monomers into the polymer network
as mentioned previously and the additivity of volumes:
Q0¼1þvw
spe
vpspe
m0
p;w
m0p
� 1
!(1)
with vwspe the specific volume of water, vp
spe the specific volume of
the dry polymer network (vpspe ¼ 0.95 mL g�1), m0
p the mass of the
polymer network (m0p ¼ mDMA + mMBA) and m0
p,w ¼ m0p + m0
w
the total mass of the swollen gel in the preparation state with m0w
the total mass of water excluding the silica contribution as the
water does not swell the inorganic phase.
Drying
Drying experiments were carried out on various hydrogel
samples under ambient conditions by measuring the loss of water
as a function of time from a rectangular piece of swollen gel
(100 mm � 5 mm � 2 mm) exposed to air but without any
specific convective flow. For a gel sample of approximately 1 g,
the drying rate is equal to 0.1 mg min�1 cm�2.
Swelling measurements
For each sample, small pieces of hydrogel in the preparation state
were initially cut, weighed and then placed at room temperature
into a glass container with a large excess of sodium chloride 0.5
mol L�1. The solution was changed every day during 10 days and
the gels, swollen at equilibrium, were finally weighed. The equi-
librium swelling ratio (Qe) of the hydrogels in sodium chloride
0.5 mol L�1 were calculated according to eqn ({1}) taking into
account the mass of polymer network and the total mass of
absorbed solution.
Rheology
Rheological experiments were carried out at 25 �C on
a controlled stress rheometer (Haake RS600) equipped with
This journal is ª The Royal Society of Chemistry 2010
parallel roughened plates, a solvent trap filled with water for
minimizing evaporation of hydrogel sample and a Peltier device
for the temperature control. The gap between the plates was set
at 3 mm before the onset of the measurements and the sample
was coated with a thin layer of paraffin oil to prevent water
evaporation. The frequency sweep was applied at low deforma-
tion, in the linear regime, over the frequency range 0.1–10 Hz.
Mechanical testing
Tensile tests were performed on a standard tensile Instron
machine, model 5565. The device used a 10 N load cell (with
a relative uncertainty of 0.16% in the range from 0 to 0.1 N) and
a video extensometer which follows the local displacement up to
120 mm (with a relative uncertainty of 0.11% at full scale).
The gel samples (L ¼ 100 mm, w ¼ 5 mm, t ¼ 2 mm) were
prepared in triplicate and stored as previously described. The
gauge length was taken constant for all the tests (L0 � 30 mm)
and the gel strip was marked with two dots with a white marker,
for their recognition by the video extensometer. Three different
mechanical tests were carried out at room temperature: mono-
tonic tensile tests, loading-unloading cycles and stress relaxation
tests.
Tensile tests were carried out at two different strain rates: at
100 mm min�1 (nominal strain rate _3 � 0.06 s�1) for standard
conditions and at higher rate 1000 mm min�1 (nominal strain rate
_3 � 0.60 s�1).
Loading–unloading experiments were performed in order to
characterize the dissipated strain energy (hysteresis). Measure-
ments were carried out on hydrogels of similar size, using the
same conditions as reported for the tensile measurements in
standard conditions (_3 � 0.06 s�1). The loading was applied at
a main constant crosshead velocity, from 0 to the maximal strain
value, 3max and then the sample was unloaded. This loading–
unloading cycle was repeated 3 times. Two levels of maximal
Soft Matter, 2010, 6, 3619–3631 | 3621
Publ
ishe
d on
07
June
201
0. D
ownl
oade
d by
Por
tland
Sta
te U
nive
rsity
on
06/1
0/20
13 1
8:46
:08.
View Article Online
strains were investigated (3max ¼ 0.5 and 3max ¼ 5) and all the
experiments were duplicated.
Stress relaxation tests consist in maintaining a fixed strain level
and recording the stress as a function of time. The same proce-
dure was carried out for all the samples using a constant strain
level of 30 ¼ 0.5 and a fixed strain rate _3 � 0.06 s�1 to reach the
applied strain.
Results and discussion
Polymerization of DMA and self-crosslinking process
Alkyl-acrylamides like DMA or NIPA are very common water-
soluble monomers often used in the preparation of hydrophilic
macromolecular structures. One of the main advantages of DMA
compared to the well-known unsubstituted acrylamide is its high
solubility in both water and organic solvents which opens the
route to homogeneous copolymerization with hydrophobic
comonomers and the preparation of amphiphilic polymers or
networks.37–39 In the case of NIPA or more hydrophobically
substituted monomers, the use of organic solvent is also an
interesting way to prepare homogeneous networks and to avoid
any phase separation that generally occurs in aqueous media,
even at low temperature, just above the lower critical solution
temperature (LCST) of the polymer.
Polymerization in aqueous media is of course more friendly for
the environment but in that case radical initiators can induce
transfer reactions leading to nonlinear polymer structures.
Potassium persulfate can react with substituted amines and the
extent of the reaction strongly depends on the level of substitu-
tion of nitrogen.40 For instance, in the case of acetamide deriv-
atives, the reaction of KPS carried out in equimolar conditions
(0.4 mol L�1) at 85 �C in oxygen-free conditions leads to a full
transformation of N,N-dimethylacetamide, whereas only 56% of
N-methylacetamide is modified and less than 10% of acetamide.
According to the authors, KPS dealkylates N-substituted and
N,N-disubstituted amides to amides and N-substituted amides
respectively, while the dealkylated group appears in the reaction
mixture as the corresponding aldehyde or ketone. As shown in
Fig. 1A, the primary radicals issued from the thermal decom-
position of persulfate can react with substituted amides via
radical attack on the a-carbon to the amide nitrogen. If we
Fig. 1 (A) First steps of the radical attack of persufate on N,N-dimethylal
polymerization of N,N-dimethylacrylamide induced by persulfate initiator.
3622 | Soft Matter, 2010, 6, 3619–3631
assume a similar mechanism between KPS and the methyl groups
of DMA, these transfer reactions, which could occur either with
the monomer or the polymer, will give rise to the formation of
branched structures and/or three-dimensional networks with
a functionality equal to 3 (see Fig. 1B).
This very important feature of the polymerization of
N-substituted acrylamides is a major drawback if one aims to
prepare model linear polymers but could be useful if the main
goal is to develop soft materials with a solid rather than a liquid
behaviour. Self-crosslinking has been reported for instance in the
preparation of N-isopropylacrylamide gel nanospheres by Gao
and Frisken.41 On the basis of the reaction mechanism given in
Fig. 1, it appears that the viscoelastic properties of macromo-
lecular formulations would strongly depend on both monomer
and persulfate concentrations. In the framework of the present
study we have investigated the role of the monomer concentra-
tion between 0.1 and 1 mol L�1 (1 to 10 wt%) keeping the molar
ratios (DMA)/(KPS)/(TEMED) equal to 100/1/1.
At low concentrations, typically below 4 wt%, the formula-
tions obtained after polymerization remain rather inhomoge-
neous (suspension of polymer clusters) and therefore it was
difficult to get reproducible results with rheological measure-
ments. In the following, we will simply consider the data
obtained at higher concentrations. As plotted in Fig. 2, the elastic
character of the formulations obtained after polymerization
clearly increases with DMA concentration with a progressive
shift from liquid-like behaviour (G0 < G0 0 : solution) to solid-like
behaviour (G0 > G0 0 : gel).
At the gel point, both elastic and loss moduli should follow the
same frequency dependence (G0 �G0 0 � un).42 And this situation
is clearly observed in Fig. 2 at a concentration Cgel close to
4.5 wt%. Below Cgel, the formulation consists of a distribution of
finite clusters as already mentioned previously. Beyond the gel
point, the materials consists of a percolating and infinitely large
macromolecule which can swell but cannot dissolve in a solvent,
and lower molecular weight molecules (sol fraction) which are
still extractable from the gel. In the following, we will take
advantage of this self-crosslinking reaction of DMA to prepare
slightly crosslinked hybrid hydrogels.
In the work of Haraguchi et al.27 on NC gels, it was shown that
nanocomposite gels prepared at low polymer concentration
(typically below CDMA¼ 0.5 mol L�1, i.e. about 5 wt%) were very
kylamide40 and (B) extrapolation of this mechanism to self-crosslinking
This journal is ª The Royal Society of Chemistry 2010
Fig. 2 Elastic (filled symbols) and loss (hollow symbols) moduli of
PDMA formulations prepared at different DMA concentrations (without
MBA crosslinker): 4 wt%: SP0_PW0.041 (C); 4.5 wt%: SP0_PW0.047
(-); 5 wt%: SP0_PW0.052 (:) and 10 wt%: SP0_PW0.14 (;).
Publ
ishe
d on
07
June
201
0. D
ownl
oade
d by
Por
tland
Sta
te U
nive
rsity
on
06/1
0/20
13 1
8:46
:08.
View Article Online
brittle and often neither uniform nor transparent. On the other
hand, a significant increase of both modulus and strength was
observed above this concentration. This behaviour was corre-
lated to the progressive formation of a real organic network with
increasing DMA concentration (sol/gel transition) in the pres-
ence of clay nanoparticles. In the present work, this idea is
confirmed in the same concentration range without added
particles and we think that self cross linking of PDMA is an
important requirement in the formation of NC gels as it will
restrict the flow properties of polymer chains and will emphasize
their elastic behaviour.
Formulation of nano-hybrid hydrogels
Several criteria have to be considered for the design of hybrid
hydrogels.
(1) As previously reported, a critical DMA concentration is
needed to reach the percolation threshold and to prepare
a homogeneous macroscopic network. This aspect is directly
related to the weight ratio ‘‘polymer/water’’ (PWY in the
nomenclature) which is inversely proportional to the swelling Q0.
(2) In a previous study34 with the same Ludox particles, it has
been shown that small polymer chains of PNIPA and PDMA
interacted strongly and adsorbed onto silica surfaces. The
adsorption isotherms were almost the same for the two
polymers and the maximum amounts of adsorbed polymers
were Gmax y 1 mg m�2. Moreover, from calorimetric experi-
ments performed on PNIPA/silica mixtures, it was shown that
at low coverage (G < 0.5 mg m�2) the polymer chains strongly
adsorbed in a flat conformation at the surface of the particles
although at higher coverage (0.5 < G # 1 mg m�2) mainly loops
and tails were formed in the outer shell with swelling and
responsive properties.
On the basis of Ludox TM-50 particles characterized by an
average specific surface Sspe ¼ 77 m2 g�1, the maximum polymer
adsorption (Gmax ¼ 1 mg m�2) is expected for a weight ratio
‘‘silica/polymer’’ of 13, corresponding to SP13 according to the
chosen nomenclature.
This journal is ª The Royal Society of Chemistry 2010
Taking into account these two key aspects, we have tried to
explore a wide range of compositions but as shown in Table 1, we
still have worked with DMA concentrations above 10 wt%
(PWY¼ 0.10 to 0.44) in order to get hybrid hydrogels with solid-
like properties. Moreover the preparation of homogeneous
samples, starting from a well-dispersed suspension of particles
(concentration of 52 wt%), leads us to work well below Gmax
(SPX � 13), i.e. with a large excess of non-adsorbed polymer
chains.
At the end of the synthesis, a very simple comparison between
organic hydrogels of DMA and hybrid ones clearly shows that
the introduction of silica particles into the formulations gives rise
to much stiffer materials that can be easily die-cut and submitted
to tensile tests.
While hybrid hydrogels made from DMA are generally very
sticky at low concentration of silica particles (viscoelastic
behaviour), either the addition of a small amount of MBA (even
at 0.1 mol%) or silica particles strongly changes the mechanical
properties of the material that becomes less sticky but much more
brittle.
Swelling behaviour of hybrid hydrogels.
When a polymer network is immersed in a solvent, the swelling
equilibrium is reached when the osmotic pressure inside the gel
(Pgel) becomes equal to that of the surrounding medium (Pout),
as described by the following relation:
Pgel ¼ Pm + Pel + Pion ¼ Pout (2)
where Pm, Pel and Pion are, respectively, the mixing, the elastic
and the ionic contributions to the osmotic pressure.
In the case of hybrid hydrogels, the silica particles embedded
inside the network bring counter ions coming from partial
dissociation of silanol groups and the latter contribute positively
to the osmotic pressure (Pion > 0), like in polyelectrolyte gels.
With the aim of studying specifically the role of silica beads as
potential crosslinkers, all the swelling experiments have been
performed at high ionic strength (NaCl 0.5 mol L�1), in order to
screen the ionic contribution (Pion y Pout ¼ P0ion).
The general comparison between equilibrium swelling values
reported in Fig. 3 and 4, points out the influence of the polymer
concentration used in the preparation state, which determines the
level of entanglements inside the network, as well as the impact of
MBA and silica particles as crosslinkers. By comparison with
MBA, the silica particles are much less effective in limiting the
equilibrium swelling (see Fig. 4), but we have to consider that: (1)
the PDMA chains only interact with the silica surface and (2) as
opposed to organic networks which are generally considered
homogeneous, hybrid ones are heterogeneous in nature alter-
nating dense zones, where PDMA strongly interacts with silica
beads, with swollen ones which correspond to the pure PDMA
matrix. The swollen zones can therefore absorb water in large
quantities.
From a general point of view, the addition of fillers in elas-
tomer networks and the consequences on mechanical and
swelling properties is a very important issue which has been
widely studied experimentally and theoretically.43–48
Soft Matter, 2010, 6, 3619–3631 | 3623
Fig. 3 Swelling equilibrium of hybrid hydrogels in NaCl 0.5 mol L�1 as
a function of the molar percentage between MBA and DMA (Z param-
eter). The weight ratios between polymer and water at the preparation
state (Y parameter) are respectively: 0.14 (black), 0.24 (grey) and 0.44
(white).
Fig. 4 Swelling equilibrium, Qe of hybrid hydrogels SPX_PW0.14 in
NaCl 0.5 mol L�1 versus silica volume fraction (dots). Comparison with
the theoretical model of Lequeux without correction for adsorbed poly-
mer layer (solid line) and with correction (dashed line : r0 ¼ 17 nm and
t ¼ 2 nm, Qlayer ¼ 2).
Publ
ishe
d on
07
June
201
0. D
ownl
oade
d by
Por
tland
Sta
te U
nive
rsity
on
06/1
0/20
13 1
8:46
:08.
View Article Online
Swelling measurements of filled elastomers is for instance
a classical test to probe the level of interactions inside the
network. If there is no adhesion at all between the polymer
matrix and the particles, the filled polymer is expected to swell
slightly more at the equilibrium (Qe) than for the unfilled matrix
itself (Qref) due to the formation of cavities filled with solvent
around the particles (Fig. 5 (a)). Conversely, if strong interac-
tions exist between the matrix and the particles, the swelling Qe
will be lower than Qref and heterogeneous due to the constraints
at the polymer–filler interface (Fig. 5 (b)).
Using these assumptions and a continuous media mechanics
approach, Lequeux and coworkers48 have derived an analytical
expression relating the swelling properties of the filled elastomer
(Qe), the swelling of the polymer matrix alone (Qref) and the
volume fraction of particles in the initial state (f):
3624 | Soft Matter, 2010, 6, 3619–3631
Qe¼
24 Q
1=3ref þ
3ð1�Q1=3ref Þð1� nÞ
2ð1� 2nÞ þ ð1þ nÞð0:64=fð1þ t=r0Þ3Þ
!2
�f
35,ð1� fÞ
(3)
with n the Poisson coefficient, taken equal to 0.5, r0 the radius of
neat particles and t the thickness of the adsorbed polymer layer
(Fig. 5c).
Using the previous model, simply taking into account the
volume fraction of neat particles (t¼ 0), we get theoretical values
which slightly overestimate the experimental swelling of filled
network (see Fig. 4). As discussed by Lequeux and co-workers48
this general behaviour could be explained by the existence of
unswollen polymer layer strongly adsorbed at the particle
interface. The main effect is an increase of the effective volume of
hard particles which can be taken into account in the model using
an additional polymer layer of thickness, t. In the framework of
the present study, assuming that 1 mg of PDMA is adsorbed on
1 m2 of silica particles (r0 ¼ 17nm), we can estimate the thickness
of the PDMA layer t ¼ 1 – 2 nm, considering that this layer is
fully dry (Qlayer ¼ 1) or slightly swollen (Qlayer ¼ 2), respectively.
In Fig. 4, we can see that the model can be slightly improved
assuming a polymer layer of 2 nm but clearly a full agreement
cannot be obtained on the whole range of silica concentration.
One possible reason of the relatively slow decrease of the
equilibrium swelling at high silica fraction could be due to the
heterogeneous polymerization taking place in a dispersed media.
Based on strong interactions between silica and PDMA, we can
reasonably assume that during the polymerization process, there
is a competition between the kinetic chains propagating at the
interfaces and in the solution. Assuming that, at the end of the
polymerization, the total amount of adsorbed PDMA is given by
Gmax ¼ 1 mg m�2, the concentration of PDMA chains forming
the bulk matrix is obviously less concentrated than the one
prepared without silica. For example, in the case of SP5_PW0.14
with 38 wt% of DMA, is expected to polymerize and self-adsorb
on silica surfaces. The remaining 62 wt% of DMA will poly-
merize homogeneously in the solution, leading to an average
ratio polymer/water in the matrix of 0.08 instead of 0.14.
Therefore, although a constant polymer/water ratio was used for
this series, the local swelling ratio of the matrix may increase with
increasing silica content.
Mechanical properties
Tensile behaviour: initial modulus and non-linear behaviour. For
each sample, various tests have been performed in order to
ensure the reproducibility of the experiments. Typical stress–
strain curves are shown Fig. 6 for the standard nominal strain
rate _3 �0.06 s�1. Results and particularly the initial part of the
curve (i.e. initial stiffness) are highly reproducible for a given
formulation.
Self-crosslinked hybrid hydrogels are seen to be highly
deformable, with average strains above 1000%. However, the
unfilled samples SP0 samples are too soft and sticky to be
handled and only the modulus can be measured by rheometry (at
1 Hz: E0 ¼ E0 ¼ 3G0 ¼ 1.05 kPa).
The tensile properties of nanosilica PDMA gels strongly
depend on silica content, but the general tensile behaviour is
This journal is ª The Royal Society of Chemistry 2010
Fig. 5 Schematic representation of the swelling behaviour of a polymer layer (initial radius R0) surrounding a hard sphere (r0) : (a.) swelling without
adhesion between the particle and the polymer matrix, swelling with adhesion without (b) or with (c) an unswollen polymer layer.49
Fig. 6 Typical tensile stress-strain curves at _3 � 0.06 s�1: effect of filler content for SP0.5_PW0.14, SP1_PW0.14, SP2_PW0.14 and SP5_PW0.14.
Publ
ishe
d on
07
June
201
0. D
ownl
oade
d by
Por
tland
Sta
te U
nive
rsity
on
06/1
0/20
13 1
8:46
:08.
View Article Online
similar for all the samples. The behaviour is highly non-linear
and shows high levels of strain at break compared to the
conventional PNIPA29 or PDMA27 crosslinked gels. In the first
part of the curve, at low strains (below 2%), the initial modulus Ei
is defined and identified as the Young’s modulus. At intermediate
and large strains a marked softening followed by a distinctive
hardening relative to standard Gaussian elasticity, is observed.
The deformation process is homogeneous during the test: no
necking process is seen in the strain range investigated.
As shown in Fig. 7, a significant increase of the initial stiffness
is observed as the volume fraction of nanoparticles increases.
According to the well-known Guth and Gold model,50 the
modulus of the reinforced material, E is given as a function of the
unfilled matrix modulus, E0 and the volume fraction of filler, f by:
E ¼ E0(1 + 2.5f + 14.1f2) (4)
This model, typically used for f <0.2, implies an incompress-
ible matrix (Poisson’s ratio n ¼ 0.5), hard spherical particles and
no interaction between the fillers and the matrix. The particle
interface is considered to be inert. As shown in Fig. 7, the
addition of silica nanoparticles leads to a high increase in the
initial modulus, much more significantly than Guth and Gold’s
prediction (represented for comparison). This reinforcement
clearly points out that the introduction of silica does not only
lead to a so-called ‘‘hydrodynamic’’ reinforcement, but obvi-
ously, interactions between PDMA and silica surface should be
taken into account.
This journal is ª The Royal Society of Chemistry 2010
In principle the average molar mass between crosslinks can be
derived from experimental swelling and elastic modulus experi-
ments. Although, due to the heterogeneity of hybrid networks,
this calculation of the average molar mass value between cross-
links remains only qualitative, it remains useful for comparative
purposes. Assuming an infinite average functionality, both affine
and phantom models converge giving rise to the following
equations:
Mc;Q ¼V1
vspe
"�Q
�2=30 Q�1=3
e
Ln�1�Q�1
e
�þQ�1
e þ c12Q�2e
#(5)
Mc;E ¼3RT
vspeQ0Ei
(6)
where V1 ¼ 18 mL g�1 is the molar volume of water, vspe ¼ 0.95
mL g�1 the specific volume of PDMA, c12 ¼ 0.48 the Flory
interaction parameter for PDMA in water,51 Q0 the swelling ratio
of the hydrogels in their preparation state, Qe their equilibrium
swelling, Ei the initial modulus, R the ideal gas constant and T
the absolute temperature.
The molar masses obtained from swelling experiments and
tensile tests are compared in Table 2.
As expected from the previous relations, the molar mass Mc
decreases with increasing modulus or decreasing swelling at
equilibrium. The agreement between Mc values obtained from
tensile tests and swelling can be considered as rather good
Soft Matter, 2010, 6, 3619–3631 | 3625
Fig. 7 Effect of silica volume fraction on initial modulus ( _3 � 0.06 s�1).
Guth and Gold’s model is represented as dash line with E0 ¼ 1.05 kPa,
obtained from rheology.
Publ
ishe
d on
07
June
201
0. D
ownl
oade
d by
Por
tland
Sta
te U
nive
rsity
on
06/1
0/20
13 1
8:46
:08.
View Article Online
(Mc,E/Mc,Q y 1 � 1.5) for most of the samples except those
which are physically crosslinked with a large amount of silica
(Mc,E/Mc,Q < 0.5). As previously mentioned, the main difficulty
for the calculation of Mc arises from the heterogeneity of hybrid
networks. The discrepancy observed for SP2_PW0.14 and
SP5_PW0.14 demonstrates that the introduction of large
amounts of silica particles, which effectively behave as physical
crosslinkers, has a stronger impact on mechanical properties
than on the swelling equilibrium. The close correlation between
mechanical and swelling properties, obtained for hybrid
networks additionally crosslinked with MBA, could be explained
by a better homogeneity of network with chemical crosslinker.
In Fig. 8a, the initial modulus, Ei versus Q0 for two series of
hydrogels (SP2 and SP5) shows the same scaling behaviour :
E � Q�2.60 . This result is in good agreement with the theoretical
prediction of Obukhov et al. (E � Q�2.250 ) for lightly crosslinked
hydrogels in good solvent conditions, where modulus and
swelling are dominated by trapped entanglements.52
The modulus obtained for the SP5 series are systematically
higher than those of SP2 gels at the same Q0 reflecting a higher
content of silica particles and higher degree of physical cross-
linking. Interestingly we can show in Fig. 8b that a master curve
can be obtained by plotting the normalized modulus [(Ei/(Q0)�2.6]
versus the weight ratio silica/PDAM (X). This plot, which gathers
all the results obtained with hybrid hydrogels (without MBA),
clearly shows the linear dependence of the reduced modulus with
the concentration of silica particles. Taking into account entan-
glements and physical crosslinks, the Young modulus can be
written as:
Table 2 Comparison between Mc values calculated from (a) initial modulusperformed in NaCl 0.5 M (index Q)
Hydrogel SPX_PWY-RZ Ei/kPa Mc,E/kg mol�1
SP0_PW0.14 1.05 705SP0.5_PW0.14 1.7 540SP1_PW0.14 2.9 317SP2_PW0.14 6.5 143SP5_PW0.14 22 42SP2_PW0.14-R0.1 15 62SP2_PW0.14-R0.2 21 44SP2_PW0.14-R0.5 40 23
3626 | Soft Matter, 2010, 6, 3619–3631
E � XQ�2.60 � X.Y2.6 (7)
According to this relation we can see that for a given silica/
polymer ratio, X, the modulus only depends on trapped entan-
glements (E � Q�2.250 � Y2.25), while for a given concentration of
trapped entanglements (Q0 y Y�1), the modulus will scale with
the number of physical crosslinkers and X is the right variable in
that case. This result differs from classical theory of chemically
crosslinked entangled networks where the contribution of
chemical crosslinks and entanglements are additive as:53
E � CRT
�1
Mc
þ 1
Me
�
where Mc and Me are the average molar mass strands between
crosslinks and entanglements, respectively, and C is the polymer
concentration.
Contrary to model networks, where physical and chemical
crosslinks are expected to be randomly distributed, it is not the
case with hybrid gels where the silica nanoparticles could intro-
duce a high level of heterogeneity in the distribution of cross-
links. From the results plotted in Fig. 8b we can infer that the
elastic modulus of hybrid networks is originally controlled by the
level of entanglements between PDMA chains and that silica
nanoparticles, which strongly interact with the polymer matrix,
dramatically increase or amplify the connectivity of the network.
A schematic picture is given in Fig. 9, where the adsorption of
PDMA chains on silica surfaces create domains of high polymer
density which are connected to each other through the entangled
PDMA matrix.
The important stiffening parameter in hybrid networks is not
the absolute concentration of silica particles but the weight ratio
silica/polymer that is proportional to the fraction of PDMA
which effectively interacts with silica surfaces. This result can be
also closely compared with the data obtained by Haraguchi and
coworkers on NC PDMA and PNIPA gels.27,30 In the case of NC
PDMA hydrogels the modulus and the ultimate tensile strength
was shown to increase almost proportionally to the clay content,
for a given polymer concentration. From a more detailed study,
performed by neutron scattering and mechanical testing on NC
PNIPA gels, they conclude that the number of PNIPA chains
bridging clay platelets per unit volume (Nchain), increases with the
concentration of inorganic particles as: Nchain � [clay]4/3. In that
case, the additional [clay]1/3 dependence of Nchain is ascribed to
the fact that crosslinks are not dispersed in a three-dimensional
space in the gel but are localized on a two-dimensional space
(planar crosslinking), i.e., on the surface of clay platelets.
Ei obtained at standard strain rate (index E) and (b) swelling experiments
Qe Mc,Q/kg mol�1 Mc,E/Mc,Q
122 605 1.293 370 1.584 306 1.182 296 0.555 141 0.337 65 1.027 34 1.317 13 1.7
This journal is ª The Royal Society of Chemistry 2010
Fig. 8 (a) Variation of the Young modulus versus the initial swelling of hybrid hydrogels SP2 and SP5. (b) Master curve of the normalized Young
modulus [(Ei/Q0)�2.6] versus the weight ratio silica/polymer (X).
Fig. 9 Schematic representation of the structure of hybrid hydrogels
with silica nanoparticles covered by PDMA layer embedded into an
entangled PDMA matrix with additional chemical crosslinks.
Fig. 10 Reduced stress plotted as a function of reciprocal l for PW0.14
hydrogels.
Publ
ishe
d on
07
June
201
0. D
ownl
oade
d by
Por
tland
Sta
te U
nive
rsity
on
06/1
0/20
13 1
8:46
:08.
View Article Online
While many publications note that the ‘‘mechanical proper-
ties’’ can be greatly improved by a proper design of the gel
network, most publications simply report the elastic modulus
and the nominal strain at break. The non-linearities in the
behaviour (i.e. non-linear viscoelasticity, hysteretic behaviour,
residual strain) are rarely investigated but can provide mean-
ingful understanding on micro-mechanisms of deformation or on
damage processes. For weakly crosslinked systems, the nonlinear
behaviour in uniaxial extension is very sensitive to subtle changes
in structure and can be used to gain insight on the organization of
the gel.54 It is the purpose of this section to investigate in more
detail the large strain behaviour of the hybrid hydrogels.
The stress-strain curves shown in Fig. 6 are markedly non
linear and significantly deviate from the classical rubber elasticity
prediction. These deviations from the rubber elasticity model are
generally represented in a Mooney diagram that considers the
variation of the reduced stress versus the reciprocal elongation, l
� 1.55,56 In this representation the reduced stress (s*) is given by:
s* ¼ F/A0(l � 1/l2) (8)
with F the force, A0 the cross-sectional area of the undeformed
sample (at Q0).
The advantage of this type of representation for soft non linear
materials is that the softening or hardening relative to the
This journal is ª The Royal Society of Chemistry 2010
Gaussian elasticity prediction appears simply as a downwards or
an upwards slope as a function of 1/l. As shown in Fig. 10,
a softening is observed for all our samples but the amplitude of
the softening increases with the fraction of silica particles. From
a molecular point of view, this softening behaviour can be caused
by different mechanisms: the reversible orientation of entangle-
ments in the tensile direction,57 the viscoelastic relaxation of the
non trapped entanglements or the desorption of polymer chains
from the silica particles.
At high extension (low 1/l) the upturn emphasizes a hardening
behaviour, usually attributed in rubbers either to finite chain
extensibility or to other processes such as strain-induced crys-
tallisation commonly observed in natural rubber.58,59 Looking
at the minimum of the curves, there is a clear correlation between
the amount of added chemical crosslinker (R ¼ 0.1 mol%) and
the reduction of chain extensibility, shorter values of Mc leading
to lower chain extensibility, since lmaxf M1/2c , see Table 2.
If we compare the influence of added silica for the self-cross-
linked samples, the minimum in the reduced stress curves occurs
Soft Matter, 2010, 6, 3619–3631 | 3627
Publ
ishe
d on
07
June
201
0. D
ownl
oade
d by
Por
tland
Sta
te U
nive
rsity
on
06/1
0/20
13 1
8:46
:08.
View Article Online
for higher values of l for higher amounts of added particles
which it is quite surprising if we consider that the interactions
between silica and PDMA are responsible for the formation of
‘‘transient crosslinks’’ which should decrease the extensibility of
the chains. At very high strains, the upturn appears sharper with
increasing amounts of added particles. The silica particles
essentially add a significant concentration of elastically active
chains with a relatively narrow distribution of distances between
particles.
Fig. 12 First loading-unloading cycle for SP0.5_PW0.14, SP1_PW0.14,
Dissipative behaviour: strain rate effect and hysteresis
Qualitatively, the impact of the strain rate on the mechanical
behaviour is shown in Fig. 11, with a significant increase of the
strain rate dependence with increasing amounts of added parti-
cles. This suggests that the fraction of the softening due to
viscoelastic relaxation processes is greatly increased as the
particles are added. Furthermore the characteristic relaxation
times introduced by the presence of particles are obviously in the
range of the applied strain rates.
Three loading–unloading cycles have been repeated with two
levels of maximal strains investigated, respectively 3max¼ 0.5 and
3max ¼ 5. The nominal strain rate has been mainly kept constant
during the test (i.e. _3� 0.06 s�1). After a loading–unloading cycle,
the dissipated energy has been calculated integrating the area
under the loop. The dissipated energy, Ed calculated for each first
cycle has been normalized by Ea, the applied strain energy
(i.e. the area under the loading curve). The instantaneous
residual deformation, 3res has been determined as soon as the
sample has been unloaded. Moreover, in order to look at the
possible damage in the structure during the cycle, the initial
moduli have been determined for the different repeated loading.
Fig. 12 compares the first cycle for 3max ¼ 5.
These relative values, plotted in Fig. 13, clearly indicate that
dissipative mechanisms increase with increasing amounts of silica
nanoparticles. The normalized dissipated energy is seen to
depend on the silica volume fraction, independently of the strain
level, both at intermediate strains (below 50%) and at large
Fig. 11 Stress–strain curves for SP1_PW0.14 (sparse marker, B), and
SP5_PW0.14 (sparse marker, ,) at nominal _3 ¼ 0.06 s�1 (solid line) and
nominal _3 ¼ 0.6 s�1 (broken line).
3628 | Soft Matter, 2010, 6, 3619–3631
strains (up to 500%). Interestingly, the extrapolation of the
results at f/0, seems in contradiction with the unfilled hydrogel
SP0_PW0.14 formulation which is very soft and too sticky to be
handled for tensile tests. Addition of chemical crosslinks leads to
a significant decrease in relative disspated mechanisms.
We can see that the addition of particles also implies a higher
part of irreversible processes in mechanisms of deformation,
increasing the instantaneous residual strain, 3res. A similar
behaviour has been reported by Haraguchi et al.60 with NC
PNIPA gels prepared with a high clay content. Nevertheless, the
process here is seen to be very fast since the areas obtained
remain very low regarding the range of silica concentrations
investigated (silica/PDMA ¼ X ¼ 0.5 to 5) compared to weight
ratios clay/PNIPA ¼ 0.3 to 1.5.60
The observed increase in residual strain due to the presence of
silica is consistent with the hypothesis that PDMA and silica
nanoparticles form non-permanent and reversible interactions.
SP2_PW0.14, SP5_PW0.14, respectively for 3max ¼ 5 at nominal
_3 ¼ 0.06s�1.
Fig. 13 Normalized dissipated strain energy and residual strain for the
first cycle for two maximal applied strain 50% and 500%.
This journal is ª The Royal Society of Chemistry 2010
Publ
ishe
d on
07
June
201
0. D
ownl
oade
d by
Por
tland
Sta
te U
nive
rsity
on
06/1
0/20
13 1
8:46
:08.
View Article Online
By comparison, both dissipated energy and residual strain
decrease with the introduction of a small proportion of chemical
crosslinker (comparing SP2_PW0.14 vs. SP2_PW0.14-R0.1).
This implies that the dissipative mechanisms are not only due to
the presence of particles, as an analogy with Mullins effect, but
they should be also due to the ‘bulk’ material properties and to its
very low crosslinking density.
Moreover, looking at the second and third applied cycles,
attention has been paid on the evolution of the initial moduli
during consecutive loadings, respectively noted E2ndi and E3rd
i . By
comparison with NC gels prepared with anisotropic clay plate-
lets, the effect of residual orientation that has been reported60 is
not expected in the case of silica nano-particles.
The loss in initial stiffness can be simply interpreted as a global
estimate of irreversible processes. A damage parameter, Dnth is
defined as follows:
Dnth ¼ 1� Enth
i
E1st
i
(9)
with Enth
i the initial modulus during the nth loading cycle and E1st
i
the initial modulus during the first loading cycle.
This parameter has been calculated for cycles at intermediate
strains (below 50%) and for large strains cycles (up to 500%). As
shown in Table 3 for intermediate strains, there is no significant
Table 3 Damage parameter for the second and third cycles at 50% and500% deformation
Sample
3max ¼ 0.5 3max ¼ 5
D2nd D3rd D2nd D3rd
SP0.5_PW0.14 0.07 � 0.01 0.07 � 0.01 �0.2 � 0.19 0.04 � 0.08SP1_PW0.14 0.01 � 0.03 0.03 � 0.02 0.27 � 0.03 0.24 � 0.09SP2_PW0.14 0.02 � 0.02 0.02 � 0.02 0.40 � 0.01 0.37 � 0.00SP5_PW0.14 0.06 � 0.02 0.05 � 0.02 0.58 � 0.05 0.66 � 0.07SP2_PW0.14-R0.1 0.07 � 0.00 0.07 � 0.00 – –
Fig. 14 Stress relaxation behavio
This journal is ª The Royal Society of Chemistry 2010
change in the initial stiffness with a quantitative recovery inde-
pendent of the particle content.
At large strains, damage appears more markedly with
increasing amount of silica particles. No significant increase in
damage has been observed between the second cycle and the
third one, since D2nd are nearly equivalent to D3rd.
Long time behaviour: stress relaxation. The stress relaxation
behaviour is plotted in Fig. 14, using the normalized stress
s(t)/smax, with smax equal to the maximum stress value. The time
reference is taken to be equal to zero when the imposed strain
level, 30 ¼ 0.5 is attained (i.e. for s ¼ smax).
For all the samples, a viscoelastic effect is pointed out, since
stress relaxation is observed as a function of time. For a suffi-
ciently long time, above 3 min, the stress is stabilized at a given
level and it is noticeable that the amount of silica strongly
influences the relaxation process, clearly showing that the life-
time of silica–PDMA interactions is a key-aspect of their
properties.
Moreover, as it has been shown by Adler and coworkers,32,61
the relaxation amplitude is also dependant on inherent properties
of polymer chains. In our case, the crosslinker addition,
comparing SP2_PW0.14 vs. SP2_PW0.14-R0.1 relaxation stress
behaviours, leads to a reduction of the viscoeslastic component.
As discussed by Haraguchi et al.,27–29,33,60,62 the exceptional
properties of nanocomposite hydrogels mainly originate from
the very large size of elastically active chains which are assumed
to be connected through inorganic particles. The situation of
silica/PDMA hybrid networks is very close even if we assume
some additional chemical crosslinks issued from the synthesis.
Taking into account the strong adsorption behaviour of PDMA
on silica surfaces, we can schematically picture the structure of
hybrid hydrogels as spherical particles covered by an adsorbed
polymer layer; the connectivity of the network being provided by
polymer chains bridging particles through an entangled polymer
matrix (Fig. 9).
ur of hybrid gels for 30 ¼ 0.5.
Soft Matter, 2010, 6, 3619–3631 | 3629
Publ
ishe
d on
07
June
201
0. D
ownl
oade
d by
Por
tland
Sta
te U
nive
rsity
on
06/1
0/20
13 1
8:46
:08.
View Article Online
Conclusions
In this paper, we have developed a simple synthetic strategy to
obtain highly extensible and yet mostly elastic nano-composite
‘self-crosslinked’ hydrogels. Taking advantage from side reac-
tions during homopolymerization of DMA in water with clas-
sical monomer/initiators, the ‘self-crosslinked’ gels reported here
are made from relatively high amounts of silica nano-particles
(silica/polymer weight ratio from 0.5 to 5, i.e. volume fractions
from 0.03 to 0.21).
The improvement in properties relies on the synergy between
a lightly swollen crosslinked polymer matrix and strongly inter-
acting silica nanoparticles creating a hydrogel with combination
of physical and chemical crosslinks. Several important conclu-
sions can be drawn from our results.
The samples obtained from the polymerization of DMA
without any added crosslinker were lightly crosslinked and had
a solid-like character above 5 wt% polymer in water as shown by
rheological measurements. This behaviour, unreported so far, is
consistent with the existence of transfer reaction to the monomer
and justifies the use of the term ‘‘self-crosslinked’’ to refer to the
gels. At those polymer concentrations, the self-crosslinked gels
made from PDMA only were however too soft and sticky to be
mechanically tested as self-standing films. The addition of a small
amount of silica particles led to a dramatic change in macro-
scopic mechanical properties and in particular to a great increase
in the elastic character. This improvement of mechanical prop-
erties did not result in any loss in transparency.
From mechanical tests performed with gels in the preparation
state, experiments showed that small strain modulus is controlled
by the product of the density of entanglements by the weight
ratio of silica nanoparticles, a remarkable result suggesting that
each silica particle contributes an additional density of elastic
chains which is proportional to the existing entanglement density
of the polymer in water.
The strong adsorption of the PDMA polymer on each particle
led up to a factor of 20 in elastic modulus increase while retaining
an extensibility of 1000%, a remarkable result. Instantaneous
recovery in strain is seen to be quite high for loading/unloading
cycles at 0.5 strain, inferior to 0.08 strain for a silica volume
fraction of 0.10. However, after higher strain levels (loading–
unloading cycle at 5 in strain) beyond about 0.05 volume fraction
of silica, the gel starts to show a significant level of hysteresis and
a non negligible residual deformation.
Tensile, hysteresis and relaxation experiments all point to the
following physical picture: at a few percent of silica volume
fraction, the viscoelastic poorly crosslinked matrix become
physically crosslinked by the particles and behaves as an elastic
gel with very high extensibility and a modulus of the order of
2–3 kPa. When additional silica is added the resulting interac-
tions between silica particles become comparatively more
important, resulting in a modulus increasing up to 20–40 kPa at
20–25 vol% silica but the gel demonstrates also an increasingly
significant strain rate dependence, hysteresis and residual
deformation.
In summary our results demonstrate that hybrid hydrogels,
with mechanical properties similar to the clay-filled hydrogels,
can be obtained with silica particles provided that the polymer
strongly adsorbs on the surface of the particles. These results are
3630 | Soft Matter, 2010, 6, 3619–3631
very promising and further optimization of the mechanical
properties can most likely be obtained by tuning the ratio of
chemical vs. physical crosslinks as well as the level of particle–
matrix interaction.
Acknowledgements
The authors would like to acknowledge Wei Fan, Freddy
Martin, Ludovic Olanier, Dr Wei-Chun Lin, Dr Tetsuharu
Narita and Dr Guylaine Ducouret for their help and advice in
synthesis and mechanical characterization of hydrogels. We
would like to specifically thank Prof Costantino Creton for many
helpful discussions.
References
1 F. L. Buchholz and A. T. Graham, Modern Superabsorbent PolymerTechnology, Wiley-VCH, 1997.
2 W. Oppermann, B. Lindemann and B. Vogerl, in Polymer Gels:Fundamentals and Applications, ed. H. B. Bohidar, P. Dubin andY. Osada, ACS Symposium Series 833, Oxford University Press,2003, vol. 833, pp. 37–50.
3 N. A. Peppas, Hydrogels in Medicine and Pharmacy, CRC Press, BocaRaton, FL, 1986.
4 O. Wichterle, Soft Contact Lenses, Wiley, New York, 1978.5 M. Haider, Z. Megeed and H. Ghandehari, J. Controlled Release,
2004, 95, 1–26.6 B. Jeong and A. Gutowska, Trends Biotechnol., 2002, 20, 305–360.7 J. Kopecek, Eur. J. Pharm. Sci., 2003, 20, 1–16.8 R. Langer and D. A. Tirrell, Nature, 2004, 428, 487–492.9 Z. Megeed, J. Cappello and H. Ghandehari, Adv. Drug Delivery Rev.,
2002, 54, 1075–1091.10 S. Varghese and J. Elisseeff, Adv. Polym. Sci., 2006, 203, 95–144.11 T. Ikeda, M. Nakano, Y. L. Yu, O. Tsutsumi and A. Kanazawa, Adv.
Mater., 2003, 15, 201–210.12 I. C. Kwon, Y. H. Bae and S. W. Kim, Nature, 1991, 354, 291–293.13 Y. Osada, H. Okuzaki and H. Hori, Nature, 1992, 355, 242–244.14 M. Shibayama, F. Ikkai, S. Inamoto, S. Nomura and C. C. Han,
J. Chem. Phys., 1996, 105, 4358–4366.15 A. Suzuki and T. Tanaka, Nature, 1990, 346, 345–347.16 D. Szabo, G. Szeghy and M. Zrinyi, Macromolecules, 1998, 31, 6541–
6548.17 T. Tanaka, Phys. Rev. Lett., 1978, 40, 820–823.18 R. Yoshida, K. Uchida, Y. Kaneko, K. Sakai, A. Kikuchi, Y. Sakurai
and T. Okano, Nature, 1995, 374, 240–242.19 A. S. Huffman, A. Afrassiabi and L. C. Dong, J. Controlled Release,
1986, 4, 213–222.20 Y. Okumura and K. Ito, Adv. Mater., 2001, 13, 485.21 C. M. Zhao, Y. Domon, Y. Okumura, S. Okabe, M. Shibayama and
K. Ito, J. Phys.: Condens. Matter, 2005, 17, S2841–S2846.22 T. Matsunaga, T. Sakai, Y. Akagi, U. I. Chung and M. Shibayama,
Macromolecules, 2009, 42, 6245–6252.23 J. P. Gong, Y. Katsuyama, T. Kurokawa and Y. Osada, Adv. Mater.,
2003, 15, 1155.24 Y. Tanaka, R. Kuwabara, Y. H. Na, T. Kurokawa, J. P. Gong and
Y. Osada, J. Phys. Chem. B, 2005, 109, 11559–11562.25 R. E. Webber, C. Creton, H. R. Brown and J. P. Gong,
Macromolecules, 2007, 40, 2919–2927.26 K. Haraguchi, Curr. Opin. Solid State Mater. Sci., 2007, 11, 47–54.27 K. Haraguchi, R. Farnworth, A. Ohbayashi and T. Takehisa,
Macromolecules, 2003, 36, 5732–5741.28 K. Haraguchi and T. Takehisa, Adv. Mater., 2002, 14, 1120–1124.29 K. Haraguchi, T. Takehisa and S. Fan, Macromolecules, 2002, 35,
10162–10171.30 S. Miyazaki, T. Karino, H. Endo, K. Haraguchi and M. Shibayama,
Macromolecules, 2006, 39, 8112–8120.31 M. Shibayama, T. Karino, S. Miyazaki, S. Okabe, T. Takehisa and
K. Haraguchi, Macromolecules, 2005, 38, 10772–10781.32 M. F. Zhu, Y. Liu, B. Sun, W. Zhang, X. L. Liu, H. Yu, Y. Zhang,
D. Kuckling and H. J. P. Adler, Macromol. Rapid Commun., 2006,27, 1023–1028.
This journal is ª The Royal Society of Chemistry 2010
Publ
ishe
d on
07
June
201
0. D
ownl
oade
d by
Por
tland
Sta
te U
nive
rsity
on
06/1
0/20
13 1
8:46
:08.
View Article Online
33 K. Haraguchi, H. J. Li, K. Matsuda, T. Takehisa and E. Elliott,Macromolecules, 2005, 38, 3482–3490.
34 L. Petit, L. Bouteiller, A. Brulet, F. Lafuma and D. Hourdet,Langmuir, 2007, 23, 147–158.
35 D. Portehault, L. Petit, N. Pantoustier, G. Ducouret, F. Lafuma andD. Hourdet, Colloids Surf., A, 2006, 278, 26–32.
36 W.-C. Lin, W. Fan, A. Marcellan, D. Hourdet and C. Creton,Macromolecules, 2010, 43, 2554–2563.
37 X. Y. Xie and T. E. HogenEsch, Macromolecules, 1996, 29, 1734–1745.
38 L. Guillaumont, G. Bokias and I. Iliopoulos, Macromol. Chem. Phys.,2000, 201, 251–260.
39 S. L. Cram, H. R. Brown, G. M. Spinks, D. Hourdet and C. Creton,Macromolecules, 2005, 38, 2981–2989.
40 H. L. Needles and R. E. Whitfield, J. Org. Chem., 1964, 29, 3632.41 J. Gao and B. J. Frisken, Langmuir, 2003, 19, 5212–5216.42 H. H. Winters and M. Mours, Adv. Polym. Sci., 1997, 134, 165.43 A. R. Payne, J. Appl. Polym. Sci., 1962, 6, 57–63.44 G. Kraus, J. Appl. Polym. Sci., 1963, 7, 861–871.45 J. Rehner, Reinforcement of Elastomers Interscience, New York, 1965.46 S. S. Sternstein and A. J. Zhu, Macromolecules, 2002, 35, 7262–7273.47 G. Huber and T. A. Vilgis, Macromolecules, 2002, 35, 9204–9210.48 J. Berriot, H. Montes, F. Lequeux, D. Long and P. Sotta,
Macromolecules, 2002, 35, 9756–9762.
This journal is ª The Royal Society of Chemistry 2010
49 J. Berriot, F. Lequeux, H. Montes and H. Pernot, Polymer, 2002, 43,6131–6138.
50 E. Guth, J. Appl. Phys., 1945, 16, 20–25.51 N. Gundogan, O. Okay and W. Oppermann, Macromol. Chem. Phys.,
2004, 205, 814–823.52 S. P. Obukhov, M. Rubinstein and R. H. Colby, Macromolecules,
1994, 27, 3191–3198.53 S. F. Edwards, Proc. Phys. Soc., 1967, 92, 9.54 F. Deplace, C. Carelli, S. Mariot, H. Retsos, A. Chateauminois,
K. Ouzineb and C. Creton, J. Adhes., 2009, 85, 18–54.55 M. Mooney, J. Appl. Phys., 1940, 11, 582.56 R. S. Rivlin, Philos. Trans. R. Soc. London, Ser. A, 1948, 241, 379–
397.57 M. Rubinstein and S. Panyukov, Macromolecules, 2002, 35, 6670–
6686.58 J. E. Mark, Rubber Chemestry and Technology, 1975, 48, 495–512.59 L. R. G. Treloar, in The physics of rubber elasticity, Clarendon Press,
Oxford, 1975, pp. 210–310.60 K. Haraguchi and H. J. Li, Macromolecules, 2006, 39, 1898–1905.61 Y. Liu, M. F. Zhu, X. L. Liu, Y. M. J. Ang, Y. Ma, Z. Y. Qin,
D. Kuckling and H. J. P. Adler, Macromolecular Symposia, 2007,353–360.
62 K. Haraguchi, H.-J. Li and L. Song, J. Colloid Interface Sci., 2008,326, 41–50.
Soft Matter, 2010, 6, 3619–3631 | 3631
