Network structure and compositional effects on tensile mechanical properties of hydrophobic association hydrogels with high mechanical strength

Article abstract of DOI:10.1016/j.polymer.2010.01.061

Hydrophobic association hydrogels (HA-gels) were successfully prepared through micellar copolymerization of acrylamide (AM) and a small amount of octylphenol polyoxyethylene acrylate (OP-4-AC) in an aqueous solution containing sodium dodecyl sulfate (SDS)

Full text of DOI:10.1016/j.polymer.2010.01.061

Polymer 51 (2010) 1507–1515
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Network structure and compositional effects on tensile mechanical properties
of hydrophobic association hydrogels with high mechanical strength
,
,
Guoqing Jiang a b, Chang Liu a, Xiaoli Liu a, Qingrui Chen a, Guohui Zhang a, Meng Yang a, Fengqi Liu a
*
a College of Chemistry, Jilin University, Changchun 130012, China
b Exploration and Development Research Institute, Daqing Oilfield Company Limited, Daqing 163453, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydrophobic association hydrogels (HA-gels) were successfully prepared through micellar copolymeri-
zation of acrylamide (AM) and a small amount of octylphenol polyoxyethylene acrylate (OP-4-AC) in an
aqueous solution containing sodium dodecyl sulfate (SDS). HA-gels exhibited excellent mechanical
properties and transparency. Especially, HA-gels possessed the capability of re-forming, such as self-
healing and molding. From Fourier transform infrared, swelling behavior and re-forming capability of
HA-gels, the network structure was established. On the basis of the micellar copolymerization theory, the
statistical molecular theory of rubber elastic, and using uniaxial stretching data, the length of the
hydrophobic microblocks, the effective network chain density and the molecular weight of the chain
length between cross-linking points were evaluated for all HA-gels; furthermore, they were also eval-
uated for the region of medium deformation by the Mooney-Rivlin theory. For HA-gels, we investigated
in detail the effects of the content of compositions in the initial solution, OP-4-AC, SDS and AM, on their
tensile mechanical properties on the basis of the proposed network structure. The results clearly indicate
their tensile strength, fracture energy, elastic modulus, and elongation strongly depended on their
composition content.
Received 5 July 2009
Received in revised form
24 December 2009
Accepted 29 January 2010
Available online 4 February 2010
Keywords:
Network structure
Mechanical properties
Hydrophobic association hydrogel
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
the binary structure, this network can suppress the stress concen-
trations to improve mechanical strength of hydrogels. NC gels are
Hydrogels have been widely applied to various fields for several
decades. However, from the viewpoint of materials, conventional
chemically cross-linked hydrogels have very limited applicability
due to their poor mechanical strength. Therefore, in recent years,
many efforts have been focused on the enhancement of mechanical
strength of hydrogels. As a result, four types of novel hydrogels with
high mechanical strength have been developed: topological
hydrogels (TP gels) [1], double network hydrogels (DN gels) [2],
nanocomposite hydrogels (NC gels) [3], macromolecular micro-
sphere composite hydrogels (MMC gels) [4]. TP gels possess figure-
of-eight cross-linkers that can slide along polymer chains. Due tothe
sliding mode, the tension among the polymer chains can be equal-
ized, therefore TP gels can be highly stretched without fracture to
almost 20 times its original length. DN gels have double network
with a high molar ratio of the first network to the second network,
and the second network (linear or loosely cross-linked) is inter-
penetrated in the first network (highly cross-linked) to form an
inhomogeneity network structure. Because of a nonlinear effect of
composed of specific polymers, a water-swellable inorganic clayand
water, in which the exfoliated clay acts as an effective multifunc-
tional cross-linker. As a result of the organic/inorganic network
structure, NC gels have the high functionality of the rigid cross-
linked points and a fairly narrow distribution of chain length,
which is the main origin that this material possesses high
mechanical strength. For the preparation of MMC gels, a peroxidized
macromolecular microspheres (MMSs) acts as both an initiator and
a cross-linker and two neighboring MMSs are joined by a grafted
polymer chain, which endows MMC gels with high mechanical
strength. On the basis of the four types of hydrogels, some novel
hydrogels are also prepared, which can provide special performance
besides mechanical strength, such as improved response rate [5] and
imparting a low surface sliding friction [6]. All these hydrogels are
possessed of good mechanical strength and rubberlike properties.
However, the lackof the capability of re-forming resulting from their
permanent cross-linking structure, the complex synthesis process
and a narrow range of reactor monomer selectivity restrict their
industrial and biomedical applications.
The hydrophobically modified polyacrylamide (HMPAM) has
been extensively studied over the past two decades. There has been
an increasing interest into the design of chemical structure of
* Corresponding author. Tel.: D86 431 85153800; fax: þ86 431 85168868.
E-mail address: liufengqi@jlu.edu.cn (F. Liu).
0032-3861/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.01.061

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G. Jiang et al. / Polymer 51 (2010) 1507–1515
hydrophobic monomers and the solution properties of HMPAM [7–
16]. In addition, considerable attention has also been focused on
hydrophobically modified hydrogels containing HMPAM [17–21]. In
hydrophobically modified hydrogels, a large number of hydrophobic
association domains are formed by hydrophobic associations of
hydrophobic groups belonging to two or more polymer chains, and
these hydrophobic association domains act as the physical cross-
linking points in a network of hydrogels. Consequently, the three-
dimensional network of hydrophobically modified hydrogels could
be constructed. The hydrophobic associations in hydrophobically
modified hydrogels has been thoroughly investigated, such as
swelling [22], the spin probe technique [18], small angle neutron
scattering [23,24], 13C NMR [24] and fluorescence [25]. These results
clearly demonstrate that hydrophobic groups can form hydrophobic
association domains in the network of hydrophobically modified
hydrogels. Moreover, hydrophobically modified hydrogels are of
significant interest because their properties can be tuned by
changing the structure, location, or concentration of the hydro-
phobic group [26]. Therefore, this kind of hydrogel is widely used for
drug release [26–28] and proteincarriers [29]. Although a significant
amount of literature is dedicated to the synthesis and the hydro-
phobic associations of hydrophobically modified hydrogels and to
the control of their release properties, much less attention has been
paid to their bulk hydrogels with good mechanical strength. In order
to overcome the disadvantages of the above-mentioned four types
of hydrogels, we have designed a new type of physicallycross-linked
hydrogel with good mechanical properties on the basis of the
intermolecular hydrophobic associations of HMPAM and surfactant
in an aqueous solution.
China. Acryloyl chloride (AC) was purchased from Shanghai Haiqu
Chemical Co., China. The above reagents were used without further
purification. AM and potassium persulfate (K2S2O8), provided by
Tianjin Fuchen Chemical Reagent Factory, China, were recrystal-
lized from distilled water before use and dried under vacuum at
room temperature. Other reagents were purchased from Beijing
Chemical Works, China, and used without further purification.
2.2. Synthesis of OP-4-AC
The OP-4-AC was synthesized as follows: first, a transparent
aqueous solution consisting of 38.20 g (0.10 mol) OP-4, 12.14 g
(0.12 mol) triethylamine (TEA), and 80 mL tetrahydrofuran (THF)
was prepared. This solution was added to a three-neck flask
equipped with electromagnetic stirrer in an ice-water bath. Next,
10.86 g (0.12 mol) AC in 20 mL of THF was added dropwise to the
former solution under the stirring conditions, and the water bath
temperature remained under 5 ꢀC. After dropwise addition was
complete, the mixture was further stirred for 5 h. Then, to separate
triethylamine hydrochloride that was the sediment of the reaction
system, the appropriate amount of acetone was added to the
mixture, and the upper clear liquid containing OP-4-AC was
distilled under reduced pressure to remove THF and the acetone.
Finally, the residual triethylamine hydrochloride was separated
from the product by centrifugation and the final product, OP-4-AC,
was dried to constant weight in vacuum at 40 ꢀC. The reaction
scheme is shown in Scheme 1.
2.3. Sample nomenclature and synthesis of HA-gels
Recently, the desired hydrogel was successfully prepared in our
laboratory [30]. The hydrogel is hydrophobic association hydrogels
(HA-gels) prepared by micellar copolymerization [8,13,16,31] of
acrylamide (AM) and a small amount of octylphenol polyoxy-
ethylene acrylate (OP-4-AC) as the hydrophobic monomer in an
aqueous solution containing sodium dodecyl sulfate (SDS). In
contrast with the above-mentioned four types of hydrogels, not
only did HA-gels exhibit outstanding transparency and mechanical
properties, but also possessed the capability of re-forming due to
dissociation and reassociation of cross-linking points, such as self-
healing and molding. In addition, dried HA-gels, which were
prepared by stretching HA-gels to a certain elongation for a period
of time in the air, can be used as shrinkable or thermal sensitivity
materials. Especially, it should be noted that the preparation
process of HA-gels was very simple; no external cross-linker was
used for the formation of their network structure and HA-gels have
a broad selectivity for component. In the present work, the network
structure for HA-gels is investigated in terms of their swelling
behavior and re-forming capability. Because the tensile test is
always an effective method to investigate mechanical properties of
various materials [3,20,32–36], we study the effects of composi-
tions of HA-gels on mechanical properties by uniaxial tensile test.
In this study, the compositions for HA-gels refer to OP-4-AC, SDS
and AM in the initial reaction solution.
HA-gels were prepared using initial reaction solutions consist-
ing of AM, OP-4-AC, SDS, solvent (H2O) and initiator (K2S2O8). In
order to prepare various HA-gels with different compositions, the
monomer content in the initial reaction solution was varied over
a wide range. For comparison, polyacrylamide hydrogel (PAM-gel)
was also prepared.
2.3.1. Sample nomenclature
In the present study, according to composition variable in the
initial reaction solution, HA-gels are expressed as F-C-X% gels,
where C stands for the composition variable. (i) When OP-4-AC was
variable, weight concentrations of SDS and AM were 3 wt% and 10
wt%, respectively, HA-gels are designated F-OP4-X% gels, where X
stands for molar percentage of OP-4-AC relative to AM. (ii) When
SDS was variable, weight concentration of AM was 10 wt% and
molar percentage of OP-4-AC relative to AM was 1 mol%, HA-gels
are designated F-SDS-X% gels in which X stands for weight
concentration of SDS. (iii) When AM was variable, weight concen-
tration of SDS was 3 wt% and molar percentage of OP-4-AC relative
to AM was 1 mol%, HA-gels are designated F-AM-X% gels, where X
stands for weight concentration of AM. For all HA-gels, nomen-
clature and the content of compositions in the initial reaction
solution are listed in Table 1.
2. Experimental
2.3.2. Synthesis of HA-gels
HA-gels were prepared by micellar copolymerization. The total
mass of the initial reaction solution was 30.00 g. For example, the
experimental procedure used for F-OP4-1% gel was as follows:
3.00 g (0.042 mol) AM, 0.18 g (0.00042 mol) OP-4-AC, 0.90 g
(0.0031 mol) SDS, and 24.33 g distilled water were added to
2.1. Materials
Octylphenol polyoxyethylene ether (OP-4) and SDS were
provided by Tianjin Guangfu Fine Chemical Research Institute,
Scheme 1. The reaction scheme of OP-4-AC synthesis.

G. Jiang et al. / Polymer 51 (2010) 1507–1515
1509
sectional area (38.48 mm2) was used for calculating tensile stress
and elastic modulus which was calculated from the increase in load
detected between elongations of 100% and 200%.
Table 1
Nomenclature and the content of compositions in initial reaction solution for
HA-gels.
HA-gels
AM (wt%)a
OP-4-AC (mol%)b
SDS (wt%)a
K2S2O8 (%)c
F-OP4-X%
F-SDS-X%
F-AM-X %
10
10
X
X
1
1
3
X
3
0.5
0.5
0.5
2.5.2. Swelling behavior
Swelling experiments were performed by immersing as-
prepared F-OP4-1% gel, F-OP4-2% gel and F-OP4-5% gel (initial
a
Weight concentration.
Molar percentage of OP-4-AC relative to AM.
Weight percentage of K2S2O8 relative to AM and OP-4-AC.
size of 7 mm
4
ꢁ 5 mm length) in an excess of distilled water at
b
c
room temperature and water was replaced everyday. The swelling
time of the former was 75 h and that of other the samples was 35
days. In each measurement, the samples were took about 10 s from
water and weighed after removing excess water from their face.
Swelling ratio was expressed by the ratio of weight of the swollen
hydrogel (Wgel) to its theoretical dried gel weight (Wdry).
a beaker. The mixture was treated with ultrasonic until a homoge-
neous solution was prepared. Then, 1.59 mL of the aqueous solution
of initiator K2S2O8 (0.01 g/mL) was added to the former solution.
Afterwards, the solution was added to a test tube and purged with
N2 for 10 min. Immediately, the tube was sealed. After being placed
at room temperature for 1 h, micellar copolymerization was
allowed to proceed in a water bath at 50 ꢀC for 5 h, and then HA-
gels with excellent mechanical properties were prepared, see
Fig. 1. In addition, under the same experimental conditions, we also
synthesized PAM-gel. For the compositions of the initial reaction
solution, PAM-gel and F-OP4-X% gels were the same except for
OP-4-AC.
3. Results and discussion
3.1. IR analysis of OP-4-AC
Fig. 2 shows the IR spectra of OP-4-AC and OP-4. From the IR
spectrum of OP-4, it is obvious that OP-4 shows a broad peak at
3450 cmꢂ1 corresponds to stretching vibration of –OH. However,
on the IR spectrum of OP-4-AC, the above-mentioned peak disap-
pears. Moreover, it can be seen that the peak at 1640 cmꢂ1 corre-
sponds to stretching vibration of C ¼ C, and the peak at 1733 cmꢂ1
corresponds to stretching vibration of C ¼ O. The peak at 986 cmꢂ1
corresponds to out-of-plane bending vibration of ¼ CH, whereas
the peak at 1400 cmꢂ1 corresponds to the shear-vibration of ¼ CH2.
The results demonstrate that OP-4-AC was successfully
synthesized.
2.4. Characterization
Fourier transform infrared (FTIR) was performed using OP-4-AC,
dried HA-gels and PAM-gel in order to analyze their compositions
and structures. Dried HA-gels and PAM-gel were prepared by
drying purified HA-gels and PAM-gel in vacuum at 60 ꢀC to
constant weight. FTIR spectra were obtained using an Avatar 360
FTIR spectrophotometer (Nicolet Inc., Madison, Wis.) with milled
dried HA-gels and PAM-gel by the conventional KBr disk tablet
method.
3.2. Preparation and IR analysis of HA-gels
HA-gels were prepared by a rather simple method. After
HMPAM was synthesized by micellar copolymerization of AM and
OP-4-AC, the hydrophobic monomer, OP-4-AC, was distributed
along the copolymer backbone in a microblock manner [13,37], and
hydrophobic associations of SDS and hydrophobic groups
belonging to two or more HMPAM chains resulted in the formation
of hydrophobic association domains [9,38]; these hydrophobic
association domains acted as cross-linking points and promoted
HMPAM chain cross-linking, so three-dimensional polymer
network were constructed. Fig. 3 shows the IR spectra of HA-gel
(F-OP4-3% gel) and PAM-gel. On the IR spectrum of HA-gel, the
peak at 1512 cmꢂ1 corresponds to the skeleton vibration of benzene
ring. The peak at 1035 cmꢂ1 corresponds to asymmetrical stretch-
ing vibration of aromatic ether bond. The peak at 900 cmꢂ1
corresponds to out-of-plane bending vibration of C-H of aromatics.
2.5. Measurements
2.5.1. Tensile mechanical properties
The measurements of tensile mechanical properties were per-
formed on HA-gels of the same size (7 mm
a Shimadzu Autograph AG-I with a 1 KN load cell (Shimadzu Corp.,
Kyoto, Japan) at 25 ꢀC. Here,
represents a diameter. The sample
length between jaws was 25 mm and the tensile experiment was
carried out at crosshead speed of 100 mm/min. Initial cross-
4
ꢁ 70 mm length) using
4
Fig. 1. Photographs of HA-gels with excellent mechanical toughness. F-OP4-2% gel can
exhibit high level of deformation, such as (a) elongation, (b) bending, (c) knotting and
(d) stretching after being knotted.
Fig. 2. FTIR spectra of OP-4-AC and OP-4.

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G. Jiang et al. / Polymer 51 (2010) 1507–1515
of the maximum of swelling shown in Fig. 5a, the swelling test of F-
OP4-1% gel within 75 h was performed, which indicated that the
phenomenon in Fig. 5a was reliable, see Fig. 5b. During the whole
swelling course in which HA-gels began to swell rapidly and then
self-deswell until swelling equilibrium, the swelling ratio of HA-
gels containing high OP-4-AC content was always greater than
that of HA-gels containing low OP-4-AC content. The phenomenon
is consistent with the conclusion that cross-linking densities for
HA-gels enhanced with increasing OP-4-AC content in HA-gels.
It is clear that the water molecules penetrated into HA-gels at
the beginning of swelling test, so that the volume of HA-gels
underwent rapid expansion. After about one day, however, the
swelling degree decreased rapidly with increasing swelling time
before the steady state reached, which means that the network
structure of HA-gels must be changed. We think that this self-
deswelling behavior for HA-gels should be mainly owed to the
rearrangement of their hydrophobic association structure. That is,
because of the restriction of network structure, there were some
hydrophobic groups which were free or only had weak inter-
reaction in the original HA-gels. When HA-gels were further
swollen, the mobility of the polymer chains as well as the non-
associating hydrophobic groups was enhanced, which resulted in
reassociation of the hydrophobic groups on their own or with other
hydrophobic association domains. As a result, cross-linking density
increased and hydrophobic association domains in the network
structure were more stable, which led to the self-deswelling
behavior of HA-gels. With increasing swelling time, the network
structure of HA-gels did not change and the swelling ratio of HA-
gels would reach the steady value at the end of this reassociation.
With increasing OP-4-AC content in HA-gels, the number of the
non-associating hydrophobic groups would increase, on the other
hand, cross-linking density also increased which led to being more
difficult for the polymer chains motion between the cross-linking
points; therefore the time required for self-deswelling would be
protracted. This deduction was verified by the swelling experiment.
As shown in Fig. 5, the time of self-deswelling lasted 32 h for
F-OP4-1% gel, 2 days for F-OP4-2% gel and 5 days for F-OP4-5% gel,
respectively.
Fig. 3. FTIR spectra of HA-gel (F-OP4-3% gel) and PAM-gel.
However, on the IR spectrum of PAM-gel, the above-mentioned
characteristic peaks do not exist. Therefore, the appearance of the
above three characteristic peaks indicates OP-4-AC was incorpo-
rated into the main chain of the copolymer and HMPAM was
formed by micellar copolymerization.
3.3. Network structure of HA-gels
The simple structural model for HA-gels is depicted in Fig. 4. The
unique network structure was first proposed on the basis of
hydrophobic associations of the HMPAM and SDS in the aqueous
solution. In this study, the evidence of HMPAM prepared is
provided by IR spectroscopy, see Fig. 3. For a meaningful compar-
ison of modified and unmodified samples, PAM-gel was also
synthesized in the presence of SDS under the same experimental
conditions, but the product could not form bulk hydrogels. This
result clearly confirmed that the gelation resulted from the cross-
linking effects of the hydrophobic associations rather than simple
entanglement of the polymer chains.
All HA-gels showed high transparency, irrespective of the
composition content. The phenomenon indicates that the size of
hydrophobic association domains as the cross-linking points in HA-
gels was smaller than the wavelength of visible light. In order to
verify the proposed structural model, the swelling behavior and the
re-forming capability of HA-gels are discussed. Fig. 5 shows that the
swelling ratio of HA-gels increased strongly with increasing
swelling time and attained a maximum value in about one day.
Surprisingly, with increasing swelling time, the swelling ratio
exhibited a rapid decrease and then reached a steady value after
a period of time. To further confirm the correctness and reliability
The property of structural reorganization of HMPAM [9,11,13]
endows HA-gels with the capability of re-forming. As shown in
Fig. 6, the molding experiment indicates that HA-gels can creep
within a long period of time due to the reversible hydrophobic
associations in hydrogels. Moreover, we found that this kind of
hydrogels possessed self-healing capability. The self-healing
mechanism, the degree of recovery and the tensile mechanical
properties of the self-healed specimen for HA-gels had been dis-
cussed elsewhere [39]. Obviously, on the basis of the analyses of
Fig. 4. Schematic illustration of the structural model with associating networks in the HA-gels.

G. Jiang et al. / Polymer 51 (2010) 1507–1515
1511
hydrophobic monomer per micelle) [10–13,16,40] that is calculated
according to equation (1) [12,16,20,40]:
½MHꢃNagg
NH
¼
(1)
½Sꢃ ꢂ CMC
Here, [MH] is the initial molar concentration of hydrophobic
monomer, [S] is the molar concentration of surfactant, CMC is its
critical micellar concentration and Nagg is its aggregation number.
In this study, because of the polymerization temperature was 50 ꢀC,
CMC was taken to be 9.2 ꢁ 10ꢂ6 mol/mL, and Nagg ¼ 60 was
assumed for SDS [10,12,13,40,41]. The values calculated of NH for
HA-gels are listed in Table 2.
3.4.2. The effective network chain density and the molecular weight
of the chain length between cross-linking points
From the beginning of the science of polymer networks, it has
been a major goal to control the effective network chain density
(number of effective cross-linked chains per unit volume) and the
molecular weight of the chain length between cross-linking points
[3]. Similar to NH, they are also indispensable network structural
parameters to the analysis of mechanical properties for HA-gels.
Subsequently, we will evaluate the effective network chain
density and the molecular weight of the chain length between
cross-linking points for small deformation and medium deforma-
tion of HA-gels in terms of the statistical molecular theory of rubber
elastic and the Mooney-Rivlin theory, respectively.
First, we evaluated the effective network chain density y0 and
the molecular weight of the chain length between cross-linking
points Mc on the basis of the statistical molecular theory of
rubber elastic. The y0 is evaluated from the uniaxial stretching data
on the assumption of affine deformation and incompressible
volume [42–47]:
Fig. 5. Time dependence of swelling ratio (Wgel/Wdry) measured for HA-gels. (a) The
swelling behavior of F-OP4-2% gel and F-OP4-5% gel during 35 days. (b) The swelling
behavior of F-OP4-1% gel during 75 h.
compositions, structure, swelling behavior and re-forming capa-
bility for HA-gels, all results indicate that the proposed network
structure was reasonable.
ꢀ
ꢁ
s
¼
y0kT
l
ꢂ
lꢂ2
(2)
Here,
l
is relative extension (
l
¼ L/L0),
s
is calculated with the
original section area of the HA-gels and y0 is calculated from
equation (2) by using at elongation of
[42,43,46,47], k is Boltzmann constant, T is the absolute tempera-
ture. Furthermore, y0 is related approximately to the Mc by [42]:
3.4. Parameters of network structure for HA- gels
s
l
¼ 2 (strain ¼ 100%)
3.4.1. The length of hydrophobic microblocks
In an aqueous solution, the hydrophobic associations of HMPAM
and SDS are strongly influenced by the size of the hydrophobic
blocks than by their effective number on the chain [12]. Therefore,
length of the hydrophobic microblocks of HMPAM is one of the
most important network structural parameters for HA-gels and has
a critical influence on their mechanical properties. The length of the
hydrophobic microblocks can be expressed as NH (the number of
ꢂ
ꢃ
r
NA
2Mc
Mn
y0
¼
1 ꢂ
(3)
Mc
Table 2
Network structural parameters of HA-gels with different composition content.
HA-gelsa
NH
y0
Mc
y0*
Mc*
b
(mol/m3)
(104 g/mol)
(mol/m3)
(104 g/mol)
F-OP4-1%
F-OP4-2%
F-OP4-3%
F-OP4-4%
F-OP4-5%
F-SDS-1%
F-SDS-2%
F-SDS-3%
F-SDS-4%
F-SDS-5%
F-SDS-6%
F-AM-8%
F-AM-10%
F-AM-12%
F-AM-15%
8.96
1.61
2.95
3.41
3.69
3.74
2.40
2.03
1.61
1.42
1.27
1.20
1.07
1.61
2.21
4.19
8.10
4.41
3.81
3.53
3.49
5.43
6.41
8.10
9.14
10.28
10.89
9.74
8.10
7.06
4.66
0.69
1.36
1.49
1.53
1.65
2.03
1.39
0.69
0.50
0.40
0.31
0.52
0.69
0.96
1.78
18.75
9.60
8.72
8.53
8.06
17.85
26.78
35.71
44.63
34.85
13.82
8.96
6.42
5.13
4.25
7.23
6.42
9.37
18.75
26.01
32.25
41.35
20.16
18.75
16.26
10.94
8.96
10.87
13.43
Fig. 6. HA-gels exhibit the capability of re-forming. (a1) The original sample of F-OP4-
3% gel. (a2) The original sample was pressed into a 2.5 mL syringe for molding. (a3) The
sample after molding at room temperature for 5 days.
a
F-OP4-1% gel, F-SDS-3% gel and F-AM-10% gel are the same.
b
Using Eq. (1) with CMC ¼ 9.2 ꢁ 10ꢂ6 mol/mL and Nagg ¼ 60.

1512
G. Jiang et al. / Polymer 51 (2010) 1507–1515
Table 3
Here,
r is the density of the polymer in the swollen hydrogel, NA is
Mechanical properties of HA-gels with different composition content.
the Avogadro’s number, Mn is the molecular weight of the primary
molecular chains, 2Mc/Mn is a correction for free chain ends which
can not effectively tied in cross-linked network of HA-gels, and
these free chain ends may be neglected because their number is
small relative to cross-linked chains [42]. Moreover, assuming the
conversion of the monomer to HMPAM is complete, using the
density of polyacrylamide (1.302 g/cm3) [48], the polymer densities
in the swollen hydrogel is approximately equal to 0.1302 g/cm3 for
F-AM-10% gel, F-OP4-X% gels and F-SDS-X% gels, 0.1042 g/cm3
for F-AM-8% gel, 0.1562 g/cm3 for F-AM-12% gel and 0.1953 g/cm3
for F-AM-15% gel, respectively. As a result, the values calculated of
y0 and Mc for HA-gels are listed in Table 2.
For medium deformation, we also evaluated the effective
network chain density y*0 and the molecular weight of the chain
length between cross-linking points M*c through the Mooney-Rivlin
theory. The equation of uniaxial stretching of the Mooney-Rivlin
theory is expressed as [49]:
HA-gelsa
Mechanical
strength (KPa) energy (J) modulus
(KPa)
Fracture
Elastic
Elongation
at break (%)
C1 (KPa)
F-OP4-1%
F-OP4-2%
F-OP4-3%
F-OP4-4%
F-OP4-5%
F-SDS-1%
F-SDS-2%
F-SDS-3%
F-SDS-4%
F-SDS-5%
F-SDS-6%
F-AM-8%
F-AM-10%
F-AM-12%
F-AM-15%
86.82
179.67
212.79
210.22
174.93
116.94
93.84
86.82
62.00
–
0.60
1.00
1.01
0.90
0.64
0.63
0.54
0.60
0.40
–
2.39
4.68
4.98
5.20
5.31
5.92
4.25
2.39
1.77
1.54
1.34
1.66
2.39
3.71
6.55
1683.21
1460.39
1281.41
1159.23
982.01
1292.47
1445.08
1683.21
1737.03
–
0.86
1.68
1.85
1.89
2.00
2.51
1.72
0.86
0.62
0.50
0.39
0.64
0.86
1.19
2.21
–
–
–
77.98
86.82
91.33
126.53
0.49
0.60
0.59
0.71
1828.36
1683.21
1657.04
1329.85
a
F-OP4-1% gel, F-SDS-3% gel and F-AM-10% gel are the same.
ꢀ
ꢁ
s
¼ 2ðC1 þ C2=
l
Þ
l
ꢂ
lꢂ2
(4)
Here, C1 and C2 are elastic parameters. According to equation (4), if
distance Dic is equivalent to the distance of neighboring cross-
linking points. If the polymer chain conformations are consid-
ered, Dic can be converted to Mc* [3]. Therefore, this indicates that Dic
for HA-gels decreases with increasing NH. The following discussion
attempts to analyze the effects of compositions of HA-gels on their
tensile mechanical properties by using network structural param-
eters (NH, y*0, Mc* and Dic).
the s/2(l-l l
ꢂ2) is plotted versus ꢂ1, Mooney-Rivlin curves will be
obtained, see Fig. 7. Therefore, C1 is evaluated from the straight
lines part of Mooney-Rivlin curves at the intercept point of the Y
axis and is listed in Table 3. Equation (2) and equation (4) may be
combined to form the well-known expression:
1
C1
¼
y*0kT
(5)
2
Here, y*0 is calculated from equation (5) by using C1 and M*c is also
calculated from equation (3) by using y*0. The evaluated y0* and M*c
values for HA-gels are also listed in Table 2. The data obtained from
two methods exhibit entirely similar tendencies with varying
composition content for HA-gels. Nevertheless, y0 from the statis-
tical molecular theory of rubber elastic is much greater than the
corresponding y*0 from the Mooney-Rivlin theory. It is clear that free
or weak interreaction hydrophobic groups in the region of small
deformation were much more than that in the region of medium
deformation. Because y*0 and M*c are from the medium deformation,
we think that using them in later discussion will be more
reasonable.
3.5. Tensile mechanical properties of HA-gels
HA-gels exhibited outstanding mechanical properties. HA-gels
were very tough and could withstand high level of deformation
such as stretching (Fig. 1a), bending (Fig. 1b), knotting (Fig. 1c) and
As shown in Table 2, for F-OP4-X% gels and F-AM-X% gels, y0*
increase with increasing the content of OP-4-AC and AM due to an
increase in the junction number; however, for F-SDS-X% gels, y0*
decreases with increasing the content of SDS due to a decrease in
the junction number. According to Table 2, if y*0 and Mc* are plotted
versus NH, respectively, y*0 increases and M*c decreases with
increasing NH for all HA-gels, see Fig. 8. The inter-crosslinking
Fig. 7. The Mooney-Rivlin curves of parts of F-SDS-X% gel.
Fig. 8. Changes of y*0 as a function of NH (a) and Mc* as a function of NH (b) for HA-gels.

G. Jiang et al. / Polymer 51 (2010) 1507–1515
1513
energy of F-OP4-X% gels first increased and then decreased with
increasing OP-4-AC content, and showed a maximum at F-OP4-3%
gel.
According to equation (1) and Table 1, throughout the range of
OP-4-AC used in the initial reaction solution, with increasing
OP-4-AC content, the [MH] enhanced, but [S] was constant. As
a consequence, the NH increased with increasing [MH], in other
words, NH enhanced with increasing OP-4-AC content, see Table 2.
In Fig. 8, with increasing NH, the y*0 increased and the M*c
decreased. Therefore, with increasing OP-4-AC content, Dic for F-
OP4-X% gels decreased. It is clear that conformation changes of
polymer chains would be more difficult with decreasing Dic.
Namely, the increase in elastic modulus mainly results from the
orientation of polymer chains. Because of this, for F-OP4-X% gels,
elastic modulus increased with increasing OP-4-AC content, see
Fig. 10a. In addition, elongations at break commonly decreases
with decreasing Dic due to the stress concentrations in the shorter
chains present at any instant during the uniaxial stretching.
Therefore, elongations at break showed decreases with increasing
OP-4-AC content, see Fig. 10b. In Fig. 10c–d, with increasing OP-4-
AC content, it is noted that the tensile strength and fracture energy
reached a plateau value and began to decrease, although the
elastic modulus showed increases. This stems from the fact that
the orientation on stretching of polymer chains in cross-linked
network got more difficult and the number of polymer chains
that bore maximum stress force also began to decrease from F-
OP4-3% gel.
3.6.1. Effect of SDS content
Fig. 9b exhibits stress-strain curves of F-SDS-X% gels with
widely different SDS content, where the tensile mechanical
properties of F-SDS-X% gels can be varied greatly by changing
SDS content in the range of SDS used. According to equation (1)
and Table 1, throughout the range of SDS used in the initial
reaction solution, with creasing SDS content, the [S] enhanced,
but [MH] was constant. As a result, the NH decreased with
increasing [S], see Table 2. As seen in Fig. 8, with decreasing NH,
the y*0 decreased and the M*c increased. Therefore, with increasing
SDS content, Dic for F-SDS-X% gels increased. This clearly dictates
the restriction of conformation changes of polymer chains for
F-SDS-X% gels decreased with increasing SDS content in the
range of SDS used. As a result, with increasing SDS content,
tensile strength and elastic modulus decreased, elongations at
break showed increases, and fracture energy showed general
decreases, see Table 3.
Fig. 9. \Stress-strain curves of HA-gels with different composition content. (a) F-OP4-X%
gels, (b) F-SDS-X% gels and (c) F-AM-X% gels.
stretching after being knotted (Fig. 1d). These results indicate that
there are many sufficiently long and flexible polyacrylamide chains
between neighboring cross-linking points, therefore HA-gels can be
deformed to a great extent on macroscopic deformation. Fig. 9
shows the tensile stress-strain curves measured for HA-gels. The
most remarkable conclusion was that the elongations at break
reached were greater than 1100% except for F-OP4-5% gel. However,
the tensile mechanical properties of HA-gels strongly depended on
the content of compositions in initial reaction solution. Tensile
strength, fracture energy, elastic modulus, and elongations at break
for HA-gels are summarized in Table 3, where the fracture energy
for each HA-gel was calculated from the areas under the stress-
strain curves.
3.6.2. Effect of AM content
Fig. 9c shows stress-strain curves of F-AM-X% gels with
different AM content. As listed in Table 1, molar percentage of
OP-4-AC relative to AM was 1 mol% for F-AM-X% gels, so OP-4-AC
content enhanced with increasing AM content and SDS content
was constant in initial reaction solution all the same. According to
the above discussion and equation (1), throughout the range of
AM used in the initial reaction solution, with increasing AM
content, the [MH] enhanced and [S] was constant, so the NH
increased, see Table 2. As showed in Fig. 8, with increasing NH, the
y*0 increased and the M*c decreased. Therefore, as similar to F-OP4-
AC gels, with increasing AM content, Dic for F-AM-X% gels
decreased. As a result, this clearly dictates the restriction of
conformation changes of polymer chains for F-AM-X% gels
increased with increasing AM content in the range of AM used.
Therefore, with increasing AM content, tensile strength and
elastic modulus increased, elongations at break exhibited
decreases, and the general trend of fracture energy showed
increases, see Table 3.
3.6. Effect of OP-4-AC content
With widely different OP-4-AC content for F-OP4-X% gels,
Fig. 9a shows tensile stress-strain curves and Fig. 10 illustrates
change curves of tensile mechanical properties. As shown in
Fig. 10a-b, with an increase in OP-4-AC content, elastic modulus
exhibited increases; however the elongations at break showed
monotonous decreased. In Fig. 10c–d, tensile strength and fracture

1514
G. Jiang et al. / Polymer 51 (2010) 1507–1515
Fig. 10. Changes of tensile mechanical properties with altering OP-4-AC content, such as (a) modulus, (b) elongations at break, (c) tensile strength, and (d) fracture energy of F-OP4-
X% gels.
The qualitative interpretation mentioned above on the effects of
OP-4-AC, SDS and AM will be further explained by additional
investigates on the tensile mechanical properties of HA-gels with
high AM content and HA-gels with low SDS content, the network
formation mechanism and other mechanical behaviors, such as
stress relaxation and reversibility of deformation.
and elastic modulus decreased, elongations at break showed
increases, and the general trend of fracture energy exhibited
decreases. For F-AM-X% gels, with increasing AM content in the
range of AM used, tensile strength and elastic modulus increased,
elongations at break decreased, and the general trend of fracture
energy exhibited increases.
4. Conclusions
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