Facile fabrication method of hydrophobic-associating cross-linking hydrogel with outstanding mechanical performance and self-healing property in the absence of surfactants

Article abstract of DOI:10.1016/j.polymer.2013.07.079

A cationic surfactant monomer, dimethyldodecyl(2-acrylamidoethyl)ammonium bromide (AMQC₁₂) was synthesized. A family of hydrophobic-associating cross-linking hydrogels (HAC-gels) fabricated via the self-assembly of amphiphilic multiblock copolymers of acrylamide and AMQC₁₂ can be synthesized by free-radical aqueous solution micelle copolymerization in the absence of surfactants using the one-pot method. The HAC-gels possessed outstanding mechanical performance, with optimal tensile strength, compressive strength, and elongation at break of 250 kPa, 14 MPa, and 1850%, respectively. Meanwhile, the HAC-gels exhibited self-healing property, and tetrahydrofuran (THF) significantly accelerated their self-healing process. The recovery hysteresis of hydrophobic-associating hydrogels prepared in the presence of surfactants can be eliminated because of homogeneity of the hydrogel network and the dynamic and mobile properties of physical cross-linking junctions. Investigations on the mechanical property and structure evolution of hydrogels revealed that the hydrophobic-associating interaction was the driving force of self-assembly of amphiphilic multiblock copolymers. Furthermore, spherical micelles and macroscopic cross-linking network can be easily switched reversibly by regulating copolymers concentration.

Full text of DOI:10.1016/j.polymer.2013.07.079

Polymer 54 (2013) 5665e5672
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Facile fabrication method of hydrophobic-associating cross-linking
hydrogel with outstanding mechanical performance and self-healing
property in the absence of surfactants
,
a
,
,
,
Kun Xu a, Huiyong An b , Cuige Lu c, Ying Tan a, Pengchong Li a c, Pixin Wang a
*
*
a Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
b School of Chemistry and Materials Science, Liaoning Shihua University, Fushun 113001, PR China
c Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A cationic surfactant monomer, dimethyldodecyl(2-acrylamidoethyl)ammonium bromide (AMQC12) was
synthesized. A family of hydrophobic-associating cross-linking hydrogels (HAC-gels) fabricated via the
self-assembly of amphiphilic multiblock copolymers of acrylamide and AMQC12 can be synthesized by
free-radical aqueous solution micelle copolymerization in the absence of surfactants using the one-pot
method. The HAC-gels possessed outstanding mechanical performance, with optimal tensile strength,
compressive strength, and elongation at break of 250 kPa, 14 MPa, and 1850%, respectively. Meanwhile,
the HAC-gels exhibited self-healing property, and tetrahydrofuran (THF) significantly accelerated their
self-healing process. The recovery hysteresis of hydrophobic-associating hydrogels prepared in the
presence of surfactants can be eliminated because of homogeneity of the hydrogel network and the
dynamic and mobile properties of physical cross-linking junctions. Investigations on the mechanical
property and structure evolution of hydrogels revealed that the hydrophobic-associating interaction was
the driving force of self-assembly of amphiphilic multiblock copolymers. Furthermore, spherical micelles
and macroscopic cross-linking network can be easily switched reversibly by regulating copolymers
concentration.
Received 23 April 2013
Received in revised form
2 July 2013
Accepted 31 July 2013
Available online 8 August 2013
Keywords:
Hydrophobic-associating cross-linking
hydrogels
Mechanical strength
Recovery hysteresis
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
formed through electrostatic force [13,14], hydrogen bonding [15],
pe
p
stacking interaction [16] and hydrophobic interaction [17].
Recently, hydrogels have attracted considerable attention
because of their challenging applications in controlled release [1],
tissue engineering [2,3], chemical separation [4], and intelligent
sensors [5]. Moreover, regulating the subtle structure and sup-
pression of network heterogeneity can improve the fragile of
hydrogels and provide them with multifunctionality.
The fabrication methods of hydrogels with excellent perfor-
mance can be divided into two categories. The first category pro-
duces chemical cross-linking hydrogels, including topological
hydrogels (TP gels) [6,7], double network hydrogels (DN gels) [8],
nanocomposite hydrogels (NC gels) [9,10], and macromolecular
microsphere composite hydrogels (MMC gels) [11,12]. The second
category produces physical cross-linking hydrogels, including those
Amphiphilic multiblock copolymers with hydrophobic and hy-
drophilic block segments can form unique micelle structures with
hydrophobic cores decorated by petals of hydrophilic blocks (i.e.,
flower-likes micelles) in dilute aqueous solution [18]. Theoretically,
a string of flowers, even macroscopic network with hydrophobic
cores as cross-linking points, can occur by increasing the concen-
tration of amphiphilic multiblock copolymers [19] Taking advantage
of the concept of the self-assembly of amphiphilic multiblock
copolymer, some supermolecular gels based on low-molecular-
weight block copolymer gelators and physical cross-linking hydro-
gel via hydrophobic-associating interaction in the presence of sur-
factants (HA gels) have been successfully realized [20,21]. However,
the complicated synthesis process of multiblock copolymers and the
extremely poor mechanical property of macroscopic self-assembled
hydrogels limit their development. Meanwhile, the presence of
surfactants causes the intensive chain transfer effects and purifica-
tion problems [22,23], and the residual surfactants in the gel
structure elicit adverse effects on gel properties.
* Corresponding authors.
E-mail addresses: ahy46@sina.com (H. An), pxwang@ciac.jl.cn (P. Wang).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.07.079

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K. Xu et al. / Polymer 54 (2013) 5665e5672
Recently, Friedrich [24,25] has focused on the synthesis of a dual
cross-linking (containing hydrophobic-associating physical and
chemical cross-linking) hydrogel via copolymerization of NIPAAm
andsurfactantmonomer(surfmer)underg irradiationintheabsence
of surfactants. The synthesized hydrogel displayed good mechanical
performance. However, massive grafting reaction points in the
free-radical micelle copolymerization in the absence of surfactants
in aqueous solution using the one-pot method. AAm and AMQC12
with AMQC12-to-AAm ratios of 1.5/98.5, 2.0/98.0, and 2.5/97.5 mol/
mol were dissolved completely in deionized water by agitation. The
total monomer concentration was controlled at 20 wt% to
ensure sufficient molecular weight of amphiphilic multiblock co-
polymers. Then, the mixture solution was poured into plastic
tubes (5 mL centrifuge tubes for tensile sample preparation and
10 mL plastic syringes for compressive ones) and then placed in an
ice bath. After the initiator VA-044 (0.5 mM) was added using
a syringe and stirred evenly, the reaction was streamed into ni-
trogen for at least 10 min and then sealed. Finally, micelle copoly-
merization was conducted in a 30 ꢀC water bath for 24 h
and hydrogels with excellent mechanical property were prepared.
The hydrogels were named as HAC-x-y, where x denotes the
AMQC12 content (mol %) and y denotes the total monomers
concentration.
The as-synthesized HAC-gels were insoluble in large amounts of
water for several months. However, they were dissolved in large
quantities of formamide for several days because of the strong
polarity of formamide that destroyed the hydrophobic-associating
interaction of amphiphilic multiblock copolymers. Thus, the HAC-
gels were diluted in 5 wt% with formamide and agitated until
they were completely. Then, a large amount of acetone was
employed to precipitate the copolymers. Finally, white powder was
obtained after vacuum drying at 30 ꢀC for 24 h.
backbone of the polymer were induced by
g irradiation, and a
chemical cross-linking network structure appeared. This chemical
cross-linking structure inevitably affected the mechanical perfor-
mance of the hydrogel because it destroyed the structure homoge-
neityof the hydrophobic-associating physicalcross-linking hydrogel.
In this study, a water-soluble cationic surfmer dimethyldo-
decyl(2-acrylamidoethyl)ammonium bromide(AMQC12) contain-
ing an amphiphilic structure and polymerizable reactive groups
was synthesized to avoid the use of surfactants and to raise the
hydrophobic moiety level of amphiphilic multiblock copolymers.
Subsequently, hydrophobic-associating cross-linking hydrogels
(HAC-gels) were prepared via radical copolymerization of acryl-
amide (AAm) and cationic surfmer in the absence of surfactants
using the one-pot method, in which the synthesis and self-
assembly of amphiphilic multiblock copolymers were realized
simultaneously. Meanwhile, spherical micelles and macroscopic
hydrogels were easily switched reversibly by regulating copolymer
concentration. The produced HAC-gels possess excellent mechan-
ical performance and self-healing property. Meanwhile, the re-
covery hysteresis of the hydrophobic-associating hydrogels
prepared in the presence of surfactants can be eliminated.
2.3. Characterization
2. Experimental section
2.3.1. Polymer composition
The proton nuclear magnetic resonance (1H NMR) spectra of
AMQC12 and amphiphilic multiblock copolymers were obtained
using a Bruker AV 600 NMR spectrometer (Bruker Group Company,
Germany) in deuterium oxide.
2.1. Materials
N, N-dimethylethylenediamine (98%) and 1-bromododecane
(99%) were purchased from Aladdin Co. (Shanghai, P. R. China)
and used as received. Acryloyl chloride from Shanghai RC Chem-
icals Co., Ltd. (Shanghai, P. R. China) was vacuum distilled to remove
any inhibitor. AAm (Aldrich Co.) was recrystallized twice using
acetone and then vacuum dried at 30 ꢀC. 2,20-Azobis[2-(2-
imidazolin-2-yl)propane] dihydrochloride (VA-044, AR grade)
was purchased from Wako Pure Chemicals Industries Ltd. (Osaka,
Japan) and used as the initiator without further purification.
Formamide (AR grade) and tetrahydrofuran (THF, AR grade) from
Beijing Chemical Works (Beijing, P. R. China) were used as received.
Water was purified by a Millipore Milli-Q system (Millipore, USA).
2.3.2. Determination of critical gel concentration (CGC)
The zero-shear viscosity
(h0) of the aqueous solution of
amphiphilic multiblock copolymers with various concentrations
was measured via rheological experiments. Steady-state shear flow
experiments were performed using a Physica MCR300 stress-
controlled rheometer (Physica, Company, Germany) with
a
concentric cylindrical geometry (CC17, cup radius ¼ 9.33 mm and
bob radius ¼ 8.33 mm). All measurements were performed at 30 ꢀC
using the TEZ180-C cylinder system in rotation Mode. The h0 values
were obtained and used to investigate the self-assembly structure
evolution of amphiphilic multiblock copolymers.
2.2. Synthesis
2.2.1. Preparation of surfmer
2.3.3. Measurement of micellar size
AMQC12 was synthesized as follows according to a previously
published method [26,27] (Scheme 1).
The size of the micelles was determined by dynamic light
scattering (DLS) using a vertically polarized HeeNe laser (DAWN
EOS, Wyatt Technology, U.S.A.). The scattering angle was fixed at
90ꢀ and the measurement was carried out at 25 ꢀC.
The morphology of the micelles was characterized by trans-
mission electron microscopy (TEM, Hitachi H-800) at an accelera-
tion voltage of 100 kV. A suspension droplet was made to drip on a
2.2.2. Synthesis and self-assembly of amphiphilic multiblock
copolymers
A macroscopic hydrogel constructed by the self-assembly
of amphiphilic multiblock copolymers can be synthesized via
Scheme 1. Synthesis route of AMQC12
.

K. Xu et al. / Polymer 54 (2013) 5665e5672
5667
Table 1
Reaction parameters and mechanical properties for HAC-gels.
Specimen
Feed composition/mol%
cma/wt%
Tensile properties
Compressive properties
AM
AMQC12
Et/kPab
st/kPa
εt/%
Ec/MPac
sc/MPa
εc/%
HAC-1.5-20
HAC-2.0-20
HAC-2.5-20
98.5
98.0
97.5
1.5
2.0
2.5
20
20
20
8.25
8.33
8.60
111.83
152.25
246.89
1461.7
1514.0
1849.5
0.169
0.186
0.189
10.31
12.47
13.98
84.2
89.0
93.8
a
cm refers to the concentration of monomer.
Et is the tensile elastic modulus for the elongation 100%e200%.
Ec is the compressive elastic modulus for the compression 10%e30%.
b
c
copper grid for 1 min. The liquid was blotted off using a filter paper,
and the grid was air dried before measurement.
strength and elongation at break of the healed samples, and the
morphology of the sections was recorded by an optical camera.
The hydrogels were freeze-dried for 48 h and their morphology
was investigated using a JSM-840 scanning electron microscope
(JEOL Company, Japan).
3. Results and discussion
3.1. Mechanical performance of HAC-gels
2.3.4. Tensile tests
Tensile measurements were performed on cylindrical hydrogel
specimens (10 mm in diameter and 30 mm in length) using a
Shimadzu Autograph AG-I with a 1 kN load cell (Shimadzu Corp.,
Kyoto, Japan) at room temperature. The sample length between
jaws was 15 mm, the crosshead speed was 100 mm/min, and the
initial cross section (78.54 mm2) was used to calculate the tensile
stress and elastic modulus. The initial cross section was obtained
from the increment in the load detected between elongations of
100% and 200%.
In our proposal, the water-soluble cationic surfmer AMQC12 [the
critical micelle concentration of AMQC12 is only 4.9 ꢁ 10ꢂ3 mol/L
(25 ꢀC) [28]] that contains amphiphilic structure and polymerizable
double bond was designed to avoid the use of surfactants and to
increase the hydrophobic moiety level of amphiphilic multiblock
copolymers. The solubility of AMQC12 can reach 2.5 g (25 ꢀC),
stemming from the quaternary ammonium group in its structure.
The structure of AMQC12 was confirmed via 1H NMR
(Supplementary Information S1). Based on the synthesis of
AMQC12
, the HAC-gels constructed via the self-assembly of
2.3.5. Compressive tests
amphiphilic multiblock copolymers of AAm and AMQC12 can be
synthesized by free-radical aqueous solution micelle copolymeri-
zation in the absence of surfactants using the one-pot method.
During copolymerization, the total monomer concentration was
controlled at 20 wt% to prevent sufficiently high molecular weights
of amphiphilic multiblock copolymers. The structure of the copol-
ymer was characterized via 1H NMR (Supplementary Information
S2). Then, the mechanical performance of the HAC-gels was
investigated and the results accompanying the reaction parameters
of those were listed in Table 1 and Fig. 1.
To the best of our knowledge, hydrophobe level is a key factor
affecting the mechanical properties of the HAC-gels because
hydrophobic-associating microdomains are developed to fabricate
a cross-linking network. In the present study, the mechanical
properties of the HAC-gels with various hydrophobe contents (from
1.5 mol% to 2.5 mol%) are shown in Fig. 1. Fig. 1a shows that the
tensile strength and elongation at break of the HAC-gels
Compressive tests were performed using an Instron 5869 with a
5 kN load cell (Instron Corp., Boston, USA). Cylindrical specimens
with a diameter of 11 mm and a thickness of 14 mm (the thickness-
to-diameter ratio was more than 1.2) were used. The crosshead
speed was 2 mm/min, and the initial cross section (94.99 mm2) was
used to calculate compressive stress. The initial compressive elastic
moduli were calculated using the ratio of the compressive stress
and strain in the compression range of 10%e30%.
2.3.6. Self-healing property tests
The as-prepared cylindrical hydrogels with a diameter of 10 mm
and a length of 30 mm were cut into two parts. A small quantity of
solvent was dropped onto the specimen sections. Tensile tests of
the healed samples at various healing times were performed using
a Shimadzu Autograph AG-I with a 1 kN load cell (Shimadzu Corp.,
Kyoto, Japan). Self-healing efficiency was evaluated by the tensile
Fig. 1. Effect of hydrophobe content on the mechanical properties of HAC-gels. (a) Tensile stressestrain curves. (b) Compressive stressestrain curves. Insert shows the determi-
nation method of the fracture point during the compression test.

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K. Xu et al. / Polymer 54 (2013) 5665e5672
significantly increased with hydrophobe content, reaching the
maximum values of 247 kPa and 1850%, respectively, at 2.5 mol%
hydrophobe content (HAC-2.5-20).
hydrophobe level suggests higher cross-linking density and better
mechanical performance of the HAC-gels. On the other hand, a fact
should be faced that uneven porous structure of all HAC-gels, as
shown in Fig. 2, hints that heterogeneous distribution of hydro-
phobe in HAC-gels in nature state. However, the dynamic and
mobility of hydrophobic-associating physical cross-linking junc-
tions can absorb and dissipate large amount of energy when HAC-
gels surfers from deformation. Thus, the mechanical performance
of HAC-gels can be significantly proved by introduction of
hydrophobe.
Recovery hysteresis is still one of the serious defects in
hydrophobic-associating hydrogels [31], which limits their appli-
cation. However, HAC-gels can recover to their original shape
immediately after the removal of the load, which exhibits obvious
elastic deformation feature. In the present study, HAC-2.5-20 first
stretched to 1600% strain at 2 mm/min and then recovered
immediately to its original condition. Then, it immediately
stretched until it fractured. As shown in Fig. 3a, the two stresse
strain curves almost overlapped in the strain range of 0%e1600%,
and the breaking strength and elongation at break of the specimen
were close to those of HAC-2.5-20 (Fig. 3a). This finding suggests
that the first stretch did not damage the inner structure of the
specimen.
Similar results were also obtained in repeated compression
measurement of HAC-2.5-20 (Fig. 3b). The first three measure-
ments were terminated at 80% strain, and the specimen maintained
the compressed state for 10 min. After the gel was released, it
immediately recovered to its original shape. Finally, the gel was
compressed until it fractured. The stressestrain curves of this
repeated compression process exhibited almost the same trend
(inset of Fig. 3b). This finding indicates that the original state of the
gel’s structure was maintained during compression.
Therefore, we can speculate a structure evolution mechanism
of the HAC-gels under deformation. When hydrogels carry
external load, the deformation and disassociation of hydrophobic-
associating microdomains and homogeneous dynamic cross-linking
Fig.1b shows the typical compressive stressestrain curves of the
HAC-gels. For all specimens, the compressive stresses gradually
increased with increasing strain until the strain crossed 70% and
then abruptly increased. In the stressestrain curves of each gel, an
extremely minor fall (inset of Fig. 1b) followed by a sharp rise was
observed. The vertex before the second rise was identified as the
fracture point. Then, the corresponding stress and strain were
considered as the breaking strength and deformation at break,
respectively. Similar to the results of the tensile test, the breaking
strength and deformation at break of the HAC-gels during the
compression measurement increased with increasing hydrophobe
content. The optimal values were approximately 14 MPa and 94%,
respectively, at 2.5 mol% hydrophobe content (HAC-2.5-20).
Considering the cross-linking effect of the hydrophobic micro-
domains on the hydrogel structure, higher hydrophobe level in-
dicates higher cross-linking density and better mechanical
performance of the HAC-gels. As the driving force of the self-
assembly of the amphiphilic multiblock copolymers, the
hydrophobic-associating interaction of the hydrophobic micro-
domains can significantly improve the homogeneity of the hydro-
gel network and provide dynamic and mobile properties to the
hydrogels [29]. When the hydrogels are subjected to external force,
the hydrophobic microdomains can deform at a large scale or even
dissociate into many small-scale microdomains to dissipate energy
[30]. After the removal of imposed forces, small-scale hydrophobic
microdomains aggregate again because of the dynamic feature of
hydrophobic-associating interaction. Thus, the HAC-gels exhibit
excellent tensile and compressive performance.
Corresponding to their mechanical property development, the
morphological evolution of the HAC-gels was evident because of
the presence of a hydrophobe. As shown in Fig. 2, all gel specimens
exhibited similar porous structures, and the mean aperture
declined with increasing hydrophobe content. Therefore, higher
Fig. 2. SEM images of the HAC-gels with various hydrophobe contents. (a and b) HAC-1.5-20, (c and d) HAC-2.0-20, and (e and f) HAC-2.5-20.

K. Xu et al. / Polymer 54 (2013) 5665e5672
5669
Fig. 3. Perfect elastic recovery of HAC-2.5-20. (a) Stressestrain curves of the tension in the repeated tensile experiments. (b) Stressestrain curves of the compression in the repeated
compression tests.
network ensure that the gels possess excellent mechanical strength
and deformation capacity. After the external load was removed, the
uniformly distributed hydrophobic-associating junctions returned to
their original position, as confirmed by the complete coincidence of
the stressestrain curves of the tensile and compression tests. Thus,
the HAC-gels exhibit outstanding mechanical performance and per-
fect elastic recovery capacity.
strength and elongation at break of the healed gel were approxi-
mately 55 kPa and 850%, respectively. When the healing time was
increased, the healing efficiency of the gel obviously increased.
When the healing time was 7 d, the tensile strength and elongation
at break reached 93 kPa and 1360%, respectively, with recovery
ratios of approximately 38% and 74%, respectively. In our design,
THF was employed to control the self-healing of the HAC-gels
because it can rapidly induce more hydrophobe onto the surface
of the fractured gel and reform more hydrophobic microdomains.
However, the self-healing efficiency of the HAC-gels must be
improved further to satisfy practical applications.
3.2. Self-healing property of HAC-gels
The HAC-gels were formed via the self-assembly of amphiphilic
multiblock copolymers. The hydrophobic-associating interaction of
the hydrophobic multiblock was the driving force of the formation
of macroscopic cross-linking network. Thus, the self-healing
feature of the HAC-gels can be expected from the dynamicity and
mobility of physical cross-linking junctions. In the present study,
the self-healing capacity of HAC-2.5-20 was assessed by the tensile
test of the healed gel. First, HAC-2.5-20 was cut into two parts, and
a small amount of THF was dropped onto the sections of the
specimen. Then, tensile tests were performed under various heal-
ing times (Fig. 4). When THF was used to induce the self-healing of
HAC-2.5-20 gel, the gel can self-repair and recover its high me-
chanical properties. When the healing time was only 1 d, the tensile
3.3. Fabrication mechanism of HCA-gels
Finally, the fabrication mechanism of the HCA-gels, i.e., the
mechanism of self-assembly structure evolution of amphiphilic
multiblock copolymers, was assumed to follow the results of the
aforementioned analysis. When AAm, cationic surfmers, and initi-
ator were dissolved in aqueous solution, the surfmers formed mi-
celles in the scale of tens of nanometers, which can ensure the
formation of amphiphilic multiblock copolymers following the
mechanism of micellar copolymerization (Scheme 2a) [32].
Meanwhile, the microblock length of amphiphilic multiblock
Fig. 4. Self-healing measurements of HAC-2.5-20. (a) Specimen being cut in two parts. (b) Healed specimen induced by THF. (c) Tensile properties of the specimen; the “scar” on the
section is indicated by the red circle. (d) Tensile stressestrain curves of the specimen under various healing times. (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)

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K. Xu et al. / Polymer 54 (2013) 5665e5672
Scheme 2. Mechanism of the micelle polymerization and self-assembly structure evolution of amphiphilic multiblock copolymers.
copolymer can be adjusted by the content of cationic surfmers.
During copolymerization, amphiphilic multiblock copolymers
appeared and exhibited individual extended chain configuration
[33] in the dilution (Scheme 2b). As the copolymerization
continued, amphiphilic multiblock copolymers with sufficient
molecular weight gradually appeared and formed various self-
assembly structures. Moreover, the self-assembly structure of
amphiphilic multiblock copolymers from the spherical micelles
(Scheme 2c, the TEM photograph is supplied in Supplementary
Information S2) to the macroscopic cross-linking network of the
Fig. 5. Solegel transition of HCA-gels triggered by the stimuli of copolymer concentration. (a) Free-flow viscous liquid. (b) Free-standing gel. (c) TEM image of spherical micelles and
particle size analysis. (d) SEM image of HAC-gel morphology.

K. Xu et al. / Polymer 54 (2013) 5665e5672
5671
various copolymer concentrations. First, h0 increased with copol-
ymer concentration and then sharply increased when the concen-
tration crossed the CGC. The CGC of amphiphilic multiblock
copolymers obviously shifted to the lower concentration region
with increasing hydrophobic moiety level. The CGCs of HAC-2.5-20,
HAC-2.0-20, and HAC-1.5-20 were 0.3, 0.8, and 1.0 g/dL, respec-
tively. This evolutionary trend, which is similar to the critical as-
sociation concentration of the hydrophobically associating water-
soluble polymers [35], suggests that a transition from intra-
molecular association to intermolecular interaction occurs when
copolymer concentration increases. Moreover, this result shows
that the hydrophobic-associating interaction of amphiphilic mul-
tiblock copolymers is the driving force of the fabrication of their
various self-assembly structures containing spherical micelles and
macroscopic cross-linking network. These two types of self-
assembly structures of amphiphilic multiblock copolymers, which
correspond to a solegel transition, can be reversibly switched only
by regulating copolymer concentration.
Fig. 6. Determination of the CGC of HAC-gels via the zero-shear viscosity (h0) of
various amphiphilic multiblock copolymer solutions.
4. Conclusions
HAC-gels (Scheme 2d) can be easily switched reversibly by regu-
lating copolymer concentration. Herein the hydrophobic effect of
dynamic hydrophobic-associating microdomains is major driving
force of the formation of HAC-gels network due to the absence of
cross-linking agent. The sufficient flexibility of the bridge hydro-
philic polymer chains and the excellent homogeneity of the
hydrogel network can significantly improve the mechanical prop-
erty of hydrogels. Thus, the HAC-gels exhibit outstanding
compressive and tensile performance. Meanwhile, the mechanical
performance of HAC-gels can be regulated by the change of cationic
surfmers level during the synthesis of HAC-gels in that the cross-
linking density of HAC-gels network can be considered as a func-
tion of cationic surfmers level. The HAC-gels also possessed self-
healing property owing to the dynamic reversibility of the
hydrophobic-associating interaction, which is the driving force of
the self-assembly of amphiphilic multiblock copolymers and the
formation of HAC-gels cross-linking network.
In this paper, a facile fabrication method of HAC-gels with
outstanding mechanical performance and self-healing property
was described based on the self-assembly of amphiphilic multi-
block copolymers induced via the hydrophobic-associating inter-
action of hydrophobic microdomains. The synthesis of amphiphilic
multiblock copolymers and the fabrication of macroscopic cross-
linking network can be realized by the one-pot method in
absence of surfactants. During the mechanical property test of
hydrogels, the optimal tensile strength, compressive strength, and
elongation at break reached 250 kPa, 14 MPa, and 1850%, respec-
tively. Meanwhile, the recovery hysteresis of the HAC-gels prepared
in the presence of surfactants can be eliminated owing to the ho-
mogeneity of the hydrogel network and the dynamic and mobile
properties of physical cross-linking junctions.
The investigation on the HAC-gel fabrication mechanism and
self-assembly structure evolution of amphiphilic multiblock co-
polymers shows that spherical micelles and macroscopic cross-
linking network can be reversibly switched by regulating copol-
ymer concentration, which corresponds to a solegel transition.
The excellent properties and peculiar structure evolution of the
HAC-gels provide them with challenging application prospects and
open an avenue in materials science. However, although the self-
healing process of the HAC-gels can be accelerated using THF, the
issue on the development of HAC-gels to satisfy practical applica-
tion requirements remains challenging.
3.4. Self-assembly structure evolution of amphiphilic multiblock
copolymers
As aforementioned, the HAC-gels were fabricated based on the
hydrophobic-associating interaction of amphiphilic multiblock
copolymers. Thus, they can be dissolved into strong polar solvents
(such as formamide). These copolymers can be employed to
investigate the self-assembly structure evolution of amphiphilic
multiblock copolymers by controlling their concentration. When
the concentration of copolymers was much lower than the CGC,
free-flowing liquid was observed (Fig. 5a). The results of TEM and
DLS also showed the presence of considerable particles at the
40 nme60 nm scales (Fig. 5b and inset of Fig. 1b). This result sug-
gests that spherical micelles were fabricated via the self-assembly
of amphiphilic multiblock copolymers in dilute aqueous solution.
However, after the concentration of copolymers crossed the CGC,
free-standing gel appeared (Fig. 5c). This observation suggests that
a macroscopic network was produced because of the cross-linking
effect of hydrophobic-associating microdomains (Fig. 5d).
Acknowledgements
This work was financially supported by National Natural Science
Foundation of China (grant no. 21004065, 51021003, 51073153 and
51103150).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2013.07.079.
To demonstrate further the self-assembly structure evolution of
amphiphilic multiblock copolymers, the h0 (zero-shear viscosity)
values of the copolymer solution, which was obtained from a
platform of the rheological curve [34], were investigated as a
function of copolymer concentration (Fig. 6). Fig. 6 shows that the
viscosity evolutionary trends of amphiphilic multiblock co-
polymers containing various hydrophobe levels were similar under
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