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polymers
Article
High-Strength Nanocomposite Hydrogels with
Swelling-Resistant and Anti-Dehydration Properties
Bo Xu 1 , Yuwei Liu 1 , Lanlan Wang 1 , Xiaodong Ge 1 , Min Fu 2 , Ping Wang 1 and Qiang Wang 1, *
1 Key Laboratory of Eco-Textile, Ministry of Education, College of Textile and Clothing, Jiangnan University,
Wuxi 214122, China; boxu@jiangnan.edu.cn (B.X.); 18861822651@163.com (Y.L.);
15852776701@163.com (L.W.); 18861870698@163.com (X.G.); wxwping@163.com (P.W.)
2 College of Chemical and Environmental Engineering, Shandong University of Science and Technology,
Qingdao 266590, China; fuminis@163.com
* Corresponding: qiang_wang@163.com; Tel.: +86-510-8591-2005
Received: 2 July 2018; Accepted: 13 September 2018; Published: 14 September 2018
Abstract: Hydrogels with excellent mechanical properties have potential for use in various fields.
However, the swelling of hydrogels under water and the dehydration of hydrogels in air severely
limits the practical applications of high-strength hydrogels due to the influence of air and water on
the mechanical performance of hydrogels. In this study, we report on a kind of tough and strong
nanocomposite hydrogels (NC-G gels) with both swelling-resistant and anti-dehydration properties
via in situ free radical copolymerization of acrylic acid (AA) and N-vinyl-2-pyrrolidone (VP) in the
water-glycerol bi-solvent solutions containing small amounts of alumina nanoparticles (Al2 O3 NPs)
as the inorganic cross-linking agents. The topotactic chelation reactions between Al2 O3 NPs and
polymer matrix are thought to contribute to the cross-linking structure, outstanding mechanical
performance, and swelling-resistant property of NC-G gels, whereas the strong hydrogen bonds
between water and glycerol endow them with anti-dehydration capacity. As a result, the NC-G
gels could maintain mechanical properties comparable to other as-prepared high-strength hydrogels
when utilized both under water and in air environments. Thus, this novel type of hydrogel would
considerably enlarge the application range of hydrogel materials.
Keywords: nanocomposite hydrogels; alumina; high-strength; anti-dehydration; swelling resistance
1. Introduction
Hydrogels are polymeric materials that consist of three-dimensional (3D) networks and a large
amount of water [1]. Because of their well-known hygroscopicity, hydrophilicity, biocompatibility,
and similarity to native tissues, hydrogels have been widely used in a wide range of applications,
including as super-absorbents, tissue engineering, drug delivery, wound dressing, as well as cell
culture [2–6]. Recently, hydrogels with excellent mechanical properties have attracted increasing
attention due to their potential for use in artificial tissues, bio-actuators, soft robots, and so forth [7–11].
Thus, massive efforts have been made to improve the mechanical properties of hydrogels [12]. Despite
several tough and strong hydrogels being fabricated via different strategies [13–18], the practical
applications of these hydrogels are still experiencing challenges. On the one hand, as a kind of “soft
and wet” material, hydrogels are usually utilized in under water environments. Unfortunately, most
hydrogels would absorb water to swell as a result of the difference in osmotic pressure [19]. After
swelling, the strength and toughness of hydrogels are drastically weakened. On the other hand,
when used in air environments, water is inevitably lost from the hydrogels by evaporation, leading to
the drying-out of hydrogels, which also causes degradation of the softness of hydrogels. Therefore,
developing hydrogel materials with excellent mechanical properties as well as swelling-resistance
Polymers 2018, 10, 1025; doi:10.3390/polym10091025 www.mdpi.com/journal/polymers
Polymers 2018, 10, 1025 2 of 12
and anti-hydration properties is an attractive but highly challenging task. Recently, several research
groups have made some outstanding contributions to this area. For example, Sakai et al. reported
a high-strength and non-swelling hydrogel with a homogeneous network structure by random
copolymerization of hydrophilic and thermoresponsive monomers with tetra-armed polyethylene
glycol (PEG) backbone [19]. When the thermoresponsive segment ratio is 0.4, the resultant
hydrogels swelled at 10 ◦ C but could recover its original shape at around 37 ◦ C. In another study,
Jin et al. fabricated a type of dual-cross-linked (DC) hydrogels composed of tannin acid (TA)
cross-linked poly(vinyl alcohol) and chemically cross-linked polyacrylamide. A strong multiple
polymer—TA hydrogen bonds not only endow hydrogels with tough mechanical properties, but
also suppress the swelling of hydrogels, resulting in good swelling resistance [20]. Our previous
studies also demonstrated that hydrogels with low swelling ratios could be realized using titania
(TiO2 ) and alumina (Al2 O3 ) nanoparticles as inorganic cross-linking agents [21,22]. The low swelling
ratios of these gels were supposed to contribute to the strong complexation interaction between
nanoparticles and carboxyl groups on the polymer chains. Although these above-mentioned hydrogels
achieved non-swelling or low swelling via different mechanisms, few reports have concentrated on
hydrogels with anti-dehydration properties, not to mention that hydrogels possess both of these
unique properties.
In this study, we successfully developed a novel high-strength nanocomposite hydrogels
(NC-G gels) that simultaneously possess anti-dehydration and swelling-resistant properties, via
in situ free-radical copolymerization of acrylic acid (AA) and N-vinyl-2-pyrrolidone (VP) in a
water-glycerol bi-solvent containing small amounts of alumina nanoparticles (Al2 O3 NPs) as the
inorganic cross-linking agents. The obtained hydrogels exhibited excellent mechanical properties
comparable to previously reported nanocomposite hydrogels. More importantly, the introduction
of glycerol endows the obtained hydrogels with anti-dehydration ability in air due to the strong
hydrogen bonding interactions between water and glycerol, which help to lock the water inside the
polymer networks. Besides, slight shrinkage instead of swelling was observed when the NC-G gels
were immersed in the water environment, which probably contributed to the increased cross-linking
density of hydrogels. Due to unique anti-dehydration and swelling-resistant properties, the NC-G
gels maintained outstanding softness and high mechanical properties both under water and in air
environments, which could greatly expand the practical applications of hydrogels.
2. Materials and Methods
2.1. Materials
Acrylic acid (AA, >99%) and N-vinyl-2-pyrrolidone (VP, 99%) were purchased from Aladdin
Reagent Co., Ltd., Shanghai, China. Glycerol (99.5%) was obtained from Innochem Co., Ltd., Beijing,
China. 2-hydroxy-40 -(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959, 98%) was purchased
from Energy Chemical Co., Ltd., Shanghai, China. A colloid solution of Al2 O3 NPs with particle size of
10–20 nm and solid content around 10% was provided by Wanjing New Materials Co., Ltd., Hangzhou,
China. Organic cross-linker N,N 0 -methylenebis-(acrylamide) (MBA, 98%) was obtained from Alfa
Aesar Co., Ltd., Shanghai, China. All the reagents were used as received without further treatment.
2.2. Preparation of Hydrogels
The hydrogels were prepared by photo-initiated in situ free radical polymerization of AA and
VP in the mixed solutions containing water, glycerol, and small amounts of Al2 O3 NPs, following the
procedure similar to our previous report [22]. Firstly, homogeneous precursor solutions were obtained
by mixing a certain amount of raw Al2 O3 colloid solution, deionized water, glycerol, and 20 mg
Irgacure 2959 in a 20 mL glass bottle under a nitrogen atmosphere. Then, the precursor solutions were
transferred into different molds after being degassed by a vacuum. Finally, free radical polymerization
was conducted under 365 nm ultraviolet (UV) light (Scientz03-II, Xinzhi Co., Ltd., Ningbo, China) with
Polymers 2018, 10, 1025 3 of 12
the intensity of 60 mW/cm2 for 30 min. The resultant hydrogels were termed as NC-Gx gels, in which
x% represents the mass percentage of glycerol in the total water-glycerol solution. The molar ratio of
AA to VP was fixed at 3:7 based on our experiment, since the hydrogel prepared with the monomer
ratio of 3:7 exhibited the best comprehensive mechanical properties compared to its counterparts
obtained with other monomer ratios. Unless otherwise specified, the total monomer content and
the Al2 O3 concentration in the water-glycerol solutions were fixed at 3 mol/L and 2%, respectively,
although they can be altered within a large range in order to meet the requirements of different
applications. For comparison, the conventional hydrogel cross-linked by 30 mg organic cross-linker
MBA was also prepared and named OR-G gels. The detailed sample names and the corresponding
compositions are listed in Table 1.
Table 1. Compositions of nanocomposite hydrogels (NC-G gels) and OR-G gel.
Sample Glycerol (g) Water (g) AA (g) VP (g) Al2 O3 (g) MBA (mg)
NC-G0 0 10 0.63 2.33 0.2 0
NC-G20 2 8 0.63 2.33 0.2 0
NC-G40 4 6 0.63 2.33 0.2 0
NC-G60 6 4 0.63 2.33 0.2 0
NC-G80 8 2 0.63 2.33 0.2 0
OR-G60 6 4 0.63 2.33 0 30
Note: 20 mg Irgacure 2959 was used in all the samples as the photo-initiator; AA: acrylic acid;
VP: N-vinyl-2-pyrrolidone; MBA: N,N 0 -methylenebis-(acrylamide).
2.3. Evaluation of Anti-Dehydration and Swelling-Resistant Properties
The anti-dehydration and swelling-resistant properties of the hydrogels were evaluated in air
atmosphere (the temperature and humidity were fixed at 27 ◦ C and 60%, respectively) and pure water
(water was changed every 12 h). The sheet-like samples with size of 10 × 10 × 2 mm were placed in
the above-mentioned circumstances and weighed after 10 days. The weight variations were used to
evaluate their anti-dehydration and swelling-resistant properties. The weight variation was calculated
as follows:
Weight variation (%) = (W 0 - W 10 )/W 0 × 100% (1)
where W 0 and W 10 are the weights of the samples before measurement and after 10 days, respectively.
Three measurements were conducted for each hydrogel, and the average values were used
for discussion.
2.4. Mechanical Tests of Hydrogels
The mechanical properties of hydrogels were tested on a MIT-1 universal test machine
(Sanfeng Co., Changzhou, China) with a 50 N load cell. The as-prepared sheet-like samples with
size of 10 × 50 × 2 mm were clamped between two clamps with a gauge length of 10 mm. Then
the tensile tests were conducted at a speed of 100 mm/min until fracture under ambient conditions
(about 25 ◦ C). The fracture stress was calculated by the force at the fracture point divided by the initial
cross-sectional area of the hydrogel sample (ca. 20 mm2 ). The elastic modulus was calculated from
the slope over 50–100% of strain on the stress-strain curve. The fracture stress, elongation at break,
and elastic modulus of hydrogels were used for discussion. As for the samples placed in air and
under water environments, the as-prepared samples with the size mentioned above were placed under
certain conditions described in Section 2.3. followed by the mechanical tests, and their values for
mechanical properties were calculated based on their actual sizes after 10 days.
2.5. Characterization of Hydrogels
Fourier transform infrared (FTIR) spectra of Al2 O3 NPs, neat copolymer of AA and VP, as well
as NC-G0 gel were obtained using an IRAffinity-1S (Shimadzu Co., Kyoto, Japan) FTIR spectrometer
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with an attenuated total reflectance (ATR) accessory. All of the samples were dried in a 95 °C oven
with an attenuated total reflectance (ATR) accessory. All of the samples were dried in a 95 ◦ C oven
before conducting the FTIR characterization. Scanning electron spectroscopy (SEM) was conducted
before conducting the FTIR characterization. Scanning electron spectroscopy (SEM) was conducted to
to observe the microstructures of the NC-G gels. The as-prepared hydrogels were first placed in water
observe the microstructures of the NC-G gels. The as-prepared hydrogels were first placed in water
(water was changed every 12 h) for 10 days to replace the glycerol with water. Then, the samples
(water was changed every 12 h) for 10 days to replace the glycerol with water. Then, the samples were
were immersed in liquid nitrogen and followed by freeze-dry (FD-1A-50, Boyikang Co., Beijing,
immersed in liquid nitrogen and followed by freeze-dry (FD-1A-50, Boyikang Co., Beijing, China) for
China) for 48 h at −48 °C. The fractured surfaces were observed using a Quanta 250 (FEI Co.,
48 h at −48 ◦ C. The fractured surfaces were observed using a Quanta 250 (FEI Co., Hillsboro, OR, USA)
Hillsboro, OR, USA) instrument at an acceleration voltage of 20 kV after coating with gold. The
instrument at an acceleration voltage of 20 kV after coating with gold. The thermal gravimetric analysis
thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurement of NC-
(TGA) and differential scanning calorimetry (DSC) measurement of NC-G gels were performed on a
G gels were performed on a Pyris 1 (Perkin Elmer Co., Waltham, MA, USA) and DSC 8000 (Perkin
Pyris 1 (Perkin Elmer Co., Waltham, MA, USA) and DSC 8000 (Perkin Elmer Co., Waltham, MA, USA)
Elmer Co., Waltham, MA, USA) instruments, respectively. For both of the characterizations, the
instruments, respectively. For both of the characterizations, the heating rate was set at 10 ◦ C/min from
heating rate was set at 10 °C/min from 30 to 300 °C under N2 atmosphere.
30 to 300 ◦ C under N2 atmosphere.
3.3.Results
Resultsand
andDiscussion
Discussion
3.1.Formation
3.1. FormationofofNanocomposite
NanocompositeHydrogels
Hydrogels(NC-G
(NC-GGels)
Gels)
Hydrogelshave
Hydrogels havebeenbeenextensively
extensivelyapplied
appliedininvarious
variousareas
areasdueduetototheir
theirspecific
specificphysicochemical
physicochemical
characteristics [23–25]. However, the poor mechanical properties
characteristics [23–25]. However, the poor mechanical properties of hydrogels has severely of hydrogels has severelylimited
limited
their application range. Although some tough and strong hydrogels have
their application range. Although some tough and strong hydrogels have been prepared via different been prepared via different
strategies,their
strategies, theiroutstanding
outstandingmechanical
mechanicalperformance
performance can canonly
onlybe beexhibited
exhibitedinintheir
theiras-prepared
as-prepared
states. Notably, hydrogels are generally utilized either in air or water
states. Notably, hydrogels are generally utilized either in air or water environments. Under these environments. Under these
circumstances,hydrogels
circumstances, hydrogelswould wouldloseloseororabsorb
absorbaalarge largeproportion
proportionofofwaterwaterwithin
withintheirtheirpolymer
polymer
networks,which
networks, which inevitably
inevitably influence
influence their mechanical
their mechanical properties,properties,
limiting theirlimiting
practical their practical
applications.
applications.
To address thisTo addressinthis
problem, thisproblem,
study, we in developed
this study, awe developed
novel aluminaa cross-linked
novel alumina cross-linked
bi-solvent NC-G bi-
solvent
gel, whichNC-G gel, which
possesses both possesses both anti-dehydration
anti-dehydration and swelling
and swelling resistant resistant
properties. properties.
The NC-G gels The NC-
were
prepared by a feasible photo-initiated free radical copolymerization of AA and VP in mixtures of waterin
G gels were prepared by a feasible photo-initiated free radical copolymerization of AA and VP
mixtures
and of containing
glycerol water and glycerol containing
small amounts of Alsmall amounts of Al2O3 NPs as illustrated in Scheme 1. It
2 O3 NPs as illustrated in Scheme 1. It was found that all
ofwas
thefound
obtainedthatNC-G
all of the
gels,obtained
regardlessNC-G gels,glycerol
of their regardless of their
content, glycerol
were content,
uniform were uniform
and stable, and couldand
stable, and could not be dissolved in water, indicating the formation
not be dissolved in water, indicating the formation of effective cross-linking structures. To confirm of effective cross-linking
structures.
this To confirm
hypothesis, SEM was this hypothesis,
conducted andSEM was conducted
the results are shown andin the results
Figure 1. Itare
canshown
be seeninthat
Figure 1. It
all the
can begels
NC-G seen that
and all the NC-G
OR-G60 gels and
gel possess OR-G60 gel
well-defined possess
porous well-defined porous
microstructures with themicrostructures
pore size around with
the pore
several size aroundregardless
micrometers, several micrometers,
of the glycerol regardless
content.ofThese
the glycerol content. These
results confirmed that results confirmed
the introduction
ofthat the introduction
glycerol in the precursorsof glycerol
wouldinnottheinfluence
precursors the would
monomer notpolymerization
influence the monomer
as well aspolymerization
the formation
as well as the formation of cross-linked
of cross-linked 3D polymer network structures. 3D polymer network structures.
Scheme 1. Preparation and cross-linking mechanism of NC-G gels. (a) Materials for the preparation of
Scheme 1. Preparation and cross-linking mechanism of NC-G gels. (a) Materials for the preparation
NC-G gels. Digital pictures of (b) hydrogel precursor and (c) NC-G gels. (d) Illustration of the network
of NC-G gels. Digital pictures of (b) hydrogel precursor and (c) NC-G gels. (d) Illustration of the
structure NC-G gels. (e) Proposed cross-linking mechanism of NC-G gels.
network structure NC-G gels. (e) Proposed cross-linking mechanism of NC-G gels.
–COOH) as well as the weakening of peak centered at 3441 cm−1 (characteristic band of –OH) in the
spectrum of NC-G0 gel imply that a strong interaction exists between the carboxyl groups and Al2O3
NPs. According to the literature [22,26–28], these interactions probably correspond to the topotactic
chelation reactions between the dissociated carboxyl groups (–COO–) on the polymer chains and the
aluminum atoms on the surface of Al2O3 NPs (Scheme 1e), which contributed to the cross-linking
Polymers 2018, 10, 1025
structures of NC-G gels. This result verifies that the Al2O3 NPs acted as cross-linking agents, since5 of 12
extra organic cross-linkers were involved in the preparation procedure [22].
Polymers 2018, 10, x FOR PEER REVIEW 5 of 12
Figure 2 displays the ATR-FTIR spectra of Al2O3 NPs, neat copolymer of AA and VP, as well as
dried NC-G0 gel. The disappearance of the band at 1680 cm-1 (characteristic band of vinyl groups) in
the spectra of neat copolymer and NC-G0 gel indicates that almost all the monomers participated in
the polymerization during the UV irradiation. In addition, this result also verifies that the
incorporation of Al2O3 NPs would not hinder the copolymerization of AA and VP, which is an
important prerequisite for the formation of hydrogels. Furthermore, compared to the spectra of neat
copolymer and that of Al2O3 NPs, the disappearance of the peak of 1712 cm−1 (characteristic band of
–COOH) as well as the weakening of peak centered at 3441 cm−1 (characteristic band of –OH) in the
spectrum of NC-G0 gel imply that a strong interaction exists between the carboxyl groups and Al2O3
NPs. According to the literature [22,26–28], these interactions probably correspond to the topotactic
chelation reactions between the dissociated carboxyl groups (–COO–) on the polymer chains and the
aluminum atoms on the surface of Al2O3 NPs (Scheme 1e), which contributed to the cross-linking
structures of NC-G gels. This result verifies that the Al2O3 NPs acted as cross-linking agents, since
extra organic cross-linkers were involved in the preparation procedure [22].
Figure
Figure 1. The1. typical
The typical scanning
scanning electron
electron microscopy(SEM)
microscopy (SEM)images
images of
of NC-G
NC-G and
and OR-G
OR-Ggels.
gels.(a)(a)NC-
NC-G0,
G0, (b) NC-G20, (c) NC-G40, (d) NC-G60, (e) NC-G80, and
(b) NC-G20, (c) NC-G40, (d) NC-G60, (e) NC-G80, and (f) OR-G60. (f) OR-G60.
Figure 2 displays the ATR-FTIR spectra of Al2 O3 NPs, neat copolymer of AA and VP, as well as
dried NC-G0 gel. The disappearance of the band at 1680 cm-1 (characteristic band of vinyl groups) in
the spectra of neat copolymer and NC-G0 gel indicates that almost all the monomers participated in the
polymerization during the UV irradiation. In addition, this result also verifies that the incorporation of
Al2 O3 NPs would not hinder the copolymerization of AA and VP, which is an important prerequisite
for the formation of hydrogels. Furthermore, compared to the spectra of neat copolymer and that of
Al2 O3 NPs, the disappearance of the peak of 1712 cm−1 (characteristic band of –COOH) as well as
the weakening of peak centered at 3441 cm−1 (characteristic band of –OH) in the spectrum of NC-G0
gel imply that a strong interaction exists between the carboxyl groups and Al2 O3 NPs. According to
the literature [22,26–28], these interactions probably correspond to the topotactic chelation reactions
between the dissociated carboxyl groups (–COO– ) on the polymer chains and the aluminum atoms on
the surface of Al2 O3 NPs (Scheme 1e), which contributed to the cross-linking structures of NC-G gels.
This resultFigure
verifies
Figure 2. that
1. The the
typical
Fourier Al 2 O3 infrared
NPs
scanning
transform acted
electron as cross-linking
microscopy
(FTIR) spectra(SEM) agents,
images
of alumina since
of NC-G extra
and
nanoparticles OR-Gorganic
(Al2O gels. (a)cross-linkers
NC-
3 NPs), neat
G0,
were involved (b) NC-G20,
in the
copolymer (c) NC-G40,
andpreparation (d) NC-G60,
NC-G0 gel. procedure [22]. (e) NC-G80, and (f) OR-G60.
2. Fourier
FigureFigure transform
2. Fourier transforminfrared
infrared(FTIR)
(FTIR)spectra
spectra of
of alumina nanoparticles(Al(Al
alumina nanoparticles 2O32 O 3 NPs),
NPs), neatneat
copolymer and NC-G0
copolymer gel.gel.
and NC-G0
Polymers 2018, 10, 1025 6 of 12
Polymers 2018, 10, x FOR PEER REVIEW 6 of 12
3.2.Anti-Dehydration
3.2. Anti-Dehydrationand
andSwelling
SwellingResistant
Resistant Properties
Properties of of NC-G
NC-G Gels
Gels
Theevaporation
The evaporationofofwater waterininairairand
andabsorbing
absorbingofofwaterwaterininwater
waterenvironments
environmentsare aretwo
twomajor
major
and long-lasting problems that limit the practical application of hydrogels
and long-lasting problems that limit the practical application of hydrogels because of their great because of their great
influence
influence on onthe the mechanical
mechanical properties
properties of hydrogels.
of hydrogels. Our previous
Our previous report demonstrated
report demonstrated that hydrogels that
hydrogels with low swelling ratios can be obtained by using
with low swelling ratios can be obtained by using AA and N,N-dimethylacrylamide (DMAA) as AA and N,N-dimethylacrylamide
(DMAA)
the as the co-monomer,
co-monomer, and Al2 O3 NPs andasAlthe2O3 NPs as the inorganic cross-linkers due to the strong chelation
inorganic cross-linkers due to the strong chelation reactions
between polymer chains and nanoparticlesnanoparticles
reactions between polymer chains and [22]. Recently,[22]. Lu etRecently,
al. reportedLu etthatal.the
reported that the
incorporation
of glycerol in hydrogels could prevent the loss of water and maintain the properties of the hydrogelof
incorporation of glycerol in hydrogels could prevent the loss of water and maintain the properties
inthe hydrogel
a wide in a wide
temperature temperature
range from −20range to 60from −20 to 60
◦ C because of °C
thebecause of the strong
strong hydrogen hydrogen
bonding bonding
interactions
interactions between water and glycerol [29]. Based on these reports,
between water and glycerol [29]. Based on these reports, we thought that the newly prepared NC-G we thought that the newly
prepared NC-G gels could possess both swelling resistant and anti-dehydration
gels could possess both swelling resistant and anti-dehydration properties. To verify this hypothesis, properties. To verify
this hypothesis,
hydrogels hydrogels
with different with content
glycerol differentwere glycerol
placedcontent
in airwere placedwater
and under in airfor
and10under
days to water for 10
observe
days to observe their changes in volume and weight. Figure 3a shows
their changes in volume and weight. Figure 3a shows the digital pictures of as-prepared NC-G gels the digital pictures of as-
with different glycerol contents. The NC-G0 gel, which contained no glycerol, is opaque, which is is
prepared NC-G gels with different glycerol contents. The NC-G0 gel, which contained no glycerol,
opaque,from
different which theishydrogels
different from the hydrogels
prepared form AAprepared
and DMAA form[22].
AAItand hasDMAA [22]. Itacknowledged
been widely has been widely
acknowledged that the carbonyl groups on the VP segment
that the carbonyl groups on the VP segment could form hydrogen bonds with the carboxyl could form hydrogen bonds groups
with the
carboxyl groups
introduced by AA introduced
[30,31]. These by hydrogen
AA [30,31]. These
bonds hydrogen
would probably bondscausewould probably of
the formation cause
small the
formation of small polymer aggregates within hydrogels, which leads
polymer aggregates within hydrogels, which leads to the phase separation, thus resulting in the to the phase separation, thus
resulting
opaque in the opaque
appearance of NC-G0appearance
gels [32]. of NC-G0
This can begels [32]. by
verified This
thecan be verified
transparent by the transparent
appearance of Al2 O3
appearance of Al 2O3 cross-linked hydrogels using AA and DMAA as the monomer with the same
cross-linked hydrogels using AA and DMAA as the monomer with the same monomer ratio, in which
monomer
the hydrogenratio, in which
bonding the hydrogen
between the polymer bonding
chainsbetween the polymer
is much lower than that chains
in theisNC-G0
much lower than that
gel, probably
in the NC-G0 gel, probably due to the steric hindrance of the two methyl
due to the steric hindrance of the two methyl groups. However, with the incorporation of glycerol, groups. However, with the
incorporation
a well-known of glycerol,
plasticizer, a well-known
into the hydrogel system, plasticizer, into the
the hydrogels hydrogel
gradually system, the
transformed fromhydrogels
opaque
to totally transparent. This is because the glycerol was able to destroy the hydrogen bondswas
gradually transformed from opaque to totally transparent. This is because the glycerol able to
between
destroychains,
polymer the hydrogen
preventing bonds between ofpolymer
the formation polymerchains,
aggregates.preventing
Therefore, thetheformation
transparence of polymer
of the
aggregates. Therefore, the transparence
hydrogels increased with increasing glycerol content. of the hydrogels increased with increasing glycerol content.
Figure 3. The digital pictures of NC-G gels. (a) As-prepared, (b) immersed in water for 10 days,
(c)Figure
placed3.inThe digital
air for pictures
10 days, of NC-G
(d) weight gels. (a)ofAs-prepared,
variation NC-G gels. (b) immersed in water for 10 days, (c)
placed in air for 10 days, (d) weight variation of NC-G gels.
Distinct from most of the hydrogels that would absorb water to swell under aqueous
environments,
Distinct thefrom NC-Gmostgels
of not
the only did notthat
hydrogels swell, but even
would slightly
absorb watershrunk after under
to swell immersion
aqueousin
water for 10 days,
environments, theasNC-G
revealedgelsinnot
Figure
only3b.didThe
not results fromeven
swell, but the quantitative
slightly shrunkmeasurements
after immersionshowin
that all of
water forthe
10hydrogels shrank about
days, as revealed 10% (w/w)
in Figure 3b. Thecompared to their
results from the original weight
quantitative (Figure 3d). This
measurements show
phenomenon
that all of thecan be explained
hydrogels shrankbasedabouton 10%the(w/w)
cross-linking
comparedmechanism of Alweight
to their original 2 O3 NPs cross-linked
(Figure 3d). This
hydrogels.
phenomenon As can
we discussed
be explained previously,
based onthe thecross-linkage
cross-linkingwithin NC-Gofgels
mechanism Al2O contributed to the
3 NPs cross-linked
interactions – groups on polymer chains and Al O NPs. In our experiment, the Al O
hydrogels.between
As we –COOdiscussed previously, the cross-linkage2 within 3 NC-G gels contributed to2 the
3
NPs were dispersed
interactions between in –COO
acidic media
– groups toon
result in a transparent
polymer chains andAl Al22O
O33 colloid
NPs. Insolution with a pH
our experiment, thevalue
Al2O3
around four.dispersed
NPs were After polymerization,
in acidic media thetoliquid
resultphase within the hydrogels
in a transparent Al2O3 colloidwassolution
also acidic,
withwith
a pHavalue
pH
value
around four. After polymerization, the liquid phase within the hydrogels was also acidic, with athe
lower than the pKa (4.75) of –COOH. Under this condition, only part of the –COOH on pH
polymer chainsthan
deprotonated to –COO – , which could interact with Al O NPs to form cross-linkage.
value lower the pKa (4.75) of –COOH. Under this condition, only 2 3 part of the –COOH on the
polymer chains deprotonated to –COO–, which could interact with Al2O3 NPs to form cross-linkage.
Polymers 2018, 10, x FOR PEER REVIEW 7 of 12
Polymers 2018, 10, 1025 7 of 12
However, when NC-G gels were immersed in neutral deionized water, more –COO– groups can be
generatedwhen
However, to react
NC-G with Alwere
gels 2O3 NPs during in
immersed theneutral
solventdeionized
exchangewater,process,
morewhich
–COO – groups
could increase
can bethe
cross-linking
generated density
to react withofAlhydrogels,
O
2 3 NPs leading
during theto the
solventshrinkage
exchange of the hydrogels.
process, which Furthermore,
could increase from
the
Figure 3b, alldensity
cross-linking the hydrogels became
of hydrogels, opaque
leading again
to the after immersion
shrinkage in water Furthermore,
of the hydrogels. for 10 days. This fromis
because
Figure 3b, the replacement
all the of glycerol
hydrogels became opaque withagainwater
after eliminated
immersion in the plasticizing
water for 10 days.effect
Thisofis because
glycerol;
therefore,
the replacementthe phase separation
of glycerol occurred
with water again and
eliminated the hydrogel
the plasticizing turned
effect opaque.therefore, the phase
of glycerol;
Figure
separation 3c displays
occurred againtheandpictures of NC-G
the hydrogel gels opaque.
turned placed in air after 10 days. It can be seen that the
NC-G0
Figure gel3cshrank
displays significantly
the pictures dueof to the evaporation
NC-G gels placed in of air
water.
afterHowever,
10 days. It when
can beglycerol was
seen that
incorporated, the volume change became increasingly less obvious.
the NC-G0 gel shrank significantly due to the evaporation of water. However, when glycerol was According to the quantitative
data shown inthe
incorporated, Figure
volume3d, change
the weight variation
became gradually
increasingly lessdecreased
obvious.from about 47.5%
According to the to 1.2% as the
quantitative
glycerol
data shown content increased
in Figure 3d, thefrom 0% to
weight 60%. Asgradually
variation for the NC-G80
decreased gel,from
the sample even absorbed
about 47.5% to 1.2% aswaterthe
from the environment. This result confirms that the incorporation of
glycerol content increased from 0% to 60%. As for the NC-G80 gel, the sample even absorbed waterglycerol could indeed inhibit the
water
from theevaporation
environment. of This
waterresult
within hydrogels
confirms that due to the strongofhydrogen
the incorporation bonding
glycerol could interactions
indeed inhibit
between
the water glycerol
evaporation and water,
of water as within
well as hydrogels
the lower volatility
due to the and a higher
strong boilingbonding
hydrogen point of interactions
glycerol [29].
To proveglycerol
between this conclusion,
and water, TGA andas
as well DSC thewere
lowerused to characterize
volatility and a higherthe boiling
NC-G gelspointandof the results
glycerol are
[29].
shown
To proveinthisFigure 4. It canTGA
conclusion, be seenandfrom
DSCFigure
were used4a thatto the NC-G0 gelthe
characterize lost moregels
NC-G thanand40%theofresults
its original
are
weight at 100 °C due to the evaporation of water. However, when glycerol
shown in Figure 4. It can be seen from Figure 4a that the NC-G0 gel lost more than 40% of its original was introduced, higher
weight at 100 ◦were
temperatures C dueneeded to remove the
to the evaporation of liquid
water. from
However,the NC-G
whengels. Fromwas
glycerol the introduced,
DSC curves higher (Figure
4b), the endothermic
temperatures were needed peaks
to shifted
remove from 118.7from
the liquid °C tothe 156.1
NC-G°C gels.
as theFrom
glycerol content
the DSC increased
curves (Figure from
4b),
0% to 80%. All these results confirm that◦ the ◦
introduction of glycerol
the endothermic peaks shifted from 118.7 C to 156.1 C as the glycerol content increased from 0% toin the hydrogel systems could
delay
80%. Allthe
these evaporation
results confirmof thethat liquid from the ofhydrogels,
the introduction glycerol in thus imparting
the hydrogel the could
systems hydrogelsdelayanti-
the
dehydration properties.
evaporation of the liquid from the hydrogels, thus imparting the hydrogels anti-dehydration properties.
Figure4.4.(a)
Figure (a)Thermal
Thermalgravimetric
gravimetricanalysis
analysis(TGA)
(TGA)and
and(b)
(b)differential
differentialscanning
scanningcalorimetry
calorimetry(DSC)
(DSC)
curves
curvesofofNC-G
NC-Ggels.
gels.
3.3. Mechanical Properties of NC-G Gels
3.2. Mechanical Properties of NC-G Gels
The mechanical properties of hydrogels play a key role in their practical applications. The currently
The mechanical properties of hydrogels play a key role in their practical applications. The
reported NC-G gels with dehydration and swelling-resistant properties also had outstanding
currently reported NC-G gels with dehydration and swelling-resistant properties also had
mechanical performance. As displayed in Figure 5, the as-prepared NC-G60 gel could withstand
outstanding mechanical performance. As displayed in Figure 5, the as-prepared NC-G60 gel could
large-scale deformations, e.g., stretch after knotting and compression without breaking. Furthermore,
withstand large-scale deformations, e.g., stretch after knotting and compression without breaking.
aFurthermore,
hydrogel strip with a diameter
a hydrogel strip withof a5.5 mm could
diameter of 5.5lift
mm upcould
to a 1.0
lift kg
up weight, showing
to a 1.0 kg weight,excellent
showing
mechanical strength. In contrast, the OR-G60 gel cross-linked by organic cross-linker
excellent mechanical strength. In contrast, the OR-G60 gel cross-linked by organic cross-linker with the same
with
monomer and glycerol content as NC-G60 gel were fragile and brittle, which was unable
the same monomer and glycerol content as NC-G60 gel were fragile and brittle, which was unable toto withstand
the deformations
withstand mentioned above.
the deformations mentioned above.
Polymers 2018, 10, 1025 8 of 12
Polymers2018,
Polymers 2018,10,
10,xxFOR
FORPEER
PEERREVIEW
REVIEW 88 of
of 12
12
Figure5.5.Illustration
Figure Illustration
Illustrationofofthe
the outstanding
theoutstanding mechanicalperformance
outstandingmechanical performance
performanceof
ofNC-G
NC-Ggels. (a)Knotted,
gels.(a) Knotted,(b)
(b)stretch
stretch
after
afterknotting,
knotting,(c)(c)before
beforecompression, (d)under
compression,(d) undercompression, (e)after
compression,(e) aftercompression,
compression,and
and(f)(f)lift
liftup
upaa
1.0 kg weight.
1.0 kg weight.
To quantitatively
To quantitatively
quantitatively measure measure the the mechanical
mechanical propertiesproperties of of NC-G
NC-G gels, gels, uniaxial
uniaxial tensile
tensile teststests were
were
conducted
conductedand
conducted andthe thestress
stressand
stress andstrain
and straincurves
strain curves
curves ofofhydrogels
of hydrogels
hydrogels areare
shown
are shown
shown in Figure
inFigure
in Figure6. The results
6.6.The
The indicate
results
results that
indicate
indicate
all
thattheallprepared
the NC-G
prepared gels
NC-G could
gels be stretched
could be more
stretched than
more seven
than
that all the prepared NC-G gels could be stretched more than seven times their original length with times
seven their
times original
their length
original with
length a tensile
with
astrength
atensile betweenbetween
tensilestrength
strength 0.26 to 0.61
between 0.26
0.26 MPa,
to0.61
to which
0.61 MPa,
MPa, iswhich
comparable
whichisiscomparable to nanocomposite
comparable tonanocomposite
to hydrogels
nanocomposite cross-linked
hydrogels
hydrogels by
cross-
cross-
other
linkedinorganic
linked by othernanomaterials
by other inorganic [14,33,34]. Here,
inorganic nanomaterials
nanomaterials it is worth
[14,33,34].
[14,33,34]. Here,
Here, noting that
itit isis worth
worth thenoting
hydrogels
noting thatprecursors
that the hydrogels
the only
hydrogels
contained
precursors2%
precursors onlyAlcontained
only O3 NPs, so
2contained 2%the
2% Almechanical
Al O33 NPs,
22O NPs, so soperformance
the mechanical
the mechanical of NC-Gperformance
performance gels could be
of NC-G
of NC-G furthergelsenhanced
gels could be
could be
by increasing
further
further enhanced
enhanced the Al by
by 2O 3 concentration
increasing
increasing the Al
the in22O
Al the
O precursor solution.
concentration
33 concentration in the
in theBesides,
precursor
precursor the solution.
glycerol
solution.content Besides,
Besides, could
the
the
affect
glycerol
glycerol thecontent
mechanical
content couldproperties
could affect theof
affect the hydrogels.properties
mechanical
mechanical As shownof
properties in the figure, the
of hydrogels.
hydrogels. Astensile
As shownstrength
shown in the
in gradually
the figure,
figure, the
the
increased
tensilestrength
tensile as the
strength glycerolincreased
gradually
gradually content increased
increased asthe
as theglycerolfromcontent
glycerol 0% to increased
content 40%,
increased afterfrom which
from 0%the
0% to40%,
to strength
40%, afterwhich
after started
whichthe to
the
decrease.
strength The reason
strength started
started to for thisThe
to decrease.
decrease. phenomenon
The reason for
reason for may be explained may
this phenomenon
this phenomenon as
may follows. (1) As as
be explained
be explained mentioned
as follows.
follows. (1) above,
(1) AsAs
the incorporation
mentioned
mentioned above,the
above, of
theglycerol in theof
incorporation
incorporation hydrogel
of glycerolsystems
glycerol inthe
in could destroy
thehydrogel
hydrogel systems
systems the hydrogen
could
could destroy
destroy bonds between
thehydrogen
the hydrogen
polymer
bondsbetween
bonds chains,polymer
between which increases
polymer chains,which
chains, the flexibility
which increasesof
increases the
the the polymerof
flexibility
flexibility ofchains.
thepolymer
the (2) It has
polymer been(2)
chains.
chains. reported
(2) ItIthas
hasbeenthat
been
the glycerol-water
reported
reported thatthe
that mixtures exhibit
theglycerol-water
glycerol-water stronger
mixtures
mixtures interactions
exhibit
exhibit strongerwith
stronger the polymers
interactions
interactions withthe
with than
the pure water,
polymers
polymers thanwhich
than pure
pure
could
water,introduce
water, which could
which more
could noncovalent
introduce
introduce more
more interactions
noncovalent
noncovalent in interactions
the polymer in
interactions networks
in the
the polymer actingnetworks
polymer as sacrificial
networks bonds
acting
acting as
as
to improve
sacrificial
sacrificial the toughness
bonds
bonds totoimprove
improve ofthe
NC-G
the gels [29].
toughness
toughness (3)
ofNC-G
of NC-G Thegels glycerol
gels [29].(3)
[29]. could
(3) Theform
The hydrogen
glycerol
glycerol couldform
could bonds
formhydrogenwith the
hydrogen
carboxyl
bondswith
bonds withgroups
the on thegroups
thecarboxyl
carboxyl polymer
groups onchain,
on thepolymer
the whichchain,
polymer wouldwhich
chain, lead would
which to the reduced
would leadto
lead tothecross-linking
the reducedcross-linking
reduced density of
cross-linking
hydrogels,
density of
density since
of hydrogels,the cross-linkage
hydrogels, since the
since of NC-G gelsof
the cross-linkage
cross-linkage contributed
of NC-G gels
NC-G gelstocontributed
the chelation
contributed toreactions
to the chelation
the between
chelation Al2 O3
reactions
reactions
NPs
between
between and Alcarboxyl
Al22O
O33 NPs
NPs groups.
and These three
and carboxyl
carboxyl factors
groups.
groups. could
These
These influence
three
three factors
factors thecouldmechanical
could influence
influence properties of NC-G
the mechanical
the mechanical
gels. Whenof
properties
properties the
of glycerol
NC-G
NC-G gels.
gels.content
Whenwas
When the under
the glycerol
glycerol 40%, the strong
content
content interactions
was under
was under 40%, 40%, the of
thethe glycerol-water
strong
strong interactions
interactions mixtures
of the
of the
and polymer networks
glycerol-water
glycerol-water mixtureswere
mixtures and dominant;
and polymer
polymer networks therefore,
networks the mechanical
were
were dominant; therefore,
dominant; strength increased
therefore, the mechanical
the with increasing
mechanical strength
strength
glycerol
increasedcontent.
increased with However,
withincreasing
increasing whencontent.
glycerol
glycerol the glycerol
content. However,
However, content when
when was thehigher
the glycerol
glycerol than 40%,was
content
content the higher
was hydrogen
higherthan thanbonds
40%,
40%,
between
thehydrogen
the glycerol
hydrogenbonds and
bondsbetween carboxyl
betweenglycerolgroups
glyceroland caused
andcarboxyl the decrease
carboxylgroups of
groupscaused the
causedthe cross-linking
thedecrease
decreaseof density
ofthe of NC-G
thecross-linking
cross-linking gels.
As a result,
density
density ofNC-G
of the mechanical
NC-G gels.As
gels. properties
Asaaresult,
result, decreased accordingly.
themechanical
the mechanical propertiesdecreased
properties decreasedaccordingly.
accordingly.
Figure6.6.Stress-strain
Figure Stress-straincurves
curvesof
ofNC-G
NC-Ggels
gelswith
withdifferent glycerolcontent.
differentglycerol content.
Polymers 2018, 10, 1025 9 of 12
Due to the anti-dehydration and swell-resistant properties as well as the excellent mechanical
properties of their as-prepared states, we thought that the NC-G gels could also possess considerable
mechanical performance both in air and water environments. Therefore, mechanical tests were
conducted on hydrogels after being placed either in air for 10 days or immersed in water for 10 days.
As shown in Figure 7, the NC-G0 gels exhibited excellent mechanical toughness after immersion in
water for 10 days, and could withstand twisting without breaking. However, after being placed in air
for 10 days, the hydrogel became stiff and brittle because of the loss of water within the hydrogels.
Thus, the hydrogel was easily fractured upon deformation. In contrast, the NC-G60 gel maintained
its outstanding mechanical performance both in air and water, which simultaneously overcame the
two long-lasting problems inhibiting the practical applications of hydrogel materials. Based on these
findings, we also measured the mechanical properties of hydrogels under different conditions, and the
results are summarized in Figure 8. All of the NC-G gels demonstrated similar mechanical properties
after immersion in water for 10 days. This is because the glycerol within hydrogels was replaced
with water, which causes the different NC-G gels to have similar compositions after immersion in
water. Furthermore, as discussed above, the volumes of the NC-G gels shrank 10% after immersion
in water due to the increased cross-linking density. Therefore, the tensile strength and elastic moduli
of the hydrogels significantly increased, while the elongations at break slightly decreased. Notably,
the tensile strength of the hydrogels immersed in water were as high as 1 MPa, making these gels
prospective for application in load-bearing systems. As for the hydrogels placed in air for 10 days, their
mechanical properties revealed a significant relationship with their glycerol content. For the NC-G0
gel, most of the water evaporated during the 10 days. Thus, no mechanical data could be obtained due
to its stiff and fragile nature. When 20% of the water was replaced by glycerol, the NC-G20 gel was
still soft after being placed in air for 10 days. It could be stretched more than 200% (234 ± 10.2%) its
original length before fracture because of the reduced water loss from the hydrogels, showing tensile
strength and elastic modulus around 0.91 ± 0.05 MPa and 520.2 ± 25.6 kPa, respectively. Further
Polymers 2018, 10, x FOR PEER REVIEW 10 of 12
increasing the glycerol content led to the increasing in the softness and extensibility of the resultant
hydrogels, whereas the elastic modulus gradually decreased. In addition, the tensile strength of
addition, the tensile strength of NC-G40 gel, NC-G60 gel, and NC-G80 gel after being placed in air
NC-G40 gel, NC-G60 gel, and NC-G80 gel after being placed in air for 10 days were measured to be
for 10 days were measured to be 0.64 ± 0.04 MPa, 0.53 ± 0.03 MPa, and 0.29 ± 0.01 MPa, respectively.
0.64 ± 0.04 MPa, 0.53 ± 0.03 MPa, and 0.29 ± 0.01 MPa, respectively. These values are almost the same
These values are almost the same as those recorded in their as-prepared states and comparable to
as those recorded in their as-prepared states and comparable to other high-strength hydrogels reported
other high-strength hydrogels reported in the literature [33–35]. In general, the data displayed in
in the literature [33–35]. In general, the data displayed in Figure 8 strongly verified that the NC-G gels
Figure 8 strongly verified that the NC-G gels could be utilized in air and water environments because
could be utilized in air and water environments because of their excellent mechanical performance
of their excellent mechanical performance under these conditions.
under these conditions.
Figure 7. Illustration of NC-G gels at different conditions.
Figure 7. Illustration of NC-G gels at different conditions.
Polymers 2018, 10, 1025
Polymers x FOR PEER REVIEW 10 of
10 of 12
Figure 8. Comparison of mechanical properties of NC-G gels placed in air and under water for 10 days.
Figure 8. Comparison of mechanical properties of NC-G gels placed in air and under water for 10
(a) Elongation at break; (b) Tensile strength; (c) Elastic modulus.
days. (a) Elongation at break; (b) Tensile strength; (c) Elastic modulus.
4. Conclusions
4. Conclusions
In summary, we prepared a novel type of high-strength nanocomposite hydrogel with
In summary, and
anti-dehydration we prepared a novel type
swelling-resistant of high-strength
properties. nanocomposite
The hydrogels were preparedhydrogel
by inwith
situanti-
free
dehydration and swelling-resistant
radical copolymerization of AA and properties. The hydrogels
VP in mixtures of water were prepared
and glycerol by inAlsitu
using 2 O3free
NPsradical
as the
copolymerization of AAagents.
inorganic cross-linking and VP Thein topotactic
mixtures of water and
chelation glycerol
reactions using Al
between Al22O33 NPs as
andthetheinorganic
polymer
cross-linking agents. The topotactic chelation reactions between Al O NPs and the
matrix not only contributed to the outstanding mechanical properties of hydrogels, but also endowed
2 3 polymer matrix
not
them only contributed
with to the outstanding
swelling resistance. mechanical
Simultaneously, theproperties of hydrogels,
strong hydrogen bonding butinteractions
also endowed them
between
with
waterswelling resistance.
and glycerol Simultaneously,
were responsible for the the strong hydrogen
anti-dehydration bonding
capacity interactions
of the between water
resultant hydrogels. Based
and glycerol
on these were
unique responsible
properties, thefor the anti-dehydration
prepared capacityhigh
hydrogels maintained of the resultant hydrogels.
mechanical performance Based
bothon
in
these
air andunique
waterproperties,
environments,the which
prepared hydrogels
would largelymaintained
enhance thehigh mechanical
practical performance
applications both in
of hydrogels.
air and water environments, which would largely enhance the practical applications of hydrogels.
Author Contributions: B.X., P.W. and Q.W. conceived and designed the experiments. Y.L., L.W. and X.G.
performed
Author the experiments.
Contributions: B.X.,B.X. andand
P.W. M.F.Q.W.
analyzed data. B.X.
conceived andand Y.L. wrote
designed the the paper. All authors
experiments. discussed
Y.L., L.W. the
and X.G.
results and improved the final text of the paper.
performed the experiments. B.X. and M.F. analyzed data. B.X. and Y.L. wrote the paper. All authors discussed
Acknowledgments:
the results and improved This the
research wasoffinancially
final text the paper. supported by the National Natural Science Foundation of
China (Grant No. 31771039); the Program for Changjiang Scholars and Innovative Research Teams in Universities
Acknowledgments:
(Grant No. IRT_15R26) Thisandresearch was financially
Fundamental supported
Research Funds byCentral
for the the National Natural
Universities Science
(Grant No. Foundation
JUSRP115A03,of
JUSRP51717A).
China (Grant No. 31771039); the Program for Changjiang Scholars and Innovative Research Teams in
Universities
Conflicts of (Grant No.
Interest: TheIRT_15R26) and Fundamental
authors declare no conflict of Research
interest. Funds for the Central Universities (Grant No.
JUSRP115A03, JUSRP51717A).
References
Conflicts of Interest: The authors declare no conflict of interest.
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