Hydrophobic association hydrogels based on N-acryloyl-alanine and stearyl acrylate using gelatin as emulsifier

Article abstract of DOI:10.1039/c6ra04762a

Poly(N-acryloyl-alanine)-based hydrophobic association hydrogels were prepared through free radical polymerization of N-acryloyl-alanine (NAA) with ammonium persulfate as initiator, in which hydrophobic monomer stearyl acrylate underwent simultaneous mice

Full text of DOI:10.1039/c6ra04762a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Hydrophobic association hydrogels based on N-
acryloyl-alanine and stearyl acrylate using gelatin
as emulsifier†
Cite this: RSC Adv., 2016, 6, 38957
ac
bc
Zhao Cui,ac Ru Cheng,ac Jie Liu,ac Youping Wu
*
*
and Jianping Deng
Poly(N-acryloyl-alanine)-based hydrophobic association hydrogels were prepared through free radical
polymerization of N-acryloyl-alanine (NAA) with ammonium persulfate as initiator, in which hydrophobic
monomer stearyl acrylate underwent simultaneous micellar polymerization in the presence of gelatin as
emulsifier. Fourier transform infrared spectra and scanning electron microscopy demonstrated the
formation and microscopic structure of the resulting hydrogels. The hydrophobic association enabled
the hydrogels to exhibit desirable toughness and could be molded into diverse shapes without breaking.
Uniaxial tensile test and cyclic tensile test also demonstrated the high toughness of the hydrogels. The
chiral monomer (NAA)-derived polymer chains rendered the hydrogels with optical activity, according to
circular dichroism spectra. More fascinatingly, the hydrogels demonstrated shape memory behavior due
to the hydrophobic poly(stearyl acrylate) domains. Therefore the tough chiral hydrogels are expected to
find significant applications in tissue engineering and other biomedical fields.
Received 23rd February 2016
Accepted 12th April 2016
DOI: 10.1039/c6ra04762a
www.rsc.org/advances
A facile way to dissipate the energy is to build reversible
crosslinks instead of permanent crosslinks inside hydrogel
1. Introduction
networks. Upon application of external stress, reversible cross-
links in a hydrogel can move inside its networks and balance
polymer chains to dissipate the stress.24,25 Hydrophobic inter-
actions have been proved to be one of the effective ways to
toughen hydrogels.26,27 Reportedly, hydrophobically modied
hydrogels were prepared by copolymerization of hydrophilic
monomer with a relatively small amount of hydrophobic
comonomer with the aid of emulsier via the so-called free
radical micellar polymerization.28,29 Micellar polymerizations
are favorable for preparing hydrogels whose properties can be
easily tuned by controlling the hydrophobic components
formed inside micelles. The resulting hydrogels are of signi-
cant promise because their properties could be readily adjusted
just by changing the ratio of hydrophobic and hydrophilic
monomers. Jiang et al.30,31 synthesized tough hydrogels by the
Hydrogels have received growing attention due to their biode-
gradability, intelligence, and ability to absorb and retain a large
amount of water.1–3 These features make hydrogels quite similar
to human tissues4,5 and have been investigated as biomaterials
(e.g. for bio-adhesives,6 tissue engineering,7,8 cartilage repair,9
drug delivery,10 embolic agents,11 and injectable hydrogels12,13).
However, generally hydrogels demonstrate poor mechanical
strength and lack stretchability, which are critical for practical
applications.14,15 For example, the hydrogels need to afford
enough stress without being damaged when used as tissue
engineering scaffolds. They should withstand long-term
repeated use when utilized as absorbents. Therefore, in recent
years a number of techniques have been proposed for tough-
ening hydrogels,16,17 by which a variety of hydrogels have been
reported, e.g. double network hydrogels,18 topological hydro-
gels,19 nanocomposite hydrogels,20,21 and macromolecular
microsphere composite hydrogels.22 The essential idea in these
methods is to dissipate the energy and thereby to improve the
strength and toughness of hydrogels.23
copolymerization of hydrophilic acrylamide and
a small
amount of hydrophobic octylphenol polyoxyethylene acrylate
with sodium dodecyl sulfate (SDS) as emulsier. The as-
obtained hydrogels possessed remarkable mechanical
strength. Okay et al.32–34 prepared a series of tough hydrogels
based on hydrophobic interaction which even provided some
fascinating properties e.g. self-healing and shape memory
properties. Herein, we hope to point out that in the above re-
ported hydrogels, small molecular emulsier, e.g. SDS was used,
and it might be toxic and thus produce some unfavorable
inuence on practical applications.27 To circumvent this
problem, biomacromolecules may be a good choice instead of
SDS.35 Among the widely used biomacromolecules, gelatin is
aState Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical
Technology, Beijing 100029, China. E-mail: wuyp@mail.buct.edu.cn; Fax: +86-10-
6445-6158; Tel: +86-10-6444-2621
bState Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, Beijing 100029, China. E-mail: dengjp@mail.buct.edu.cn
cCollege of Materials Science and Engineering, Beijing University of Chemical
Technology, Beijing 100029, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra04762a
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 38957–38963 | 38957

View Article Online
RSC Advances
Paper
ampholytic and has been extensively used in pharmaceutical concentration was xed at 20% w/v and the content of the
and food industries. Especially, gelatin macromolecule chains hydrophobic monomer in the total monomers was 2%, 3% and
could stabilize the oil phase in water and thus could be used as 4% (in mol), and the corresponding hydrogels were denoted as
emulsier.36,37
HAG2%, HAG3% and HAG4%, respectively. The detailed prep-
In the present study, we prepared tough, chiral hydrogels for aration of the hydrogels is stated below by taking HAG2% as
the rst time through simultaneous free radical polymerization example. Gelatin (0.75 g) was dissolved in 5.3 mL deionized
of chiral hydrophilic monomer N-acryloyl-alanine (NAA) and water at 40 ꢀC and stirred for 2 h to obtain a transparent solution
free radical micellar polymerization of hydrophobic monomer and then hydrophobic monomer C18 (0.204 mmol) was added
stearyl acrylate (C18) in aqueous media at 60 ꢀC by using gelatin in the solution. Aer C18 was uniformly emulsied by gelatin
as emulsier and ammonium persulfate as initiator. Gelatin under stirring, NAA solution (9.996 mmol NAA in 2 mL NaOH
was used as emulsier for performing the micellar polymeri- solution) was added in the mixture. Then initiator APS was
zation of C18 and it is also expected to improve the biocom- added and the reaction emulsion was bubbled with N2 for 10
patibility of the hydrogels. The as-obtained hydrophobic min. The mixture solution was transferred quickly to a plastic
association hydrogels (HAGs) demonstrated toughness due to tube (6 mm diameter for tensile test and 10 mm diameter for
the hydrophobic interactions among stearyl acrylate-derived manual compression) and sealed immediꢀately. The sealed
polymer chains. The chiral monomer NAA derived from L- plastic tube was immediately placed in a 60 C water bath and
alanine and acryloyl chloride endowed the hydrogels with then polymerization began and lasted for 24 h. Hydrogels with
intriguing optical activity. Herein, it is important to point out excellent mechanical property were thus obtained. For
that the major driving force for us to use NAA as starting comparison, hydrogel samples without gelatin and C18 were
material lies in that amino acid-derived polymers (e.g. alanine- also prepared under comparable conditions (see Table 1).
based polymers38–40) have constituted a unique class of polymers
currently drawing increasing attention. The study along this 2.4 Characterization of the emulsions
direction may provide novel chiral polymer materials as
demonstrated by our earlier studies.41,42
ꢀ
The emulsions (C18 in gelatin aqueous solution at 40 C) were
observed by optical microscopy equipped with a Canon digital
camera to get photos of the droplets.44 Specically, a drop of
emulsion was placed on a piece of glass slide and then covered
with another piece of cover glass. Aer setting the magnication
as 400 and regulating the knob to nd the clear photograph, the
morphology could be observed by the digital camera. To further
demonstrate the emulsication property of gelatin, the solution
only containing C18 and water was also viewed by the optical
microscopy.
2. Experimental section
2.1 Materials
L-Alanine and acryloyl chloride were purchased from Aladdin
(China). Gelatin (type A, from porcine skin with a gel strength
about 300 g Bloom) and stearyl acrylate (C18) were obtained
from Sigma-Aldrich Company. Ethyl acetate, anhydrous
ethanol, sodium hydroxide (NaOH), hydrochloric acid (HCl),
and ammonium persulfate (APS) were purchased from Beijing
Chemical Reagents Company. All the reagents obtained were of
analytic grade and used without further purication.
2.5 Characterization of the hydrogels
Fourier transform infrared spectra (FT-IR) were recorded on
a TENSOR 27 spectrophotometer (KBr tablet for powder spec-
imen and ATR for dried hydrogels). Circular dichroism (CD)
and UV-vis absorption spectra were measured on JASCO J-
810 spectropolarimeter at room temperature, by placing
a certain amount of water-swollen hydrogels between two pieces
of quartz glass. The structure of hydrogels was observed by
scanning electron microscopy (SEM, Hitachi S-4800). For this
purpose, the swollen hydrogels were frozen in liquid nitrogen
and then freeze-dried in the freeze direr for 48 h before
observation.
2.2 Synthesis of chiral monomer NAA
The chiral monomer NAA was synthesized according to our
earlier investigations.43 The major procedure is stated as below.
2.784 g L-alanine was added in 12.5 mL 0.2 mol Lꢁ1 NaOH
aqueous solution at 25 ꢀC; and then 2.5 mL acryloyl chloride
was dropwise added in the reaction system in an ice-bath aer
bubbled with N2 for 20 min. The white precipitate appeared by
adding 10 mL 2 mol Lꢁ1 HCl aqueous solution aer reacting for
2 h in ice-bath and then for another 1 h at room temperature.
Aer extracting ve times with ethyl acetate and evaporation of
the solvent, we could obtain the coarse product. Aer further
purication by recrystallization from ethanol at 0 ꢀC and dried
under 50 ꢀC vacuum oven, pure L-NAA monomer was obtained.
2.6 Tensile tests
Uniaxial tensile tests were performed on electric tensile tester
(SANS, China) with a 500 N load cell at room temperature.45 The
samples were cylindrical hydrogels of about 6 mm in diameter
and 25 mm in length, and the sample length between the
2.3 Preparation of hydrophobic association hydrogels
clamps was 14 mm and the tensile speed was 500 mm minꢁ1
.
Hydrophobic association hydrogels (HAGs) were synthesized by Cyclic tests were also conducted, and the maximum elongation
free radical copolymerization of hydrophilic NAA with hydro- was xed at 500% using HAG4% as example. When the hydrogel
phobic C18 at 60 ꢀC for 24 h by using macromolecular gelatin as was stretched to 500% elongation, the sample was unloaded
emulsier and APS as initiator. Herein, the comonomers' total until the stress became zero. The second and third loading–
38958 | RSC Adv., 2016, 6, 38957–38963
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Table 1 Parameters for preparing hydrophobic association hydrogels (HAG)a
HAG
C18 ratio (mol%)
Gelatin (g)
C18 (mmol)
NAA (mmol)
APS (mmol)
HAG2%
HAG3%
HAG4%
Blank-1
Blank-2
2%
3%
4%
3%
0
0.75
0.75
0.75
0
0.204
0.303
0.399
0.303
0.303
9.996
9.797
9.576
9.797
9.797
0.204
0.202
0.199
0.202
0.202
0.75
a
HAG2%, HAG3%, and HAG4% showed satisfactory toughness; blank-1: only sticky product rather than hydrogel formed; blank-2: the hydrogel was
too weak and could not be stretched.
unloading cycles were conducted immediately without any stay. interaction served as crosslinking points for forming the
Herein, the corresponding three loading–unloading curves were hydrogel networks.
noted as 41, 42 and 43. Meanwhile, aer a waiting time for 15
In the strategy presented in Scheme 1, the key factor for
min, the same HAG4% sample was performed for the fourth forming hydrophobic association hydrogels is the presence of
loading–unloading cycle, and we named the recorded stress– hydrophobic polymer domains derived from C18 in aqueous
strain curve as 44.
solution, with the aid of emulsifying function of gelatin. To
elucidate this hypothesis, optical microscope was used to
observe the emulsion comprising C18 and gelatin, and the
corresponding photographs are shown in Fig. 1b–d. Fig. 1a
shows the C18 droplets in the blank test only containing the
hydrophobic monomer C18 and water (without any gelatin). In
this case, the hydrophobic monomer C18 assumed a “sheet”
form and oated on water, indicating that C18 could not be
dispersed in water. However, C18 in the gelatin aqueous solu-
tion (Fig. 1b–d) became spherical and well dispersed in the
aqueous system. This is due to the emulsication effect of
gelatin. Therefore C18 could be dispersed in the gelatin
aqueous solution and stabilized by gelatin macromolecules.
Three hydrophobic association hydrogels were produced by
changing the hydrophobic/hydrophilic monomer ratio, and the
corresponding parameters and hydrogels as-formed are
summarized in Table 1. The hydrogels formed by the physical
hydrophobic interactions could be molded into arbitrary shapes
due to the reversible physical crosslinking points (Fig. 2). For
example, the hydrogels (taking HAG3% as a representative)
could be molded as a circle, a U form, and a cross without any
damage (Fig. 2a). Furthermore, the swelling ability of the three
hydrogels was measured. The swelling ratio of them was
approximately 150 (HAG2%), 130 (HAG3%), and 120 times
(HAG4%). (The swelling ratio was determined with the corre-
sponding dried hydrogel as control sample). Fig. S1 (ESI†)
presents a typical photograph for HAG2% aer swelling in
water.
2.7 Shape memory behavior
The cylindrical hydrogel sample (HAG4%) with a diameter of
6 mm and length of 6 cm was used to examine the shape
memory behavior.34,46 The hydrogel was wrapped with plastic
wrap and immersed in 60 ꢀC water for 3 h (the reason for the
usage of plastic wrap was to guarantee the hydrogel contact-
ing but not absorbing hot water), which facilitated the shape
changing of the hydrogel. Then the hydrogel was twisted on
a glass rod to form spiral shape, and this temporary spiral
shape was xed by cooling the sample to ambient tempera-
ture and further equilibrating overnight. Aer that the
deformed hydrogel sample was immersed in a water bath at
60 ꢀC for recovering to the original shape. To measure the
shape recovery ratio R, the sample with a original length of 2
cm was strentched to 7 cm, and then the sample length L was
recorded at 1 min intervals. R was calculated by the equation:
R ¼ (7 ꢁ L)/(7 ꢁ 2) ꢂ 100%.
3. Results and discussion
3.1 Strategy to prepare the hydrogels
In the present study, we designed and successfully prepared
tough chiral hydrogels via free radical (micellar) polymerization
approach. The aims are to improve the hydrogels' mechanical
properties by hydrophobic interaction and to improve the
biocompatibility by introducing biopolymer gelatin as macro-
molecular emulsier. The materials used for preparing the
hydrogels include only emulsier gelatin, hydrophilic monomer
NAA, hydrophobic monomer C18, and initiator APS. As we
know, hydrogels were three-dimensional networks crosslinked
by chemical and/or physical interactions. Due to the emulsier
property of gelatin mentioned above and the absence of any
crosslinker, the mechanism for forming the hydrogels is
considered as free radical micellar polymerization.28,29 The
preparative strategy is illustratively presented in Scheme 1,
where gelatin acted as emulsier for the hydrophobic mono-
Scheme 1 Schematic illustration for forming tough hydrogels by
mer. Herein, we highlight that the hydrophobic aggregation hydrophobic association effect.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 38957–38963 | 38959

View Article Online
RSC Advances
Paper
the hydrogels. In addition, we could compress the hydrogels
with pressure by two ngers nearly to 50% of its original height
without any visible cracks or breaks along the hydrogel
(Fig. 2c1–c3). All these properties testied the toughness (exible
and elastic) of the obtained chiral hydrogels. The tensile prop-
erties will be quantitatively discussed later on.
3.2 Characterization of the hydrogels
To preliminarily conrm the formation of hydrophobic associ-
ation hydrogels, HAG2% was taken as a representative of the
hydrogels, together with the reactants, were measured by FT-IR
spectroscopy. The FT-IR spectra are shown in Fig. 3. A compar-
ison among the spectra of gelatin (spectrum a), stearyl acrylate
(C18, spectrum b), NAA (spectrum c), and the product HAG2%
(spectrum d) provides us a strong evidence for the successful
formation of the hydrogels. Specically, the peaks at 1611 cmꢁ1
(Fig. 3b) and 1602 cmꢁ1 (Fig. 3c), which are assigned to the C]C
double bonds in hydrophobic monomer C18 and hydrophilic
monomer NAA, disappeared completely in the hydrogel network
(Fig. 3d), indicating that the double bonds were consumed and
polymerization took place in the monomers. Meanwhile, the
corresponding characteristic peaks in NAA and C18 could also
be seen in the hydrogel network. The appearance of the char-
acteristic peaks at 1732 (C]O absorption), 1651 and 1550 cmꢁ1
(amide group) of NAA and the peaks at 2925 and 2854 cmꢁ1 (C–H
stretching) of C18 demonstrated the presence of the monomer
units in the hydrogel. All the observations discussed above
further conrmed the formation of hydrogels.
Fig. 1 Optical micrographs of the emulsion droplets (a) only C18 and
water (no gelatin); (b–d) C18 in the gelatin aqueous solution with C18
content: 2, 3 and 4 mol%, respectively.
The hydrogels were immersed in water to reach water-uptake
equilibrium, and then frozen-dried in the freeze direr in order
Fig. 2 Digital photographs of HAG3% with excellent mechanical
toughness. The hydrogel exhibited high level of deformation: (a) to observe the structure of the hydrogels by SEM. The relevant
molded into arbitrary shapes, (b) stretched by hand, (c) compressed
manually to a certain degree without any damage.
SEM images are illustrated in Fig. 4. The pore structures of the
hydrogels were remained aer freeze-drying water. The pores
tended to become smaller with the increase in the hydrophobic
monomer ratio, which increased the crosslink density of the
To further testify the free radical micellar copolymerization
hydrogels.
and the hydrophobic association effect, two blank experiments
respectively named as blank-1 and blank-2 were also conducted
(Table 1). In blank-1, all the other conditions for preparing
3.3 Optical activity of the hydrogels
HAG3% kept unchanged but without adding gelatin; in blank-2,
all the other conditions for preparing HAG3% kept unchanged
but without adding C18 (the products from the two blank tests
are also named as blank-1 and blank-2, respectively). The
photographs for the products (blank-1 and blank-2) aer poly-
merization at 60 ꢀC for 24 h are shown in Fig. S2 (ESI†). For
blank-1, hydrophobic C18 could not be dispersed in aqueous
solution; and only a sticky material rather than hydrogel was
formed aer polymerization, due to the lacking of gelatin. As for
blank-2, even though hydrogel could be formed, however the
hydrogel was too weak to be stretched due to the lack of C18
which otherwise should be responsible for forming physical
crosslinks. The products in blank-1 and blank-2 clearly
demonstrate the roles played by gelatin and C18 in forming the
designed hydrogels. The C18-derived components inside the
hydrogels provide hydrophobic interactions due to their insol-
ubility in water and accordingly provide the desired strength for
The present study aimed to prepare tough chiral hydrogels
through free radical micellar polymerization. Herein, the
Fig. 3 Typical FT-IR spectra of (a) gelatin, (b) C18, (c) NAA and (d) HAG
2% (as representative for the HAGs).
38960 | RSC Adv., 2016, 6, 38957–38963
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Fig. 4 SEM images of the cross-section of hydrophobic association
hydrogels. (a) HAG2%; (b) HAG3%; (c) HAG4%.
hydrophilic monomer NAA, synthesized by the reaction
between chiral L-alanine and acryloyl chloride, in theory should
endow the hydrogels with the desired optical activity. Hence,
the water-swollen equilibrium hydrogels were placed between
two pieces of quartz glass and subjected to circular dichroism
(CD) and UV-vis absorption spectroscopy measurement at room
temperature, since CD and UV-vis spectroscopies are effective to
characterize the optical activity of materials.47–50 The hydrogels
showed considerable CD signals around 225 nm in Fig. 5a,
indicating the optical activity of the hydrogels. The corre-
sponding UV-vis spectroscopy presented in Fig. 5b also had
absorptions at wavelength below 250 nm. To acquire deep
understanding of the chiral hydrogels, we also measured the CD
and UV-vis spectra of gelatin. The related spectra are also
illustrated in Fig. 5. It shows the CD spectra of the hydrogels are
quite different from that of gelatin. Referring to our earlier
studies concerning chiral hydrogels constructed by NAA,41 we
conclude that the optical activity of the hydrogels was origi-
nated majorly in the chiral monomer NAA.
Fig. 6 Typical photographs of HAG2% before the tensile test (a1) and
just before rupture (a2); (b) stress–strain curves of the hydrogels. In (a1)
and (a2), the hydrogel sample was highlighted by red line box.
association crosslinks inside the hydrogels slided and thus
dissipated the mechanical energy inside the hydrogel
networks.13 Therefore, the hydrophobic association hydrogels
showed remarkable toughness, similar to the properties of
elastomers, which is highly desirable from the view point of
practical applications.
Cyclic tensile tests at a certain strain were also performed on
HAG4% for further understanding the hydrogels, and the ob-
tained loading/unloading curves are shown in Fig. 7. The results
show that the hydrogel exhibited evident stress soening
phenomenon, similar to elatomer materials, when the loading/
unloading tensile test was performed ongoing without any
interval time (see curves 41, 42, and 43). Interestingly, aer n-
ishing the third cyclic test and remaining a 15 min waiting time,
the obtained 44 curve (the fourth test) in Fig. 7 was close to the
rst curve 41 and the relevant values were also higher than the
last two curves (42 and 43). During the rst three tests, the
hydrogel was continuously tested without any pause, so the
hydrogel did not have time to recover to its original state. When
the hydrogel was le for a 15 min waiting time, the hydrogel
network, at least partially, recovered to its original network
(curve 44). Thus the cyclic tests conrm the energy dissipation in
the hydrogels and further the presence of reversible hydrophobic
crosslinks in the hydrogel networks.51 This consideration is
3.4 Mechanical properties of the hydrogels
The mechanical properties of the obtained hydrogels were
investigated by tensile test. Fig. 6a demonstrates the specic
photographs of the hydrogel HAG2% before the tensile test and
the just before rupture in the tensile test using electric tensile
tester. The photographs vividly manifested the mechanical
properties of the hydrophobic association hydrogels. Fig. 6b
shows the corresponding stress–strain curves of the three
hydrogels (HAG2%, HAG3%, and HAG4%, see Table 1). As the
content of C18 increased (in the order of HAG2%, HAG3%, and
HAG4%), the corresponding stress and strain increased and
reached the maximum values of 76 kPa and 750%, respectively
(HAG4%). When subjected to external load, the hydrophobic
Fig. 7 Four successive loading/unloading cycles of hydrogel HAG4%
with a fixed strain 500%. The first three curves (41, 42, and 43) were
Fig. 5 CD (a) and UV-vis (b) spectra of the hydrogels swelled in conducted immediately after the preceding tensile test. The fourth
deionized water at ambient temperature.
curve (44) was conducted after 15 min of waiting time.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 38957–38963 | 38961

View Article Online
RSC Advances
Paper
hydrogels and the results are presented in Fig. 9. It shows the
shape recovery ratio (R) of the hydrogels at different time. All the
three hydrogels showed a high shape recovery ratio (>80%).
When the temperature was above the melting temperature of
the crystalline domains formed by C18-derived polymer chains,
the crystalline domains melted. Accordingly, the physically
crosslinked network structures resulting from the hydrophobic
association enabled the hydrogels to recover nearly to the
original length.
Fig. 8 Photographs demonstrating thermal-induced shape memory
process of HAG4% from the temporary spiral shape to the permanent
rod shape.
4. Conclusion
Tough chiral hydrophobic association hydrogels were prepared
through free radical micellar polymerization of stearyl acrylate
with gelatin as emulsier, together with the radical polymeri-
zation of NAA in the same aqueous polymerization system.
Herein, the hydrophobic association domains acted as physical
crosslinking points, by which to form tough hydrogels. The
hydrogels could be strentched and compressed, and also
notably, the physically crosslinked network structures enabled
the hydrogels to exhibit interesting shape memory behavior.
The chiral monomer NAA endowed the hydrogels with optical
activity, according to CD spectra. All these remarkable advan-
tages facilitated the present hydrogels to be further investigated
for establishing advanced functional biomaterials. Especially
worthy to note is the presence of gelatin in the hydrogels, which
is expected to afford the hydrogels with desirable biocompati-
bility. The potential uses of the chiral hydrogels in chiral-related
areas also deserve much more attention. Our research is
currently undergoing along the directions.
Fig. 9 Curves of shape recovery ratio (R) vs. time of the hydrogels. The
detailed conditions are presented in Experimental section.
further manifested by the performance of shape memory of the
hydrogels, as to be reported next.
Acknowledgements
3.5 Shape memory performance of the hydrogels
This work was supported by the National Natural Science
Foundation of China (21474007, 21274008, 21174010), the
Funds for Creative Research Groups of China (51221002), and
the “Specialized Research Fund for the Doctoral Program of
Higher Education” (SRFDP 20120010130002).
The presence of hydrophobic interactions may provide the
hydrogels with some other interesting properties, such as self-
healing and shape memory behavior.31,34,46 According to the
earlier reports in literature, the hydrogels formed by stearyl
acrylate bearing long alkyl chains could be able to crystallize
upon self-assembly into hydrophobic domains. These crystal-
line domains were very stable and could be disrupted only upon
heating above the melting temperature.52 The melting temper-
ature (Tm) of the hydrophobic C18 polymer is about 50 C,53–55
and therefore the corresponding thermal-induced shape
memory behavior of HAG4% was conducted at 60 ꢀC. The
results are depicted in Fig. 8.
In Fig. 8, the permanent rod shape hydrogel was treated in 60
ꢀC water and thus could be easily deformed into the temporary
spiral shape by being wrapped on a glass rod. For the hydrogels,
the crystalline domains formed by the long alkyl groups in C18
facilitated to x the temporary shape at room temperature.34,56
The above temporary spiral shape was xed at ambient
temperature within 12 h. Aer putting it in hot water (60 ꢀC)
again, the temporary spiral shape gradually recovered to nearly
its original rod shape within 180 s. We further quantitatively
measured the recovery ratio R (as dened above) of the three
Notes and references
1 X. Du, J. Zhou, J. Shi and B. Xu, Chem. Rev., 2015, 115, 13165.
2 T. Jungst, W. Smolan, K. Schacht, T. Scheibel and J. Groll,
Chem. Rev., 2016, 116, 1496.
3 K. Yue, G. T. de Santiago, M. M. Alvarez, A. Tamayol,
N. Annabi and A. Khademhosseini, Biomaterials, 2015, 73,
254.
4 T. Billiet, M. Vandenhaute, J. Schelout, S. Van Vlierberghe
and P. Dubruel, Biomaterials, 2012, 33, 6020.
5 J. C. Cuggino, C. B. Contreras, A. J. Kairuz, B. A. Maletto and
C. I. A. Igarzabal, Mol. Pharm., 2014, 11, 2239.
6 C. Ghobril and M. W. Grinstaff, Chem. Soc. Rev., 2015, 44,
1820.
ꢀ
7 J.-F. Xing, M.-L. Zheng and X.-M. Duan, Chem. Soc. Rev.,
2015, 44, 5031.
8 H. Wang and S. C. Heilshorn, Adv. Mater., 2015, 27, 3717.
38962 | RSC Adv., 2016, 6, 38957–38963
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
9 W. Li, D. Wang, W. Yang and Y. Song, RSC Adv., 2016, 6, 32 D. C. Tuncaboylu, M. Sari, W. Oppermann and O. Okay,
20166. Macromolecules, 2011, 44, 4997.
10 E. P. da Silva, M. R. Guilherme, F. P. Garcia, C. V. Nakamura, 33 U. Gulyuz and O. Okay, So Matter, 2013, 9, 10287.
L. Cardozo-Filho, C. G. Alonso, A. F. Rubira and 34 C. Bilici and O. Okay, Macromolecules, 2013, 46, 3125.
M. H. Kunita, RSC Adv., 2016, 6, 19060.
11 X. Su, L. Bu, H. Dong, S. Fu, R. Zhuo and Z. Zhong, RSC Adv.,
2016, 6, 2904.
12 D. Maitland, S. B. Campbell, J. Chen and T. Hoare, RSC Adv.,
2016, 6, 15770.
35 X. Zhang, Y. H. Yang, J. R. Yao, Z. Z. Shao and X. Chen, ACS
Sustainable Chem. Eng., 2014, 2, 1318.
36 J. Olijve, F. Mori and Y. Toda, J. Colloid Interface Sci., 2001,
243, 476.
37 L. Lobo, J. Colloid Interface Sci., 2002, 254, 165.
13 J.-A. Yang, J. Yeom, B. W. Hwang, A. S. Hoffman and 38 K. Bauri, K. D. Sayala, R. S. Roy and P. De, Eur. Polym. J.,
S. K. Hahn, Prog. Polym. Sci., 2014, 39, 1973. 2015, 73, 237.
14 J. Y. Sun, X. H. Zhao, W. R. K. Illeperuma, O. Chaudhuri, 39 K. Bauri, A. Narayanan, U. Haldar and P. De, Polym. Chem.,
K. H. Oh, D. J. Mooney, J. J. Vlassak and Z. G. Suo, Nature,
2012, 489, 133.
15 K. R. Shull, Nature, 2012, 489, 36.
2015, 6, 6152.
40 Y. Shen, G. Li, Y. Ma, D. Yu, J. Sun and Z. Li, So Matter,
2015, 11, 7502.
16 X. J. Liu, H. Q. Li, B. Y. Zhang, Y. J. Wang, X. Y. Ren, S. Guan 41 P. Xie, X. Liu, R. Cheng, Y. P. Wu and J. P. Deng, Ind. Eng.
and G. H. Gao, RSC Adv., 2016, 6, 4850. Chem. Res., 2014, 53, 8069.
17 Y. Lin, D. He, Z. Chen, L. Wang and G. Li, RSC Adv., 2016, 6, 42 R. Cheng, J. Liu, P. Xie, Y. P. Wu and J. P. Deng, Polymer,
12479. 2015, 68, 246.
18 T. C. Suekama, J. Hu, T. Kurokawa, J. P. Gong and 43 L. Shi, P. Xie, Z. M. Li, Y. P. Wu and J. P. Deng, Macromol.
S. H. Gehrke, ACS Macro Lett., 2013, 2, 137.
Chem. Phys., 2013, 214, 1375.
19 Y. Okumura and K. Ito, Adv. Mater., 2001, 13, 485.
20 M. X. Shen, L. Li, Y. M. Sun, J. Xu, X. H. Guo and
R. K. Prud’homme, Langmuir, 2014, 30, 1636.
44 J. Surh, E. A. Decker and D. J. McClements, Food
Hydrocolloids, 2006, 20, 596.
45 A. K. Gaharwar, S. A. Dammu, J. M. Canter, C. J. Wu and
G. Schmidt, Biomacromolecules, 2011, 12, 1641.
21 Y. Piao and B. Chen, RSC Adv., 2016, 6, 6171.
22 F. Z. Jiang, T. Huang, C. C. He, H. R. Brown and H. L. Wang, 46 J. K. Hao and R. A. Weiss, ACS Macro Lett., 2013, 2, 86.
J. Phys. Chem. B, 2013, 117, 13679.
23 M. Shibayama, So Matter, 2012, 8, 8030.
47 K. Maeda, M. Muto, T. Sato and E. Yashima, Macromolecules,
2011, 44, 8343.
24 J. Q. Xu, D. A. Bohnsack, M. E. Mackay and K. L. Wooley, J. 48 W. F. Li, X. Liu, G. Y. Qian and J. P. Deng, Chem. Mater., 2014,
Am. Chem. Soc., 2007, 129, 506. 26, 1948.
25 Y. Wei, Y. Wang, C. Wei, Q. Zhao, Y. Yan, J. Yang and 49 F. Yao, D. Y. Zhang, C. H. Zhang, W. T. Yang and J. P. Deng,
J. Huang, RSC Adv., 2015, 5, 106005. Bioresour. Technol., 2013, 129, 58.
26 S. Abdurrahmanoglu, V. Can and O. Okay, Polymer, 2009, 50, 50 C. Song, C. H. Zhang, F. J. Wang, W. T. Yang and J. P. Deng,
5449. Polym. Chem., 2013, 4, 645.
27 W. B. Li, H. Y. An, Y. Tan, C. G. Lu, C. Liu, P. C. Li, K. Xu and 51 X. H. Zhao, So Matter, 2014, 10, 672.
P. X. Wang, So Matter, 2012, 8, 5078.
52 O. E. Philippova, A. S. Andreeva, A. R. Khokhlov, A. Islamov,
28 E. J. Regalado, J. Selb and F. Candau, Macromolecules, 1999,
32, 8580.
29 P. Kujawa, A. A. Hayet, J. Selb and F. Candau,
Macromolecules, 2006, 39, 384.
30 G. Q. Jiang, C. Liu, X. L. Liu, G. H. Zhang, M. Yang and
F. Q. Liu, Macromol. Mater. Eng., 2009, 294, 815.
31 G. Q. Jiang, C. Liu, X. L. Liu, Q. R. Chen, G. H. Zhang,
M. Yang and F. Q. Liu, Polymer, 2010, 51, 1507.
A. I. Kuklin and V. I. Gordeliy, Langmuir, 2003, 19, 7240.
53 A. Matsuda, J. Sato, H. Yasunaga and Y. Osada,
Macromolecules, 1994, 27, 7695.
54 F. Dutertre, P.-Y. Pennarun, O. Colombani and E. Nicol, Eur.
Polym. J., 2011, 47, 343.
55 Z. Ren, W. Hu, C. Liu, S. Li, X. Niu and Q. Pei,
Macromolecules, 2016, 49, 134.
56 K. Inomata, T. Terahama, R. Sekoguchi, T. Ito, H. Sugimoto
and E. Nakanishi, Polymer, 2012, 53, 3281.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 38957–38963 | 38963