- Email: [email protected]
Self-healable tough supramolecular hydrogels crosslinked by poly-cyclodextrin through host-guest interaction
Accepted Manuscript Title: Self-Healable Tough Supramolecular Hydrogels Crosslinked by Poly-Cyclodextrin through Host-Guest Interaction’ Authors: Tingting Cai, Shuangjun Huo, Tao Wang, Weixiang Sun, Zhen Tong PII: DOI: Reference:
S0144-8617(18)30295-9 https://doi.org/10.1016/j.carbpol.2018.03.039 CARP 13389
To appear in: Received date: Revised date: Accepted date:
12-1-2018 14-3-2018 14-3-2018
Please cite this article as: Cai, Tingting., Huo, Shuangjun., Wang, Tao., Sun, Weixiang., & Tong, Zhen., Self-Healable Tough Supramolecular Hydrogels Crosslinked by Poly-Cyclodextrin through Host-Guest Interaction’.Carbohydrate Polymers https://doi.org/10.1016/j.carbpol.2018.03.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Self-Healable Tough Supramolecular Hydrogels Crosslinked by PolyCyclodextrin through Host-Guest Interaction
Tingting Cai,1 Shuangjun Huo,1 Tao Wang, *1 Weixiang Sun,1 and Zhen Tong *1,2
Research Institute of Materials Science, South China University of Technology, Guangzhou
IP T
1
510640, China
State Key Laboratory of Luminescent Materials and Devices, South China University of
SC R
2
U
Technology, Guangzhou 510640, China
N
Corresponding author.
A
Tel: +86-020-87112886-601
M
Fax: +86-020-87112886-605
A
CC
EP
Graphical Abstract
TE D
Email: [email protected] (T. Wang), [email protected] (Z. Tong)
1
Highlights
Novel tough functional supramolecular hydrogel was prepared based on the host-guest
IP T
interaction between the polyfunctional crosslinking agents poly-cyclodextrin and adamantane.
The hydrogel showed rapid self-healing and improved healing percentage benefited from
SC R
the poly-cyclodextrin, which could provide more host sites and binding opportunities for the complexation.
Shape memory was realized through the reversible second crosslinked network by
U
M
A
N
polyacrylic acid backbone and Fe3+.
Abstract
TE D
A new supramolecular hydrogel with superior self-healing and shape memory properties was synthesized via in-situ copolymerization of the adamantane-containing monomer Nadamantylacrylamide (Ad-AAm) and monomer acrylic acid (AAc) in the polycyclodextrin
EP
(PCD) aqueous solution, where PCD served as the polyfunctional physical cross-linker.
The
CC
PCD-Ad hydrogels showed strength of 40 kPa with breaking strain of 600%, but small tensile loading-unloading hysteresis and residual deformation benefited from the reversible
A
crosslinking structure. condition.
Excellent self-healing property was realized at 70 °C and humid
The healing percentage reached over 70% after 120 min, which showed enhanced
self-healing efficiency compared with the reported supramolecular hydrogels crosslinked by CD and Ad.
This may benefit from the polyfunctional PCD crosslinking reagent, which
provided more host sites and binding opportunities for the complexation during the self-healing process.
Shape memory behavior was also realized through the reversible second 2
crosslinked network by the carboxyl groups of PAAc with Fe3+ ions.
The present study
provides a new method to improve self-healing ability of the supramolecular hydrogels, which will find the applications in self-healing coating, biomedical materials and soft actuators.
Keywords:
Supramolecular
hydrogel;
Polyfunctional
poly-cyclodextrin;
SC R
IP T
interaction; Self-healing; Shape memory.
Host-guest
1. Introduction
Hydrogels have been conspicuously used in biomedical fields, such as wound dressings and
U
hygiene products, because of their unique water containing polymer network structure similar
N
to the human tissues (Caló & Khutoryanskiy, 2015; Taylor & In het Panhuis, 2016).
The
A
mechanical strength and reliability of the hydrogels are crucial factors for the applications.
M
Self-healing ability can greatly improve the reliability of the hydrogels after damage or with
TE D
defects through the reconstitution of the reversible bonds in the hydrogel network (Brochu, Craig & Reichert, 2011; Mauldin & Kessler, 2010; Wei et al., 2014; Wu, Meure & Solomon, 2008; Yang & Urban, 2013).
Numerous approaches in constructing self-healing hydrogels
EP
have been reported, which can be classified into dynamic covalent bonds such as phenylboronate ester (Deng, Brooks, Abboud & Sumerlin, 2015), disulfide (Canadell,
CC
Goossens & Klumperman, 2011), imine (Wei, Zhao, Chen, Zhang & Zhang, 2016),
A
acylhydrazone (Kuhl et al., 2015), reversible radical reaction (Cheng et al., 2015), and reversible Diels-Alder cycloaddition (Wei et al., 2013), and non-covalent interactions. e.g., hydrophobic interactions (Tuncaboylu, Sari, Oppermann & Okay, 2011), host-guest interactions (Harada, Takashima & Nakahata, 2014), hydrogen bonds (Cui & Campo, 2012), and crystallization (Zhang, Xia & Zhao, 2012).
3
Among the non-covalent interactions to realize the self-healing capacity, the host-guest interaction has attracted much more attention, which has been widely used to prepare the selfhealing hydrogels.
The host-guest interaction is reversible and sensitive to local environment
and can be broken and easily recovered, which is based on macrocyclic recognition between cyclic hosts, e.g., cyclodextrins (CD), crown ethers, calix[n]arenes, cucurbit[n]urils,
IP T
cyclophanes, and pillararenes, and their corresponding guests (Chu & Ravoo, 2017; Nuvoli et al., 2016; Sanna et al., 2017; Yang, Yuan, Zhang & Scherman, 2014; Yang et al., 2015; Yu,
β-CD and cholic acid
SC R
Wang & Chen, 2014; Yu, Ha, Sun & Shi, 2014; Zhang et al., 2012).
host-guest interaction was used to obtain self-healing supramolecular hydrogel (Jia & Zhu, 2015).
Host-guest macromers formed by molecular self-assembly between adamantane-
U
functionalized hyaluronic acid guest polymers and monoacrylated β-cyclodextrins host
N
monomers was also applied to prepare self-healable biopolymer based freestanding Harada’s group has reported a series of self-
A
supramolecular hydrogels (Wei et al., 2016).
M
healing hydrogels using the host-guest interaction between CD and different guest molecules,
TE D
including adamantane, ferrocene, polyethylene glycol, phenolphthalein, and so on (Kakuta et al., 2013; Miyamae, Nakahata, Takashima & Harada, 2015; Nakahata, Takashima & Harada, 2016; Nakahata, Takashima, Yamaguchi & Harada, 2011; Takashima et al., 2017b).
The
EP
self-healable hydrogel using water-soluble polymer chains grafted with β-CD and polymer
CC
having adamantane (Ad) side groups showed maximum healing percentage of ~45% after healed for 24 h or even longer under humid condition (Nakahata, Takashima & Harada, 2016).
A
They also studied the relationship between the self-healing percentage of the supramolecular hydrogels and the molecular structure of the guest molecules, and reported the maximum healing percentage of ~61% for dried hydrogels after 48 h-healing under 100 °C (Takashima et al., 2017a).
High self-healing percentage along with the high toughness is still desired for
the host-guest supramolecular hydrogels.
4
Poly-cyclodextrin (PCD) is the polymerized cyclodextrin usually crosslinked by epichlorohydrin in alkaline media (Mocanu, Vizitiu & Carpov, 2001; Renard, Deratani, Volet & Sebille, 1997).
PCD has been used as the host molecules to prepare hydrogels with the
adamantane-containing guest polymers by mixing the components together, but the hydrogels were rather weak (Gosselet, Beucler, Renard, Amiel & Sebille, 1999; Koopmans & Ritter, PCD nanogel was used as macro-crosslinker to prepare the light-switchable self-
IP T
2008).
healing hydrogel with azobenzene as the guest molecule, similarly, the mechanical strength was
SC R
not desirable either (strength ~10 kPa and breaking strain ~100%) (Yang et al., 2017).
In this work, we design a new way to prepare supramolecular hydrogels by using PCD as the polyfunctional crosslinking agent to replace the traditional monofunctional CD molecules
U
grafted on the polymer chains to improve the self-healing property.
The hydrogel is
N
synthesized via in-situ copolymerization of the adamantane-containing monomer N-
A
adamantylacrylamide (Ad-AAm) and monomer acrylic acid (AAc) in the PCD aqueous solution.
M
These hydrogels exhibit high self-healing percentage and excellent shape memory behavior
2. Experimental
EP
2.1 Materials
TE D
based on the rapid recoverable inclusion complexation of PCD and Ad groups.
Monomer AAc (stabilized with 200 ppm hydroquinone methylether, Aladdin Industrial
CC
Inc.), β-CD (98%, J&K), epichlorohydrin (C3H5ClO, 99%, J&K), potassium peroxydisulfate (KPS, K2S2O8, 99%, J&K), acryloyl chloride (97%, stabilized with 400
A
ppm phenothiazine, J&K), adamantylamine (C10H17N, 98.5%, Meryer) were all used as received.
Ferric chloride hexahydrate (FeCl3⋅6H2O), hydrochloric acid (HCl), and
sodium hydroxide (NaOH) (Guangzhou Chemical Reagent Factory, China) were analytical reagents.
Pure water was obtained by deionization and filtration using a
5
Millipore purification apparatus (resistivity: 18.2 MΩ cm) and bubbled with nitrogen gas for 3 h prior to use to remove O2. 2.2 Synthesis of Polycyclodextrin (PCD) As shown in Fig. S1 in the Supplementary Information, sodium hydroxide aqueous solution (15 wt%) was mixed with β-CD and stirred for 3 h at room temperature, then
Thereafter, the above solution was added into acetone drop by drop.
IP T
epichlorohydrin was slowly added into the solution under stirring for another 3 h.
The precipitate
SC R
was filtered and dissolved in deionized water, followed by neutralization with diluted HCl and dialysis for 7 days (molecular weight cut off = 7000). was obtained by freeze-drying, and the yield was ~55%.
Finally, the product
The molecular weight of the
U
PCD was obtained as Mw ~ 13000, Mn ~ 6000 with Mw/Mn ~ 2.2 from a gel permeation
N
chromatography (GPC) calibrated by dextran standards.
Thus, the number of
A
repeating β-CD unit in one PCD molecule is ~5 based on Mn, but it is worth to note that
M
the PCD molecules are non-linear structure with relatively high polydispersity.
TE D
2.3 Synthesis of N-Adamantylacrylamide (Ad-AAm) As briefly shown in Fig. S2, adamantylamine (20 mmol, 3.025 g) was dissolved in 400 mL of THF at 60 °C for 3 h, and cooled to < 5 °C.
Trimethylamine (3.1 mL, 22 mmol)
EP
was added into the solution, and then acryloyl chloride (1.8 mL, 22 mmol) in 20 mL The solution was stirred for 6 h in an ice water
CC
THF was added dropwise in 30 min. bath.
The precipitate was removed by filtration, and the solvent in the filtrate was The Ad-AAm was obtained
A
removed by rotary evaporation to produce a yellow solid.
through a silica gel column chromatography (dichloromethane: petroleum ether = 3:7) as a white solid.
1
Yield: 2.15 g, 52.2%.
H NMR (600 MHz, DMSO, 25 °C): δ =
1.62 (m, 6H), 1.95 (m, 6H), 2.01 (m, 3H), 5.47 (dd, 1H), 6.01 (dd, 1H), 6.02 (m 1H), and 6.22 (s 1H). 2.4 Synthesis of Hydrogels
6
The PCD-Ad hydrogel was synthesized as follows.
First, desired amount of PCD and
Ad-AAm was dissolved in deionized water under stirring for 4 h at 70 °C to produce a homogeneous and transparent solution. added under stirring.
Then, monomer AAc and initiator KPS were
The solution was purged with nitrogen gas for 30 min and then
poured into a glass tube of 5 mm (diameter) × 120 mm (length) or a laboratory-made
The polymerization was conducted at 60 °C overnight.
any crosslinking reagents were used.
No
In this paper, the PCD-Ad hydrogel was referred
SC R
× 1 mm (thickness).
IP T
mold of two flat glasses separated by a rubber spacer of 80 mm (width) × 80 mm (length)
to as PCDxAdy, where x and y stood for the concentration (mM) of the β-CD and Ad units in the solution, respectively.
For example, the PCD3Ad3 gel consisted of 3 mM
N
in the solution was 2 M unless mentioned otherwise.
A
2.5 Characterization
H NMR of PCD in D2O, Ad-AAm in DMSO-d6, and 2D NOESY NMR of PCD and
M
1
The concentration of AAc
U
β-CD and 3 mM Ad-AAm in the polymerization solution.
600 MHz.
TE D
Ad-AAm complex in D2O were obtained using a Bruker Avance III HD spectrometer at The δ-scale relative to TMS was calibrated using the deuterium signal of
the solvent as an internal standard.
EP
Molecular weight was determined using a gel permeation chromatography (GPC)
CC
system (HLC-8320, Tosoh) with a refractive index detector and a combination of two columns (Shodex SB-806 HQ, Tosoh).
Dextran was used as the standard and the
A
column was eluted with H2O at 1 mL/min and 25 °C. The tensile strength was measured with a Shimadzu AG-X plus testing system at
ambient temperature on cylindrical hydrogel samples after specified treatments.
The
sample length between the jaws was 40 mm, the sample diameter was 5 mm, and the crosshead speed was 50 mm/min.
The tensile strain was taken as the length change
related to the original length, and the tensile stress was evaluated on the initial cross-
7
section of the sample.
The stretch hysteresis was measured on the samples under the
strain below 500%. Rheology measurements were carried out on the as-prepared PCD-Ad hydrogel with a stress-controlled rheometer AR-G2 (TA) using parallel plates of diameter of 50 mm. Silicone oil was laid on the edge of the fixture plates to prevent solvent evaporation.
IP T
The self-healing of blade cut as-prepared PCD-Ad hydrogel was carried out by keeping the cut surfaces in contact and healed in humid environment at room
The humid condition was realized by putting
SC R
temperature or 70 °C for different times.
the sample in a container with water but not contacting to water.
The healing
percentage was defined as the tensile strength of the self-healed hydrogel related to that
U
of the corresponding as-prepared one treated under the same condition.
N
The swelling behavior of the as-prepared PCD-Ad hydrogels was observed after
This swelling was nonequilibrium and the purpose
M
at room temperature, respectively.
A
immersing in AdCOONa solution, β-CD solution, as well as pure water for only 15 min
The swelling
TE D
was to verify the host-guest crosslinking in the PCD-Ad network.
degree was estimated as the weight of the swollen hydrogel related to the weight of the as-prepared one.
EP
The shape memory behavior of the hydrogel was investigated in the following way:
CC
the hydrogel strip was curled to the temporary shape manually and fixed by immersed in 0.06 M FeCl3/0.2 M HCl solution for a desired time; while the shape recovery was
A
realized by immersing the curled sample in 0.6 M HCl solution. 3. Results and discussion 3.1 PCD-Ad Supramolecular Hydrogels The preparation of the PCD-Ad hydrogel is schematically illustrated in Fig. 1.
Prior
to the radical copolymerization, the hydrophobic guest monomer (Ad-AAm) was
8
dissolved in the PCD aqueous solution to produce a uniform solution with the help of inclusion complexation at 70 °C (Fig. 1A-B).
Then, the copolymerization with
monomer AAc was conducted in the solution at 60 °C (Fig. 1C).
Fig. S3 shows the
2D NOESY NMR spectrum of the inclusion complex of PCD and Ad in D2O with Figs. S4 and S5 for the 1H-NMR spectra of PCD and Ad-AAm, respectively.
The existence
IP T
of the inclusion complex is confirmed by the cross-peaks of PCD (δH 3.5 ppm ~ 3.75 ppm) and adamantane (δH 1.69 ppm ~ 2.20 ppm), indicating their combination in Therefore, the copolymerization produces a polymer network crosslinked
SC R
solution.
Synthesis of PCD-Ad hydrogel by copolymerization of Ad-AAm and AAc
EP
Fig. 1
TE D
M
A
N
U
by multifunctional PCD and Ad units via host-guest interaction (Fig. 1D).
crosslinked with PCD through host-guest interaction.
Dissolving of Ad-AAm in PCD
CC
aqueous solution (A-B), PCD/Ad-AAm solution containing monomer AAc (C), the
A
crosslinked network of the PCD-Ad hydrogel after the copolymerization (D).
As shown in Fig. 2A, both the polymerization solution and hydrogel are almost transparent with slight milky white.
After copolymerization, the PCD-Ad hydrogel
can sustain large compression and elongation even with a knot (Fig. 2B and C),
9
exhibiting excellent mechanical properties with high water content of 83.8 wt%.
The
iii).
(A) Photos of the PCD3Ad3 hydrogel before (i) and after polymerization (ii, (B) The hydrogel before (i), during (ii) and after compression (iii).
U
Fig. 2
SC R
IP T
recovery to its original shape is rapid after removing the external force.
(C) The
N
original hydrogel (i), the knotted gel (ii), elongation (iii), and recovery (iv) of the knotted
M
A
gel.
TE D
The swelling of the hydrogel in the solution containing competitive host or guest was observed to confirm the crosslinking by the inclusion complex.
Fig. 3A presents the
photos of the PCD-Ad hydrogels before and after immersing in a specified solution for
EP
only 15 min, and Fig. 3B depicts that the swelling degree of the hydrogel is ~2.1 in 0.05
CC
M solution of the competitive guest AdCOONa, while only ~1.4 and ~1.3 in 0.01 M βCD solution (restricted by its low solubility in water) and pure water, respectively.
A
The larger swelling in the competitive guest AdCOONa solution is caused by the higher association of AdCOONa to β-CD than that of Ad-AAm (Kakuta et al., 2013; Rekharsky & Inoue, 1998).
Thus, the crosslinking in the PCD-Ad hydrogel is partially destroyed
by AdCOONa, leading to a large swelling degree of the hydrogel in the competitive guest solution.
This confirms the existence of the host-guest inclusion as the
crosslinking in the PCD-Ad hydrogel.
10
The mechanical properties of the PCD-Ad hydrogels with different AAc, PCD, and Ad-AAm concentrations were investigated by stretch and dynamic shearing. tensile stress-strain curves are plotted in Fig. S6.
The
The strength increases from about
14 kPa to 35 kPa but the strain at break first increases and then decreases as the AAc concentration is increased from 1 M to 4 M due to an increase of the polymer content in By simultaneously increasing the β-CD and Ad content from 0.01
IP T
the hydrogel (A).
M to 0.07 M, i.e., increasing the inclusion complex of PCD and Ad, the strength
SC R
increases from about 18 kPa to 40 kPa, while the strain at break decreases slightly (B), because of an increase in the crosslinking density of the hydrogels (Kakuta, Takashima
TE D
M
A
N
U
& Harada, 2013).
2.5
B
CC
A
Fig. 3
Swelling Degree
EP
2.0
1.5
1.0
0.5
0.0
0.05 M AdCOONa 0.01 M β-CD
Pure water
Immersion Solution
(A) Photos of the PCD3Ad3 hydrogel before and after immersing in the
indicated solution for 15 min, and (B) the corresponding swelling degree (estimated as the weight of the swollen hydrogel related to that of the as-prepared one) after immersion.
11
Energy dissipation of the PCD-Ad hydrogel during deformation was investigated by three continuous tensile loading-unloading cycles (0%-500%-0%) (Fig. 4A).
The
hysteresis percentage is about 15% with the residual strain of about 25% for the first cycle, while it decreases to about 10% in following cycles, showing low energy dissipation and low residual strain during the deformation-recovery cycle.
This result
IP T
indicates that a small part of the crosslinking points are destroyed during elongation to
dissipate the tensile energy, while the crosslinking can be reconstructed rapidly due to
SC R
the high association constant of β-CD and Ad groups (Kakuta et al., 2013; Rekharsky &
104
20
B
10 5 0
1st
2nd
N
15
A
10
20
G', G'' (Pa)
Hysteresis / %
3rd
Tensile cycle
5
M
Stress (kPa)
A 15
U
Inoue, 1998).
103
1st cycle 2nd cycle 3rd cycle
0 0
100
200
300
400
= 0.1% = 100% 102
500
0
500
TE D
Strain (%)
1000
1500
Time (s)
104
A
CC Fig. 4
G', G'' (Pa)
EP
C
G'
103
G" 2
10
,
25 oC
,
45 oC
,
65 C
,
75 C
101
102
o o
1
10 10-2
10-1
100
(rad/s)
(A) Tensile stress-strain curves of loading-unloading cycle of the PCD3Ad3
hydrogel, the insert is the corresponding hysteresis percentage (defined as the area within the cycle relative to the area below the loading stress-strain curve).
(B) Storage
modulus (filled symbols) and loss modulus (open symbols) of the PCD3Ad3 hydrogel
12
at shear strain of 0.1% and 100%, respectively at 25 °C and ω = 6.283 rad/s.
(C)
Frequency sweep of the PCD3Ad3 hydrogel at γ = 0.5% at indicated temperatures.
Fig. 4B shows the change in dynamic moduli of the PCD-Ad hydrogel with the shear strain jump between 0.1% and 100%.
The former is in the linear viscoelasticity G' and
IP T
region, and the latter locates in the nonlinear viscoelasticity region (Fig. S7A).
G" show a simultaneous jump with the strain jump, i.e., G' decreases and G" increases
SC R
suddenly at the strain of 100% and the both almost completely return to their original values upon the strain returning to 0.1% over all the cycles.
These jumps indicate that
the inclusion complex crosslinking can be partially destroyed at large strain but restored
The G' is always higher than G" means that the crosslinked network still
N
density.
U
at low strain reversibly and rapidly, manifesting a reversible change in the crosslinking
A
exists even at higher shear strain and the destruction is only partial.
M
The temperature effect on the dynamic moduli of the hydrogel was also observed from G' is almost a constant independent of
TE D
25 to 80 °C and shown in Figs. 4C and S7B.
temperature, implying that the supramolecular crosslinking by the host-guest complexation of PCD and Ad is stable at the tested temperatures.
G" slightly
EP
decreases upon heating due to the reduction in viscosity.
CC
Moreover, the polyfunctional PCD molecules are independent from the polymer chains jointed in the network, thus, their movement would be easier than the CD grafted This is also favorable for the reconstitution of the crosslinking
A
on the polymer chains.
points during the deformation and recovery process.
The inclusion between PCD and
Ad in the hydrogel with host-guest complex thus acts as “quickly recoverable sacrificial bonds” with fast recovery and small energy dissipation.
The network in the PCD-Ad
gel is, therefore tough and elastic, different from the weak hydrogel, of which the network is destroyed entirely under large amplitude shearing (Li, Yan, Yang, Chen &
13
Zeng, 2015; Tseng et al., 2015).
Furthermore, other kinds of physically crosslinked
hydrogels, for example, the UPy gels depending on the hydrogen bonding and the polyampholyte gels with electrostatic interaction, manifested large tensile hysteresis (Jeon, Cui, Illeperuma, Aizenberg & Vlassak, 2016; Sun et al., 2013), which seems to be ascribed to the long time required for the reconstitution of the sacrificial bonds in the
IP T
unloading period. 3.2 Self-Healing Behaviors
SC R
Fig. 5 demonstrates the self-healing process of the PCD3Ad3 hydrogel after cut into two pieces and contacted each other at 70 °C under a humid condition (dyed for easy
recognition), for the humid and heating condition is beneficial to the healing of hydrogels
U
(Gao, Du, Sun & Fu, 2015; Kakuta et al., 2013; Wang et al., 2014).
After 30 min, the
The self-healed hydrogel can bear a large loading without trace of damage
M
interface.
Besides, dye diffusion is observed at the healed
A
to the loss of water (Fig. 6C).
N
two pieces of the cut hydrogel are jointed together tightly with slight volume shrink due
TE D
at the junction (Figs. 6D and 6E), showing an outstanding self-healing capability for the
A
CC
EP
present PCD-Ad supramolecular hydrogel.
Fig. 5
Self-healing of the PCD3Ad3 hydrogel: (A) cut sample; (B) cut surfaces
contacted each other; (C) self-healed gel for 30 min under humid condition at 70 °C; (D) the healed hydrogel under stretching, and (E) loading weight of 80 g.
14
The self-healing efficiency was quantitatively evaluated by tensile test.
Figs. 6 and
S8 show the tensile stress-strain curves of the self-healed PCD3Ad3 hydrogel under different healing temperatures and times in the humid condition compared with that experiencing the same treatment without cut.
Fig. 7 manifests the healing percentage
as the tensile strength relative to that of the control hydrogel sample experiencing the The hydrogel treated at room temperature has low
IP T
same treatment without cut.
healing percentage of 10% after 24 h under humid condition (Fig. S8A).
As a
70 °C.
SC R
contrast, the self-healing efficiency increases greatly when the process is conducted at
The healing percentage is 16% at 15 min and raises to 53% at 30 min and 73%
at 120 min.
At the same time, the water content in the hydrogel decreases obviously
U
during the self-healing process at 70 °C (63% after 60 min, 27% after 120 min from 84%
N
for the as-prepared one) owing to the high temperature in spite of humid condition (Fig.
But the samples are still at hydrogel state after the healing process with favorable The heating and humid condition promote the molecular motion,
TE D
elasticity (Fig. 6).
M
6).
A
7B), which also contributes the increase in the hydrogel strength (see the control in Fig.
in turn accelerate the host-guest complexation and improve the self-healing (Nakahata, Takashima & Harada, 2016; Takashima et al., 2017a).
Moreover, the water loss
EP
during heating also drives the polymer chains at the interfaces to contact each other
CC
closely to form strong complexation between the cut surfaces (Gao, Du, Sun & Fu, 2015).
A
We also tried the self-healing without humid condition at room temperature or at 70
°C for different times, however, the healing effect was very poor.
Similar results were
also reported by Nakahata et al. in the β-CD-Ad crosslinking system with PAAm as the main chain polymer (Nakahata, Takashima & Harada, 2016).
Consequently, the
humid environment as well as heating are necessary for the self-healing process upon the PCD and Ad inclusion with PAAc or PAAm as the main chains.
15
On the other
hand, the β-CD-Ad dry hydrogel with poly(methyl triethylene glycol acrylate) (PTEGA, with glass-transition temperature Tg of -50 °C, much lower than PAAc and PAAm) as the polymer backbone also showed self-healing only upon heating without humid condition (Takashima et al., 2017a).
The soft polymer chains of the dry hydrogels
promoted the mobility of the host-guest molecules in the polymers.
The healing
IP T
percentage of the dry hydrogel increased as the increase of temperature and healing time, the healing percentage was ~30% at 75 °C for 48 h, while it increased to a maximum of
The present PCD-Ad hydrogel shows
SC R
61% at 100 °C for 48 h or even longer time.
improved self-healing percentage and shortened healing time, which seems to be that the polyfunctional PCD would provide more host sites and binding opportunities for the
60
300
A 50
B
control
40 30 20 10
M
15 min 70 oC humid condition water content: 81%
Stress (kPa)
250
control
TE D
Stress (kPa)
A
N
U
inclusion.
200 150
60 min 70 oC humid condition water content: 63%
100 50
self-healed
self-healed
0 0
200
400
600
0 0
800
200
Stress (kPa)
EP CC
1200
C 1000
control
120 min 70 oC humid condition water content: 27%
800 600 400
self-healed
200 0 0
600
Strain (%)
Strain (%)
A
400
200
400
Strain (%)
16
600
800
800
1000
Fig. 6
Stress-strain curves of the self-healed PCD3Ad3 gel at 70 °C for different times
(A: 15 min, B: 60 min, C: 120 min).
The control is the same hydrogel after the same
80
100
20
0 24 h
15 min 30 min 60 min 90 min 120 min
70 oC humid condition
80
IP T
40
70 C
Water content (%)
60
B
o
60 40 20 0 0
15
30
45
60
75
90
105
120
Time (min)
Healing time
(A) The healing percentage of the PCD3Ad3 hydrogel, and (B) the water content
U
Fig. 7
SC R
A Room temperature
Healing percentage (%)
treatment without cut.
M
3.3 Shape Memory Behavior
A
N
of the PCD3Ad3 hydrogel at 70 °C and humid condition varying with healing time.
TE D
The shape memory behavior has been explored on the PAAc backbone hydrogels by introducing Fe3+ to form a reversible second crosslinked network through the ionic bonding with the carboxyl groups -COO- in PAAc (Zhao, Huang, Wang, Sun & Tong, The present PCD-Ad hydrogel has the similar PAAc-co-PAM backbone, so
EP
2017).
The as-prepared
CC
that the shape memory capability is endowed to it in the similar way.
PCD3Ad3 hydrogel of straight strip as the permanent shape (Fig. 8A) is twisted and
A
immersed in 0.06 M FeCl3/0.2 M HCl solution for 2 h to fix the temporary shape (Fig. 8B).
At the same time, the hydrogel changes its color from whitish to brownish red
due to the introduction of Fe3+ to form the new crosslinking between Fe3+ and -COO(Zhao, Huang, Wang, Sun & Tong, 2017; Zhao et al., 2017).
HCl is added in the
FeCl3 solution to prevent the fast aggregation on the hydrogel surface due to the fast binding of Fe3+ to -COO-.
When the hydrogel in the helical shape is immersed in HCl,
17
the bonded Fe3+ ions are partially replaced by protons to form -COOH at low pH. Therefore, the hydrogel recovers to the permanent shape gradually (Fig. 8C), and the complete shape recovery is achieved after 3 h immersion (Fig. 8D).
Meanwhile, its
The shape memory of the PCD3Ad3 hydrogel: (A) the as-prepared gel in
N
Fig. 8
U
SC R
IP T
color also restores whitish because of the release of Fe 3+ ions.
A
permanent shape (80 mm×20 mm×1 mm); (B) the fixed temporary shape after being
M
twisted and immersed in 0.06 M FeCl3 / 0.2 M HCl solution for 2 h; (C) the partially
TE D
recovered shape after immersed in 0.6 M HCl solution for 1 h; (D) the completely recovered shape after immersed in 0.6 M HCl solution for 3 h.
EP
4. Conclusions
CC
In summary, a tough supramolecular hydrogel has been synthesized via a pre-assembled method to form PCD and Ad inclusive complex prior to the polymerization.
The
A
present PCD-Ad hydrogel crosslinked by the host-guest interaction of the polyfunctional PCD and Ad showed favorable mechanical properties with small hysteresis and residual deformation during tensile loading-unloading cycles.
The fast and efficient self-
healing of the hydrogel was realized at 70 °C in humid condition, and the healing percentage reached 73% after healing for 120 min. realized by introducing Fe3+ to the hydrogel. 18
Shape memory behavior was
Potential applications of the present
supramolecular hydrogel will be developed in the fields such as self-healing coating, biomedical materials and soft actuators. Acknowledgements The financial support from the NSFC (51573060 and 21427805), the Pearl River S&T
IP T
Nova Program of Guangzhou (201710010146), the Fundamental Research Funds for the Central Universities (2017MS081), and the Science and Technology Program of Guangzhou
SC R
(201604010016) is gratefully acknowledged. References
Brochu, A. B. W., Craig, S. L., & Reichert, W. M. (2011). Self-healing biomaterials.
U
Journal of Biomedical Materials Research Part A, 96A(2), 492-506.
N
Caló, E., & Khutoryanskiy, V. V. (2015). Biomedical applications of hydrogels: A
A
review of patents and commercial products. European Polymer Journal, 65, 252-267.
M
Canadell, J., Goossens, H., & Klumperman, B. (2011). Self-healing materials based on
TE D
disulfide links. Macromolecules, 44(8), 2536-2541. Cheng, C., Bai, X., Zhang, X., Li, H., Huang, Q., & Tu, Y. (2015). Self-healing polymers based on a photo-active reversible addition-fragmentation chain transfer
EP
(raft) agent. Journal of Polymer Research, 22(4), 46. Chu, C.-W., & Ravoo, B. J. (2017). Hierarchical supramolecular hydrogels: Self-
CC
assembly by peptides and photo-controlled release via host-guest interaction. Chemical
A
Communications, 53(92), 12450-12453. Cui, J., & Campo, A. d. (2012). Multivalent h-bonds for self-healing hydrogels. Chemical Communications, 48(74), 9302-9304. Deng, C. C., Brooks, W. L. A., Abboud, K. A., & Sumerlin, B. S. (2015). Boronic acid-based hydrogels undergo self-healing at neutral and acidic ph. ACS Macro Letters, 4(2), 220-224.
19
Gao, G., Du, G., Sun, Y., & Fu, J. (2015). Self-healable, tough, and ultrastretchable nanocomposite hydrogels based on reversible polyacrylamide/montmorillonite adsorption. ACS Applied Materials & Interfaces, 7(8), 5029-5037. Gosselet, N. M., Beucler, F., Renard, E., Amiel, C., & Sebille, B. (1999). Association of hydrophobically modified poly (n,n-dimethylacrylamide hydroxyethylmethacrylate)
IP T
with water soluble β-cyclodextrin polymers. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 155(2), 177-188.
SC R
Harada, A., Takashima, Y., & Nakahata, M. (2014). Supramolecular polymeric
materials via cyclodextrin–guest interactions. Accounts of Chemical Research, 47(7), 2128-2140.
U
Jeon, I., Cui, J., Illeperuma, W. R. K., Aizenberg, J., & Vlassak, J. J. (2016).
N
Extremely stretchable and fast self-healing hydrogels. Advanced Materials, 28(23),
A
4678-4683.
M
Jia, Y.-G., & Zhu, X. X. (2015). Self-healing supramolecular hydrogel made of
27(1), 387-393.
TE D
polymers bearing cholic acid and β-cyclodextrin pendants. Chemistry of Materials,
Kakuta, T., Takashima, Y., & Harada, A. (2013). Highly elastic supramolecular
EP
hydrogels using host–guest inclusion complexes with cyclodextrins. Macromolecules,
CC
46(11), 4575-4579.
Kakuta, T., Takashima, Y., Nakahata, M., Otsubo, M., Yamaguchi, H., & Harada, A.
A
(2013). Preorganized hydrogel: Self-healing properties of supramolecular hydrogels formed by polymerization of host–guest-monomers that contain cyclodextrins and hydrophobic guest groups. Advanced Materials, 25(20), 2849-2853. Koopmans, C., & Ritter, H. (2008). Formation of physical hydrogels via host−guest interactions of β-cyclodextrin polymers and copolymers bearing adamantyl groups. Macromolecules, 41(20), 7418-7422.
20
Kuhl, N., Bode, S., Bose, R. K., Vitz, J., Seifert, A., Hoeppener, S., Garcia, S. J., Spange, S., van der Zwaag, S., Hager, M. D., & Schubert, U. S. (2015). Self-healing materials: Acylhydrazones as reversible covalent crosslinkers for self-healing polymers Advanced Functional Materials, 25(22), 3278-3278. Li, L., Yan, B., Yang, J., Chen, L., & Zeng, H. (2015). Novel mussel-inspired
IP T
injectable self-healing hydrogel with anti-biofouling property. Advanced Materials, 27(7), 1294-1299.
SC R
Mauldin, T. C., & Kessler, M. R. (2010). Self-healing polymers and composites. International Materials Reviews, 55(6), 317-346.
Miyamae, K., Nakahata, M., Takashima, Y., & Harada, A. (2015). Self-healing,
U
expansion–contraction, and shape-memory properties of a preorganized
A
International Edition, 54(31), 8984-8987.
N
supramolecular hydrogel through host–guest interactions. Angewandte Chemie
M
Mocanu, G., Vizitiu, D., & Carpov, A. (2001). Cyclodextrin polymers. Journal of
TE D
Bioactive and Compatible Polymers, 16(4), 315-342. Nakahata, M., Takashima, Y., & Harada, A. (2016). Highly flexible, tough, and selfhealing supramolecular polymeric materials using host–guest interaction.
EP
Macromolecular Rapid Communications, 37(1), 86-92.
CC
Nakahata, M., Takashima, Y., Yamaguchi, H., & Harada, A. (2011). Redox-responsive self-healing materials formed from host–guest polymers. Nature Communications, 2,
A
511.
Nuvoli, D., Alzari, V., Nuvoli, L., Rassu, M., Sanna, D., & Mariani, A. (2016). Synthesis and characterization of poly(2-hydroxyethylacrylate)/β-cyclodextrin hydrogels obtained by frontal polymerization. Carbohydrate Polymers, 150, 166-171. Rekharsky, M. V., & Inoue, Y. (1998). Complexation thermodynamics of cyclodextrins. Chemical Reviews, 98(5), 1875-1918.
21
Renard, E., Deratani, A., Volet, G., & Sebille, B. (1997). Preparation and characterization of water soluble high molecular weight β-cyclodextrinepichlorohydrin polymers. European Polymer Journal, 33(1), 49-57. Sanna, D., Alzari, V., Nuvoli, D., Nuvoli, L., Rassu, M., Sanna, V., & Mariani, A. (2017). Β-cyclodextrin-based supramolecular poly(n-isopropylacrylamide) hydrogels
IP T
prepared by frontal polymerization. Carbohydrate Polymers, 166, 249-255.
Sun, T. L., Kurokawa, T., Kuroda, S., Ihsan, A. B., Akasaki, T., Sato, K., Haque, M.
SC R
A., Nakajima, T., & Gong, J. P. (2013). Physical hydrogels composed of
polyampholytes demonstrate high toughness and viscoelasticity. Nature Materials, 12, 932.
U
Takashima, Y., Sawa, Y., Iwaso, K., Nakahata, M., Yamaguchi, H., & Harada, A.
N
(2017a). Supramolecular materials cross-linked by host–guest inclusion complexes:
A
The effect of side chain molecules on mechanical properties. Macromolecules, 50(8),
M
3254-3261.
TE D
Takashima, Y., Yonekura, K., Koyanagi, K., Iwaso, K., Nakahata, M., Yamaguchi, H., & Harada, A. (2017b). Multifunctional stimuli-responsive supramolecular materials with stretching, coloring, and self-healing properties functionalized via host–guest
EP
interactions. Macromolecules, 50(11), 4144-4150.
CC
Taylor, D. L., & In het Panhuis, M. (2016). Self-healing hydrogels. Advanced Materials, 28(41), 9060-9093.
A
Tseng, T.-C., Tao, L., Hsieh, F.-Y., Wei, Y., Chiu, I.-M., & Hsu, S.-h. (2015). An injectable, self-healing hydrogel to repair the central nervous system. Advanced Materials, 27(23), 3518-3524. Tuncaboylu, D. C., Sari, M., Oppermann, W., & Okay, O. (2011). Tough and selfhealing hydrogels formed via hydrophobic interactions. Macromolecules, 44(12), 4997-5005.
22
Wang, T., Zheng, S., Sun, W., Liu, X., Fu, S., & Tong, Z. (2014). Notch insensitive and self-healing pnipam-pam-clay nanocomposite hydrogels. Soft Matter, 10(19), 3506-3512. Wei, K., Zhu, M., Sun, Y., Xu, J., Feng, Q., Lin, S., Wu, T., Xu, J., Tian, F., Xia, J., Li, G., & Bian, L. (2016). Robust biopolymeric supramolecular “host−guest macromer”
IP T
hydrogels reinforced by in situ formed multivalent nanoclusters for cartilage regeneration. Macromolecules, 49(3), 866-875.
SC R
Wei, Z., Yang, J. H., Du, X. J., Xu, F., Zrinyi, M., Osada, Y., Li, F., & Chen, Y. M.
(2013). Dextran-based self-healing hydrogels formed by reversible diels–alder reaction under physiological conditions. Macromolecular Rapid Communications, 34(18),
U
1464-1470.
N
Wei, Z., Yang, J. H., Zhou, J., Xu, F., Zrínyi, M., Dussault, P. H., Osada, Y., & Chen,
A
Y. M. (2014). Self-healing gels based on constitutional dynamic chemistry and their
M
potential applications. Chemical Society reviews, 43(23), 8114-8131.
TE D
Wei, Z., Zhao, J., Chen, Y. M., Zhang, P., & Zhang, Q. (2016). Self-healing polysaccharide-based hydrogels as injectable carriers for neural stem cells. Scientific Reports, 6, 37841.
EP
Wu, D. Y., Meure, S., & Solomon, D. (2008). Self-healing polymeric materials: A
CC
review of recent developments. Progress in Polymer Science, 33(5), 479-522. Yang, H., Yuan, B., Zhang, X., & Scherman, O. A. (2014). Supramolecular chemistry
A
at interfaces: Host–guest interactions for fabricating multifunctional biointerfaces. Accounts of Chemical Research, 47(7), 2106-2115. Yang, Q., Wang, P., Zhao, C., Wang, W., Yang, J., & Liu, Q. (2017). Light-switchable self-healing hydrogel based on host–guest macro-crosslinking. Macromolecular Rapid Communications, 38(6), 1600741.
23
Yang, X., Yu, H., Wang, L., Tong, R., Akram, M., Chen, Y., & Zhai, X. (2015). Selfhealing polymer materials constructed by macrocycle-based host-guest interactions. Soft Matter, 11(7), 1242-1252. Yang, Y., & Urban, M. W. (2013). Self-healing polymeric materials. Chemical Society reviews, 42(17), 7446-7467.
IP T
Yu, C., Wang, C.-F., & Chen, S. (2014). Robust self-healing host–guest gels from
magnetocaloric radical polymerization. Advanced Functional Materials, 24(9), 1235-
SC R
1242.
Yu, J., Ha, W., Sun, J.-n., & Shi, Y.-p. (2014). Supramolecular hybrid hydrogel based on host–guest interaction and its application in drug delivery. ACS Applied Materials
U
& Interfaces, 6(22), 19544-19551.
N
Zhang, H. J., Xia, H. S., & Zhao, Y. (2012). Poly(vinyl alcohol) hydrogel can
A
autonomously self-heal. ACS Macro Letters, 1(11), 1233-1236.
M
Zhang, M., Xu, D., Yan, X., Chen, J., Dong, S., Zheng, B., & Huang, F. (2012). Self-
TE D
healing supramolecular gels formed by crown ether based host–guest interactions. Angewandte Chemie, 124(28), 7117-7121. Zhao, L., Huang, J., Wang, T., Sun, W., & Tong, Z. (2017). Multiple shape memory,
EP
self-healable, and supertough PAA-GO-Fe3+ hydrogel. Macromolecular Materials and
CC
Engineering, 302(2), 1600359. Zhao, L., Huang, J., Zhang, Y., Wang, T., Sun, W., & Tong, Z. (2017). Programmable
A
and bidirectional bending of soft actuators based on janus structure with sticky tough paa-clay hydrogel. ACS Applied Materials & Interfaces, 9(13), 11866-11873.
24
S0144-8617(18)30295-9 https://doi.org/10.1016/j.carbpol.2018.03.039 CARP 13389
To appear in: Received date: Revised date: Accepted date:
12-1-2018 14-3-2018 14-3-2018
Please cite this article as: Cai, Tingting., Huo, Shuangjun., Wang, Tao., Sun, Weixiang., & Tong, Zhen., Self-Healable Tough Supramolecular Hydrogels Crosslinked by Poly-Cyclodextrin through Host-Guest Interaction’.Carbohydrate Polymers https://doi.org/10.1016/j.carbpol.2018.03.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Self-Healable Tough Supramolecular Hydrogels Crosslinked by PolyCyclodextrin through Host-Guest Interaction
Tingting Cai,1 Shuangjun Huo,1 Tao Wang, *1 Weixiang Sun,1 and Zhen Tong *1,2
Research Institute of Materials Science, South China University of Technology, Guangzhou
IP T
1
510640, China
State Key Laboratory of Luminescent Materials and Devices, South China University of
SC R
2
U
Technology, Guangzhou 510640, China
N
Corresponding author.
A
Tel: +86-020-87112886-601
M
Fax: +86-020-87112886-605
A
CC
EP
Graphical Abstract
TE D
Email: [email protected] (T. Wang), [email protected] (Z. Tong)
1
Highlights
Novel tough functional supramolecular hydrogel was prepared based on the host-guest
IP T
interaction between the polyfunctional crosslinking agents poly-cyclodextrin and adamantane.
The hydrogel showed rapid self-healing and improved healing percentage benefited from
SC R
the poly-cyclodextrin, which could provide more host sites and binding opportunities for the complexation.
Shape memory was realized through the reversible second crosslinked network by
U
M
A
N
polyacrylic acid backbone and Fe3+.
Abstract
TE D
A new supramolecular hydrogel with superior self-healing and shape memory properties was synthesized via in-situ copolymerization of the adamantane-containing monomer Nadamantylacrylamide (Ad-AAm) and monomer acrylic acid (AAc) in the polycyclodextrin
EP
(PCD) aqueous solution, where PCD served as the polyfunctional physical cross-linker.
The
CC
PCD-Ad hydrogels showed strength of 40 kPa with breaking strain of 600%, but small tensile loading-unloading hysteresis and residual deformation benefited from the reversible
A
crosslinking structure. condition.
Excellent self-healing property was realized at 70 °C and humid
The healing percentage reached over 70% after 120 min, which showed enhanced
self-healing efficiency compared with the reported supramolecular hydrogels crosslinked by CD and Ad.
This may benefit from the polyfunctional PCD crosslinking reagent, which
provided more host sites and binding opportunities for the complexation during the self-healing process.
Shape memory behavior was also realized through the reversible second 2
crosslinked network by the carboxyl groups of PAAc with Fe3+ ions.
The present study
provides a new method to improve self-healing ability of the supramolecular hydrogels, which will find the applications in self-healing coating, biomedical materials and soft actuators.
Keywords:
Supramolecular
hydrogel;
Polyfunctional
poly-cyclodextrin;
SC R
IP T
interaction; Self-healing; Shape memory.
Host-guest
1. Introduction
Hydrogels have been conspicuously used in biomedical fields, such as wound dressings and
U
hygiene products, because of their unique water containing polymer network structure similar
N
to the human tissues (Caló & Khutoryanskiy, 2015; Taylor & In het Panhuis, 2016).
The
A
mechanical strength and reliability of the hydrogels are crucial factors for the applications.
M
Self-healing ability can greatly improve the reliability of the hydrogels after damage or with
TE D
defects through the reconstitution of the reversible bonds in the hydrogel network (Brochu, Craig & Reichert, 2011; Mauldin & Kessler, 2010; Wei et al., 2014; Wu, Meure & Solomon, 2008; Yang & Urban, 2013).
Numerous approaches in constructing self-healing hydrogels
EP
have been reported, which can be classified into dynamic covalent bonds such as phenylboronate ester (Deng, Brooks, Abboud & Sumerlin, 2015), disulfide (Canadell,
CC
Goossens & Klumperman, 2011), imine (Wei, Zhao, Chen, Zhang & Zhang, 2016),
A
acylhydrazone (Kuhl et al., 2015), reversible radical reaction (Cheng et al., 2015), and reversible Diels-Alder cycloaddition (Wei et al., 2013), and non-covalent interactions. e.g., hydrophobic interactions (Tuncaboylu, Sari, Oppermann & Okay, 2011), host-guest interactions (Harada, Takashima & Nakahata, 2014), hydrogen bonds (Cui & Campo, 2012), and crystallization (Zhang, Xia & Zhao, 2012).
3
Among the non-covalent interactions to realize the self-healing capacity, the host-guest interaction has attracted much more attention, which has been widely used to prepare the selfhealing hydrogels.
The host-guest interaction is reversible and sensitive to local environment
and can be broken and easily recovered, which is based on macrocyclic recognition between cyclic hosts, e.g., cyclodextrins (CD), crown ethers, calix[n]arenes, cucurbit[n]urils,
IP T
cyclophanes, and pillararenes, and their corresponding guests (Chu & Ravoo, 2017; Nuvoli et al., 2016; Sanna et al., 2017; Yang, Yuan, Zhang & Scherman, 2014; Yang et al., 2015; Yu,
β-CD and cholic acid
SC R
Wang & Chen, 2014; Yu, Ha, Sun & Shi, 2014; Zhang et al., 2012).
host-guest interaction was used to obtain self-healing supramolecular hydrogel (Jia & Zhu, 2015).
Host-guest macromers formed by molecular self-assembly between adamantane-
U
functionalized hyaluronic acid guest polymers and monoacrylated β-cyclodextrins host
N
monomers was also applied to prepare self-healable biopolymer based freestanding Harada’s group has reported a series of self-
A
supramolecular hydrogels (Wei et al., 2016).
M
healing hydrogels using the host-guest interaction between CD and different guest molecules,
TE D
including adamantane, ferrocene, polyethylene glycol, phenolphthalein, and so on (Kakuta et al., 2013; Miyamae, Nakahata, Takashima & Harada, 2015; Nakahata, Takashima & Harada, 2016; Nakahata, Takashima, Yamaguchi & Harada, 2011; Takashima et al., 2017b).
The
EP
self-healable hydrogel using water-soluble polymer chains grafted with β-CD and polymer
CC
having adamantane (Ad) side groups showed maximum healing percentage of ~45% after healed for 24 h or even longer under humid condition (Nakahata, Takashima & Harada, 2016).
A
They also studied the relationship between the self-healing percentage of the supramolecular hydrogels and the molecular structure of the guest molecules, and reported the maximum healing percentage of ~61% for dried hydrogels after 48 h-healing under 100 °C (Takashima et al., 2017a).
High self-healing percentage along with the high toughness is still desired for
the host-guest supramolecular hydrogels.
4
Poly-cyclodextrin (PCD) is the polymerized cyclodextrin usually crosslinked by epichlorohydrin in alkaline media (Mocanu, Vizitiu & Carpov, 2001; Renard, Deratani, Volet & Sebille, 1997).
PCD has been used as the host molecules to prepare hydrogels with the
adamantane-containing guest polymers by mixing the components together, but the hydrogels were rather weak (Gosselet, Beucler, Renard, Amiel & Sebille, 1999; Koopmans & Ritter, PCD nanogel was used as macro-crosslinker to prepare the light-switchable self-
IP T
2008).
healing hydrogel with azobenzene as the guest molecule, similarly, the mechanical strength was
SC R
not desirable either (strength ~10 kPa and breaking strain ~100%) (Yang et al., 2017).
In this work, we design a new way to prepare supramolecular hydrogels by using PCD as the polyfunctional crosslinking agent to replace the traditional monofunctional CD molecules
U
grafted on the polymer chains to improve the self-healing property.
The hydrogel is
N
synthesized via in-situ copolymerization of the adamantane-containing monomer N-
A
adamantylacrylamide (Ad-AAm) and monomer acrylic acid (AAc) in the PCD aqueous solution.
M
These hydrogels exhibit high self-healing percentage and excellent shape memory behavior
2. Experimental
EP
2.1 Materials
TE D
based on the rapid recoverable inclusion complexation of PCD and Ad groups.
Monomer AAc (stabilized with 200 ppm hydroquinone methylether, Aladdin Industrial
CC
Inc.), β-CD (98%, J&K), epichlorohydrin (C3H5ClO, 99%, J&K), potassium peroxydisulfate (KPS, K2S2O8, 99%, J&K), acryloyl chloride (97%, stabilized with 400
A
ppm phenothiazine, J&K), adamantylamine (C10H17N, 98.5%, Meryer) were all used as received.
Ferric chloride hexahydrate (FeCl3⋅6H2O), hydrochloric acid (HCl), and
sodium hydroxide (NaOH) (Guangzhou Chemical Reagent Factory, China) were analytical reagents.
Pure water was obtained by deionization and filtration using a
5
Millipore purification apparatus (resistivity: 18.2 MΩ cm) and bubbled with nitrogen gas for 3 h prior to use to remove O2. 2.2 Synthesis of Polycyclodextrin (PCD) As shown in Fig. S1 in the Supplementary Information, sodium hydroxide aqueous solution (15 wt%) was mixed with β-CD and stirred for 3 h at room temperature, then
Thereafter, the above solution was added into acetone drop by drop.
IP T
epichlorohydrin was slowly added into the solution under stirring for another 3 h.
The precipitate
SC R
was filtered and dissolved in deionized water, followed by neutralization with diluted HCl and dialysis for 7 days (molecular weight cut off = 7000). was obtained by freeze-drying, and the yield was ~55%.
Finally, the product
The molecular weight of the
U
PCD was obtained as Mw ~ 13000, Mn ~ 6000 with Mw/Mn ~ 2.2 from a gel permeation
N
chromatography (GPC) calibrated by dextran standards.
Thus, the number of
A
repeating β-CD unit in one PCD molecule is ~5 based on Mn, but it is worth to note that
M
the PCD molecules are non-linear structure with relatively high polydispersity.
TE D
2.3 Synthesis of N-Adamantylacrylamide (Ad-AAm) As briefly shown in Fig. S2, adamantylamine (20 mmol, 3.025 g) was dissolved in 400 mL of THF at 60 °C for 3 h, and cooled to < 5 °C.
Trimethylamine (3.1 mL, 22 mmol)
EP
was added into the solution, and then acryloyl chloride (1.8 mL, 22 mmol) in 20 mL The solution was stirred for 6 h in an ice water
CC
THF was added dropwise in 30 min. bath.
The precipitate was removed by filtration, and the solvent in the filtrate was The Ad-AAm was obtained
A
removed by rotary evaporation to produce a yellow solid.
through a silica gel column chromatography (dichloromethane: petroleum ether = 3:7) as a white solid.
1
Yield: 2.15 g, 52.2%.
H NMR (600 MHz, DMSO, 25 °C): δ =
1.62 (m, 6H), 1.95 (m, 6H), 2.01 (m, 3H), 5.47 (dd, 1H), 6.01 (dd, 1H), 6.02 (m 1H), and 6.22 (s 1H). 2.4 Synthesis of Hydrogels
6
The PCD-Ad hydrogel was synthesized as follows.
First, desired amount of PCD and
Ad-AAm was dissolved in deionized water under stirring for 4 h at 70 °C to produce a homogeneous and transparent solution. added under stirring.
Then, monomer AAc and initiator KPS were
The solution was purged with nitrogen gas for 30 min and then
poured into a glass tube of 5 mm (diameter) × 120 mm (length) or a laboratory-made
The polymerization was conducted at 60 °C overnight.
any crosslinking reagents were used.
No
In this paper, the PCD-Ad hydrogel was referred
SC R
× 1 mm (thickness).
IP T
mold of two flat glasses separated by a rubber spacer of 80 mm (width) × 80 mm (length)
to as PCDxAdy, where x and y stood for the concentration (mM) of the β-CD and Ad units in the solution, respectively.
For example, the PCD3Ad3 gel consisted of 3 mM
N
in the solution was 2 M unless mentioned otherwise.
A
2.5 Characterization
H NMR of PCD in D2O, Ad-AAm in DMSO-d6, and 2D NOESY NMR of PCD and
M
1
The concentration of AAc
U
β-CD and 3 mM Ad-AAm in the polymerization solution.
600 MHz.
TE D
Ad-AAm complex in D2O were obtained using a Bruker Avance III HD spectrometer at The δ-scale relative to TMS was calibrated using the deuterium signal of
the solvent as an internal standard.
EP
Molecular weight was determined using a gel permeation chromatography (GPC)
CC
system (HLC-8320, Tosoh) with a refractive index detector and a combination of two columns (Shodex SB-806 HQ, Tosoh).
Dextran was used as the standard and the
A
column was eluted with H2O at 1 mL/min and 25 °C. The tensile strength was measured with a Shimadzu AG-X plus testing system at
ambient temperature on cylindrical hydrogel samples after specified treatments.
The
sample length between the jaws was 40 mm, the sample diameter was 5 mm, and the crosshead speed was 50 mm/min.
The tensile strain was taken as the length change
related to the original length, and the tensile stress was evaluated on the initial cross-
7
section of the sample.
The stretch hysteresis was measured on the samples under the
strain below 500%. Rheology measurements were carried out on the as-prepared PCD-Ad hydrogel with a stress-controlled rheometer AR-G2 (TA) using parallel plates of diameter of 50 mm. Silicone oil was laid on the edge of the fixture plates to prevent solvent evaporation.
IP T
The self-healing of blade cut as-prepared PCD-Ad hydrogel was carried out by keeping the cut surfaces in contact and healed in humid environment at room
The humid condition was realized by putting
SC R
temperature or 70 °C for different times.
the sample in a container with water but not contacting to water.
The healing
percentage was defined as the tensile strength of the self-healed hydrogel related to that
U
of the corresponding as-prepared one treated under the same condition.
N
The swelling behavior of the as-prepared PCD-Ad hydrogels was observed after
This swelling was nonequilibrium and the purpose
M
at room temperature, respectively.
A
immersing in AdCOONa solution, β-CD solution, as well as pure water for only 15 min
The swelling
TE D
was to verify the host-guest crosslinking in the PCD-Ad network.
degree was estimated as the weight of the swollen hydrogel related to the weight of the as-prepared one.
EP
The shape memory behavior of the hydrogel was investigated in the following way:
CC
the hydrogel strip was curled to the temporary shape manually and fixed by immersed in 0.06 M FeCl3/0.2 M HCl solution for a desired time; while the shape recovery was
A
realized by immersing the curled sample in 0.6 M HCl solution. 3. Results and discussion 3.1 PCD-Ad Supramolecular Hydrogels The preparation of the PCD-Ad hydrogel is schematically illustrated in Fig. 1.
Prior
to the radical copolymerization, the hydrophobic guest monomer (Ad-AAm) was
8
dissolved in the PCD aqueous solution to produce a uniform solution with the help of inclusion complexation at 70 °C (Fig. 1A-B).
Then, the copolymerization with
monomer AAc was conducted in the solution at 60 °C (Fig. 1C).
Fig. S3 shows the
2D NOESY NMR spectrum of the inclusion complex of PCD and Ad in D2O with Figs. S4 and S5 for the 1H-NMR spectra of PCD and Ad-AAm, respectively.
The existence
IP T
of the inclusion complex is confirmed by the cross-peaks of PCD (δH 3.5 ppm ~ 3.75 ppm) and adamantane (δH 1.69 ppm ~ 2.20 ppm), indicating their combination in Therefore, the copolymerization produces a polymer network crosslinked
SC R
solution.
Synthesis of PCD-Ad hydrogel by copolymerization of Ad-AAm and AAc
EP
Fig. 1
TE D
M
A
N
U
by multifunctional PCD and Ad units via host-guest interaction (Fig. 1D).
crosslinked with PCD through host-guest interaction.
Dissolving of Ad-AAm in PCD
CC
aqueous solution (A-B), PCD/Ad-AAm solution containing monomer AAc (C), the
A
crosslinked network of the PCD-Ad hydrogel after the copolymerization (D).
As shown in Fig. 2A, both the polymerization solution and hydrogel are almost transparent with slight milky white.
After copolymerization, the PCD-Ad hydrogel
can sustain large compression and elongation even with a knot (Fig. 2B and C),
9
exhibiting excellent mechanical properties with high water content of 83.8 wt%.
The
iii).
(A) Photos of the PCD3Ad3 hydrogel before (i) and after polymerization (ii, (B) The hydrogel before (i), during (ii) and after compression (iii).
U
Fig. 2
SC R
IP T
recovery to its original shape is rapid after removing the external force.
(C) The
N
original hydrogel (i), the knotted gel (ii), elongation (iii), and recovery (iv) of the knotted
M
A
gel.
TE D
The swelling of the hydrogel in the solution containing competitive host or guest was observed to confirm the crosslinking by the inclusion complex.
Fig. 3A presents the
photos of the PCD-Ad hydrogels before and after immersing in a specified solution for
EP
only 15 min, and Fig. 3B depicts that the swelling degree of the hydrogel is ~2.1 in 0.05
CC
M solution of the competitive guest AdCOONa, while only ~1.4 and ~1.3 in 0.01 M βCD solution (restricted by its low solubility in water) and pure water, respectively.
A
The larger swelling in the competitive guest AdCOONa solution is caused by the higher association of AdCOONa to β-CD than that of Ad-AAm (Kakuta et al., 2013; Rekharsky & Inoue, 1998).
Thus, the crosslinking in the PCD-Ad hydrogel is partially destroyed
by AdCOONa, leading to a large swelling degree of the hydrogel in the competitive guest solution.
This confirms the existence of the host-guest inclusion as the
crosslinking in the PCD-Ad hydrogel.
10
The mechanical properties of the PCD-Ad hydrogels with different AAc, PCD, and Ad-AAm concentrations were investigated by stretch and dynamic shearing. tensile stress-strain curves are plotted in Fig. S6.
The
The strength increases from about
14 kPa to 35 kPa but the strain at break first increases and then decreases as the AAc concentration is increased from 1 M to 4 M due to an increase of the polymer content in By simultaneously increasing the β-CD and Ad content from 0.01
IP T
the hydrogel (A).
M to 0.07 M, i.e., increasing the inclusion complex of PCD and Ad, the strength
SC R
increases from about 18 kPa to 40 kPa, while the strain at break decreases slightly (B), because of an increase in the crosslinking density of the hydrogels (Kakuta, Takashima
TE D
M
A
N
U
& Harada, 2013).
2.5
B
CC
A
Fig. 3
Swelling Degree
EP
2.0
1.5
1.0
0.5
0.0
0.05 M AdCOONa 0.01 M β-CD
Pure water
Immersion Solution
(A) Photos of the PCD3Ad3 hydrogel before and after immersing in the
indicated solution for 15 min, and (B) the corresponding swelling degree (estimated as the weight of the swollen hydrogel related to that of the as-prepared one) after immersion.
11
Energy dissipation of the PCD-Ad hydrogel during deformation was investigated by three continuous tensile loading-unloading cycles (0%-500%-0%) (Fig. 4A).
The
hysteresis percentage is about 15% with the residual strain of about 25% for the first cycle, while it decreases to about 10% in following cycles, showing low energy dissipation and low residual strain during the deformation-recovery cycle.
This result
IP T
indicates that a small part of the crosslinking points are destroyed during elongation to
dissipate the tensile energy, while the crosslinking can be reconstructed rapidly due to
SC R
the high association constant of β-CD and Ad groups (Kakuta et al., 2013; Rekharsky &
104
20
B
10 5 0
1st
2nd
N
15
A
10
20
G', G'' (Pa)
Hysteresis / %
3rd
Tensile cycle
5
M
Stress (kPa)
A 15
U
Inoue, 1998).
103
1st cycle 2nd cycle 3rd cycle
0 0
100
200
300
400
= 0.1% = 100% 102
500
0
500
TE D
Strain (%)
1000
1500
Time (s)
104
A
CC Fig. 4
G', G'' (Pa)
EP
C
G'
103
G" 2
10
,
25 oC
,
45 oC
,
65 C
,
75 C
101
102
o o
1
10 10-2
10-1
100
(rad/s)
(A) Tensile stress-strain curves of loading-unloading cycle of the PCD3Ad3
hydrogel, the insert is the corresponding hysteresis percentage (defined as the area within the cycle relative to the area below the loading stress-strain curve).
(B) Storage
modulus (filled symbols) and loss modulus (open symbols) of the PCD3Ad3 hydrogel
12
at shear strain of 0.1% and 100%, respectively at 25 °C and ω = 6.283 rad/s.
(C)
Frequency sweep of the PCD3Ad3 hydrogel at γ = 0.5% at indicated temperatures.
Fig. 4B shows the change in dynamic moduli of the PCD-Ad hydrogel with the shear strain jump between 0.1% and 100%.
The former is in the linear viscoelasticity G' and
IP T
region, and the latter locates in the nonlinear viscoelasticity region (Fig. S7A).
G" show a simultaneous jump with the strain jump, i.e., G' decreases and G" increases
SC R
suddenly at the strain of 100% and the both almost completely return to their original values upon the strain returning to 0.1% over all the cycles.
These jumps indicate that
the inclusion complex crosslinking can be partially destroyed at large strain but restored
The G' is always higher than G" means that the crosslinked network still
N
density.
U
at low strain reversibly and rapidly, manifesting a reversible change in the crosslinking
A
exists even at higher shear strain and the destruction is only partial.
M
The temperature effect on the dynamic moduli of the hydrogel was also observed from G' is almost a constant independent of
TE D
25 to 80 °C and shown in Figs. 4C and S7B.
temperature, implying that the supramolecular crosslinking by the host-guest complexation of PCD and Ad is stable at the tested temperatures.
G" slightly
EP
decreases upon heating due to the reduction in viscosity.
CC
Moreover, the polyfunctional PCD molecules are independent from the polymer chains jointed in the network, thus, their movement would be easier than the CD grafted This is also favorable for the reconstitution of the crosslinking
A
on the polymer chains.
points during the deformation and recovery process.
The inclusion between PCD and
Ad in the hydrogel with host-guest complex thus acts as “quickly recoverable sacrificial bonds” with fast recovery and small energy dissipation.
The network in the PCD-Ad
gel is, therefore tough and elastic, different from the weak hydrogel, of which the network is destroyed entirely under large amplitude shearing (Li, Yan, Yang, Chen &
13
Zeng, 2015; Tseng et al., 2015).
Furthermore, other kinds of physically crosslinked
hydrogels, for example, the UPy gels depending on the hydrogen bonding and the polyampholyte gels with electrostatic interaction, manifested large tensile hysteresis (Jeon, Cui, Illeperuma, Aizenberg & Vlassak, 2016; Sun et al., 2013), which seems to be ascribed to the long time required for the reconstitution of the sacrificial bonds in the
IP T
unloading period. 3.2 Self-Healing Behaviors
SC R
Fig. 5 demonstrates the self-healing process of the PCD3Ad3 hydrogel after cut into two pieces and contacted each other at 70 °C under a humid condition (dyed for easy
recognition), for the humid and heating condition is beneficial to the healing of hydrogels
U
(Gao, Du, Sun & Fu, 2015; Kakuta et al., 2013; Wang et al., 2014).
After 30 min, the
The self-healed hydrogel can bear a large loading without trace of damage
M
interface.
Besides, dye diffusion is observed at the healed
A
to the loss of water (Fig. 6C).
N
two pieces of the cut hydrogel are jointed together tightly with slight volume shrink due
TE D
at the junction (Figs. 6D and 6E), showing an outstanding self-healing capability for the
A
CC
EP
present PCD-Ad supramolecular hydrogel.
Fig. 5
Self-healing of the PCD3Ad3 hydrogel: (A) cut sample; (B) cut surfaces
contacted each other; (C) self-healed gel for 30 min under humid condition at 70 °C; (D) the healed hydrogel under stretching, and (E) loading weight of 80 g.
14
The self-healing efficiency was quantitatively evaluated by tensile test.
Figs. 6 and
S8 show the tensile stress-strain curves of the self-healed PCD3Ad3 hydrogel under different healing temperatures and times in the humid condition compared with that experiencing the same treatment without cut.
Fig. 7 manifests the healing percentage
as the tensile strength relative to that of the control hydrogel sample experiencing the The hydrogel treated at room temperature has low
IP T
same treatment without cut.
healing percentage of 10% after 24 h under humid condition (Fig. S8A).
As a
70 °C.
SC R
contrast, the self-healing efficiency increases greatly when the process is conducted at
The healing percentage is 16% at 15 min and raises to 53% at 30 min and 73%
at 120 min.
At the same time, the water content in the hydrogel decreases obviously
U
during the self-healing process at 70 °C (63% after 60 min, 27% after 120 min from 84%
N
for the as-prepared one) owing to the high temperature in spite of humid condition (Fig.
But the samples are still at hydrogel state after the healing process with favorable The heating and humid condition promote the molecular motion,
TE D
elasticity (Fig. 6).
M
6).
A
7B), which also contributes the increase in the hydrogel strength (see the control in Fig.
in turn accelerate the host-guest complexation and improve the self-healing (Nakahata, Takashima & Harada, 2016; Takashima et al., 2017a).
Moreover, the water loss
EP
during heating also drives the polymer chains at the interfaces to contact each other
CC
closely to form strong complexation between the cut surfaces (Gao, Du, Sun & Fu, 2015).
A
We also tried the self-healing without humid condition at room temperature or at 70
°C for different times, however, the healing effect was very poor.
Similar results were
also reported by Nakahata et al. in the β-CD-Ad crosslinking system with PAAm as the main chain polymer (Nakahata, Takashima & Harada, 2016).
Consequently, the
humid environment as well as heating are necessary for the self-healing process upon the PCD and Ad inclusion with PAAc or PAAm as the main chains.
15
On the other
hand, the β-CD-Ad dry hydrogel with poly(methyl triethylene glycol acrylate) (PTEGA, with glass-transition temperature Tg of -50 °C, much lower than PAAc and PAAm) as the polymer backbone also showed self-healing only upon heating without humid condition (Takashima et al., 2017a).
The soft polymer chains of the dry hydrogels
promoted the mobility of the host-guest molecules in the polymers.
The healing
IP T
percentage of the dry hydrogel increased as the increase of temperature and healing time, the healing percentage was ~30% at 75 °C for 48 h, while it increased to a maximum of
The present PCD-Ad hydrogel shows
SC R
61% at 100 °C for 48 h or even longer time.
improved self-healing percentage and shortened healing time, which seems to be that the polyfunctional PCD would provide more host sites and binding opportunities for the
60
300
A 50
B
control
40 30 20 10
M
15 min 70 oC humid condition water content: 81%
Stress (kPa)
250
control
TE D
Stress (kPa)
A
N
U
inclusion.
200 150
60 min 70 oC humid condition water content: 63%
100 50
self-healed
self-healed
0 0
200
400
600
0 0
800
200
Stress (kPa)
EP CC
1200
C 1000
control
120 min 70 oC humid condition water content: 27%
800 600 400
self-healed
200 0 0
600
Strain (%)
Strain (%)
A
400
200
400
Strain (%)
16
600
800
800
1000
Fig. 6
Stress-strain curves of the self-healed PCD3Ad3 gel at 70 °C for different times
(A: 15 min, B: 60 min, C: 120 min).
The control is the same hydrogel after the same
80
100
20
0 24 h
15 min 30 min 60 min 90 min 120 min
70 oC humid condition
80
IP T
40
70 C
Water content (%)
60
B
o
60 40 20 0 0
15
30
45
60
75
90
105
120
Time (min)
Healing time
(A) The healing percentage of the PCD3Ad3 hydrogel, and (B) the water content
U
Fig. 7
SC R
A Room temperature
Healing percentage (%)
treatment without cut.
M
3.3 Shape Memory Behavior
A
N
of the PCD3Ad3 hydrogel at 70 °C and humid condition varying with healing time.
TE D
The shape memory behavior has been explored on the PAAc backbone hydrogels by introducing Fe3+ to form a reversible second crosslinked network through the ionic bonding with the carboxyl groups -COO- in PAAc (Zhao, Huang, Wang, Sun & Tong, The present PCD-Ad hydrogel has the similar PAAc-co-PAM backbone, so
EP
2017).
The as-prepared
CC
that the shape memory capability is endowed to it in the similar way.
PCD3Ad3 hydrogel of straight strip as the permanent shape (Fig. 8A) is twisted and
A
immersed in 0.06 M FeCl3/0.2 M HCl solution for 2 h to fix the temporary shape (Fig. 8B).
At the same time, the hydrogel changes its color from whitish to brownish red
due to the introduction of Fe3+ to form the new crosslinking between Fe3+ and -COO(Zhao, Huang, Wang, Sun & Tong, 2017; Zhao et al., 2017).
HCl is added in the
FeCl3 solution to prevent the fast aggregation on the hydrogel surface due to the fast binding of Fe3+ to -COO-.
When the hydrogel in the helical shape is immersed in HCl,
17
the bonded Fe3+ ions are partially replaced by protons to form -COOH at low pH. Therefore, the hydrogel recovers to the permanent shape gradually (Fig. 8C), and the complete shape recovery is achieved after 3 h immersion (Fig. 8D).
Meanwhile, its
The shape memory of the PCD3Ad3 hydrogel: (A) the as-prepared gel in
N
Fig. 8
U
SC R
IP T
color also restores whitish because of the release of Fe 3+ ions.
A
permanent shape (80 mm×20 mm×1 mm); (B) the fixed temporary shape after being
M
twisted and immersed in 0.06 M FeCl3 / 0.2 M HCl solution for 2 h; (C) the partially
TE D
recovered shape after immersed in 0.6 M HCl solution for 1 h; (D) the completely recovered shape after immersed in 0.6 M HCl solution for 3 h.
EP
4. Conclusions
CC
In summary, a tough supramolecular hydrogel has been synthesized via a pre-assembled method to form PCD and Ad inclusive complex prior to the polymerization.
The
A
present PCD-Ad hydrogel crosslinked by the host-guest interaction of the polyfunctional PCD and Ad showed favorable mechanical properties with small hysteresis and residual deformation during tensile loading-unloading cycles.
The fast and efficient self-
healing of the hydrogel was realized at 70 °C in humid condition, and the healing percentage reached 73% after healing for 120 min. realized by introducing Fe3+ to the hydrogel. 18
Shape memory behavior was
Potential applications of the present
supramolecular hydrogel will be developed in the fields such as self-healing coating, biomedical materials and soft actuators. Acknowledgements The financial support from the NSFC (51573060 and 21427805), the Pearl River S&T
IP T
Nova Program of Guangzhou (201710010146), the Fundamental Research Funds for the Central Universities (2017MS081), and the Science and Technology Program of Guangzhou
SC R
(201604010016) is gratefully acknowledged. References
Brochu, A. B. W., Craig, S. L., & Reichert, W. M. (2011). Self-healing biomaterials.
U
Journal of Biomedical Materials Research Part A, 96A(2), 492-506.
N
Caló, E., & Khutoryanskiy, V. V. (2015). Biomedical applications of hydrogels: A
A
review of patents and commercial products. European Polymer Journal, 65, 252-267.
M
Canadell, J., Goossens, H., & Klumperman, B. (2011). Self-healing materials based on
TE D
disulfide links. Macromolecules, 44(8), 2536-2541. Cheng, C., Bai, X., Zhang, X., Li, H., Huang, Q., & Tu, Y. (2015). Self-healing polymers based on a photo-active reversible addition-fragmentation chain transfer
EP
(raft) agent. Journal of Polymer Research, 22(4), 46. Chu, C.-W., & Ravoo, B. J. (2017). Hierarchical supramolecular hydrogels: Self-
CC
assembly by peptides and photo-controlled release via host-guest interaction. Chemical
A
Communications, 53(92), 12450-12453. Cui, J., & Campo, A. d. (2012). Multivalent h-bonds for self-healing hydrogels. Chemical Communications, 48(74), 9302-9304. Deng, C. C., Brooks, W. L. A., Abboud, K. A., & Sumerlin, B. S. (2015). Boronic acid-based hydrogels undergo self-healing at neutral and acidic ph. ACS Macro Letters, 4(2), 220-224.
19
Gao, G., Du, G., Sun, Y., & Fu, J. (2015). Self-healable, tough, and ultrastretchable nanocomposite hydrogels based on reversible polyacrylamide/montmorillonite adsorption. ACS Applied Materials & Interfaces, 7(8), 5029-5037. Gosselet, N. M., Beucler, F., Renard, E., Amiel, C., & Sebille, B. (1999). Association of hydrophobically modified poly (n,n-dimethylacrylamide hydroxyethylmethacrylate)
IP T
with water soluble β-cyclodextrin polymers. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 155(2), 177-188.
SC R
Harada, A., Takashima, Y., & Nakahata, M. (2014). Supramolecular polymeric
materials via cyclodextrin–guest interactions. Accounts of Chemical Research, 47(7), 2128-2140.
U
Jeon, I., Cui, J., Illeperuma, W. R. K., Aizenberg, J., & Vlassak, J. J. (2016).
N
Extremely stretchable and fast self-healing hydrogels. Advanced Materials, 28(23),
A
4678-4683.
M
Jia, Y.-G., & Zhu, X. X. (2015). Self-healing supramolecular hydrogel made of
27(1), 387-393.
TE D
polymers bearing cholic acid and β-cyclodextrin pendants. Chemistry of Materials,
Kakuta, T., Takashima, Y., & Harada, A. (2013). Highly elastic supramolecular
EP
hydrogels using host–guest inclusion complexes with cyclodextrins. Macromolecules,
CC
46(11), 4575-4579.
Kakuta, T., Takashima, Y., Nakahata, M., Otsubo, M., Yamaguchi, H., & Harada, A.
A
(2013). Preorganized hydrogel: Self-healing properties of supramolecular hydrogels formed by polymerization of host–guest-monomers that contain cyclodextrins and hydrophobic guest groups. Advanced Materials, 25(20), 2849-2853. Koopmans, C., & Ritter, H. (2008). Formation of physical hydrogels via host−guest interactions of β-cyclodextrin polymers and copolymers bearing adamantyl groups. Macromolecules, 41(20), 7418-7422.
20
Kuhl, N., Bode, S., Bose, R. K., Vitz, J., Seifert, A., Hoeppener, S., Garcia, S. J., Spange, S., van der Zwaag, S., Hager, M. D., & Schubert, U. S. (2015). Self-healing materials: Acylhydrazones as reversible covalent crosslinkers for self-healing polymers Advanced Functional Materials, 25(22), 3278-3278. Li, L., Yan, B., Yang, J., Chen, L., & Zeng, H. (2015). Novel mussel-inspired
IP T
injectable self-healing hydrogel with anti-biofouling property. Advanced Materials, 27(7), 1294-1299.
SC R
Mauldin, T. C., & Kessler, M. R. (2010). Self-healing polymers and composites. International Materials Reviews, 55(6), 317-346.
Miyamae, K., Nakahata, M., Takashima, Y., & Harada, A. (2015). Self-healing,
U
expansion–contraction, and shape-memory properties of a preorganized
A
International Edition, 54(31), 8984-8987.
N
supramolecular hydrogel through host–guest interactions. Angewandte Chemie
M
Mocanu, G., Vizitiu, D., & Carpov, A. (2001). Cyclodextrin polymers. Journal of
TE D
Bioactive and Compatible Polymers, 16(4), 315-342. Nakahata, M., Takashima, Y., & Harada, A. (2016). Highly flexible, tough, and selfhealing supramolecular polymeric materials using host–guest interaction.
EP
Macromolecular Rapid Communications, 37(1), 86-92.
CC
Nakahata, M., Takashima, Y., Yamaguchi, H., & Harada, A. (2011). Redox-responsive self-healing materials formed from host–guest polymers. Nature Communications, 2,
A
511.
Nuvoli, D., Alzari, V., Nuvoli, L., Rassu, M., Sanna, D., & Mariani, A. (2016). Synthesis and characterization of poly(2-hydroxyethylacrylate)/β-cyclodextrin hydrogels obtained by frontal polymerization. Carbohydrate Polymers, 150, 166-171. Rekharsky, M. V., & Inoue, Y. (1998). Complexation thermodynamics of cyclodextrins. Chemical Reviews, 98(5), 1875-1918.
21
Renard, E., Deratani, A., Volet, G., & Sebille, B. (1997). Preparation and characterization of water soluble high molecular weight β-cyclodextrinepichlorohydrin polymers. European Polymer Journal, 33(1), 49-57. Sanna, D., Alzari, V., Nuvoli, D., Nuvoli, L., Rassu, M., Sanna, V., & Mariani, A. (2017). Β-cyclodextrin-based supramolecular poly(n-isopropylacrylamide) hydrogels
IP T
prepared by frontal polymerization. Carbohydrate Polymers, 166, 249-255.
Sun, T. L., Kurokawa, T., Kuroda, S., Ihsan, A. B., Akasaki, T., Sato, K., Haque, M.
SC R
A., Nakajima, T., & Gong, J. P. (2013). Physical hydrogels composed of
polyampholytes demonstrate high toughness and viscoelasticity. Nature Materials, 12, 932.
U
Takashima, Y., Sawa, Y., Iwaso, K., Nakahata, M., Yamaguchi, H., & Harada, A.
N
(2017a). Supramolecular materials cross-linked by host–guest inclusion complexes:
A
The effect of side chain molecules on mechanical properties. Macromolecules, 50(8),
M
3254-3261.
TE D
Takashima, Y., Yonekura, K., Koyanagi, K., Iwaso, K., Nakahata, M., Yamaguchi, H., & Harada, A. (2017b). Multifunctional stimuli-responsive supramolecular materials with stretching, coloring, and self-healing properties functionalized via host–guest
EP
interactions. Macromolecules, 50(11), 4144-4150.
CC
Taylor, D. L., & In het Panhuis, M. (2016). Self-healing hydrogels. Advanced Materials, 28(41), 9060-9093.
A
Tseng, T.-C., Tao, L., Hsieh, F.-Y., Wei, Y., Chiu, I.-M., & Hsu, S.-h. (2015). An injectable, self-healing hydrogel to repair the central nervous system. Advanced Materials, 27(23), 3518-3524. Tuncaboylu, D. C., Sari, M., Oppermann, W., & Okay, O. (2011). Tough and selfhealing hydrogels formed via hydrophobic interactions. Macromolecules, 44(12), 4997-5005.
22
Wang, T., Zheng, S., Sun, W., Liu, X., Fu, S., & Tong, Z. (2014). Notch insensitive and self-healing pnipam-pam-clay nanocomposite hydrogels. Soft Matter, 10(19), 3506-3512. Wei, K., Zhu, M., Sun, Y., Xu, J., Feng, Q., Lin, S., Wu, T., Xu, J., Tian, F., Xia, J., Li, G., & Bian, L. (2016). Robust biopolymeric supramolecular “host−guest macromer”
IP T
hydrogels reinforced by in situ formed multivalent nanoclusters for cartilage regeneration. Macromolecules, 49(3), 866-875.
SC R
Wei, Z., Yang, J. H., Du, X. J., Xu, F., Zrinyi, M., Osada, Y., Li, F., & Chen, Y. M.
(2013). Dextran-based self-healing hydrogels formed by reversible diels–alder reaction under physiological conditions. Macromolecular Rapid Communications, 34(18),
U
1464-1470.
N
Wei, Z., Yang, J. H., Zhou, J., Xu, F., Zrínyi, M., Dussault, P. H., Osada, Y., & Chen,
A
Y. M. (2014). Self-healing gels based on constitutional dynamic chemistry and their
M
potential applications. Chemical Society reviews, 43(23), 8114-8131.
TE D
Wei, Z., Zhao, J., Chen, Y. M., Zhang, P., & Zhang, Q. (2016). Self-healing polysaccharide-based hydrogels as injectable carriers for neural stem cells. Scientific Reports, 6, 37841.
EP
Wu, D. Y., Meure, S., & Solomon, D. (2008). Self-healing polymeric materials: A
CC
review of recent developments. Progress in Polymer Science, 33(5), 479-522. Yang, H., Yuan, B., Zhang, X., & Scherman, O. A. (2014). Supramolecular chemistry
A
at interfaces: Host–guest interactions for fabricating multifunctional biointerfaces. Accounts of Chemical Research, 47(7), 2106-2115. Yang, Q., Wang, P., Zhao, C., Wang, W., Yang, J., & Liu, Q. (2017). Light-switchable self-healing hydrogel based on host–guest macro-crosslinking. Macromolecular Rapid Communications, 38(6), 1600741.
23
Yang, X., Yu, H., Wang, L., Tong, R., Akram, M., Chen, Y., & Zhai, X. (2015). Selfhealing polymer materials constructed by macrocycle-based host-guest interactions. Soft Matter, 11(7), 1242-1252. Yang, Y., & Urban, M. W. (2013). Self-healing polymeric materials. Chemical Society reviews, 42(17), 7446-7467.
IP T
Yu, C., Wang, C.-F., & Chen, S. (2014). Robust self-healing host–guest gels from
magnetocaloric radical polymerization. Advanced Functional Materials, 24(9), 1235-
SC R
1242.
Yu, J., Ha, W., Sun, J.-n., & Shi, Y.-p. (2014). Supramolecular hybrid hydrogel based on host–guest interaction and its application in drug delivery. ACS Applied Materials
U
& Interfaces, 6(22), 19544-19551.
N
Zhang, H. J., Xia, H. S., & Zhao, Y. (2012). Poly(vinyl alcohol) hydrogel can
A
autonomously self-heal. ACS Macro Letters, 1(11), 1233-1236.
M
Zhang, M., Xu, D., Yan, X., Chen, J., Dong, S., Zheng, B., & Huang, F. (2012). Self-
TE D
healing supramolecular gels formed by crown ether based host–guest interactions. Angewandte Chemie, 124(28), 7117-7121. Zhao, L., Huang, J., Wang, T., Sun, W., & Tong, Z. (2017). Multiple shape memory,
EP
self-healable, and supertough PAA-GO-Fe3+ hydrogel. Macromolecular Materials and
CC
Engineering, 302(2), 1600359. Zhao, L., Huang, J., Zhang, Y., Wang, T., Sun, W., & Tong, Z. (2017). Programmable
A
and bidirectional bending of soft actuators based on janus structure with sticky tough paa-clay hydrogel. ACS Applied Materials & Interfaces, 9(13), 11866-11873.
24