- Email: [email protected]
cyclodextrin-based nanocomposite hydrogel with enhanced strength and thermo-responsive ability
Accepted Manuscript Title: Graphene/cyclodextrin-based nanocomposite hydrogel with enhanced strength and thermo-responsive ability Authors: Pang Zhu, Yonghong Deng, Chaoyang Wang PII: DOI: Reference:
S0144-8617(17)30718-X http://dx.doi.org/doi:10.1016/j.carbpol.2017.06.081 CARP 12469
To appear in: Received date: Revised date: Accepted date:
21-3-2017 20-6-2017 20-6-2017
Please cite this article as: Zhu, Pang., Deng, Yonghong., & Wang, Chaoyang., Graphene/cyclodextrin-based nanocomposite hydrogel with enhanced strength and thermo-responsive ability.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2017.06.081 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.
Graphene/cyclodextrin-based nanocomposite hydrogel with enhanced strength and thermo-responsive ability
Pang Zhua, Yonghong Dengb,*, Chaoyang Wanga,*
a
Research Institute of Materials Science, South China University of Technology, Guangzhou
510640, China b
Department of Materials Science & Engineering, South University of Science and
Technology of China, Shenzhen 518055, China
*Corresponding authors: Prof. Chaoyang Wang E-mail: [email protected]; Tel & fax: +86020-87112886 Prof. Yonghong Deng E-mail: [email protected]; Tel & fax: +860755-88015462
1
Highlights
Host-guest interaction enhanced nanocomposite hydrogels were prepared.
Physical dual crosslinked system for hydrogels was investigated.
The hydrogels had high strength and favorable thermoplastic ability.
One-pot fabrication of the high-performance hydrogels was developed.
Abstract Dual crosslinked system has been proved to be an efficient method to obtain tough and high
strength
hydrogels.
Herein,
we
synthesized
a
novel
graphene
oxide/p(acrylamide-co-poly(ethylene glycol) methyl ether methacrylate)/α-cyclodextrin (GO/P(AM-co-PEGMA)/CD) physical dual crosslinked hydrogel via copolymerization of AM and PEGMA in the α-CD/GO solution. The polymer main chains adsorb onto the GO surface resulting in the first crosslinked system and multiple hydrogen bonds between α-CDs that thread on the PEGMA side chains establish the second crosslinked system. The GO/P(AM-co-PEGMA)/CD hydrogel exhibits favorable tensile properties with fracture strain of 1800 % and high fracture stress of 660 kPa; in addition, the hydrogel can bear large compressive stress (2.7 MPa) at strain of 85 % without rupture. Furthermore, physical dual crosslinked system endows the GO/P(AM-co-PEGMA)/CD hydrogel with thermoplastic ability and thermo-responsive shape memory behavior. This facial one-pot method will contribute to design and application of high performance hydrogel.
Chemical compounds studied in this article acrylamide (AM, PubChem CID: 6579) poly(ethylene glycol) methyl ether methacrylate (PEGMA, No PubChem CID is found) 2
α-cyclodextrin (α-CD, PubChem CID: 444913) graphene oxide (GO, PubChem CID: 124202900) Keywords: hydrogel, dual crosslinked system, graphene oxide, α-cyclodextrin
1. Introduction Hydrogels, a kind of three-dimensional polymer containing amounts of water, have attracted much attention during the last two decades (Kamata, Akagi, Kayasuga-Kariya, Chung, & Sakai, 2014; Xu, Li, Wang, Zhang, & Zhang, 2013; Zhang, Liu, Huang, Wang, & Wang, 2015). Because of the favorable biocompatibility, hydrogels exhibit promising potential application in many engineering fields, such as controlled drug delivery system (Bean et al., 2014; Casolaro & Casolaro, 2015; Ding et al., 2011; Zhao et al., 2012), tissue engineering (Hong et al., 2015; Kumar et al., 2011; Ren, He, Xiao, Li, & Chen, 2015), and water treatment (Huang, Wu, Liu, Liu, & Zhang, 2013; Peng, Zhong, Ren, & Sun, 2012). However, the hydrogel usually don’t have enough strength, which has seriously limited its applications (Cong, Wang, & Yu, 2014). So various special hydrogels have been prepared to resolve the problem, for example, nanocomposite (NC) hydrogels (Gao, Du, Sun, & Fu, 2015), slide-ring hydrogels (Okumura & Ito, 2001), double network (DN) hydrogels (Chen, Zhu, Zhao, Wang, & Zheng, 2013; Gong, Katsuyama, Kurokawa, & Osada, 2003) and tetra-arm hydrogels (Sakai et al., 2008), all of them exhibited improved mechanical performance. Among them, graphene oxide (GO) NC hydrogel has witnessed a quick development of high strength hydrogel (Huang, Zhang, & Ruan, 2014; Jinchen, Zixing, Min, Hong, & Jie, 2013; Liu et al., 2012; Teng, Qiao, Wang, Jiang, & Zhu, 2016). Despite this, it is still far away from satisfying engineering demands. Therefore, many researchers have been 3
focusing on how to further improve the mechanical performance of the GO nanocomposite hydrogels. One strategy is to combine the interpenetrating network (IPN) or DN hydrogel with the graphene oxide NC hydrogel. Du et al. synthesized an inorganic/organic IPN hydrogel using one-pot method (Du et al., 2015), and the IPN GO nanocomposite hydrogel was proved to be biocompatible, which could stand a high compressive stress (22.81.5 MPa at strain of 93 %). However, to prepare the IPN hydrogel, several freeze-thawed cycles were needed, which made the synthesize process very time-consuming. Cong et al. synthesized the polyacrylamide/GO (PAM/GO) DN hydrogel (Cong et al., 2014). Even that the PAM/GO hydrogel was compressed drastically, it could recover its original shape quickly after being released, but its low fracture strength (150 kPa) was still not satisfying. In addition, the dual crosslinked (DC) system is another choice to obtain tough GO nanocomposite hydrogel. Adding very little N,N′-methylene-bis-acrylamide (BIS) as chemical crosslinked agent, Shi et al. synthesized hybrid poly(N-isopropylacrylamide)/GO dual crosslinked nanocomposite hydrogel with GO sheets acting as physical crosslinked agent, and the obtained DC hydrogel’s fracture strength was as high as 720 kPa (Shi et al., 2015). Using Fe3+ as an ionic crosslinked agent, another dual crosslinked GO/poly(acrylic acid) (GO/PAA) hydrogel exhibited excellent mechanical properties (fracture strength and strain were 777 kPa and 2980 %, respectively) (Zhong, Liu, & Xie, 2015). In this hydrogel, ionic bonds among Fe3+ and carboxyl groups of PAA chains could dissipate lots of energy via breaking and recombining dynamically, while the GO sheets on which the PAA chains absorbed could transfer stress to the whole hydrogel matrix and help to improve the 4
mechanical strength. In addition, adding another nanoparticle, such as hectorite clay, as a cooperative crosslinked agent could also effectively improve the mechanical properties of GO nanocomposite hydrogels (Zhang et al., 2014). In order to further simplify the fabrication process and avoid using toxic heavy metal ions
or
chemical
crosslinked
agents,
herein,
we
reported
a
novel
graphene
oxide/P(acrylamide-co-poly(ethylene glycol) methyl ether methacrylate)/α-cyclodextrin [GO/P(AM-co-PEGMA)/CD] dual crosslinked nanocomposite hydrogel. α-CD is a kind of cyclic oligosaccharides which consists of 6 glucose units and has a hydrophobic inner cavity and hydrophilic surface, and it is also commercially available and water soluble, what’s more, biocompatible (Loethen, Kim, & Thompson, 2007). It has been widely reported that α-CD can form interesting inclusion complex which is called rotaxane with poly(ethylene glycol) (PEG) via the host-guest interaction. And the adjacent rotaxanes tend to contact with each other closely because of hydrogen bonds between α-CDs, which has been utilized to prepare supramolecular hydrogel.(Liu, Zhang, & Li, 2011; Wenz, Han, & Muller, 2006). The GO/P(AM-co-PEGMA)/CD
dual
crosslinked
hydrogel
was
prepared
via
radical
copolymerization of AM and PEGMA in the α-CD/GO solution using one-pot method. In this DC hydrogel, polymer main chains adsorbed on the GO sheets, which formed the first crosslinked system; and multiple hydrogen bonds between neighboring α-CDs that threaded onto
the
PEGMA
side
chains
formed
the
second
crosslinked
system.
The
GO/P(AM-co-PEGMA)/CD hydrogel exhibited excellent tensile property (fracture strength and strain were 660 kPa and 1800 %, respectively) and could be compressed at a large strain of 85 % without rupture. Moreover, reversible physical crosslinked system allowed the hydrogel to be reshaped via thermocompression and endowed the hydrogel with thermal responsive shape memory behavior. We hope that this work can provide inspiration for designing and application of high strength hydrogel. 5
2. Experimental section 2.1. Materials GO was prepared via a modified Hummers method which could be found in our previous work (Zhu, Hu, Deng, & Wang, 2016), and the TEM iamge of GO nanosheets was exhibited in Fig. S1. Monomer acrylamide (AM) was obtained from Shanghai Richjoint Chemical Reagents. poly(ethylene glycol) methyl ether methacrylate (PEGMA, Mw 950 g/moL) was purchased from Shanghai Aladdin Biochemical Technology Co.,Ltd. α-cyclodextrin (α-CD) (HPLC 98 %, Mw 973 g/moL) was purchased from Shanghai Macklin Biochemical Technology Co. Ltd. Potassium peroxydisulfate (KPS) was purchased from Tianjin Fuchen Chemical Reagent Factory. Purified water with resistivity 18.0 Mcm was obtained from a Millipore purification apparatus. 2.2. Preparation of hydrogels GO/P(AM-co-PEGMA)/CD gel was prepared via one-pot in-situ radical polymerization. Typically, 2.6 g AM, 4 mL PEGMA (0.1 g/mL) and 2.9 mL H2O were added to 7.5 mL GO suspension (2 mg/mL) and the mixture was stirred in ice-water bath for 2 h (for different GO/P(AM-co-PEGMA)/CD gel synthesized here, the total concentration of AM and PEGMA was kept constantly as 20 % wt/v). Then the mixture was bubbled with N2 to extrude O2 for 15 min, and 2.86 g α-CD (the molar ratio of α-CD to PEGMA is 7:1) and 0.6 mL KPS solution (50 mg/mL) were added subsequently. The mixture was heated at 45 oC for 15 min to dissolve α-CD under stirring. At last, the pre-gel solution was injected into the glass mould and incubated at 45 oC for 24 h. Under 45 oC, KPS decomposed producing radical and
6
initiated the copolymerization of AM and PEGMA; finally, the GO/P(AM-co-PEGMA)/CD gel was obtained. PAM/GO gel for comparison was also synthesized as the following procedures. 3.0 g AM, 6.9 mL H2O and 7.5 mL GO suspension (2 mg/mL) was mixed together and the mixture was stirred in ice-water bath for 2 h and bubbled with N2 to extrude O2 for 15 min； 0.6 mL KPS solution (50 mg/mL) were added subsequently, then, the pre-gel solution was injected into glass mould and incubated at 45 oC for 24 h to initiate the polymerization of AM, and the PAM/GO gel was obtained. 2.3. Mechanical tests Mechanical tests of obtained gels were carried out using a shimadzu autograph AG-X plus 1KN system under 25 oC in air. For tensile tests, the gel was cut into 3.2 (diameter)45 (length) mm cylinder and the rate of stretch was 100 mm/min. For compress tests, the gel was cut into 13.518 mm cylinder and the compress speed was 2 mm/min. For tearing test, the gel sample was cut into 401010 mm cuboid with a notch of 20 mm. One arm of the gel sample was fixed while the other one was stretched at a speed of 50 mm/min. Silicone oil was coated on the surfaces of gel samples to prevent water evaporation. The tearing energy (T) was calculated according to: 𝑇=
2 𝐹𝑎𝑣𝑒 𝑤
Here, Fave represents the average force of the max values of the steady-state tear curve, and w is the width of the gel sample (here, w=10 mm). Tearing test curves are showed in Fig. S1. 2.4. Rheological measurements
7
Rheological measurements were conducted with stress controlled rheometer AR-G2 (TA) via a parallel plate of 40 mm. For frequency sweep measurements, the frequency ranged from 0.1 to 100 rad/s with a definite strain of 0.5 %. Temperature sweep measurements were conducted with temperature ranging from 30 to 90 oC, and the temperature increasing rate was 1 oC/min. Silicon oil was used to prevent water evaporation during the whole process. 2.5. Shape-memory behavior A straight hydrogel of 460 mm was first heated at 80 oC for 10 min, and then it was curled into the “U” shape and cooled in room temperature to fix the deformation. To demonstrate its shape-memory behavior, the deformed hydrogel was immersed into water of 80 oC, and its recovery process was recorded. 3. Result and discussion 3.1. Synthesis of GO/P(AM-co-PEGMA)/CD gel According to the previous work, rotaxane which is composed of α-CD and PEGMA tends to aggregate together and forms insoluble polyrotaxane (Loethen et al., 2007), which will result in lots of heterogeneities in the obtained hydrogel and weaken the hydrogel’s strength. So, it is necessary to restrain the host-guest interaction between α-CD and PEGMA before the copolymerization of AM and PEGMA to get high strength hydrogel. And based on the published works, the concerted effect of AM monomer and an appropriate temperature could successfully accomplish this goal (Feng, Zhou, Dai, Yasin, & Yang, 2016). Therefore, to synthesize GO/P(AM-co-PEGMA)/CD hydrogel, firstly, AM and PEGMA were added into the GO solution and stirred, during this process, AM tended to adsorb on the GO surface because of the hydrogen bond interaction between the -NH2 groups and oxygen-containing 8
groups on the GO surface (Ferse et al., 2008; Haraguchi, Li, Matsuda, Takehisa, & Elliott, 2005). Then initiator and α-CD were added and stirred at 45 oC (Fig. 1a). The pre-gel solution was pulled into the glass mould and incubated at 45 oC. With the proceeding of reaction, the growing P(AM-co-PEGMA) polymer main chains attached closely to the GO sheets owing to the physical interaction (Ferse et al., 2008; Haraguchi et al., 2005; Zhu et al., 2014), establishing the first crosslinked system (Fig. 1b). At the same time, as more and more AM monomers were fixed onto the polymer chains, in consequence, the shield effect decreased. Therefore, driven by the hydrophobic interaction between ethylene oxide group (-CH2-CH2-O-) and internal cavity of α-CD, the α-CD which located in suitable position would penetrate onto the PEGMA side chains and slide along the chains (Ceccato, Lonostro, & Baglioni, 1997). Other free α-CDs molecules would continue to thread onto the PEGMA side chains in a head-to-head or tail-to-tail orientation, as a result, the rotaxanes formed. (Loethen et al., 2007). During the process, because α-CDs threaded onto the PEGMA side chains, these PEGMA side chains were forced to unfold and stretch, and turned into elongated conformation. At last, neighboring rotaxanes interacted with each other closely and formed micro crystalline domains because of multifold hydrogen bonds between the α-CDs (Fleury et al., 2005; Wei et al., 2005), consequently, the second crosslinked system formed (Fig. 1c).
X-ray diffraction (XRD) has been widely used to investigate the structure of the inclusion complex composed of α-CDs and polymers. As exhibited in Fig. S2, a variety of sharp diffraction peaks (2: 12.118o, 14.296o, 21.665o) represented pure α-CD (Shen et al., 9
2015; Yasin et al., 2015). No peaks were found in PAM/GO gel, indicating that the polymer chains were in disorder state. For GO/P(AM-co-PEGMA)/CD gel, two diffraction peaks (2: 19.675o and 19.939o) represented the characteristic channel-type structure of inclusion complex composed of α-CDs and the PEGMA side chains (Li, He, Zhang, Tam, & Ni, 2015; Wang & Chen, 2007; Wei et al., 2005; Zhou, Ye, Liu, Hu, & Qian, 2015). However, it is possible that there are free α-CDs in the hydrogel.
3.2. Mechanical and rheological properties of GO/P(AM-co-PEGMA)/CD hydrogel As Fig. 2a exhibited, the dual crosslinked system endowed GO/P(AM-co-PEGMA)/CD hydrogel with improved strength. Single crosslinked PAM/GO exhibited favorable ductility (fracture strain of 2800 %) but very weak fracture strength of 100 kPa. It was because that the interaction between PAM chains and GO was not strong enough, so that the PAM chains could easily slide away from the GO surface under small stress. But for GO/P(AM-co-PEGMA)/CD dual crosslinked hydrogel, the microcrystalline formed by rotaxane supplied extra crosslinked points, so much larger stress was needed to destroy it. As a result, GO/P(AM-co-PEGMA)/CD dual crosslinked hydrogel possessed higher fracture strength of 660 kPa. On the other hand, as mentioned in 3.1, the PEGMA side chains turned into elongated conformation during the copolymerization process, which would make polymer chain pre-stretched and result in increased brittleness of GO/P(AM-co-PEGMA)/CD hydrogel. Therefore, the stretchable ability (fracture strain of 1800 %) of the GO/P(AM-co-PEGMA)/CD gel decreased compared to the soft PAM/GO hydrogel. In addition, the GO/P(AM-co-PEGMA) hydrogel which was kept in the sealed environment under room temperature still exhibited almost the same mechanical property with that of the fresh hydrogel (Fig. S3), proving the hydrogel’s favorable stability. 10
Tearing energy and hysteresis loop were two important indexes to characterize the toughness of hydrogel. Fig. 2b showed that GO/P(AM-co-PEGMA)/CD hydrogel exhibited much higher tearing energy (1755 J/m2) than that of PAM/GO hydrogel (152 J/m2). Fig. 2c exhibited the cycle tensile curves of PAM/GO hydrogel and GO/P(AM-co-PEGMA)/CD dual crosslinked hydrogel, and the tearing test curves of the two kinds of hydrogel were showed in Fig. S4. The dual crosslinked hydrogel demonstrated a large hysteresis loop, while the hysteresis loop of PAM/GO was very little. Correspondingly, as Fig. 2d exhibited, more energy (2799 kJ/m3) was dissipated for GO/P(AM-co-PEGMA)/CD hydrogel during the loading-unloading process. As analyzed in other papers, both of higher tearing energy and larger
hysteresis
loop
proved
an
obvious
increment
of
toughness
of
GO/P(AM-co-PEGMA)/CD hydrogel (Lin, Ma, Wang, & Zhou, 2015; Qiang et al., 2015). As demonstrated in Fig. 3a, PAM/GO gel was so soft that the compress strength was only 254.6 kPa at strain of 85 %, and it was much little than that of GO/P(AM-co-PEGMA)/CD hydrogel (2698 kPa) at the same strain. Pictures of the compress-release process of GO/P(AM-co-PEGMA)/CD hydrogel was showed in Fig. 3b. The original gel (Fig. 3b1) which was 13 mm in diameter and 22 mm in height could be compressed dramatically and without any macro rupture (Fig. 3b2). More importantly, after being released, the hydrogel could almost recover to its original shape (Fig. 3b3), which proved that the GO/P(AM-co-PEGMA)/CD hydrogel possessed a good fatigue resistance.
Ratios of α-CD to PEGMA and AM to PEGMA as well as the GO concentration were three crucial factors to determine the properties of GO/P(AM-co-PEGMA)/CD hydrogel. Fig. 11
4a showed the tensile curves of GO/P(AM-co-PEGMA)/CD hydrogel with molar ratios of α-CD to PEGMA ranging from 0:1 to 9:1. When no α-CD was added (0:1), the GO/P(AM-co-PEGMA)/CD hydrogel was single crosslinked, which showed the weakest mechanical strength and fracture strain. Increasing α-CD content could obviously improve the strength of GO/P(AM-co-PEGMA)/CD hydrogel, and the best mechanical property was obtained when the ratio of α-CD to PEGMA was 7:1. To further increase the α-CD content (9:1), the mechanical strength decreased. Fig. 4b exhibited the corresponding fracture dissipated energy of various GO/P(AM-co-PEGMA)/CD hydrogels, and it demonstrated the same trend which could be found in Fig. 4a, that was to say, the fracture dissipated energy increased firstly with increasing α-CD content, while decreased when the α-CD content was excessed. Rheological measurements were also conducted. As showed in Fig. S5, both G and G increased with α-CD content, and decreased when the ratio was larger than 7:1, which was very consistent with the trend exhibited in Fig. 4. Quantitative analysis could explain why the α-CD content had such an effect on the hydrogel precisely. The maximum amount of α-CD that threaded onto one PEGMA side chain could be estimated based on the length of completely stretched PEGMA chain and the height of the α-CD molecule (Ceccato et al., 1997). The molecular weight of PEGMA monomer used here was 950 g/moL, and taking the length of various chemical bonds (C-C, C-O, C=C) and corresponding bond angels into consideration, it could be calculated that the length of a completely stretched PEGMA chain (L) was about 5.426 nm, while the height of α-CD was 7.9 Å (H). So when one PEGMA chain was fully covered by α-CDs, the ratio of 12
α-CD to PEGMA was about 6.86 (L/H≈6.86). And this value was very much close to the best formula when α-CD: PEGMA= 7:1 as discussed above. Therefore, it could be concluded that with the increase of α-CD content, more α-CDs threaded onto the PEGMA forming rotaxane so that more hydrogen bonds formed between neighboring rotaxanes, as a result, the second crosslinked system became stronger. When α-CD: PEGMA= 7:1, the PEGMA was fully covered by α-CD and the hydrogel exhibited best mechanical property. However, when α-CD content was excessed, the shield effect of AM was unable to prevent the α-CD threading onto the PEGMA chains before copolymerization, which would result insoluble polyrotaxane in the pre-gel solution; at the same time, excess free α-CD molecules that didn’t thread onto the PEGMA side chains would also aggregate together and tend to precipitate. Both
the
two
factors
would
form
amounts
of
defects
in
the
obtained
GO/P(AM-co-PEGMA)/CD hydrogel. In fact, obvious phase-separation phenomenon could be found in the GO/P(AM-co-PEGMA)/CD hydrogel when the ratio reached to 9:1, which undoubtedly weakened the mechanical strength of the hydrogel. Further, it maybe supposed that changing the length of the PEGMA chain would be helpful to prepare tougher and stronger hydrogel. Effects of the GO concentration and molar ratio of PEGMA to AM on the GO/P(AM-co-PEGMA) hydrogel’s mechanical property were exhibited in Fig. S6 and Fig. S7. On the one hand, as the crosslinked agent, low GO concentration would result in a weak first crosslinked system, consequently, the hydrogel’s strength was low; on the other hand, excess GO tended to aggregate together, for example, there were lots of macro GO blocks in the obtained GO/P(AM-co-PEGMA)/CD hydrogel when GO concentration was 3 mg/mL, 13
which weakened the strength of the hydrogel seriously (Fig. S6). As Fig. S7 exhibited, too much PEGMA was disadvantage for the tensile performance of GO/P(AM-co-PEGMA)/CD hydrogel; it was because that excess PEGMA (ratio was 1:59) resulted in a densely second crosslinked system which markedly improved the hydrogel’s brittleness, and the fracture strain reduced obviously comparing to that of the hydrogel whose molar ratio of PEGMA to AM was 1:87. On the contrary, when the ratio was too small (1:175), the obtained hydrogel demonstrated similar tensile curve of single crosslinked PAM/GO hydrogel (Fig. 2a), which meant that limited PEGMA resulted in a very loose second crosslinked system and contributed little to improve the mechanical strength of the GO/P(AM-co-PEGMA)/CD hydrogel. And it was important to notice that if there was no PEGMA was added (PEGMA: AM=0:1), the obtained hydrogel showed extreme low fracture strain and fracture strength, as the inset picture of Fig. S7 exhibited. It proved that α-CD could not build the second crosslinked system by itself without PEGMA. Conversely, amounts of free α-CD would seriously weaken the mechanical performance of the hydrogel. 3.3. Thermoplastic property and thermo-responsive behave of GO/P(AM-co-PEGMA)/CD hydrogel It has been reported that the polymer chains could desorb from the nano-sheets surfaces when NC hydrogel was heated at a suitable temperature, making it possible to reshape the hydrogel after polymerization (Cui et al., 2015; Dai et al., 2015). As the fully physical
crosslinked
hydrogel,
Fig.
5a
showed
the
reshape
process
of
GO/P(AM-co-PEGMA)/CD hydrogel. The original hydrogel with a diameter of 13 mm and a height of 15 mm was heated at 80 oC for 10 min; then the hydrogel was compressed and fixed under pressure to keep its reshaped hydrogel. After being cooled at 25 oC, a discal reshaped 14
hydrogel (Fig. 5a2) with a diameter of 35.6 mm and a height of 2 mm was obtained. And as Fig. 5b exhibited, the reshaped hydrogel still exhibited high fracture strength (619 kPa), which was close to the original gel (660 kPa). As Fig. 6 exhibited, the temperature sweep measurement proposed a precise dynamic change process of the storage modulus (G) and the loss modulus (G) during the heating-cooling process. G decreased dramatically from 238.8 kPa to 14 kPa with increasing temperature; when being cooled to 30 oC, G recovered to 224.8 kPa. And G exhibited a similar change trend with that of the G. The decrease of G meant the crosslinked density declined, which was because that the polymer chains desorbed from GO surface and the multiple hydrogen bonds between adjacent rotaxanes were damaged. During the cooling process, the polymer chains re-adsorbed on the GO surfaces, and hydrogen bonds formed again. And it was interesting to notice that there were obvious hysteresis loops in the temperature sweep curves, especially for G. It was most likely because that both re-adsorption and re-forming of hydrogen bonds were time-consuming, which was also observed in other thermoplastic hydrogel (Feng et al., 2016). Fig. 7 exhibited the GO/P(AM-co-PEGMA)/CD hydrogel’s thermo-responsive shape memory behavior. A straight hydrogel gel was first curled into temporary “U” shape via heating-cooling process, and then the deformed hydrogel was immersed into 80 oC hot water. In the hot water, hydrogen bonds were damaged and failed to keep temporary shape(Feng et al., 2016; Yasin et al., 2015). At last, the hydrogel almost recovered to its original straight shape. 4. Conclusions In this work, we synthesized a novel GO/P(AM-co-PEGMA)/CD dual crosslinked 15
hydrogel using one-pot method. The obtained hydrogel’s fracture strain and strength were 1800 % and 660 kPa, and could be compressed at a large strain of 85 % without rupture. Ratios of α-CD to PEGMA and PEGMA to AM as well as the GO concentration were proved to have an important effect on the performance of GO/P(AM-co-PEGMA)/CD hydrogel. In addition, dual physical crosslinked system endowed the hydrogel with favorable thermoplastic ability and thermo-responsive shape memory behavior. The method avoided using chemical crosslinked agent and heavy metal ions which were harmful for human body, and the synthesize process was simple and time-saving. Therefore, it is expected that this work can provide inspiration for designing and application of high strength hydrogel. Acknowledgements This research was supported by the National Natural Science Foundation of China (21474032) and the Natural Science Foundation of Guangdong Province (2016A030311031). Appendix A. Supplementary data References Bean, J. E., Alves, D. R., Laabei, M., Esteban, P. P., Thet, N. T., Enright, M. C., et al. (2014). Triggered release of bacteriophage k from agarose/hyaluronan hydrogel matrixes by staphylococcus aureus virulence factors. Chemistry of Materials, 24, 7201-7208. Casolaro, M., & Casolaro, I. (2015). Controlled release of antidepressant drugs by multiple stimuli-sensitive hydrogels based on alpha-aminoacid residues. Journal of Drug Delivery Science and Technology, 30, 82-89. Ceccato, M., LoNostro, P., & Baglioni, P. (1997). Alpha cyclodextrin/polyethylene glycolpolyrotaxane: A study of the threading process. Langmuir, 9, 2436-2439. Chen, Q., Zhu, L., Zhao, C., Wang, Q., & Zheng, J. (2013). A robust, one-pot synthesis of highly mechanical and recoverable double network hydrogels using thermoreversible sol-gel polysaccharide. Advanced Materials, 30, 4171-4176. Cong, H.-P., Wang, P., & Yu, S.-H. (2014). Highly elastic and superstretchable graphene oxide/polyacrylamide hydrogels. Small, 3, 448-453. Cui, W., Zhang, Z.-J., Li, H., Zhu, L.-M., Liu, H., & Ran, R. (2015). Robust dual physically crosslinked hydrogels with unique self-reinforcing behavior and improved dye adsorption capacity. Rsc Advances, 65, 52966-52977. Dai, X., Zhang, Y., Gao, L., Bai, T., Wang, W., Cui, Y., et al. (2015). A mechanically strong, highly stable, thermoplastic, and self-healable supramolecular polymer hydrogel. Advanced Materials, 23, 3566-3571. 16
Ding, J., Zhuang, X., Xiao, C., Cheng, Y., Zhao, L., He, C., et al. (2011). Preparation of photo-crosslinked ph-responsive polypeptide nanogels as potential carriers for controlled drug delivery. Journal of Materials Chemistry, 30, 11383-11391. Du, G., Nie, L., Gao, G., Sun, Y., Hou, R., Zhang, H., et al. (2015). Tough and biocompatible hydrogels based on in situ interpenetrating networks of dithiol-connected graphene oxide and poly(vinyl alcohol). Acs Applied Materials & Interfaces, 5, 3003-3008. Feng, W., Zhou, W., Dai, Z., Yasin, A., & Yang, H. (2016). Tough polypseudorotaxane supramolecular hydrogels with dual-responsive shape memory properties. Journal of Materials Chemistry B, 11, 1924-1931. Ferse, B., Richter, S., Eckert, F., Kulkarni, A., Papadakis, C. M., & Arndt, K.-F. (2008). Gelation mechanism of poly(n-isopropylacrylamide)-clay nanocomposite hydrogels synthesized by photopolymerization. Langmuir, 21, 12627-12635. Fleury, G., Brochon, C., Schlatter, G., Bonnet, G., Lapp, A., & Hadziioannou, G. (2005). Synthesis and characterization of high molecular weight polyrotaxanes: Towards the control over a wide range of threaded alpha-cyclodextrins. Soft Matter, 5, 378-385. 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, 8, 5029-5037. Gong, J. P., Katsuyama, Y., Kurokawa, T., & Osada, Y. (2003). Double-network hydrogels with extremely high mechanical strength. Advanced Materials, 14, 1155-1161. Haraguchi, K., Li, H. J., Matsuda, K., Takehisa, T., & Elliott, E. (2005). Mechanism of forming organic/inorganic network structures during in-situ free-radical polymerization in pnipa-clay nanocomposite hydrogels. Macromolecules, 8, 3482-3490. Hong, S., Sycks, D., Chan, H. F., Lin, S., Lopez, G. P., Guilak, F., et al. (2015). 3d printing: 3d printing of highly stretchable and tough hydrogels into complex, cellularized structures. Advanced Materials, 27, 4034-4034. Huang, Y., Zhang, M., & Ruan, W. (2014). High-water-content graphene oxide/polyvinyl alcohol hydrogel with excellent mechanical properties. Journal of Materials Chemistry A, 27, 10508-10515. Huang, Z., Wu, Q., Liu, S., Liu, T., & Zhang, B. (2013). A novel biodegradable beta-cyclodextrin-based hydrogel for the removal of heavy metal ions. Carbohydrate Polymers, 2, 496-501. Jinchen, F., Zixing, S., Min, L., Hong, L., & Jie, Y. (2013). Mechanically strong graphene oxide/sodium alginate/polyacrylamide nanocomposite hydrogel with improved dye adsorption capacity. Journal of Materials Chemistry A, 25, 7433-7443. Kamata, H., Akagi, Y., Kayasuga-Kariya, Y., Chung, U.-i., & Sakai, T. (2014). "Nonswellable" hydrogel without mechanical hysteresis. Science, 6173, 873-875. Kumar, P. T. S., Srinivasan, S., Lakshmanan, V.-K., Tamura, H., Nair, S. V., & Jayakumar, R. (2011). Beta-chitin hydrogel/nano hydroxyapatite composite scaffolds for tissue engineering applications. Carbohydrate Polymers, 3, 584-591. Li, F., He, J., Zhang, M., Tam, K. C., & Ni, P. (2015). Injectable supramolecular hydrogels fabricated from pegylated doxorubicin prodrug and alpha-cyclodextrin for ph-triggered drug delivery. Rsc Advances, 67, 54658-54666. Lin, P., Ma, S., Wang, X., & Zhou, F. (2015). Molecularly engineered dual-crosslinked hydrogel with ultrahigh mechanical strength, toughness, and good self-recovery. Advanced Materials, 12, 2054-2059. Liu, J., Chen, C., He, C., Zhao, L., Yang, X., & Wang, H. (2012). Synthesis of graphene peroxide and its application in fabricating super extensible and highly resilient nanocomposite hydrogels. Acs Nano, 9, 17
8194-8202. Liu, K. L., Zhang, Z., & Li, J. (2011). Supramolecular hydrogels based on cyclodextrin-polymer polypseudorotaxanes: Materials design and hydrogel properties. Soft Matter, 24, 11290-11297. Loethen, S., Kim, J.-M., & Thompson, D. H. (2007). Biomedical applications of cyclodextrin based polyrotaxanes. Polymer Reviews, 3, 383-418. Okumura, Y., & Ito, K. (2001). The polyrotaxane gel: A topological gel by figure-of-eight cross-links. Advanced Materials, 7, 485-492. Peng, X.-W., Zhong, L.-X., Ren, J.-L., & Sun, R. C. (2012). Highly effective adsorption of heavy metal ions from aqueous solutions by macroporous xylan-rich hemicelluloses-based hydrogel. Journal of Agricultural and Food Chemistry, 15, 3909-3916. Qiang, C., Lin, Z., Hong, C., Hongli, Y., Lina, H., Jia, Y., et al. (2015). A novel design strategy for fully physically linked double network hydrogels with tough, fatigue resistant, and self-healing properties. Advanced Functional Materials, 10, 1598-1607. Ren, K., He, C., Xiao, C., Li, G., & Chen, X. (2015). Injectable glycopolypeptide hydrogels as biomimetic scaffolds for cartilage tissue engineering. Biomaterials, 7, 238-249. Sakai, T., Matsunaga, T., Yamamoto, Y., Ito, C., Yoshida, R., Suzuki, S., et al. (2008). Design and fabrication of a high-strength hydrogel with ideally homogeneous network structure from tetrahedron-like macromonomers. Macromolecules, 14, 5379-5384. Shen, J., Pang, J., Kalwarczyk, T., Holyst, R., Xin, X., Xu, G., et al. (2015). Manipulation of multiple-responsive fluorescent supramolecular materials based on the inclusion complexation of cyclodextrins with tyloxapol. Journal of Materials Chemistry C, 31, 8104-8113. Shi, K., Liu, Z., Wei, Y.-Y., Wang, W., Ju, X.-J., Xie, R., et al. (2015). Near-infrared light-responsive poly(n-isopropylacrylamide)/graphene oxide nanocomposite hydrogels with ultrahigh tensibility. Acs Applied Materials & Interfaces, 49, 27289-27298. Teng, C., Qiao, J., Wang, J., Jiang, L., & Zhu, Y. (2016). Hierarchical layered heterogeneous graphene-poly(n-isopropylacrylamide)-clay hydrogels with superior modulus, strength, and toughness. Acs Nano, 1, 413-420. Wang, Z., & Chen, Y. (2007). Supramolecular hydrogels hybridized with single-walled carbon nanotubes. Macromolecules, 9, 3402-3407. Wei, H. L., Yu, H. Q., Zhang, A. Y., Sun, L. G., Hou, D. D., & Feng, Z. G. (2005). Synthesis and characterization of thermosensitive and supramolecular structured hydrogels. Macromolecules, 21, 8833-8839. Wenz, G., Han, B. H., & Muller, A. (2006). Cyclodextrin rotaxanes and polyrotaxanes. Chemical Reviews, 3, 782-817. Xu, B., Li, H., Wang, Y., Zhang, G., & Zhang, Q. (2013). Nanocomposite hydrogels with high strength crosslinked by titania. Rsc Advances, 20, 7233-7236. Yasin, A., Zhou, W., Yang, H., Li, H., Chen, Y., & Zhang, X. (2015). Shape memory hydrogel based on a hydrophobically-modified polyacrylamide (hmpam)/alpha-cd mixture via a host-guest approach. Macromolecular Rapid Communications, 9, 845-851. Zhang, E., Wang, T., Zhao, L., Sun, W., Liu, X., & Tong, Z. (2014). Fast self-healing of graphene oxide-hectorite clay-poly(n,n-dimethylacrylamide) hybrid hydrogels realized by near-infrared irradiation. Acs Applied Materials & Interfaces, 24, 22855-22861. Zhang, Y., Liu, J., Huang, L., Wang, Z., & Wang, L. (2015). Design and performance of a sericin-alginate interpenetrating network hydrogel for cell and drug delivery. Scientific Reports, 5. No. 12374. Zhao, C., Chen, Q., Patel, K., Li, L., Li, X., Wang, Q., et al. (2012). Synthesis and characterization of ph-sensitive 18
poly(n-2-hydroxyethyl acrylamide)-acrylic acid (poly(heaa/aa)) nanogels with antifouling protection for controlled release. Soft Matter, 30, 7848-7857. Zhong, M., Liu, Y.-T., & Xie, X.-M. (2015). Self-healable, super tough graphene oxide-poly(acrylic acid) nanocomposite hydrogels facilitated by dual crosslinked effects through dynamic ionic interactions. Journal of Materials Chemistry B, 19, 4001-4008. Zhou, M., Ye, X., Liu, K., Hu, J., & Qian, X. (2015). Tunable thermo-responsive supramolecular hydrogel: Design, characterization, and drug release. Journal of Polymer Research, 9, 1-8. Zhu, P., Hu, M., Deng, Y., & Wang, C. (2016). One-pot fabrication of a novel agar-polyacrylamide/graphene oxide nanocomposite double network hydrogel with high mechanical properties Advanced Engineering Materials, 10, 1799-1807. Zhu, Z., Song, G., Liu, J., Whitten, P. G., Liu, L., & Wang, H. (2014). Liquid crystalline behavior of graphene oxide in the formation and deformation of tough nanocomposite hydrogels. Langmuir, 48, 14648-14657.
19
b
a
c
Fig. 1. Schematic of the preparation process and dual crosslinked mechanism of GO/P(AM-co-PEGMA)/CD hydrogel.
20
1800 800
a
1500
Tearing energy (J/m2)
PAM/GO gel GO/P(AM-co-PEGMA)/CD gel
Stress (kPa)
600
400
200
0
0
500
1000
1500
2000
2500
3000
Strain (%) PAM/GO gel GO/P(AM-co-PEGMA)/CD gel
1200 900 600 300 0
PAM/GO gel
GO/P(AM-co-PEGMA)/CD gel
3000
c
2500
400
Uhys (kJ/m3)
Stress (kPa)
600
b
200
d
2000 1500 1000 500
0 0
500
0
1000
PAM/GO gel
GO/P(AM-co-PEGMA)/CD gel
Strain (%)
Fig. 2. (a) Tensile curves, (b) tearing energy, (c) loading-unloading curves at strain of 1000 % and (d) corresponding dissipated energy of PAM/GO gel, and GO/P(AM-co-PEGMA)/CD gel, respectively.
21
3000
a
PAM/GO gel GO/P(AM-co-PEGMA)/CD gel
Stress (kPa)
2500 2000 1500 1000 500 0
0
20
40
60
80
100
Strain (%)
b1
b3
b2
Fig. 3 (a) Compression curves of PAM/GO gel and GO/P(AM-co-PEGMA)/CD gel; (b)
750
0:1 1:1 3:1 5:1 7:1 9:1
Stress (kPa)
600
450
a
300
150
0
0
500
1000
1500
2000
Fracture dissipated energy (kJ/m 3)
pictures of (b1) original, (b2) compressed and (b3) released GO/P(AM-co-PEGMA)/CD gel.
1000
b 800
600
400
200
0
0:1
1:1
3:1
5:1
7:1
9:1
Strain (%)
Fig. 4. (a) Tensile curves and (b) fracture dissipated energy of GO/P(AM-co-PEGMA)/CD gel with different molar ratios of α-CD to PEGMA; the concentration of GO 1mg/mL and the 22
molar ratio of PEGMA to AM was 1:87.
a1
a2 Original gel
Stress (kPa)
600
Reshaped gel
b
400
Reshaped gel 200
0
0
200
400
600
800
Strain (%)
Fig.
5
(a)
Thermoplastic
property
and
GO/P(AM-co-PEGMA)/CD hydrogel
23
(b)
tensile
curve
of
reshaped
250
ing at He
150
G' G''
ng oli Co
G' & G'' (kPa)
200
100 50 0 30
Heating Cooling 40
50
60
70
80
90
Temperature (°C)
Fig. 6. Temperature sweep of GO/P(AM-co-PEGMA)/CD gel from 30-90 oC.
5s
0s
11 s
Fig. 7. Pictures of thermo-responsive shape memory behavior of GO/P(AM-co-PEGMA)/CD gel
24
S0144-8617(17)30718-X http://dx.doi.org/doi:10.1016/j.carbpol.2017.06.081 CARP 12469
To appear in: Received date: Revised date: Accepted date:
21-3-2017 20-6-2017 20-6-2017
Please cite this article as: Zhu, Pang., Deng, Yonghong., & Wang, Chaoyang., Graphene/cyclodextrin-based nanocomposite hydrogel with enhanced strength and thermo-responsive ability.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2017.06.081 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.
Graphene/cyclodextrin-based nanocomposite hydrogel with enhanced strength and thermo-responsive ability
Pang Zhua, Yonghong Dengb,*, Chaoyang Wanga,*
a
Research Institute of Materials Science, South China University of Technology, Guangzhou
510640, China b
Department of Materials Science & Engineering, South University of Science and
Technology of China, Shenzhen 518055, China
*Corresponding authors: Prof. Chaoyang Wang E-mail: [email protected]; Tel & fax: +86020-87112886 Prof. Yonghong Deng E-mail: [email protected]; Tel & fax: +860755-88015462
1
Highlights
Host-guest interaction enhanced nanocomposite hydrogels were prepared.
Physical dual crosslinked system for hydrogels was investigated.
The hydrogels had high strength and favorable thermoplastic ability.
One-pot fabrication of the high-performance hydrogels was developed.
Abstract Dual crosslinked system has been proved to be an efficient method to obtain tough and high
strength
hydrogels.
Herein,
we
synthesized
a
novel
graphene
oxide/p(acrylamide-co-poly(ethylene glycol) methyl ether methacrylate)/α-cyclodextrin (GO/P(AM-co-PEGMA)/CD) physical dual crosslinked hydrogel via copolymerization of AM and PEGMA in the α-CD/GO solution. The polymer main chains adsorb onto the GO surface resulting in the first crosslinked system and multiple hydrogen bonds between α-CDs that thread on the PEGMA side chains establish the second crosslinked system. The GO/P(AM-co-PEGMA)/CD hydrogel exhibits favorable tensile properties with fracture strain of 1800 % and high fracture stress of 660 kPa; in addition, the hydrogel can bear large compressive stress (2.7 MPa) at strain of 85 % without rupture. Furthermore, physical dual crosslinked system endows the GO/P(AM-co-PEGMA)/CD hydrogel with thermoplastic ability and thermo-responsive shape memory behavior. This facial one-pot method will contribute to design and application of high performance hydrogel.
Chemical compounds studied in this article acrylamide (AM, PubChem CID: 6579) poly(ethylene glycol) methyl ether methacrylate (PEGMA, No PubChem CID is found) 2
α-cyclodextrin (α-CD, PubChem CID: 444913) graphene oxide (GO, PubChem CID: 124202900) Keywords: hydrogel, dual crosslinked system, graphene oxide, α-cyclodextrin
1. Introduction Hydrogels, a kind of three-dimensional polymer containing amounts of water, have attracted much attention during the last two decades (Kamata, Akagi, Kayasuga-Kariya, Chung, & Sakai, 2014; Xu, Li, Wang, Zhang, & Zhang, 2013; Zhang, Liu, Huang, Wang, & Wang, 2015). Because of the favorable biocompatibility, hydrogels exhibit promising potential application in many engineering fields, such as controlled drug delivery system (Bean et al., 2014; Casolaro & Casolaro, 2015; Ding et al., 2011; Zhao et al., 2012), tissue engineering (Hong et al., 2015; Kumar et al., 2011; Ren, He, Xiao, Li, & Chen, 2015), and water treatment (Huang, Wu, Liu, Liu, & Zhang, 2013; Peng, Zhong, Ren, & Sun, 2012). However, the hydrogel usually don’t have enough strength, which has seriously limited its applications (Cong, Wang, & Yu, 2014). So various special hydrogels have been prepared to resolve the problem, for example, nanocomposite (NC) hydrogels (Gao, Du, Sun, & Fu, 2015), slide-ring hydrogels (Okumura & Ito, 2001), double network (DN) hydrogels (Chen, Zhu, Zhao, Wang, & Zheng, 2013; Gong, Katsuyama, Kurokawa, & Osada, 2003) and tetra-arm hydrogels (Sakai et al., 2008), all of them exhibited improved mechanical performance. Among them, graphene oxide (GO) NC hydrogel has witnessed a quick development of high strength hydrogel (Huang, Zhang, & Ruan, 2014; Jinchen, Zixing, Min, Hong, & Jie, 2013; Liu et al., 2012; Teng, Qiao, Wang, Jiang, & Zhu, 2016). Despite this, it is still far away from satisfying engineering demands. Therefore, many researchers have been 3
focusing on how to further improve the mechanical performance of the GO nanocomposite hydrogels. One strategy is to combine the interpenetrating network (IPN) or DN hydrogel with the graphene oxide NC hydrogel. Du et al. synthesized an inorganic/organic IPN hydrogel using one-pot method (Du et al., 2015), and the IPN GO nanocomposite hydrogel was proved to be biocompatible, which could stand a high compressive stress (22.81.5 MPa at strain of 93 %). However, to prepare the IPN hydrogel, several freeze-thawed cycles were needed, which made the synthesize process very time-consuming. Cong et al. synthesized the polyacrylamide/GO (PAM/GO) DN hydrogel (Cong et al., 2014). Even that the PAM/GO hydrogel was compressed drastically, it could recover its original shape quickly after being released, but its low fracture strength (150 kPa) was still not satisfying. In addition, the dual crosslinked (DC) system is another choice to obtain tough GO nanocomposite hydrogel. Adding very little N,N′-methylene-bis-acrylamide (BIS) as chemical crosslinked agent, Shi et al. synthesized hybrid poly(N-isopropylacrylamide)/GO dual crosslinked nanocomposite hydrogel with GO sheets acting as physical crosslinked agent, and the obtained DC hydrogel’s fracture strength was as high as 720 kPa (Shi et al., 2015). Using Fe3+ as an ionic crosslinked agent, another dual crosslinked GO/poly(acrylic acid) (GO/PAA) hydrogel exhibited excellent mechanical properties (fracture strength and strain were 777 kPa and 2980 %, respectively) (Zhong, Liu, & Xie, 2015). In this hydrogel, ionic bonds among Fe3+ and carboxyl groups of PAA chains could dissipate lots of energy via breaking and recombining dynamically, while the GO sheets on which the PAA chains absorbed could transfer stress to the whole hydrogel matrix and help to improve the 4
mechanical strength. In addition, adding another nanoparticle, such as hectorite clay, as a cooperative crosslinked agent could also effectively improve the mechanical properties of GO nanocomposite hydrogels (Zhang et al., 2014). In order to further simplify the fabrication process and avoid using toxic heavy metal ions
or
chemical
crosslinked
agents,
herein,
we
reported
a
novel
graphene
oxide/P(acrylamide-co-poly(ethylene glycol) methyl ether methacrylate)/α-cyclodextrin [GO/P(AM-co-PEGMA)/CD] dual crosslinked nanocomposite hydrogel. α-CD is a kind of cyclic oligosaccharides which consists of 6 glucose units and has a hydrophobic inner cavity and hydrophilic surface, and it is also commercially available and water soluble, what’s more, biocompatible (Loethen, Kim, & Thompson, 2007). It has been widely reported that α-CD can form interesting inclusion complex which is called rotaxane with poly(ethylene glycol) (PEG) via the host-guest interaction. And the adjacent rotaxanes tend to contact with each other closely because of hydrogen bonds between α-CDs, which has been utilized to prepare supramolecular hydrogel.(Liu, Zhang, & Li, 2011; Wenz, Han, & Muller, 2006). The GO/P(AM-co-PEGMA)/CD
dual
crosslinked
hydrogel
was
prepared
via
radical
copolymerization of AM and PEGMA in the α-CD/GO solution using one-pot method. In this DC hydrogel, polymer main chains adsorbed on the GO sheets, which formed the first crosslinked system; and multiple hydrogen bonds between neighboring α-CDs that threaded onto
the
PEGMA
side
chains
formed
the
second
crosslinked
system.
The
GO/P(AM-co-PEGMA)/CD hydrogel exhibited excellent tensile property (fracture strength and strain were 660 kPa and 1800 %, respectively) and could be compressed at a large strain of 85 % without rupture. Moreover, reversible physical crosslinked system allowed the hydrogel to be reshaped via thermocompression and endowed the hydrogel with thermal responsive shape memory behavior. We hope that this work can provide inspiration for designing and application of high strength hydrogel. 5
2. Experimental section 2.1. Materials GO was prepared via a modified Hummers method which could be found in our previous work (Zhu, Hu, Deng, & Wang, 2016), and the TEM iamge of GO nanosheets was exhibited in Fig. S1. Monomer acrylamide (AM) was obtained from Shanghai Richjoint Chemical Reagents. poly(ethylene glycol) methyl ether methacrylate (PEGMA, Mw 950 g/moL) was purchased from Shanghai Aladdin Biochemical Technology Co.,Ltd. α-cyclodextrin (α-CD) (HPLC 98 %, Mw 973 g/moL) was purchased from Shanghai Macklin Biochemical Technology Co. Ltd. Potassium peroxydisulfate (KPS) was purchased from Tianjin Fuchen Chemical Reagent Factory. Purified water with resistivity 18.0 Mcm was obtained from a Millipore purification apparatus. 2.2. Preparation of hydrogels GO/P(AM-co-PEGMA)/CD gel was prepared via one-pot in-situ radical polymerization. Typically, 2.6 g AM, 4 mL PEGMA (0.1 g/mL) and 2.9 mL H2O were added to 7.5 mL GO suspension (2 mg/mL) and the mixture was stirred in ice-water bath for 2 h (for different GO/P(AM-co-PEGMA)/CD gel synthesized here, the total concentration of AM and PEGMA was kept constantly as 20 % wt/v). Then the mixture was bubbled with N2 to extrude O2 for 15 min, and 2.86 g α-CD (the molar ratio of α-CD to PEGMA is 7:1) and 0.6 mL KPS solution (50 mg/mL) were added subsequently. The mixture was heated at 45 oC for 15 min to dissolve α-CD under stirring. At last, the pre-gel solution was injected into the glass mould and incubated at 45 oC for 24 h. Under 45 oC, KPS decomposed producing radical and
6
initiated the copolymerization of AM and PEGMA; finally, the GO/P(AM-co-PEGMA)/CD gel was obtained. PAM/GO gel for comparison was also synthesized as the following procedures. 3.0 g AM, 6.9 mL H2O and 7.5 mL GO suspension (2 mg/mL) was mixed together and the mixture was stirred in ice-water bath for 2 h and bubbled with N2 to extrude O2 for 15 min； 0.6 mL KPS solution (50 mg/mL) were added subsequently, then, the pre-gel solution was injected into glass mould and incubated at 45 oC for 24 h to initiate the polymerization of AM, and the PAM/GO gel was obtained. 2.3. Mechanical tests Mechanical tests of obtained gels were carried out using a shimadzu autograph AG-X plus 1KN system under 25 oC in air. For tensile tests, the gel was cut into 3.2 (diameter)45 (length) mm cylinder and the rate of stretch was 100 mm/min. For compress tests, the gel was cut into 13.518 mm cylinder and the compress speed was 2 mm/min. For tearing test, the gel sample was cut into 401010 mm cuboid with a notch of 20 mm. One arm of the gel sample was fixed while the other one was stretched at a speed of 50 mm/min. Silicone oil was coated on the surfaces of gel samples to prevent water evaporation. The tearing energy (T) was calculated according to: 𝑇=
2 𝐹𝑎𝑣𝑒 𝑤
Here, Fave represents the average force of the max values of the steady-state tear curve, and w is the width of the gel sample (here, w=10 mm). Tearing test curves are showed in Fig. S1. 2.4. Rheological measurements
7
Rheological measurements were conducted with stress controlled rheometer AR-G2 (TA) via a parallel plate of 40 mm. For frequency sweep measurements, the frequency ranged from 0.1 to 100 rad/s with a definite strain of 0.5 %. Temperature sweep measurements were conducted with temperature ranging from 30 to 90 oC, and the temperature increasing rate was 1 oC/min. Silicon oil was used to prevent water evaporation during the whole process. 2.5. Shape-memory behavior A straight hydrogel of 460 mm was first heated at 80 oC for 10 min, and then it was curled into the “U” shape and cooled in room temperature to fix the deformation. To demonstrate its shape-memory behavior, the deformed hydrogel was immersed into water of 80 oC, and its recovery process was recorded. 3. Result and discussion 3.1. Synthesis of GO/P(AM-co-PEGMA)/CD gel According to the previous work, rotaxane which is composed of α-CD and PEGMA tends to aggregate together and forms insoluble polyrotaxane (Loethen et al., 2007), which will result in lots of heterogeneities in the obtained hydrogel and weaken the hydrogel’s strength. So, it is necessary to restrain the host-guest interaction between α-CD and PEGMA before the copolymerization of AM and PEGMA to get high strength hydrogel. And based on the published works, the concerted effect of AM monomer and an appropriate temperature could successfully accomplish this goal (Feng, Zhou, Dai, Yasin, & Yang, 2016). Therefore, to synthesize GO/P(AM-co-PEGMA)/CD hydrogel, firstly, AM and PEGMA were added into the GO solution and stirred, during this process, AM tended to adsorb on the GO surface because of the hydrogen bond interaction between the -NH2 groups and oxygen-containing 8
groups on the GO surface (Ferse et al., 2008; Haraguchi, Li, Matsuda, Takehisa, & Elliott, 2005). Then initiator and α-CD were added and stirred at 45 oC (Fig. 1a). The pre-gel solution was pulled into the glass mould and incubated at 45 oC. With the proceeding of reaction, the growing P(AM-co-PEGMA) polymer main chains attached closely to the GO sheets owing to the physical interaction (Ferse et al., 2008; Haraguchi et al., 2005; Zhu et al., 2014), establishing the first crosslinked system (Fig. 1b). At the same time, as more and more AM monomers were fixed onto the polymer chains, in consequence, the shield effect decreased. Therefore, driven by the hydrophobic interaction between ethylene oxide group (-CH2-CH2-O-) and internal cavity of α-CD, the α-CD which located in suitable position would penetrate onto the PEGMA side chains and slide along the chains (Ceccato, Lonostro, & Baglioni, 1997). Other free α-CDs molecules would continue to thread onto the PEGMA side chains in a head-to-head or tail-to-tail orientation, as a result, the rotaxanes formed. (Loethen et al., 2007). During the process, because α-CDs threaded onto the PEGMA side chains, these PEGMA side chains were forced to unfold and stretch, and turned into elongated conformation. At last, neighboring rotaxanes interacted with each other closely and formed micro crystalline domains because of multifold hydrogen bonds between the α-CDs (Fleury et al., 2005; Wei et al., 2005), consequently, the second crosslinked system formed (Fig. 1c).
X-ray diffraction (XRD) has been widely used to investigate the structure of the inclusion complex composed of α-CDs and polymers. As exhibited in Fig. S2, a variety of sharp diffraction peaks (2: 12.118o, 14.296o, 21.665o) represented pure α-CD (Shen et al., 9
2015; Yasin et al., 2015). No peaks were found in PAM/GO gel, indicating that the polymer chains were in disorder state. For GO/P(AM-co-PEGMA)/CD gel, two diffraction peaks (2: 19.675o and 19.939o) represented the characteristic channel-type structure of inclusion complex composed of α-CDs and the PEGMA side chains (Li, He, Zhang, Tam, & Ni, 2015; Wang & Chen, 2007; Wei et al., 2005; Zhou, Ye, Liu, Hu, & Qian, 2015). However, it is possible that there are free α-CDs in the hydrogel.
3.2. Mechanical and rheological properties of GO/P(AM-co-PEGMA)/CD hydrogel As Fig. 2a exhibited, the dual crosslinked system endowed GO/P(AM-co-PEGMA)/CD hydrogel with improved strength. Single crosslinked PAM/GO exhibited favorable ductility (fracture strain of 2800 %) but very weak fracture strength of 100 kPa. It was because that the interaction between PAM chains and GO was not strong enough, so that the PAM chains could easily slide away from the GO surface under small stress. But for GO/P(AM-co-PEGMA)/CD dual crosslinked hydrogel, the microcrystalline formed by rotaxane supplied extra crosslinked points, so much larger stress was needed to destroy it. As a result, GO/P(AM-co-PEGMA)/CD dual crosslinked hydrogel possessed higher fracture strength of 660 kPa. On the other hand, as mentioned in 3.1, the PEGMA side chains turned into elongated conformation during the copolymerization process, which would make polymer chain pre-stretched and result in increased brittleness of GO/P(AM-co-PEGMA)/CD hydrogel. Therefore, the stretchable ability (fracture strain of 1800 %) of the GO/P(AM-co-PEGMA)/CD gel decreased compared to the soft PAM/GO hydrogel. In addition, the GO/P(AM-co-PEGMA) hydrogel which was kept in the sealed environment under room temperature still exhibited almost the same mechanical property with that of the fresh hydrogel (Fig. S3), proving the hydrogel’s favorable stability. 10
Tearing energy and hysteresis loop were two important indexes to characterize the toughness of hydrogel. Fig. 2b showed that GO/P(AM-co-PEGMA)/CD hydrogel exhibited much higher tearing energy (1755 J/m2) than that of PAM/GO hydrogel (152 J/m2). Fig. 2c exhibited the cycle tensile curves of PAM/GO hydrogel and GO/P(AM-co-PEGMA)/CD dual crosslinked hydrogel, and the tearing test curves of the two kinds of hydrogel were showed in Fig. S4. The dual crosslinked hydrogel demonstrated a large hysteresis loop, while the hysteresis loop of PAM/GO was very little. Correspondingly, as Fig. 2d exhibited, more energy (2799 kJ/m3) was dissipated for GO/P(AM-co-PEGMA)/CD hydrogel during the loading-unloading process. As analyzed in other papers, both of higher tearing energy and larger
hysteresis
loop
proved
an
obvious
increment
of
toughness
of
GO/P(AM-co-PEGMA)/CD hydrogel (Lin, Ma, Wang, & Zhou, 2015; Qiang et al., 2015). As demonstrated in Fig. 3a, PAM/GO gel was so soft that the compress strength was only 254.6 kPa at strain of 85 %, and it was much little than that of GO/P(AM-co-PEGMA)/CD hydrogel (2698 kPa) at the same strain. Pictures of the compress-release process of GO/P(AM-co-PEGMA)/CD hydrogel was showed in Fig. 3b. The original gel (Fig. 3b1) which was 13 mm in diameter and 22 mm in height could be compressed dramatically and without any macro rupture (Fig. 3b2). More importantly, after being released, the hydrogel could almost recover to its original shape (Fig. 3b3), which proved that the GO/P(AM-co-PEGMA)/CD hydrogel possessed a good fatigue resistance.
Ratios of α-CD to PEGMA and AM to PEGMA as well as the GO concentration were three crucial factors to determine the properties of GO/P(AM-co-PEGMA)/CD hydrogel. Fig. 11
4a showed the tensile curves of GO/P(AM-co-PEGMA)/CD hydrogel with molar ratios of α-CD to PEGMA ranging from 0:1 to 9:1. When no α-CD was added (0:1), the GO/P(AM-co-PEGMA)/CD hydrogel was single crosslinked, which showed the weakest mechanical strength and fracture strain. Increasing α-CD content could obviously improve the strength of GO/P(AM-co-PEGMA)/CD hydrogel, and the best mechanical property was obtained when the ratio of α-CD to PEGMA was 7:1. To further increase the α-CD content (9:1), the mechanical strength decreased. Fig. 4b exhibited the corresponding fracture dissipated energy of various GO/P(AM-co-PEGMA)/CD hydrogels, and it demonstrated the same trend which could be found in Fig. 4a, that was to say, the fracture dissipated energy increased firstly with increasing α-CD content, while decreased when the α-CD content was excessed. Rheological measurements were also conducted. As showed in Fig. S5, both G and G increased with α-CD content, and decreased when the ratio was larger than 7:1, which was very consistent with the trend exhibited in Fig. 4. Quantitative analysis could explain why the α-CD content had such an effect on the hydrogel precisely. The maximum amount of α-CD that threaded onto one PEGMA side chain could be estimated based on the length of completely stretched PEGMA chain and the height of the α-CD molecule (Ceccato et al., 1997). The molecular weight of PEGMA monomer used here was 950 g/moL, and taking the length of various chemical bonds (C-C, C-O, C=C) and corresponding bond angels into consideration, it could be calculated that the length of a completely stretched PEGMA chain (L) was about 5.426 nm, while the height of α-CD was 7.9 Å (H). So when one PEGMA chain was fully covered by α-CDs, the ratio of 12
α-CD to PEGMA was about 6.86 (L/H≈6.86). And this value was very much close to the best formula when α-CD: PEGMA= 7:1 as discussed above. Therefore, it could be concluded that with the increase of α-CD content, more α-CDs threaded onto the PEGMA forming rotaxane so that more hydrogen bonds formed between neighboring rotaxanes, as a result, the second crosslinked system became stronger. When α-CD: PEGMA= 7:1, the PEGMA was fully covered by α-CD and the hydrogel exhibited best mechanical property. However, when α-CD content was excessed, the shield effect of AM was unable to prevent the α-CD threading onto the PEGMA chains before copolymerization, which would result insoluble polyrotaxane in the pre-gel solution; at the same time, excess free α-CD molecules that didn’t thread onto the PEGMA side chains would also aggregate together and tend to precipitate. Both
the
two
factors
would
form
amounts
of
defects
in
the
obtained
GO/P(AM-co-PEGMA)/CD hydrogel. In fact, obvious phase-separation phenomenon could be found in the GO/P(AM-co-PEGMA)/CD hydrogel when the ratio reached to 9:1, which undoubtedly weakened the mechanical strength of the hydrogel. Further, it maybe supposed that changing the length of the PEGMA chain would be helpful to prepare tougher and stronger hydrogel. Effects of the GO concentration and molar ratio of PEGMA to AM on the GO/P(AM-co-PEGMA) hydrogel’s mechanical property were exhibited in Fig. S6 and Fig. S7. On the one hand, as the crosslinked agent, low GO concentration would result in a weak first crosslinked system, consequently, the hydrogel’s strength was low; on the other hand, excess GO tended to aggregate together, for example, there were lots of macro GO blocks in the obtained GO/P(AM-co-PEGMA)/CD hydrogel when GO concentration was 3 mg/mL, 13
which weakened the strength of the hydrogel seriously (Fig. S6). As Fig. S7 exhibited, too much PEGMA was disadvantage for the tensile performance of GO/P(AM-co-PEGMA)/CD hydrogel; it was because that excess PEGMA (ratio was 1:59) resulted in a densely second crosslinked system which markedly improved the hydrogel’s brittleness, and the fracture strain reduced obviously comparing to that of the hydrogel whose molar ratio of PEGMA to AM was 1:87. On the contrary, when the ratio was too small (1:175), the obtained hydrogel demonstrated similar tensile curve of single crosslinked PAM/GO hydrogel (Fig. 2a), which meant that limited PEGMA resulted in a very loose second crosslinked system and contributed little to improve the mechanical strength of the GO/P(AM-co-PEGMA)/CD hydrogel. And it was important to notice that if there was no PEGMA was added (PEGMA: AM=0:1), the obtained hydrogel showed extreme low fracture strain and fracture strength, as the inset picture of Fig. S7 exhibited. It proved that α-CD could not build the second crosslinked system by itself without PEGMA. Conversely, amounts of free α-CD would seriously weaken the mechanical performance of the hydrogel. 3.3. Thermoplastic property and thermo-responsive behave of GO/P(AM-co-PEGMA)/CD hydrogel It has been reported that the polymer chains could desorb from the nano-sheets surfaces when NC hydrogel was heated at a suitable temperature, making it possible to reshape the hydrogel after polymerization (Cui et al., 2015; Dai et al., 2015). As the fully physical
crosslinked
hydrogel,
Fig.
5a
showed
the
reshape
process
of
GO/P(AM-co-PEGMA)/CD hydrogel. The original hydrogel with a diameter of 13 mm and a height of 15 mm was heated at 80 oC for 10 min; then the hydrogel was compressed and fixed under pressure to keep its reshaped hydrogel. After being cooled at 25 oC, a discal reshaped 14
hydrogel (Fig. 5a2) with a diameter of 35.6 mm and a height of 2 mm was obtained. And as Fig. 5b exhibited, the reshaped hydrogel still exhibited high fracture strength (619 kPa), which was close to the original gel (660 kPa). As Fig. 6 exhibited, the temperature sweep measurement proposed a precise dynamic change process of the storage modulus (G) and the loss modulus (G) during the heating-cooling process. G decreased dramatically from 238.8 kPa to 14 kPa with increasing temperature; when being cooled to 30 oC, G recovered to 224.8 kPa. And G exhibited a similar change trend with that of the G. The decrease of G meant the crosslinked density declined, which was because that the polymer chains desorbed from GO surface and the multiple hydrogen bonds between adjacent rotaxanes were damaged. During the cooling process, the polymer chains re-adsorbed on the GO surfaces, and hydrogen bonds formed again. And it was interesting to notice that there were obvious hysteresis loops in the temperature sweep curves, especially for G. It was most likely because that both re-adsorption and re-forming of hydrogen bonds were time-consuming, which was also observed in other thermoplastic hydrogel (Feng et al., 2016). Fig. 7 exhibited the GO/P(AM-co-PEGMA)/CD hydrogel’s thermo-responsive shape memory behavior. A straight hydrogel gel was first curled into temporary “U” shape via heating-cooling process, and then the deformed hydrogel was immersed into 80 oC hot water. In the hot water, hydrogen bonds were damaged and failed to keep temporary shape(Feng et al., 2016; Yasin et al., 2015). At last, the hydrogel almost recovered to its original straight shape. 4. Conclusions In this work, we synthesized a novel GO/P(AM-co-PEGMA)/CD dual crosslinked 15
hydrogel using one-pot method. The obtained hydrogel’s fracture strain and strength were 1800 % and 660 kPa, and could be compressed at a large strain of 85 % without rupture. Ratios of α-CD to PEGMA and PEGMA to AM as well as the GO concentration were proved to have an important effect on the performance of GO/P(AM-co-PEGMA)/CD hydrogel. In addition, dual physical crosslinked system endowed the hydrogel with favorable thermoplastic ability and thermo-responsive shape memory behavior. The method avoided using chemical crosslinked agent and heavy metal ions which were harmful for human body, and the synthesize process was simple and time-saving. Therefore, it is expected that this work can provide inspiration for designing and application of high strength hydrogel. Acknowledgements This research was supported by the National Natural Science Foundation of China (21474032) and the Natural Science Foundation of Guangdong Province (2016A030311031). Appendix A. Supplementary data References Bean, J. E., Alves, D. R., Laabei, M., Esteban, P. P., Thet, N. T., Enright, M. C., et al. (2014). Triggered release of bacteriophage k from agarose/hyaluronan hydrogel matrixes by staphylococcus aureus virulence factors. Chemistry of Materials, 24, 7201-7208. Casolaro, M., & Casolaro, I. (2015). Controlled release of antidepressant drugs by multiple stimuli-sensitive hydrogels based on alpha-aminoacid residues. Journal of Drug Delivery Science and Technology, 30, 82-89. Ceccato, M., LoNostro, P., & Baglioni, P. (1997). Alpha cyclodextrin/polyethylene glycolpolyrotaxane: A study of the threading process. Langmuir, 9, 2436-2439. Chen, Q., Zhu, L., Zhao, C., Wang, Q., & Zheng, J. (2013). A robust, one-pot synthesis of highly mechanical and recoverable double network hydrogels using thermoreversible sol-gel polysaccharide. Advanced Materials, 30, 4171-4176. Cong, H.-P., Wang, P., & Yu, S.-H. (2014). Highly elastic and superstretchable graphene oxide/polyacrylamide hydrogels. Small, 3, 448-453. Cui, W., Zhang, Z.-J., Li, H., Zhu, L.-M., Liu, H., & Ran, R. (2015). Robust dual physically crosslinked hydrogels with unique self-reinforcing behavior and improved dye adsorption capacity. Rsc Advances, 65, 52966-52977. Dai, X., Zhang, Y., Gao, L., Bai, T., Wang, W., Cui, Y., et al. (2015). A mechanically strong, highly stable, thermoplastic, and self-healable supramolecular polymer hydrogel. Advanced Materials, 23, 3566-3571. 16
Ding, J., Zhuang, X., Xiao, C., Cheng, Y., Zhao, L., He, C., et al. (2011). Preparation of photo-crosslinked ph-responsive polypeptide nanogels as potential carriers for controlled drug delivery. Journal of Materials Chemistry, 30, 11383-11391. Du, G., Nie, L., Gao, G., Sun, Y., Hou, R., Zhang, H., et al. (2015). Tough and biocompatible hydrogels based on in situ interpenetrating networks of dithiol-connected graphene oxide and poly(vinyl alcohol). Acs Applied Materials & Interfaces, 5, 3003-3008. Feng, W., Zhou, W., Dai, Z., Yasin, A., & Yang, H. (2016). Tough polypseudorotaxane supramolecular hydrogels with dual-responsive shape memory properties. Journal of Materials Chemistry B, 11, 1924-1931. Ferse, B., Richter, S., Eckert, F., Kulkarni, A., Papadakis, C. M., & Arndt, K.-F. (2008). Gelation mechanism of poly(n-isopropylacrylamide)-clay nanocomposite hydrogels synthesized by photopolymerization. Langmuir, 21, 12627-12635. Fleury, G., Brochon, C., Schlatter, G., Bonnet, G., Lapp, A., & Hadziioannou, G. (2005). Synthesis and characterization of high molecular weight polyrotaxanes: Towards the control over a wide range of threaded alpha-cyclodextrins. Soft Matter, 5, 378-385. 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, 8, 5029-5037. Gong, J. P., Katsuyama, Y., Kurokawa, T., & Osada, Y. (2003). Double-network hydrogels with extremely high mechanical strength. Advanced Materials, 14, 1155-1161. Haraguchi, K., Li, H. J., Matsuda, K., Takehisa, T., & Elliott, E. (2005). Mechanism of forming organic/inorganic network structures during in-situ free-radical polymerization in pnipa-clay nanocomposite hydrogels. Macromolecules, 8, 3482-3490. Hong, S., Sycks, D., Chan, H. F., Lin, S., Lopez, G. P., Guilak, F., et al. (2015). 3d printing: 3d printing of highly stretchable and tough hydrogels into complex, cellularized structures. Advanced Materials, 27, 4034-4034. Huang, Y., Zhang, M., & Ruan, W. (2014). High-water-content graphene oxide/polyvinyl alcohol hydrogel with excellent mechanical properties. Journal of Materials Chemistry A, 27, 10508-10515. Huang, Z., Wu, Q., Liu, S., Liu, T., & Zhang, B. (2013). A novel biodegradable beta-cyclodextrin-based hydrogel for the removal of heavy metal ions. Carbohydrate Polymers, 2, 496-501. Jinchen, F., Zixing, S., Min, L., Hong, L., & Jie, Y. (2013). Mechanically strong graphene oxide/sodium alginate/polyacrylamide nanocomposite hydrogel with improved dye adsorption capacity. Journal of Materials Chemistry A, 25, 7433-7443. Kamata, H., Akagi, Y., Kayasuga-Kariya, Y., Chung, U.-i., & Sakai, T. (2014). "Nonswellable" hydrogel without mechanical hysteresis. Science, 6173, 873-875. Kumar, P. T. S., Srinivasan, S., Lakshmanan, V.-K., Tamura, H., Nair, S. V., & Jayakumar, R. (2011). Beta-chitin hydrogel/nano hydroxyapatite composite scaffolds for tissue engineering applications. Carbohydrate Polymers, 3, 584-591. Li, F., He, J., Zhang, M., Tam, K. C., & Ni, P. (2015). Injectable supramolecular hydrogels fabricated from pegylated doxorubicin prodrug and alpha-cyclodextrin for ph-triggered drug delivery. Rsc Advances, 67, 54658-54666. Lin, P., Ma, S., Wang, X., & Zhou, F. (2015). Molecularly engineered dual-crosslinked hydrogel with ultrahigh mechanical strength, toughness, and good self-recovery. Advanced Materials, 12, 2054-2059. Liu, J., Chen, C., He, C., Zhao, L., Yang, X., & Wang, H. (2012). Synthesis of graphene peroxide and its application in fabricating super extensible and highly resilient nanocomposite hydrogels. Acs Nano, 9, 17
8194-8202. Liu, K. L., Zhang, Z., & Li, J. (2011). Supramolecular hydrogels based on cyclodextrin-polymer polypseudorotaxanes: Materials design and hydrogel properties. Soft Matter, 24, 11290-11297. Loethen, S., Kim, J.-M., & Thompson, D. H. (2007). Biomedical applications of cyclodextrin based polyrotaxanes. Polymer Reviews, 3, 383-418. Okumura, Y., & Ito, K. (2001). The polyrotaxane gel: A topological gel by figure-of-eight cross-links. Advanced Materials, 7, 485-492. Peng, X.-W., Zhong, L.-X., Ren, J.-L., & Sun, R. C. (2012). Highly effective adsorption of heavy metal ions from aqueous solutions by macroporous xylan-rich hemicelluloses-based hydrogel. Journal of Agricultural and Food Chemistry, 15, 3909-3916. Qiang, C., Lin, Z., Hong, C., Hongli, Y., Lina, H., Jia, Y., et al. (2015). A novel design strategy for fully physically linked double network hydrogels with tough, fatigue resistant, and self-healing properties. Advanced Functional Materials, 10, 1598-1607. Ren, K., He, C., Xiao, C., Li, G., & Chen, X. (2015). Injectable glycopolypeptide hydrogels as biomimetic scaffolds for cartilage tissue engineering. Biomaterials, 7, 238-249. Sakai, T., Matsunaga, T., Yamamoto, Y., Ito, C., Yoshida, R., Suzuki, S., et al. (2008). Design and fabrication of a high-strength hydrogel with ideally homogeneous network structure from tetrahedron-like macromonomers. Macromolecules, 14, 5379-5384. Shen, J., Pang, J., Kalwarczyk, T., Holyst, R., Xin, X., Xu, G., et al. (2015). Manipulation of multiple-responsive fluorescent supramolecular materials based on the inclusion complexation of cyclodextrins with tyloxapol. Journal of Materials Chemistry C, 31, 8104-8113. Shi, K., Liu, Z., Wei, Y.-Y., Wang, W., Ju, X.-J., Xie, R., et al. (2015). Near-infrared light-responsive poly(n-isopropylacrylamide)/graphene oxide nanocomposite hydrogels with ultrahigh tensibility. Acs Applied Materials & Interfaces, 49, 27289-27298. Teng, C., Qiao, J., Wang, J., Jiang, L., & Zhu, Y. (2016). Hierarchical layered heterogeneous graphene-poly(n-isopropylacrylamide)-clay hydrogels with superior modulus, strength, and toughness. Acs Nano, 1, 413-420. Wang, Z., & Chen, Y. (2007). Supramolecular hydrogels hybridized with single-walled carbon nanotubes. Macromolecules, 9, 3402-3407. Wei, H. L., Yu, H. Q., Zhang, A. Y., Sun, L. G., Hou, D. D., & Feng, Z. G. (2005). Synthesis and characterization of thermosensitive and supramolecular structured hydrogels. Macromolecules, 21, 8833-8839. Wenz, G., Han, B. H., & Muller, A. (2006). Cyclodextrin rotaxanes and polyrotaxanes. Chemical Reviews, 3, 782-817. Xu, B., Li, H., Wang, Y., Zhang, G., & Zhang, Q. (2013). Nanocomposite hydrogels with high strength crosslinked by titania. Rsc Advances, 20, 7233-7236. Yasin, A., Zhou, W., Yang, H., Li, H., Chen, Y., & Zhang, X. (2015). Shape memory hydrogel based on a hydrophobically-modified polyacrylamide (hmpam)/alpha-cd mixture via a host-guest approach. Macromolecular Rapid Communications, 9, 845-851. Zhang, E., Wang, T., Zhao, L., Sun, W., Liu, X., & Tong, Z. (2014). Fast self-healing of graphene oxide-hectorite clay-poly(n,n-dimethylacrylamide) hybrid hydrogels realized by near-infrared irradiation. Acs Applied Materials & Interfaces, 24, 22855-22861. Zhang, Y., Liu, J., Huang, L., Wang, Z., & Wang, L. (2015). Design and performance of a sericin-alginate interpenetrating network hydrogel for cell and drug delivery. Scientific Reports, 5. No. 12374. Zhao, C., Chen, Q., Patel, K., Li, L., Li, X., Wang, Q., et al. (2012). Synthesis and characterization of ph-sensitive 18
poly(n-2-hydroxyethyl acrylamide)-acrylic acid (poly(heaa/aa)) nanogels with antifouling protection for controlled release. Soft Matter, 30, 7848-7857. Zhong, M., Liu, Y.-T., & Xie, X.-M. (2015). Self-healable, super tough graphene oxide-poly(acrylic acid) nanocomposite hydrogels facilitated by dual crosslinked effects through dynamic ionic interactions. Journal of Materials Chemistry B, 19, 4001-4008. Zhou, M., Ye, X., Liu, K., Hu, J., & Qian, X. (2015). Tunable thermo-responsive supramolecular hydrogel: Design, characterization, and drug release. Journal of Polymer Research, 9, 1-8. Zhu, P., Hu, M., Deng, Y., & Wang, C. (2016). One-pot fabrication of a novel agar-polyacrylamide/graphene oxide nanocomposite double network hydrogel with high mechanical properties Advanced Engineering Materials, 10, 1799-1807. Zhu, Z., Song, G., Liu, J., Whitten, P. G., Liu, L., & Wang, H. (2014). Liquid crystalline behavior of graphene oxide in the formation and deformation of tough nanocomposite hydrogels. Langmuir, 48, 14648-14657.
19
b
a
c
Fig. 1. Schematic of the preparation process and dual crosslinked mechanism of GO/P(AM-co-PEGMA)/CD hydrogel.
20
1800 800
a
1500
Tearing energy (J/m2)
PAM/GO gel GO/P(AM-co-PEGMA)/CD gel
Stress (kPa)
600
400
200
0
0
500
1000
1500
2000
2500
3000
Strain (%) PAM/GO gel GO/P(AM-co-PEGMA)/CD gel
1200 900 600 300 0
PAM/GO gel
GO/P(AM-co-PEGMA)/CD gel
3000
c
2500
400
Uhys (kJ/m3)
Stress (kPa)
600
b
200
d
2000 1500 1000 500
0 0
500
0
1000
PAM/GO gel
GO/P(AM-co-PEGMA)/CD gel
Strain (%)
Fig. 2. (a) Tensile curves, (b) tearing energy, (c) loading-unloading curves at strain of 1000 % and (d) corresponding dissipated energy of PAM/GO gel, and GO/P(AM-co-PEGMA)/CD gel, respectively.
21
3000
a
PAM/GO gel GO/P(AM-co-PEGMA)/CD gel
Stress (kPa)
2500 2000 1500 1000 500 0
0
20
40
60
80
100
Strain (%)
b1
b3
b2
Fig. 3 (a) Compression curves of PAM/GO gel and GO/P(AM-co-PEGMA)/CD gel; (b)
750
0:1 1:1 3:1 5:1 7:1 9:1
Stress (kPa)
600
450
a
300
150
0
0
500
1000
1500
2000
Fracture dissipated energy (kJ/m 3)
pictures of (b1) original, (b2) compressed and (b3) released GO/P(AM-co-PEGMA)/CD gel.
1000
b 800
600
400
200
0
0:1
1:1
3:1
5:1
7:1
9:1
Strain (%)
Fig. 4. (a) Tensile curves and (b) fracture dissipated energy of GO/P(AM-co-PEGMA)/CD gel with different molar ratios of α-CD to PEGMA; the concentration of GO 1mg/mL and the 22
molar ratio of PEGMA to AM was 1:87.
a1
a2 Original gel
Stress (kPa)
600
Reshaped gel
b
400
Reshaped gel 200
0
0
200
400
600
800
Strain (%)
Fig.
5
(a)
Thermoplastic
property
and
GO/P(AM-co-PEGMA)/CD hydrogel
23
(b)
tensile
curve
of
reshaped
250
ing at He
150
G' G''
ng oli Co
G' & G'' (kPa)
200
100 50 0 30
Heating Cooling 40
50
60
70
80
90
Temperature (°C)
Fig. 6. Temperature sweep of GO/P(AM-co-PEGMA)/CD gel from 30-90 oC.
5s
0s
11 s
Fig. 7. Pictures of thermo-responsive shape memory behavior of GO/P(AM-co-PEGMA)/CD gel
24