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f Sci

Keywords:Self-healing

ge(Dol

multi-stimuli reversible responsiveness, such as thermo-, acid/base-, and chemo- induced gelesol

ns havry, ander mat

polymers after being damaged or healing agents [14e16] to formnew polymeric materials in the destroyed area. Moreover, much ofthe reportedmaterials needed long time to heal the materials [5,17]and the number of breaking and healing cycles was limited [14].Although signicant progress has been made in this area, it is stillurgently to develop a systemwith intrinsic self-healing properties.

and functionalitynon-covalent in-idate to constructecently, Guan [33]onding to prepareproperties and re-eal. Becauseof the, hosteguest in-being widely used

in the construction of smart materials with stimuli-responsiveness[34e38] and shape-memory [39,40] properties for their diversebinding selectivity and reversible stimuli responsiveness. Especially,the construction of supramolecular polymer gels, which are pre-cisely designed physical gels brought together by reversible sec-ondary interactions to form three dimensional networks of meltmacromolecules [41], has attracted considerable attention. How-ever, most of them focused on their stimuli-responsiveness and fewof them showed the self-healing property [42,43]. Moreover, the

* Corresponding author.

Contents lists availab

ym

els

Polymer 54 (2013) 6929e6935E-mail address: cchen@iccas.ac.cn (C.-F. Chen).sources wasted and environment pollutions resulted by its nature.However, polymers with self-healing properties could signicantlyextend the lifetimes of materials [1], which would thus cut the costand correspondingly reduce demand on resources and environ-mental impact. During the past decades, scientists and engineershavepaidmoreandmoreattentions to constructmaterialswith self-healing properties [2,3]. Most of the reported approaches requiredeither an input of external energy in the form of heating [4e9] orlight [10e13] to induce reformation of the covalent bonds between

external stimuli [32] but also retained stabilitysimilar to covalently bonded systems. Thus,teractions could be considered as a good candmaterials with intrinsic self-healing properties. Rand co-workers reported the use of hydrogen-bself-repairing polymers with good mechanicalquiresnoexternal inputof energyormaterials tohcombination of multiple non-covalent bondsteractions have drawn considerable interest andtogether by irreversible covalent bonds, are usually unable to self-repaired under continuous sustaining damage and causing re-

inherent reversible properties of these weak interactions, materialslinked by non-covalent bond not only exhibited responsiveness toHosteguest interactionSupramolecular polymer gel

1. Introduction

Materials with advanced functiocontemporary technology and industour daily life. Conventional polym0032-3861/$ e see front matter 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymer.2013.10.048transitions. Furthermore, the result of rheological measurements showed the gel has an intrinsic self-healing property, and the thixotropic process could be repeated at least three times. Interestingly,when doped with spiropyran molecules, the supramolecular polymer gel could also be employed aserasable materials. Thus, these results could be highly anticipated to benet for further construction ofsmart materials with high healing efciency, and ultimately be used in practical application.

2013 Elsevier Ltd. All rights reserved.

e been widely used inhave a great impact onerials, which are held

During the past decade, supramolecular chemistry has beenattractive in materials science, especially on construction of thematerials based on non-covalent interactions including hydrogenbonding [18e21], metaleligand interactions [22e28], ionic in-teractions [29e31] and pep interactions [8,9]. Owing to theAvailable online 1 November 2013

Accepted 28 October 2013

by the H NMR spectroscopy and solution viscometry, and we also obtained a colorless and transparentsupramolecular polymer gel at high concentration. Moreover, the supramolecular polymer gel showedSupramolecular polymer gel with multihealing and erasable properties generat

Fei Zeng a, Ying Han a, Zhi-Chao Yan b, Chen-Yang LaBeijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Rof Sciences, Beijing 100190, ChinabCAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy o

a r t i c l e i n f o

Article history:Received 22 August 2013Received in revised form19 October 2013

a b s t r a c t

A supramolecular polymertaining dibenzylammoniumthe formation of the supram

1

Pol

journal homepage: www.All rights reserved.imuli responsive, self-by hosteguest interactions

b, Chuan-Feng Chen a,*

nition and Function, Institute of Chemistry, Chinese Academy

ences, Beijing 100190, China

l formed between triptycene-based bis(crown ether) and copolymer con-BA) moieties by hosteguest interactions was described. We demonstratedecular polymer networks between the bis(crown ether) and the copolymer

le at ScienceDirect

er

evier .com/locate/polymer

er 54materials that showed bothmulti-stimuli reversible responsivenessand intrinsic fast self-healing properties had rarely been reported.

Herein, we report the formation of supramolecular polymer gelby hosteguest interactions between a triptycene-based bis(crownether) host 1 and copolymer 2 containing dibenzylammoniummoieties (Fig. 1). We found that this gel showed multi-stimulireversible responsiveness such as thermo-, pH-, and chemo-induced gelesol transitions. Especially, this material representedfast self-healing property without input of external energy orhealing agents, and the breaking and healing cycles could maintainat least three times. Moreover, the supramolecular polymer geldoped with spiropyran molecules could also be used as an erasablematerial. It is highly anticipated that the results presented herewould benet for the further construction of materials with highhealing efciency and ultimately be used in practical application.

2. Experimental

2.1. Characterization methods

1H NMR and 13C NMR spectra were recorded on a BrukerDMX300 NMR spectrometer. The Mn and polydispersity index ofcopolymer 2 were determined by gel permeation chromatography(GPC) (Waters Co.) using polystyrene (PS) as standard and dime-thylformamide (DMF) as eluent. The reduced viscosities of copol-ymer 2with host 1 and copolymer 2with DB24C8 at (25 0.05) Cwere performed on the Ubbelohde viscometer. Field emissionscanning electron micrography (FE-SEM, Hitachi S-4800) was usedto observe the morphologies of the xerogels at an acceleratingvoltage of 10 kV. Before the experiment, samples were prepared bydrop casting the suspension of free-dried gels on silicon substratesthen coating with Pt. Rheological dates were obtained from anAR2000ex strain-controlled rheometer using 20 mm parallel plategeometry at 25 C and analyzed with rheology advantage dataanalysis software. The samples were placed between the para-plateand the platformwith special care to avoid evaporation of solvents.The recovery properties of the samples in response to applied shearforces were followed the following programmed procedure(applied shear force, expressed in terms of strain; duration in pa-rentheses): 0.1% (300 s)/ 100% (300 s)/ 0.1% (600 s)/ 100%(300 s)/ 0.1% (600 s)/ 100% (300 s)/ 0.1% (1200 s).

2.2. Synthesis of copolymer 2

The mixture of monomer 3 (0.74 g, 1.56 mmol), methyl acrylate(4.03 g, 46.8 mmol) and AIBN (15 mg, 0.09 mmol) were dissolved inDMF (6 mL) in a Schlenk tube, and the mixture solution wasdegassed by bubbling nitrogen for 20 min. Then, the Schlenk tubewas sealed and heated in an oil bath set to 65 C for 12 h. Aftercooling to room temperature, the polymer was precipitation fromcold diethyl ether as colorless viscous liquid. Discarding the upperuid, and then vacuum drying afforded the copolymer 2 as acolorless and transparent solid (3.10 g, 65%). 1H NMR (300 MHz,CDCl3, 295 K): d 7.99e7.34 (br, 9H, DBA aryl-H), 7.13e6.92 (br, 4H,St-H), 4.98 (br, 2H, St-CH2), 4.21 (br, 4H, DBA-CH2), 3.66e3.17 (br,27H, ester-OCH3), 1.81e1.21 (br, 39H, polymer backbone).

3. Results and discussion

3.1. Synthesis and characterization of copolymer 2

The synthesis of copolymer 2 bearing pendant dibenzylammo-nium moiety was achieved by free-radical copolymerization ofmonomer 3 [36] and methyl acrylate in DMF in the presence of

F. Zeng et al. / Polym6930AIBN (Scheme 1). The molar ratio of DBA to ester moiety in thecopolymer estimated from the 1H NMR spectroscopy was 1:9. Fromthe gel permeation chromatography (GPC) measurement, thenumber-average molecular weight (Mn) of the copolymer is64.8 kDa, and the polydispersity index (PDI) value is 1.18 (Fig. S2,Supporting Information).

3.2. Self-assembly of host 1 and copolymer 2

In the previous work [44], we reported a novel triptycene-basedbis(crown ether) host 1 incorporating two dibenzo-24-crown-8ethermoieties, and found that it could form a 1:2 complexwith twomolecules of dibenzy-lammonium (DBA) salt 4, and the complex-ation process was also well established. As copolymer 2 bearingdibenzylammonium moieties, complexation between host 1 andcopolymer 2 could be occurred and 1H NMR spectroscopy was usedto investigate the complexation process. As shown in Fig. 2, afterthe mixture of Host 1 and copolymer 2 at a 1:2 M ratio of host 1/DBA units in CDCl3/CD3CN (1:1, v/v), the signals from protons H1, H2and H3 of Host 1 shifted upeld, and the signals corresponding tothe methylene protons H4 and H5 of copolymer 2 showed signi-cant downeld shift. These observations suggested that the sec-ondary ammonium salt moiety of copolymer 2 was located in thecenter of the dibenzo-24-crown-8 ether of host 1, which couldresult in the formation of cross-linked supramolecular polymernetworks ultimately. Moreover, it was also found that the protonsof H1-5 showed two sets of signals, corresponding to the complexedand uncomplexed molecules, which indicated that the complexa-tion between host 1 and the DBA units of copolymer 2 was a slowexchange process on the NMR spectroscopy timescales. These re-sults are consistent with the complexation between host 1 and thedibenzylammonium salts as reported before [44].

Self-assembly of host 1 and copolymer 2 to form supramolecularpolymer networks in solution was also studied by viscometry. Asshown in Fig. 3, no obvious changes of reduced viscosity withvaried concentrations of dibenzo-24-crown-8 (DB24C8) andcopolymer 2 at a 1:1 M ratio of DB24C8/DBA in CHCl3/CH3CN wereobserved, indicating that no signicant physical entanglementsoccurred. However, the reduced viscosity of host 1 and copolymer 2at a 1:2 M ratio of host 1/DBA units mixed in CHCl3/CH3CNincreased exponentially with varied concentrations, which sug-gested the self-assembly process occurred and ultimately formedthe cross-linked supramolecular polymer networks. Hence, it hasbeen proved that the self-assembly of host 1 and the DBA units ofcopolymer 2 is undoubtedly dispensable for the formation of thecross-linked supramolecular polymer networks.

3.3. Gelation and multi-stimuli reversible responsivity of thesupramolecular polymer gel

Interestingly, it was found that a colorless and transparentorganogel could be formed when host 1 and copolymer 2 weremixed in CHCl3/CH3CN (v/v 1:1) at high concentration (Fig. S3).Such phenomenon represents the direct support for the formationof supramolecular polymer networks and thereby entraps a largeamount of solvent. Also, scanning electron microscope was used toinvestigate the microstructure of supramolecular gel, and the re-sults showed that the porous structure could be observed, whichconrmed the formation of supramolecular polymer networks(Fig. S4).

Hosteguest interactions with multiple non-covalent bondingonly require relatively low activation energy to be broken, whichmade the gel be sensitive to temperature. As expected, heating thesupramolecular polymer gel in the vial above 60 C led to the for-mation of colorless uid solution. Subsequent cooling to the room

(2013) 6929e6935temperature restored the gel state immediately (Fig. 4b). Thus, the

Scheme 1. Synthesis of copolymer 2.

Fig. 1. a) The proton designations of host 1 and formation of a 1:2 complex between hostsupramolecular polymer gel between host 1 and copolymer 2.

F. Zeng et al. / Polymer 54 (2013) 6929e6935 6931reversible gelesol transition induced by temperature could beachieved. It was known that the association and disassociation ofthe complex between DB24C8 and secondary ammonium saltscould be chemically controlled by pH, which inspired us to furthercarried out the pH-induced reversible gelesol transition. As shownin Fig. 4d, the colorless uid solution was observed when 2.2 equiv(per host 1) of triethylamine (TEA) was added to the gel, owing tothe deprotonation of the DBA salts and ultimately decomplexationof the complex. As soon as the addition of 2.4 equiv (per host 1) oftriuoroacetic acid (TFA) to the above solution, the supramolecularpolymer gel could be formed again due to the reformation of thesupramolecular polymer networks after protonation of the sec-ondary amine in the copolymer. The 1H NMR experiment alsodemonstrated this reversible process. Upon the addition of 2.2equiv. (per host 1) of triethylamine (TEA) to the solution of host 1and copolymer 2 in CDCl3/CD3CN, all the complexed signals dis-appeared. After 2.4 equiv. of triuoroacetic acid were added, theproton signals for complexation were observed again (Fig. S5).

Furthermore, the reversible solegel transition could be trig-ged by chemical stimuli. Since DB24C8 showing higher bindingafnity toward potassium cation K than secondary ammonium

1 and guest 4. b) Cartoon representations of copolymer 2 and host 1. c) Formation of

from the paraquat in the form of use the gel as drug delivery.

host

F. Zeng et al. / Polymer 546932salts [40], adding 4.0 equiv. (per host 1) of KPF6 to the supra-molecular gel led to a transparent colorless solution. When 6.0equiv. (per host 1) of 18-crown-6 was added, which couldbind K more tightness than DB24C8, the reformation of the gelwould appear (Fig. 4d). Protecting groups such as (Boc)2O arealso used to investigate the reversible solegel transition. Afteraddition of 4.0 equiv. (per host 1) of (Boc)2O and catalytic amountof DMAP to the gel, it resulted in the collapse of the supramo-lecular gel. The subsequent addition of acetic acid could restorethe gel state. More interestingly, the reversible solegel transitioncould also be achieved by the competitive guest. The apparentassociation constant of host 1 and paraquat (Ka,exp 1.63(0.3) 104 M1) was higher than the constant between host 1

Fig. 2. Partial 1H NMR spectra (300 MHz, CDCl3:CD3CN 1:1, 295 K) of a) free host 1, b)uncomplexed moieties, respectively), and c) free copolymer 2. [1]0 4.0 mM.and DBA (Kap,1 720 (180) M1 and Kap,2 77 (22) M1) [44].Thus, adding of 1.0 equiv. (per host 1) of paraquat to the gel couldcause gel collapse immediately, due to the formation of more

0 5 10 15 20 250

50

100

150

200

250

300

copolymer 2+host 1 copolymer 2+DB24C8

re

d / m

l g-1

C / g L-1

Fig. 3. Variation of reduced viscosities (CHCl3/CH3CN 1:1, 25 C) as a function of totalconcentrations obtained for supramolecular polymer networks of 1 and copolymer 2(1:2 M ratio for host 1/DBA in 2), and the control mixture of DB24C8 and copolymer 2(1:1 M ratio for DB24C8/DBA in 2).3.4. Rheological measurements and self-healing property ofsupramolecular gel

Rheological experiments were further carried out to investigatethe viscoelastic properties of supramolecular polymer gel. Asstable complex between host 1 and paraquat. When 2.0 equiv ofhost 1 was added, the supramolecular polymer gel could bereformed. The reversible solegel transition induced by compet-itive guest has rarely been reported, and it would be found po-tential applications in the cure of patients that could be suffered

1 and copolymer 2 (molar ratio 1:2 host 1/DBA units, c and uc denote complexed and

(2013) 6929e6935shown in Fig. 5a, the oscillatory rheological measurements showedthat at lower scanning frequency, G0 (storage modulus) is smallerthan G00 (loss modulus), which suggested the viscous property ofthe gel. While at higher scanning frequency, G0 is larger than G00,and the elastic property of the samplewas demonstrated. As shownin Fig. 5b, strain amplitude sweeps of the samples demonstrated anelastic response typical of gel. The value of G0 rapidly decreasesabove the critical strain region (g 26.0%), indicating a collapse ofthe gel state to a quasi-liquid state. Moreover, the gel can fastrecover its mechanical strength without either input of externalenergy or healing agents after destroyed by large-amplitude oscil-latory, known as thixotropy. Once a large-amplitude oscillatoryforcewas applied (g 100%), the value of G0 decreased from 1341 to404 Pa (Pa), resulting in the collapse of the gel. The value of G0

immediately recovers to nearly 90% of its initial value once the highstrain was canceled. Furthermore, the thixotropic process could berepeated several times (Fig. 5c). This excellent self-healing propertyof the supramolecular gel might be resulted from the introducing ofhosteguest interactions to the materials. The resulted present herewould provide an opportunity for the construction of materialswith self-healing properties.

The self-healing property of the supramolecular polymer gel canalso be observed in sight. Considering the observation of the self-healing property of the gel visually and directly, we placed thegel in the bottom of the vial and damaged it with a knife thenturned it upside down. As shown in Fig. 6, it was found that the gelcould repair itself in a short time after the destroyed.

Fig. 4. Supramolecular polymer gel responding to external stimulus (16.37 mg host 1 and 40.4 mg copolymer 2 in 1.1 mL CHCl3/CH3CN 1/1). a) Guest-induced gelesol transition,b) thermo-induced gelesol transition, c) chemical-induced gelesol transition, d) K-induced gelesol transition, and e) acid/base-induced gelesol transition.

0.1 1 10 1000.1

1

10

100

1000

10000

G' G''

G', G

'' (Pa

)

Frequency (rad/s)

0 500 1000 1500 2000 2500 3000 3500 400010

100

1000

10000 G' G''

G', G

'' (Pa

)

Time (s)0.1 1 10 100

0.1

1

10

100

1000

10000

G', G

'' (Pa

)

G' G''

Frequency (rad/s)

0.1 1 10 100

100

1000 G' G''

G', G

'' (Pa

)

strain %

Fig. 5. Rheological properties of the supramolecular polymer gel. (a) G0 and G00 values of the supramolecular polymer gel on frequency sweep. (b), (c) G0 and G00 values of thesupramolecular gel on strain sweep (b) and in continuous step strain measurements (c). (d) G0 and G00 values of the supramolecular gel on frequency sweep after several damagetest.

F. Zeng et al. / Polymer 54 (2013) 6929e6935 6933

3.5. Supramolecular polymer gel used as erasable material

Spiropyran (SP) molecules [45] are an important class ofphotochromic compounds and have been investigated extensivelyfor optical memories, switches, and displays [46,47], owing to the

reversible changes between open form and the closed form undervisible or UV light irradiation. SP-functionalized macromoleculargels [48] and low-molecular-weight gelators (LMWGs) with the SPmoiety have been reported [49e51]. However, they always havesynthetic difculties encountered in the preparation of SP-

Fig. 6. Photographs of the supramolecular gel with self-healing process.

F. Zeng et al. / Polymer 54 (2013) 6929e69356934Fig. 7. a) Photochromic reactions of spiropyran molecule under UV and visible light irradexposure to UV light (365 nm) for 5.0 min, and erasing the recorded pattern by exposurespiropyran molecule between UV and Visible light.iation. b) Photographs of the supramolecular gel recording of ICCAS characters byto visible light for 1.0 min c) The changes of maximum absorption (561 nm) of the

functionalized molecules. Here, we used the doping method(described in the Supporting Information) conveniently to pre-pared supramolecular polymer gel containing spiropyran mole-cules and showed well photochromic property. As shown in Fig. 7a,the colorless transparent gel was covered with a black paperengraved with the characters ICCAS and these characters turn

[3] Ghosh SK. Self-healing materials: fundamental, design strategies, and appli-cations. Weinheim: Wiley-VCH; 2009.

[4] Chen X, Dam MA, Ono K, Mal A, Shen H, Nutt SR, et al. Science 2002;295:1698e702.

[5] Liu YL, Hsieh CY. J Polym Sci Part A Polym Chem 2006;44:905e13.[6] Zhang Y, Broekhuis AA, Picchioni F. Macromolecules 2009;42:1906e12.[7] Cordier P, Tournilhac F, Souli-Ziakovic C, Leibler L. Nature 2008;451:977e80.[8] Burattini S, Greenland BW, Merino DH, Weng W, Seppala J, Colquhoun HM,

et al. J Am Chem Soc 2010;132:12051e8.

F. Zeng et al. / Polymer 54 (2013) 6929e6935 6935into pink after exposure to UV light for 5.0 min. While under thevisible light, the characters disappeared within 1.0 min. Moreover,these changes could also be monitored by the changes of the ul-traviolet absorption spectrum of the supramolecular polymer gel.As shown in Fig. 7b, the absorbance of the supramolecular polymergel decreased from 0.275 to 0.006 under visible light for 1.0 min,after exposure to the UV light for 5.0 min the value reached 0.27.Furthermore, this process could be repeated several times. Thus, wecould draw a conclusion that our supramolecular gel could be usedas a material to data writing or erasing.

4. Conclusion

In summary, we have described the construct of supramolecularpolymer gel by the self-assembly of triptycene-based bis(crownether) host 1 and copolymer 2 containing DBA moieties via hosteguest interactions. Interestingly, it was found that the supramo-lecular polymer gel showed multi-stimuli reversible responsive-ness such as thermo-, acid/base-, and chemo- induced gelesoltransitions. Especially, the reversible solegel transition thatinduced by competitive guest has been reported and might providean approach to use the gel as drug delivery to cure of patients thatsuffered from the paraquat. Moreover, this material exhibitedintrinsic self-healing property and the thixotropic process could berepeated at least three times. Furthermore, it was found that thesupramolecular polymer gel could be successfully employed aserasable material when it was doped with spiropyran molecule.The results presented here indicated our supramolecular gel couldbe an ideal candidate for the development of intelligent materialwith desired functionalities. Our future works will focus onexpanding the application of the materials and using other noveltriptycene-derived synthetic hosts with multiple cavities [52e54]to construct materials with desired functionalities and ultimatelybeing used in practical, which is underway in our laboratory.

Acknowledgment

We thank the National Natural Science Foundation of China(21332008, 91127009), and the National Basic Research Program(2011CB932501) for nancial support.

Appendix A. Supplementary data

Supplementary data related to this article can be found online athttp://dx.doi.org/10.1016/j.polymer.2013.10.048.

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Supramolecular polymer gel with multi stimuli responsive, self-healing and erasable properties generated by hostguest inte ...1 Introduction2 Experimental2.1 Characterization methods2.2 Synthesis of copolymer 2

3 Results and discussion3.1 Synthesis and characterization of copolymer 23.2 Self-assembly of host 1 and copolymer 23.3 Gelation and multi-stimuli reversible responsivity of the supramolecular polymer gel3.4 Rheological measurements and self-healing property of supramolecular gel3.5 Supramolecular polymer gel used as erasable material

4 ConclusionAcknowledgmentAppendix A Supplementary dataReferences