Ultrastiff and Tough Supramolecular Hydrogels with a Dense and

Subscriber access provided by UNIV OF BARCELONA

Article

Ultrastiff and Tough Supramolecular Hydrogels with a Dense and Robust Hydrogen Bond Network Yan Jie Wang, Xin Ning Zhang, Yihu Song, Yiping Zhao, Li Chen, Fengmei Su, Liangbin Li, Zi Liang Wu, and Qiang Zheng Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05262 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Ultrastiff and Tough Supramolecular Hydrogels with a Dense and Robust Hydrogen Bond Network Yan Jie Wang, †,‡,# Xin Ning Zhang,†,# Yihu Song,† Yiping Zhao,‡ Li Chen,‡,§ Fengmei Su,∥,& Liangbin Li,∥ Zi Liang Wu, †,* Qiang Zheng† †

Ministry of Education Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China; ‡ State Key Laboratory of Separation Membranes and Membrane Processes, School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China; §School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China; ∥National Synchrotron Radiation Lab and Chinese Academy of Sciences Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei 230026, China; & National Engineering Research Center for Advanced Polymer Processing Technology, Ministry of Education Key Laboratory of Materials Processing and Mold, Zhengzhou University, Zhengzhou 450002, China. #

These authors contributed equally to this work.

*Corresponding author. E-mail: [email protected] Abstract: Design of tough hydrogels has made great progress in the past two decades. However, the synthetic tough gels are usually much softer than some biotissues (e.g., skins with modulus up to 100 MPa). Here we report a new class of ultrastiff and tough supramolecular hydrogels facilely prepared by copolymerization of methacrylic acid and methacrylamide. The gels with water content of approximately 50–70 wt% possessed remarkable mechanical properties, with Young's modulus of 2.3–217.3 MPa, tensile breaking stress of 1.2–8.3 MPa, breaking strain of 200-620%, and tearing fracture energy of 2.9–23.5 kJ/m2, superior to most existing hydrogels, especially in terms of modulus. Typical yielding and crazing were observed in the gel under tensile loading, indicating the forced elastic deformation of these hydrogels in a glassy state, as confirmed by dynamic mechanical analysis. The ultrahigh stiffness was attributed to the dense crosslinking and reduced segmental mobility caused by the robust intra- and inter-chain hydrogen bonds. Because of the dynamic nature of noncovalent bonds, these supramolecular gels also showed rate-dependent mechanical performances, along with good shape memory and recyclability. This strategy should be applicable for other systems toward robust mechanical properties, versatile functionalities, and promising applications of hydrogel materials as structural elements.

1 ACS Paragon Plus Environment

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. Introduction Soft biotissues such as tendons, cartilages, and meniscuses are in a gel state with water content of 50–75 wt% and possess excellent mechanical performances,1-3 which inspire scientists to design hydrogels with comparable properties for load-bearing applications.4-6 In the last two decades, various tough hydrogels, including nanocomposite gels, double-network gels, and dual-crosslink gels, have been developed by designing the network structure and imparting an energy dissipation mechanism.4-12 Besides these gels with permanent or quasi-permanent networks, tough supramolecular hydrogels have also been prepared, in which polymer chains are solely crosslinked by noncovalent bonds.13-19 The mechanical toughening of supramolecular gels is attributed to the wide distribution of bonding strength; under loading, the relatively weak bonds or their clusters break to dissipate energy, whereas strong ones survive to maintain the integrity of hydrogel.13 Some synthetic hydrogels are even tougher than soft biotissues. For example, the fracture energy of alginate/polyacrylamide hydrogels was 9 kJ/m 2, much higher than the value (~1 kJ/m2) of native cartilages.12 However, in terms of stiffness, the tough hydrogels (Young’s modulus E: 0.01–1 MPa)7-19 are much softer than the cartilages and skins (E up to 100 MPa).1-3 Although soft hydrogels can achieve high strength and toughness via high stretchability,20 certain applications of tough gels as structural elements still require high stiffness. For instance, using low-modulus tough hydrogels as artificial cartilages will lead to mechanical mismatch, especially the undesired large displacement under loading. Therefore, designing tough and stiff hydrogels has both fundamental and practical significances. In recent years, several hydrogels have been developed with Young's modulus up to tens or hundreds of megapascals.21-27 For example, Tiller and co-workers have prepared ultrastiff hydrogels containing percolated minerals.25 These hybrid hydrogels possess Young's modulus up to 440 MPa, yet their tensile breaking strain and fracture energy were only 17% and 800 J/m 2, respectively. Sheiko and co-workers have developed hydrogen bond-reinforced hydrogels of poly(N,N-dimethylacrylamide-co-methacrylic acid) (P(DMAA-co-MAAc)) with high Young's modulus and fracture energy up to 28 MPa and 9 kJ/m2, respectively, when incubated in acidic condition (pH = 3).22 However, in pure water (pH ~6), the gel became highly swollen and very weak, with tensile breaking stress of 17 kPa, breaking strain of 160%, and Young's modulus of 20 2 ACS Paragon Plus Environment

Page 2 of 24

Page 3 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kPa. In addition, the existence of chemical crosslinking results in poor processability and recyclability of aforementioned tough gels.22,25-27 To obtain supramolecular hydrogels with combined high stiffness, strength, and fracture energy, as well as good stability and recyclability, still remains a big challenge. As mentioned above, the P(DMAA-co-MAAc) hydrogels are very tough and stiff in acidic condition, yet dramatically weakened in neutral or alkaline condition, due to the dissociation of hydrogen bonds.22 Although hydrogen bonds are usually unstable in aqueous environment, they can be effectively stabilized when located in hydrophobic pockets.28-30 For example, Meijer and co-coworkers have synthesized a multi-segmented amphiphilic copolymer with long-chain poly(ethylene glycol) and ureidopyrimidinone (UPy) unit connected via short-chain hydrophobic oligo-methylene spacer, and developed tough supramolecular hydrogels by swelling the bulk material in water.28 Since the hydrophobic spacers effectively protected the hydrogen bonding unit against the attack of water molecules, the self-complementary UPy units formed stable and strong dimers in the aqueous environment, affording high strength and toughness of the hydrogels. However, laborious organic synthesis is inevitable in the molecular design. It's highly desired to develop stiff and tough physical hydrogels via a facile approach based on the formation of stable and robust hydrogen bonds. In actual, the simplest hydrophobic motif on polymer chain is methyl group, which may stabilize the hydrogel by jacketing the hydrogen bonds to avoid the attack of water molecules. Herein, we report a series of ultrastiff and tough supramolecular hydrogels of poly(methacrylamide-co-methacrylic

acid)

(P(MAAm-co-MAAc))

facilely

prepared

by

free-radical copolymerization of the aqueous precursor solution. The equilibrated hydrogels with water content q of 50–70 wt% showed excellent mechanical properties, with Young's modulus E of 2.3–217.3 MPa, tensile breaking stress σb of 1.2–8.3 MPa, breaking strain εb of 200–620%, and tearing fracture energy G of 2.9–23.5 kJ/m2, superior to most existing hydrogels, especially in terms of the modulus. Typical yielding and crazing were observed during the tensile loading, indicating the forced elastic deformation of the glassy hydrogels. The high stiffness was attributed to the dense crosslinking and reduced segmental mobility caused by the compact hydrogen bonds between the carboxylic acid and amide groups, which were stabilized by the 3 ACS Paragon Plus Environment


hydrophobic methyl motifs. As a consequence, the tough supramolecular hydrogels were stable over a wide range of pH (≤ 9.6). Owing to the dynamic nature of hydrogen bonds, the hydrogels also showed rate- and temperature-dependent mechanical properties, along with shape memory behavior and gel-to-sol transition. The fragmented gels can be dissolved and reconstructed into tough hydrogels with mechanical properties comparable to the original one. Such tough supramolecular hydrogels with high stiffness, shape memory property, and recyclability should be an ideal structural material with potential applications in biomedical and engineering fields. 2. Experimental Section Materials. Methacrylamide (MAAm), methacrylic acid (MAAc), acrylamide (AAm), acrylic acid (AAc), and potassium persulfate (KPS) were used as received from Aladdin Chemistry Co., Ltd. N, N'-methylenebis(acrylamide) (MBAA) and N,N,N',N'-tetramethylethylenediamine (TMEDA) were used as purchased from Sigma-Aldrich. Millipore deionized water was used in all the experiments. Synthesis of Hydrogels. P(MAAm-co-MAAc) hydrogels were synthesized by free-radical copolymerization of MAAm and MAAc. Prescribed amounts of MAAm, MAAc, and KPS (used as initiator) were dissolved in water. After degassing, the reaction accelerator, TMEDA, was added to the precursor solution, which was injected in a reaction cell consisting of a pair of glass substrates separated with 1 mm-thick silicone spacer. The sample was kept at room temperature for 24 h to complete the polymerization. The as-prepared hydrogel was swelled in a large amount of water to remove the residuals and achieve the equilibrium state. The water was exchanged every day for one week. The recipe of precursor solutions for gel synthesis was listed in Table S1. The concentration of KPS and TMEDA was kept as 0.5 mol% (relative to the total monomers) and 0.1 vol% (relative to the total volume of the precursor solution), respectively. The hydrogels are coded as MM- fam-Cm, in which fam and Cm are the molar fraction of MAAm and the concentration of total monomers in M (i.e. mol/L), respectively. Chemically crosslinked PMAAc hydrogel and PMAAm hydrogel were also prepared by free-radical polymerization. The aqueous precursor solution containing 2 M monomer (MAAc or MAAm), 1 mol% MBAA, and 0.5 mol% KPS (relative to the monomer) was injected into the reaction cell, which was kept in an oven at 70 oC for 6 h to complete the reaction. These gels were placed in a watch glass to slowly 4 ACS Paragon Plus Environment

Page 4 of 24

Page 5 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


evaporate the solvent at room temperature or partially swelled by stepwise adding prescribed amount of water for several days to achieve the equilibrium state. The hydrogels with controlled water content were used for the tensile tests. Characterizations. Fourier transform infrared spectroscopy (FTIR) was performed at room temperature to the PMAAm, PMAAc, and P(MAAm-co-MAAc) hydrogels by using a Nicolet iS10 FTIR spectrometer (Thermo Scientific, USA). All the spectra were obtained with 32 scans and a resolution of 2 cm-1 in the range of 4000-400 cm-1. Thermal behavior of the gel in wet and dry states was measured by using differential scanning calorimeter (DSC 25; TA instruments, USA) equipped with a cooling system. The sample with a mass of ~10 mg was sealed in an aluminium DSC pan and scanned under a nitrogen atmosphere from 10 to 90 °C with a heating rate of 5 °C/min. The water content of the hydrogel, q, was calculated by q = (ws – wd)/ws, in which ws and wd are the mass of the gel in the swollen and dried states, respectively. The relative swelling ratio in length, S, at different pH was calculated by S = L/L0, in which L and L0 are the diameter of disc gel incubated in acidic/alkaline solution and in pure water at room temperature (20 °C), respectively. The pH value was controlled by the addition of hydrochloric acid and sodium hydroxide. S of the hydrogel at different temperature was measured in a similar way, in which L and L0 are the diameter of disc gel at a certain temperature and at 20 °C, respectively. Dynamic mechanical analysis (DMA) was performed to the hydrogel (MM-0.2-6) using a DMA Q800 equipment (TA Instruments). A rectangular sample (dimensions: 17.5 mm × 12.7 mm × 2 mm) was cut from the bulk hydrogel. The test was run in a single cantilever beam mode with a frequency of 1 Hz when the temperature increased from 20 to 90 oC (heating rate: 5 oC/min). The surface of hydrogel was painted by silicone oil to prevent the solvent evaporation during the test. Rheological behaviors of the hydrogels were investigated using an AR-G2 rheometer (TA instruments, USA). The disc-shaped hydrogel (MM-0.2-6) with thickness of ~1 mm and diameter of 20 mm was adhered to the plates with glue and surrounded by silicone oil. Strain-sweep of the gel was performed to the sample at 20 oC from 0.0006% to 0.06% at a frequency of 1 Hz. Temperature-sweep was performed to the sample from 20 to 90 oC (heating rate: 5 oC/min) at a strain amplitude of 0.05% (in the linear region) and a frequency of 1 Hz. Frequency-sweeps were performed in the range of 0.1-100 Hz to the sample at different temperature with a strain 5 ACS Paragon Plus Environment


amplitude of 0.05%. Master curves of storage modulus G', loss modulus G'', and loss factor tanδ were obtained by time-temperature superposition shifts at a reference temperature of 70 oC. Based on the Arrhenius plot of temperature-dependent shift factors, apparent activation energy Ea was calculated from the slope of the curve. Small- and wide-angle X-ray scattering (SAXS/WAXS) measurements were carried out at room temperature on the BL19U beamline of Shanghai Synchrotron Radiation Facility (SSRF). The wavelength of radiation source is 0.103 nm. To perform SAXS/WAXS measurements to the hydrogel at different tensile strain, the dumbbell-shaped sample was clamped on the two drums through two thin steel stripes, which were fixed with two fine screws at the ends. Two-dimensional (2D) patterns were collected by a Mar165 CCD detector with 2048 × 2048 pixels and a pixel size of 80 × 80 μm2. Sample-to-detector distance was 2829 mm for the SAXS measurement and 165 mm for the WAXS measurement. The 2D data was converted to one-dimensional (1D) profile by integration with the Fit2D software from European Synchrotron Radiation Facility. Mechanical properties of the hydrogels were measured by tensile tests using a commercial tensile tester (Instron 3343). The samples were cut from the hydrogel sheets into a dumbbell shape with initial gauge length of 12 mm and width of 2 mm. The nominal stress-strain curves were recorded, and the Young's modulus was calculated from the initial slope of the curve with a strain below 8%. The tensile tests at different temperature or different stretch rate, as well as the cyclic tensile tests, were performed to the samples in a water bath. Tearing tests were also performed at room temperature to characterize the fracture energy of the gels. The gels were cut into rectangular samples (35 mm × 8 mm) with 10 mm initial notch at the middle of the short edge. Two arms of the specimen were clamped, and the upper arm was pulled upward at a constant velocity of 100 mm/min. The variation of tearing force with the displacement was recorded. The tearing energy, G, was calculated by G = 2F/d, where F and d are the steady tearing force and the gel thickness, respectively.13 The shape memory properties were characterized as follows. A gel strip (MM-0.2-6) was stretched to a strain ε0 of 50% at 60 ºC and then transformed to 10 ºC water bath. After removing the external force, the strain was ε1. Then, the sample was put in 60 ºC water bath, and the gel 6 ACS Paragon Plus Environment

Page 6 of 24

Page 7 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


recovered to a strain of ε2. The shape fixity ratio, Rf, and shape recovery ratio, Rr, are calculated by Rf = ε1/ε0 × 100% and Rr = (ε0 −ε2)/ε0 × 100%, respectively. The procedure was repeated for three times to investigate the reversibility of shape memory behaviors.

3. Results and Discussion 3.1. Synthesis of Hydrogels. The P(MAAm-co-MAAc) hydrogels were prepared by polymerization of the aqueous precursor solution containing prescribed amounts of MAAm and MAAc (Figure 1a). The resultant hydrogels were immersed in a large amount of water to achieve the equilibrium state. The appearance and dimensions of the hydrogels rarely changed during the swelling process. The samples are coded as MM-fam-Cm, in which fam and Cm are the molar fraction of MAAm and the concentration of total monomers in M, respectively. Mechanically robust hydrogels were obtained over a wide range of compositions, yet intact hydrogels cannot be obtained when fam ≤ 0.05 or Cm ≤ 3 M due to the insufficient crosslinking via inter-chain hydrogen bonding or topological entanglement. As fam increased, the appearance of hydrogels changed from transparent to translucent and then to opaque (Figure 1b); the boundary moved to high fam as Cm increased, i.e., transparent hydrogels were only obtained at relatively low fam and high Cm (Figure S1). For example, the transmittance of MM-0.2-6 gel with 1 mm thickness was over 80%. The obtained transparent or turbid hydrogels showed extraordinarily high strength and stiffness, as demonstrated in Figure 1c.



Figure 1. (a) Schematic for the synthesis of stiff and tough supramolecular hydrogels based on formation of robust hydrogen bonds. (b) Appearance of the hydrogels (MM-fam-5) synthesized with different fraction of MAAm, fam. (c) Photos to demonstrate the high strength (i) and high stiffness (ii) of the equilibrated hydrogel (MM-0.2-6) at room temperature. The thickness of plate hydrogel was 1 mm, and the diameter of cylinder hydrogel was 2 mm. The robust hydrogels were stable in water, yet they can be dissolved in NaOH solution (Figure S2), indicating the supramolecular nature of gel matrix.18 Apparently, the formation of stable supramolecular network was associated with hydrogen bonding between the carboxylic acid group of MAAc units and the amine motif of MAAm units. As shown in Figure 2, the peaks of carbonyl stretching resonances in the FTIR spectra of PMAAm and PMAAc hydrogels were 1613 and 1690 cm-1, respectively, which shifted to 1620 and 1686 cm-1 in the spectrum of P(MAAm-co-MAAc) hydrogel, indicating the formation of intra- and inter-chain hydrogen bonds between the carboxylic acid and amine groups.26,31,32


Page 8 of 24

Page 9 of 24

PMAAm

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PMAAc

1613

MM-0.2-6

1690

1620 1686

4000

2000 1600

1200

-1

800

400

Wavenumber (cm )

Figure 2. FTIR spectra of PMAAm, PMAAc, and P(MAAm-co-MAAc) (MM-0.2-6) hydrogels. D2O was used as the solvent of the hydrogels.

3.2. Mechanical Properties of Gels. Both as-prepared and equilibrated hydrogels with moderate water content, q, of 46-70 wt% showed remarkable mechanical properties (Figure S3). The equilibrated hydrogels (MM- fam-5) with fam of 0.1–0.35 had σb of 1.9–5.2 MPa, εb of 300–615%, E of 2.3–174.8 MPa, and toughness We (i.e. the extension work) of 6.5–17.5 kJ/m3 (Figures 3a, 3b and Table S2). The fracture energy G, as determined by tearing tests, ranged from 3.4 to 13.8 kJ/m2. For the gels (MM-0.3-Cm) with different Cm, σb, εb, E, We, and G increased from 1.2 MPa, 198%, 11.5 MPa, 2.0 kJ/m3, and 2.9 kJ/m2 to 8.0 MPa, 418%, 217.3 MPa, 27.9 kJ/m3, and 23.5 kJ/m2, respectively, as Cm increased from 3.5 to 6 M (Figures 3c, 3d and Table S2). Therefore, these supramolecular hydrogels simultaneously possessed high stiffness and toughness, which were tunable over a wide range and superior to most existing hydrogels and soft biotissues, especially in terms of the modulus (Figures 3e and 3f).1-3,7-19



E

G

100

70 60 50

3

40

800

Strain (%) Cm (M) 3.5 4 4.5 5 5.5 6

8 6 4

0.10

d)

0.15

0.20

0.25

0.30

0

30

35

90

30

80

25

70

0.35

fam

320

b

240

E

G

q

160

2 0

20

80

15

100

200

300

400

Strain (%)

500

10

40

5

5

30

0

20

600

6

10

3.5

f)

this work

5

10

50

10

0 0

60

4.0

5

10

4.5

5.0

Cm (M)

5.5

q (wt%)

600

2

400

b, E (MPa)

200

10

Stress (MPa)

12

6

1 0

6.0

this work 4

10 2

skin

3

10

B-DN gel

PVA gel

2

10

polyampholyte gel

1

10

0

20

40

skin

3

10

DN gel

2

10

PVA gel

1

10

tetra-PEG gel alginate-PAAm gel

0

G (J/m )

4

10

10

80

5

0

e)

15

9

50

3

c)

90

2

150

2

18 q

G (kJ/m )

Stress (MPa)

4

200

b, E (MPa)

0.1 0.15 0.2 0.25 0.3 0.35

6

b

q (wt%)

b) 250

fam

G (kJ/m )

8

a)

E (kPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 24

60

80

0

100

10

0

20

q (wt%)

40

60

80

100

q (wt%)

Figure 3. (a-d) Tensile stress-strain curves (a,c) and corresponding mechanical parameters (b,d) of the equilibrated gels with different fam (MM-fam-5) (a,b) and different Cm (MM-0.3-Cm) (c,d). Tearing fracture energy, G, and water content, q, of the gels were also presented in (b) and (d). Tensile and tearing tests were performed at room temperature with a stretch rate of 100 mm/min. (e,f) Material property charts of Young's modulus (e) and fracture energy (f) versus water content for various soft materials. Materials include the P(MAAm-co-MAAc) gel in this work, nanocomposite (NC) gel,8 tetra-arm polyethylene glycol (tetra-PEG) gel,9 poly(vinyl alcohol) (PVA) gel,17 double-network (DN) gel,11 alginate-polyacrylamide (alginate-PAAm) gel,12 PVA-PAAm gel,21 polyampholyte gel,13 poly(N-acryloyl glycinamide) (PNAGA) gel,18 semicrystalline gel,34 block-copolymer DN (B-DN) gel,31 alginate gel,12 and PAAm gel,12 as well as native cartilage and skin.2,3 We should note that the mechanical properties of obtained hydrogels had good reproducibility (Figure S4), and the ultrahigh stiffness of the hydrogels mainly resulted from the 10 ACS Paragon Plus Environment

Page 11 of 24

formation of a dense and robust hydrogen bond network, although the stiffness of gels also increased with the decrease in water content. As shown in Figure 3e, the P(MAAm-co-MAAc) hydrogels had much higher E than other tough gels with similar water content.13,33,34 q of our gels is 46-70 wt%, beyond the minimum content of water, ~10-30 wt%, to induce glassy-to-soft transition of conventional hydrophilic polymers.33 As shown in Figure 4a, the single network hydrogels of PMAAm and PMAAc with q of 50 wt% were still soft and elastic. However, the P(MAAm-co-MAAc) gel (MM-0.2-6) with the same water content possessed much higher σb, εb, and E. The influence of q on E of these hydrogels was shown in Figure 4b, in which q was tuned by solvent evaporation or controlled swelling of the as-prepared gels before the tensile tests. At high water content (q > 80 wt%), the three hydrogels were soft (E < 0.1 MPa). However, as q decreased, E of P(MAAm-co-MAAc) hydrogel drastically increased, with the value of 42 MPa at q = 60 wt% and 110 MPa at q = 50 wt%. In contrast, E of PMAAm and PMAAc hydrogels was only 0.3 and 7.6 MPa, respectively, at q = 50 wt%. The distinct behaviors resulted from the different density and strength of hydrogen bonds in the hydrogels. In P(MAAm-co-MAAc) gels with moderate water content, dense and robust hydrogen bonds should be formed between MAAm and MAAc groups. 12

b) 200

q = 50 wt%

9

P(MAAm-co-MAAc) PMAAc PMAAm

150

6

P(MAAm-co-MAAc)

E (MPa)

a) Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 50

3 PMAAc

PMAAm

0

0 0

100

200

300

Strain (%)

400

500

40

50

60

70

80

90

100

q (wt%)

Figure 4. (a) Tensile stress-strain curves of chemically crosslinked PMAAm gel and PMAAc gel, as well as the supramolecular P(MAAm-co-MAAc) gel (MM-0.2-6) with identical q of 50 wt%. (b) Variations of Young's modulus, E, of the three kinds of gels as a function of q. 3.3. Forced Elastic Deformation and Glassy State of Gel. During the tensile loading of P(MAAm-co-MAAc) hydrogels, typical yielding phenomenon was usually observed. As shown in Figure 5a, necking appeared accompanying with localized stress-whitening in the original transparent gel (MM-0.2-6) at a small strain. It is interesting that micron-sized crazing lines were 11 ACS Paragon Plus Environment


observed on the surface of the gel at a strain of 10%, i.e. the onset of yielding (Figures 5b and 5c). Further loading resulted in strain-softening and appearance of shear band at the necking zone with crazing lines ±45º to the elongation direction. The stretching resulted in drastic molecular orientation, as revealed by the appearance of strong birefringence (inset of Figure 5c) and anisotropic SAXS patterns (Figure S5).35,36 Such necking phenomenon with clear crazing and shear band is common in glassy thermoplastics,37 yet it is observed for the first time in hydrogels to our knowledge, indicating the plastic deformation of this hydrogel with high stiffness.

c)

a)

1) ε=0

2) 15%

3)

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 24

4)

50%

140%

5)

10 8

i)

ε=0

500 μm

3)

4)

iii) iv)

4

ε = 5%

2

A

1) i)

0 0

Zʹ

1 mm

P

100

200

300

400

Strain (%)

ε = 10%

ii)

5)

2)

6

300%

b)

ii)

ε = 50%

ε = 100%

iv)

iii)

Figure 5. (a,b) Photos (a) and micrographs (b) to show the yielding behavior of MM-0.2-6 gel during the tensile loading. The sample was stretched to a certain strain for the microscopy observation at room temperature. (c) The stress-strain curve of MM-0.2-6 gel. The strains corresponding to the images in (a) and (b) were noted on the curve as triangle and circle, respectively. Inset: Polarizing micrograph of the sample with a strain of 5%; P: polarizer, A: analyzer, Z': slow axis of λ-plate. At room temperature, the deformed hydrogel can only partially recover to its original state upon unloading (Figure 6). The hydrogel showed significant hysteresis in the first loading-unloading curve with a maximum strain of 100%, indicating that a large amount of energy was dissipated during the loading process. During the cyclic tensile tests, the residual strain decreased with the waiting time tw and reached constant value of ~15%, when tw ≥ 12 h. However, the gel readily recovered to its original length after incubation in hot water (60 oC) for 3 min. After cooling back to room temperature, the hydrogel showed the same mechanical 12 ACS Paragon Plus Environment

Page 13 of 24

properties as the original one (Figure 6). This result, together with the yielding phenomena, indicated that the ultrastiff hydrogel experienced forced elastic deformation at room temperature, which was usually found in glassy or semicrystalline polymers.37,38 12 10

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8

Virgin o 60 C for 3 min

Waiting time (h) 0 3 12 24

40

80

6 4 2 0 0

20

60

100

120

Strain (%)

Figure 6. Cyclic tensile stress-strain curves of MM-0.2-6 hydrogel. The tests were performed at 20 ºC. Loading-unloading was performed again to the sample at 20 ºC (red curve), after incubating it at 60 ºC for 3 min and then at 20 ºC for 3 h. X-ray scattering, DSC, and DMA were applied to characterize the state of the tough hydrogel with ultrahigh stiffness. As shown in Figure S6, there was no sharp scattering in WAXS pattern of the gel (MM-0.2-6) in wet state or in X-ray diffraction (XRD) pattern of the gel in dry state, indicating that the gel was amorphous without crystalline structure.13,38 On the other hand, glass transition was observed in the DSC thermograms of the gel in wet and dry states.38 The glass transition temperature, Tg, of the dry gel was ~190 oC (Figure S7). In the presence of water, Tg, of the wet gel shifted to ~55 oC (Figure 7a). Tg of the wet hydrogel was also confirmed by DMA.39 As shown in Figure 7b, a glassy plateau region below 40 oC was followed by the transition region with decreased storage modulus E' and loss modulus E''. As a consequence, the loss factor tanδ had a broad peak at ~63 oC, corresponding to Tg of the wet gel, which was comparable to the value found by DSC. Tg of the MM-fam-6 hydrogels with different fam gradually shifted from 28 to 78 oC as fam increased from 0.1 to 0.35 (Figure S8), probably because of the increased strength and amount of hydrogen bond clusters. Therefore, it's rational that the glassy hydrogel showed forced elastic deformation at room temperature, several tens degree lower than its Tg.37


a)

Page 14 of 24

0

heating

-30 -60 -90 10

20

30

40

50

60

70

o

80

90

Temperature ( C) 10

10

10

9

10

8

10

7

10

6

0.6 E'

0.5

E''

0.4

tan

b) E', E'' (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Heat flow (mW/g)


0.3 tan

0.2 0.1

20

30

40

50

60

70

o

80

90

Temperature ( C)

Figure 7. DSC thermogram (a) and DMA spectra (b) of the MM-0.2-6 hydrogel during the heating process from 20 to 90 ºC. Heating rate: 5 ºC/min. DMA test was performed in a single cantilever beam mode with a frequency of 1 Hz. 3.4. Dynamic Mechanical Properties and Stability. Owing to the dynamic nature of hydrogen bonds, the mechanical properties of P(MAAm-co-MAAc) hydrogels depended on the stretch rate and test temperature (Figures 8a and 8c). The tensile tests were performed in a water bath at a certain temperature, and no evident volume inflation of the gel was observed even at 70 oC (Figure S9). σb and E increased, whereas εb decreased, with the increase in stretch rate or the decrease in test temperature (Figure S10). The yielding stress linearly increased with log(strain rate) or linearly decreased with the test temperature (Figures 8b and 8d), which can be described by Eyring model (eq. 1) for the systems with mechanically induced dissociation of noncovalent bonds.34,40 (1) σy,  , Ey, and Va are the tensile yielding stress, deformation rate, activation energy of yielding, and effective activation volume, respectively. The values of Ey and Va extracted by data fitting are 122 kJ/mol and 8.4 nm3, respectively. These values are much larger than those of individual 14 ACS Paragon Plus Environment

Page 15 of 24

hydrogen bond (12–20 kJ/mol and ~0.1 nm3), indicating the existence of cooperative hydrogen bond clusters.41,42 Stretch rate (mm/min)

Stress (MPa)

12

1 5 10 50 100 500 1000

9 6 3

b) 12 Yielding stress (MPa)

a) 15

0 600

400

200

c)

12

o

Temp. ( C) 10 20 30 40 50 55 60 70

10 8 6 4

9

6

3

800

Strain (%)

2 0

10

-3

10

-2

10

-1

10

-1

0

Strain rate (s )

d) 12 Yielding stress (MPa)

0

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9 6 3 0 -3

0

200

400

600

800

1000

280

Strain (%)

290

300

310

320

Temperature (K)

330

Figure 8. Tensile stress-strain curves of MM-0.2-6 gel at different stretch rate (a), different temperature (c) and the variations of corresponding yielding stress (b,d). Test temperature in (a) and stretch rate in (c) were 20 ºC and 100 mm/min, respectively. The dynamic properties of the gels were further investigated by rheological measurements. Temperature sweep of MM-0.2-6 gel showed that the storage modulus G' and loss modulus G'' decreased with the temperature, and the loss factor tanδ' had a broad peak at ~65 ºC (Figure 9a), corresponding to Tg of the gel. In addition, the spectra of frequency sweeps of the gel at different temperatures followed the principle of time-temperature superposition (Figure 9b). Arrhenius plot for the shift factor αT of the master curve provided two apparent activation energy values Ea of 130 and 259 kJ/mol (Figure 9c). The wide range of Ea was attributed to the wide distribution of the strength of hydrogen bonds and their clusters, probably originating from the randomly dispersed MAAm and MAAc repeat units in the copolymers.13,43 During the stretching, the weak hydrogen bond clusters ruptured to dissipate energy, whereas the strong ones maintained to resist the deformation. It is rational that the minimum value of Ea was close to the value of Ey, indicating that the yielding of the glassy hydrogel under loading essentially resulted from the 15 ACS Paragon Plus Environment


breaking of relatively weak hydrogen bond clusters. 8

10

7

1.5

G' 1.0

G'' 0.5

10

6

10

5

Tan '

Tan '

G', G'' (Pa)

a) 10

0.0 -0.5 20

30

40

50

60

70

80

90

10

10

Temperature ( C)

7

6

10

5

10

4

2.0

o

o

60 C o 70 C o 80 C o 90 C

55 C o 65 C o 75 C o 85 C o 95 C

G' 1.5

G''

Tan '

b) G', G'' (Pa)

o

1.0

Tan '

0.5

10

-3

10

-2

10

-1

10

0

10

1

10

2

10

3

0.0

Frequency (Hz) 6

c)

4 2

lnT

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 24

0

Ea = 259 kJ/mol

-2 -4 Ea = 130 kJ/mol

-6 2.7

2.8

2.9

-3 -1

3.0

3.1

1/T (10 K )

Figure 9. (a) Temperature-sweep of MM-0.2-6 hydrogel from 20 to 90 ºC at a frequency of 1 Hz and strain amplitude of 0.05%. Heating rate: 5 ºC/min. (b,c) Master curves of frequency dependence of the storage modulus G', loss modulus G'', and loss factor tanδ of the gel by time-temperature superposition shifts at a reference temperature of 70 ºC (b), and Arrhenius plot for the temperature dependent shift factors αT (c). The apparent activation energy Ea was calculated from the slop of the curve. The robust cooperative hydrogen bonds afforded the tough and stiff gels with good stability in neutral or weakly alkaline condition (pH ≤ 9.6) (Figure 10). However, they became highly swollen or even dissolved in strongly alkaline condition. In comparison, the as-prepared 16 ACS Paragon Plus Environment

Page 17 of 24

poly(acrylamide-co-acrylic acid) (P(AAm-co-AAc)) gel was dissolved after being swelled in water. The P(DMAA-co-MAAc) hydrogels developed by Sheiko and coworkers were also highly swollen and dramatically weakened, when pH increased from 3 to 6.22 The high stability of hydrogen bonds in P(MAAm-co-MAAc) gels was related to the shielding effect of the hydrophobic backbone with methyl groups, which protected the hydrogen bonds against the attack of water molecules.28-30 The robust hydrogen bonding reduced the segmental mobility to form compact gel matrix in a glassy state and thus resulted in high stiffness of the tough hydrogels.44 The pendant methyl groups should also decrease the rotation flexibility of chain segments to some extent and thus shift the glass transition temperature to above room temperature.45,46 The influence of methyl group on the properties of hydrogels was also reflected in the distinct stability and mechanical performances of the gels prepared by copolymerization of AAc (or MAAc) and AAm (or MAAm). P(AAm-co-AAc) gel was not stable in water; P(MAAm-co-AAc) and P(AAm-co-MAAc) gels were stable, yet their mechanical properties, especially E, were much lower than those of P(MAAm-co-MAAc) gel. Detailed results will be

9 6 3 0.2 0.0

600

250

2.8

12

500

200

2.4

9

400

150

2.0

6

300

100

3

200

50

1.2

0

100

0

0.8

0

-50

0.4

b

b

S

E

-3 0

100

200

300

400

500

0

2

4

6

8

10

12

E (MPa)

1.6 3.6 5.6 8.6 9.6 10.6

12

b) 15

b (%)

pH

b (MPa)

a) 15

1.6

S

reported in a forthcoming paper.

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14

pH

Strain (%)

Figure 10. Tensile stress-strain curves of MM-0.2-6 hydrogel equilibrated at 20 ºC in the aqueous solutions with different pH (a) and corresponding mechanical parameters (b). The swelling ratio in length, S, with pH = 5.6 as the reference was also shown in (b). 3.5. Shape Memory and Recyclability. Beyond the mechanical properties, the dynamic nature of hydrogen bonds also imparted versatile functions to these physical hydrogels. The hydrogel became soft and showed good self-recovery ability at elevated temperature, as demonstrated by the cyclic tensile tests with maximum strain of 200% at 60 oC (Figure S11). Although a large residual strain (~50%) was observed after the first unloading, the residual strain εr gradually 17 ACS Paragon Plus Environment


decreased and the loading-unloading loop approached the first one after waiting for a while. εr almost fully disappeared and hysteresis ratio Rh (calculated as the area ratio of the second hysteresis to the first) was 96% after waiting for 3 min, indicating the fast self-recovery of the hydrogel. The self-recovery was related to the reformation of hydrogen bonds, affording the gels high fatigue resistance that is important for practical applications. The temperature-mediated drastic variation of mechanical properties endowed the gels with shape memory capacity (Figure 11a).47 To quantitatively characterize the shape memory property, a slender gel strip was softened and stretched to a strain of 50% in hot water (60 ºC) and then fixed in cold water (10 ºC) (Figure 11b). The gel readily recovered to the original shape after being placed in hot water. The fixity ratio, Rf, and recovery ratio, Rr, of this hydrogel were further measured by cyclic shape fixation and recovery. Rf and Rr were as high as 93% and 99%, respectively (Figure 11c). We should note that the birefringence of the shape memorized gel in Figure 11b(ii') was opposite to that in Figure 5c, probably because of the different stress conditions. Under loading, the polymer chains should orient along the tensile direction and the hydrogel showed blue birefringence. However, the gel contracted to some extent (~7%) after removing the external force, which might result in preferential orientation of polymer chains perpendicular to the contraction direction.48 Further extension of the shape memorized hydrogel led to reversion of the birefringence (Figure S12). i)

c) 120

ii)

i)

ii)

iii) 1 cm

A

i') A

P

Zʹ P

ii')

240

E

Rr

200

100

1 cm

b)

Rf

160

80

120 60 80 40

40

20

iii')

0

0

-40 10

1 mm

E (MPa)

a)

Rf , Rr (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 24

60

10

60

o

10

60

Temperature ( C)

Figure 11. (a,b) Shape memory behavior of the gel. The MM-0.2-6 gel (i) was deformed in a helix structure (a) or stretched to a strain of 50% (b) in hot water (60 ºC) and then transformed into cold water (10 ºC) for 1 h to fix the shape after removing the external force (ii). The gel recovered to its original shape after being kept in hot water for 3 min (iii). Corresponding polarizing micrographs were shown in the bottom. (c) Shape fixity ratio Rf, recovery ratio Rr and elastic modulus E of the gel during the cyclic shape memory and recovery. 18 ACS Paragon Plus Environment

Page 19 of 24

The tough supramolecular hydrogels also had good recyclability. As shown in Figure 12a, the fragmented MM-0.2-6 hydrogels were completely dissolved in 0.5 M NaOH solution, which was used to fabricate a film by solution casting. The casted film was swelled in 1 M HCl solution and then transformed to a large amount of water. The reconstructed hydrogel film showed good mechanical properties, comparable to those of original gel (Figure 12b). These versatile properties should expand the applications of these tough and stiff hydrogels as structural elements with defined structures.

a)

i)

ii)

b)

12 10

iv)

iii)

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Virgin

8 6 Reconstructed

4 2 0 0

100

200

300

400

Strain (%)

Figure 12. (a) Reconstruction of hydrogel film by dissolving the fragments of MM-0.2-6 gel (i) in 0.5 M NaOH solution (ii), casting into film (iii), and then sequentially swelling it in 1 M HCl solution and pure water (iv). (b) Tensile stress-curves of the original and reconstructed hydrogels. Tensile tests were performed at room temperature with a stretch rate of 100 mm/min.

4. Conclusions In summary, we have demonstrated a novel strategy to develop stiff and tough hydrogels based on the formation of dense and robust hydrogen bond network. The supramolecular gels were in a glassy state and showed typical temperature- and stretch rate-dependent forced elastic deformation with specific yielding and crazing. These hydrogels with moderate water content possessed excellent mechanical properties with σb of 1.2–8.3 MPa, εb of 198–615%, E of 2.3–217.3 MPa, We of 2.0–27.9 kJ/m3, and G of 2.9–23.5 kJ/m2, superior to most existing hydrogels and comparable to tough soft biotissues, especially in terms of the modulus. Owing to the dynamic nature of hydrogen bonds, the hydrogels also showed shape memory property. Although the P(MAAm-co-MAAc) hydrogels are analogous to tough biotissues in terms of the water content and Young's modulus, they cannot fully mimic the mechanical performances of 19 ACS Paragon Plus Environment


Page 20 of 24

biotissues such as the human back skin, which has significant stiffening effect at large strain.2,49 However, the development of tough hydrogels with ultrahigh stiffness should broaden the applications of gel materials as structural elements in biomedical and engineering fields. Compared to other stiff hydrogels developed by forming percolated minerals via mineralization or forming dense hydrophobic domains via solvent exchange,25,26,50,51 the hydrogels in this work were prepared by a more facile and versatile approach. The resultant hydrogels had good stability and recyclability. Therefore, making hydrogels in the glassy state should be an effective approach to simultaneously achieve high toughness and stiffness. Furthermore, the strategy by incorporating hydrophobic motifs to stabilize the hydrogen bonds should be applicable to other materials toward robust mechanical properties and versatile functionalities. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Recipe of precursor solutions for gel synthesis, transmission spectra, SAXS/WAXS data, tensile stress-strain curves, DMA spectra, and polarizing micrographs of the hydrogels ( PDF) Acknowledgments We thank Prof. Yoshihito Osada, Prof. Stephen Z. D. Cheng, and Prof. Costantino Creton for the helpful discussions. This work was supported by National Natural Science Foundation of China (51773179), Fundamental Research Funds for the Central Universities of China, and Thousand Young Talents Program of China. References (1) Gartner, L. P.; Hiatt, J. L. Color Textbook of Histology, 2nd ed.; Saunders: Philadelphia, 2001. (2) Wegst, U. G. K.; Ashby, M. F. The mechanical efficiency of natural materials. Phil. Mag. 2004, 84, 2167–2181. (3) Little, C. J.; Bawolin, N. K.; Chen, X. Mechanical properties of natural cartilage and tissue-engineered constructs. Tissue Eng. Part B Rev. 2011, 17, 213–227. (4) Zhao, X. Multi-scale multi-mechanism design of tough hydrogels: building dissipation into stretchy networks. Soft Matter 2014, 10, 672–687. (5) Zhang, Y. S.; Khademhosseini, A. Advances in engineering hydrogels. Science 2017, 356, eaaf3627. (6) Creton, C. 50th anniversary perspective: networks and gels: soft but dynamic and tough. 20 ACS Paragon Plus Environment

Page 21 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Macromolecules 2017, 50, 8297−8316. (7) Okumura, Y.; Ito, K. The polyrotaxane gel: a topological gel by figure-of-eight cross-links. Adv. Mater. 2001, 13, 485–487. (8) Haraguchi, K.; Takehisa, T. Nanocomposite hydrogels: a unique organic-inorganic network structure with extraordinary mechanical, optical, and swelling/de-swelling properties. Adv. Mater. 2002, 14, 1120–1124. (9) Sakai, T.; Matsunaga, T.; Yamamoto, Y.; Ito, C.; Yoshida, R.; Suzuki, S.; Sasaki, N.; Shibayama, M.; Chung, U. Design and fabrication of a high-strength hydrogel with ideally homogeneous network structure from tetrahedron-like macromonomers. Macromolecules 2008, 41, 5379–5384. (10) Yang, Y.; Wang, X.; Yang, F.; Shen, H.; Wu, D. A universal soaking strategy to convert composite hydrogels into extremely tough and rapidly recoverable double-network hydrogels. Adv. Mater. 2016, 28, 7178–7184. (11) Gong, J. P.; Katsuyama, Y.; Kurokawa, T.; Osada, Y. Double-network hydrogels with extremely high mechanical strength. Adv. Mater. 2003, 15, 1155–1158. (12) Sun, J.-Y.; Zhao, X.; Illeperuma, W. R. K.; Chaudhuri, O.; Oh, K. H.; Mooney, D. J.; Vlassak, J. J.; Suo, Z. Highly stretchable and tough hydrogels. Nature 2012, 489, 133–136. (13) Sun, T. L.; Kurokawa, T.; Kuroda, S.; Ihsan, A. B.; Akasaki, T.; Sato, K.; Haque, M. A.; Nakajima, T.; Gong, J. P. Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity. Nat. Mater. 2013, 12, 932–937. (14) Henderson, K. J.; Zhou, T. C.; Otim, K. J.; Shull, K. R. Ionically cross-linked triblock copolymer hydrogels with high strength. Macromolecules 2010, 43, 6193–6200. (15) Hao, J.; Weiss, R. A. Viscoelastic and mechanical behavior of hydrophobically modified hydrogels. Macromolecules 2011, 44, 9390–9398. (16) Tuncaboylu, D. C.; Sari, M.; Oppermann, W.; Okay, O. Tough and self-healing hydrogels formed via hydrophobic interactions. Macromolecules 2011, 44, 4997–5005. (17) Zhang, L.; Zhao, J.; Zhu, J.; He, C.; Wang, H. Anisotropic tough poly(vinyl alcohol) hydrogels. Soft Matter 2012, 8, 10439–10447. (18) Dai, X.; Zhang, Y.; Gao, L.; Bai, T.; Wang, W.; Cui, Y.; Liu, W. A Mechanically Strong, Highly Stable, Thermoplastic, and self-healable supramolecular polymer hydrogel. Adv. Mater. 2015, 27, 3566–3571. (19) Chen, H.; Yang, F.; Chen, Q.; Zheng, J. A novel design of multi-mechanoresponsive and mechanically strong hydrogels. Adv. Mater. 2017, 29, 1606900. (20) Zhao, X. Designing toughness and strength for soft materials. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 8138–8140. (21) Li, J.; Suo, Z.; Vlassak, J. J. Stiff, strong, and tough hydrogels with good chemical stability. J. Mater. Chem. B 2014, 2, 6708–6713. (22) Hu, X.; Vatankhah-Varnoosfaderani, M.; Zhou, J.; Li, Q.; Sheiko, S. S. Weak hydrogen bonding enables hard, strong, tough, and elastic hydrogels. Adv. Mater. 2015, 27, 6899–6905. (23) Zheng, S. Y.; Ding, H.; Qian, J.; Yin, J.; Wu, Z. L.; Song, Y.; Zheng, Q. Metal-coordination complexes mediated physical hydrogels with high toughness, stick-slip tearing behavior, and good processability. Macromolecules 2016, 49, 9637–9646. (24) Tian Y.; Wei, X.; Wang, Z. J.; Pan, P.; Li, F.; Ling, D.; Wu, Z. L.; Zheng, Q. A facile approach to prepare tough and responsive ultrathin physical hydrogel films as artificial muscles. ACS Appl. Mater. Interfaces 2017, 9, 34349–34355. 21 ACS Paragon Plus Environment


Page 22 of 24

(25) Rauner, N.; Meuris, M.; Zoric, M.; Tiller, J. C. Enzymatic mineralization generates ultrastiff and tough hydrogels with tunable mechanics. Nature 2017, 543, 407–410. (26) Zhang, X. N.; Wang, Y. J.; Sun, S.; Hou, L.; Wu, P.; Wu, Z. L.; Zheng, Q. A tough and stiff hydrogel with tunable water content and mechanical properties based on the synergistic effect of hydrogen bonding and hydrophobic interaction. Macromolecules 2018, 51, 8136−8146. (27) Wang, Y. J.; Li, C. Y.; Wang, Z. J.; Zhao, Y.; Chen, L.; Wu, Z. L.; Zheng, Q. Hydrogen bond-reinforced double-network hydrogels with ultrahigh elastic modulus and shape memory property. J. Polym. Sci. Part B: Polym. Phys. 2018, 56, 1281−1286. (28) Guo, M.; Pitet, L. M.; Wyss, H. M.; Vos, M.; Dankers, P. Y. W.; Meijer, E. W. Tough stimuli-responsive supramolecular hydrogels with hydrogen-bonding network junctions. J. Am. Chem. Soc. 2014, 136, 6969–6977. (29) Cui, Y.; Tan, M.; Zhu, A.; Guo, M. Non-covalent interaction cooperatively induced stretchy, tough and stimuli-responsive polyurethane–urea supramolecular (PUUS) hydrogels. J. Mater. Chem. B 2015, 3, 2834–2841. (30) Jeon, I.; Cui, J.; Illeperuma, W. R. K.; Aizenberg, J.; Vlassak, J. J. Extremely stretchable and fast self-healing hydrogels. Adv. Mater. 2016, 28, 4678–4683. (31) Zhang, H. J.; Sun, T. L.; Zhang, A. K.; Ikura, Y.; Nakajima, T.; Nonoyama, T.; Kurokawa, T.; Ito, O.; Ishitobi, H.; Gong, J. P. Tough physical double-network hydrogels based on amphiphilic triblock copolymers. Adv. Mater. 2016, 28, 4884–4890. (32) Ding, H.; Zhang, X. N.; Zheng, S. Y.; Song, Y.; Wu, Z. L.; Zheng, Q. Hydrogen bond reinforced poly (1-vinylimidazole-co-acrylic acid) hydrogels with high toughness, fast self-recovery, and dual pH-responsiveness. Polymer 2017, 131, 95–103. (33) Itagaki, H.; Kurokawa, T.; Furukawa, H.; Nakajima, T.; Katsumoto, Y.; Gong, J. P. Water-induced brittle-ductile transition of double network hydrogels. Macromolecules 2010, 43, 9495–9500. (34) Bilici, C.; Ide, S.; Okay, O. Yielding behavior of tough semicrystalline hydrogels. Macromolecules 2017, 50, 3647–3654. (35) Wu, Z. L.; Sawada, D.; Kurokawa, T.; Kakugo, A.; Yang, W.; Furukawa, H.; Gong, J. P. Strain-induced molecular reorientation and birefringence reversion of a robust, anisotropic double-network hydrogel. Macromolecules 2011, 44, 3542–3547. (36) Yang, H.; Liu, D.; Ju, J.; Li, J.; Wang, Z.; Yan, G.; Ji, Y.; Zhang, W.; Sun, G.; Li. L. Chain deformation on the formation of Shish nuclei under extension flow: an in situ SANS and SAXS study. Macromolecules 2016, 49, 9080–9088. (37) Lazurkin, J. S. Cold-drawing of glass-like and crystalline polymers. J. Polym. Sci. 2003, 30, 595–604. (38) Yanagisawa, Y.; Nan, Y.; Okuro, K.; Aida, T. Mechanically robust, readily repairable polymers via tailored noncovalent cross-linking. Science 2018, 359, 72–76. (39) Bain, E. D.; Long, T. Y.; Beyer, F. L.; Savage, A. M.; Dadmun, M. D.; Martin, H.; Lenhart, J. L.; Mrozek, R. A. Tough, rapidly swelling thermoplastic elastomer hydrogels for hemorrhage control. Macromolecules 2018, 51, 4705–4717. (40) McCrum, N. G.; Buckley, C. P.; Bucknall, C. B. Principles of Polymer Engineering, Oxford University Press: New York, 1997. (41) Song, G.; Zhang, L.; He, C.; Fang, D.-C.; Whitten, P. G.; Wang, H. Facile fabrication of 22 ACS Paragon Plus Environment

Page 23 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


tough hydrogels physically cross-linked by strong cooperative hydrogen bonding. Macromolecules 2013, 46, 7423–7435. (42) Hu, X.; Zhou, J.; Daniel, W. F. M.; Vatankhah-Varnoosfaderani, M.; Dobrynin, A. V.; Sheiko, S. S. Dynamics of dual networks: strain rate and temperature effects in hydrogels with reversible H-bonds. Macromolecules 2017, 50, 652–659. (43) Luo, F.; Sun, T. L.; Nakajima, T.; King, D. R.; Kurokawa,T.; Zhao, Y.; Ihsan, A. B.; Li, X.; Guo, H.; Gong, J. P. Strong and tough polyion-complex hydrogels from oppositely charged polyelectrolytes: a comparative study with polyampholyte hydrogels. Macromolecules 2016, 49, 2750–2760. (44) Soutar, I.; Swanson, L.; Thorpe, F. G.; Zhu, C. Fluorescence studies of the dynamic behavior of poly (dimethylacrylamide) and its complex with poly (methacrylic acid) in dilute solution. Macromolecules 1996, 29, 918–924. (45) Rubinstein, M.; Colby, R. H. Polymer Physics, Oxford University Press: New York, 2003. (46) Liu, G.; Meng, Z.; Wang, W.; Zhou, Y.; Zhang, L. Sequence distribution effects on glass transition temperatures of copolymers: an extended Gibbs-DiMarzio equation in view of bond rotation flexibility. J. Phys. Chem. B 2008, 112, 93–99. (47) Lӧwenberg, C.; Balk, M.; Wischke, C.; Behl, M.; Lendlein, A. Shape-memory hydrogels: evolution of structural principles to enable shape switching of hydrophilic polymer networks. Acc. Chem. Res. 2017, 50, 723–732. (48) Takahashi, R.; Wu, Z. L.; Arifuzzaman, M.; Nonoyama, T.; Nakajima, T.; Kurokawa, T.; Gong, J. P. Control superstructure of rigid polyelectrolytes in oppositely charged hydrogels via programmed internal stress. Nat. Commun. 2014, 5, 4490. (49) Vatankhah-Varnosfaderani, M.; Keith, A. N.; Cong, Y.; Liang, H.; Rosenthal, M.; Sztucki, M.; Clair, C.; Magonov, S.; Ivanov, D. A.; Dobrynin, A. V.; Sheiko, S. S. Chameleon-like elastomers with molecularly encoded strain-adaptive stiffening and coloration. Science 2018, 359, 1509−1513. (50) Jiao, C.; Chen, Y.; Liu, T.; Peng, X.; Zhao, Y.; Zhang, J.; Wu, Y.; Wang, H. Rigid and strong thermoresponsive shape memory hydrogels transformed from poly(vinylpyrrolidone-co-acryloxy acetophenone) organogels. ACS Appl. Mater. Interfaces 2018, 10, 32707–32716. (51) Sato, K.; Nakajima, T.; Hisamatsu, T.; Nonoyama, T.; Kurokawa, T.; Gong, J. P. Phase-seperation-induced anomalous stiffening, toughening, and self-healing of polyacrylamide gels. Adv. Mater. 2015, 27, 6990−6998.



Table of Contents


Page 24 of 24

Ultrastiff and Tough Supramolecular Hydrogels with a Dense and

Recommend Documents