Ultrastiff, Tough, and Healable IonicâHydrogen Bond Cross-Linked

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Ultrastiff, Tough and Healable Ionic-Hydrogen Bond Cross-linked Hydrogels and Their Uses as Building Blocks to Construct Complex Hydrogel Structures Yongzhi Liang, Jinqiao Xue, Binyang Du, and Jingjing Nie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20520 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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ACS Applied Materials & Interfaces

Ultrastiff, Tough and Healable Ionic-Hydrogen Bond Cross-linked Hydrogels and Their Uses as Building Blocks to Construct Complex Hydrogel Structures Yongzhi Liang,† Jinqiao Xue, ‡ Binyang Du‡* and Jingjing Nie †* †Department ‡MOE

of Chemistry, Zhejiang University, Hangzhou 310027, China

Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer

Science & Engineering, Zhejiang University, Hangzhou 310027, China

ABSTRACT We

report

the

ultrastiff

and

tough

poly(acrylamide-co-acrylic

acid)/Na-alginate/Fe3+

(P(AM-co-AA)/Na-alginate/Fe3+) hydrogel via the formation of hybrid ionic-hydrogen bond cross-linking networks.

The optimal P(AM-co-AA)/Na-alginate/Fe3+ hydrogel possessed super

high elastic modulus (~24.6 MPa), tensile strength (~10.4 MPa), compression strength (~44 MPa), and toughness (~4800 J/m2). The P(AM-co-AA)/Na-alginate/Fe3+ hydrogel was highly stable and maintained its superior mechanical properties in 0.5-2 M NaCl solution, aqueous solution with pH ranging from 4 to 10. The ionic cross-linking networks of the P(AM-co-AA)/Na-alginate/Fe3+ hydrogels can be locally and selectively dissociated by treating with NaOH aqueous solution with pH of 13 for 1 min and reformed by locally adding the additional Fe3+ solutions, making the hydrogels healable and cohesive. The healed hydrogels from the cutting surfaces can bear a tensile strength up to 7.1 MPa. Various complex hydrogel structures can be post constructed by using the P(AM-co-AA)/Na-alginate/Fe3+ hydrogels as building blocks via the adhesion of virgin prepared hydrogels. Keywords: ionic cross-linked hydrogels, ultrastiff, saline-resistance, healable, complex hydrogel structure

*Corresponding

author. E-mail: [email protected]. [email protected] 1

ACS Paragon Plus Environment

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INTRODUCTION Hydrogels, as a kind of soft wet material, bearing fascinating properties such as high water content, biocompatibility, and permeability, have been extensively investigated for a variety of applications in biomedical fields1-4. However, the conventional hydrogels have been limited in practical application owing to their weak and brittle nature. Over the past decades, based on introducing effective energy dissipation mechanism or overcoming their inherent structural inhomogeneity, many efforts have been devoted to develop strong and tough hydrogels, such as double-network hydrogels5,

nanocomposite

microgel-reinforced

hydrogels6,

hydrogels9,

slide-ring

polyampholytes

hydrogels7,

hydrogels10,

tetra-PEG

hydrogels8,

macromolecular

microphere

composite (MMC) hydrogels11, and the like. Currently, engineering stiff and tough hydrogels to meet much harsher condition such as substitutes for cartilage12 and skin13 is still a big challenge. To date, the tensile stress and elastic modulus of synthetic hydrogels can hardly surpass ten megapascals (MPa) 14-15, while the value of the natural counterparts can reach dozens of MPa. Suo et al.1 first reported the formation of tough hydrogels via the strong interaction between Ca2+ and carboxyl group of alginate. The obtained double network hydrogels containing a chemically cross-linked PAM network and Ca2+ ionic cross-linked alginate network exhibited excellent mechanical properties with the fraction energy of 9000J/m2, the elastic modulus of around 50-100 KPa, and the tensile strength of about 200 KPa. Lin et al14. reported the strong double network hydrogels with tensile strength of about 6 MPa, elastic modulus of about 4 MPa, and the fracture strain of about 700%, which consisted of a chemically cross-linked PAM network and an ionic cross-linked AA/Fe3+ network. Gong et al.

16

synthesized a stiff hydrogel containing a

polyacrylamide (PAM) network and an amphiphilic triblock copolymer, which can reach a tensile stress of ~10.5 MPa with elastic modulus of ~2.2 MPa or a tensile modulus of ~14.2 MPa with stress of ~5.5 MPa, depending on the composition of the hydrogels. However, the water contents of these hydrogels were less than 50 wt% and slightly lower than the requirement for using as 2


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biomimetic materials. Recently, a new type of tough hydrogels generated by enzymatic mineralization can modulate the stiffness up to 440 MPa, which was several times higher than that of cartilage materials17. However, the hydrogels can only be stretched to about several percent of the original length17, which limited their potential applications. The self-recovery property of hydrogels is also important to improve their fatigue resistance and extend their service life. The energy dissipation mechanisms of double network hydrogels with dual chemically cross-linked networks5 are mostly attributed to the break of sacrificial bonds in the first brittle network, which can then protect the second network and toughen the hydrogels. However, the fracture of covalent bonds is permanent and irreversible so that the hydrogels cannot recover from damage. To improve the self-recovery property of hydrogels, introduction of reversible physical bonds such as ionic bonds into the hydrogel networks was one possible method to remedy this defect1,10,14,18. For example, the polyampholyte hydrogels cross-linked by ionic bonds exhibited a fast-recovery property at room temperature10. However, up to now, the reported ionic bonds of hydrogel were unstable in saline solution and both the modulus and tensile strength of the obtained hydrogels decreased dramatically in saline solution, which limited their application in physiological circumstance10.

The saline-resistant property of hydrogels is crucial for the possible prospect of

hydrogels as cartilage substitute materials.

It will be thus very interesting to fabricate hydrogels

cross-linked with strong ionic bonds, which can be stable in saline solution. Furthermore, the incorporation of reversible physical bonds also renders the hydrogels healable10,14,16,19,20. The self-healing between two cut hydrogel surfaces was mostly reported, which was attributed to the reformation of physical bonds at the cut surfaces when they were brought into contact. The highest fracture stress that the healed hydrogels can withstand was 2.3 MPa as reported by Gong et al. for a tough physical double-network hydrogels cross-linked by strong hydrophobic association and weak hydrogen bonds of amphiphilic triblock copolymers16. Although such hydrogels were not automatic self-healable at room temperature, of which the authors attributed to the delayed recovery of strong hydrophobic associations and unfully contact of the cut surfaces due to the high stiffness of the gels, 3



Page 4 of 41

the cut surfaces of the hydrogels can be healed by first treating with N, N-dimethylformamide (DMF), then heating at 60 °C for 60 min, and finally immering in water to extract the DMF

16.

However, to our knowledge, the formation of noncolvent bonds between two virgin hydrogel surfaces or one virgin surface and one cut surface is not yet reported. The adhesion or cohesion of hydrogels can be utilized to construct hierarchical hydrogel structures that were difficult synthesized in-situ, which might expand the applications of hydrogels especially in the biomedical related fields. Herein,

we

report

a

poly(acrylamide-co-acrylic

acid)/Na-alginate/Fe3+

(P(AM-co-AA)/Na-alginate/Fe3+) hydrogel via the formation of hybrid ionic-hydrogen bond cross-linking networks, which was ultrastiff, tough, saline and pH resistant, good self-recovery, healable and cohesive. Note that no chemical cross-linker was used in the present work. The optimal P(AM-co-AA)/Na-alginate/Fe3+ hydrogel possessed super high elastic modulus (~24.6 MPa), tensile strength (~10.4 MPa), compression strength (~44 MPa), and toughness (~4800 J/m2). The hydrogel was highly stable and maintains its superior mechanical properties in 0.5-2 M NaCl solution, aqueous solution with pH ranging from 4 to 10. The healed hydrogels from the cutting surfaces can bear a fracture stress up to 7.1 MPa, which was, to the best of our knowledge, the highest value reported for healed hydrogels to date. We demonstrate for the first time that various complex hydrogel structures can be post constructed by using the P(AM-co-AA)/Na-alginate/Fe3+ hydrogels as building blocks via the formation of ionic bonds between the virgin prepared hydrogels.

EXPERIMENTAL SECTION Materials. Acrylamide (99%, AM), sodium alginate (at very low viscosity, Na-alginate), sodium alginate (at low viscosity, Na-alginate)

and α-ketoglutaric (98%) were purchased from J&K

Chemical. Acrylic acid (99%, AA) and alginic acid sodium salt powder (Na-alginate) were purchased from Sigma-Aldrich Chemicals. Iron(III) chloride hexahydrate (98%, FeCl3 • 6H2O), 4


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calcium chloride (CaCl2,  96%), aluminium chloride (AlCl3 • 6H2O,  97%), sodium chloride (99.8%, NaCl), formamide (97%), acetic acid (96%) and acetone(96%) were purchased from Sinopharm Chemical Reagent. The phosphate-buffered saline solutions (PBS 0.01 M, pH 7.4, 6.8, 5.0) were prepared according to the standard protocols. Deionized water was used in all the experiments. All the reagents were used as received. The viscosities of 1% Na-alginate, Na-alginate and Na-alginate in H2O at 25 oC were about ≤10 cp, 15-25 cp and 59.6 cp, respectively, according to the information provided in the webpage of manufactories. Table S1 summarizes the product information of the three sodium alginates used in the present work. Figure S1 shows the photos of the three sodium alginates, namely Na-alginate, Na-alginate and Na-alginate. Preparation of hydrogels. The hydrogels were prepared via a three-step procedure. First, we dissolved given amounts of Na-alginate (0.3 g), AA/AM (2.49 g in total) with various AA/AM molar ratios (0.05, 0.1, 0.15, and 0.2), and UV-initiator α-ketoglutaric (10 mg) at 40 oC in 7.2 g H2O to form a homogeneous solution. After degassing the solution with nitrogen gas for 10 min, the solution was transferred to a poly(methyl methacrylate) (PMMA) mold. The photo-initiation was conducted under UV light (365 nm) for 30 min to give h-Gels. The water content of h-Gels was fixed at 72 wt%. In the second step, the as-prepared h-Gels were removed carefully from the PMMA mold and immersed into the aqueous solution of ferric chloride with various concentrations (0.06 M, 0.12 M, and 0.3 M) for 24 h to give the d-Gels.

In the third step, the obtained d-Gels

were further immersed in a physiology saline solution (0.1536 M NaCl aqueous solution) for 24 h to remove the superfluous ferric ions and hence give the s-Gels. In order to investigate the difference of ionic bonds formed between ferric ions and carboxyl groups (COO-) of acrylic acid or sodium alginate, several controlled hydrogels were synthesized by using the similar preparation procedure described above. Firstly, PAM/Na-alginate-s-Gel with similar amount of AM and Na-alginate but without the addition of AA (i.e. 2.5 g AM and 0.3 g 5



Page 6 of 41

Na-alginate, corresponding 1.52 mmol COO-) and P(AM-co-AA)-s-Gel-I with 2.5 g AM and 1.52 mmol AA were prepared, respectively. In such conditions, the concentrations of COO- group in PAM/Na-alginate-s-Gel and P(AM-co-AA)-s-Gel-I were the same. However, the COO- group in PAM/Na-alginate-s-Gel came from Na-alginate, whereas the COO- group in P(AM-co-AA)-s-Gel-I came from AA.

The P(AM-co-AA)-s-Gel-II was prepared without the use of Na-alginate, of

which the AA molar concentration was equal to the summation of AA molar concentration and Na-alginate molar concentration in the s-Gel with AA/AM molar ratio of 0.2. In other word, the concentrations of COO- group in P(AM-co-AA)-s-Gel-II and s-Gel with AA/AM molar ratio of 0.2 were the same, but higher than those of PAM/Na-alginate-s-Gel and P(AM-co-AA)-s-Gel-I. The P(AM-co-AA)-s-Gel-III with AA/AM molar ratio of 0.2 and without the use of Na-alginate was also prepared. Note that the composition of AM and AA in P(AM-co-AA)-s-Gel-III was identical with that of s-Gel with AA/AM molar ratio of 0.2. Furthermore, in order to study the effects of different sodium alginates on the mechanical properties of resultant s-Gels, the s-Gel and s-Gel with AA/AM molar ratio of 0.2 were also prepared with the addition of Na-alginate and Na-alginate, respectively, by using the similar procedure as described above for the preparation of s-Gel. Preparation of linear PAM, PAA, P(AM-co-AA) and P(AM-co-AA)/Na-alginate.

Linear

polymers polyacrylamide (PAM) and poly(acrylic acid) (PAA) as well as the copolymer P(AM-co-AA) were synthesized by photo initiated free radical polymerization. The free radical copolymerization of AM and AA in H2O without the presence of Na-alginate was carried out under the same copolymerization condition as those of h-Gels. Briefly, the total mass fraction of monomers was 25 wt% and the molar ratios of AA to AM were 0.05, 0.1, 0.15 and 0.2, respectively.

Note that the h-Gels gave polymer mixture of P(AM-co-AA)/Na-alginate with

various AA/AM ratios. The yields of P(AM-co-AA) with various AA/AM molar ratios were measured to be larger than 95% and the yields P(AM-co-AA)/Na-alginate with various AA/AM molar ratios were about 98% as determined by 1H NMR and shown in Table S2. The 1H NMR 6


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spectra of AM, AA, Na-alginate, P(AM-co-AA) and P(AM-co-AA)/Na-alginate with AA/AM molar ratio of 0.2 were shown as Figure S2 in the supporting information.

For 1H NMR

measurements, the as-prepared P(AM-co-AA) and P(AM-co-AA)/Na-alginate (the h-Gel) were directly dissolved in D2O without any further treatment.

We also used acetone to wash out the

unreacted acrylamide (AM) and acrylic acid (AA) for five days from the h-Gels with AA/AM molar ratios of 0.05, 0.1, 0.15 and 0.2, respectively. The yield was calculated as (mass of purified h-Gel – mass of alginate)/mass of the monomers. The obtained yield was about 99.6%, 99%, 100%, and 99.6%, respectively, for the h-Gels with AA/AM molar ratios of 0.05, 0.1, 0.15 and 0.2. These results were consistent with the yields of about 98% as determined by

1H

NMR for

P(AM-co-AA)/Na-alginate with various AA/AM molar ratios. The results obtained from 1H-NMR measurements were considered to be more accurate than those determined from a mass balance. Furthermore, water cannot be used as solvent to wash out the unreacted AM and AA because the P(AM-co-AA) copolymers and alginates with small molecular weights will be washed away or dialyzed away. We had tested the dialysis of alginates in deionized water by using a dialysis tube with molecular weight cutoff of 3500 for three days. It was found that about 9% of alginates with low molecular weight were lost. We had also tested the dialysis of h-Gels in deionized water and found that significant amounts of product were dialyzed out, indicating that there were also a lot of P(AM-co-AA) copolymers with molecular weight smaller than 3500. The yields were larger than 0.95 for all P(AM-co-AA) copolymers with and without the presence of Na-alginate, which indicated that the copolymerization of AM and AA was almost completed regardless of the AA/AM molar ratios. It has been known that the hydroxyl groups of alginate can be attacked by the free radicals and hence alginate macro-radicals will be generated, resulting in the formation of grafted alginates.21,22 In order to check whether the P(AM-co-AA) grafted alginate will be formed during the free radical polymerization, the h-Gel with AA/AM molar ratio of 0.2 was freeze-dried and pulverized to powders. The dried h-Gel powders were then purified with a formamide/acetic acid mixture (1:1 7



Page 8 of 41

v/v) to remove the P(AM-co-AA) copolymers.22 Note that alginate or grafted alginate cannot dissolve in formamide/acetic acid mixed solvent but P(AM-co-AA) copolymers can. The formamide/acetic acid mixture was changed once a day and the insoluble products were separated and collected by suction filtration. The filter liquor was also collected and then precipitate in acetone in order to check whether there were P(AM-co-AA) copolymers in the liquor. Note that P(AA-co-AM) copolymers cannot dissolve in acetone. Large amount of precipitates were observed, indicating that there were a lot of P(AM-co-AA) copolymers extracted from the dried h-Gel powders by the formamide/acetic acid mixed solvent.

After purification in the formamide/acetic

acid mixture for three days, there was not P(AM-co-AA) copolymers in the filter liquor and the final insoluble products was collected and dried in oven. The grafting efficiency (%E) of grafted alginate in h-Gel with AA/AM molar ratio of 0.2 was then calculated as %E = (Wt.final-alginate – Wt.added-alginate)/Wt.added-monomers *100. Characterization of linear polymers and copolymers. 1H NMR spectra were recorded at room temperature on a Bruker 400 MHz Avance III spectrometer by using tetramethylsilane as the internal standard and D2O as solvent. The glass transition temperature (Tg) of the freeze-dried PAM, PAA, Na-alginate, P(AM-co-AA) and P(AM-co-AA)/Na-alginate with various AA/AM molar ratios were measured by using differential scanning calorimetry (DSC) on a DSC Q20 (TA Instruments) calorimeter with a heating rate of 20 oC/min under nitrogen atmosphere. To eliminate the thermal history of the samples, the samples were first heated from 40 oC to 140 oC and then cooled back to 40 oC at the rate of 20 oC/min. The pure PAA sample was heated again from 40 oC to 150 oC at the rate of 20 oC/min in order to determine the Tg. The other samples were heated again from 40 oC to 220 oC at the rate of 20 oC/min in order to determine the Tg. Fourier transform infrared

(FT-IR)

spectra

of

freeze-dried

PAM,

PAM/Na-alginate,

P(AM-co-AA)

and

P(AM-co-AA)/Na-alginate were recorded on a Bruker Vector 22 spectrometer with KBr pellets. Measurement of mechanical properties of the hydrogels. All mechanical properties of the hydrogels were measured by using an Instron model 3343 testing machine with a 1 KN load cell 8


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(Instron Corporation, MA) at room temperature. For tensile test and the cyclic tensile test, hydrogels with a thickness of 1.5-2.5 mm were cut into dumbbell-shaped samples with length of 20 mm and width of 4 mm (GB/T-528-2008). The rate of extension was fixed at 100 mm/min for tensile and loading-unloading tests. For the loading-unloading test, silicone oil was coated on the hydrogel surface to prevent water from evaporating from the samples. The elastic modulus was calculated from the slope at the strain ranging 5~15% of the stress-strain curve. In the compression test, samples with cylinder shapes (~ 5 mm in diameter and 2.5–4.5 mm in initial thickness) were used and the crosshead speed was set at 5 mm/min. A pure shear test was used to characterize the toughness of the hydrogels according to the method reported by Suo et al.1. Two different samples, notched and unnotched of the same hydrogels, were used to measure the fracture energy T. The rectangular shape with a length of 40 mm, width of 15 mm (a0) and thickness of 1.5~2.0 mm (b0) was clamped on two sides, and the distance between the two clamps was 20 mm. For the notched samples, an initial notch of 5 mm in length was cut using a razor blade at the middle. The rate of extension was also fixed at 100 mm/min. When the crack of notched sample started to propagate, the critical tensile distance between two clamps was recorded as L0. The area beneath the Force-Distance curves of unnotched samples U(L0) were then calculated, and the fracture energy was calculated as T = U(L0)/(a0 ×b0). At least three specimens were tested for each hydrogel. Determination of water contents of the hydrogels. The hydrogels were weighed on a balance and recorded as mwet. Afterwards, the hydrogels were dried in an oven at 120 oC until the weight kept constant. The weight was recorded as mdry. The water content was calculated as (mwet-mdry)/mwet. The average values were calculated from at least four independent data for each hydrogel.

RESULTS AND DISCUSSION Synthesis of P(AM-co-AA)/Na-alginate/Fe3+) Hydrogels. As shown in Scheme 1, the 9



Page 10 of 41

preparation of P(AM-co-AA)/Na-alginate/Fe3+ hydrogel was via a simple three-step procedure. In the first step, we synthesized the soft and sticky poly(acrylamide-co-acrylic acid)/Na-alginate (P(AM-co-AA)/Na-alginate) hydrogels by photo-initiated free radical copolymerization of acrylamide (AM) and acrylic acid (AA) with the presence of Na-alginate. Note that Na-alginate was used for the preparation of hydrogels in the present work if no otherwise stated. The amide group of AM, carboxyl group of AA and carboxylic groups of Na-alginate can form hydrogen bond with each other, leading to the formation of cross-linking network and hence the formation of weak hydrogels. The as-prepared hydrogels were named as h-Gel, where h referred to hydrogen bonding. Alginate is a linear copolymer that comprises mannuronic acid (M unit) and guluronic acid (G unit) associating with carboxylic groups on both units of alginate chains, which can form ionic bonds with trivalent ions such as ferric ions23. Furthermore, P(AM-co-AA) chains, which contain carboxyl groups, can also form ionic coordination interactions with ferric ions.

Therefore, in the second

step, we immersed the h-Gels into the aqueous solutions of ferric chloride with concentration of 0.3 M if no otherwise specified for 24 h to form ionic cross-linking between ferric ions and both alginate and P(AM-co-AA) chains, giving P(AM-co-AA)/Na-alginate/Fe3+ hydrogels. The obtained hydrogels were coded as d-Gel, where d presented dual cross-linked networks. The single carboxyl group COO- is a bidentate ligand and the coordination number of Fe3+ is 6

24.

For P(AM-co-AA)

and alginate molecular chains with multiple carboxyl groups, they can be considered as polydentate ligands. Furthermore, Fe3+ can also coordinate with free H2O molecules. Therefore, there were many possible complexed structures between ferric ions (Fe3+) and carboxyl groups (COO-), including uncombined ferric ions, monodentate coordination, bidentate chelating, bidentate bridging and cross-linking complexed structures in the hydrogel network, as shown in Scheme 1. Some of the possible complexed structures were shown Scheme 1b. Note that the oxygen atom of H2O molecules can also coordinate with Fe3+ ion, which was not shown in Scheme 1b. It can be seen that the complexed structures of (i) to (iv) did not contribute to the formation of cross-linked network of the hydrogel. Only the complexed structures formed between interchain carboxyl groups 10


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and ferric ions such as (v) to (viii) contributed the formation of cross-linked network and the mechanical properties of the hydrogels. In the third step, the d-Gels were soaked in physiological saline

solution

(0.1536

M

NaCl

aqueous

solution)

for

24

h

to

give

the

final

P(AM-co-AA)/Na-alginate/Fe3+ hydrogels, which were named as s-Gels, where s meant the treatment of saline solution. After immersing process, the free and weak coordinated Fe3+ ions were washed out and more cross-linking complexed structures like (v)-(viii) might be formed between ferric ions (Fe3+) and interchain carboxyl groups (COO-) in the s-Gel network as indicated in Scheme 1a.

All the hydrogels contain 50-80 wt% water, which were comparable to the water

contents of cartilage 12 and skin 13.

Scheme

1.

(a)

The

synthesis

route

of

hybrid

ionic-hydrogen

bond

cross-linked

P(AM-co-AA)/Na-alginate/Fe3+ hydrogels and the possible cross-linking structures. (b) Some possible complexed structures between COO- and Fe3+. (i) monodentate coordination, (ii) bidentate chelating, forming a chelate ring, (iii) bidentate bridging, (iv) intrachain bidentate chelating, forming two chelate rings, (v) cross-linking structure via monodentate coordination of two COOgroups, (vi) cross-linking structure via bidentate chelating, (vii) cross-linking structure via bidentate bridging, (viii) cross-linking structure via monodentate coordination of four COO- groups.

11



Page 12 of 41

To verify the proposed formation mechanism of s-Gels shown in Scheme 1, the yields of copolymer P(AM-co-AA) with various AA/AM molar ratios with and without the presence of Na-alginate were first investigated. The molar ratio of copolymers and residual monomers can be accurately determined by 1H-NMR spectrum.

Figure S2 shows the 1H-NMR spectra of monomer

AM and AA, Na-alginate, P(AM-co-AA) and P(AM-co-AA)/Na-alginate in D2O. For the monomers AM and AA, the characteristic chemical shifts of hydrogen atoms of carbon-carbon double bonds were in the range of 5.7 to 6.5 ppm. However, the chemical shift related with the active hydrogen of –NH2 and –COOH groups can be hardly observed due to the deuterium 12


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exchange between the monomers (AM and AA) and solvent (D2O). The chemical shifts of hydrogen atoms of Na-alginate appeared in the range of 3.7-5.0 ppm. For P(AM-co-AA) linear copolymer, the chemical shifts of hydrogen atoms on the backbone of copolymer chain appeared at 1.5-1.8 ppm and 2.1-2.4 ppm. Therefore, the presence of Na-alginate did not interfere with the chemical shifts of P(AM-co-AA) and the residual monomers. As a result, the ratio between the peak area of 1.5-1.8 ppm and 2.1-2.4 ppm and the summation of peak area of 1.5-1.8 ppm, 2.1-2.4 ppm and 5.7-6.5 ppm gave the yield of P(AM-co-AA). The yields of P(AM-co-AA) with various AA/AM molar ratios were then determined to be larger than 95% and the yields P(AM-co-AA)/Na-alginate with various AA/AM molar ratios were about 98%, as shown in Table S2. These results indicated that the copolymerization of AM and AA was almost completed regardless of AA/AM molar ratios and the presence of Na-alginate. Secondly, the glass transition temperatures (Tgs) of poly(acrylic acid) (PAA), polyacrylamide (PAM), Na-alginate, P(AM-co-AA) and P(AM-co-AA)/Na-alginate with AA/AM molar ratio of 0.05, 0.1, 0.15 and 0.2 were measured by DSC, respectively. For AA/AM aqueous solutions with concentration of 25 wt%, the pH value was 2.2, 2.1, 2.1 and 2.0 for AA/AM molar ratio of 0.05, 0.1, 0.15 and 0.2, respectively. After adding the sodium alginate (Na-alginate, 0.3 g), the pH value of the mixed aqueous solutions changed to be 3.8, 3.6, 3.5 and 3.4 for the solutions with AA/AM molar ratio of 0.05, 0.1, 0.15 and 0.2, respectively. Indeed, the addition of Na-alginate can slightly neutralize some of the acrylic acid, leading to the conversion of certain amount of sodium alginate to alginic acid. According to the report by Caraness et al.25, the reactivity ratios of acrylic acid and acrylamide were about 0.6 and 0.6 under acid condition (pH ~ 3.8) and the reactivity ratios of acrylic acid and acrylamide were about 1.7 and 0.5 under strong acid condition (pH ~ 2.2). Thus, it was speculated that the resultant P(AM-co-AA) copolymer might have a long block of polyacrylamide

chain.

To

address

this

speculation,

Tgs

of

P(AM-co-AA)

and

P(AM-co-AA)/Na-alginate were measured by DSC. Figure 1 shows the second heating traces of PAM, P(AM-co-AA) and P(AM-co-AA)/Na-alginate with AA/AM molar ratio of 0.2. Note that Tg 13



Page 14 of 41

of pure poly(acrylic acid) (PAA) and Na-alginate were measured to be about 121.1oC and 158.5oC, respectively, as shown in Figure S3. The Tg of pure poly(acrylamide) (PAM) was measured to be about 103.6oC, as shown in Figure1. Only one Tg of 105.8oC was observed for P(AM-co-AA) with AA/AM molar ratio of 0.2, which suggested that the obtained P(AM-co-AA) was a random copolymer. If the obtained P(AM-co-AA) copolymer had a long block of polyacrylamide, two Tgs might be expected. Nesrinne et al. also reported26 that the co-polymer systems poly(AM-co-AA) exhibited only one Tg, which had a value between the Tg of the individual homopolymers, namely PAM

and

PAA.

Our

results

were

consistent

with

Nesrinne

et

al.’s

report.

For

P(AM-co-AA)/Na-alginate with AA/AM molar ratio of 0.2, the Tg of 110.5oC was observed, and the Tg of Na-alginate cannot observed because of the low content of Na-alginate. This result further indicated that the addition of Na-alginate made the Tg of P(AM-co-AA) increased. It can be explained by the fact that the hydrogen bonds between the P(AM-co-AA) chains and Na-alginate chains limited the movements of polymer chains. Similar results were obtained for P(AM-co-AA) and P(AM-co-AA)/Na-alginate with AA/AM molar ratio of 0.05, 0.1, and 0.15 (data not shown). Therefore, it can be safely deduced that the P(AM-co-AA) copolymers obtained during the preparation of h-Gels were random copolymers. Furthermore, the reactivity ratios of acrylic acid and acrylamide reported by Caraness et al.25 were measured with diluted copolymerization system. However, for the preparation of h-Gels, the concentration of AM and AA was 25 wt%, which was much higher than those usually used for solution polymerization. In such high concentrated copolymerization system, the free radical might easily initiate the adjacent monomers regardless of AM or AA, leading to the random copolymerization and random copolymer P(AM-co-AA). Although the addition of Na-alginate slightly changed the pH value of the reaction system, it did not significantly affect the free radical copolymerization of AM and AA in these highly concentrated systems. Furthermore, P(AM-co-AA) grafted alginates might be also formed during the free radical copolymerization because it has been known that the hydroxyl groups of alginate can be attacked by the free radicals and hence alginate macro-radicals will be generated. The alginate macro-radicals 14


Page 15 of 41

will lead to the formation of P(AM-co-AA) grafted alginates. The amount of P(AM-co-AA) grafted alginates in the h-Gel with AA/AM molar ratio of 0.2 was then measured by purifying the dried h-Gel powders with the formamide/acetic acid mixed solvent as described in the experimental section. The grafting efficiency (%E) of grafted alginate in h-Gel with AA/AM molar ratio of 0.2 was determined to be 5.2%.

In other word, about 5.2 percent of monomers (AM and AA) formed

grafted P(AM-co-AA) chains onto alginate molecular chains. These results indicated that P(AM-co-AA) grafted alginates were indeed formed during the preparation of h-Gel via free radical polymerization and the h-Gel consisted of P(AM-co-AA) grafted alginates, ungrafted alginates and

Heat Flow

P(AM-co-AA) copolymers.

103.6oC 110.5oC 105.8oC

Endo

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


PAM P(AM-co-AA) P(AM-co-AA)/Na-alginate

50

100

150

o

200

Temperature ( C) Figure 1. The DSC curves of PAM, P(AM-co-AA) and P(AM-co-AA)/Na-alginate with AA/AM molar ratio of 0.2.

Thirdly, the types of hydrogen bonds formed in the Gels were investigated by FT-IR measurements. Figure 2 shows the FT-IR spectra of polyacrylamide (PAM), copolymer P(AM-co-AA), mixtures of PAM/Na-alginate and P(AM-co-AA)/Na-alginate. The absorption band at 3175 cm−1 was assigned to the symmetrical stretching of N-H27. The bands at 2931 and 1452 cm−1 were assigned to C-H stretching and bending vibration, respectively28. The band at 1669 cm-1 15



Page 16 of 41

was assigned to the stretching vibration of C=O group connected to the amide group, which was the characteristics of the acrylamide unit25, and the band at 1613 cm−1 was attributed to the bending vibrations of N-H. For P(AM-co-AA), PAM/Na-alginate and P(AM-co-AA)/Na-alginate, the band at 3175 cm−1 shifted to 3188, 3188 and 3189 cm−1, respectively. Furthermore, the band at 1669 cm−1 shifted to 1672, 1673 and 1677 cm−1 for P(AM-co-AA), PAM/Na-alginate and P(AM-co-AA)/Na-alginate, respectively. These results indicated that the strong hydrogen bonds between –CONH2 and –COO- were formed. Furthermore, the band at 1615 cm-1 in P (AM-co-AA) and PAM/Na-alginate can attributed to the asymmetrical stretching27 of –COO-. However, this band shifted to 1616 cm-1 for P(AM-co-AA)/Na-alginate, which indicated that the hydrogen bonds of carboxylic acid dimers were formed and the amount of such hydrogen bonds were less than those hydrogen bonds between amide and carboxylic acid. It was reasonable because the molar fraction of AM in the system was much larger than those of AA and Na-alginate. The higher concentration of AM will lead to the formation of more hydrogen bonds between amide and carboxylic acid than those of carboxylic acid dimers.

P(AM-co-AA)/Na-alginate 3189 2931

PAM/Na-alginate P(AM-co-AA) PAM

1677/1616

3188

2931

3188

2931

1452

1452 1673/1615 1452 1672/1615

3175 2931

1452 1669

4000

3500

3000

1500

1000

Wave number (cm -1) Figure

2.

The

FT-IR

spectra

of

PAM,

P(AM-co-AA),

P(AM-co-AA)/Na-alginate.

16


PAM/Na-alginate

and

Page 17 of 41 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


Finally, the contents of Fe3+ in the d-Gels and s-Gels were measured by weighting the Gels before and after immersing into the FeCl3 and NaCl aqueous solutions. After soaking the h-Gels in FeCl3 aqueous solution, the increasing mass in dried d-Gels can be attributed to the adsorption of Fe3+ and Cl-, which can be calculated by subtracting the mass of dried h-Gels from the mass of dried d-Gels. Because the conversion of AM and AA in h-Gels was about 98% (cf. Table S2), the effect of residual monomers could be neglected. Furthermore, the Fe3+ may displace the Na+ on the alginate and all Na+ ions may be displaced by Fe3+ if excess FeCl3 was used. However, it was difficult to measure the amount of Na+ in the resultant d-Gels and s-Gels. Therefore, we calculated the molar ratio of COO- and Fe3+ under two extreme conditions. One was that none Na+ ion was displaced by Fe3+ and the other was that all Na+ ions were displaced by Fe3+. The obtained molar ratio of COO- and Fe3+ in d-Gels and s-Gels was summarized in Table S3. For example, n(COO-)/n(Fe3+) was about 1.72 for the d-Gel with AA/AM molar ratio of 0.2 by assuming that none Na+ ion was displaced by Fe3+, whereas n(COO-)/n(Fe3+) was about 1.64 if all Na+ ions were assumed to be displaced by Fe3+. These results suggested that the replacement of Na+ by Fe3+ did not significantly affected the molar ratio of n(COO-)/n(Fe3+) of d-Gel. Similar results were also obtained for s-Gel. For s-Gel with AA/AM molar ratio of 0.2, n(COO-)/n(Fe3+) was in the range of 2.16~2.28. The n(COO-)/n(Fe3+) of s-Gels was much higher than those of corresponding d-Gels, which indicated that Fe3+ ions in the d-Gels indeed diffused out when immersing d-Gels in physiological saline solution (0.1536 M NaCl aqueous solution). These results suggested that one Fe3+ ion will coordinate with more carboxyl groups in s-Gels than those in d-Gels. The obtained n(COO-)/n(Fe3+)s for s-Gels with AA/AM molar ratios of 0.1, 0.15 and 0.2 were less than 3, which suggested that there were still enough Fe3+ for the formation of strong ionic bonds between Fe3+ and COO- group in the Gels. Therefore, the extent of ionic cross-linking did not reduce after immersing the d-Gels in saline solution. More cross-linking complexed structures like (v)-(viii) shown in Scheme 1b might be formed between ferric ions (Fe3+) and carboxyl groups (COO-) in the s-Gel network, leading to the increase of cross-linking density. The increase of cross-linking density of s-Gels was further 17



Page 18 of 41

confirmed by the decrease of the water content and the increase of mechanical properties (see below). High Mechanical Properties of P(AM-co-AA)/Na-alginate/Fe3+ Hydrogels. The typical strain-stress profiles of h-Gel, d-Gel and s-Gel prepared with AA/AM molar ratio of 0.2 are shown in Figure 3a and the corresponding elastic moduli are shown in Figure 3b. It can be seen that the tensile strength and elastic modulus of h-Gel were only ~30 KPa and ~100 KPa, respectively, which indicated the inherent weakness and softness of hydrogen-bond cross-linked h-Gel. In contrast, after immersing h-Gel in 0.3 M ferric solution for 24 h, the obtained d-Gel became strong and stiff with tensile strength of ~7.4 MPa and elastic modulus of ~8.5 MPa, which were about 247 times and 85 times higher than the corresponding values of h-Gel. The drastic promotion of the mechanical performance of d-Gel demonstrated that the introduction of ionic coordinate interaction was significant for the formation of tough hydrogels. Furthermore, when further soaking d-Gel in physiological saline solution for 24 h, the elastic modulus of resultant s-Gel increased up to ~24.6 MPa, which was about 3 times higher than that of corresponding d-Gel. strength of s-Gel can exceed 10 MPa.

Besides, the tensile

After soaking process, it was found that the transparent

saline solution turned to be faint yellow, showing the presence of ferric ions in the saline solution. This result suggested that when immersing d-Gels in saline solution, the superfluous Fe3+ ions diffused out from d-Gels and more cross-linking complexed structures between Fe3+ ions and carboxy groups were thus formed, leading to the formation of s-Gels with dramatically improved stiffness and strength.

After diffusion equilibrium, the stiff and strong hydrogels with

saline-resistance were obtained.

The compressive strain-stress curves of the three kinds of

hydrogels are shown in Figure S4.

The compression strength of h-Gels was only ~3 MPa at the

strain of 90%. However, the compression strength of s-Gels can reach ~44 MPa, which was about 14.7 times higher than that of h-Gels.

18


a h-Gel d-Gel s-Gel

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 6 4 2 0 0

50

100

150

200

Strain (%)

250

b Elastic modulus (MPa)

Page 19 of 41

300

25 20 15 10 5 0

h-Gel

d-Gel

s-Gel

Figure 3. Tensile stress-strain curves (a) and elastic modulus (b) of h-Gel, d-Gel and s-Gel. The inset of (a) is the images of dumbbell-shaped hydrogel samples, i.e. h-Gel (left), d-Gel (middle) and s-Gel (right).

The molar ratio of AA/AM was a crucial factor that can affect the mechanical performance of the resultant hydrogels.

Figure 4 shows the stress profiles, elastic moduli and fracture energies of

d-Gels and s-Gels obtained with AA/AM molar ratios of 0.05, 0.1, 0.15, and 0.2, respectively. It can be clearly observed that increasing the AA/AM molar ratio, the tensile strength of d-Gels increased along with the decrease of the fracture strain. The fracture strength of s-Gels followed the same rule with relatively low strain. The elastic moduli of d-Gels with AA/AM molar ratios of 0.05, 0.1, 0.15, and 0.2 were 1.34, 1.11, 3.09 and 9.82 MPa, respectively. The elastic moduli of s-Gels with AA/AM ratios of 0.05, 0.1, 0.15, and 0.2 were about 1.2, 2.4, 2.5 and 2.9 times higher than those of the corresponding d-Gels, respectively. It was understandable because more AA render the d-Gels with more carboxy groups and hence more cross-linking complexed structures between Fe3+ ions and carboxy groups can be formed after immersing in saline solution.

It can be also illustrated by

the change of water content between d-Gels and s-Gels, as shown in Figure S5. With AA/AM molar ratio of 0.05, the water content of d-Gels increased from 77.5 wt% to 79.4 wt% for the corresponding s-Gels.

Interestingly, with higher AA/AM molar ratios, the d-Gels slightly shrunk

after immersing in saline solution. The water content of s-Gels with AA/AM molar ratio of 0.15 was about 58.5 wt% as compared to that of 64.6 wt% for the corresponding d-Gels. The decrease of 19



water content suggested that the cross-linking density of obtained s-Gels was larger than that of corresponding d-Gels. Figure 4d shows that all the d-Gels and s-Gels prepared herein exhibit super toughness associated with ultra-high fracture energies as measured by pure shear test (supporting information, S-VIDEO 1 and 2). The fracture energies of d-Gels and s-Gels prepared with AA/AM molar ratio of 0.05 were ~2800 J/m2 and ~2600 J/m2, respectively, which were superior to that of most reported hydrogels. The fracture energies of d-Gels increased with increasing the molar ratio of AA/AM. For d-Gel and s-Gel with AA/AM molar ratio of 0.15, the fracture energies were ~6100 J/m2 and ~7100 J/m2, respectively. Especially, d-Gel with AA/AM molar ratio of 0.2 can reached remarkably ~11700 J/m2, which was comparable to that of natural rubbers29. However, the fracture energy of the corresponding s-Gels was only ~ 4800 J/m2.

It can be inferred from the pure

shear curves (Figure S6) that the ultra-toughness of the d-Gels and s-Gels can be attributed to the cooperative effects of super stiffness and stretchability. The appropriate association of stiffness and stretchability endowed d-Gel with AA/AM molar ratio of 0.2 the rubber-level toughness. However, the dramatic increase of stiffness for the corresponding s-Gel, i.e. elastic modulus of ~24.6 MPa, leaded to relatively low fracture energy of ~ 4800 J/m2.

Nevertheless, the above results indicated

that it was possible to engineer stiff and tough hydrogels by hybrid ionic-hydrogen bond cross-linked networks.

8

b

0.05 0.10 0.15 0.20

d-Gel

6

4

2

0

0

100

200

300

400

500

600

12 0.05 0.10 0.15 0.20

s-Gel 10

Stress (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

Page 20 of 41

8 6 4 2 0

700

Strain (%)

0

100

20


200

300

400

Strain (%)

500

600

Page 21 of 41

d

25 20

d-Gel s-Gel

2

Fracture energy ( J/m )

c Elastic modulus (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


15 10 5 0

0.05

0.10

0.15

AA/AM molar ratio

12000

9000

6000

3000

0

0.20

d-Gel s-Gel

0.05

0.10

0.15

AA/AM molar ratio

0.20

Figure 4. Stress-strain curves of d-Gels (a) and s-Gels (b), the elastic moduli (c) and fracture energies (d) of d-Gels and s-Gels with different molar ratios of AA/AM.

The effects of ferric concentration and the soaking times on the mechanical properties of the resultant d-Gels were further investigated.

We soaked the h-Gels with AA/AM molar ratio of 0.2

into FeCl3 aqueous solutions with concentrations of 0.06, 0.12 and 0.3 M, respectively, for 24 h.

It

was found that increasing the concentration of ferric chloride will obviously result in the increase of the elastic modulus and tensile strength of the resultant d-Gels along with the decrease of water contents, as shown in Figure S7. The strength of d-Gels soaking in 0.12 M Fe3+ was about twice higher than that of d-Gels soaking in 0.06 M Fe3+. Increasing the concentration of Fe3+ from 0.12 to 0.3 M, the tensile strength of d-Gels further increased ~1.2 times. It was understandable that more ionic coordinate interactions can be formed with higher concentration of Fe3+. Figure S8a shows the tensile stress profiles of the resultant d-Gels obtained by immersing the as-prepared h-Gels with AA/AM molar ratio of 0.2 into 0.3 M Fe3+ solutions for various times. Clearly, increasing the soaking times in Fe3+ solutions will enhance the mechanical properties of the resultant d-Gels. Meanwhile, the water content of d-Gels decreased with increasing the soaking time in Fe3+ solution, as shown in Figure S8b. The mechanical properties and water contents of d-Gels were stable when the soaking time exceeds 18 h.

The immersing time of d-Gels in physiology saline solution also

affected the mechanical properties of resultant s-Gels. Figure S9 shows that the tensile strength and modulus of s-Gels increase with increasing immersing time up to 24 h. After 24 h, the tensile 21



Page 22 of 41

strength and modulus of s-Gels remained almost unchanged even when immersing in saline solution for five days.

Figure 5.

Photos of s-Gels to demonstrate its excellent mechanical properties. (a) A 1 kg steel

block is lifted up by using a dumbbell s-Gel with AA/AM molar ratio of 0.2. (b) s-Gel with AA/AM molar ratio of 0.2 before compression (b1), immediately after compression (b2) and 30 min after compression (b3). (c) the good stretchability of s-Gel with AA/AM molar ratio of 0.15.

The super high toughness and notch-insensitivity of s-Gels with AA/AM molar ratio of 0.2 are demonstrated in Figure 5a. A dumbbell s-Gel sample with thickness of 1.8 mm, width of 4.0 mm and a hole with 5 mm in diameter at the bottom can easily lift up a 1 kg steel block without fracture at the middle or hole position. Figure 5b illustrates the outstanding compression properties and self-recovery of the s-Gels. Figures 5b1-b3 are the cylinder s-Gel sample before compression, immediately compression at the strain of 90 % and 30 min after release of compression, respectively. It can be seen that the s-Gel sample recovers to 88% of its original height after releasing the compression for 30 min. Figure 5c displays the good stretchability of s-Gels with AA/AM molar ratio of 0.15, which can be easily stretched to large extent. Various tough hydrogels formed by utilizing the strong interaction between ferric ion and the 22


Page 23 of 41 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


carboxyl group have been reported in literature. Lin et al14. reported the strong double network hydrogels, which consisted of a chemically cross-linked PAM network and an ionic cross-linked AA/Fe3+ network. This double network hydrogels exhibited a tensile strength of about 6 MPa, elastic modulus of about 4 MPa, and the fracture strain of about 700%.14 The P(AM-co-AA)/clay/Fe3+ hydrogels prepared by Hu et al.30 possessed the tensile strength of about 3.5 MPa, elastic modulus of about 0.6 MPa, and the fracture strain of about 2100%. The cross-linked network of the P(AM-co-AA)/clay/Fe3+ hydrogels was made up by the hydrogen bonds between clay and P(AM-co-AA) chains and the ionic coordinates between Fe3+ and −COO− groups of P(AM-co-AA)30. The P(AM-co-AA)/quaternized tunicate cellulose nanocrystals/Fe3+ hydrogels reported by Zhang et al.31 exhibited a tensile strength of 7.78 MPa, an elastic modulus of 12.66 MPa,

and

the

fracture

strain

of

300%.

The

P(AM-co-AA)/2-vinyl-

4,6-diamino-2-vinyl-1,3,5-triazine (VDT)/Fe3+ hydrogels, which were formed via the hydrogen bonds between VDT and P(AM-co-AA) chains and the ionic coordinates between Fe3+ and −COO− groups of P(AM-co-AA), exhibited a tensile strength of about 4.3 MPa, an elastic modulus of about 0.9 MPa, and the fracture train of 1760%.32 However, the role of each component played in the mechanical properties of the resultant hydrogels was not systematically investigated yet. In order to illustrate the contribution of AM, AA and Na-alginate to the mechanical properties of s-Gels in the present work, several control hydrogels, namely PAM/Na-alginate-s-Gel, P(AM-co-AA)-s-Gel-I, P(AM-co-AA)-s-Gel-II, and P(AM-co-AA)-s-Gel-III, were prepared as described in the experimental section.

Note that the concentration of COO- groups in PAM/Na-alginate-s-Gel and

P(AM-co-AA)-s-Gel-I was the same. However, the COO- group in PAM/Na-alginate-s-Gel came from Na-alginate, whereas the COO- group in P(AM-co-AA)-s-Gel-I came from AA. By using AA as the sole monomer, the as-prepared poly(acrylic acid) (PAA) cannot form solid gel because the water content was up to 93%. However, P(AM-co-AA)-s-Gel-I and P(AM-co-AA)-s-Gel-II were successfully obtained, which indicated that the presence of AM was important for the formation of cross-linking network via hydrogen bonding. Since the concentration of AM was much higher than 23



Page 24 of 41

that of AA, the AM contributed to the formation of main body of h-Gel. Figure S10 shows the strain-stress curves of PAM/Na-alginate-s-Gel and P(AM-co-AA)-s-Gel-I. It can be seen that PAM/Na-alginate-s-Gel exhibited a yield phenomenon at about 20% strain and the fracture strain at ~400%. The tensile strength and elastic modulus were about 0.23 and 1.23 MPa for PAM/Na-alginate-s-Gel. The water content of PAM/Na-alginate-s-Gel was about 83.0 wt%. No yield phenomenon was observed for P(AM-co-AA)-s-Gel-I. P(AM-co-AA)-s-Gel-I showed similar tensile strength and fracture strain as those of PAM/Na-alginate-s-Gel. However, the elastic modulus of P(AM-co-AA)-s-Gel-I was significantly smaller than that of PAM/Na-alginate-s-Gel. The yield phenomenon in PAM/Na-alginate gel might cause a better stiffening effect than AA. Secondly, we compared the mechanical properties of s-Gel with AA/AM molar ratio of 0.2 and P(AM-co-AA)-s-Gel-II. The concentration of COO- in P(AM-co-AA)-s-Gel-II and s-Gel with AA/AM molar ratio of 0.2 was the same, but higher than those in PAM/Na-alginate-s-Gel and P(AM-co-AA)-s-Gel-I. Figure S10b shows that the elastic modulus of P(AM-co-AA)-s-Gel-II was about 18.9 MPa, which was smaller than that of s-Gel with AA/AM molar ratio of 0.2 (i.e. ~24.6 MPa). The fracture strain of P(AM-co-AA)-s-Gel-II was about 308%, which was higher than that of s-Gel with AA/AM molar ratio of 0.2, i.e. about 206%. These results further indicated that Na-alginate had a better stiffening effect on the resultant s-Gel than AA. We further prepared P(AM-co-AA)-s-Gel-III with AA/AM molar ratio of 0.2, of which the composition of AM and AA was identical with that of s-Gel with AA/AM molar ratio of 0.2. The strain-stress curve of P(AM-co-AA)-s-Gel-III is given in Figure S10c. The tensile strength and modulus of P(AM-co-AA)-s-Gel-III were about 9.7 and 18.7 MPa, which were about 42 and 15 times higher than those of PAM/Na-alginate-s-Gel as shown in Figure S10a. The water content of P(AM-co-AA)-s-Gel-III was about 50.5 wt%. The s-Gel with AA/AM molar ratio of 0.2 exhibited a tensile strength of 10.4 MPa and elastic modulus of ~24.6 MPa, which were higher than those of P(AM-co-AA)-s-Gel-III.

From the above results, the roles of AA, AM and Na-alginate played in

the s-Gels can be deduced as following: (1) the role of AA was to form the main ionic cross-linking 24


Page 25 of 41 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


point between Fe3+ and carboxyl groups and bear the main mechanical performance, (2) the role of Na-alginate was to stiffen the gels via the formation of ionic interaction between Fe3+ and alginate chains and P(AM-co-AA) chains, and (3) the role of AM was to form the main scaffold of the s-Gels. As mentioned above, after soaking the weak h-Gels in FeCl3 aqueous solution, the obtained d-Gels were strong and tough. To further study the role of Fe3+ played in s-Gels, various s-Gels with AA/AM molar ratio of 0.2 were also prepared by using other species of ions, i.e. Na+, Ca2+, Al3+. It was found that the as-prepared h-Gel was dissolved after immersing it in 0.3 M NaCl aqueous solution. Furthermore, after immersing the as-prepared h-Gel in 0.3 M CaCl2 aqueous solution, the resultant d-Gel-Ca2+ can only maintain its shape in solution and the mechanical properties of d-Gel-Ca2+ were very weak. The obtained d-Gel-Ca2+ cannot even bear its own weight, as shown in Figure S11. After immersing the h-Gel in 0.3 M AlCl3 aqueous solution, the tensile strength and fracture strain of the resultant d-Gel-Al3+ were about 0.22 MPa and 817%, respectively, as shown in Figure S12. The water content of d-Gel-Al3+ was about 84.0 wt%. The s-Gel-Al3+ obtained by further immersing d-Gel-Al3+ in saline solution exhibited a tensile strength of 0.05 MPa and fracture strain of 435%. These results indicated that the d-Gels and s-Gels physically cross-linked with Ca2+ or Al3+ were much weaker than those physically cross-linked with Fe3+. The effects of types of sodium alginate on the mechanical properties of resultant hydrogels were also investigated. The s-Gel and s-Gel were synthesized by using the identical procedure of s-Gel but with different sodium alginates, namely Na-alginate and Na-alginate, as described in the experimental section. Note that the viscosities of 1% Na-alginate, Na-alginate and Na-alginate in H2O at 25 oC were about ≤10 cp, 15-25 cp and 59.6 cp, respectively, according to the information provided by the manufactories. In another word, the order of molecular weight of the three sodium alginates was Na-alginate < Na-alginate < Na-alginate. Figure 6 shows the stress-strain curves of s-Gel, s-Gel and s-Gel with AA/AM molar ratio of 0.2.

It can be seen that s-Gel exhibited an

25



elastic modulus of 31.3 MPa and tensile strength of 11.3MPa. The water content of s-Gel was about 50.4 wt%. The elastic modulus and tensile strength of s-Gel were about 33.2 MPa and 11.0 MPa, respectively, with the water content of 48.6 wt%. These results indicated that s-Gel, s-Gel and s-Gel had enhanced mechanical properties regardless of the type of sodium alginates used. The mechanical properties of s-Gel and s-Gel were even better than those of s-Gel. The sodium alginates with higher viscosity will lead to the higher elastic modulus and tensile strength of the resultant s-Gel and s-Gel. However, the higher viscosities of Na-alginate and Na-alginate will make the reaction solutions more viscous so that small bubbles in the reaction solutions were difficult to be exhausted, which will affect the preparation of hydrogels.

s-Gel s-Gel' s-Gel''

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

Page 26 of 41

8 6 4 2 0

0

50

100

150

200

250

Strain (%) Figure 6. The stress-strain curves of s-Gel, s-Gel and s-Gel with AA/AM molar ratio of 0.2 prepared with Na-alginate, Na-alginate and Na-alginate, respectively.

Hysteresis and Self-Recovery of P(AM-co-AA)/Na-alginate/Fe3+ Hydrogels. The hysteresis behaviors of d-Gels and s-Gels were further investigated via loading-unloading cycles. Apparent hysteresis loops were observed for d-Gels and s-Gels with various AA/AM molar ratios, as shown in Figure S13, indicating that d-Gels and s-Gels can effectively dissipate energy. When increasing the AA content, the area of hysteresis loop raised significantly. The toughness of the hydrogels can 26


Page 27 of 41

be represented by the energy dissipation, which can be calculated from the area of hysteresis loop. The d-Gels and s-Gels with AA/AM molar ratio of 0.2 can dissipate energy for ~2.8 MJ/m3 and ~5.0 MJ/m3, respectively, even at the strain of 100%, showing a super toughness.

The energy

dissipation of s-Gels was about twice of that of corresponding d-Gels, which can be attributed to the increment of elastic modulus for s-Gels.

b

d-Gel Origin 0 min 5 min 15 min 30 min

6 5 4

Hysteresis ratio (%)

Stress (MPa)

a

3 2 1 0

20

40

60

Strain (%)

80

20

20 40 10

20

0

5

10

15

20

25

0

30

100

40

Hysteresis ratio (%)

s-Gel

2

0

60

0

d

4

0

30

Wating time (min)

s-Gel Origin 0 min 5 min 30 min

6

d-Gel 80

100

10 8

40

40

60

Strain (%)

80

100

80

30

60

20 40

10

20 0

0

5

10

15

20

25

30

Residual strain (%)

Stress (MPa)

c

0

100

Residual strain (%)

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


0

Waiting time (min)

Figure 7. Cyclic tensile loading-unloading curves of d-Gel (a) and s-Gel (c) with various waiting times. The residual strain and hysteresis ratio of d-Gel (b) and s-Gel (d) with various the waiting times.

The self-recovery of d-Gel and s-Gel with AA/AM molar ratio of 0.2 was studied at room temperature. Figure 7 shows that both d-Gel and s-Gel exhibit good self-recovery at the strain of 100%. The hysteresis ratio and residual strain of d-Gel and s-Gel for different waiting times after the first loading-unloading cycle are shown in Figures 7b and 7d, respectively. For d-Gel, the 27



Page 28 of 41

hysteresis ratio increased from 23.7 % with the waiting time of 0 min to 63.7% with the waiting time of 5 min, while the residual strain decreased from 12.6% to 7.14%, indicating large self-recovery at initial stage (Figure 7b). Furthermore, the hysteresis ratio reached 76.8% at the waiting time of 30 min with residual strain recovering to 4.2%. Lin et al.14 reported the synthesis of hybrid ionic-covalent bonding P(AM-co-AA)/Fe3+ hydrogels with N,N’-methylenebis-acrylamide as the chemical cross-linker and Fe3+ as the ionic cross-linker, of which the hysteresis ratio can reach ~60% with the waiting time of 30 min and 87.6% after 4 h. Gong et al.16 synthesized physical double-network hydrogels with strong hydrophobic association of amphiphilic triblock copolymer poly(butyl methacrylate)-b-poly(methacrylic acid)-b-poly(butyl methacrylate) and hydrogen bonds between PAM and hydrophilic poly(methacrylic acid), the hydrogel with water content of ~44% exhibited a recovery efficiency of ~85% after a waiting time of 5 min. For s-Gels, when the second test was conducted immediately, the hysteresis ratio and residual strain are 21.7% and 16.0%, respectively. With waiting times of 5 min and 30 min, the hysteresis ratio can recover to 53.3 % and 57.1%, respectively, indicating fast recovery rate of ferric coordination interactions. These results indicated that the strong ionic coordination bonds in d-Gel and s-Gel networks can sever as the reversible bonds, which were broken to dissipate energies when external force was applied, while the dissociated ionic bonds can rebuild rapidly without any external stimuli after the loading was removed.

Table S4 summaries the tensile strength, elastic modulus, fracture energy and strain at

break of cartilage and various hydrogels reported in literature and the present work.

It can be seen

that the combination mechanical properties of s-Gels were excellent and most close to those of cartilage.

Stability of P(AM-co-AA)/Na-alginate/Fe3+ Hydrogels in Saline, PBS and Aqueous Solution with Various pH Values. The P(AM-co-AA)/Na-alginate/Fe3+ hydrogels exhibited highly stability in saline solutions with various concentration, including physiological saline concentration. As shown in Figures 8a and 8b, the tensile strength and elastic modulus of s-Gels with AA/AM molar 28


Page 29 of 41 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


ratio of 0.2 remain nearly unchanged after soaking in NaCl solutions with concentration of 0.5, 1 and 2 M for one week, respectively. The elastic moduli of s-Gels were 28.81, 27.97 and 28.62 MPa, respectively, after soaking in NaCl solutions with concentration of 0.5, 1 and 2 M for one week. However, the fracture strains decreased slightly with the increase of saline concentration.

The

corresponding water content of s-Gels changed from the original value of 51.4 wt% to 50, 49.1 and 46.8 wt%, respectively. The slightly decrease of water content in high concentration NaCl solution might be due to the salting out effect, leading to the enhancement of cross-linking networks. To our knowledge, this is for the first time that the physical ionic cross-linked hydrogels with excellent stability and mechanical properties in concentrated saline solutions were achieved. The stability and mechanical properties of P(AM-co-AA)/Na-alginate/Fe3+ hydrogels in 0.01 M phosphate-buffered saline (PBS) solutions with pH 7.4, 6.8 and 5.0 were further tested. It was found that s-Gels with AA/AM molar ratio of 0.2 swollen in PBS solutions, of which the water contents increased to be 65.4，61, and

66.5 wt% after soaking in PBS solutions with pH of 7.4, 6.8 and 5.0

for one week, respectively. The swollen s-Gels resulted in the decrease of tensile strength and elastic modulus. The corresponding tensile strength of the swollen s-Gels decreased to be 6.11, 7.2 and 5.36 MPa, respectively, as shown in Figure 8c.

Figure 8d shows that the corresponding

elastic modulus of the swollen s-Gels decreases to be 6.08, 7.98 and 3.75 MPa, respectively. However, the elongation at break of the swollen s-Gels increased to be 440%, 422%, and 538%, respectively.

Nevertheless, the swollen s-Gels still exhibited good mechanical properties in PBS

solutions although the decline of mechanical properties was observed because of the swelling of the hydrogels. The stabilities of P(AM-co-AA)/Na-alginate/Fe3+ hydrogels in acidic and base aqueous solutions were also investigated in the pH range of 1 to 13. Figure 8e shows the stress-strain curves of s-Gels with the AA/AM molar ratio of 0.2 after immersing in aqueous solutions with pH value of 1, 4 and 10 for one week, respectively. It can be seen that the s-Gels immersing in aqueous solutions with pH of 4 and 10 exhibited the tensile strength of 9.11 and 9.05 MPa, respectively. The corresponding 29



Page 30 of 41

elastic moduli of the soaked s-Gels were 21.07±3.85 MPa and 22.36±0.62 MPa (Figure 8f), which were similar with the original modulus of 22.67±2.02 MPa. The corresponding water contents of s-Gels were 55.2 and 55.3 wt%, respectively. These results indicated that the s-Gels were stable in the aqueous solutions with pH of 4 and 10.

However, the extreme acid and base solutions will

greatly affect the mechanical properties of s-Gels. After soaking in HCl aqueous solution with pH of 1 for one week, the tensile strength of s-Gels decreased to 3.74 MPa along with the increase of fractured strain to 750%, which was about 3.6 times higher than the original value (i.e. 209%). The corresponding elastic modulus also declined to be ca. 1.01 MPa, which was only 0.041 times of original value (i.e. ~24.6 MPa). The corresponding water content increased to 76.9 wt%, indicating that the s-Gels was strongly swollen in aqueous solution with pH = 1.

Furthermore, the three

dumbbell-shape s-Gels were completely disintegrated after immersing in NaOH aqueous solution with pH of 13 for one week, forming a very weak hydrogel, as shown in Figure S14. This phenomenon might be attributed to the fact that the high concentration OH- will capture Fe3+ ions, thus destroying the ionic cross-linking points of the hydrogels. The exposed hydrophilic network chains will gradually dissolve and swell in 0.1 M NaOH aqueous solution. In strong basic condition (pH = 13) with high concentration of OH-, the ferric ions Fe3+ will react with OH-, leading to the formation of Fe(OH)3. As a consequence, the ferric ions failed to form the strong ionic bonds with COO- groups of P(AM-co-AA) and Na-alginate, resulting in the degradation of s-Gels. Furthermore, the amide groups of P(AM-co-AA) might hydrolyze in strong basic condition. We thus performed FT-IR measurements for pure polyacrylamide before and after immersing in NaOH aqueous solutions with pH = 10 and 13 for one week, respectively. Figure S15 shows the FT-IR spectra of polyacrylamide before and after immersing in NaOH aqueous solution. A characteristic peak at 1560 cm-1 appeared for polyacrylamide after immersing in NaOH aqueous solution with pH = 13 for one week, which might be attributed to the stretching vibration of carboxylate group (– COO-).24 However, strong characteristic peaks associated with the vibration of amide groups were still observed, as shown in Figure S15. Therefore, it can be deduced that the amide group (– 30


Page 31 of 41

CONH2) can partially hydrolyze after immersing in NaOH aqueous solution with pH = 13 for one week.

However, no hydrolysis of polyacrylamide was observed after immersing in NaOH

aqueous solution with pH = 10 for one week. These results were consistent with the fact that s-Gels maintained its original form and mechanical properties after immersing in aqueous solution with pH = 10 for one week.

a

b

10

30

Elastic modulus (MPa)

Stress (MPa)

8 6 4 2 2M

0

0

50

100

1M

0.5M

150

25 20 15 10 5 0

200

0.5M

Strain (%) 8

pH 7.4 pH 5.0

4

2

0

0

100

200

300

400

2M

d

pH 6.8

6

1M

Concentration of NaCl solution


c 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


500

8

6

4

2

0

600

pH 7.4

Strain (%)

pH 6.8

pH of PBS solution

31


pH 5.0


f

10 pH 1.0 pH 4.0 pH 10.0

8


e

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

6 4 2 0

0

100

200

300

400

500

600

700

25 20 15 10 5 0

800

Page 32 of 41

Strain (%)

pH 1.0

pH 4.0

pH 10.0

pH of aqueous solution

Figure 8. Stress-strain curves (a) and the corresponding elastic modulus (b) of s-Gels with AA/AM molar ratio of 0.2 after soaking in NaCl solutions with concentration of 0.5, 1 and 2 M for one week, respectively. Stress-strain curves (c) and the corresponding elastic modulus (d) of s-Gels with AA/AM molar ratio of 0.2 after soaking in PBS solutions with pH of 7.4, 6.8 and 5.0 for one week, respectively. Stress-strain curves (e) and the corresponding elastic modulus (f) of s-Gels with AA/AM molar ratio of 0.2 after immersing in aqueous solutions with pH of 1, 4 and 10 for one week, respectively.

High

Healing

Efficiency

of

P(AM-co-AA)/Na-alginate/Fe3+

Hydrogels.

Although

P(AM-co-AA)/Na-alginate/Fe3+ hydrogels were cross-linked via absolute physical interactions of ionic and hydrogen bonds, the automatic self-healing between two cut surfaces was not observed. The possible reason might be due to the strong ionic interactions between ferric ions and carboxyl groups of polymer chains, which can hardly offer excess bonding points between the fractured surfaces. As mentioned above, the ionic bonds of the hydrogel can be dissociated by high concentration NaOH solution. Taking advantage of this property, we can heal the fractured hydrogels via the dissociation and re-association of ionic bonds. The cut surfaces are treated locally with 0.1 M NaOH solution (pH = 13) for 1 min to dissociate the local ionic bonds, brought in contact, and treated with 0.3 M ferric ions solution to reform the ionic bonds, and finally immersed in the ferric ions solution, leading to the completed formation of ionic bonds. 32


Although the above

Page 33 of 41

discussion and Figure S15 indicated that the amide group (–CONH2) can partially hydrolyze after immersing in NaOH aqueous solution with pH = 13 for one week, it was unlikely that amide groups in the s-Gels will hydrolyze when treating with 0.1 M NaOH solution (pH = 13) for 1 min. Figures 9a and 9b show the images of a healed s-Gel with AA/AM molar ratio of 0.2 and the tensile test of the healed and original s-Gels, respectively. The healed s-Gel can be bended without crack. Furthermore, a healed dumbbell sample with thickness of 1.93 mm, width of 4.0 mm and a hole with 5 mm in diameter at the bottom can easily lift up a 1 kg steel block without fracture at healed joint (Figure 9a).

The healed surface can bear a tensile strength up to 7.1 MPa with high healing

efficiency, which was about 3 times higher than that of 2.3MPa previously reported by Gong et al. 16 and

the new highest fractured stress reported for healed hydrogels to date.

b

Origin

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 6 4

Healed

2 0

0

50

100

150

Strain (%)

200

33


250


Page 34 of 41

Figure 9. (a) Photos of healing procedure of s-Gel. The healed s-Gel can be bended without crack at healed joint. Furthermore, a healed dumbbell sample with thickness of 1.93 mm, width of 4.0 mm and a hole with 5 mm in diameter at the bottom can easily lift up a 1 kg steel block. The red arrow and ellipse indicate the healed joints. (b) Stress-strain curves of healed and orgrinal s-Gels with AA/AM molar ratio of 0.2.

P(AM-co-AA)/Na-alginate/Fe3+ Hydrogels as Building Blocks. Moreover, the healing of s-Gels was not limited between two cut surfaces. The virgin surfaces of s-Gels can be adhered together by the dissociation and re-association of ionic bonds via the similar treating procedure of 0.1 M NaOH solution and 0.3 M ferric ions solution as described above. This interesting character rendered the virgin prepared s-Gels usable as building blocks for post constructing the sophisticated hydrogel structures, which can hardly achieve in direct gelation process. By localized dissociation and re-association of ionic bonds, the adhesion of s-Gels with different shape and dimension can be realized. Figure 10a shows a simple pyramid hydrogel pattern made by conjunction via adhesion of the ends of three virgin cylindrical s-Gels. Figure 10b shows a cross formed by the surfaces of two virgin s-Gel cylinders, indicating that the s-Gels can be adhered together firmly even with such small contact area. Figures 10c and 10d show that a sophisticated Chinese character and a park bench can be constructed by virgin s-Gels. The hydrogel park bench made by the s-Gel sheets and cylinders can bear 1 kg weight on its top, as shown in Figure S16. To our knowledge, this is for the first

time

that

the

adhesion

of

virgin

hydrogel

surfaces

is

reported.

Such

P(AM-co-AA)/Na-alginate/Fe3+ hydrogels with excellent healing and adhesive properties regardless of the cut or uncut surfaces might find interesting potential applications in various technical fields.

34


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Figure 10. Photos of hydrogel structures made via the adhesion of virgin prepared s-Gels with AA/AM molar ratio of 0.2. (a) A simple pyramid pattern, (b) a cross of two s-Gel cylinders, (c) a sophisticated Chinese character, and (d) a park bench.

CONCLUSION In this work, we report the P(AM-co-AA)/Na-alginate/Fe3+ hydrogels formed via strong ionic bonds and weak hydrogen bonds. The synergistic physical bonds between Fe3+, P(AM-co-AA) and alginate chains resulted in high stiffness, toughness, fatigue resistance, and saline resistance for the hydrogels. The optimal P(AM-co-AA)/Na-alginate/Fe3+ hydrogel possessed super high elastic modulus (~24.6 MPa), tensile strength (~10.4 MPa), compression strength (~44 MPa), and toughness (~4800 J/m2). The s-Gels were highly stable and maintain its superior mechanical properties in concentrated saline solution (0.5-2 M NaCl solution), acidic and basic aqueous 35



Page 36 of 41

solutions in the pH range of 4 to 10. The ionic cross-linking networks of the s-Gels can be locally and selectively dissociated and reformed via simple treatment of basic solution with pH 13 for 1 min and subsequent Fe3+ solution, making the hydrogels healable, adhesive and suitable as building blocks for the construction of complex hydrogel structures.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XX.XXXX/acsami.XXXXXXX. Additional stress-strain curves, pure shear curves, water contents of h-Gels, d-Gels, and s-Gels, 1H-NMR

spectra of AM, AA, Na-alginate, P(AM-co-AA), and P(AM-co-AA)/Na-alginate,

additional FT-IR spectra and DSC curves, Video of pure shear tests of d-Gels and s-Gels, additional photo of complex hydrogel structure.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. [email protected] ORCID Binyang Du: 0000-0002-5693-0325 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (No. 21674097), the second level of 2016 Zhejiang Province 151 Talent Project, and Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry (201601), Changchun Institute of Applied 36


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Chemistry, Chinese Academy of Sciences for financial support.

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Ultrastiff, Tough, and Healable IonicâHydrogen Bond Cross-Linked

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