High-Strength, Tough, Fatigue Resistant, and Self-Healing Hydrogel

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High-strength, tough, fatigue resistant and self-healing hydrogel based on dual physically cross-linked network Zhengyu Gong, Guoping Zhang, Xiaoliang Zeng, Jinhui Li, Gang Li, Wangping Huang, Rong Sun, and Ching-Ping Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05627 • Publication Date (Web): 22 Aug 2016 Downloaded from http://pubs.acs.org on August 24, 2016

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ACS Applied Materials & Interfaces

High-strength, tough, fatigue resistant and self-healing hydrogel based on dual physically cross-linked network Zhengyu Gong†,‡, Guoping Zhang†§*, Xiaoliang Zeng†,‡, Jinhui Li†,‡, Gang Li†,‡, Wangping Huang†, Rong Sun †, Chingping Wong§, ║ †

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, 518055, China. ‡ Shenzhen College of Advanced Technology, University of Chinese Academy of Sciences, Shenzhen, 518055, China. E-mail: [email protected] or [email protected]; Fax: +86-755-86392299; Tel: +86-755-86392104 § Department of Electronics Engineering, The Chinese University of Hong Kong, Hong Kong, China. ║ School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, Georgia, 30332, United States.

Abstract: Hydrogels usually suffer from low mechanical strength, which largely limit their application in many fields. In this paper, we prepared a dual physically cross-linked hydrogel composed of poly (acrylamide-co-acrylic acid) (PAM-co-PAA) and polyvinyl alcohol (PVA) by simple two-steps methods of copolymerization and freezing/thawing. The hydrogen bond-associated entanglement of copolymer chains formed as crosslinking points to construct the first network. After being subjected to the freezing/thawing treatment, PVA crystalline domains were formed to serve as knots of the second network. The hydrogels were demonstrated to integrate strength and toughness (1230±90 kPa and 1250 kJ/m3) by the introduction of second physically cross-linked network. What`s more, the hydrogels exhibited rapid recovery, excellent fatigue resistance, and self-healing property. The dynamic property of the dual physically cross-linked network contributes to the excellent energy dissipation and self-healing property. Therefore, this work provides a new route to understand the toughness mechanism of dual physically cross-linked hydrogels, hopefully promoting current hydrogel research and expanding their applications. KEYWORDS: H-bond, self-healing, toughness, dual physically cross-linked network, hydrogel

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1. INTRODUCTION Polymer hydrogels, containing plenty of water as dispersion medium, are a class of condensed matter cross-linked with covalent or non-covalent bonds.1 They are featuring a three-dimensional polymer network, which have been widely used for sensing,2 actuation,3 tissue engineering,4 and drug delivery carriers.5 The physical interactions for hydrogels with impermanent network include hydrophobic, ionic, and hydrogen bonding as well as crystalline domains and chain entanglement.6-10 Compared with chemically cross-linked hydrogels, the hydrogels based on physical nodes are characterized by their dynamic property. By exploiting non-covalent dynamics in hydrogel network, mechanical toughening of hydrogel materials have been reported.11-13 In addition, it may be an effective way to design and prepare self-healing gels by introducing physical nodes due to their dynamic property.14-15 Therefore, it`s available to increase the service life of as mentioned physically cross-linked hydrogel materials with self-healing ability when they suffer fatal damage, aiming to reach sustainable development in the long term. However, the hydrogels composed of just one physically cross-linked network are still limited to low mechanical strength, poor toughness, and poor recoverability, which greatly hinder their application for some fields requiring excellent integrated properties. Recently, the emerging double network (DN) hydrogels have attracted much attention because of their excellent mechanical properties.16-18 Two contrasting network, separately cross-linked by covalent or non-covalent bonds, act cooperatively with each other to enhance the toughness and strength of DN hydrogels. Upon deformation, the first network broke and dissipated the energy, while the second network works to retain its integrity. However, the hydrogels featuring both excellent mechanical properties and self-healing ability are few reported. Okay and coworkers prepared a hydrogel composed of a combination of a physically cross-linked network formed via reversible hydrophobic associations and a covalent network.19 When the molar ratio of covalent cross-linker content to the monomer reached 0.01, the hydrogels could withstand 8.1±0.1 MPa compressive stress which was more than 24


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and 4 times higher than that of the individual chemical or physical gels. However, its energy dissipation value and self-healing efficiency were decreasing obviously. A hybrid hydrogel reported by Li et al., consisting of PAM and partly crystallized PVA, achieved an elastic modulus of 5 MPa and strength of 2.5 MPa. This hydrogel wouldn`t rupture in concentrated electrolyte solutions, but no self-healing ability was mentioned.

20

Gong et al. reported a tough double network hydrogel consisting of

methacrylated chondroitin sulfate (MCS) and polyacrylamide (PAAm), which demonstrated a strength over 20 times greater than the single network (SN) of either MCS or PAAm, but no shelf-healing phenomena was mentioned.21 Recently, Chen et al. developed a new method to use the hydrogen-bond cross-linked agar gel as the first network and the hydrophobically cross-linked polyacrylamide gel (HPAAm gel) as the second network for a fully physically cross-linked DN gel, which demonstrated excellent mechanical properties and notable self-healing property via reassembly of the reversible network.22 However, it can’t realize high recovery efficiency of mechanical properties as the broken agar network couldn’t automatically recover through the reassembly of agar helical bundles at room temperature. Generally，most reported DN hydrogels are chemically cross-linked or hybrid-linked which usually had poor toughness and fatigue resistance, and lack of recovery mechanism restricted their application. In this work, we prepared double network hydrogels composed of polyvinyl alcohol (PVA) and poly (acrylamide-co-acrylic acid) (PAM-co-PAA) without any chemical cross-linkers which constructed the dual physically cross-linked network. Therefore, this work provides a new route to understand the toughness mechanism of dual physically cross-linked hydrogels, hopefully promoting current hydrogel research and expanding their applications. Induced by the hydrogen bond between the carboxyl group and acylamide group (-C=O···H-NH-C=O), the entanglement of copolymer chains was formed as cross-linking points to construct the first three-dimentional network. PVA crystalline domains were generated through freezing/thawing procedure which acted as knots of the second network. The PVA hydrogels fabricated through freezing/thawing method



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have been extensively investigated.23-26 Compared with the traditional full chemically or hybrid physically-chemically cross-linked DN gels, this full physically crossed DN design hopefully provides the hydrogel additional platform to bear stress, dissipate energy, and reconstruct the network upon deformation, helping to promote strength, toughness,

fatigue

resistant.

And

the

hydrogen

bond

between

poly

(acrylamide-co-acrylic acid) and polyvinyl alcohol offer the gels self-healing ability. What`s more, the mechanical strength of the hydrogels can easily be tuned by varying the copolymer content and the number of freezing/thawing cycles. 2. EXPEIMENTAL METHOD Materials.

Polyvinyl

alcohol

powder

(PVA,

Mw

89000–98000,

hydrolysis>99%), was purchased from Sinopharm Chemical Reagent Co.,Ltd. Acrylamide (AM, reagent grade, 99%), acrylic acid (AA, GC, >99%) , Ammonium persulfate (AR, ≥98%), N,N′-Methylenebis(acrylamide) (96%) were purchased from Aladdin. All reagents were used as received. Preparation of PVA/PAM-co-PAA double network hydrogel. The polyvinyl alcohol/poly(acrylamide-co-acrylic acid) (PVA/CP, CP means copolymer) double network hydrogel was prepared as the following process. Firstly, PVA powder was dissolved in distilled water at 90 °C and the mixture was stirred to be homogeneous. Then, acrylamide and acrylic acid were added into the PVA solution. After the mixture being de-oxygen for 20 min with nitrogen gas, 1 wt% initiator ammonium persulfate (APS) was sequentially feeding. The content of PVA/(PVA+H2O) was fixed at 15 wt%. Unless otherwise stated, the weight ratio of copolymer/(copolymer+H2O) was 45 wt%, molar ratio of AM/AA was 1:1, and one freezing/thawing cycle was proceeding. At last, the resultant mixture was transferred into a glass mold, and kept at 60 °C for 6 h to complete the reaction. After that, the samples were frozen at −25 °C for 4 h, which was followed by thawing at room temperature for 10 h at least. For control tests, the as-mentioned hydrogels without freezing/thawing procedure (single physically cross-linked network) were also prepared, marked as “PVA/CP SN gel”, and the hydrogels consisting of


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physically–chemically hybrid cross-linked network, marked as “PVA/CP-C DN gel”, were prepared by adding N,N′-Methylenebis(acrylamide) as chemical cross-linking agent (0.1% molar ratio of comonomers), then applied to the freezing/thawing procedure.

Figure 1. Schematic illustration for the preparation of fully physically cross-linked PVA/CP DN hydrogel. Characterization. Wide-angle X-ray diffraction (XRD) patterns were recorded on Rigaku D/MAX-2500/PC X-ray powder diffractometer with Cu Kα radiation (λ=0.15406 nm); the range of the diffraction angle (2θ) was 5–50° at a scanning speed of 6° min-1. Scanning electronic micrographs (SEM, Nova NanoSEM 450) was used to characterize the morphology of hydrogel samples after freezing/drying process. A microcomputer controlled electronic universal testing machine (AG-X Plus, SHIMADZU) equipped with a force sensor (100 N) was used to measure tensile mechanical properties of hydrogels, with a gauge length of 30mm and a drawing rate of 100 mm/min. The samples were cut to size (50 mm×10 mm×1 mm). The gel samples were coated with a thin layer of silicon oil to prevent the evaporation of water and each sample test was repeated at least five times to ensure reproducibility. The hysteresis energy, △Ui, in the ith loading-unloading cycle is defined as: ∆Ui = ∫ σdε

………………………..(1)

where σ is stress, and ε is strain. The energy loss coefficient, η, measures how efficiently a material dissipates energy and is calculated as shown below:



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η= Ui = ∫

∆Ui Ui

σ max

0

σdε

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………………………..(2)

………………………..(3)

Ui is the elastic energy stored in the materials when it is loaded elastically to a stress σmax in the ith cycle. The dynamic modulus of the hydrogels was measured on an Anton Paar MCR 302 rheometer. The frequency scan from 1 Hz to 100 Hz was conducted at room temperature by using the shear mode. The gap at the apex of the para-plate was set to be 2–3 mm. 3. RESULTS AND DISCUSSION

Figure 2. WAXD patterns of the gel before (PVA/CP SN gel) and after (PVA/CP DN gel) various cycles of freezing/thawing. Synthesis of PVA/CP DN gel. The general procedure to prepare PVA/CP DN hydrogel through two steps is illustrated in Figure 1. Firstly, all the monomers including AM, AA and initiator are dissolved in PVA solution and heated to complete the copolymerization of AM and AA, obtaining PVA/CP SN gel. The weak cooperative H-bonding between acrylamide and acrylic acid lead them to aggregate into

clusters.27

After

polymerization,

the

hydrogen

bond-associated chain

entanglement of PAM-co-PAA serves as cross-linking points to form the first network of the DN gel. It can be seen from Figure 1 that the homogeneous solution get gelation after being heated for 6 h. Secondly, the resulting gel is subjected to different


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freezing/thawing cycles, which generates PVA crystalline domains for constructing the second network of the DN gel. The 101ത reflection of typical PVA crystalline domain for PVA/CP DN gel is showed as a sharp diffraction peak at 2θ=19.7° from the X-ray diffraction measurement results in Figure 2, but no characteristic diffraction peak of crystallized PVA can be observed before freezing/thawing.28 Additionally, the diffraction intensity increase as the hydrogel is subjected to more freezing/thawing cycles (in Figure 2, two and three cycles), which means a higher crystallinity and more stable crystalline microdomains are formed. Compared with traditional chemically cross-linked hydrogels, no extra cross-linking agent is introduced into this DN hydrogel system. Therefore, the fully physically cross-linked PVA/CP DN hydrogel can be fabricated via this facile method. All the PVA/CP DN gels with varying CP contents were subjected to freeze-drying procedure to fabricate samples for SEM characterization and the typical SEM images of cross-section of different PVA/CP samples were shown in Figure 3. It can be seen that the profile of pure PVA without any CP is nearly smooth and there is no obvious pore in the microstructure, because of pores shrinkage during freeze-drying which is consistent with previous reports.29-30 However, with increasing the CP content, there are more pores formed and the average pore size was also increasing. When the CP content is increased to 45 wt%, average size of the pores in the PVA/CP DN hydrogel increases from 1.13 µm to 3.65 µm. This may be explained by two reasons: on one hand, copolymer chain entanglement domain severs as cross-link points and polymer segments act as skeleton to construct the gel filling with liquid water as dispersion medium; on the other hand, addition of PAM-co-PAA prevents the PVA polymer chains from being close to form hydrogen bonding, so that the density of PVA crystalline domains is decreased, leading to a more loose structure (Figure 3d). 29 Mechanical properties of PVA/CP DN gel. Taking advantage of the fully physically interpenetrating DN architecture, the as-prepared PVA/CP gel possesses excellently mechanical strength. The typical stress–strain curves of PVA/CP DN gel, PVA/CP-C DN gel and PVA/CP SN gel were characterized as shown in Figure 4. It is



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seen that the tensile strength (σmax) of PVA/CP SN gels is only 360 kPa at the strain of 442% (Figure 4a). In contrast, the stress of PVA/CP DN gels can increase up to 990 kPa, which is nearly 3 times higher than that of PVA/CP SN gels , and the corresponding ultimate elongation (εmax) is 534%, also a little larger than that of

Figure 3. SEM images of PVA/CP DN hydrogel with different CP contents: a) 0 wt%; b) 15 wt%; c) 30 wt%; d) 45 wt%.

PVA/CP SN gels (502%). These results suggest that the introduction of another cross-linking network can significantly improve the mechanical strength of gels. Furthermore, PVA/CP DN gels also exhibit slighter better tensile properties than PVA/CP-C DN gels (σmax of 830 kPa, εmax of 442%). The second physically cross-linked network additionally contributes to dissipate energy, bear stress, and reconstruct the network upon deformation. Therefore, compared with single physical network or physically-chemically hybrid network hydrogel, the double physically cross-linked network makes the hydrogel exhibit high mechanical strength. For PVA/CP DN gels, further investigation was performed to study the effect that the copolymer content and number of freezing/thawing cycles had on tensile mechanical properties. As shown in Figure 4b, with the increasing content of PAM-co-PAA, the mechanical properties of the PVA/CP DN gel are significantly enhanced. The fracture tensile strength of pure PVA gel (15 wt%), prepared via one freezing/thawing cycle, gets only 32 kPa, while it climbs to 980 kPa when the content


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of copolymer is increased up to 45 wt%. Similar to the trend of fracture tensile strength, the young`s modulus (E，calculated in the initial linear range from the stress–strain curves) also elevate from 0.019 kPa to 268 kPa when CP content increases from 0 wt% to 45 wt%, demonstrating superior deformation resistance of PVA/CP DN gel. The enhancement of mechanical properties can be ascribed that a more stable network is constructed as more cross-linking points being formed when

Figure 4. a) Tensile stress–strain curves of PVA/CP DN gel, PVA/CP-C DN gel and PVA/CP SN gel; b) Tensile stress–strain curves of PVA/CP DN gel with one cycle of freezing/thawing procedure at different copolymer contents and c) with different cycles of freezing/thawing procedure at 35 wt% copolymer.

Table 1. Tensile strength and young`s modulus of PVA/CP DN gel with different copolymer contents and freezing/thawing cycles. freezing/thawing cycle number

15wt% σmax(kPa)

E(kPa)

30wt% σmax(kPa)

E(kPa)

45wt% σmax(kPa)

E(kPa)

1

298±30

70±5

470±37

230±20

980±72

268±25

2

650±55

120±20

760±60

280±30

1020±88

306±34

3

980±72

130±24

1130±75

320±35

1230±90

364±38

the copolymer content increasing. As shown in Figure 4c, by subjecting the PVA/CP DN gel to various cycles of freezing/thawing, the tensile strength is further improved with more freezing/thawing cycle procedure. The hydrogel with one, two, and three



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cycle(s) number of the freezing/thawing procedure have a tensile strength up to 481 kPa, 771 kPa and 1223 kPa, respectively. The enhanced network of PVA with more stable crystalline micro-domains should be responsible for that improvement. However, the hybrid cross-linked gels reported by Li et al outperform PVA/CP DN gels in the view of E (5 MPa) and σmax (2.5 MPa) since they were prepared through annealing process which promotes crystallization at the cost of water reduction. 20 In addition, for PVA/CP DN gel, the ultimate elongation also increases with increasing the number of freezing/thawing cycles, which is similar to the results of Li`s work.31 The detailed data of tensile tests can be found in Table 1 and their statistical analysis can be found in Figure S1 and S2. All these results suggest that the fully physically cross-linked network acts cooperatively with each other to enhance the mechanical strength of the hydrogels. It can be explained that enhanced physically cross-linked network is able to better sustain the stress resulting from stretching, making the failure occur at a greater elongation of the sample.

Figure 5. Angular frequency dependence of storage modulus and loss modulus at 25 °C for PVA/CP DN hydrogels. a, b) With different CP contents proceeding one freezing/thawing cycle; c, d) With different freezing/thawing cycles (35 wt% of CP).

It`s sensitive to the internal structure of bulk polymers for polymer viscoelasticity,


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so that rheological characterization can provide insight into the properties of hydrogel.32-33 Figure 5 illustrates the frequency dependence of the storage modulus (G`) and loss modulus (G``) for PVA/CP DN gels with different CP contents and being subjected to various cycles of freezing/thawing. It can be seen from Figure 5a-d that G` remains much higher than G`` over the entire frequency range, which indicate that all these hydrogels have three-dimentional network formed inherently and presented a dominant elastic property. It is worth noting that the G` is nearly unchanged over the surveyed frequency at low CP content while it`s increasing distinctly at high CP content. We presume that with the CP content increasing, the physically cross-linked network of PVA resulting from chain crystallization is less powerful to provide a stable network for hydrogel that restricts the entangling copolymer chain motions. It can be further confirmed through Figure 5c that with being subjected to more freezing/thawing cycles, the G` tends to vary little over the entire frequency range, which can be explained that gels build up more PVA crystalline domains, forming more stable network, to further restrict the chain motions. Anyway, increasing copolymer contents and numbers of freezing/thawing cycles make G` higher, which agree to the above results of tensile tests. Toughness of PVA/CP DN gel. Besides, toughness of hydrogels usually offers remarkable dissipation of energy, which is of great importance to be practical application.34 Therefore, loading-unloading tests were performed to evaluate the energy dissipation and energy dissipation coefficient of PVA/CP DN gel as shown in Figure 6, which could be calculated from the hysteresis curves as mentioned above. These independent PVA/CP DN gel samples were firstly loaded with different strain and then unloaded to zero force. The pronounced hysteresis in Figure 6a suggest that PVA/CP DN gels could effectively dissipate energy, and the hysteresis loop become bigger at high strain which is essential for tough hydrogels. A quantified datum for the energy dissipation and energy dissipation coefficient of PVA/CP DN gel are demonstrated in Figure 6b. It clearly shows that the as-prepared hydrogel can dissipate energy (Uhys) as much as 1250 kJ/m3 at the strain 500% with a corresponding coefficient reaching 66.5%.



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Figure 6c compares the stress-strain curves of PVA/CP DN gel, PVA/CP-C DN gel and PVA/CP SN gel during a loading-unloading cycle at the same strain (300%) to reveal their energy dissipation capacity. Obvious hysteresis loops are observed in all three types of hydrogels, but PVA/CP DN gel has the largest hysteresis loop. The corresponding dissipation energy of PVA/CP DN gel is 390 kJ/m3, while PVA/CP-C DN and PVA/CP SN gel have comparable smaller dissipated energy of 200 and 240 kJ/m3, respectively. These results support that PVA/CP DN gel dissipates energy more efficiently than PVA/CP-C DN gel and PVA/CP SN gel under deformation, thus leading to high mechanical strength and toughness, for which the cooperation of two

Table 2. Dissipated energy, young`s modulus and their recovery of PVA/CP DN gel, PVA/CP-C DN gel, and PVA/CP SN gel after successive two loading–unloading cycles.

Hydrogel samples

1st loading-unloading cycle

2nd loading-unloading cycle

recovery

Uhys(kJ/m3)

E(kPa)

Uhys(kJ/m3)

PVA/CP DN gel

390±35

503±40

150±37

250±20

38

49.7

PVA/CP-C DN gel

200±20

420±20

140±25

220±30

70

52.3

320±40

139±26

130±35

PVA/CP SN gel

240±24

E(kPa)

Uhys(%)

58

E(%)

40.6

physically cross-linked network plays a key role. After the first loading-unloading cycle, the second loading cycle was applied to the same gel specimens without rest. As shown in Figure 6d, all three types of gels exhibit very small hysteresis loops, which represent limited energy dissipation in the second loading cycle. Detailed data can be found in Table 2. Quantitatively, PVA/CP DN gel, PVA/CP-C DN gel and PVA/CP SN gel loss their 62%, 30%, and 42% of toughness, and their 50%, 48%, and 60% of elastic modulus, respectively. Regardless of different network, structures, and interactions, all the three gels are soften and weaken after the first loading. Different from hybrid-linked PVA/CP-C DN gel and single linked PVA/CP SN gel, both


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physically cross-linked network of PVA/CP DN gel contribute to energy dissipation via chain dissociations when PVA/CP DN gel is stretched, leading to a large hysteresis of DN gels. When the hydrogel suffers tensile deformation, the copolymer chains transfer from an entangled state to a disentangled state as shown in Figure 7, and the destruction of the H-bond between PAM-co-PAA chains happens meanwhile. At the same time, the PVA crystalline domains partly unzip under deformation with dissipating energy, which offer the DN gel additional toughness.

Figure 6. a) Loading–unloading tests of PVA/CP DN gels under different strain (50%, 100%, 200%, 300%, 400%, 500%); b) The calculated energy dissipated and energy dissipation coefficient of PVA/CP DN gels during the loading–unloading tests under different strains; c) The first and d) the second, without rest, loading-unloading stress-strain curves of PVA/CP DN gel, PVA/CP-C DN gel and PVA/CP SN gel under 300% strain.

After deformation, the disentangled copolymer chains thermally tend to curl again and unzipped crystallites can also recover at room temperature.20, 35 Based on the dynamic property of the dual physically cross-linked network, PVA/CP DN gels complete the disassociation and association performance accompanied with applied energy dissipated and self-recovery.



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Figure 7. Scheme of tensile deformation and self-recovery mechanism of PVA/CP DN gels. Anti-fatigue and self-healing property of PVA/CP DN gel. Considering that the PVA/CP DN gels was based on dual physically cross-linked network, we conducted the consecutive loading–unloading cycle tests at a maximum strain of 300%, 5 min of resting between two tests, to test their anti-fatigue property. As shown in Figure 8a, the areas of hysteresis loops of PVA/CP DN gel almost remain constant for the following consecutive cycle tests except some decrease occurring after the first cycle. As mentioned above, it`s impossible for the copolymer entanglement to fully recover, which leading to the decrease in toughness of PVA/CP DN gel after the first cycle. Even so, the dissipated energy (285 kJ/m3) after the first cycle achieves about 73% of that of the first cycle (390 kJ/m3), suggesting PVA/CP DN gel have great fatigue resistance. In addition, compared with the dissipated energy (~150 kJ/m3) obtained from immediately second cycle, the PVA/CP DN gel also shows a good recovery rate.


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Figure 8. a) Five successive cyclic tensile loading–unloading curves of PVA/CP DN gel at a strain of 300% with 5 min resting between two tests; b) Dissipated energy of PVA/CP DN gel calculated from the area of their hysteresis loops.

In addition, we further challenged our PVA/CP DN gel for its self-healing property. Firstly, the gel sample with a copolymer monomer molar ratio of 1:1 was cut into half, and the fracture surfaces were physically put together at 37 oC (temperature of human body) for 12 h. Unfortunately, we found that the healed gel was easily broken down at the fracture place when stretch stress was applied. We presumed that there were not enough H-bond formed to bridge the fracture face. Therefore, we increased the PAA chain segment content in the copolymer by tuning the ratio of AM/AA to 2:8, hoping that more H-bond would be formed between free PAA segment and PVA chain. As shown in Figure 9 a1-a3, the healed gel can hold its own weight and bending deformation. The same gel healed for 12 h can withstand a rather large stretching deformation of 200% (Figure 9. a4-a7). The healing efficiency of PVA/CP DN gels is quantitatively evaluated by comparing the elongation at break for original and healed gel samples. As shown in Figure 9b, the healed gel with monomer ratio of 2:8 achieves the ultimate elongation of 245%, while the healed gel with monomer ratio of 1:1 shows a much lower elongation at break of 89%. The self-healing efficiency of PVA/CP DN gel, with different monomer mole ratio, 1:1 and 2:8, are determined and corresponded respectively to 16% and 37% (Figure 9c). Further investigation of factors on self-healing property has been carried out and specific self-healing mechanism will be discussed in the future work.



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Figure 9. a) Self-healing properties of PVA/CP DN gel: a1-a3) The healed gel at room temperature in 2 min (a1), can bear its weight (a2) can be bended without break (a3); The specimen after self-healing at 37 °C for 12 h can be stretched up to more than 200% in length (a4–a7); b) Tensile stress–strain curves of healed PVA/CP DN gel with different monomer ratio; c) Healing efficiency of PVA/CP DN gel with different monomer ratio.

Comparison with other self-healing double-network hydrogels. In Figure 10, we present a chart for compare the various self-healing double-network hydrogels, including

physically-chemically

cross-linked

double

network

and

physically-physically cross-linked double network. In terms of strength and toughness, our PVA/CP DN gels outperform most hydrogels. For silica nanoparticle/PDMAA composite

hydrogels36,

graphene

oxide/PAACA composite

hydrogels37

and

DN-HPAAm hydrogels38, they all have tensile strength below 100 kPa and dissipation energy below 100 kJ/m3. HAPAM/LAPONITE® composite hydrogels39 and graphene oxide/HAPAM composite hydrogels40 make their elongation at break be several thousand times over their own initial length, but still have relatively low strength and are incapable to dissipation energy sufficiently. Our PVA/CP DN gels integrate strength and toughness (1230±90 kPa and 1250 kJ/m3), and also have medium


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elongation at break (~550%). Anyway, our self-healing double-network hydrogels are more promising to be used in the practical application.

Figure 10. Tensile strength versus dissipation energy charts for comparison of various self-healing double-network hydrogels, including 1) silica nanoparticle/PDMAA composite hydrogel (P-C)36; 2) graphene oxide /PAACA composite hydrogel (P-P)37; 3) DN-HPAAm (P-C) hydrogel38; 4) HAPAM/LAPONITE® composite hydrogel (P-P)39; 5) graphene oxide /HAPAM composite hydrogel (P-P)40 and 6) our PVA/CP DN hydrogel (P-P) in this work. P-C means physically-chemically cross-linked double network; P-P means physically-physically cross-linked double network.

4. CONCLUSIONS In this work, we developed a new type of dual physically cross-linked hydrogel composed of PVA and PAM-co-PAA via free radical polymerization and freezing/thawing procedure. The copolymer chain entanglement through H-bond and PVA crystalline domains serve as cross-linking points of two network, which effectively enhance the mechanical properties of the gels(a strength of 1230±90 kPa, modulus of 364 kPa, toughness of 1250 kJ/m3), comparable to hybrid-network DN gels and superior to the conventional single network gels. In addition, the characteristic dual physically cross-linked network enable the PVA/CP DN gels quickly reassemble the network of the gel, resulting in rapid self-recovery. The PVA/CP DN gels also feature self-healing ability through the H-bonds formed between the polymers. More importantly, our design strategy may not only provide



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novel hydrogel with excellent properties but also help understanding the toughness mechanism of the gels featuring dual physically cross-linked network.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ■ ACKNOWLEDGEMENTS This work was financially supported by National Natural Science Foundation of China (Grant No. 21201175), Guangdong and Shenzhen Innovative Research Team Program (No.2011D052, KYPT20121228160843692), R&D Funds for basic Research Program of Shenzhen (Grant No. JCYJ20150401145529012), and Key Laboratory of Guangdong Province (2014B030301014).

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(graphical abstract)


High-Strength, Tough, Fatigue Resistant, and Self-Healing Hydrogel

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