Dual Physically Cross-Linked Nanocomposite Hydrogels Reinforced

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Dual Physically Crosslinked Nanocomposite Hydrogels Reinforced by Tunicate Cellulose Nanocrystals with High Toughness and Good Self-recoverability Tiantian Zhang, Tao Zuo, Danning Hu, and Chunyu Chang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06219 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on June 27, 2017

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ACS Applied Materials & Interfaces

Dual Physically Crosslinked Nanocomposite Hydrogels Reinforced by Tunicate Cellulose Nanocrystals with High Toughness and Good Self-recoverability Tiantian Zhang,† Tao Zuo, ‡ Danning Hu,† Chunyu Chang*,†

†

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072,

China ‡

Yangtze Valley Water Environment Monitoring Center.

Corresponding author: Chunyu Chang (Email: [email protected])

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ACS Paragon Plus Environment


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ABSTRACT The weak mechanical properties of hydrogels usually limited their application in biomedical and industrial fields. Herein, we reported a nanocomposite network of poly (acrylic acid-co-acrylamide) (PAAAM) sequentially crosslinked by quaternized tunicate cellulose nanocrystals (Q-TCNCs) and Fe3+. Q-TCNCs acted as both interfacial compatible reinforcements and cross-linkers in the nanocomposite hydrogels to form loose crosslinking, whereas compact crosslinking was built by ionic coordination between Fe3+ and –COO- of PAAAM. The toughness of dual crosslinked hydrogel (D-Gel) was 340 times of that of mono crosslinked hydrogel (m-Gel) which was 10 times of PAAAM hydrogel. Moreover, the nanocomposite hydrogels exhibited excellent self-recoverability after treating the stretched samples in FeCl3 aqueous solution. This work provided a universal strategy for construction of tough nanocomposite hydrogel reinforced with cellulose nanocrystals.

KEYWORDS: High toughness, dual physically crosslinked hydrogel, electrostatic interaction, ionic coordination interaction, tunicate cellulose nanocrystals

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1. INTRODUCTION Nanocomposite materials prepared by incorporating nanoparticles in polymer matrices have been investigated in academia and applied in industry for decades.1 Agglomeration of nanoparticles in the polymeric networks usually impair the mechanical properties of materials. Therefore, the key point to construct nanocomposite materials with high performance is to improve the interfacial compatibility between nanoparticles and polymers.2 Hydrogels, as a kind of soft wet materials, are comprised of crosslinked hydrophilic polymers, which can be widely used in sensing, actuation, tissue engineering and drug delivery, due to their high swelling ratio, biocompatibilities, and stimuli-responsive properties.3-6 Owing to the poor mechanical properties of hydrogels which limit their practical application, a great deal of strategies have been established to develop tough hydrogels by forming ideal hydrogel networks, such as topological hydrogels, nanocomposite hydrogels, and double network hydrogels.7-9 In the nanocomposite hydrogels, nanoparticles participated in the formation of strong networks as multifunctional crosslinking agents. Various forms of nanoparticles, including silica nanoparticles, montmorillonite, graphene oxide, and titanate nanosheet, have been employed to fabricate nanocomposite hydrogels.10-13 Tunicate cellulose nanocrystals (TCNCs) with high modulus (~140 GPa) and high aspect ratio (~72) were prepared by acid hydrolysis of the tunicate mantles,14,15 and widely used to reinforce various polymers.16-25 The mechanism of TCNCs reinforced nanocomposites was the formation of load-bearing percolating architecture 3



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through the hydrogen bonding interaction among TCNCs in the host polymer matrices.26 To the best of our knowledge, however, TCNCs reinforced hydrogels were rarely reported, since the above mentioned reinforcing mechanism was inapplicable in nanocomposite materials with high water content. Despite CNCs from other sources participated in the construction of nanocomposite hydrogel and the interfacial compatibility between CNCs and polymeric matrix was well improved by using organic siloxane, the reported nanocomposite hydrogels did not exhibit impressive mechanical properties.27-30 The current research status encouraged us to design tough hydrogels with unique networks by combination of TCNCs and polymers with good interfacial compatibility and explore the corresponding mechanism of reinforcement. Recently, dual crosslinked hydrogels have drawn much attention due to their excellent mechanical performance, including dual chemically crosslinked hydrogels,31 dual physically crosslinked hydrogels,32 and hybrid crosslinked hydrogels.33 In the hydrogel networks, chemical crosslinking points were achieved by covalent bonds that could maintain the elasticity of hydrogels, whereas physical crosslinking points displayed various forms, such as ionic interactions,34 crystalline domains,35 hydrogen bonds,36 hydrophobic associations,37 and host-guest interactions.38 Compared to chemical crosslinking points, physical crosslinking points were reversible, which allowed hydrogels to recover or self-heal after large deformation or disruption. The goal of this work was to create a tough nanocomposite hydrogel with dual physically crosslinked networks. Q-TCNCs acted as both interfacial compatible reinforcements and multifunctional crosslinking agents to form loosely crosslinked 4


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network through the electrostatic attraction between positive charges on the surface of Q-TCNCs and the negative charges on the side chains of PAAAM. Sequentially, Fe3+ were introduced into the loosely crosslinked networks and interacted with –COO- to obtain compact crosslinked network through ionic coordination. Although tough hydrogels with dual physically crosslinked networks have been widely reported, this is the first time that TCNCs as crosslinking agents participated in the dual crosslinked hydrogel networks. The key roles of Q-TCNCs in the nanocomposite hydrogels were as follows: (i) the surface modified TCNCs greatly improved the interfacial compatibility between the fillers and polymer matrices; (ii) the reversible electrostatic interaction between Q-TCNCs and PAAAM maintained the elasticity and improved the toughness of hydrogels; (iii) the pulling out of Q-TCNCs from polymer matrices also dissipated mechanical energy in the deformed hydrogels.

2. MATERIALS AND METHODS 2.1 Materials. Tunicate (Halocynthia roretzi Drasche) was received from Weihai Evergreen Marine science and technology Co. Ltd (Shandong, China). Acrylic acid (AA), acrylamide (AM), potassium persulfate (KPS), ferric chloride hexahydrate (FeCl3·6H2O) were purchased from Sinocharm Chemical Reagent Co. Ltd, China. (2,3-Epoxypropyl) trimethylammonium chloride (EPTAC) was obtained from Shanghai Dibo Chemical Technology Co. Ltd, China. All the reagents were used without further purification except AA which was pretreated by vacuum distillation to remove inhibitor. Other reagents were analytical pure grade. 5



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2.2 Fabrication of hydrogels. Isolation of tunicate cellulose from Halocynthia roretzi Drasche, preparation of TCNCs as well as the modification of TCNCs were conducted according to our previous work,39 and the experiment detail was supplied in Supporting Information. For the preparation of PAAAM hydrogel, monomers (AA and AM, 2 g), KPS (40 mg), and water were mixed to obtain a 10 g solution. After mechanical stirring (3 min) and ultrasonic treatment (20 min), polymerization was conducted in the solution under nitrogen atmosphere at 60 °C for 5 h to form PAAAM gels. Nanocomposite hydrogels were fabricated as follows, firstly, monomers (AA and AM, 2 g), KPS (40 mg) were added into Q-TCNCs suspension (8 g) under mechanical stirring for 3 min, and treated with ultrasonic for 20 min. Then, the resulting suspension was transferred in to glass mold, and polymerization was conducted at 60 °C for 5 h under the protection of nitrogen to form mono crosslinked hydrogels (m-Gels). The AA/AM molar ratio was 0.15, 0.25, 0.5, and 1, respectively, while the content of Q-TCNCs in the dried gels was 0, 4, and 8 wt%, respectively. Then, the m-Gels were soaked into FeCl3 solution (0.06, 0.1, and 0.6 mol L-1) for 5 h, respectively, to obtain original dual crosslinked hydrogels (d-Gels). Finally, the d-Gels were immersed into distilled water for 48 h to remove superfluous Fe3+ and get dual physically crosslinked hydrogels (D-Gels). Unless special emphasis, D-Gel was fabricated by using AA/AM molar ratio was 0.25, Q-TCNCs content was 8 wt%, and Fe3+ concentration was 0.1 mol L-1. 2.3 Characterization. Fourier transfer infrared spectroscopy (FTIR) spectrogram was carried out by NICOLET 5700 FTIR spectrometer (Thermo Fisher Scientific, USA). 6


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The freeze-dried samples were cut into powder and then dried in a vacuum oven at 60 °C for 12 h before analyzed in KBr disks in the range of 4000-400 cm-1. Scanning electron microscope (SEM) images were taken with the field emission scanning electron microscopy (FESEM, Zeiss, Germany) at an accelerating voltage of 5 kV. The hydrogels were swollen to equilibrium in distilled water at 25 °C for 24 h, frozen in liquid nitrogen, snapped immediately, and freeze-dried. Cross-section of specimens were coated with gold vapor, observed, and photographed. Transmission electron microscope (TEM) observation of nanocomposite hydrogels were performed by using a JEM-2010 transmission electron microscope (JEOL, Japan) with an acceleration voltage at 200 kV. After embedded in epoxy resin, sample was prepared by slicing for ultrathin section. 2.4 Mechanical measurement. The tensile behavior of nanocomposite hydrogels were performed by a universal material testing machine with a 1000 N load cell (Instron 5967, USA) at room temperature. For the tensile tests and loading-unloading tests, the initial length of hydrogels samples between two clamps was 20 mm, and the cross-head velocity was 20 mm min-1. For each hydrogel samples, five duplicates were measured and the average values were calculated. The elastic modulus was calculated by the slope of initial linear region of the stress-strain curves, the toughness was determined by the area below the stress-strain curve, and the dissipated energy was estimated by the area between the loading-unloading profiles. For recoverability tests, the stretched hydrogels were recovered under different conditions (air, water, and FeCl3 aqueous solution) and measured after different recovery time intervals (0 h, 7



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0.5 h, 2 h, and 5 h). The recoverability of hydrogel samples were calculated by the area ratio between loading-unloading tests.

3. RESULTS AND DISSCUSION

Figure 1. Schematic illustrations for the preparation of dual crosslinked hydrogels: (a) In situ polymerization of AA and AM monomers in the presence of Q-TCNCs to form mono crosslinked hydrogel (m-Gel) networks; (b) Fe3+ interacted with –COO- groups in the m-Gel to get dual crosslinked hydrogel (d-Gel) networks, and (c) Reorganization of ionic coordination after removal of excess and unstable Fe3+ to obtain final hydrogel (D-Gel).

3.1 Fabrication and appearance of nanocomposite hydrogels. For construction of the nanocomposite hydrogel, acrylic acid and acrylamide monomers were polymerized firstly in the presence of Q-TCNCs which were employed as multifunctional crosslinking agents to form mono crosslinked hydrogel (m-Gel) networks (Figure 1a). The slight modification of TCNCs (0.22 AGU unit-1) resulted 8


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in the loosely crosslinked networks of m-Gel. The electrostatic attraction between surface modified CNCs and matrices was widely employed to construct nanocomposite materials.40 After removal of residual monomers by distilled water, m-Gel was immersed into ferric chloride solution for the loading of Fe3+ into hydrogel networks (Figure 1b), where electrostatic attraction between Fe3+ and –COO- of PAAAM was built, leading to the formation of densely crosslinked networks. Finally, dual crosslinked hydrogel (D-Gel) was obtained by eliminating excess and unstable Fe3+ for the reorganization of ionic coordination (Figure 1c). Owing to both the –N(CH3)3+ of Q-TCNCs and Fe3+ could interact with the –COO- of PAAAM, there are two kind of physically crosslinked networks in the nanocomposite hydrogels.

Figure 2. Photographs of hydrogels PAAAM gel (a), m-Gel (b), and D-Gel (c, d, and e).

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Figure 2a-c show the photographs of hydrogels subjected to stretching. PAAAM gel could not withstand the stretching of 100 g weight, whereas m-Gel could endure 100 g weight and exhibited high ductility due to the reinforcement of TCNCs which acted as both physical cross-linkers and nanofillers. After further treatment with Fe3+, D-Gel could bear the loading weight of 2.5 kg, displaying its high stiffness and toughness. These results indicated that the introduction of Fe3+ significantly improved the toughness of nanocomposite hydrogels. Furthermore, D-Gel maintained integrity under various deformations such as weaving (Figure 2d) and crimping (Figure 2e), revealing that the dual physically crosslinked hydrogel had excellent mechanical performance, which will be systematically investigated in the section of mechanical properties of nanocomposite hydrogels. 3.2 Structure and Morphology of nanocomposite hydrogels. FTIR spectra of PAAAM hydrogel, m-Gel, and D-Gel are displayed in Figure S1. The absorption bands at 3438 cm-1 for stretching vibration of O-H and N-H could be observed in the spectra of all samples. Compared to the spectrum of PAAAM gel, the absorption band broadened for m-Gel due to the introduction of Q-TCNCs which contained a large number of hydroxyl groups, whereas it became narrow for D-Gel because the formation of ionic coordination reduced the -OH of carboxyl groups. On the other hand, the shoulder peak at 1726 cm-1 in the spectrum of PAAAM gel attributed to the stretching vibration of C=O (carboxyl groups that associated with amide groups through hydrogen bonding in the hydrogel networks) disappeared in the spectra of m-Gel and D-Gel. This result can be explained by the reorganization of carboxyl 10


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groups through the interaction with Q-TCNCs and Fe3+, where the corresponding peaks overlapped with absorption bands of amide I at 1650 cm-1.

Figure 3. SEM and TEM images of hydrogel samples: SEM images of PAAAM gel (a), m-Gel (b), and D-Gel (c), and TEM image of D-Gel (d).

Due to the difference of hydrogel network architectures, the water contents of PAAAM gel, m-Gel, and D-Gel were 99.6%, 99.5%, and 66%, respectively. It is well known that the morphology of hydrogels are usually dependence on the water contents of hydrogels. As shown in Figure 3a-c, all hydrogel samples had porous structures with different morphologies. PAAAM gel exhibited a smooth failure surface morphology composed of a large amount of nanofibers, which could be assigned to the presence of hydrogen bonds among the PAAAM chains. However, the 11



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nanofibers in the m-Gel disappeared and its average pore size decreased from 1.54 µm (PAAAM gel) to 1.23 µm because of the introduction of Q-TCNCs in the hydrogel networks. After crosslinked by Fe3+, the average pore size of D-Gel further decreased to be 0.72 µm, and the nanofibers appeared again due to the formation of ionic coordination. From the TEM image (Figure 3d) of D-Gel, we can see that Q-TCNCs uniformly distributed in the hydrogel matrix without obvious aggregation. The width of Q-TCNCs was about 24 ± 12 µm in D-Gel sample, which was consistent with that of the as-prepared Q-TCNCs (27 ± 10 µm) (Figure S2). Moreover, the boundary between Q-TCNCs and PAAAM was unclear, revealing the good interfacial compatibility between nanofillers and matrix.

3.3 pH-sensitive behaviors and mechanical properties of nanocomposite hydrogels. The influences of pH on the swelling behaviors of various nanocomposite hydrogels and pure PAAAM hydrogel were investigated in the solution with same ionic strength (0.1 M) (Figure S3). The swelling ratio of all hydrogels strongly depended on the pH values of solution. PAAAM and m-Gel were significantly shrinking at pH=1 and swelling at pH=13, which indicated that the protonation of carboxylic ions in acidic medium (pH=1) made dense hydrogel network via hydrogen bonds resulted in low swelling ratio of hydrogel, whereas strong electrostatic repulsion of COO-/COO- and COO-/OH- at alkaline condition (pH=13) extended the hydrogel networks, leading to high swelling ratio of hydrogels. However, d-Gel and 12


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D-Gel had higher swelling ratio in acidic medium in comparison with that in neutral solution, where the brownish red of dual physical crosslinked hydrogels disappeared in acidic medium, revealing that the COO-/Fe3+ ionic coordination in dual physical crosslinked hydrogels was replaced by COO-/H+ interaction and the crosslinking density of hydrogel decreased, resulting in the high swelling ratio of hydrogels. Q-TCNCs as multifunctional cross-linkers and nanofillers could significantly reinforce the mechanical properties of hydrogels compared with PAAAM hydrogel. As shown in Figure 4a, the tensile strength (σ) and elongation at break (ε) of PAAAM hydrogel were 4.5 kPa and 407%, respectively. After the introduction of Q-TCNCs, both σ and ε values of hydrogels increased as the increase of Q-TCNC contents. For instance, the σ and ε values of m-Gel with 8 wt% Q-TCNCs could reach up to 28.8 kPa and 589%. Furthermore, the elastic modulus and toughness of PAAAM hydrogel was 10 kPa and 13 kJ m-3, whereas those of m-Gel with 8 wt% Q-TCNCs increased to be 37.5 kPa and 134.2 kJ m-3, respectively (Figure 4b). These results indicated that the incorporation of Q-TCNCs into PAAAM hydrogel networks not only enhanced their tensile strength and elongation at break, but also improved their elastic modulus and toughness, revealing that m-Gels could effectively dissipate mechanical energy and transfer stress with the help of Q-TCNCs. The same results were also obtained by measuring the dual crosslinked hydrogels with different Q-TCNC contents (Figure S4).

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Figure 4. Stress-strain curves (a, c), elastic modulus and toughness (b, d) of m-Gels: (a, b) Effects of TCNC contents and (c, d) effects of AA/AM molar ratio.

The chemical composition of PAAAM chains was another important influence factor of network architecture of m-Gels, since PAA segments not only provided negative charges for the interaction with Q-TCNCs but also acted as supply for ionic coordination with Fe3+. Therefore, the effect of AA/AM molar ratio on the mechanical properties of m-Gels was systematically investigated. As shown in Figure 4c, all mono crosslinked hydrogels were very soft and flexible. With increasing the molar ratio from 0.15 to 0.25, the σ of m-Gels increased from 14.4 kPa to 28.8 kPa, and the corresponding ε also improved from 293% to 589%, respectively. When the molar ratio further increased from 0.25 to 0.5, σ further increased to 32.7 kPa, but the ε value decreased to 436%, but the σ of m-Gels sharply decreased to be 15.3 kPa with AA/AM molar ratio of 1. Meanwhile, the elastic modulus and toughness of m-Gels 14


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increased first and then decreased along with the raise of AA/AM molar ratio (Figure 4d). The maximal values of elastic modulus and toughness were 37.5 kPa and 134.2 kJ m-3, respectively, as AA/AM molar ratio was 0.25. According to above results, we employed Q-TCNC content of 8 wt% and AA/AM molar ratio of 0.25 for the construction of m-Gel. After treated m-Gels with ferric chloride solution, d-Gels were obtained through the coordination between –COO- and Fe3+. Compared to the σ values of m-Gels, the second crosslinking network could significantly improve the mechanical strength of hydrogels as shown in Figure 5a and b. The σ, elastic modulus, and toughness of d-Gel were two orders of those of m-Gel. As the increase of ferric ion concentration from 0.06 M to 0.6 M, the σ values of hydrogel samples increased from 5.4 MPa to 7.7 MPa, while their elastic moduli raised from 8.9 MPa to 15.6 MPa. It implied that high concentration of Fe3+ could enhance the total crosslinking density of hydrogel, leading to larger tensile strength and elastic modulus. However, both the elongation at break and toughness of hydrogels decreased when the concentration of Fe3+ was as high as 0.6 M. So the maximal toughness of hydrogels was 18.9 MJ m-3 when the m-Gel was treated by Fe3+ aqueous solution with a concentration of 0.1 M. Both the tensile strength and toughness of the dual crosslinked hydrogels were far higher than those of other reported cellulose nanocrystal reinforced hydrogels.27-30

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Figure 5. Stress-strain curves (a, c), elastic modulus, and toughness (b, d) of dual crosslinked hydrogels: Effect of Fe3+ concentration before and after rinse with water.

For the reorganization of Fe3+ in the networks, hydrogel samples were immersed in distilled water to remove excess and unstable Fe3+, where carboxylate-Fe3+ coordinates in the forms of monodentate or bidentate were converted into tridentate coordinates. When the Fe3+ concentrations were 0.06 and 0.1 M, the mechanical properties of hydrogels changed slightly before and after rinse water (Figure 5c). However, the σ value of D-Gel obtained in 0.6 M Fe3+ aqueous solution increased from 7.7 to 9.9 MPa after soaking in distilled water, revealing that more tridentate coordinates formed in D-Gel resulted in superior mechanical properties after soaking in distilled water. Figure 5d displays the effects of Fe3+ concentration on the elastic modulus and toughness of D-Gels. The elastic moduli of D-Gels increased as the increase of Fe3+ concentration, revealing the improvement of the stiffness of 16


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hydrogels, whereas the maximal toughness of D-Gels was obtained by treating with 0.1 M Fe3+ aqueous solution, due to the sharp reduction of ε value of D-Gel prepared in Fe3+ aqueous solution with high concentration. The change tendencies of elastic modulus and toughness for D-Gels were same as those of d-Gels.

Figure 6. Self-recovery of dual crosslinked hydrogels: (a, b) d-Gel and (c, d) D-Gel. Loading-unloading curves of dual crosslinked hydrogels at different recovery times (a, c) and the time-dependent recovery of hysteresis loop (b, d).

3.4 Recoverability of nanocomposite hydrogels. The self-recoverability of nanocomposite hydrogels were investigated under various conditions including air, distilled water, and FeCl3 solution. Figure 6a depicts the loading-unloading curves of d-Gel with different recovery times. The noticeable hysteresis loops of d-Gel were observed due to the energy dissipation via the fracture of sacrificial bonds and the 17



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pullout of Q-TCNCs from polymeric matrix, where the sacrificial bonds mainly included

–N(CH3)3+/–COO-

electrostatic

interactions and –COO-/Fe3+ ionic

coordination. The recovery of hydrogel increased with the prolongation of recovery time, and reached 89.6 ± 2.7% after 5 h recovery (Figure 6b). The recoverability of hydrogel was mainly attributed to the presence of dynamic reversible bonds in the hydrogel networks. To better understand the effects of unstable Fe3+ on the recovery of hydrogel, loading-unloading curves of D-Gels is shown in Figure 6c. The tendency for the recovery of D-Gel was same as that of d-Gel, but the recovery of D-Gels was higher than that of d-Gels under same recover time (Figure 6d). These results indicated that the presence of unstable Fe3+ in the sample networks could not promote the recovery of hydrogels.

Figure 7. Self-recovery of D-Gel treated with distilled water (a, b) and 0.1 M FeCl3 solution (c, d). Loading-unloading curves of D-Gel at different recovery times (a, c) 18


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and the time-dependent recovery of hysteresis loop (b, d).

To further understand the self-recovery mechanism of hydrogel, we conducted the recovery process of hydrogel in water and FeCl3 aqueous solution instead of air. Figure 7a shows the loading-unloading curves of D-Gel under various recovery times in water, where the recovery of D-Gel increased from 32.7 ± 2.4% to 84.9 ± 6.4% with prolongation of recovery time from 0 to 5 h, as shown in Figure 7b. The recovery (84.9 ± 6.4%) of hydrogel treated in water for 5 h was lower than that (89.6 ± 2.7%) of hydrogel treated in air. The above results should be ascribed to the fact that the stretched D-Gel contained unstable Fe3+ which reorganized networks when the hydrogel recovered in air but removed from networks when the hydrogel recovered in water. Interestingly, the recoverability of hydrogel was greatly enhanced when the hydrogel was treated in FeCl3 aqueous solution (Figure 7c), where the recovery of hydrogel reached 98.4 ± 4.7% after 2 h treatment in FeCl3 aqueous solution. After 5 h treatment, the recovery of hydrogel further increased to be 102.3 ± 4.3% (Figure 7d), indicating that more Fe3+ entered hydrogel networks, interacted with carboxyl groups, and improved the recoverability of hydrogel.

4. CONCLUSIONS We designed and fabricated a new series of dual physically crosslinked hydrogels based on PAAAM and Q-TCNCs. In the hydrogel networks, Q-TCNCs were acted as both multifunctional cross-linkers and reinforcements which participated in 19



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Page 20 of 27

construction of first crosslinked network through electrostatic interaction between –N(CH3)3+ and –COO-. Then, Fe3+ was used for further crosslinking of PAAAM by formation of –COO-/Fe3+ ionic coordinates. Since these physical bonds (electrostatic interactions and coordination bonds) served as the reversible sacrificial bonds which could rupture to effectively dissipate energies, dual crosslinked hydrogels exhibited high

tensile

strength,

high

ductility

and

high

toughness.

Importantly,

self-recoverability of hydrogel was strongly dependent on the recovery conditions: FeCl3 aqueous solution > air > water. The as-prepared hydrogel could recover to the original state in FeCl3 aqueous solution within 2 h. These TCNCs reinforced hydrogels with excellent mechanical properties would largely expand application potential of hydrogels in biomedical fields.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publish website at DOI: … … Isolation of cellulose from tunicate; Preparation of tunicate cellulose nanocrystals; Modification of tunicate cellulose nanocrystals; pH-sensitive behaviors of hydrogels; FTIR of hydrogel samples; TEM image of Q-TCNCs; Effect of Q-TCNC contents on the mechanical properties of d-Gels and D-Gels.

AUTHOR INFORMATION 20


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Corresponding author Phone: 86-27-87219274. E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21304021), Hubei Province Science Foundation for Youths (2015CFB499), Jiangsu Province Science Foundation for Youths (BK20150382), Pearl River S&T Nova Program of Guangzhou (201506010101), and Guangzhou science and technology project (201510010221).

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Dual Physically Cross-Linked Nanocomposite Hydrogels Reinforced

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