Article pubs.acs.org/Macromolecules
Low Chemically Cross-Linked PAM/C-Dot Hydrogel with Robustness and Superstretchability in Both As-Prepared and Swelling Equilibrium States Meng Hu,† Xiaoyu Gu,† Yang Hu,*,†,‡ Tao Wang,† Jian Huang,† and Chaoyang Wang*,† †
Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, China Institute of Biomaterials, College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China
‡
S Supporting Information *
ABSTRACT: Superior mechanical, recoverable, and swelling properties are important for the application practice of hydrogel. However, most of the hydrogels do not possess those three features at the same time. Herein, we have prepared a novel low chemical cross-linked polyacrylamide (PAM)/carbon nanodot (C-dot) hydrogel by introducing the C-dot into low chemically cross-linked PAM network. C-dot acts as both a physical cross-linker and lubricant in the low chemical cross-linked PAM network, and the synergistic effect between C-dot and PAM chains endows the hydrogel with extraordinary mechanical, recoverable, and swelling properties. The as-prepared hydrogel can be stretched over 3700% with fracture strength as high as 166 kPa, and it can keep high recoverability even when it is stretched up to 500% (more than 97% recovery ratio). Furthermore, the highest swelling ratio of the hydrogel is up to 235 times, which is much higher than that of the conventional PAM hydrogel. Moreover, even in the swelling equilibrium state, the hydrogel can be stretched up to 650% and almost completely recover once the stress is removed. The hydrogel with such an excellent mechanical property in both as-prepared and swollen states is barely reported and can greatly extend its potential application in biomedical fields. Okumura et al.22 prepared a kind of TP hydrogel by introducing a figure-of-eight cross-linker. Because of the mobilizable cross-linking points, polymer chains in TP hydrogel are flexible and extensible, which benefits the cross-linked network stretching and stress dissipating, thus endowing TP hydrogel with excellent water absorption and stretch ability. Haraguchi et al.23,24 used the homogeneously dispersed inorganic nanoparticles as physical cross-linkers to prepare NC hydrogels. In NC hydrogels, the flexible polymer chains are linked by homogeneously dispersed inorganic nanoparticles, constituting a strong hybrid polymer network.25−33 And thus the NC hydrogels usually exhibited ultrahigh elongation (more than 1000%) at low inorganic content (≤5 wt %).2,23,24,28,34−36 The mechanical properties of DN, TP, and NC hydrogels could be greatly enhanced. However, some deficiencies still exist. For example, DN hydrogels usually show very poor recovery property because the high chemical cross-linked first network is irreversibly damaged during stretching process. Besides, the high chemical cross-linked first network in DN hydrogels would seriously reduce the swelling extent of the hydrogels, leading to poor water absorption.19,37−39 TP hydrogel is not tough enough because of the low and flexible cross-links.39,40 NC hydrogels usually have low recovery
1. INTRODUCTION Polymer hydrogels are soft-wet materials composed of a large amount of water and three-dimensional networks of polymer chains cross-linked by strong covalent or physical bonds.1−3 The special soft-wet property endows hydrogels with excellent biocompatibility, enabling them to be extensively studied and widely applied in biomedical field, such as tissue engineering scaffolds, drug release vehicles, biosensors, and cell modulating substances.4−16 However, conventional chemical cross-linked hydrogels are usually very brittle, with poor mechanical property, which seriously limits their industrial and biomedical applications.17,18 Therefore, improving the mechanical properties of hydrogel has been an important topic in the field of hydrogel science. Until now, several new types of polymer hydrogels, such as double-network (DN), topological (TP) cross-linking, and nanocomposite (NC) hydrogels, have been widely studied for the preparation of hydrogels with excellent mechanical property. Herein, Gong et al.19 pioneered the concept of DN hydrogel as a route to improve the mechanical property of hydrogel. They prepared DN hydrogels by combining a stiff and brittle high chemical cross-linked firstnetwork with a soft and ductile loosely chemical cross-linked second-network. When the DN hydrogel was stretched, the stiff and brittle high chemical cross-linked first-network unzipped, and dissipated the stress, while the soft and ductile loosely chemical cross-linked second-network remained intact, causing the DN hydrogel to be tough and fracture resistant.19−21 © XXXX American Chemical Society
Received: October 28, 2015 Revised: March 28, 2016
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DOI: 10.1021/acs.macromol.5b02352 Macromolecules XXXX, XXX, XXX−XXX
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Co. Ltd. (China). N,N′-Methylenebis(acrylamide) (MBA) was purchased from Tianjian Kermel Chemical Reagent Company (China), N,N,N′,N′-tetramethylethylenediamine (TEMED) used as accelerator was purchased from J&K Chemicals (China), β-cyclodextrin was purchased from Aladin Ltd. (Shanghai, China), and hydrochloric acid was purchased from Guangzhou Chemical Factory (China). Here, AM was purified by recrystallization from deionized water and dried in a vacuum at room temperature before use. APS, cyclodextrin, and hydrochloric acid were used as received without further purification. All other chemicals were analytical grade reagents and purified with the standard methods. Pure water was produced by deionization and filtration using a Millipore apparatus (resistivity ≥18.2 MΩ cm). 2.2. Preparation of C-Dots. A certain amount of β-cyclodextrin (5 g) and hydrochloric acid (30 mL, 18−19 wt %) mixed solution was kept at 70 °C for 4 h. Then, the obtained brown solution was made a rotary evaporation at 60 °C to get rid of most water and hydrochloric acid, while the pH was adjusted to 7−8 with Na2CO3 (2.5 mol/L). After that the neutral brown solution was centrifuged (at 15 000 rpm) for 25 min to separate big particles; the supernatant was further dialyzed with a dialysis bag (retained molecular weight 500 Da) for 24 h to purify the C-dots aqueous suspension. Finally, the as-prepared pure C-dots aqueous suspension was freeze-dried to power. 2.3. Preparation of Hybrid Hydrogels. The C-dot/PAM hydrogel was synthesized by in situ polymerization of monomer AM in the aqueous suspension of C-dots. The MBA was first dispersed with deionized water to a desired concentration (0.01 mol/L). Here, 1.35 g of AM monomer and a certain volume of MBA (0.1−0.55 mL) were added into a certain volume of C-dots suspension (4.7−4.25 mL) with designed concentration. Afterward, the mixture suspension was treated ultrasonically in an ice−water bath for 10 min to dissolve the AM and treated with nitrogen for 15 min to remove most of oxygen. Finally, 0.2 mL of APS (25 mg/mL) and 24 μL of TEMED were added under stirring and ultrasonic bath condition. Then, the mixture suspension was rapidly injected into cylinder molds (4.0 mm diameter × 150 mm length), sealed with plastic wrap, and degassed for 1−2 min by ultrasonic in an ice−water bath. The polymerization was conducted at 20 °C for 48 h to produce the C-dot/PAM hydrogel. In this work, the hybrid hydrogels were designated as ABmCn, where subscripts m and n stand for the weight ratios of MBA/AM and Cdots/water, respectively. Unless specifically noted, the concentration of AM monomer was fixed at 27% (weight ratio of AM/water), and the weight ratio of APS/AM was kept at 0.37%. For example, AB0.029C1 means the hydrogel consisted of 10 g of water, 2.7 g of AM, 0.385 mg of MBA, and 0.1 g of C-dots. 2.4. Characterization of C-Dots and Hydrogels. The particle diameter and morphology of the obtained C-dots were characterized by a transmission electron microscope (TEM, JEOL JEM-2100F), with an accelerating voltage of 200 kV. The Fourier transform infrared (FT-IR) spectra of C-dots and hydrogels were recorded on a German Vector-33 IR instrument in the range of 400−4000 cm−1 at room temperature, using a KBr pellet. The dynamic mechanical properties of the ABmCn hydrogel were detected with the strain-controlled rheometer ARES-RFS. The frequency sweep was performed over the range of 0.01−100 rad/s. The silicone oil was used to prevent water evaporation by being laid on the edge of the fixture plates. The swelling ratio was determined by the weight ratio. For the weight ratio, Qw = Ws/Wd, was used to evaluate the swelling ratio, where Ws is the weight of the swollen hydrogel and Wd is the weight of the corresponding dry hydrogel. For Qw, the hydrogel samples of 4.0 mm diameter and about 10 mm length were immersed in a large amount of water at 25 °C with daily replacement until the hydrogels reached equilibrium swelling. The hydrogels were measured after removing the excess water from the surface by filter papers. The glass transition temperature (Tg) of the dry hydrogels was carried out with a Netzsch differential scanning calorimetry (DSC) 204 F1 calorimeter on heating from 60 to 220 °C at the rate of 10 °C/min under nitrogen protection. 2.5. Mechanical Property Measurements. Mechanical properties of the as-prepared and swollen hydrogels were measured by a Shimadzu Autograph AG-Xplus 50N system at room temperature. The
property in the case of high stretching ratio and even lost mechanical strength in the high swollen condition.18,26,27,41 Therefore, it is very imperative to develop effective methods for the preparation of novel hydrogels with extraordinary comprehensive performance of excellent mechanical property, water-absorbing ability, and recovery property in both asprepared and even swelling equilibrium states. Recently, the combination of the covalent bonds and physical bonds has been proved as an ideal method to prepare hydrogel with high strength, elongation, and excellent recovery properties.38,42,43 For example, Suo et al.38 prepared a new type of double network hydrogel with high stretching ratio (more than 2000%), toughness, and excellent recovery property through coexisting networks of polyacrylamide (covalent) and alginate (ionic). Here, we speculate that combination of covalent bonds from chemical cross-linked hydrogel and physical bonds from NC hydrogel together is also a viable method to prepare hydrogel with extraordinary comprehensive performance. Carbon nanodots (C-dots) as a new class of carbon nanomaterials have been widely reported in the field of biological labeling, catalysis, medical diagnosis, and optoelectronics because of their several desirable properties, such as high stability, low toxicity, and excellent biocompatibility.44−48 Typically, C-dots are spherical carbon nanoparticles with sizes below 10 nm. Besides, there are plenty of functional groups, such as hydroxyl, epoxide, and carboxyl groups on C-dots surface, endowing them with a good water dispersibility.44,49,50 Those characteristics of C-dots are similar to clay and graphene nanoparticles.25,36 Therefore, based on these similar characteristics, C-dots also have great potential to be used as physical cross-linker in NC hydrogels. In this work, we have prepared a novel low chemically crosslinked polyacrylamide (PAM)/C-dot hydrogel by in situ polymerization of monomer acrylamide (AM) in the C-dot aqueous suspension. In this hydrogel, spherical nanosize C-dots interacted with low chemically cross-linked PAM chains formed another physical network, which caused the coexistence of the physical network and the covalent network. Based on the coexisting high density physical bonds, stress could be well dissipated by unzipping physical bonds during deformation process. As a result, this novel low chemical cross-linked PAM/ C-dots hydrogel could be stretched over 3700% with fracture strength as high as 166 kPa, which was much better than those in the previous studies through various chemical and physical bonds for enhancing the mechanical properties.51,52 Besides, as the stress could be well dissipated by unzipping physical bonds, the elastic chemical network would keep intact, which could provide strong resilience for hydrogel and lead to an excellent recovery property. Moreover, the low chemical cross-linked network could keep hydrogel intact during swelling process, which was in favor of improving the swelling property of hydrogel.22,37,39 In addition, the physical network could keep intact and coexist with chemical network even in the swollen state. As a result, the hydrogel could keep excellent mechanical property even in swelling equilibrium state (stretching over 850%, with fracture strength as high as 6 kPa even with the swelling ratios as high as 67 times). Hydrogels with such an excellent mechanical property at so high water absorption ratio have rarely been reported.
2. EXPERIMENTAL SECTION 2.1. Materials. Ammonium persulfate (APS) and monomer acrylamide (AM) were purchased from Shanghai Rich Joint Reagents B
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Figure 1. (a−g) Photographs demonstrating the excellent mechanical behavior of as-prepared AB0.029C2.5 hydrogel: (a−c) the hydrogel was compressed 90% and quickly recover after releasing the compression; (d, e) the hydrogel was stretched to over 10 times its initial length; (f, g) the hydrogel was stretched to over 10 times its initial length in the torsion state.
Figure 2. Mechanical properties of AB0.029Cn hydrogels with different C-dot contents: (a) tensile stress−strain curves and (b) tensile stresses and strains as a function of the C-dot content; (c) compressive stress−strain curves and (d) compressive stresses as a function of the C-dot content at 85% strain. as-prepared and swollen hydrogel samples for tensile testing were rodlike shapes with 20 mm length, while the diameter of the as-prepared hydrogel samples was 4 mm, while the diameter of the swollen samples was determined individually using a vernier caliper. Besides, the crosshead speed during the experiment was 100 mm/min. The tensile hysteresis of the as-prepared and swollen hydrogel samples was also measured at the same conditions. The tensile strain was taken as the length change related to the initial length, and the tensile stress was evaluated on the cross section of the initial sample. For the compression testing, cylindrical samples of the as-prepared hydrogel were used with dimensions of 13 mm diameter and 15 mm height. The crosshead speed was 5 mm/min, and the compression strength was obtained using the stress at deformation of 85%. The compression hysteresis of the as-prepared hydrogel samples was also measured at the same conditions using the stress at deformation of 80%. The recovery ratio of hydrogel is calculated as a ratio of strain: Rr = (Ts − Rs)/Ts × 100%, where Rr is the recovery ratio, Ts is the tensile strain (here, the tensile strain are 500% and 800%, respectively), and Rs is the residual strain.18,38
3. RESULTS AND DISCUSSION 3.1. Characterization of C-Dots and As-Prepared ABmCn Hydrogels. TEM images of the obtained C-dots demonstrated that C-dots used for preparing ABmCn hydrogels were spherical nanoparticles with narrow particle size distribution (Figure S1a), and the average particle size of Cdots was 3.5 ± 0.7 nm (Figure S1b), which was estimated by measuring more than 100 particles from TEM images using the Nano Measurer 1.2 software. Besides, the aqueous suspension (0.05 g/mL) of the obtained C-dots could be stored for two months without precipitation (Figure S1c,d), indicating an excellent aqueous dispersibility of the obtained C-dots. As shown in Figure 1, the as-prepared AB0.029C2.5 hydrogel displayed uniformity light yellow and transparency when viewed with the naked eye, indicating that C-dots were well dispersed in ABmCn hydrogels. In addition, different from brittle conventional chemically cross-linked hydrogels, the as-prepared C
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Figure 3. Robust properties of AB0.029C2.5 hydrogel: (a) tensile stress−strain curves with a stretch of 800% and (b) compressive stress−strain curves at a 80% strain for the different loading−unloading cycles.
Figure 4. Mechanical properties of ABmC2.5 hydrogels with different MBA contents: (a) tensile stress−strain curves and (b) tensile stresses and strains as a function of the MBA content; (c) compressive stress−strain curves and (d) compressive stresses as a function of the MBA content at a 85% strain.
cross-linked hydrogels. Indeed, even compared with those of clay/PAM NC hydrogels24,28,35,55,56 and graphene/PAM NC hydrogels,2,36,42,43 the ultrahigh elongation at break of the asprepared ABmCn hydrogels are obviously higher. Here, with increasing concentration of C-dots, both the stress and strain showed local maxima. This was because with increasing concentration of C-dots, the physical cross-linked points in hydrogel increased, which could improve the stress of hydrogels. Meanwhile, because C-dots were very small nanospheres, they might move along with the PAM chains and act as lubricants in hydrogel stretching process, making PAM chains have larger movement space and easier to movement. As a result, with increasing concentration of Cdots, the PAM chains would be more flexible, have larger movement space to form a relatively thick and loose layer of PAM chains adsorbed on the C-dot surface, and increase the number density of the elastically effective PAM chains, which could great increment of the dissipation in stretching process,55,56 leading to the excellent strain and stress of AB0.029Cn. However, when the concentration of C-dot was too high (higher than 2.5 wt %), they might be aggregated in the hydrogel, and in turn the number of the contact points between C-dots and PAM chains would decrease, which decreased the density of physical cross-link points in the hydrogel, leading to the decrease of the Young modulus and stress of AB0.029Cn. Besides the tensile test, the compression properties of as-
ABmCn hydrogels exhibited a high performance in tensile strength and compressibility. They were robust enough to withstand high deformations without obvious damages. As shown in Figure 1a−c, the as-prepared AB0.029C2.5 hydrogel could be compressed to 90% without any damage and rapidly recover to its original shape as soon as the compression force was removed. Besides, Figure 1d,e showed that the as-prepared AB0.029C2.5 hydrogel could be stretched to over 10 times its initial length and does not fracture. Moreover, the as-prepared AB0.029C2.5 could withstand a high level of torsion, be knotted, and stretched to the same ratio with its unknotted sample without broken (Figure 1f,g). All the above results revealed that the as-prepared ABmCn hydrogel was robust and highly deformable. To further quantitatively investigate the mechanical property of as-prepared ABmCn hydrogels, the tensile and compressive tests have been conducted. As shown in Figure 2a,b and Table S3, compared with the poor stretching ratio (less than 800%) and fracture strength (less than 60 kPa) of pure low crosslinked PAM hydrogel (AB0.029C0), the stretching ratio and fracture strength of as-prepared AB0.029Cn hydrogels have been greatly enhanced. In particular, the as-prepared AB0.029C2.0 hydrogel could be stretched to 3529% ± 108% with fracture strength as high as 163 ± 15 kPa. Besides, the tensile elongation at break of as-prepared AB0.029C3.0 hydrogel even surpassed 4200%, which had barely been reported for chemical D
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As reported, the presence of high elasticity chemical crosslinked network was in favor of improving the restorability of hydrogels.38,43,57 Therefore, the as-prepared ABmCn hydrogel with chemical cross-linked network should also exhibit excellent recovery property. In order to prove this, the cyclic tensile testing of as-prepared ABmCn hydrogels was carried out (the maximum stretching ratio was 500% and 800% for AB0.029Cn and ABmC2.5 hydrogels, respectively), and the recovery ratio of hydrogel was calculated as a ratio of strain.18,38 As shown in Figure S2a, all of the as-prepared AB0.029Cn and ABmC2.5 hydrogels displayed outstanding elastic resilience. Table 1
prepared ABmCn hydrogels were also investigated (Figure 2c,d and Table S3). Here, different from clay/PAM NC hydrogels and graphene/PAM NC hydrogels whose compression stresses were enhanced along with the increase content of clay/ graphene,28,42 the as-prepared ABmCn hydrogels became softer after the addition of C-dots. As shown in Figure 2d, the compression stress of as-prepared ABmCn hydrogels decreased along with the increase of C-dots content. That was because the sheet structure of clay and graphene made them easily stack together in the compression process and limited the polymer chains movement, leading to a high compressive stress, while the smaller nanosize (smaller than 5 nm) and spherical structure of C-dots endowed the hydrogels with greater mobility in the compression process, which was in favor of avoiding stress concentration. Besides, the moving C-dots could act as lubricants in the hydrogel, which caused PAM chains more flexible in deformation process, and thus decreased the compressive stress. Significantly, as a result of the existing of chemical cross-linker, strong chemical cross-linked network was formed in AB0.029Cn hydrogels, leading to the robust property of AB0.029Cn hydrogels. As shown in Figure 3a,b, the obvious hysteresis loops of the AB0.029C2.5 hydrogel were barely unchanged during the multiple loading−unloading cyclic stretching and compression experiments, fully demonstrating that the ABmCn hydrogel was very robust and could suffer multiple loading−unloading cyclic stretching and compression without damage. Meanwhile, the almost unchanged hysteresis loops also indicated that during the multiple loading− unloading cyclic stretching and compression experiments stress was well dissipated by the physical coactions during C-dots and PAM chains unzipping process, while the chemical cross-linked network of the hydrogel could keep intact and strong. In this experiment, the content of chemical cross-linker MBA also had great influence on the mechanical property of the hydrogel. As shown in Figure 4a−d and Table S4, as same as the conventional chemical cross-linked hydrogel, the stretching ratio of as-prepared ABmC2.5 (with different content of MBA) hydrogels sharply decreased along with the increase of crosslinkers (MBA) content. However, other than conventional high cross-linked chemical hydrogels, which always displayed brittle and poor mechanical property,2,43 the as-prepared ABmC2.5 hydrogel exhibited excellent mechanical property (with the fracture strength of 102 kPa at a stretch ratio of 950%) even in high chemical cross-linked condition (AB0.063C2.5). The excellent mechanical property of high chemical cross-linked ABmC2.5 hydrogel demonstrated that although chemical crosslinked network would seriously restrict the flexible of PAM chains, the movable physical cross-link points (C-dots) and the reversible physical coaction between C-dots and PAM chains could still dissipate the stress efficiently, endowing the asprepared ABmC2.5 hydrogel with excellent mechanical property. Besides, the compression testing (Figure 4c,d and Table S4) showed that different from conventional high cross-linked chemical hydrogels which were easy to be crushed,2,26−31,42,33 the as-prepared ABmC2.5 hydrogels even could be compressed over 85% without crush, further confirming that the reversible physical coaction between movable C-dots and PAM chains greatly improved the toughness of as-prepared ABmCn hydrogel. Furthermore, because of the hydrogel would not be crushed at 85% strain, the compress stresses of hydrogel at 85% strain could increase along with the increase of MBA content (Figure 4d).
Table 1. Elastic Recoveries of the As-Prepared ABmCn Hydrogels during Cyclic Tensile Tests sample (AB0.029Cn)
elastic recovery at 500% stretching (%)
sample (ABmC2.5)
elastic recovery at 800% stretching (%)
AB0.029C0 AB0.029C1.5 AB0.029C2.5 AB0.029C3.5
98.4 98.3 97.1 95.6
AB0.029C2.5 AB0.040C2.5 AB0.051C2.5 AB0.063C2.5
90.6 98.3 98.6 99
shows that the instantaneous elastic recovery ratios of asprepared AB0.029Cn hydrogels were over 95%, and the recovery ratio (500% strain) of the as-prepared AB0.029Cn could reach above 99.5% within 4 h (Figure S3a,b), much higher than the recovery ratios of the common NC hydrogel,18 which proved that the presence of chemical cross-linked network could endow as-prepared ABmCn hydrogels with an excellent recovery property. Besides, Figure S2b and Table 1 show that the recovery ratios of ABmC2.5 hydrogels increased along with the increase of cross-linker MBA content, further confirming that chemical cross-linked network is in favor of improving the restorability of ABmCn hydrogels. Moreover, when the content of chemical cross-linker MBA increased to 0.063 wt %, the elastic recovery ratio of the as-prepared ABmCn hydrogel was as high as 99% (Table 1). As far as we know, hydrogel with such high recovery ratio after being stretched 800% has rarely been reported. The above mechanical property test results revealed that Cdots could greatly improve the stretching ratio and fracture stress of the as-prepared ABmCn hydrogel. To elucidate the mechanism, a series of characterization of the as-prepared ABmCn hydrogel had been carried out. As shown in Figure 5a, the dynamic rheology of as-prepared ABmCn NC hydrogel were plotted against the angular frequency ω. The result indicated a elastic response of the as-prepared ABmCn, with storage modulus G′ (elastic) much higher than the loss modulus G″ (viscous) over the entire frequency range.39 Besides, the extent of viscous dissipation of the as-prepared ABmCn hydrogel was quantified in terms of the loss tangent tan δ (tan δ = G″/G′). From Figure 5a, we have calculated that the tan δ of AB0.029C0, AB0.029C1.5, AB0.029C2.5, and AB0.029C3.5 was around 0.037, 0.061, 0.109, and 0.115 respectively. The tan δ of ABmCn (that contain C-dot) was much higher than that of AB0.029C0, suggesting that C-dot could increase the viscous character of ABmCn, which was important for the high extensibility and robustness of ABmCn.39 Besides, the DSC test showed that other than dried clay/ graphene NC hydrogel, whose glass transition temperature (Tg) is higher than that of pure PAM and almost independent of clay/graphene content,2,28 the Tg of dried AB0.029Cn hydrogels decreased obviously along with the increase of C-dots content (Figure 5b and Table S5), demonstrating that addition of CE
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Figure 5. (a) Dynamic rheology of ABmCn hydrogels: elastic modulus G′ (solid symbols) and viscous modulus G″ (open symbols) as a function of frequency ω. (b) DSC of PAM hydrogel and ABmCn hydrogels. (c) FTIR spectra of C-dots powder, PAM hydrogel, and ABmCn hydrogels.
Scheme 1. (a) Schematic Demonstration of the ABmCn Hydrogel with Cross-Linking Networks in the Hybrid Hydrogel Including Chemically Cross-Linked PAM Network (Green) and Hydrogen Bonds between C-Dots and PAM (Red); (b) Stretch Model for the ABmCn Hydrogel
CO stretching (of the −CO−NH2), and NH2 in-plane rocking in PAM were shifted to 3428, 1645, and 1117 cm−1, respectively, in AB0.029C2.5 hydrogel. The shifts of the characteristic peaks in the FTIR spectra fully implied that there were strong hydrogen-bonding interactions between the amides groups of PAM chains and the oxygen-containing groups of C-dots in ABmCn hydrogels.2,42 Moreover, different from chemically cross-linked conventional hydrogel with barely self-healing property, ABmCn hydrogels displayed slight selfhealing property. As shown in Figure S4, the cutting hydrogels self-healed in a short time when putting the incision stick together (Figure S4a). Besides, the cut hydrogel could be stretched to 350% after self-healing at room temperature for 12 h. As we know, NC hydrogels always displayed excellent selfhealing property because of the strong and recoverable physical interaction between long flexible polymer chains and physical cross-linkers.29 Therefore, the specially self-healing property of ABmCn hydrogels further confirmed that in ABmCn hydrogels Cdots could make PAM chains more flexible and at the same time maintain strong interaction with them. All of above results revealed that ABmCn hydrogels consisted of the strong low cross-linked chemical network and intensive
dots nanoparticles could lead to the PAM chains conformation conversion easier. That may be because the mobility of the spherical C-dot nanoparticles brought in larger space for free movement of PAM chains, which caused that the polymer molecules were free and highly flexible, taking nearly random conformations between the C-dots. Furthermore, the FTIR characterizations of C-dot, PAM, chemical cross-linked hydrogel (AB0.029C0), and AB0.029C2.5 hydrogel were carried out. As shown in Figure 5c, the FTIR characterization of C-dot exhibited clear peaks at 3368, 1642, and 1030 cm−1, which were attributed to the stretching vibrations of O−H and CO as well as asymmetric and symmetric stretching vibrations of C− O in the C−O−C group, respectively. As for the pure PAM hydrogel, a series of characteristic bands at 3434, 1654, and 1124 cm−1 corresponded to N−H stretching, CO stretching, and NH2 in-plane rocking, respectively. For the FTIR spectra of AB0.029C0, the characteristic peaks belonging to PAM were nearly unchanged. In the FTIR spectrum of the AB0.029C2.5 hydrogel, the characteristic peaks belonging to C-dots powder decreased or even disappeared because of the low content of Cdots in the NC hydrogel. However, the characteristic peaks at 3434, 1652, and 1122 cm−1 corresponding to N−H stretching, F
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hydrogel was not strong enough to keep figure in high swelling ratio condition. In that situation, the network collapsed, leading to the decrement of swelling ratio and mechanical property of swollen hydrogel. However, when the content of MBA was higher than 0.017%, the swelling ratio of ABmC2.5 hydrogels decreased along with the increase of MBA content, confirming that the movement of polymer chains in high chemical crosslinked network would be seriously limited and lead to the decrement of swelling ratio of hydrogel.19,37 In addition, the swelling ratio of AB0.029C2.5 hydrogel at different pH aqueous solution (Figure 6c and Figure S5e−g) showed that the swelling ratio of ABmCn hydrogels greatly increased in alkaline conditions. The biggest swelling ratio of AB0.029C2.5 hydrogel was 457 times (at pH = 11.5 condition), which was 6.8 times higher than the swelling ratio in pure aqueous. That might be because although the PAM polymer itself was neutral and expected to be not so sensitive to pH, there were lot of carboxyl groups (sensitive to pH) on C-dot surface (which had been proven by XPS and FT-IR analysis). In alkaline conditions the carboxyl groups of C-dot surface will become a carboxylic acid ion and form an electric double layer on C-dot surface, which could both increase the osmotic pressure of hydrogel and weaken the hydrogen-bond interaction between C-dot and PAM chains, leading to the ABC hydrogel having a greater swelling ratio in alkaline conditions. This result maybe meant that even in swelling equilibrium states (in pure aqueous), the cross-linked networks (both chemical and physical) in ABmCn hydrogels still remained strong and to be stretchable. In order to confirm whether the swollen ABmCn hydrogel still exhibited rubber elasticity under deformation, a series of mechanics performance test had been done. As shown in Figure 7, the swollen AB0.029C2.5 hydrogel could suffer from a high
physical cross-linked network (consisted of C-dots and chemical cross-linked PAM chains) (Scheme 1a). When the ABmCn hydrogels was subjected to stretch, the strong and low cross-linked chemical network deformed, which enabled the PAM chains to move, and led to the intensive physical crosslinked network unzipped. The unzipping of the intensive physical cross-linked network, in its turn, reduced the stress concentration of the chemical cross-linked network.38 Besides, as we mentioned above, the spherical morphology and nanometer scale make C-dots easy to move; therefore, C-dots would move with PAM chains during the stretching process (because there was the strong physical interactions between PAM chains and C-dots). That collaborative mobility of PAM chains and C-dots would make PAM chains more flexible, leading to the formation of a relatively thick and loose layer of PAM chains adsorbed on C-dot surface, which was in favor of the stress dissipating more effectively during the stretching process,55,56 so that the chemical cross-linked network could endure bigger deformation (Scheme 1b). As a result, when ABmCn hydrogels were stretched, the physical cross-linked network unzipped for stress dissipating, while the chemical cross-linked network stabilized the deformation and remained intact. Besides, when the stress was removed, the stretched chemical network would recover to its initial state while the unzipped physical cross-links could be recombined, leading to excellent mechanical and recovery property of ABmCn hydrogels. Herein, the swelling behavior of the ABmCn hydrogels was also investigated. As shown in Figure 6, for AB0.029Cn hydrogels,
Figure 6. Swelling ratios of (a) AB0.029Cn hydrogels, (b) ABmC2.5 hydrogels, and (c) AB0.029C2.5 hydrogel at different pH values.
the swelling ratios increased along with the increase of C-dots content (Figure 6a). That was the possible reason that with the increase of C-dot content, more hydrophilic group, especially carboxyl group, would be introduced into the hydrogel (it had been proved that there were plenty of hydrophilic groups, such as hydroxyl and carboxyl groups on C-dot surfaces), which improved the water absorbing capacity (mixing energy) of the hydrogel, leading to the increase of the swelling ratio of ABmCn hydrogels along with the increase of C-dot content. For ABmC2.5 hydrogels (Figure 6b), when MBA content was low (0.017%), the swelling ratio (236 times) was much higher than that of common PAM/NC hydrogels.2,27,34−36,41 That was because compared with the physical cross-linked network, the stronger low chemical cross-linked network could suffer bigger expansion in swelling process,22,39 leading to a higher swelling ratio of the hydrogel. Besides, Figure S5a-d shows that when the content of chemical cross-linker MBA was too low (0.011%), the formed chemical cross-linked network in
Figure 7. (a−f) Photographs demonstrating the excellent mechanical behavior of the swollen AB0.029C2.5 hydrogels. (a) The swollen hydrogel was compressed (b) and (c) quickly recoveried after the compression was released. (d) The swollen hydrogel could be stretched (e) to over 3.5 times of its initial length and (f) quickly recovered after the stress was released.
compression without any damage and quickly recover to its initial state when compression stress was released (Figure 7a− c). Besides, Figure 7d−f shows that the swollen AB0.029C2.5 hydrogel could be stretched to over 3.5 times its initial length without fracture and quickly recover once the stress was released. G
DOI: 10.1021/acs.macromol.5b02352 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 8. Mechanical properties of swollen ABmCn hydrogels. (a) Tensile stress−strain curves of swollen AB0.029Cn hydrogels and (b) tensile stress, strain, and water content as a function of the C-dot content. (c) Tensile stress−strain curves of swollen ABmC2.5 hydrogels and (d) tensile stress, strain, and water content as a function of the MBA content.
of MBA (at 0.011% and 0.017%); however, in such a high swollen condition, the formed cross-linked network could not still keep intact and strong (Figure S5a−d), leading to a poor mechanical property of swollen hydrogels (AB0.011C2.5 and AB0.017C2.5). As the content of MBA increased, stronger and more intensive chemical cross-linked network were formed in ABmC2.5 hydrogels, which seriously limited the swelling property of the ABmC2.5 hydrogel (decreasing the swelling ratios), improving the mechanical property of swollen ABmC2.5 hydrogels (Figure 8d). Figure 8c,d and Table S7 show that the swollen AB0.063C2.5 hydrogel with 95.5% water containing (the swelling ratio as high as 21.2) was still very tough and could be stretched to 734% with 22.75 kPa fracture stress. A hydrogel with such an excellent mechanical property at such a high swelling ratio was barely reported. The cyclic tensile test of the swollen AB0.029C2.5 hydrogel displayed an obvious hysteresis loop and excellent recovery property (Figure S6a), which confirmed that PAM chains still kept strong interaction with Cdots in swollen AB0.029C2.5 hydrogel, leading to the stress being well dissipated during stretching process, while keeping chemical cross-linked network intact. Besides, the repeating tensile test of swollen AB0.029C2.5 hydrogel showed that AB0.029C2.5 hydrogel could be repeatedly stretched to 650% with tensile curves nearly unchanged, which further confirmed that the chemical cross-linked network was intact during stretching process (because the stress could be well dissipated by the recoverable interaction between C-dots and PAM chains), leading to an excellent recovery property of swollen ABmCn hydrogel. As we know, keeping excellent mechanical property in an in vivo environment is very important for the biomedical application of hydrogel. To further confirm whether the ABmCn hydrogel could still exhibit excellent mechanical property in an in vivo environment, the equilibrium swelling ratio and mechanical property of AB0.029C2.5 hydrogel in physiological saline solution had been measured. As shown in Figure 9, compared to the equilibrium swelling ratio and mechanical property of AB0.029C2.5 hydrogel in water, similar equilibrium swelling ratio and tensile stress−strain curve were
In addition, by tensile test, the mechanical property of the swollen ABmCn hydrogel had been further quantitatively investigated. As shown in Figure 8 as well as Tables S6 and S7, ABmCn hydrogels with different content of C-dots (AB0.029Cn) and MBA (ABmC2.5) displayed different mechanical properties. Figure 8a shows that all of the swollen AB0.029Cn hydrogels could be stretched, exhibiting excellent mechanical property. As we know, higher swelling ratio meant bigger volume expanding of the cross-linked network in hydrogel, which caused polymer chain disentanglement to occur and strained polymer chains, thus decreasing the flexibility of polymer chains.37,39,41 As the flexibility of polymer chains decreased, less stress could be dissipated by the interaction of polymer chains in stretching process. Therefore, hydrogel with higher swelling ratio always displayed worse mechanical property.39,41 However, as shown in Figures 8a,b and 6a, compared to pure chemical cross-linked swollen hydrogel (AB0.029C0), swollen AB0.029C1 hydrogel displayed higher swelling ratio/water content (the swelling ratios were 32 and 42 for swollen AB0.029C0 and AB0.029C1, respectively) and exhibited better mechanical property, indicating that the stress could be better dissipated in swollen AB0.029C1 hydrogel during stretching process. That may be because although PAM chains were more stretching in swollen AB0.029C1 hydrogel, they still kept strong interactions with C-dots. As a result, during stretching process, the strong interactions between C-dots and PAM chains unzipped progressively, leading to the good stress dissipation. Besides, as mentioned above, C-dots had synergic movement with PAM chains during stretching process, leading to PAM chains being more flexible. In that case, more stress was dissipated and led to better mechanical property. Moreover, comparing experiment data with theoretical prediction data of the strain for the swollen ABCn hydrogel further confirms that although the heterogeneity of the network structure in hydrogel became pronounced accompanied by the swelling, the swollen ABCn hydrogel still could keep excellent mechanical properties (Table S8 and Figure S7). For the swollen ABmC2.5 hydrogel, as shown in Figure 6b, Figure 8c,d, and Table S7, the swelling ratios were very high at low content H
DOI: 10.1021/acs.macromol.5b02352 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Basic Research Program of China (973 Program, 2012CB821500), the National Natural Science Foundation of China (21274046 and 21474032), and the Fundamental Research Funds for the Central Universities (2015ZM158).
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Figure 9. Tensile stress−strain curves of AB0.029C2.5 hydrogel swollen in pure water and physiological saline solution.
observed in physiological saline solution. These results confirmed that the ABmCn hydrogel could also exhibit excellent mechanical property even in a physiological environment, which would largely expand the application potential of hydrogels in biomaterials.
4. CONCLUSION In summary, we had proposed a simple way to prepare a kind of novel low chemically cross-linked PAM/C-dot hydrogel by combination of the physical network (physical bonds) and chemical network (covalent bonds) together. In this hydrogel, C-dots interacted with the chemical cross-linked PAM chains to form the physically cross-linked network of the hydrogel. The unzipping of the intensive physical cross-linked network could well disperse the stress concentration and keep chemical crosslinked network intact during deformation process, resulting in toughness and superstretchable property of this novel hydrogel. The as-prepared hydrogel could be stretched up to 3700% with strength as high as 166 kPa. Besides, the low chemical crosslinked network greatly improved the recovery property of hydrogel: the recovery ratio of hydrogel was more than 97%, even at the stretching ratio of 500%. Moreover, the best swelling ratio (in pure water) of the low chemical cross-linked PAM/C-dot hydrogel was more than 235 times, much higher than conventional chemical cross-link PAM hydrogel, indicating very excellent water absorption. In addition, the mechanical test of the swollen ABmCn hydrogels showed that hydrogels could keep excellent mechanical and recovery property even in swelling equilibrium state, which had been rarely reported. The swollen ABmCn hydrogels could be stretched more than 650% and almost completely recovery once the stress was removed. The excellent comprehensive properties of hydrogels in both the as-prepared and swollen states would largely expand the application potential of hydrogel in tissue engineering scaffolds, drug release vehicles, biosensors, and cell modulating substances.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02352. Characterization of C-dots and composites, property testing of ABmCn hydrogels (PDF)
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