Dual Physically Cross-Linked Double Network Hydrogels with High

Research Article www.acsami.org

Dual Physically Cross-Linked Double Network Hydrogels with High Mechanical Strength, Fatigue Resistance, Notch-Insensitivity, and Self-Healing Properties Ningxiao Yuan, Lu Xu, Hualiang Wang, Youpeng Fu, Zhe Zhang, Lan Liu, Cuiling Wang, Jianhao Zhao, and Jianhua Rong* Department of Materials Science and Engineering, College of Science and Engineering, Jinan University, Guangzhou 510632, P. R. China S Supporting Information *

ABSTRACT: Double-network (DN) hydrogels with high strength and toughness have been developed as promising materials. Herein, we explored a dual physically cross-linked polyacrylamide/xanthan gum (PAM/XG) DN hydrogel. The nonchemically cross-linked PAM/XG DN hydrogels exhibited fracture stresses as high as 3.64 MPa (13 times higher than the pure PAM single network hydrogel) and compressive stresses at 99% strain of more than 50 MPa. The hydrogels could restore their original shapes after continuously loading−unloading tensile and compressive cyclic tests. In addition, the PAM/XG DN hydrogels demonstrated excellent fatigue resistance, notch-insensitivity, high stability in different harsh environments, and remarkable selfhealing properties, which might result from their distinctive physical-cross-linking structures. The attenuated total reflectance infrared spectroscopy (ATR-IR) and dynamic thermogravimetric analysis (TGA) results indicated that there were no chemical bonds (only hydrogen bonds) between the XG and PAM networks. The PAM/XG DN hydrogel synthesis offers a new avenue for the design and construction of DN systems, broadening current research and applications of hydrogels with excellent mechanical properties. KEYWORDS: double network hydrogels, self-healing, fatigue resistance, high mechanical strength, notch-insensitivity

■

INTRODUCTION

DN hydrogels are composed of two different polymer networks with asymmetric structures: a rigid and brittle first network (for example alginate, PAMPS), which provides reversible sacrificial bonds and dissipates energy under large deformation, and a soft and stretchable second network (for example, PAM), which provides elasticity to the hydrogel.12,13 At present, most DN hydrogels share a common structural feature of both networks being chemically cross-linked. The interpenetration of the two contrasting networks makes the chemically cross-linked DN hydrogels both tough and soft; they contain approximately 90 wt % water and possess elastic moduli of 0.1−1.0 MPa, failure tensile stresses of 1−10 MPa at strains

Hydrogels, a kind of smart soft material that consists of water and distinctive 3D cross-linked network structures, have been widely used in many fields, such as drug delivery, tissue engineering, cartilage repair, and biosensor.1−7 However, the applications of hydrogels are often limited by their poor mechanical properties (strength, toughness, stiffness, and selfrepairing capability).8 Developing hydrogel materials with both high mechanical strength and self-healing properties is necessary for practical applications. As one kind of high mechanical strength hydrogel, double network (DN) hydrogels have attracted considerable attention due to their extraordinary mechanical properties and good biocompatibilities, which make them promising candidates as supporting and load-bearing material.9−11 © 2016 American Chemical Society

Received: September 26, 2016 Accepted: November 18, 2016 Published: November 18, 2016 34034

DOI: 10.1021/acsami.6b12243 ACS Appl. Mater. Interfaces 2016, 8, 34034−34044

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic diagram representation of the preparation of the PAM/XG hydrogel and a possible network structure of the PAM/XG hydrogel.

cellulose. Every second glucose unit carries a side chain consisting of a trisaccharide composed of (β-1,4) mannose, glucuronic acid and (β-1,2) mannose, which attach to alternate glucose residues in the main chain by α-1,3 linkages.33 The average molecular weight ranges from 2 × 106 to 20 × 106 Da and can be controlled by the biosynthesis conditions.34 In aqueous solutions, two disaccharide units in different xanthan chains can form ionic cross-links through divalent cations (for example Ca2+ or Pb2+), resulting in a network in water (xanthan hydrogel; see Figure 1). In this study, we used Ca2+ cross-linked XG as the first network in the DN hydrogel. Compared with other natural macromolecules (such as chitosan and alginate), XG has many advantages, for instance, high molecular weight and plentiful side chains, which can help to improve the strength of the hydrogels by increasing the entanglement degree of the networks and intramolecular cross-linking. Surfactant (SDS) containing PAM, the second network of the DN hydrogel, was physical cross-linked by hydrophobic interactions instead of chemical bonds, in which hydrophobic monomer stearyl methacrylate (SMA) was used as the hydrophobic comonomer. 35−37 During the formation of the hydrogel, the comonomer SMA was first dissolved in the reaction solution by the solubilization of SDS to form micelles. The polymerizable methylacrylyl groups of SMA were exposed to the outside of the micelles, while the hydrophobic stearyl groups were inside the micelles. After the copolymerization of SMA and AM, these micelles connect different polymer chains together to form PAM network. These micelles are reversible, which could reassemble under certain conditions after being broken. Therefore, in addition to the high strength and toughness, self-healing properties also appeared in the PAM/ XG DN hydrogels because of the physical cross-linking of both networks. Especially, the key of the good self-healing properties of our DN hydrogels was the reversible breakable cross-links of hydrophobic association surrounded by SDS surfactant micelles.

of 1000−2000%, failure compressive stresses of 20−60 MPa at strains of 90−95%, and tearing fracture energies of 100−1000 J/m2.14−19 However, once a DN hydrogel suffers damage at high strain, the fracture of the first network will result in irreversible and permanent bond breakage, leading the hydrogel to lose most of its mechanical properties.20−22 To improve the fatigue resistance and self-recovery properties of the hydrogels, reversible physical bonds, including hydrogen bonds,23,24 hydrophobic interactions, 25 π−π stacking, 26 or ionic bonds,27,28 were introduced to the hydrogel networks.29 The physically cross-linked bonds in the first network could unzip and dissipate energy upon deformation. Once the stress was removed, the physical bonds could reform.8,30 Sun et al. designed a hybrid DN hydrogel with alginate cross-linked with divalent cations (Ca2+) as the first network and covalently cross-linked polyacrylamide as the second network, which showed excellent stretchability (more than 20 times their initial lengths) and notch-insensitivity. However, the DN hydrogel has a lower strength with a fracture stress of ∼160 kPa and slowly recoverable properties (∼74% work from the first loading at 80 °C for 24 h).8 We recently synthesized hybrid polyacrylamide/bacterial cellulose nanofiber cluster (PAM/ BCNC) hydrogels consisting of hydrogen bond cross-linked BCNC networks and covalently cross-linked PAM networks. The hybrid PAM/BCNC hydrogels exhibited superior mechanical properties of elongation at break of 2200%, fracture stress of 1.35 MPa, toughness of 8.7 MJ/m3. Additionally, the gels can reach a strain of approximately 99% without break (compressive stress at 99% of more than 30 MPa). However, poor recoverable properties were observed even after waiting more than 48 h, and it could not self-heal after being damaged.31 To the best of our knowledge, hydrogels with both high mechanical strength and self-healing properties are quite rare in the previously reported literature. Xanthan gum (XG) is a high molecular weight polysaccharide produced mainly by the bacterium Xanthomonas campestris.32 The main chain consists of glucose molecules connected by β-1,4 glycosidic bonds, which is similar to 34035


Research Article


■

weighed at specific time intervals until they reached swelling equilibrium. Afterward, the hydrogels were dried in a vacuum oven at 70 °C until a constant weight was obtained. The swelling ratio and equilibrium water contents38,39 of the hydrogels were calculated using the following equations:

EXPERIMENTAL SECTION

Materials. Xanthan gum (JL-SH, Inner Mongolia, China) was donated by Inner Mongolia Jianlong Biochemistry Co., Ltd. Acrylamide (AM), stearyl methacrylate (SMA), sodium dodecyl sulfate (SDS), and calcium sulfate (CaSO4·2H2O) were purchased from Aladdin Industrial Corporation. Ammonium persulfate (APS) and sodium chloride (NaCl) were purchased from Fuchen Chemical Regent Factory (Tianjin, China). The APS was recrystallized in deionized water and dried at 25 °C under vacuum. The other regents were used as received. Synthesis of the PAM/XG Double Network (DN) Hydrogels. The PAM/XG DN hydrogels were prepared via in situ polymerization of acrylamide in xanthan gum solution using hydrophobic associations and bivalent cations as the cross-linking agents of the first network and second network, respectively. Typically, xanthan gum (0.1, 0.2, 0.3, or 0.4 g) was first dissolved in 7 wt % SDS/0.9 M NaCl aqueous solution (10 mL) under vigorous stirring for 12 h at room temperature. AM (4.0 g) and SMA (0.381 g, 2 mol % of AM) were then added into this hybrid solution while stirring for 1 h at 45 °C. The total monomer concentration was fixed at 30 w/v% in all samples. After adding APS (0.044 g, 1 wt % of total monomer) and CaSO4·2H2O (0.0184 g), the hybrid solution was quickly degassed and then poured into the space between two glass plates with a 1.8 mm-thick silicon rubber spacer and plastic molds (10 mm in diameter and 10 ± 1 mm in height); the solutions were left at 70 °C for 48 h for polymerization. The obtained hybrid gels were denoted as PAM/XG-n, where n stands for the weight percentage of XG relative to AM (n = 2.5, 5, 7.5, and 10) of the hydrogel. The as-prepared hydrogel samples were directly used for mechanical tests without any purification. Mechanical Test. The mechanical properties of all of the asprepared hydrogels were measured using an electronic universal testing machine (Zwick/Roell Z005, Germany) at room temperature. For the tensile tests, the hydrogel samples with thicknesses of 1.8 mm were sliced into rectangles (35 mm in length and 4 mm in width). The velocity of the tensile and loading−unloading tests was fixed at 100 mm/min. The Young’s moduli were calculated from the slope of the linear regions (εt = 5−10%) of the stress−strain curves. The toughnesses of the hydrogels were represented by the total work required for the fracture of a unit volume of material (W). The W of the hydrogels was calculated from the following equation: W = U/V, where U is the universal work done by the applied force and V is the tested volume of the hydrogel. For the compression tests, cylindrical hydrogel samples (10 mm in diameter and 7 ± 1 mm in height) were used in a BL-GRW005 K electronic universal testing machine (Zwick/Roell, Germany) at a constant testing speed of 2 mm/min in compression and cyclic tests. All of the mechanical tests were carried out at room temperature. Each data point was the average of at least 3 measurements. The error bars were obtained from their standard deviations. Self-Healing Experiment. The PAM and PAM/XG series hydrogels were tested in self-healing experiments. Two disc specimens (PAM/XG-10) of each hydrogel were cut into two pieces, respectively. For better visualization, one disc was colored with rhodamine B. Two semicircular hydrogels with different colors were coated with DI water and connected by putting two fresh cuts together, which were sealed in a polyethylene bag and submerged in a 70 °C water bath. To observe the self-healing process, a PAM/XG-10 hydrogel film with a thickness of 1.8 mm was broken using a razor blade. Optical microscopy images were taken to record the self-healing process of the hydrogel at various time intervals. For the recovery tests of the mechanical properties, PAM and PAM/XG series hydrogel specimens were first cut into two pieces, the fresh cut surfaces of which were coated with water and then placed in contact to heal for a certain time at a certain temperature. Tensile tests of the healed samples were performed at 100 mm/min. Swelling Behavior. The water swelling ratio, equilibrium water contents, and the swelling kinetics of PAM and PAM/XG series hydrogels were measured using a gravimetric method at room temperature as follows. The as-prepared cylindrical hydrogel samples were fully immersed in deionized water. The swollen samples were

swelling ratio =

Wsw Wo

(1)

where Wsw is the swollen weight and Wo is the weight of the asprepared gel.

equilibrium water contents =

Wsw − Wdry Wsw

× 100%

(2)

where Wdry is the dry weight of the cylindrical hydrogel sample. To evaluate the swelling rate of PAM and PAM/XG DN (series) hydrogels, first-order kinetics Voigt model was used.

Wt = We(1 − e−t / τ )

(3)

where Wt (g/g) is the swollen weight at time t (h), We stands for the equilibrium swelling (g/g), and τ is the rate parameter of swelling. Additionally, PAM and PAM/XG-10 hydrogels were immersed in deionized water and acidic (pH 2.0) and alkaline (pH 12.0) solutions. The equilibrated hydrogels were sliced into rectangles (35 mm in length and 4 mm in width), which were used in the tensile tests. The stretching speed of the test was fixed at 100 mm/min. Characterization. The as-prepared PAM, XG, and PAM/XG hydrogels were frozen at −20 °C and then freeze-dried for 24 h. The obtained freeze-dried samples were cut into small pieces, the fresh fractures of which were coated with Au−Pd alloy in a sputter coater under an argon atmosphere. The morphologies of the coated samples were investigated using an Ultra55 field emission scanning electron microscope (SEM; Zeiss, Germany). For the measurements of attenuated total reflectance infrared spectroscopy (ATR-IR), 1H NMR, spectra and dynamic thermogravimetric (DTG), the as-prepared hydrogels were dialyzed in a large excess of water to remove the mobile monomers and ions from the hydrogels and were then freeze-dried. The ATR-IR spectra of these freeze-dried samples were recorded over the scanning range between 4000 and 550 cm−1 with a resolution of 4 cm−1 and averaged over 64 scans in a VERTEX70 FT-IR spectrometer (Bruker, Germany) at room temperature. 1H NMR spectra (in d6-DMSO) and solid state 13 C NMR spectra of PAM hydrogel without SMA, PAM (with SMA), and PAM/XG hydrogels (with SMA) were recorded (at 250 MHz) using a UX-400 NMR spectrometer (Bruker, Germany). TGA measurements of the samples were performed using an NETZSCH TG 209 F3 Tarsus instrument. The heating rate for the analysis was set at 10 °C/min under a N2 atmosphere and a temperature range of 40−750 °C. For degradation kinetics study, nonisothermal TGAs were conducted at three different rates of 5, 10, and 20 °C/min.

■

RESULTS AND DISCUSSION Structures and Morphologies of the Hydrogels. Figure 1 shows a schematic representation of a possible network structure of the as-prepared PAM/XG hydrogel. There were several kinds of physical cross-linking bonds in the PAM/XG double network (DN) hydrogels, one of which was the divalent cation (Ca2+) cross-linking via the carboxyl groups in the XG networks. Another one was the hydrophobic micelles (SMA dissolved in the SDS micelles) serving as the cross-linking group in which SMA copolymerized with acrylamide monomers formed the second network. Moreover, some hydrogen bonds existed between PAM and XG networks. The ATR-IR spectra of the XG, PAM, and PAM/XG hydrogels are shown in Figure 2. The characteristic peak of XG at 1720 cm−1 corresponded to CO stretching vibration of carboxylic groups. The PAM gel exhibited characteristic peaks 34036


Research Article


ranging from 33 to 45 ppm was assigned to methylene C1 and methane C2, signals at 180 ppm was related to carbon atoms of −CONH2 group (C3). For PAM and PAM/XG-10 (with SMA), the new peaks at 32 and 14 ppm could be assigned to the methylene carbons C8−C24 and the methyl carbons C6 and C25 of SMA units, respectively. These summative NMR date suggested the copolymerization of AM and SMA. The microstructures of the XG, PAM, and PAM/XG hydrogels were observed by FE-SEM. For the pure XG and PAM gels, the pores were very large. The walls of the pores appeared to be quite thin (Figure 4a,b) especially in XG gel. The PAM/XG gels exhibited different morphologies due to the formation of the double network. As the amount of XG increased, the pore size of the PAM/XG hydrogel decreased significantly, and the pore distribution was more uniform (Figure 4c−f), suggesting the formation of more packed and uniform structures. Mechanical Properties of PAM and PAM/XG Hydrogels. The PAM/XG DN hydrogel exhibited superior mechanical strength, stretchability, and notch-insensitivity. As shown in Figure 5a, a knotted PAM/XG-10 hydrogel sheet could be stretched up to several times its initial length. Moreover, this gel was so tough that it had a high resistance against defects (could hang a 1 kg steel block via a hole in this hydrogel). Figure 5b shows the typical tensile stress−strain curves of PAM/XG hydrogel samples. Because of the effect of the micelles, the PAM hydrogel without XG was soft (elongation at break: 1540%, fracture stress: 0.28 MPa), which were much larger than traditional chemically cross-linked PAM hydrogels.8,21 The pure Ca2+ cross-linked XG hydrogel without PAM was so weak that it failed to withstand tester clamping. In contrast, the fracture stress of PAM/XG-10 could reach 3.64 MPa, which was 13 times higher than that of the pure PAM single network (SN) hydrogel. The corresponding elongation of PAM/XG-10 at break was 1575%, slightly larger than that of the pure PAM hydrogel. The concentration of XG affected the mechanical properties of hydrogels greatly. The higher the concentration of XG, the higher the strength of the hydrogel. The Young′s modulus and toughness also increased with increasing XG (see Table 1), indicating that the first network of divalent cations (Ca 2+) cross-linked XG chains could significantly improve the mechanical strength. Hysteresis loops were observed in the loading−unloading cycle tests (Figure 5c). The PAM/XG-10 hydrogel showed

Figure 2. ATR-IR spectra the essential peaks of XG, PAM, and PAM/ XG hydrogels.

at 3342 and 3197 cm−1 for the stretching vibration of N−H, 1659 cm−1 for CO stretching, and 1608 cm−1 for N−H deformation. Compared with the absorption peaks of pure PAM and XG hydrogels, the PAM/XG DN hydrogel did not have any new peaks, indicating that there were no new chemical bonds between the two networks. However, the shift of the CO stretching peaks from 1659 to 1653 cm−1 could be attributed to the formation of intermolecular hydrogen bonds between XG and PAM chains. In order to confirm the copolymerization reactions of AM and SMA, PAM hydrogel without SMA was also prepared. Figures S1 (Supporting Information) and 3 showed the ATRIR, 1H NMR, and 13C NMR spectra of PAM and PAM/XG hydrogels (with SMA) and PAM hydrogel without SMA. Both PAM and PAM/XG-10 hydrogels (Figure S1) exhibited the characteristic bands at 2918 and 2851 cm−1 owing to the stretching of the methylene groups of SMA units, which were absent in PAM hydrogel without SMA. In the 1H NMR spectra (Figure 3a), all hydrogels exhibited the characteristic peak b at 1.24 ppm which was attributed to the protons on methylene chain, while the peak b in PAM and PAM/XG hydrogels were much stronger than that in PAM without SMA because there are more methylene groups in the former. The characteristic peak a at 0.86 ppm, assigned to the methyl protons of − (CH2)17CH3 of SMA units, was absent in the sample of PAM hydrogel without SMA. The 13C NMR spectra of samples were shown in Figure 3b. For PAM (without SMA), a broad signal

Figure 3. 1H NMR (a) and solid state 13C NMR (b) spectra of the PAM hydrogel without SMA, PAM, and PAM/XG-10 hydrogels. 34037


Research Article


Figure 4. SEM images exhibiting the microstructure of XG (a), PAM (b), PAM/XG-2.5 (c), PAM/XG-5 (d), PAM/XG-7.5 (e), and PAM/XG-10 (f).

notable hysteresis loops in the first loading−unloading cycle, indicating that DN gels dissipated energy effectively. After the first cycle, a 49% residual strain was observed. However, the residual strain decreased with increasing waiting time and disappeared after a certain waiting time (300 min; Figure 5d). The second loading−unloading curve gradually recovered back to the first loading−unloading curve with the increase of the waiting time when the residual strain disappeared, which indicated nearly full-recovery (hysteresis ratio = 93%) of the PAM/XG-10 hydrogel structure. We think that the nearly full recovery of the PAM/XG hydrogel is similar to that of traditional DN hydrogels. The first network (Ca2+ cross-linked XG chains) is broken during deformation, but no breaking of primary chains occurs whereas the XG chains might be unzipped and the Ca2+ pulled out. After the force is released, the Ca2+ will rapidly return to their original positions to reform the cross-link structure. Generally, the second network (hydrophobically cross-linked PAM) serves as the permanent cross-linkers, imparting elasticity, whereas the first network (divalent cation cross-linked XG chains) serves as the reversible cross-linkers that break and reform via deformation to dissipate energy as reversible sacrificial bonds. In order to confirm the effect of the micelles in hydrogels, four samples were prepared by (I) polymerizing acrylamide in the xanthan/SDS solution without SMA; (II) polymerizing acrylamide in xanthan/SMA solution without SDS; (III) polymerizing acrylamide in xanthan solution without both SMA and SDS; (IV) polymerizing acrylamide in the xanthan solution with both SMA and SDS. Figure S2 (Supporting Information) showed that the tough PAM/XG hydrogels formed only when both SDS and SMA were used (hydrogel IV). The other three hydrogels were soft and crackly, which demonstrated that the hydrophobic micelles (composed of SDS and SMA) in the hydrogel acted a role as cross-linker. We also investigated the crack resistance of the DN hydrogel by stretching two samples, one single-edge notched and one central notched PAM/XG-10 hydrogel (the length of the notch ∼6 mm). The results are shown in Figure 5e,f. After being stretched up to 7 and 6.5 times, respectively, both of the notches remained stable, indicating that there was no stress concentration in the front of the notch top.40 According to Gong′s reports, crack blunting would occur when the σf/E > 2 (σf, fracture stress; E, modulus).41 The σf/E of the PAM/XG-n

(n = 2.5, 5, 7.5, and 10) range from 6.56 to 7.88, which are much larger than 2, suggesting the excellent notch-insensitivity of the DN hydrogels. The behavior might result from the synergetic interaction of the divalent cation (Ca2+) cross-linked XG network and the hydrophobically cross-linked PAM network. When a notched hydrogel was stretched, the XG network dissipated energy around the crack and transferred stress to the adjacent large zone. Meanwhile, the PAM network bridged the crack and stabilized deformation, eliminating the stress concentration and resisting crack propagation. The typical compressive stress−strain curves of the pure PAM and PAM/XG DN hydrogels are shown in Figure 6a. The compressive stress gradually increased with increasing strain until the compressive strain was approximately 70%; above this point, it suddenly increased. The inset of Figure 6a shows that the pure PAM hydrogel broke at compressive strain 44%. The other four PAM/XG hydrogels did not break even when the compressive strain increased up to 99%. PAM/XG-10 exhibited remarkable stress (more than 50 MPa) when the compressive strain = 99%. The compressive modulus and compressive stresses at 90% and 95% strains of the DN hydrogels were summarized in Table 1. To investigate the shape recoverability, cyclic compression tests (strain from 0 to 90%) were carried out with the PAM/ XG-10 DN hydrogel. The stress-time curves in Figure 6b show that there was only a small decrease in the stress after every cycle, suggesting that no significant irrecoverable damage had been done to the DN hydrogel even at such a high strain. The inset in Figure 6b shows that the hydrogel sample could almost restore its original shape after ten loading−unloading cyclic tests, suggesting that the hydrogel had good fatigue resistance. Previously, Harrass et al. reported a DN hydrogel composed of cross-linked six arm star-shaped poly(ethylene oxide-statpropylene oxide) and polyacrylamide with fully reversible behavior under cyclic compression experiments. However, their highest compression strength was only 5.6 MPa.42 Self-Healing Behaviors. According to the previous reports of Okay’s group, when a hydrophobic cross-linked hydrogel was cut into two pieces, the broken hydrophobic associations on the cut surfaces could reform after the two fresh cuts were put together.43 The self-healing behavior of our hydrogels was studied as shown in Figure 7. In Figure 7a, two disk specimens of the PAM/XG-10 hydrogel (one of them was colored by 34038


Research Article


Figure 5. (a) Knotted hydrogel sheet (width: 4 mm, thickness: 1.8 mm) PAM/XG-10 could bear stretching, and a 1 kg steel block was hung from the PAM/XG-10 gel (thickness: 1.8 mm) via a hole (made by a piercer); (b) tensile stress−strain curves of hydrogels with five different xanthan gum contents; (c) recovery curves of PAM/XG-10 gel samples with different wait times after the first loading−unloading; (d) time-dependent recovery of residual strain and hysteresis loop of PAM/XG-10 hydrogel; (e, f) crack resistance by stretching the notched PAM/XG-10 hydrogel sample.

Table 1. Mechanical Properties of PAM and PAM/XG DN Hydrogels tensile samples

elongation at break (%)

fracture stress (MPa)

PAM PAM/XG-2.5 PAM/XG-5 PAM/XG-7.5 PAM/XG-10

1540 ± 73 2300 ± 185 1960 ± 94 1790 ± 108 1575 ± 91

0.291 1.702 2.041 2.540 3.639

± ± ± ± ±

0.021 0.127 0.151 0.172 0.207

compressive

Young’s modulus (MPa) 0.072 0.216 0.309 0.387 0.464

± ± ± ± ±

0.006 0.013 0.021 0.028 0.037

toughness (MJ/ m3 )

fracture stress (MPa)

± ± ± ± ±

0.083 ± 0.006

1.013 17.70 19.15 22.14 27.64

0.017 0.092 0.119 0.153 0.141

strength at 90% strain (MPa) 9.059 13.79 15.20 19.05

± ± ± ±

0.092 0.138 0.174 0.143

strength at 95% strain (MPa) 21.05 26.41 29.20 34.54

± ± ± ±

0.094 0.130 0.169 0.151

generated by a razor blade. The results indicated that the notch on the PAM/XG-10 hydrogel film could autonomously self-heal within 10 h at ambient temperature without any healing agent. We further measured the healing efficiency (σself‑healed/σoriginal) of PAM and PAM/XG series hydrogels using tensile tests. The hydrogel specimens were first cut into two pieces. The fresh cut surfaces were coated with DI water and then placed into

rhodamine B) were cut in half, respectively. The fresh surfaces with different colors were placed in contact. Then, they were sealed in a polyethylene bag and submerged in a 70 °C water bath. After healing for 10 h, the self-healed hydrogel sample could be stretched by hand without breaking. Additionally, PAM/XG-10 hydrogel films could also exhibit self-healing behavior. Figure 7b shows optical microscopy images recording the self-healing process of the incision 34039


Research Article


Figure 6. (a) Compressive stress−strain curves of pure PAM and DN hydrogels with different xanthan gum content; (b) cyclic stress−time compression tests of the PAM/XG-10 hydrogel.

Figure 7. (a) Photographs demonstrating the self-healing behavior of PAM/XG-10 hydrogels. (a1) Four pieces of original hydrogel samples with or without red dye. (a2) The two different colored semicircles were placed together for 10 h in a sealed polyethylene bag and submerged in a 70 °C water bath. (a3) The self-healed hydrogel could sustain deformation without breakage. (b) Optical microscopy images that recorded changes of the incision on the PAM/XG-10 hydrogel over time at ambient temperature; (c) stress−strain curves of the original (solid lines) and self-healed (dash lines) hydrogels for 48 h at 70 °C; (d) healing efficiencies of the corresponding self-healed hydrogels in (c); (e) healing efficiencies of the PAM/XG2.5 hydrogel self-healed for different time at different temperatures.

contact to heal for 48 h at 70 °C. Figure 7c shows that all of the curves of the self-healed hydrogels were completely coincident with the original samples except for the smaller elongation at break. By comparing the tensile strengths before and after selfhealing, the healing efficiencies were 71%, 54%, 49%, 43%, and 38% (Figure 7d), for PAM, PAM/XG-2.5, PAM/XG-5, PAM/ XG-7.5, and PAM/XG-10 hydrogels, respectively. These results indicated that the stiffness of hydrogels was not changed, but the strength and toughness decreased. The reason might be due to the recovery of the microscopic chains aggregation structure but the fractured chains could not reform. Additionally, we evaluated the influences of temperature and time on self-healing efficiency of hydrogels (Figures 7e and S3−S5, Supporting Information). The results indicated that the self-healing

efficiency increased with increasing temperature and time, which could be attributed to increased solubility of hydrophobic parts in these hydrogels and increased the chains mobility under the higher temperature and longer time.44,45 So the polymer chains on the two cut surfaces could diffuse easier from one side to the other, and the hydrophobic associations across the fracture surfaces become more accessible to each other. However, the self-healing efficiency of the PAM/XG series hydrogels gradually decreased with the increase of the XG content, which might be attributed to the restriction of the chain movements by XG. In a series of self-healing experiments, we found that coating the cut surfaces with DI water before putting them together increased self-healing efficiency in PAM/XG series hydrogels. 34040


Research Article


Figure 8. Schematic representation of the self-healing mechanism of the PAM/XG hydrogel.

Figure 9. (a) Time dependences of the swelling ratios for the PAM and PAM/XG hydrogels; (b) equilibrium water contents; (c) tensile stress− strain and (d) loading−unloading cycle test curves of PAM/XG-10 hydrogel in different media. Error bars represent mean ± SD (n = 3).

Swelling Behavior and Stability of the PAM and PAM/ XG Hydrogels. The swelling ratios of the PAM and PAM/XG DN hydrogels were measured at different times by soaking the as-prepared hydrogels in water. Figure 9a shows that the swelling ratios of the hydrogels obviously increased during the initial 72 h and reached a steady value after 100 h. The swelling ratio of the PAM hydrogel was much larger than that of the PAM/XG hydrogels. The swelling ratios and equilibrium water contents of the hydrogels (shown in Figure 9b) obviously decreased with increasing XG content. The possible reason was that the formation of the double cross-linked network made the

For example, the self-healing efficiency of PAM/XG-10 hydrogel was 27% at 70 °C (48 h) without coating water, but it could reach 38% after coating water on the cut surfaces. We deduced the possible process, as shown in Figure 8. When a PAM/XG hydrogel was cut into two pieces, the hydrophobic associations on the cut surfaces were broken. After being coated with water, the SDS on the cut surfaces disassembled into the thin water film. After the sections were put together and with the evaporation and infiltration of water, the disassembled SDS would reassemble together with stearyl group of SMA in the side chain of PAM, to form the hydrophobic aggregates.46 34041


Research Article


Figure 10. Thermogravimetric analysis (TGA) (a) and thermal decomposition rate (b) curves of the PAM, XG, and PAM/XG series hydrogels.

the dry PAM is stable until the temperature up to 285 °C. So we think that the first step of mass degradation of the PAM hydrogel below 285 °C could be attributed to the evaporation of the bound water in PAM network.49−51 The second degradation step ranged from 285 to 500 °C. During this stage, one ammonia molecule was first liberated for every two amide groups and formed one imide group (285 to 400 °C), following with the decomposition of imides and the breakage of the PAM backbone (400 to 500 °C).8,49,52 The PAM/XG hydrogels also clearly showed that two pyrolysis stages. Compared with PAM, PAM/XG hydrogel presented a higher residual mass and the mass-loss rate peak of the first step shifted to higher temperature, which might be caused by the formation of intermolecular hydrogen bonds between the PAM and the XG chains. All the results indicated that the thermal stability of PAM/XG hydrogel was visibly higher than that of PAM. In order to investigate the thermal degradation kinetics of the hydrogels, we selected PAM and PAM/XG-10 hydrogels to conduct TG experiments with various heating rates (5, 10, and 20 °C/min). The TG curves and the analysis of degradation kinetics of PAM and PAM/XG-10 hydrogels are shown in Figures S6, S7, and S8 (Supporting Information), respectively. Figure S7 showed that the Arrhenius plots of PAM and PAM/ XG-10 hydrogels based on Kissinger−Akahira−Sunose (KAS) method (details are shown in the Supporting Information). The values of Ea were calculated by taking into account the conversion rate interval between 0.2 and 0.8, where their values were nearly constant. Figure S8 (Supporting Information) showed that the activation energies (Ea) of the degradation of PAM and PAM/XG-10 hydrogels were 40.32 and 42.17 kJ/ mol, respectively. Generally, the higher value of Ea means slower reaction rate and more stable structure.53 Therefore, the comparison also confirmed that the thermal stability of PAM/ XG is better than that of PAM.

hydrogel structure more compact and prevented the penetration of water molecules. The rate parameters τ calculated via eq 3 were 0.047, 0.097, 0.129, 0.139, and 0.159 h for PAM, PAM/XG-2.5, PAM/XG-5, PAM/XG-7.5, and PAM/XG-10, respectively. The swelling rates of PAM/XG hydrogels were slower than pure PAM hydrogel because of the higher network density and the more hydrogen bonds leading to the whole gel network more compact in DN hydrogel. The decreasing of the swelling rates of PAM/XG hydrogels with increasing XG amount is due to the same reason. The swelling ratio, time to reach 50% of total swelling (T50), and rate parameter (τ) were calculated and reported in Table S1. We further tested the mechanical properties of the swelling gels equilibrated in water with different pH values. The results are shown in Figure 9c,d. The PAM/XG-10 hydrogel still exhibited excellent mechanical properties (e.g., fracture stress >1 MPa, elongation at break >900%) in all solutions (pH 2.0, 7.0, and 12.0). The corresponding recovery properties (Figure 9d) remained relatively stable, which were still comparable to and even exceeded some soft load-bearing tissues.22 All of these mechanical test results suggests the high stability of the PAM/ XG hydrogels even in a harsh environment, which has rarely been seen in the literature. However, it is worth noting that the fracture stresses of both hydrogels equilibrated in pH 2 and 12 solutions are lower than that in neutral solution. We think the difference might results from the change of cross-link density of the first network, Ca2+ cross-linked XG, in different conditions. In the lower pH solution, the ion exchange between H+ and Ca2+ would decrease the amount of Ca2+ linked with XG chains. In the higher pH solution, the equilibrium reaction of Ca2+ and OH− would also decrease the amount of Ca2+ linked with XG chains leading to lower cross-link density of the first network. From the results in Figure 9c, it seems that the latter made more Ca2+ leaving the XG networks which resulted in the lowest strength of hydrogel in alkaline solution. Additionally, the PAM/XG hydrogels after water-swelling measurement lose their self-healing property (not shown in the paper). Thermostability of the PAM and PAM/XG Hydrogels. TG results and thermal decomposition rate (dw/dt) of the PAM, XG, and PAM/XG series hydrogels were presented in Figure 10. For XG, the initial mass loss (below 180 °C) was owing to the removal of moisture as hydrogen bound water to the saccharide structure,47,48 and the major weight loss ranging from 220 to 800 °C was due to chain degradation.32 The PAM hydrogel showed two main degradation stages. Minsk49 and Silva50 reported that the water is the only product below 200 °C by checking the obtained vapors via mass spectrometry, and

■

CONCLUSIONS In this work, we designed and fabricated a new type of dual physically cross-linked double network (DN) hydrogel, consisting of divalent cation (Ca2+) cross-linked XG chains as the rigid and brittle first network and hydrophobically crosslinked PAM as the second network. The first network provided reversible sacrificial bonds and dissipates energy, whereas the second network provided elasticity to the DN hydrogel. Because of the distinct dual physically cross-linked structure, the PAM/XG DN hydrogels showed notably mechanical properties. The tensile fracture stresses and compressive stresses of the PAM/XG hydrogels could be as high as 3.64 34042


Research Article


(5) Roy, I.; Gupta, M. N. Smart Polymeric Materials: Emerging Biochemical Applications. Chem. Biol. 2003, 10, 1161−1171. (6) Qiu, Y.; Park, K. Environment-Sensitive Hydrogels for Drug Delivery. Adv. Drug Delivery Rev. 2001, 53, 321−339. (7) Lee, K. Y.; Mooney, D. J. Hydrogels for Tissue Engineering. Chem. Rev. 2001, 101, 1869−1879. (8) Sun, J. Y.; Zhao, X.; Illeperuma, W. R.; Chaudhuri, O.; Oh, K. H.; Mooney, D. J.; Vlassak, J. J.; Suo, Z. Highly Stretchable and Tough Hydrogels. Nature 2012, 489, 133−136. (9) Haque, M. A.; Kurokawa, T.; Gong, J. P. Super Tough Double Network Hydrogels and Their Application as Biomaterials. Polymer 2012, 53, 1805−1822. (10) Tanaka, Y.; Abe, H.; Kurokawa, T.; Furukawa, H.; Gong, J. P. First Observation of Stick-Slip Instability in Tearing of Poly(vinyl alcohol) Gel Sheets. Macromolecules 2009, 42, 5425−5426. (11) Kaneko, D.; Tada, T.; Kurokawa, T.; Gong, J. P.; Osada, Y. Mechanically Strong Hydrogels with Ultra-Low Frictional Coefficients. Adv. Mater. 2005, 17, 535−538. (12) Gong, J. P. Materials both Tough and Soft. Science 2014, 344, 161−162. (13) Chen, Q.; Zhu, L.; Zhao, C.; Wang, Q. M.; Zheng, J. A Robust, One-Pot Synthesis of Highly Mechanical and Recoverable Double Network Hydrogels Using Thermoreversible Sol-Gel Polysaccharide. Adv. Mater. 2013, 25, 4171−4176. (14) Nakajima, T.; Furukawa, H.; Tanaka, Y.; Kurokawa, T.; Osada, Y.; Gong, J. P. True Chemical Structure of Double Network Hydrogels. Macromolecules 2009, 42, 2184−2189. (15) Huang, M.; Furukawa, H.; Tanaka, Y.; Nakajima, T.; Osada, Y.; Gong, J. P. Importance of Entanglement between First and Second Components in High-Strength Double Network Gels. Macromolecules 2007, 40, 6658−6664. (16) Na, Y. H.; Tanaka, Y.; Kawauchi, Y.; Furukawa, H.; Sumiyoshi, T.; Gong, J. P.; Osada, Y. Necking Phenomenon of Double-Network Gels. Macromolecules 2006, 39, 4641−4645. (17) Tsukeshiba, H.; Huang, M.; Na, Y. H.; Kurokawa, T.; Kuwabara, R.; Tanaka, Y.; Furukawa, H.; Osada, Y.; Gong, J. P. Effect of Polymer Entanglement on the Toughening of Double Network Hydrogels. J. Phys. Chem. B 2005, 109, 16304−16309. (18) Tanaka, Y.; Kuwabara, R.; Na, Y. H.; Kurokawa, T.; Gong, J. P.; Osada, Y. Determination of Fracture Energy of High Strength Double Network Hydrogels. J. Phys. Chem. B 2005, 109, 11559−11562. (19) Na, Y. H.; Kurokawa, T.; Katsuyama, Y.; Tsukeshiba, H.; Gong, J. P.; Osada, Y.; Okabe, S.; Karino, T.; Shibayama, M. Structural Characteristics of Double Network Gels with Extremely High Mechanical Strength. Macromolecules 2004, 37, 5370−5374. (20) Chen, Q.; Zhu, L.; Chen, H.; Yan, H. L.; Huang, L. N.; Yang, J.; Zheng, J. A Novel Design Strategy for Fully Physically Linked Double Network Hydrogels with Tough, Fatigue Resistant, and Self-Healing Properties. Adv. Funct. Mater. 2015, 25, 1598−1607. (21) Chen, Q.; Chen, H.; Zhu, L.; Zheng, J. Fundamentals of Double Network Hydrogels. J. Mater. Chem. B 2015, 3, 3654−3676. (22) Gong, J. P. Why Are Double Network Hydrogels So Tough? Soft Matter 2010, 6, 2583−2590. (23) Gao, H.; Wang, N.; Hu, X. F.; Nan, W. J.; Han, Y. J.; Liu, W. G. Double Hydrogen-Bonding pH-Sensitive Hydrogels Retaining HighStrengths Over a Wide pH Range. Macromol. Rapid Commun. 2013, 34, 63−68. (24) Cordier, P.; Tournilhac, F.; Soulie-Ziakovic, C.; Leibler, L. SelfHealing and Thermoreversible Rubber from Supramolecular Assembly. Nature 2008, 451, 977−980. (25) Hao, J. K.; Weiss, R. A. Viscoelastic and Mechanical Behavior of Hydrophobically Modified Hydrogels. Macromolecules 2011, 44, 9390−9398. (26) Burattini, S.; Colquhoun, H. M.; Fox, J. D.; Friedmann, D.; Greenland, B. W.; Harris, P. J. F.; Hayes, W.; Mackay, M. E.; Rowan, S. J. A Self-Repairing, Supramolecular Polymer System: Healability as a Consequence of Donor-Acceptor pi-pi Stacking Interactions. Chem. Commun. 2009, 6717−6719.

MPa and more than 50 MPa, respectively. They could restore their original shapes after continuous loading−unloading compressive cyclic tests, demonstrating their superior fatigue resistance. Single-edge and central notched PAM/XG hydrogel samples still showed ∼7 times elongation at break. Furthermore, the PAM/XG hydrogels exhibited remarkable self-healing properties and high stability in different harsh environments. In addition, we confirmed the copolymerization reactions of acrylamide and stearyl methacrylate via ATR-IR, 1H NMR, and 13 C NMR. TGA and ATR-IR results suggested the formation of hydrogen bonds between the PAM and XG networks. TGA kinetics results revealed that thermal stability of PAM/XG was better than that of PAM. We also explored the self-healing mechanism of the PAM/XG hydrogels and thought that their self-healing capability was mainly derived from the selfassembly of the hydrophobic associations. The combination of high mechanical strength and toughness, fatigue resistance, stability in harsh environments, and selfhealing properties, along with an easy synthetic method, make this type of material an ideal candidate for supporting and loadbearing material. We believe that the new DN hydrogel with both good mechanical strength and self-healing properties will open a new avenue for hydrogel exploration.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12243. Gelation results, healing efficiencies of the PAM and PAM/XG-10 hydrogels, swelling kinetic parameters, and TGA kinetics (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jianhua Rong: 0000-0003-1261-3851 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 20604010, 51173070, and 21344001).

■

REFERENCES

(1) Zheng, W. J.; Liu, Z. Q.; Xu, F.; Gao, J.; Chen, Y. M.; Gong, J. P.; Osada, Y. In Vitro Platelet Adhesion of PNaAMPS/PAAm and PNaAMPS/PDMAAm Double-Network Hydrogels. Macromol. Chem. Phys. 2015, 216, 641−649. (2) Li, Z.; Wei, Z.; Xu, F.; Li, Y. H.; Lu, T. J.; Chen, Y. M.; Zhou, G. J. Novel Phosphorescent Hydrogels Based on an IrIII Metal Complex. Macromol. Rapid Commun. 2012, 33, 1191−1196. (3) Luo, Z. L.; Yue, Y. Y.; Zhang, Y. F.; Yuan, X.; Gong, J. P.; Wang, L. L.; He, B.; Liu, Z.; Sun, Y. L.; Liu, J.; Hu, M. F.; Zheng, J. Designer D-Form Self-Assembling Peptide Nanofiber Scaffolds for 3-Dimensional Cell Cultures. Biomaterials 2013, 34, 4902−4913. (4) El-Mohdy, H. L. A.; Safrany, A. Preparation of Fast Response Superabsorbent Hydrogels by Radiation Polymerization and Crosslinking of N-Isopropylacrylamide in Solution. Radiat. Phys. Chem. 2008, 77, 273−279. 34043


Research Article

ACS Applied Materials & Interfaces (27) Henderson, K. J.; Zhou, T. C.; Otim, K. J.; Shull, K. R. Ionically Cross-Linked Triblock Copolymer Hydrogels with High Strength. Macromolecules 2010, 43, 6193−6201. (28) Hunt, J. N.; Feldman, K. E.; Lynd, N. A.; Deek, J.; Campos, L. M.; Spruell, J. M.; Hernandez, B. M.; Kramer, E. J.; Hawker, C. J. Tunable, High Modulus Hydrogels Driven by Ionic Coacervation. Adv. Mater. 2011, 23, 2327−2331. (29) Luo, F.; Sun, T. L.; Nakajima, T.; Kurokawa, T.; Bin Ihsan, A.; Li, X. F.; Guo, H. L.; Gong, J. P. Free Reprocessability of Tough and Self-Healing Hydrogels Based on Polyion Complex. ACS Macro Lett. 2015, 4, 961−964. (30) Stevens, L.; Calvert, P.; Wallace, G. G.; Panhuis, M. I. H. IonicCovalent Entanglement Hydrogels from Gellan Gum, Carrageenan and an Epoxy-Amine. Soft Matter 2013, 9, 3009−3012. (31) Yuan, N. X.; Xu, L.; Zhang, L.; Ye, H. W.; Zhao, J. H.; Liu, Z.; Rong, J. H. Superior Hybrid Hydrogels of Polyacrylamide Enhanced by Bacterial Cellulose Nanofiber Clusters. Mater. Sci. Eng., C 2016, 67, 221−230. (32) Bueno, V. B.; Takahashi, S. H.; Catalani, L. H.; de Torresi, S. I. C.; Petri, D. F. S. Biocompatible Xanthan/Polypyrrole Scaffolds for Tissue Engineering. Mater. Sci. Eng., C 2015, 52, 121−128. (33) Petri, D. F. S. Xanthan Gum: A Versatile Biopolymer for Biomedical and Technological Applications. J. Appl. Polym. Sci. 2015, 132, n/a. (34) Palaniraj, A.; Jayaraman, V. Production, Recovery and Applications of Xanthan Gum by Xanthomonas Campestris. J. Food Eng. 2011, 106, 1−12. (35) Tuncaboylu, D. C.; Sari, M.; Oppermann, W.; Okay, O. Tough and Self-Healing Hydrogels Formed via Hydrophobic Interactions. Macromolecules 2011, 44, 4997−5005. (36) Leyrer, R. J.; Machtle, W. Emulsion Polymerization of Hydrophobic Monomers Like Stearyl Acrylate with Cyclodextrin as a Phase Transfer Agent. Macromol. Chem. Phys. 2000, 201, 1235− 1243. (37) Tuncaboylu, D. C.; Argun, A.; Sahin, M.; Sari, M.; Okay, O. Structure Optimization of Self-Healing Hydrogels Formed via Hydrophobic Interactions. Polymer 2012, 53, 5513−5522. (38) Dai, X. Y.; Zhang, Y. Y.; Gao, L. N.; Bai, T.; Wang, W.; Cui, Y. L.; Liu, W. G. A Mechanically Strong, Highly Stable, Thermoplastic, and Self-Healable Supramolecular Polymer Hydrogel. Adv. Mater. 2015, 27, 3566−3571. (39) Wang, T.; Sun, W. X.; Liu, X. X.; Wang, C. Y.; Fu, S. Y.; Tong, Z. Promoted Cell Proliferation and Mechanical Relaxation of Nanocomposite Hydrogels Prepared in Cell Culture Medium. React. Funct. Polym. 2013, 73, 683−689. (40) Yu, Q. M.; Tanaka, Y.; Furukawa, H.; Kurokawa, T.; Gong, J. P. Direct Observation of Damage Zone around Crack Tips in DoubleNetwork Gels. Macromolecules 2009, 42, 3852−3855. (41) Luo, F.; Sun, T. L.; Nakajima, T.; Kurokawa, T.; Zhao, Y.; Bin Ihsan, A.; Guo, H. L.; Li, X. F.; Gong, J. P. Crack Blunting and Advancing Behaviors of Tough and Self-healing Polyampholyte Hydrogel. Macromolecules 2014, 47, 6037−6046. (42) Harrass, K.; Kruger, R.; Moller, M.; Albrecht, K.; Groll, J. Mechanically Strong Hydrogels with Reversible Behaviour under Cyclic Compression with MPa Loading. Soft Matter 2013, 9, 2869− 2877. (43) Gulyuz, U.; Okay, O. Self-Healing Polyacrylic Acid Hydrogels. Soft Matter 2013, 9, 10287−10293. (44) Volpert, E.; Selb, J.; Candau, F. Associating Behaviour of Polyacrylamides Hydrophobically Modified with Dihexylacrylamide. Polymer 1998, 39, 1025−1033. (45) Gulyuz, U.; Okay, O. Self-Healing Poly(acrylic acid) Hydrogels with Shape Memory Behavior of High Mechanical Strength. Macromolecules 2014, 47, 6889−6899. (46) Jeon, I.; Cui, J. X.; Illeperuma, W. R. K.; Aizenberg, J.; Vlassak, J. J. Extremely Stretchable and Fast Self-Healing Hydrogels. Adv. Mater. 2016, 28, 4678−4683.

(47) Darzi, H. H.; Larimi, S. G.; Darzi, G. N. Synthesis, Characterization and Physical Properties of a Novel Xanthan Gum/ Polypyrrole Nanocomposite. Synth. Met. 2012, 162, 236−239. (48) Zohuriaan, M. J.; Shokrolahi, F. Thermal Studies on Natural and Modified Gums. Polym. Test. 2004, 23, 575−579. (49) Minsk, L. M.; Kotlarchik, C.; Meyer, G. N.; Kenyon, W. O. Imidization During Polymerization of Acrylamide. J. Polym. Sci., Polym. Chem. Ed. 1974, 12, 133−140. (50) Silva, M. E. S. R. E.; Dutra, E. R.; Mano, V.; Machado, J. C. Preparation and Thermal Study of Polymers Derived from Acrylamide. Polym. Degrad. Stab. 2000, 67, 491−495. (51) Grassie, N.; McNeill, I. C.; Samson, J. N. R. The Thermal Degradation of Polymethacrylamide and Copolymers of Methacrylamide and Methyl Methacrylate. Eur. Polym. J. 1978, 14, 931−937. (52) Burrows, H. D.; Ellis, H. A.; Utah, S. I. Adsorbed Metal Ions as Stabilizers for the Thermal Degradation of Polyacrylamide. Polymer 1981, 22, 1740−1744. (53) Ma, Z. Q.; Chen, D. Y.; Gu, J.; Bao, B. F.; Zhang, Q. S. Determination of Pyrolysis Characteristics and Kinetics of Palm Kernel Shell Using TGA-FTIR and Model-Free Integral Methods. Energy Convers. Manage. 2015, 89, 251−259.

34044


Dual Physically Cross-Linked Double Network Hydrogels with High

Recommend Documents