Research Article pubs.acs.org/journal/ascecg
One-Pot Assembly of Microfibrillated Cellulose Reinforced PVA− Borax Hydrogels with Self-Healing and pH-Responsive Properties Beili Lu,§,† Fengcai Lin,§,† Xin Jiang,† Jiajia Cheng,‡ Qilin Lu,† Jianbin Song,† Chong Chen,† and Biao Huang*,† †
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College of Material Engineering, Fujian Agriculture and Forestry University, No. 15 Shangxiadian Road, Cangshan District, Fuzhou City, Fujian Province 350002, China ‡ College of Chemistry, Fuzhou University, No. 2 Xueyuan Road, Minhou District, Fuzhou City, Fujian Province 350116, China ABSTRACT: A facile, environmentally benign approach has been developed for the preparation of dynamic, multiresponsive, and self-healing hydrogels from inexpensive bamboo pulp, poly(vinyl alcohol) (PVA), and borax. The microfibrillated cellulose (MFC) reinforced PVA−borax hydrogels were produced through a onepot route in conjunction with ball milling and physical blending in tandem in aqueous medium. In this way, MFC particles could be efficiently generated and well-dispersed in a polymer matrix, and they have been verified by scanning electron microscopy. The rheology analysis indicated a close relationship between the mechanical strength and the MFC loading and ball milling time. Due to the dynamic equilibrium of the didiol−borax linkages and the reinforcement of MFC fibers, the hydrogels showed enhanced self-healing behavior and mechanical stiffness, which was also supported by rheology analyses. In addition, the hydrogels were found to be sensitive to the pH value. The hydrogels present a solvent or gel state with the change of pH value, and this sol−gel transfer can be repeated while maintaining the shape, further demonstrating the dynamic reversible behavior of the hydrogels. KEYWORDS: Microfibrillated cellulose, One-pot route, Hydrogels, Self-healing, pH-responsive
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such as drug delivery,12 sensors,13 superabsorbent materials,14 tissue engineering,15 and biomaterials.16 Notably, thanks to the existence of large amounts of solvent inside, the polymer chain segments of hydrogels could diffuse more easily across the wounded interfaces and rearrange to heal the scratches, facilitating the self-healing efficiency of materials.17 Therefore, the excellent polymer chain mobility of hydrogels makes them an ideal candidate for self-healing materials. Recently, the rising need in biomedical and pharmaceutical fields spurred considerable interest in developing efficient methods for the preparation of nontoxic, environmentally friendly, and biodegradable hydrogels as replacements for synthetic materials.18 To address these concerns, poly(vinyl alcohol) (PVA), a biocompatible and water-soluble polymer, has been widely used as matrix polymer for hydrogels.19 For example, Zhao et al. have achieved autonomous self-healing hydrogels from physical cross-linked PVA by a cyclic freezing− thawing method.20 However, the high PVA content, low transparency, and time-consuming process, coupled with high energy consumption, severely hindered its further application. Another practical method for the preparation of PVA hydrogels
INTRODUCTION Over the past decades, smart materials have attracted widespread interest from the academic and industrial areas by their unique applications in a diverse range of fields.1 Selfhealing materials, one of the most promising areas of smart materials, have become a research hotspot recently.2 The external stimuli-responsive materials possess the ability to recover their original structures after damage.3 Such recovery can occur automatically or in response to the changes in the environment, including light,4 temperature,5 pH value,6 electricity,7 magnetic field,8 and so on. Owing to the ability to undergo self-repair from external damage upon the application of a stimulus, self-healing materials effectively extend their lifetime and reduce the cost of the systems, which makes these smart materials exhibit significant advantages over traditional materials and have emerging wide applications in industry and biomedical materials.9 Hydrogels are soft and hydrophilic materials, containing cross-linked (physically and/or chemically) three-dimensional polymeric networks.10 Stimuli-responsive hydrogels are particularly attractive because, in addition to their reasonable hydrophilicity and permeability, they perform a sol−gel transition in response to the specific external triggers, including temperature, pH, and light.11 The “smart” behavior opens a wide range of applications for stimuli-responsive hydrogels, © 2016 American Chemical Society
Received: September 20, 2016 Revised: November 12, 2016 Published: November 19, 2016 948
DOI: 10.1021/acssuschemeng.6b02279 ACS Sustainable Chem. Eng. 2017, 5, 948−956
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flow properties of the PVA−borax cross-linked system.34 Nevertheless, in this study MFCs need to be premanufactured through mechanical treatment, involving refining, grinding, sonication, homogenization, microfluidization, and electrospinning.35 One-pot assembly of MFC reinforced PVA−borax hydrogels from cheap and easily available raw materials remains a challenge and has not yet been reported. In the present work, with cheap bamboo pulp and commercially available PVA and borax, an efficient one-pot route in conjunction with ball milling and physical blending in tandem was utilized to prepare multifunctional MFC reinforced PVA−borax hydrogels. The prepared composites displayed significant improvement in mechanical strength compared to the neat PVA−borax hydrogels. In addition, the automatically self-healing property and pH-responsive ability were integrated into one single system to obtain biocompatible and nontoxic smart hydrogels under mild conditions. The effects of different concentrations and dimensional characteristics of MFCs on the dynamic rheological properties of these hydrogels are investigated. Moreover, the morphology study, self-healing process analysis, and sol−gel transition performance of the hydrogels all indicate our success in developing a simple environmentally friendly one-pot tandem method to prepare dynamic MFC−PVA−borax hydrogels.
is introducing a reversible cross-linker such as borax.21 When hydrolyzed in water, borax easily dissociated into boric acid and borate ions, which form a boric/borate buffer solution at a pH around 9 (eqs 1 and 2).22 An aqueous solution of PVA can
form hydrogels easily through cross-linking with borax ions. The cross-linking mechanism is divided into two steps, monodiol complexation (eq 3) followed by a cross-linking reaction to form the didiol−borax complex (eq 4). The presence of interchain dynamic didiol−borax complexation as well as hydrogen bonding between hydroxyl groups on PVA chains led to the gelation of PVA solution.23,24 Although its malleability and ductility is significantly enhanced, the thermoreversible equilibrium cross-linking between PVA and borax still leads to weak mechanical properties, finite lifetime, and low stability of the hydrogels.25 As such, ways of enhancing the performance of PVA−borax hydrogels are highly desirable, as they may improve its application scope. Nanocellulose derivatives have been readily utilized in polymer modification26 due to their excellent mechanical properties, relatively reactive surface, superior biocompatibility, and biodegradability. The reinforcing effects of nanocellulose derivatives on the properties of hydrogels, to impart mechanical strength, encourage cross-linking, and provide stimuli-responsive behavior, have also been studied.27 However, due to the poor solubility of nanocellulose derivatives in normal aqueous solution, the combination of them with PVA−borax hydrogels is still rare.28 Wu et al. recently found that, by respectively incorporating three types of cellulose nanoparticles (CNPs) (two types of cellulose nanocrystals (CNCs) and one kind of cellulose nanofibers (CNFs)) to the PVA−borax system, the reinforced hydrogels presented self-healing ability, with enhanced mechanical strength and toughness, respectively.29,30 However, the CNPs need to be prepared beforehand and the production of CNPs involves strong acid hydrolysis, complex synthetic steps, a lengthy separation process, and a timeconsuming purification procedure.31 More importantly, the homogeneous dispersion within a polymeric matrix was usually a formidable challenge for the application of cellulose derivatives in composite polymers.32 As one type of cellulose morphology which has been widely used as a reinforcing agent in various materials, microfibrillated cellulose (MFC) enjoys the following advantages: high aspect ratio with diameter of 10−100 nm and length of several micrometers, excellent mechanical properties (high strength and stiffness), web-like structures, and ready biodegradability.33 In 2014, Seppälä et al. studied the reinforcing effect of nanofibrillated cellulose (NFC) in PVA-borax hydrogels and observed the improvement of the non-Newtonian behavior and
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EXPERIMENTAL SECTION
Materials. Bamboo pulp provided by Nanping Paper Co., Ltd. (Nanping, Fujian, China) was prepared from Moso bamboo (Phyllostachys heterocycla cv. pubescens) using a kraft pulping process followed by hypochlorite bleaching. Bleached bamboo pulp was cut into pieces and beaten to form powdered cellulose with a highefficiency pulverizer. Poly(vinyl alcohol) (PVA) (DS = 1750 ± 50, over 99.0% purity, Mw = 75,000−80,000 g/mol) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Borax (sodium tetraborate decahydrate, over 99.5% purity, Na2B4O7·10H2O, Mw = 381.37 g/mol) was obtained from Aladdin Industrial Corporation (Shanghai, China). All reagents used in this work were of analytical grade, and deionized water was used for the preparation of aqueous solutions. Preparation of Hydrogels. MFC−PVA−borax hydrogels with 2.0 wt % of MFC, 4.0 wt % of PVA, and 0.4 wt % of borax were prepared via one-pot tandem reactions as follows (Figure 1). Mechanical pretreatment of the cellulose pulp was performed within the planetary ball mill equipped with two agate jars, each of which was 100 cm3 in volume and loaded with 20 agate balls. For each experiment, a mixture of 2.0 g of cellulose pulp, 25 mL of deionized water, and 20 5 mm agate balls was added into the agate jar and milled at a rotational speed of 650 rpm for several hours. After milling, the samples were introduced into a 200 mL beaker, with 0.8 g of borax powder and 45 mL of deionized water added. Meanwhile, 8.0 g of PVA powder was dissolved in 92 mL of deionized water with continuous magnetic stirring at 90 °C in an oil bath until the PVA was completely dissolved. Subsequently, the preheating MFC/borax aqueous suspensions (90 °C) were slowly added to the PVA solution. The mixture was stirred vigorously at a speed of 200 rpm until a homogeneous solution with well-dispersed MFC was formed. It should be noted that, to avoid the evaporation of the water, all the heating and stirring process was conducted in a hermetically sealed beaker. As the temperature slowly dropped down, the viscoelastic properties of the mixture were gradually presented. When the solution was further cooled to room temperature, homogeneously stable MFC−PVA−borax hydrogels were finally formed. All the other samples were prepared according to the conditions processed for the MFC−PVA−borax hydrogels. The hydrogels with 4 wt % of PVA and 0.4 wt % of borax (without MFC) were utilized as a control sample (designated as PB). The hydrogels with 0.5 wt %, 1.0 wt %, 2.0 wt %, 3.0 wt %, 4.0 wt %, and 5.0 wt % of MFC were labeled as PB-MFC-0.5, PB-MFC-1.0, PB-MFC-2.0, PB949
DOI: 10.1021/acssuschemeng.6b02279 ACS Sustainable Chem. Eng. 2017, 5, 948−956
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could effectively improve the efficiency and increase the yield of products. Characterization. To observe the morphology, one drop of deionized water diluted suspension of MFCs collected at different milling times was deposited on a carbon coated Cu grid and allowed to dry. TEM images were obtained with a JEOL JEM-1010 transmission electron microscope (Japan Electronics Co., Ltd., Japan) operated at a 120 kV accelerating voltage. The untreated bamboo pulp was freezedried and coated with gold for field emission scanning electron microscopy (FESEM) analysis. To observe the dispersion of MFC in PVA−borax, the cross section was exposed by fracturing the composites in liquid nitrogen. The fracture surfaces were then coated with gold before observation. All the samples were photographed by SU8010 FE-SEM (Hitachi, Japan) at an accelerating voltage of 1.0 kV. FTIR spectra of the samples were studied with a Nicolet 380 FTIR spectrometer (Thermo Electron Instruments Co., Ltd., USA) in the frequency range of 4000−400 cm−1 with a total of 32 scans and a resolution of 4 cm−1. Prior to analysis, each sample was first ground with KBr and pressed into thin pellets. The thermal stability of the hydrogels was characterized with a thermal gravimetric analyzer (NETZSCH STA 449 F3 Jupiter). The samples were heated from 25 to 600 °C at a heating rate of 10 °C/min under nitrogen atmosphere with a flow rate of 25 mL/min. The rheological behaviors of MFC−PVA−borax hydrogels were analyzed with a stress controlled rheometer Rotational Rheometer MARS III Haake (Thermo Scientific, Germany) by using a parallel plate geometry with a diameter of 35 mm. Samples were prepared in the form of disks with a diameter of 30 mm and a thickness of 3 mm and then subjected to a strain sweep test in which they were deformed at different shear strains (the modulus G was independent of the applied strain). To ensure each measurement was determined in the linear viscoelastic region, a deformation of 10% was chosen in the
Figure 1. Schematic illustration of the preparation and synthesis process of the MFC−PVA−borax hydrogels via one-pot tandem reactions. MFC-3.0, PB-MFC-4.0, and PB-MFC-5.0, respectively, and these samples were collectively referred to as MFC-reinforced PVA−borax hydrogels. Alternatively, a series of reference variant hydrogels composed of 3.0 wt % MFC for different ball milling times (1 h, 3 h, 5 h, 7 h, and 9 h) were designated as PB-MFC 1h, PB-MFC 3h, PBMFC 5h, PB-MFC 7h, and PB-MFC 9h, respectively. In this work, by applying a one-pot protocol for the preparation of MFC−PVA−borax hydrogels, a lengthy separation and purification process of the intermediate chemical compounds was avoided, which
Figure 2. FESEM image of bamboo pulp (a) and TEM micrographs of MFC (b−f): (b) 1 h ball milling, (c) 3 h ball milling, (d) 5 h ball milling, (e) 7 h ball milling, and (f) 9 h ball milling. 950
DOI: 10.1021/acssuschemeng.6b02279 ACS Sustainable Chem. Eng. 2017, 5, 948−956
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Figure 3. Field emission scanning electron microscopy (FESEM) micrographs of the fractured cross sections of pure PVA−borax hydrogels (a) and PVA−borax hydrogels reinforced by 3.0 wt % MFC for different ball milling times: (b) 1 h, (c) 3 h, (d) 5 h, (e) 7 h, and (f) 9 h. tests. Oscillatory frequency sweeps were performed from 0.01 to 10 Hz at 30 °C under each gel condition. Analyses of the self-healing process of MFC−PVA−borax hydrogels were also carried out by a rheology study. The hydrogel PB-MFC-3.0 (3.0 wt % of MFC, 3 h ball milling) was cut into 8 pieces with a scalpel on the plate, and the G′ and G″ versus time of the broken gel were recorded at a frequency of 1.0 Hz and strain of 10%. The G′ and G″ dependence on time in the continuous step strain for PB-MFC-3.0 were measured with the change of amplitude oscillatory force from a strain of 20% to 1% under the same frequency of 1.0 Hz. All the above processes were repeated twice.
bamboo pulp is anticipated to add into PVA−borax hydrogels with good dispersion to improve the low mechanical strength.36 Fracture Morphology Analysis. The morphologies of the fractured cross sections of the PVA−borax hydrogel and its composite containing 3.0 wt % MFC prepared for different milling times were observed by field emission scanning electron microscopy (FESEM) respectively (Figure 3). It can be clearly seen from Figure 3a that the fracture surface of a pure PVA− borax hydrogel presents a relatively smooth texture, while addition of the MFC to the PVA−borax hydrogel results in a close network structure in all samples (Figure 3b−3f). Due to the presence of the hydroxyl group on the MFC and their higher specific surface area, the mechanical properties of the composite hydrogels could be effectively enhanced through Hbonding and physical interactions between filler and PVA matrix. Moreover, the milling time of MFC has an important effect on the hydrogel properties with the 3 h milling time to the optimum as shown by the FESEM micrographs. When the milling time is 1 or 3 h, MFC does not flocculate and is well dispersed in the PVA−borax hydrogel. However, with the milling time extended to 5 h, 7 h, or longer, a certain extent of aggregations was observed. This is possibly due to the fact that extensive fibrillation results in a smaller particle size and a higher tendency to collide, which may induce a higher probability to form strong hydrogen bonds between the hydroxyl groups of adjacent polymer strands.37,38 Agglomeration is the result of the formation of additional hydrogen bonds between the cellulosic particles during the ball-milling process.39 The morphologies of the fractured cross sections of PVA− borax hydrogel and its composite containing different amounts
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RESULTS AND DISCUSSION Morphology Analysis. Figure 2 shows the field emission scanning electron microscopy (FESEM) image of untreated bamboo pulp and transmission electron micrograph (TEM) images of MFC samples for different ball milling times. As is shown in Figure 2a, the untreated bamboo pulp fibers are dispersed as micron-sized bands or ribbons (with width of 20− 45 μm) with a rather smooth surface. After a short ball milling time of 1 h, the sizes of the cellulose fibers (Figure 2b) are apparently reduced compared to the untreated bamboo pulp fibers, indicating that the mechanical treatment has affected the structure. With the extension of the ball milling time, the diameter of the fibers becomes smaller and the surface of fibers becomes rougher (Figure 2c−2f). As revealed by the enlarged TEM image in Figure 2b-2f; all the samples have been disintegrated into nanoscale microfibrils after a few hours of ball milling time. In addition, a tight network structure of MFC is formed during the ball milling process, which indicates the micronization of fibers into their constituent particles. Based on this result, the MFC formed after one-pot ball milling from 951
DOI: 10.1021/acssuschemeng.6b02279 ACS Sustainable Chem. Eng. 2017, 5, 948−956
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Figure 4. Field emission scanning electron microscopy (FESEM) micrographs of the fractured cross sections of PVA−borax hydrogels reinforced with different amounts of MFCs by 3 h ball milling: (a) 0.0%, (b) 1.0%, (c) 2.0%, (d) 3.0%, (e) 4.0%, and (f) 5.0%.
Figure 5. (a) FTIR spectra, (b) TG (dotted line) and DTG (solid line) curves of MFC, PVA−borax hydrogel (PB), and PVA−borax hydrogel reinforced by 3.0 wt % MFC (PB-MFC-3.0).
cross-linking (bridged by borax) and hydrogen bonding.40 Therefore, self-aggregation of MFC was effectively avoided and the enhanced interactions between MFC and the matrix lead to superior performance for the one-pot prepared MFC reinforced PVA−borax hydrogels. FTIR and Thermogravimetric Analysis. Figure 5a shows the FTIR spectra of MFC, PB, and PB-MFC-3.0. In the FTIR spectra, MFC shows some characteristic peaks at 3416 cm−1 (hydrogen bond O−H stretching vibration), 2900 cm−1 (the C−H symmetric stretching vibration), 1639 cm−1 (O−H bending of the absorbed water), 1059 cm−1 (the C−O stretching vibration), and 895 cm−1 (the asymmetric out-ofplane ring stretching).41 While the PB hydrogel displays several
of MFCs were also observed by FESEM respectively, as is shown in Figure 4. As revealed before, the fractured structure of pure PVA−borax hydrogel is relatively smooth (Figure 4a). In contrast, the MFC reinforced PVA−borax hydrogels present a rougher fracture surface in all samples (see Figure 4b, 4c, 4d, 4e, and 4f). The white dots are MFC fibers that are randomly organized but uniformly distributed throughout the hydrogels at the fracture surface. In addition, with higher MFC loading, the density of white dots increases, while no significant aggregation of MFCs in any samples is detected. This result indicates the good MFC dispersion in the polymer matrix for composite materials, probably due to the fact that both PVA and MFC are hydrophilic and can form reversible covalent 952
DOI: 10.1021/acssuschemeng.6b02279 ACS Sustainable Chem. Eng. 2017, 5, 948−956
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Figure 6. Storage modulus G′ and loss modulus G″ of MFC−PVA−borax hydrogels versus frequency: (a) using different concentrations of MFCs, (b) using MFCs prepared at different ball-milling times. Tensile experiment by hydrogels dyed by rhodamine B: (c) hydrogel prepared by the standard one-pot process with 3.0 wt % MFC, and (d) hydrogel prepared by mixing PVA and borax aqueous without MFC.
Figure 7. (A) Demonstration of the self-healing ability for PB-MFC-3.0 under room temperature. Rheology analyses of the self-healing process of PB-MFC-3.0 under room temperature: (B1−B2) G′ and G″ versus time during the self-healing process (frequency: 1.0 Hz; strain: 10%) (B1), the G′ and G″ dependence on time in continuous step strain measurements for PB-MFC-3.0 with an alternate small oscillation force (strain = 1%, frequency = 1.0 Hz) and a large one (strain = 20%, frequency = 1.0 Hz) (B2).
Representative TG and DTG curves of MFC, PB, and PBMFC-3.0 are presented in Figure 5b. The initial thermal degradation temperature of PB-MFC-3.0 increased from 280 °C to about 331 °C in comparison with neat PB. Moreover, the DTG curves show that the thermal decomposition peaks of the maximum weight loss appear at 312 °C for the neat PB and at 366 °C for PB-MFC-3.0. All of the above results indicate the beneficial effect of MFC on the thermal stability of PB-MFC
characteristic peaks of borax and borate, i.e. 1429 and 1334 cm−1 (asymmetric stretching relaxation of B−O−C), 829 cm−1 (B−O stretching from residual B(OH)4−), and 657 cm−1 (bending of B−O−B linkages within borate networks).42 In the spectra of PB-MFC hydrogel, most characteristic peaks of cellulose overlap with others. However, the existence of MFC in the PB-MFC hydrogel can be confirmed from the absorption band at about 896 and 1059 cm−1. 953
DOI: 10.1021/acssuschemeng.6b02279 ACS Sustainable Chem. Eng. 2017, 5, 948−956
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Figure 8. pH-responsive performance of MFC-reinforced PVA−borax hydrogels: (a) original hydrogel, (b) sol formed after adding HCl, (c) hydrogel reformed after adding NaOH, (d) sol formed again after adding HCl in the hydrogel depicted in part c, and (e) hydrogel after five cycles.
hydrogels, resulting in the improvement of the heat resistant properties of composite materials.43 Rheology Analysis. To gain insight into the gelation process, the dynamic rheology of PVA−borax hydrogels was investigated with various concentration and dimensional characteristics of MFC, respectively. The changes in the storage modulus G′ and loss modulus G″ of the hydrogels in the frequency range of 0.01−10 Hz were depicted in Figure 6a−6b. All samples exhibited viscous-like behavior at low frequency (G′ < G″) and elastic-like behavior (G′ > G″) at high frequency, indicating the existence of a typical reversible crosslinked network in the one-pot prepared MFC−PVA−borax hydrogels.44 As is expected, owing to the entanglement and reversible cross-linking performance of MFC in the PVA−borax hydrogles, the G′ and G″ values increased significantly with MFC concentration increasing, reaching the maximum value of 11380 Pa (G′) and 4036 Pa (G″), respectively (Figure 6a). The crossover of G′ and G″ occurred at lower frequency with higher MFC loading, indicating the enhanced mechanical strength and stiffness of the hydrogels. However, considering the inconvenience caused by the thick hydrogel with higher content of MFC, the hydrogels with 3.0 wt % of MFC (PB-MFC-3.0) were used to further test other mechanical properties and stimuli-responsive behaviors. In order to investigate the mechanical properties of the hydrogels obtained at different ball-milling times, the dynamic rheology of different PVA− borax hydrogels was studied and the results are shown in Figure 6b. With the increase of ball milling time, the G′ and G″ values increased significantly, confirming the progressive reinforcement of MFC in the PVA−borax hydrogels. However, extending the ball milling time from 3 to 5 and 7 h resulted in a decrease of the G′ and G″ values at both low and high frequency, possibly due to the self-association of the MFC particles, which would influence the interaction of the MFC with matrix polymers.45 This result is also in good agreement with the SEM analysis as shown in Figure 3. The toughness of MFC reinforced PVA−borax hydrogels (hydrogel prepared by the standard one-pot process with 3.0 wt % MFC) was demonstrated by the tensile experiment in Figure 6c−6d. Two samples of dyed cylindrical hydrogels (length of about 1 cm) which were manufactured by the one-pot method with or without MFC, were stretched by hand. For hydrogel containing MFC, more than 30 cm of elongation without fracture was observed. In contrast, hydrogel without MFC broke easily in the process of stretch. Self-healing Analysis. In the following section, the selfhealing behavior of the hydrogel was investigated, as depicted in Figure 7A. Two freshly prepared hydrogels with different colors (the red one was treated with rhodamine B) were cut into two pieces, respectively. The separate semicircles from two
different original hydrogels were brought into contact (Figure 7a−c). After contact for 10 min, the fractured surfaces adhered to each other and healed to form one single hydrogel autonomously without any stimulus or external force, meaning the fast and mild self-healing ability exhibited in the one-pot manufactured hydrogel (Figure 7a−d). Slight dye diffusion from the red half to the white half was observed in the process, resulting in a blurred interface (Figure 7a−d). The self-healed hydrogel could be firmly stretched by hand without any crack at the interface (Figure 7a−e). These results suggested that the 3D-network structure and mechanical strength of the hydrogels were both recovered.46 The stimulus-free coalescence and selfrecovery of hydrogels were probably attributed to the flexibility and the hydrophilic nature of polymer chains, facilitating the reformation of hydrogen bonds across the interface and a reversible didiol−borax complex in the network. The self-healing process was also quantitatively studied by carrying out rheology analyses. After the hydrogel (3.0 wt % MFC, 3 h ball milling) was cut into 8 pieces, the loss modulus G″ of the hydrogels quickly recovered to a similar value to that of the original hydrogel (Figure 7B1). The storage moduli G′ of the broken hydrogel exhibited lower (∼200 Pa) than the original one (∼2030 Pa) at first. However, the G′ value rose with the extending of time and finally reached almost the same value (∼2016 Pa) as the original hydrogel, suggesting the complete recovery of the inner structure of the hydrogel.47 The high-water-content hydrogels were then operated with a large amplitude oscillatory collapse to study the autonomous recovery of their mechanical characteristics. As shown in Figure 7B2, the hydrogels were first deformed with a small-amplitude oscillatory force (γ = 1.0%, frequency = 1.0 Hz), and PB-MFC3.0 (3.0 wt % of MFC, 3 h ball milling) presents a solid nature with a G′ of 2.3 KPa and a G″ of 1.8 KPa (tan δ = G″/G′ ≈ 0.78). While applying a large-amplitude oscillatory force (γ = 20%, frequency =1.0 Hz) to the hydrogels, the G′ value and G″ value decreased to ∼1.4 KPa and 1.3 KPa, respectively, resulting in a quasi-liquid state of the hydrogel (tan δ = G″/G′ ≈ 0.93). When the amplitude was decreased once again (γ = 1%, frequency = 1.0 Hz), the G′ and G″ immediately returned to the initial values, suggesting the quick recovery of the internal network (tan δ = 2.3 kPa/1.8 kPa ≈ 0.78). This process could be repeated several times, and all the results support the thixotropic nature, elastic response, and self-healing ability of the dynamic hydrogels.48 pH-Responsive Analysis. MFC-reinforced PVA−borax hydrogels manufactured through the one-pot method also exhibit a pH-responsive property with the change of pH value. As is shown in Figure 8, adding HCl aqueous solution (5 μL, 3 M) to the original hydrogel (1 g) gives rise to the transformation of hydrogel into solvent quickly (about 3 min, 954
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Natural Science Foundation of China (No. 21402027, 31370560), the Natural Science Foundation of Fujian Province (No. 2015J05046), the Education Department of Fujian Province (No. JA14108), open fund of Guangxi Key Laboratory of Chemistry and Engineering of Forest Products (No. GXFC 14-03) and Chemicals and Science Foundation for Distinguished Young Scholars of Fujian Agriculture and Forestry University (No. xjq201503).
with approximately pH of 3), while the addition of an equivalent NaOH aqueous solution (5 μL, 3 M) leads to the reformation of the hydrogel. The results suggest that this sol− gel transfer can be successfully accomplished repeatedly, demonstrating the dynamic reversible behavior of the hydrogels. Since only a small amount of HCl aqueous solution (5 μL) was added and the experiment was performed at room temperature, hydrolysis of MFC was not possible and the MFC maintained the original micron-sized structure among the hydrogel sol−gel conversion processes. The pH-sensitive sol− gel transition behavior is probably due to the reversible didiol complexation between hydroxyl groups of PVA or MFC and borate ions, the dynamic balance of which could be shifted by changing the pH value of the solution.49 The basic conditions could facilitate the formation of a hydrogel by efficiently stabilizing the tetrahedral borate, while the acid led to the dissociation of the didiol−borate complex.50,51 Therefore, the original hydrogel was prepared under basic conditions offered by borax aqueous solution with pH around 9.5.
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CONCLUSIONS In conclusion, a facile approach is presented to prepare dynamic hydrogels by using inexpensive bamboo pulp, poly(vinyl alcohol) (PVA), borax, and water as the main components. The microfibrillated cellulose (MFC) reinforced PVA−borax−water system was successfully fabricated through an efficient one-pot route in conjunction with ball milling and physical blending in tandem in an aqueous medium. The welldispersed MFC in the polymer matrix acted as multifunctional reversible cross-linking agents and nanofillers to enhance the interactions between MFC and the matrix, while efficiently avoiding aggregation of MFC. As in rheology analysis, higher MFC loading leads to the improved mechanical strength and stiffness of the composite hydrogels. However, extending the ball milling time resulted in a decrease of mechanical property with the 3 h ball milling time to be the optimum. The hydrogel network was built through the formation of interchain dynamic didiol−borax complexation as well as hydrogen bonding between the B(OH)4− and the OH groups on the sides of the PVA/MFC. The reversible, exchangeable, and dynamic nature of the didiol−borax linkages in the 3D network gave rise to the hydrogels exhibiting automatically self-healing ability and pH-responsive property. This method provides a valuable and environmentally friendly alternative to the preparation of dynamic, multiresponsive hydrogels from cheap and readily available materials, which is expected to be applied in many biomedical areas in the future.
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REFERENCES
(1) Song, Y.; Wei, W.; Qu, X. Colorimetric biosensing using smart materials. Adv. Mater. 2011, 23, 4215−4236. (2) Yang, Y.; Urban, M. W. Self-healing polymeric materials. Chem. Soc. Rev. 2013, 42, 7446−7467. (3) Liu, F.; Urban, M. W. Recent advances and challenges in designing stimuli-responsive polymers. Prog. Polym. Sci. 2010, 35, 3− 23. (4) Burnworth, M.; Tang, L.; Kumpfer, J. R.; Duncan, A. J.; Beyer, F. L.; Fiore, G. L.; Rowan, S. J.; Weder, C. Optically healable supramolecular polymers. Nature 2011, 472, 334−337. (5) Feng, C.; Lü, S.; Gao, C.; Wang, X.; Xu, X.; Bai, X.; Gao, N.; Liu, M.; Wu, L. “Smart” fertilizer with temperature- and pH-responsive behavior via surface-initiated polymerization for controlled release of nutrients. ACS Sustainable Chem. Eng. 2015, 3, 3157−3166. (6) Roberts, M. C.; Hanson, M. C.; Massey, A. P.; Karren, E. A.; Kiser, P. F. Dynamically restructuring hydrogel networks formed with reversible covalent crosslinks. Adv. Mater. 2007, 19, 2503−2507. (7) Qi, H.; Mäder, E.; Liu, J. Electrically conductive aerogels composed of cellulose and carbon nanotubes. J. Mater. Chem. A 2013, 1, 9714−9720. (8) Xu, F.; Wu, C. A. M.; Rengarajan, V.; Finley, T. D.; Keles, H. O.; Sung, Y.; Li, B.; Gurkan, U. A.; Demirci, U. Three-dimensional magnetic assembly of microscale hydrogels. Adv. Mater. 2011, 23, 4254−4260. (9) Billiet, S.; Hillewaere, X. K. D.; Teixeira, R. F. A.; Du Prez, F. E. Chemistry of crosslinking processes for self-healing polymers. Macromol. Rapid Commun. 2013, 34, 290−309. (10) Elisseeff, J. Hydrogels: structure starts to gel. Nat. Mater. 2008, 7, 271−273. (11) Koetting, M. C.; Peters, J. T.; Steichen, S. D.; Peppas, N. A. Stimulus-responsive hydrogels: theory, modern advances, and applications. Mater. Sci. Eng., R 2015, 93, 1−49. (12) Hoffman, A. S. Biomedical applications and challenges for clinical translation. Adv. Drug Delivery Rev. 2013, 65, 10−16. (13) Buenger, D.; Topuz, F.; Groll, J. Hydrogels in sensing applications. Prog. Polym. Sci. 2012, 37, 1678−1719. (14) Nykänen, V. P. S.; Nykänen, A.; Puska, M. A.; Silva, G. G.; Ruokolainen, J. Dual-responsive and super absorbing thermally crosslinked hydrogel based on methacrylate substituted polyphosphazene. Soft Matter 2011, 7, 4414−4424. (15) Drury, J. L.; Mooney, D. J. Hydrogels for tissue engineering: scaffold design variables and applications. Biomaterials 2003, 24, 4337−4351. (16) Peppas, N. A.; Hilt, J. Z.; Khademhosseini, A.; Langer, R. Hydrogels in biology and medicine: from molecular principles to bionanotechnology. Adv. Mater. 2006, 18, 1345−1360. (17) Wei, Z.; Yang, J. H.; Zhou, J.; Xu, F.; Zrínyi, M.; Dussault, P. H.; Osada, Y.; Chen, Y. M. Self-healing gels based on constitutional dynamic chemistry and their potential applications. Chem. Soc. Rev. 2014, 43, 8114−8131. (18) Li, Y.; Rodrigues, J.; Tomas, H. Injectable and biodegradable hydrogels: gelation, biodegradation and biomedical applications. Chem. Soc. Rev. 2012, 41, 2193−2221. (19) Lozinsky, V. I.; Galaev, I. Y.; Plieva, F. M. Polymeric cryogels as promising materials of biotechnological interest. Trends Biotechnol. 2003, 21, 445−451. (20) Zhang, H.; Xia, H.; Zhao, Y. Poly(vinyl alcohol) hydrogel can autonomously self-heal. ACS Macro Lett. 2012, 1, 1233−1236.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Beili Lu: 0000-0002-7957-1986 Author Contributions §
B.L. and F.L. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the generous financial support of the State Forestry Administration 948 project (No. 2014-4-30), National 955
DOI: 10.1021/acssuschemeng.6b02279 ACS Sustainable Chem. Eng. 2017, 5, 948−956
Research Article
ACS Sustainable Chemistry & Engineering (21) Sinton, S. Complexation chemistry of sodium borate with poly(vinyl alcohol) and small diols: a boron-11 NMR study. Macromolecules 1987, 20, 2430−2441. (22) Schultz, R. K.; Myers, R. R. The Chemorheology of poly (vinyl alcohol)-borate gels. Macromolecules 1969, 2, 281−285. (23) Keita, G.; Ricard, A.; Audebertt, R.; Pezron, E.; Leibler, L. The poly(vinyl alcohol)-borate system: influence of polyelectrolyte effects on phase diagrams. Polymer 1995, 36, 49−54. (24) Kanaya, T.; Takahashi, N.; Nishida, K.; Seto, H.; Nagao, M.; Takeda, T. Neutron spin-echo studies on dynamic and static fluctuations in two types of poly(vinyl alcohol) gels. Phys. Rev. E 2005, 71, 011801. (25) Lin, H. L.; Liua, Y. F.; Yua, T. L.; Liua, W. H.; Rwei, S. P. Light scattering and viscoelasticity study of poly(vinyl alcohol)−borax aqueous solutions and gels. Polymer 2005, 46, 5541−5549. (26) Klemm, D.; Kramer, F.; Moritz, S.; Lindström, T.; Ankerfors, M.; Gray, D.; Dorris, A. Nanocelluloses: a new family of nature-based materials. Angew. Chem., Int. Ed. 2011, 50, 5438−5466. (27) McKee, J. R.; Hietala, S.; Seitsonen, J.; Laine, J.; Kontturi, E.; Ikkala, O. Thermoresponsive nanocellulose hydrogels with tunable mechanical properties. ACS Macro Lett. 2014, 3, 266−270. (28) Chang, C.; Lue, A.; Zhang, L. Effects of crosslinking methods on structure and properties of cellulose/PVA hydrogels. Macromol. Chem. Phys. 2008, 209, 1266−1273. (29) Han, J.; Lei, T.; Wu, Q. Facile preparation of mouldable polyvinyl alcohol-borax hydrogels reinforced by well-dispersed cellulose nanoparticles: physical, viscoelastic and mechanical properties. Cellulose 2013, 20, 2947−2958. (30) Han, J.; Lei, T.; Wu, Q. High-water-content mouldable polyvinyl alcohol-borax hydrogels reinforced by well-dispersed cellulose nanoparticles: dynamic rheological properties and hydrogel formation mechanism. Carbohydr. Polym. 2014, 102, 306−316. (31) Kobayashi, H.; Ito, Y.; Komanoya, T.; Hosaka, Y.; Dhepe, P. L.; Kasai, K.; Hara, K.; Fukuoka, A. Synthesis of sugar alcohols by hydrolytic hydrogenation of cellulose over supported metal catalysts. Green Chem. 2011, 13, 326−333. (32) Tomé, L. C.; Pinto, R. J.; Trovatti, E.; Freire, C. S.; Silvestre, A. J.; Neto, C. P.; Gandini, A. Transparent bionanocomposites with improved properties prepared from acetylated bacterial cellulose and poly(lactic acid) through a simple approach. Green Chem. 2011, 13, 419−427. (33) Siró, I.; Plackett, D. Microfibrillated cellulose and new nanocomposite materials: a review. Cellulose 2010, 17, 459−494. (34) Spoljaric, S.; Salminen, A.; Luong, N. D.; Seppälä, J. Stable, selfhealing hydrogels from nanofibrillated cellulose, poly(vinyl alcohol) and borax via reversible crosslinking. Eur. Polym. J. 2014, 56, 105−117. (35) Lavoine, N.; Desloges, I.; Dufresne, A.; Bras, J. Microfibrillated cellulose−its barrier properties and applications in cellulosic materials: a review. Carbohydr. Polym. 2012, 90, 735−764. (36) Qi, X.; Yang, G.; Jing, M.; Fu, Q.; Chiu, F. C. Microfibrillated cellulose-reinforced bio-based poly(propylene carbonate) with dual shape memory and self-healing properties. J. Mater. Chem. A 2014, 2, 20393−20401. (37) Vartiainen, J.; Pöhler, T.; Sirola, K.; Pylkkänen, L.; Alenius, H.; Hokkinen, J.; Tapper, U.; Lahtinen, P.; Kapanen, A.; Putkisto, K.; Hiekkataipale, P.; Eronen, P.; Ruokolainen, J.; Laukkanen, A. Health and environmental safety aspects of friction grinding and spray drying of microfibrillated cellulose. Cellulose 2011, 18, 775−786. (38) Fortunati, E.; Armentano, I.; Zhou, Q.; Iannoni, A.; Saino, E.; Visai, L.; Berglund, L. A.; Kenny, J. M. Multifunctional bionanocomposite films of poly(lactic acid), cellulose nanocrystals and silver nanoparticles. Carbohydr. Polym. 2012, 87, 1596−1605. (39) Khoshkava, V.; Kamal, M. R. Effect of drying conditions on cellulose nanocrystal (CNC) agglomerate porosity and dispersibility in polymer nanocomposites. Powder Technol. 2014, 261, 288−298. (40) Lu, J.; Wang, T.; Drzal, L. T. Preparation and properties of microfibrillated cellulose polyvinyl alcohol composite materials. Composites, Part A 2008, 39, 738−746.
(41) Lu, J.; Askeland, P.; Drzal, L. T. Surface modification of microfibrillated cellulose for epoxy composite applications. Polymer 2008, 49, 1285−1296. (42) Ramadevudu, G.; Lakshmi Srinivasa Rao, S.; Shareeffuddin, M.; Narasimha Chary, M.; Lakshmipathi Rao, M. FTIR and optical absorption studies of new magnesium lead borate glasses. Global. J. Sci. Frontier Res. Phys. Space. Sci. 2012, 12, 41−46. (43) Niu, Y.; Zhang, X.; He, X.; Zhao, J.; Zhang, W.; Lu, C. Effective dispersion and crosslinking in PVA/cellulose fiber biocomposites via solid-state mechanochemistry. Int. J. Biol. Macromol. 2015, 72, 855− 861. (44) Carretti, E.; Grassi, S.; Cossalter, M.; Natali, I.; Caminati, G.; Weiss, R. G.; Baglioni, P.; Dei, L. Poly(vinyl alcohol)-borate hydro/ cosolvent gels: viscoelastic properties, solubilizing power, and application to art conservation. Langmuir 2009, 25, 8656−8662. (45) Nair, S. S.; Zhu, J. Y.; Deng, Y.; Ragauskas, A. J. Hydrogels prepared from cross-linked nanofibrillated cellulose. ACS Sustainable Chem. Eng. 2014, 2, 772−780. (46) Deng, G.; Tang, C.; Li, F.; Jiang, H.; Chen, Y. Covalent crosslinked polymer gels with reversible sol-gel transition and self-healing properties. Macromolecules 2010, 43, 1191−1194. (47) Zhang, Y.; Tao, L.; Li, S.; Wei, Y. Synthesis of multiresponsive and dynamic chitosan-based hydrogels for controlled release of bioactive molecules. Biomacromolecules 2011, 12, 2894−2901. (48) Wang, Q.; Mynar, J. L.; Yoshida, M.; Lee, E.; Lee, M.; Okuro, K.; Kinbara, K.; Aida, T. High-water-content mouldable hydrogels by mixing clay and a dendritic molecular binder. Nature 2010, 463, 339− 343. (49) Lin, H. L.; Yu, T. L.; Cheng, C. H. Reentrant behavior of poly(vinyl alcohol)-borax semidilute aqueous solutions. Colloid Polym. Sci. 2000, 278, 187−194. (50) Chung, W. Y.; Lee, S. M.; Koo, S. M.; Suh, D. H. Surfactant-free thermochromic hydrogel system: PVA/borax gel networks containing pH-sensitive dyes. J. Appl. Polym. Sci. 2004, 91, 890−893. (51) Audebeau, E.; Oikonomou, E. K.; Norvez, S.; Iliopoulos, I. Onepot synthesis and gelation by borax of glycopolymers in water. Polym. Chem. 2014, 5, 2273−2281.
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