A non-covalent strategy for montmorillonite/xylose self-healing hydrogels

Abstract

The self-healing capability of hydrogels has become a hot topic in the area of hydrogel research. An economical, convenient, eco-friendly, and reproducible approach for the preparation of self-healing xylose-based hydrogels is introduced in this article. First, methylguanidine hydrochloride was grafted onto the backbone of xylose using ethylene glycol as a crosslinking agent, then xylose with guanidinium ion pendants on its peripheries was entangled with exfoliated layered anionic montmorillonite (MMT) clay nanoplatelets under the dispersion of sodium polyacrylate (PAAS), thus forming xylose-based hydrogels, which were connected by hydrogen bonds and displayed intermolecular adsorption because of their internal spongy porous structure. The synthesized xylose-based hydrogels had a rapid self-healing ability and showed good swelling property. The structure and morphology of the composite hydrogels were characterized using FT-IR and SEM. The compression stress–strain results suggest that the elasticity of the xylose-based hydrogels increased with the increase of modified xylose solution, and the compression stress increased with the increasing concentration of modified xylose. Thermal gravimetric analysis (TGA) indicated that the composite hydrogels had a good heat resistant property due to the added inorganic MMT. All these properties demonstrate that the composite hydrogels have potential applications such as water absorbents, flame retardants, and as other functional materials.

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Introduction

With the decrease in fossil-fuels, the world has been paying attention to the investigations on new biodegradable materials that are much less reliant on petroleum products and more on traditional materials.^1,2 Therefore, lignocellulosic biomass has become a new focus of research in recent years. It mainly consists of three parts: lignin, cellulose, and hemicelluloses, which are all inexhaustible sources of energy and are renewable, non-toxic and biodegradable. Xylose is a pentitol, which is obtained from the xylan-rich portion of hemicelluloses from plants.³ It is the most important raw material in the manufacture of xylitol. Xylose can be used as a food additive on account of its health functions⁴ and is also used as a no-calorie sweetener, because it cannot be absorbed by the body.⁵ In addition, xylose can be used as a chemical platform for the production of industrially important chemicals.⁶

A prosperous research field has been formed in biomass-based hydrogels as a result of their wide potential applications such as super absorptive materials, immobilization of enzymes, and vehicles for drug delivery in pharmaceutical and medical science.^7,8 Furthermore, because of their good biocompatibility and ability to realize some of the functions of human tissues, some hydrogels have been applied to biomedical applications.^9,10 Given the environmental deterioration, it is reasonable to replace some petroleum-based plastics with biomass-based gels. Hydrogels are three-dimensional networks of crosslinked molecules and the vast majority of their mass consists of water, however they still exhibit solid-like mechanical properties.¹¹ Hydrogels can be classified into two categories depending on whether or not their crosslinked gel networks use the covalent or non-covalent approach.¹² Covalent hydrogels are rather fragile, having poor transparency and unable to autonomously heal once their network structure is broken because their polymer chains are interconnected by permanent non-reversible bonds.¹³ To overcome these limitations, hydrogels formed through non-covalent and physical associations arising from hydrogen bonds and intermolecular interactions, instead of covalent crosslinks, have attracted significant interest recently. A new class of “aqua materials” has demonstrated that significant improvement in the mechanical properties of hydrogels is possible.¹² Aqua materials have good mechanical properties with a high content of water and ultralow content of components; in addition, they can be molded into self-standing objects. More recently, hybridized polymers with clay nanosheets (CNSs), so-called nanocomposite hydrogels, are capable of displaying good mechanical properties and optical transparency.¹⁴ Montmorillonite (MMT), which is a natural mineral, is composed of stacked layers of aluminum octahedrons and silicon tetrahedrons.¹⁵ These layered silicates contain dangling hydroxyl end groups that can form hydrogen bonds with water, which make MMT highly hydrophilic.¹⁶ Furthermore, MMT can be used as a reinforcing agent because of its good capability of exfoliation and dispersion in polymer matrices.^17,18

Herein, we report a strategy for the formation of non-covalent hydrogels, on account of its outstanding features for convenient, quick, and reproducible preparation and the capability of self-healing. Guanidinium ions were grafted onto the backbone of xylose to obtain a modified xylose. Sodium polyacrylate (PAAS) was used as the dispersant for the MMT suspension. The xylose-based hydrogels were prepared by mixing the modified xylose with the MMT/PAAS suspension, and the properties of the composite hydrogels were further investigated in this study.

Materials and methods

Materials

Xylose was purchased from Beijing Aoboxing Bio-tech Co., Ltd. Methylguanidine hydrochloride was obtained from Tokyo Chemical Industry Co., Ltd. MMT, which is a naturally occurring mineral salt with a layered structure, was obtained from Alfa Aesar. PAAS (molecular weight ≥ 3 × 10⁷) was purchased from Sinopharm Chemical Reagent Co. Ltd. Analytical grade ethylene glycol was purchased from Beijing Chemical Works. All the reagents mentioned above were directly used without further purification.

Preparation of modified xylose

First, xylose (4.0 g) was dissolved in 20 mL of distilled water with vigorous mechanical stirring at 60 °C for 20 min. When xylose was dissolved, aqueous ammonia was added to the solution to adjust the pH to 11; then the crosslinking agent, 0.5 mL of ethylene glycol, was added to the solution, and it was left heating at 60 °C for 25 min. After that, 1.5 g of methylguanidine hydrochloride was added to the solution and the mixture was continued to be heated with stirring at 60 °C for 2.5 h. A desired amount of ethylene glycol (0.5 mL) was added again and the mixture was stirred for another 2 h. Finally, the reaction mixture was settled down for volatilization for one week at room temperature to obtain the modified xylose crystals. Fig. 1 shows the synthetic mechanism of the modified xylose.

Fig. 1 Synthetic mechanism of modified xylose for grafting methylguanidine hydrochloride.

Preparation of MMT suspension

MMT (2.0 g) was first dispersed in distilled water (38 mL) with vigorous mechanical stirring at room temperature for 30 min, followed by ultrasonication for 15 min, and then continuous stirring for 30 min. This process was repeated four times. After storage of the MMT solution overnight, the undissolved solids were removed by centrifugation to obtain a homogeneous suspension of MMT (3.98 wt%), which was used for the preparation of the composite hydrogels.

Preparation of xylose-based self-healing hydrogels

In a typical hydrogel preparation, 5 mg mL⁻¹ of PAAS solution was prepared by dissolving a certain amount of PAAS in an aqueous solution. 1.0 mL of PAAS solution was added to a stirred suspension of MMT (3.0 mL) at room temperature. After about 10 min, the mixed suspension turned to a viscous solution owing to the exfoliation of MMT. Then, 0.1 mL of aqueous solution of modified xylose (50 mg mL⁻¹) was added dropwise using an injector while stirring. The mixed solution became stiff immediately, forming a self-standing object, which was labeled as G-0.1. The other hydrogels were prepared similarly as mentioned above, using different volumes of modified xylose solution, such as 0, 0.3, 0.5, and 0.7 mL, which were coded as G-0, G-0.3, G-0.5, and G-0.7, respectively (Table 1). In addition, the hydrogels with different concentrations of modified xylose solution were also prepared, such as 30, 50, 70, and 90 mg mL⁻¹, which were labeled as H-30, H-50, H-70, and H-90, respectively (Table 2). To explore the properties of the hydrogel without MMT, 0.4 mL of modified xylose solution (90 mg mL⁻¹) was directly added to a stirred 3 mL of PAAS solution without MMT; however, the composite hydrogel was not formed due to the absence of MMT.

Table 1 Hydrogels with different volumes of modified xylose^a

Sample code	MMT V (mL)	PAAS V (mL)	Modified xylose V (mL)
a The concentration of MMT, PAAS, and modified xylose solution was 3.98%, 5 mg mL^−1, and 50 mg mL^−1, respectively.
G-0	3	1	0
G-0.1	3	1	0.1
G-0.3	3	1	0.3
G-0.5	3	1	0.5
G-0.7	3	1	0.7

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Table 2 Hydrogels with different concentrations of modified xylose^a

Sample code	MMT C (wt%)	PAAS C (mg mL^−1)	Modified xylose C (mg mL^−1)
a The volume of MMT, PAAS, and modified xylose solution was 3, 1, and 0.4 mL, respectively.
H-30	3.98	5	30
H-50	3.98	5	50
H-70	3.98	5	70
H-90	3.98	5	90

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FT-IR spectroscopy

FT-IR spectra of the samples were obtained using a Thermo Scientific Nicolet iN 10 FT-IR microscope with an FT-IR spectrometer from 4000 to 650 cm⁻¹ at a resolution of 4 cm⁻¹ and 128 scans per sample.

Swelling capacity measurements

The freeze-dried composite hydrogels were weighed and then immersed into distilled water to test their swelling capacity at room temperature. The swollen hydrogels were taken out and filtered with a nylon fabric bag filter for 20 min until no free water dripped, then weighed at regular intervals until the composite hydrogels reached the swelling equilibrium. The equilibrium water absorption was calculated using the following equation:

Q_eq = (W₂ − W₁)/W₁

where Q_eq is the equilibrium water absorption, which is defined as grams of water per gram of sample. W₁ and W₂ are the mass of hydrogel samples before and after swelling, respectively.

SEM analysis

The sectional structures of the composite hydrogel were measured by a scanning electron microscope (SEM, HitachiS-3400NII) to observe its internal microstructure. Images were obtained that were dependent on the feature being traced.

Mechanical test

The composite hydrogels were shaped to cylindrical samples, and compression stress–strain tests were recorded using a compressive tester (CTM6503, Shenzhen SANS Technology Co., Ltd. China) at a compression speed of 5 mm min⁻¹.

Thermal analysis

The thermal property analysis curves of the xylose-based self-healing hydrogels were obtained using thermal gravimetric analysis (TGA) with the temperature ranging from 20 to 600 °C at a ramp rate of 20 °C min⁻¹.

Results and discussion

The reaction mechanism

The reaction mechanism of hydrogelation is illustrated in Fig. 2. Firstly, guanidinium ion pendants were introduced to the backbone of the xylose chain by a cross-linking reaction, which could be used for the preparation of hydrogels with MMT, because the surfaces were full of anions.¹² MMT is a type of natural mineral with a 2 : 1 layered silicate crystal structure, and its interlayer cations are easily exchanged with inorganic or organic cations (Fig. 2a).¹⁹ When MMT is mixed with PAAS in water, they become highly entangled together and dispersed because of mutual repulsion, which results from the possible site-specific wrapping of their positive-charged edge parts with anionic PAAS (Fig. 2b).²⁰ The modified xylose is a dendritic molecule that contains multiple guanidinium ion pendants on its peripheries. Therefore, the modified xylose, which is dispersed with the MMT/PAAS suspension, can strongly interact due to the non-covalent interactions between the guanidinium ion pendants and oxyanion surface groups, including hydrogen bonds and electrostatic interactions.^21–23 Then, the good mechanical and self-healing hydrogel is formed quickly with a well-developed network (Fig. 2c).

Fig. 2 Schematic of the mechanism of hydrogelation.

FT-IR analysis

FT-IR spectroscopy was carried out to monitor the crosslinking process and the resulting products. Fig. 3 shows the FT-IR spectra of xylose (spectrum (a)), methylguanidine hydrochloride (spectrum (b)), and the modified xylose (spectrum (c)). In Fig. 3a, the absorption peak at 3362 cm⁻¹ corresponds to the –OH stretching vibration. The band at 1092 cm⁻¹ originates from the C–O–C stretching vibration.²⁴ In the FT-IR spectrum of methylguanidine hydrochloride (spectrum (b)), the bands at 3378, 1650, and 1070 cm⁻¹ are assigned to –NH, –NH₂, and C–NH₂ vibrations, respectively. In the FT-IR spectrum of the modified xylose, the graft of xylose with methylguanidine hydrochloride, is shown in Fig. 3c. The observed broad absorption band at 3279 cm⁻¹ is due to the overlap of the –OH stretching of xylose and the –NH stretching of methylguanidine hydrochloride.²⁵ The signals at 1650 cm⁻¹ are assigned to the vibration of –NH₂, which is consistent with the absorption peaks of methylguanidine hydrochloride. Moreover, the new absorption peak that appears at 1186 cm⁻¹ is attributed to the stretching vibration of the new bond, C–C–N, between xylose and methylguanidine hydrochloride. Therefore, the results from the spectra indicate that xylose was modified successfully.

Fig. 3 FTIR spectra of xylose (a), methylguanidine hydrochloride (b) and modified xylose (c).

The FT-IR spectra of modified xylose, PAAS, MMT, hydrogels G-0, and G-0.5 are shown in Fig. 4. In the spectrum of PAAS (Fig. 4b), the frequency of the vibrational band at 2919 cm⁻¹ is assigned to the –CH₂ stretching. The two different stretching vibrations at 1590 and 1440 cm⁻¹ correspond to carboxylate (COO⁻). In the spectrum of MMT (Fig. 4c), the peak at 3620 cm⁻¹ is attributed to the Al–OH stretching vibration.²⁶ The strong peak at 1634 cm⁻¹ is assigned to the –OH bending vibration. The characteristic peaks at 991 and 834 cm⁻¹ are indicative of the stretching vibration of Si–O–Si and Si–O–Al, respectively.²⁷

Fig. 4 FTIR spectra of modified xylose (a), PAAS (b) and MMT (c), hydrogels G-0 (d), and G-0.5 (e).

The FT-IR spectra of hydrogels G-0 and G-0.5 are shown in spectrum (d) and spectrum (e) in Fig. 4, respectively. The absorption peaks at 3620, 1634 cm⁻¹ in both hydrogels G-0 and G-0.5 are consist with the absorption peaks of MMT, and the signals at 1560 and 1440 cm⁻¹, which both exist in hydrogels G-0 and G-0.5, originate from the carboxylate (COO⁻) vibration of PAAS. These featured bands suggest that the hydrogel G-0 was just formed by physical crosslinking by PAAS entangled with MMT. Furthermore, in the spectrum of hydrogel G-0.5, a new broad absorption band at 3380 cm⁻¹ is assigned to the overlap of the –OH stretching vibration of xylose and the –NH stretching vibration of methylguanidine hydrochloride. The vibration at 1653 cm⁻¹ is due to –NH₂ of methylguanidine hydrochloride. Based on the analysis mentioned above, the results indicate that carboxyl and guanidinium ions were introduced into the backbone of hydrogel G-0.5, which are beneficial for the formation of abundant of hydrogen bonds.^28,29 The internal hydrogel G-0.5 are crosslinked by noncovalent interactions, which endow the xylose-based hydrogel with rapid self-healing functionality.³⁰

Self-healing process of xylose-based hydrogel

The process of self-healing for the xylose-based hydrogel is illustrated in Fig. 5. As is shown in Fig. 5a, it was found that the composite hydrogel had good plasticity and could be molded into a certain shape. The prepared intact hydrogel was cut into two parts as shown in Fig. 5b. Then, the ruptured surfaces were pressed together (Fig. 5c), and the fractured pieces merged autonomously into a single piece within a few minutes, which was attributed to plenty of reversible non-covalent bonds of hydrogen bonds and electrostatic adsorption. When the interface of the two recombined sections was dangled, the self-healing composite hydrogel could withstand its self-weight and the self-healing junction surface of the hydrogel could be found clearly in Fig. 5d. These illustrations reveal that the xylose-based hydrogels exhibit self-healing ability and mechanical properties reversibility.

Fig. 5 Self-healing process of xylose-based hydrogel.

Swelling behaviors of xylose-based hydrogels

Swelling capacity is a reflection of hydrogel crosslinking density and internal structure,^31,32 and the swelling behavior was detected in order to further investigate the structural features of the xylose-based hydrogels. The swelling capacity of xylose-based hydrogels H-70 and H-90 are shown in Fig. 6. It was found that the swelling ratio gradually increased from 0 to 14 h and reached equilibrium after immersion for 16 h, which indicated that the composite hydrogels achieved the swelling equilibrium in a relatively short period of time. Moreover, hydrogels H-70 and H-90 demonstrated similar swelling curves within the first few hours, but the hydrogel H-90 showed a better swelling equilibrium with the increase in swelling time on account of more guanidinium ions were added in this sample, thus forming more hydrophilic groups with the MMT/PAAS suspension.

Fig. 6 Swelling capacity of hydrogels H-70 and H-90.

Morphological analysis

The interior crosslinked structure of the xylose-based hydrogel was characterized using scanning electron microscopy (SEM). Fig. 7 shows the SEM images of the hydrogel H-90. Images (a), (b), (c) and (d) were obtained at the magnifications of 200×, 500×, 800×, 1200×, respectively. As is shown in image (a), it is clearly found that the interior of the hydrogel H-90 are composed of plentiful honeycomb structures, which resulted in a composite hydrogel that is soft and flexible. Moreover, in images (b) and (c), the morphology of hydrogel H-90 consisted of abundant micropores and lamellar structures with a tight network, and these porous structures were presumably the hydrophilic groups. The MMT particles were not found on the surface of the gel structures, which suggested that the MMT nanosheets were completely dispersed into the hydrogels without aggregation because of the interactions between the anion groups of MMT and guanidinium ion pendants of the modified xylose.³³ In addition, the tight network among the molecular chains of the crosslinked polymer was caused by the crosslinking reaction, which resulted in pores that became small and a dense interconnected network. These porous and spongy dense structures were probably responsible for swelling and rapid self-healing of the xylose-based hydrogels.

Fig. 7 SEM images of hydrogel H-90 at magnifications of 200× (a), 500× (b), 800× (c), 1200× (d).

Mechanical properties of the xylose-based hydrogel

The cylindrical composite hydrogel samples were tested to record compression stress–strain curves. The compression stress–strain curves on behalf of the mechanical properties of hydrogels G-0, G-0.1, G-0.3, G-0.5, and G-0.7 are shown in Fig. 8. The compression stress of all the composite hydrogels increased slightly with the increasing deformation and all five samples showed remarkably similar curves despite the different volumes of modified xylose solution. This indicates that these composite hydrogels have good elasticity and ductile property because of hydrogen bonds and electrostatic interactions. When the strain exceeded 50%, the entire compression stress-curves increased sharply. In addition, compared with the different compression stress-curves, the strain increased with the increasing volume of modified xylose at the same compression stress. At the compression stress of 100 kPa, the strain of the hydrogel G-0 was 58.7%, and the strains of the other composite hydrogels were 64.2% (G-0.1), 65.1% (G-0.3), 67.1% (G-0.5), and 87.6% (G-0.7), which indicate that the plasticity of the prepared hydrogels increased with the added volume of modified xylose.

Fig. 8 Compression stress–strain curves of hydrogels G-0, G-0.1, G-0.3, G-0.5, and G-0.7.

The compression stress–strain curves of hydrogels H-30, H-50, H-70, and H-90 are shown in Fig. 9. As can be seen from Fig. 9, the strain increased with the decreasing concentration of modified xylose from H-90 to H-30 at the same compression strength. However, the compression stress increased from H-30 to H-90 at the same strain. At the strain of 75%, the compression strength of the hydrogels was 130 (H-30), 154 (H-50), 272 (H-70), and 336 kPa (H-90). The reason for this was that higher the concentration of the modified solution, more hydrogen bonds and electrostatic interactions were formed during the preparation of the hydrogels, which suggest that the interior network structure of the xylose-based hydrogels was well crosslinked.

Fig. 9 Compression stress–strain curves of hydrogels H-30, H-50, H-70, and H-90.

Comparing Fig. 8 with Fig. 9, it was found that the strain of the composite hydrogels increased with the increased volume of modified xylose solution from G-0 to G-0.7, as seen in Fig. 8. However, the strain of the composite hydrogels decreased with the increasing concentration of modified xylose from H-30 to H-90, as seen in Fig. 9. The obvious differences in the stress–strain of the hydrogels were possibly caused by the different water contents in the composite hydrogels because a high water content in hydrogels can promote the elasticity of the composite hydrogels. Modified xylose was firstly dissolved in distilled water and then stirred with an MMT/PAAS suspension to form the hydrogels. The water content of the composite hydrogels increased with the increase of modified xylose solution. Therefore, it led to the increase in the elasticity of the hydrogels from G-0 to G-0.7, whereas the hydrogels H-30, H-50, H-70, and H-90 had the same water content, thus more hydrogen bonds and electrostatic interactions were formed with the increasing concentration of modified xylose. This formed a tighter and tougher network structure during the preparation of the composite hydrogels, which resulted in the decreasing elasticity and the increasing compression stress of the hydrogels from H-30 to H-90.

Thermal stability

Thermal gravimetric analysis (TGA) was carried out to investigate the thermal properties of the materials and composite hydrogels. The TGA curves of xylose (a), methylguanidine hydrochloride (b), modified xylose (c), MMT (d) and hydrogel (e) are illustrated in Fig. 10. As is shown in the TGA curve of xylose, it started degrading when the temperature reached about 220 °C, and the weight loss of xylose mainly occurred at 220–500 °C. Methylguanidine hydrochloride possessed thermal stability below 300 °C (Fig. 10b). However, its weight declined sharply by 96% from 280 to 370 °C. When the temperature continued to increase to 500 °C, the methylguanidine hydrochloride sample was decomposed completely. The curve of the modified xylose is introduced in Fig. 10c. Compared with the curves of xylose (a) and methylguanidine hydrochloride (b), more residual weight was observed in the curve of the modified xylose (c). This suggests that methylguanidine hydrochloride was grafted onto xylose successfully forming new bonds between xylose and methylguanidine hydrochloride with a relatively high molecular weight. Therefore, the thermal properties of the modified xylose were better than xylose and methylguanidine hydrochloride. As can be seen from the curve of MMT, the initial low temperature weight loss (<100 °C) corresponded to the free water evaporation. In addition, 90% residue weight of MMT still remained when the temperature reached 600 °C, which indicates that MMT had excellent thermal stability, which is beneficial to the flame retardant property of the composite hydrogels.²⁶ From the curve of hydrogel, the weight of the hydrogel dropped slowly in the range of 20–600 °C, whereas the weight only decreased by 33% during the whole process of thermal decomposition. This indicates that the reagent materials are dispersed homogeneously and the thermal stability of the hydrogel is far greater than that of the modified xylose due to the addition of the inorganic phase, MMT.

Fig. 10 TGA curves of xylose (a), methylguanidine hydrochloride (b), modified xylose (c), MMT (d), and hydrogel (e).

Conclusions

A convenient, quick, and inexpensive method has been applied for the fabrication of hydrogels, using xylose as the main subject for crosslinking with MMT and PAAS. The prepared xylose-based hydrogels are reproducible and biodegradable. The hydrogels with an internal tight porous network structure were prepared with the use of hydrogen bonds and intermolecular interactions instead of the traditional covalent bond, which result in the hydrogels having a rapid self-healing ability and show good swelling performance. From the results of compression stress–strain, it was found that the elasticity of the xylose-based hydrogels increased with the increasing volume of modified xylose solution, and the compression stress increased with the increasing concentration of modified xylose. Furthermore, the xylose-based hydrogels exhibited admirable thermal stability on account of the added MMT nanoplatelets as an inorganic phase. These research results reveal that xylose-based hydrogels have promising applications in many fields. The self-healing property of the hydrogels can prolong the lifespan of materials and reduce the replacement costs due to damage. They can also be used as flame retardant wrapping materials because of their excellent thermal heat resistance. Moreover, the composite hydrogel can be used as a soil and water humectant on account of its good swelling performance. Thus, the composite hydrogels have a wide range of applications in environmentally friendly materials.

Acknowledgements

This study was supported by the State Forestry Administration (201404617), the National Natural Science Foundation of China (21406014), the Ministries of Education (NCET-13-0670), and the author of National Excellent Doctoral Dissertations of China (201458).

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