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Aqueous compatible boron nitride nanosheets for highperformance hydrogels Received 00th January 20xx, Accepted 00th January 20xx
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Xiaozhen Hu, Jiahui Liu, Qiuju He, Yuan Meng, Liu Cao, Ya-Ping Sun, Jijie Chen, Fushen Lu
*a
DOI: 10.1039/x0xx00000x www.rsc.org/
Hexagonal boron nitride nanosheet (BNNS) possesses ultimate thermal and chemical stabilities and mechanical strengths. However, the unmodified BNNS is hydrophobic and insoluble in water, which hinders its uses in many technological areas requiring aqueous compatibility. In this work, h-BN was treated with molten citric acid to produce aqueous dispersible boron nitride sheets (ca-BNNSs). The resultant ca-BNNSs were used to fabricate ca-BNNS/polyacrylamide (i.e., BNNS2.5/PAAm) nanocomposite hydrogels, targeting high water retentivity and flexibility. The BNNS2.5/PAAm hydrogel (initially swelling in water) largely remained swollen (water content ~94 wt%) even after one-year storage under ambient conditions. Importantly, the swollen BNNS2.5/PAAm hydrogel (water content ~95 wt%) was highly flexible. Its elongation and compressive strength exceeded 10,000% and 8 MPa at 97% strain, respectively. Moreover, the aforementioned hydrogel recovered upon the removal of compression force, without obvious damage. The substantially improved water retentivity and flexibility revealed that BNNSs served as a promising new platform in the development of highperformance hydrogels.
Introduction Hydrogels are 3-dimentional (3D) polymeric networks that contain a large amount of water and they have been extensively utilized in diverse fields, such as biomedical 1-7 8-9 10-11 applications, soft machines, and actuators. Hydrogels of high water content yet mechanically strong are in demand for many technological applications. Among effective strategies for robust hydrogels is the improvement of network structures through the incorporation of multifunctional cross12-18 linkers into hydrogels. For example, Haraguchi et al. dispersed exfoliated clays into poly(N-isopropyl acrylamide) for a nanocomposite hydrogel exhibiting favorable mechanical, 18 optical, and swelling-deswelling properties. Similarly, many 19-22 other nanoscale materials including graphene oxides, 23 24 25 carbon nanotubes, layered double hydroxide, silica, and 26 ferritin particles have been explored for the preparation of nanocomposite hydrogels. Hexagonal boron nitride nanosheet (BNNS) is often considered as a structural and isoelectronic analog of graphene, a.
Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Guangdong 515063, P. R. China. E-mail: [email protected] b. Beijing Key Laboratory of Bioprocess, School of Life Science and Technology, Beijing University of Chemical Technology, Beijing 100871, P. R. China. c. Department of Chemistry and Laboratory for Emerging Materials and Technology, Clemson University, Clemson, South Carolina 29634, USA. † Electronic Supplementary Information (ESI) available: [Raman spectra and TGA curves of pristine h-BN and ca-BNNS; photographs, SEM images, swelling curves and compression stress of PAAm and/or BNNS0.1-1.5/PAAm hydrogels]. See DOI: 10.1039/x0xx00000x
possessing ultimate thermal and chemical stabilities and 27-28 mechanical strengths. Unlike the extremely widely studied graphene materials, however, BNNSs are largely unexplored despite their unique properties, especially in terms of their uses in nanocomposite hydrogels. In general, the fabrication of nanocomposite hydrogels requires the nanoscale fillers as cross-linkers to be highly hydrophilic and well soluble in aqueous media. However, hexagonal boron nitride (h-BN) and derived BNNSs are generally insoluble not only in water but 29 also in common organic solvents, so that their modifications have been explored for improved solubility characteristics. More specifically, covalent or non-covalent functionalization schemes have been employed in the solubilization of BNNSs, 30 including the defect-site derivatization, radical covalent 31-32 33 functionalization, Lewis acid-base interaction, or π34 stacking. For example, pristine h-BN powder was exfoliated and functionalized with the use of amine-terminated 35 36 poly(ethylene glycol), poly(sodium 4-styrenesulfonate), or 37 ionic surfactants for the modified BNNSs to be more watersoluble. As a significant limitation in these functionalization schemes, however, the agents used for the modification and dispersion became essentially unwanted “contamination” in the resulting samples, making them unsuitable for the 36 preparation of high-performance nanocomposite hydrogels. New functionalization schemes that would lead to aqueous compatible BNNSs free from unwanted impurities are in demand for the desired hydrogels and beyond. In work reported here we treated pristine h-BN powder with molten citric acids and produced a stable BNNS aqueous solution (ca-BNNS). The solubilization of BNNSs was likely due
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Experimental ca-BNNS Citric acid (33.3 g) and h-BN (500 mg) were homogeneously ground and added to a flask. The mixture was vigorously stirred and heated to 170 ºC for 8 days under the argon protection. The resulting solid was suspended in water, filtered over a porous cellulose ester membrane, and then repeatedly washed with water and ethanol to completely remove the free citric acid. The obtained ca-BNNS sample was redispersed in water and allowed to stand undisturbed overnight. The concentration of aqueous ca-BNNS dispersion was estimated -1 to be 1.1 mg·mL .
(Wt – Wd)/Wd and WC = (Wt – Wd)/Wt × 100%, where WtOnline and View Article DOI: 10.1039/C5NR07578E Wd were the weights of the swollen hydrogel at prescribed time intervals and the corresponding dried hydrogel, respectively.
Results and Discussion The treatment of h-BN with molten citric acid was for the exfoliation to yield BNNSs of a few layers, which was enabled by the functionalization as well as insertion of citric acid molecules. The resulting nanosheets were still functionalized with citric acid, thus denoted as ca-BNNS. Unlike the h-BN precursor, the ca-BNNS sample is highly soluble or dispersible in water and other polar organic solvents. The concentrated ca-BNNS aqueous dispersion appeared milky, but upon dilution the dispersion became transparent, and both concentrated and dilute dispersions exhibited the Tyndall effect (Fig. 1). The ca-BNNS dispersions were stable over an extended period of -1 time. For example, the dispersion of 0.25 mg·mL in concentration was largely unchanged after being stored for 9 months under ambient conditions. (a)
ca-BNNS/Polyacrylamide Hydrogel
0.00
(b)
Concentration (mg/mL) 0.02 0.04 0.06 4 3 2 1
Absorption
ca-BNNS powder (25 mg) was dispersed in the nitrogen-purged water (10 mL) with the aid of mild ultrasonication. Acrylamide monomer, ammonium peroxydisulfate (APS, 10 mg) and N,N,N',N'-tetramethyl-ethylenediamine (TMEDA, 5 μL) were successively added to the aqueous ca-BNNS dispersion in an ice bath, followed by the vigorous stirring for 15 min. The ice bath was replaced by a water bath (25 ºC) to initiate the free radical polymerization. When the reaction mixture became viscous and lost mobility, it was transferred to a cylindrical or tubular mold, allowing a complete polymerization process in 48 h. The resulting hydrogel (denoted as BNNS2.5/PAAm) was immersed in fresh water (100 mL × 10) for 2 days to get rid of unreacted reagents and residual impurities. Polyacrylamide (PAAm) hydrogel was prepared with the identical procedure for the BNNS2.5/PAAm hydrogel, except that N,N'-methylenebisacrylamide (BIS) was used as the crosslinker. The hydrogels (pre-dried in vacuum oven, 0.1 – 0.4 g) were immersed in water at different temperatures (25, 45 and 60 ºC) or pH values (1.3, neutral, and 12.1) to reach swelling equilibrium. The hydrogels were weighed at prescribed time intervals after removing the excess water on the hydrogel surface with a filter paper. The swelling ratio (SR) and water content (WC) were calculated by the following equations, SR =
0
200
300
400 500 600 Wavelength (nm)
700
800
Fig. 1 (a) Concentrated (left) and dilute (middle) showing a Tyndall effect upon irradiation and water (right) being presented for comparison; (b) UV-vis absorption of aqueous ca-BNNS dispersion (Inset: correlation of concentration and absorbance of ca-BNNS dispersions at 206 nm).
The aqueous ca-BNNS dispersion was found to be largely clear over the visible spectrum, as expected, exhibiting some absorption in the UV (peaking at ~206 nm). The observed
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to the hydrophilic –NH2 and –OH groups generated during acid treatment rather than linking substantial amounts of citric acid molecules onto BNNS surface. The hydrophilicity and cleanness of ca-BNNS sample allowed the fabrication of caBNNS/polyacrylamide (i.e., BNNS2.5/PAAm) nanocomposite hydrogels with remarkably enhanced swelling behaviors and mechanical properties. The water content of a swollen BNNS2.5/PAAm hydrogel remained as high as 94 wt% even after storage under ambient conditions for more than a year. Moreover, the BNNS2.5/PAAm hydrogel containing ~95 wt% water withstood 10,000% elongation and 8 MPa compression stress, without rupture. To the best of our knowledge, the performance of BNNS2.5/PAAm hydrogel (especially for water retentivity) is superior or comparable to those of previously reported nanocomposite hydrogels with similar nanomaterials loading and water content.
Normalized Absorption
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ca-BNNS
h-BN
BNNS2.5/PAAm hydrogels 1614
3190 3407
(b)
1661
PAAm hydrogels 3415
3193
1619 1667
4000 3500 3000 2500 2000 1500 1000 -1 Wavenumbers (cm ) 200 nm
100 nm
(c)
(d)
1 m
Fig. 3 FT-IR spectra of ca-BNNS (Olive), pristine h-BN (Black), BNNS2.5/PAAm hydrogels (Red), and PAAm hydrogels (Blue) (Inset: enlarged ca-BNNS spectrum showing a double peak).
4 0 0.0
0.2
500
0.4 0.6 Position (m)
0.8
1.0
Fig. 2 (a) and (b) TEM images of a ca-BNNS sample at different magnifications; (c) a typical AFM image of a ca-BNNS sample; (d) a zoomed-in image of the indicated area in (c) and a height profile alone the line.
The functionalization of BNNS by citric acid molecules on the sheet surfaces or edges in ca-BNNS was probed by using FT-IR, in comparison with the spectrum of the precursor h-BN (Fig. 3). Both ca-BNNS and h-BN samples showed two prominent -1 -1 peaks at ~1380 cm and ~800 cm , corresponding to the characteristics of in-plane B-N ring stretching vibration and out-of-plane B-N-B bending vibration, respectively. The peaks of ca-BNNS were narrower than those of pristine h-BN likely 38 due to the smaller thickness for the former sample. The ca-1 BNNS sample showed additional peaks centered at 3450 cm -1 and 1640 cm , which might be assigned to the stretching and bending vibration of a primary amine. A shoulder peak at -1 39 ~3200 cm might resulted from the B–OH groups, which was -1 consistent with the appearance of a Raman peak at ~870 cm 31 (Fig. S1, ESI†). The negligible peaks, if any, in the regions of -1 -1 2950 – 2850 cm and 1700 – 1750 cm for the stretching mode of –CH2 and –COOH in the citric acid molecules, indicated that the ca-BNNS sample contained only a relatively small amount of citric acid moieties. The treatment of h-BN
The TGA results further confirmed that the amount of citric acid and hydrophilic groups (–NH2 and –OH) linked onto BN sheets accounted for about 2.2% of the sample weight (Fig. S2, ESI†). Therefore, it is remarkable that such a relatively low level of functionalization of BNNS by citric acid could substantially improve the aqueous compatibility of the nanosheets for stable dispersions. It might also suggest that the nanosheet configuration is in itself "water-friendly", only requiring some help from the functionalization moieties to enhance the aqueous dispersion and prevent from aggregation and/or re-stacking. The minimal content of functionalization moieties in the ca-BNNS sample is very favorable to its uses as nano-fillers for the fabrication of high-performance nanocomposite hydrogels. Owing to the high hydrophilicity and the thin lamellar structure, the ca-BNNS was successfully incorporated into hydrogels in which acrylamide monomers were polymerized and cross-linked with ca-BNNS in situ. The interactions between ca-BNNS and polyacrylamide were preliminarily characterized by using FT-IR. The characteristic peaks of amide group (N-H and C=O) are redshifted in the FT-IR spectrum of a BNNS2.5/PAAm hydrogel as compared with those in a PAAm -1 hydrogel (Fig. 3). Specifically, The IR peaks at 3415 cm , 3193 -1 -1 cm and 1619 cm for PAAm hydrogel, corresponding to the -1 N–H stretching and bending vibration, shifted to 3407 cm , -1 -1 3190 cm and 1614 cm in the spectrum of BNNS2.5/PAAm sample, respectively. Meanwhile, the stretching mode of C=O -1 in the amide groups displayed the similar redshift (1667 cm -1 for PAAm hydrogels versus 1661 cm for BNNS2.5/PAAm hydrogel). The redshifts might be attributed to the hydrogen bonds between –OH and/or –NH2 groups on the boron nitride
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(a)
with molten citric acid likely broke B-N bonds and Viewgenerated Article Online DOI: 10.1039/C5NR07578E hydrophilic –NH2 and –OH groups at the defective sites and edges rather than linked massive citric acid molecules onto the BNNS surface.
Transmittance (%)
absorbance was linearly correlated with the ca-BNNs concentration (obeying the Lambert-Beer's law), suggesting the homogenous dispersion of ca-BNNS (Fig. 1). The estimated -1 -1 extinction coefficient of ca-BNNS is ~56 mL·mg ·cm at 206 nm. The ca-BNNSs were mostly round or elliptic in shape, with lateral sizes of a few hundred nanometers according to results from microscopy characterizations (Fig. 2). The thickness of the nanosheets was around 5 nm based on the height profile in the AFM images. Both TEM and AFM results suggested that the ca-BNNSs were predominantly few-layered boron nitride nanosheets, consistent with the expectation that the treatment of h-BN with molten citric acid resulted in the exfoliation of and peeling off nanosheets from the precursor hBN.
Height (nm)
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Table 1 Formulations of ca-BNNS/polyacrylamide hydrogels. a
Hydrogel
Acrylamide
Cross-linker
(g)
BIS (mg)
ca-BNNS (mg)
BNNS2.5
1
0
25
BNNS1.5
1
10
15
BNNS1.0
1
15
10
BNNS0.5
1
20
5
BNNS0.1
1
24
1
PAAm
1
25
0
a The subscripts denote ca-BNNS concentration (mg·mL-1) during gelation
When the weight ratio of ca-BNNS to acrylamide monomer decreased from 2.5% to 2%, the obtained hydrogel was fluidic though it was very viscous. For comparison purpose, hydrogels with reduced ca-BNNS contents (BNNS0.1-1.5/PAAm) were prepared with the identical procedure for the BNNS2.5/PAAm hydrogels, in which the ca-BNNS was partially replaced by N,N'-methylenebisacrylamide (BIS) and the total concentration -1 of ca-BNNS and BIS was constant (2.5 mg·mL ) in every hydrogel. It should be noted that the effective crosslinking groups (-OH, -NH2 and –COOH) on ca-BNNS surface (around 0.5 – 1.3 mmol/g) were one order of magnitude lower than those in BIS crosslinker (around 13 mmol/g). The cocrosslinked hydrogels with various ca-BNNS and BIS contents were listed in Table 1 and Fig. S3 (ESI†). (b)
(a)
50 μm
100 μm
(c)
(d)
100 μm
50 μm
Fig. 4 Typical SEM images of lyophilized xerogels. (a) BNNS2.5/PAAm sample with a hierarchical pore structure and (b) a zoomed-in image of the indicated area in (a); PAAm sample at low (c) and high (d) magnifications.
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10.1039/C5NR07578E The hydrogels with and without ca-BNNSDOI: were lyophilized and visualized by SEM. The BNNS2.5/PAAm hydrogel exhibited a hierarchical and uniform pore structure (Fig. 4). The first-level (large) pores were hundreds of microns in diameter and their walls consisted of second-level (small) pores with sizes of tens of microns. Therefore, the large pores were interconnected by small pores, generating a foam-like 3-dimentional network. On the contrary, no hierarchical pore structure was found in the PAAm hydrogel (without ca-BNNS) and the hydrogels with reduced ca-BNNS content (BNNS0.1-1.5/PAAm hydrogels) where the pores were largely isolated and non-uniform in sizes (Fig. 4 and Fig. S4, ESI†). The swelling behaviors of dry hydrogels were systematically investigated under variant conditions. The swelling curves (viz. swelling ratio versus time) for all dry hydrogels (with and without ca-BNNS) were steep in the beginning and tended to tail off and flatten (Fig. 5 and S5, ESI†). The equilibrium swelling ratio in pure water at 25 ºC was determined to be 10.4 for PAAm hydrogel and it slightly increased with the increment of ca-BNNS loading for the co-crosslinked hydrogels (BNNS0.11.5/PAAm). Once the hydrogel was solely cross-linked by caBNNS (i.e., BNNS2.5/PAAm), the equilibrium swelling ratio dramatically increased to 74, which was more than four times larger than that of BNNS1.5/PAAm hydrogel. The equilibrium swelling ratio was temperature and pH dependent for both PAAm and BNNS2.5/PAAm hydrogels. For example, the equilibrium swelling ratio of BNNS2.5/PAAm hydrogel increased with the raise of swelling temperature and it reached 120 after swelling in pure water for ~30 h at 60 ºC (Fig. 5). Moreover, the equilibrium swelling ratios of BNNS2.5/PAAm and PAAm hydrogels measured in NaOH solution (pH = 12.1) were nearly one order of magnitude larger than those measured in HCl (pH = 1.3) and NaCl (0.9 wt%) solutions. It was worth pointing out that the equilibrium swelling ratios of BNNS2.5/PAAm hydrogels were consistently larger than those of PAAm samples in all the tested systems. Most importantly, the water retentivity of BNNS2.5/PAAm hydrogel is superior to those of previously reported 41-45 hydrogels. Specifically, after equilibrium swelling in pure water at 25 ºC, all the hydrogles (PAAm and BNNS0.1-2.5/PAAm) were stored in air to evaluate the water retentivity under ambient conditions. The BNNS2.5/PAAm hydrogel remained swollen and its water content was estimated to be still as high as 94 wt% after one-year storage (Fig. 5 and Fig. S6, ESI†). The water retentivities of co-crosslinked hydrogels (BNNS0.11.5/PAAm) were better than that of PAAm as observed in the deswelling measurements, but they were still considerably lower than that of BNNS2.5PAAm hydrogel. The significantly enhanced water retentivity and swelling performance were probably ascribed to the hierarchical pore structures of BNNS2.5/PAAm hydrogels. Supposedly, the hierarchical pores enabled BNNS2.5/PAAm hydrogel to expand and thus generated plenty of hydrophilic and capacious cavies for water suction. Meanwhile, the hydrophilic environment inside hierarchical pores minimized the escape of water from BNNS2.5/PAAm hydrogels.
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sheets and amide groups attached to the polyacrylamide 40 mainchains.
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Swelling Ratio (g/g)
90 60
25 °C 45 °C 60 °C
30
0
15
30 45 Time (h)
60
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3 mm
(b)16
120
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DOI: 10.1039/C5NR07578E
12 8
25 °C 45 °C 60 °C
4
0
75
(c)
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30 45 Time (h)
60
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400
100
in HCl in NaCl in NaOH
100
0
(e) PAAm
10
20 30 Time (h)
BNNS0.1
40
BNNS0.5
(b)
60 in HCl in NaCl in NaOH
40 20 0
BNNS1.0
10
20 30 Time (h)
BNNS1.5
40
BNNS2.5
Fig. 5 Swelling ratio–time curves of BNNS2.5/PAAm (left panel) and PAAm (right panel) dry hydrogels in water at different temperatures (a and b) and in aqueous solution with different pH values (c and d); (e) photograph of BNNS2.5/PAAm hydrogel showing its extraordinary water retentivity (water content is as high as 94 wt%) after storage under ambient conditions for more than a year. Photographs of the hydrogels with different ca-BNNS contents were shown for comparison.
The swollen BNNS2.5/PAAm hydrogel showed excellent flexibilities as well, which might be due to its hierarchical pore structure again. The BNNS2.5/PAAm hydrogel was able to withstand compression, tensile, knotting and torsion even under high degree of deformations (Fig. 6). The elongation and compressive strength of a BNNS2.5/PAAm hydrogel (water 46 content ~95%) exceeded 10,000% and 8 MPa at 97% strain, respectively (Fig. 7). Importantly, the hydrogels quickly recovered without any obvious damage once the external forces were removed. The elastic modulus of BNNS2.5/PAAm hydrogel (water content ~98%) was measured to be 1.4 kPa. In sharp contrast, the PAAm hydrogel containing ~92 wt% water is fragile in nature. It ruptured when the compression strength reached 0.04 MPa or fractured when a tensile stress was loaded. Similarly, all the BNNS0.1-1.5/PAAm hydrogels containing 92-93 wt% water, where the ca-BNNS was partially replaced with covalent cross-linker (BIS), ruptured during compression tests. Although the compression strength of BNNS0.1-1.5/PAAm hydrogels gradually improved with the increase of ca-BNNS content and finally reached 0.21 MPa for the BNNS1.5/PAAm hydrogel, these values were still substantially lower than that of BNNS2.5/PAAm hydrogel (Fig. S7, ESI†).
(e)
(d)
(f)
Fig. 6 Flexibility of BNNS2.5/PAAm and PAAm hydrogels. (a) complete recovery of BNNS2.5/PAAm sample from compression, in comparison with (b) fracture of PAAm sample under compression; BNNS 2.5/PAAm hydrogels (c) in a elongation measurement and (d, e and f) withstand various deformations. 8
6
4
2
0 0
20
40 60 Strain (%)
80
100
Fig. 7 Compression stress–strain curve of BNNS2.5/PAAm hydrogel .
Conclusions In summary, h-BN was treated with molten citric acid to produce aqueous dispersible boron nitride sheets (ca-BNNSs). Characterization data indicated that the treatment likely generated hydrophilic –NH and –OH groups at the edges or defect sites of boron nitride sheets instead of linking massive amounts of citric acid molecules onto sheet surfaces. The resultant ca-BNNSs exhibited desirable aqueous compatibility and enabled the fabrication of high-performance hydrogels, targeting high water retentivity and flexibility. The
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200
80
Stress (MPa)
300
Swelling Ratio (g/g)
Swelling Ratio (g/g)
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BNNS2.5/PAAm hydrogel (initially swelling in water) largely remained swollen (water content ~94 wt%) even after oneyear storage under ambient conditions. Importantly, the swollen BNNS2.5/PAAm hydrogel (water content ~95 wt%) was highly flexible. Its elongation and compressive strength exceeded 10,000% and 8 MPa at 97% strain, respectively. Moreover, the aforementioned hydrogel recovered upon the removal of compression force, without obvious damage. The substantially improved water retentivity and flexibility revealed that ca-BNNSs served as a highly promising new platform in the development of high-performance nanocompoiste hydrogels for many technological applications.
Acknowledgements This work was supported by the NSFC (51272152) and Guangdong Natural Science Foundation (S2013010014171). Additional supports from Guangdong Province (2012KJCX0053, 2014KCXTD012 and 2013A061401016) and SRF for ROCS (SEM) were also acknowledged. Jijie Chen, Weixiong Liang and Zhou Lu participated in this project through an undergraduate research program.
Notes and references 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
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20 J. Wu, A. Chen, M. Qin, R. Huang, G. Zhang, B. Xue, J. Wei, Y. View Article Online Li, Y. Cao and W. Wang, Nanoscale, 2015, 1655. DOI:7,10.1039/C5NR07578E 21 L. Sun, N. Hu, J. Peng, L. Chen and J. Weng, Adv. Funct. Mater., 2014, 24, 6897. 22 N. Zhang, R. Li, L. Zhang, H. Chen, W. Wang, Y. Liu, T. Wu, X. Wang, W. Wang, Y. Li, Y. Zhao and J. Gao, Soft Matter, 2011, 7, 7231. 23 X. Zhang, C. L. Pint, M. H. Lee, B. E. Schubert, A. Jamshidi, K. Takei, H. Ko, A. Gillies, R Bardhan, J. J. Urban, M. Wu, R. Fearing and A. Javey, Nano Lett., 2011, 11, 3239. 24 Z. Hu and G. Chen, Adv. Mater., 2014, 26, 5950. 25 N. S. Kehr, E.A. Prasetyanto, K. Benson, B. Ergün, A. Galstyan, H.-J. Galla, Angew. Chem. Int. Ed., 2013, 52, 1156. 26 M. K. Shin, G. M. Spinks, S. R. Shin, S. I. Kim, S. J. Kim, Adv. Mater., 2009, 21, 1712. 27 A. Pakdel, Y. Bando and D. Golberg, Chem. Soc. Rev., 2014, 43, 934. 28 F. S. Lu, F. Wang, W. H. Gao, X. C. Huang, X. Zhang and Y. L. Li, Nanosci. Nanotechnol. Lett., 2012, 4, 949. 29 T. Ikuno, T. Sainsbury, D. Okawa, J. M. J. Fréchet and A. Zettl, Solid State Commun., 2007, 142, 643. 30 Y. Lin, T. V. Williams, W. Cao, H. E. Elsayed-Ali and J. W. Connell, J. Phys. Chem. C, 2010, 114, 17434. 31 T. Sainsbury, A. Satti, P. May, Z. Wang, I. McGovern, Y. K. Gun’ko and J. Coleman, J. Am. Chem. Soc., 2012, 134, 18758. 32 T. Sainsbury, A. Satti, P. May, A. O’Neill, V. Nicolosi, Y. K. Gun’ko and J. N. Coleman, Chem. Eur. J., 2012, 18, 10808. 33 A. Nag, K. Raidongia, K. P. S. S. Hembram, R. Datta, U. V. Waghmare and C. N. R. Rao, ACS Nano, 2010, 4, 1539. 34 D. Lee, S. H. Song, J. Hwang, S. H. Jin, K. H. Park, B. H. Kim, S. H. Hong and S. Jeon, Small, 2013, 9, 2602. 35 Y. Lin, T. V. Williams and J. W. Connell, J. Phys. Chem. Lett., 2010, 1, 277. 36 F. S. Lu, F. Wang, W. H. Gao, X. C. Huang, X. Zhang and Y. L. Li, Mater. Express, 2013, 3, 1. 37 R. J. Smith, P. J. King, M. Lotya, C. Wirtz, U. Khan, S. De, A. O’Neil, G. S. Duesberg, Jaime. C. Grunlan, G. Moriarty, J. Chen, J. Wang, A. I. Minett, V. Nicolosi and J. N. Coleman, Adv. Mater., 2011, 23, 3944. 38 M. Du, Y. Wu and X. Hao, CrystEngComm, 2013, 15, 1782. 39 A. S. Nazarov, V. N. Demin, E. D. Grayfer, A. I. Bulavchenko, A. T. Arymbaeva, H.-J. Shin, J.-Y. Choi and V. E. Fedorov, Chem.-Asian J., 2012, 7, 554. 40 R. Liu, S. Liang, X. Tang, D. Yan, X. Li and Z. Yu, J. Mater. Chem., 2012, 22, 14160. 41 X. Hu, W. Wei, X. Qi, H. Yu, L. Feng, J. Li, S. Wang, J. Zhang and W. Dong, J. Mater. Chem. B, 2015, 3, 2685. 42 G. Pizzorusso, E. Fratini, J. Eiblmeier, R. Giorgi, D. Chelazzi, A. Chevalier and P. Baglioni, Langmuir, 2012, 28, 3952. 43 J. Shen, B. Yan, T. Li, Y. Long, N. Li and M. Ye, Soft Matter, 2012, 8, 1831. 44 Y. Bai, B. Chen, F. Xiang, J. Zhou, H. Wang and Z. Suo, Appl. Phys. Lett., 2014, 105, 151903. 45 Y. Huang, M. Zhang and W. Ruan, J. Mater. Chem. A, 2014, 2, 10508. 46 When the compress strength reached 8 MPa, the BNNS2.5/PAAm hydrogel has been highly compacted.
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Aqueous compatible boron nitride nanosheets for highperformance hydrogels Received 00th January 20xx, Accepted 00th January 20xx
a
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a
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a
Xiaozhen Hu, Jiahui Liu, Qiuju He, Yuan Meng, Liu Cao, Ya-Ping Sun, Jijie Chen, Fushen Lu
*a
DOI: 10.1039/x0xx00000x www.rsc.org/
Hexagonal boron nitride nanosheet (BNNS) possesses ultimate thermal and chemical stabilities and mechanical strengths. However, the unmodified BNNS is hydrophobic and insoluble in water, which hinders its uses in many technological areas requiring aqueous compatibility. In this work, h-BN was treated with molten citric acid to produce aqueous dispersible boron nitride sheets (ca-BNNSs). The resultant ca-BNNSs were used to fabricate ca-BNNS/polyacrylamide (i.e., BNNS2.5/PAAm) nanocomposite hydrogels, targeting high water retentivity and flexibility. The BNNS2.5/PAAm hydrogel (initially swelling in water) largely remained swollen (water content ~94 wt%) even after one-year storage under ambient conditions. Importantly, the swollen BNNS2.5/PAAm hydrogel (water content ~95 wt%) was highly flexible. Its elongation and compressive strength exceeded 10,000% and 8 MPa at 97% strain, respectively. Moreover, the aforementioned hydrogel recovered upon the removal of compression force, without obvious damage. The substantially improved water retentivity and flexibility revealed that BNNSs served as a promising new platform in the development of highperformance hydrogels.
Introduction Hydrogels are 3-dimentional (3D) polymeric networks that contain a large amount of water and they have been extensively utilized in diverse fields, such as biomedical 1-7 8-9 10-11 applications, soft machines, and actuators. Hydrogels of high water content yet mechanically strong are in demand for many technological applications. Among effective strategies for robust hydrogels is the improvement of network structures through the incorporation of multifunctional cross12-18 linkers into hydrogels. For example, Haraguchi et al. dispersed exfoliated clays into poly(N-isopropyl acrylamide) for a nanocomposite hydrogel exhibiting favorable mechanical, 18 optical, and swelling-deswelling properties. Similarly, many 19-22 other nanoscale materials including graphene oxides, 23 24 25 carbon nanotubes, layered double hydroxide, silica, and 26 ferritin particles have been explored for the preparation of nanocomposite hydrogels. Hexagonal boron nitride nanosheet (BNNS) is often considered as a structural and isoelectronic analog of graphene, a.
Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Guangdong 515063, P. R. China. E-mail: [email protected] b. Beijing Key Laboratory of Bioprocess, School of Life Science and Technology, Beijing University of Chemical Technology, Beijing 100871, P. R. China. c. Department of Chemistry and Laboratory for Emerging Materials and Technology, Clemson University, Clemson, South Carolina 29634, USA. † Electronic Supplementary Information (ESI) available: [Raman spectra and TGA curves of pristine h-BN and ca-BNNS; photographs, SEM images, swelling curves and compression stress of PAAm and/or BNNS0.1-1.5/PAAm hydrogels]. See DOI: 10.1039/x0xx00000x
possessing ultimate thermal and chemical stabilities and 27-28 mechanical strengths. Unlike the extremely widely studied graphene materials, however, BNNSs are largely unexplored despite their unique properties, especially in terms of their uses in nanocomposite hydrogels. In general, the fabrication of nanocomposite hydrogels requires the nanoscale fillers as cross-linkers to be highly hydrophilic and well soluble in aqueous media. However, hexagonal boron nitride (h-BN) and derived BNNSs are generally insoluble not only in water but 29 also in common organic solvents, so that their modifications have been explored for improved solubility characteristics. More specifically, covalent or non-covalent functionalization schemes have been employed in the solubilization of BNNSs, 30 including the defect-site derivatization, radical covalent 31-32 33 functionalization, Lewis acid-base interaction, or π34 stacking. For example, pristine h-BN powder was exfoliated and functionalized with the use of amine-terminated 35 36 poly(ethylene glycol), poly(sodium 4-styrenesulfonate), or 37 ionic surfactants for the modified BNNSs to be more watersoluble. As a significant limitation in these functionalization schemes, however, the agents used for the modification and dispersion became essentially unwanted “contamination” in the resulting samples, making them unsuitable for the 36 preparation of high-performance nanocomposite hydrogels. New functionalization schemes that would lead to aqueous compatible BNNSs free from unwanted impurities are in demand for the desired hydrogels and beyond. In work reported here we treated pristine h-BN powder with molten citric acids and produced a stable BNNS aqueous solution (ca-BNNS). The solubilization of BNNSs was likely due
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Experimental ca-BNNS Citric acid (33.3 g) and h-BN (500 mg) were homogeneously ground and added to a flask. The mixture was vigorously stirred and heated to 170 ºC for 8 days under the argon protection. The resulting solid was suspended in water, filtered over a porous cellulose ester membrane, and then repeatedly washed with water and ethanol to completely remove the free citric acid. The obtained ca-BNNS sample was redispersed in water and allowed to stand undisturbed overnight. The concentration of aqueous ca-BNNS dispersion was estimated -1 to be 1.1 mg·mL .
(Wt – Wd)/Wd and WC = (Wt – Wd)/Wt × 100%, where WtOnline and View Article DOI: 10.1039/C5NR07578E Wd were the weights of the swollen hydrogel at prescribed time intervals and the corresponding dried hydrogel, respectively.
Results and Discussion The treatment of h-BN with molten citric acid was for the exfoliation to yield BNNSs of a few layers, which was enabled by the functionalization as well as insertion of citric acid molecules. The resulting nanosheets were still functionalized with citric acid, thus denoted as ca-BNNS. Unlike the h-BN precursor, the ca-BNNS sample is highly soluble or dispersible in water and other polar organic solvents. The concentrated ca-BNNS aqueous dispersion appeared milky, but upon dilution the dispersion became transparent, and both concentrated and dilute dispersions exhibited the Tyndall effect (Fig. 1). The ca-BNNS dispersions were stable over an extended period of -1 time. For example, the dispersion of 0.25 mg·mL in concentration was largely unchanged after being stored for 9 months under ambient conditions. (a)
ca-BNNS/Polyacrylamide Hydrogel
0.00
(b)
Concentration (mg/mL) 0.02 0.04 0.06 4 3 2 1
Absorption
ca-BNNS powder (25 mg) was dispersed in the nitrogen-purged water (10 mL) with the aid of mild ultrasonication. Acrylamide monomer, ammonium peroxydisulfate (APS, 10 mg) and N,N,N',N'-tetramethyl-ethylenediamine (TMEDA, 5 μL) were successively added to the aqueous ca-BNNS dispersion in an ice bath, followed by the vigorous stirring for 15 min. The ice bath was replaced by a water bath (25 ºC) to initiate the free radical polymerization. When the reaction mixture became viscous and lost mobility, it was transferred to a cylindrical or tubular mold, allowing a complete polymerization process in 48 h. The resulting hydrogel (denoted as BNNS2.5/PAAm) was immersed in fresh water (100 mL × 10) for 2 days to get rid of unreacted reagents and residual impurities. Polyacrylamide (PAAm) hydrogel was prepared with the identical procedure for the BNNS2.5/PAAm hydrogel, except that N,N'-methylenebisacrylamide (BIS) was used as the crosslinker. The hydrogels (pre-dried in vacuum oven, 0.1 – 0.4 g) were immersed in water at different temperatures (25, 45 and 60 ºC) or pH values (1.3, neutral, and 12.1) to reach swelling equilibrium. The hydrogels were weighed at prescribed time intervals after removing the excess water on the hydrogel surface with a filter paper. The swelling ratio (SR) and water content (WC) were calculated by the following equations, SR =
0
200
300
400 500 600 Wavelength (nm)
700
800
Fig. 1 (a) Concentrated (left) and dilute (middle) showing a Tyndall effect upon irradiation and water (right) being presented for comparison; (b) UV-vis absorption of aqueous ca-BNNS dispersion (Inset: correlation of concentration and absorbance of ca-BNNS dispersions at 206 nm).
The aqueous ca-BNNS dispersion was found to be largely clear over the visible spectrum, as expected, exhibiting some absorption in the UV (peaking at ~206 nm). The observed
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to the hydrophilic –NH2 and –OH groups generated during acid treatment rather than linking substantial amounts of citric acid molecules onto BNNS surface. The hydrophilicity and cleanness of ca-BNNS sample allowed the fabrication of caBNNS/polyacrylamide (i.e., BNNS2.5/PAAm) nanocomposite hydrogels with remarkably enhanced swelling behaviors and mechanical properties. The water content of a swollen BNNS2.5/PAAm hydrogel remained as high as 94 wt% even after storage under ambient conditions for more than a year. Moreover, the BNNS2.5/PAAm hydrogel containing ~95 wt% water withstood 10,000% elongation and 8 MPa compression stress, without rupture. To the best of our knowledge, the performance of BNNS2.5/PAAm hydrogel (especially for water retentivity) is superior or comparable to those of previously reported nanocomposite hydrogels with similar nanomaterials loading and water content.
Normalized Absorption
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ca-BNNS
h-BN
BNNS2.5/PAAm hydrogels 1614
3190 3407
(b)
1661
PAAm hydrogels 3415
3193
1619 1667
4000 3500 3000 2500 2000 1500 1000 -1 Wavenumbers (cm ) 200 nm
100 nm
(c)
(d)
1 m
Fig. 3 FT-IR spectra of ca-BNNS (Olive), pristine h-BN (Black), BNNS2.5/PAAm hydrogels (Red), and PAAm hydrogels (Blue) (Inset: enlarged ca-BNNS spectrum showing a double peak).
4 0 0.0
0.2
500
0.4 0.6 Position (m)
0.8
1.0
Fig. 2 (a) and (b) TEM images of a ca-BNNS sample at different magnifications; (c) a typical AFM image of a ca-BNNS sample; (d) a zoomed-in image of the indicated area in (c) and a height profile alone the line.
The functionalization of BNNS by citric acid molecules on the sheet surfaces or edges in ca-BNNS was probed by using FT-IR, in comparison with the spectrum of the precursor h-BN (Fig. 3). Both ca-BNNS and h-BN samples showed two prominent -1 -1 peaks at ~1380 cm and ~800 cm , corresponding to the characteristics of in-plane B-N ring stretching vibration and out-of-plane B-N-B bending vibration, respectively. The peaks of ca-BNNS were narrower than those of pristine h-BN likely 38 due to the smaller thickness for the former sample. The ca-1 BNNS sample showed additional peaks centered at 3450 cm -1 and 1640 cm , which might be assigned to the stretching and bending vibration of a primary amine. A shoulder peak at -1 39 ~3200 cm might resulted from the B–OH groups, which was -1 consistent with the appearance of a Raman peak at ~870 cm 31 (Fig. S1, ESI†). The negligible peaks, if any, in the regions of -1 -1 2950 – 2850 cm and 1700 – 1750 cm for the stretching mode of –CH2 and –COOH in the citric acid molecules, indicated that the ca-BNNS sample contained only a relatively small amount of citric acid moieties. The treatment of h-BN
The TGA results further confirmed that the amount of citric acid and hydrophilic groups (–NH2 and –OH) linked onto BN sheets accounted for about 2.2% of the sample weight (Fig. S2, ESI†). Therefore, it is remarkable that such a relatively low level of functionalization of BNNS by citric acid could substantially improve the aqueous compatibility of the nanosheets for stable dispersions. It might also suggest that the nanosheet configuration is in itself "water-friendly", only requiring some help from the functionalization moieties to enhance the aqueous dispersion and prevent from aggregation and/or re-stacking. The minimal content of functionalization moieties in the ca-BNNS sample is very favorable to its uses as nano-fillers for the fabrication of high-performance nanocomposite hydrogels. Owing to the high hydrophilicity and the thin lamellar structure, the ca-BNNS was successfully incorporated into hydrogels in which acrylamide monomers were polymerized and cross-linked with ca-BNNS in situ. The interactions between ca-BNNS and polyacrylamide were preliminarily characterized by using FT-IR. The characteristic peaks of amide group (N-H and C=O) are redshifted in the FT-IR spectrum of a BNNS2.5/PAAm hydrogel as compared with those in a PAAm -1 hydrogel (Fig. 3). Specifically, The IR peaks at 3415 cm , 3193 -1 -1 cm and 1619 cm for PAAm hydrogel, corresponding to the -1 N–H stretching and bending vibration, shifted to 3407 cm , -1 -1 3190 cm and 1614 cm in the spectrum of BNNS2.5/PAAm sample, respectively. Meanwhile, the stretching mode of C=O -1 in the amide groups displayed the similar redshift (1667 cm -1 for PAAm hydrogels versus 1661 cm for BNNS2.5/PAAm hydrogel). The redshifts might be attributed to the hydrogen bonds between –OH and/or –NH2 groups on the boron nitride
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(a)
with molten citric acid likely broke B-N bonds and Viewgenerated Article Online DOI: 10.1039/C5NR07578E hydrophilic –NH2 and –OH groups at the defective sites and edges rather than linked massive citric acid molecules onto the BNNS surface.
Transmittance (%)
absorbance was linearly correlated with the ca-BNNs concentration (obeying the Lambert-Beer's law), suggesting the homogenous dispersion of ca-BNNS (Fig. 1). The estimated -1 -1 extinction coefficient of ca-BNNS is ~56 mL·mg ·cm at 206 nm. The ca-BNNSs were mostly round or elliptic in shape, with lateral sizes of a few hundred nanometers according to results from microscopy characterizations (Fig. 2). The thickness of the nanosheets was around 5 nm based on the height profile in the AFM images. Both TEM and AFM results suggested that the ca-BNNSs were predominantly few-layered boron nitride nanosheets, consistent with the expectation that the treatment of h-BN with molten citric acid resulted in the exfoliation of and peeling off nanosheets from the precursor hBN.
Height (nm)
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Table 1 Formulations of ca-BNNS/polyacrylamide hydrogels. a
Hydrogel
Acrylamide
Cross-linker
(g)
BIS (mg)
ca-BNNS (mg)
BNNS2.5
1
0
25
BNNS1.5
1
10
15
BNNS1.0
1
15
10
BNNS0.5
1
20
5
BNNS0.1
1
24
1
PAAm
1
25
0
a The subscripts denote ca-BNNS concentration (mg·mL-1) during gelation
When the weight ratio of ca-BNNS to acrylamide monomer decreased from 2.5% to 2%, the obtained hydrogel was fluidic though it was very viscous. For comparison purpose, hydrogels with reduced ca-BNNS contents (BNNS0.1-1.5/PAAm) were prepared with the identical procedure for the BNNS2.5/PAAm hydrogels, in which the ca-BNNS was partially replaced by N,N'-methylenebisacrylamide (BIS) and the total concentration -1 of ca-BNNS and BIS was constant (2.5 mg·mL ) in every hydrogel. It should be noted that the effective crosslinking groups (-OH, -NH2 and –COOH) on ca-BNNS surface (around 0.5 – 1.3 mmol/g) were one order of magnitude lower than those in BIS crosslinker (around 13 mmol/g). The cocrosslinked hydrogels with various ca-BNNS and BIS contents were listed in Table 1 and Fig. S3 (ESI†). (b)
(a)
50 μm
100 μm
(c)
(d)
100 μm
50 μm
Fig. 4 Typical SEM images of lyophilized xerogels. (a) BNNS2.5/PAAm sample with a hierarchical pore structure and (b) a zoomed-in image of the indicated area in (a); PAAm sample at low (c) and high (d) magnifications.
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10.1039/C5NR07578E The hydrogels with and without ca-BNNSDOI: were lyophilized and visualized by SEM. The BNNS2.5/PAAm hydrogel exhibited a hierarchical and uniform pore structure (Fig. 4). The first-level (large) pores were hundreds of microns in diameter and their walls consisted of second-level (small) pores with sizes of tens of microns. Therefore, the large pores were interconnected by small pores, generating a foam-like 3-dimentional network. On the contrary, no hierarchical pore structure was found in the PAAm hydrogel (without ca-BNNS) and the hydrogels with reduced ca-BNNS content (BNNS0.1-1.5/PAAm hydrogels) where the pores were largely isolated and non-uniform in sizes (Fig. 4 and Fig. S4, ESI†). The swelling behaviors of dry hydrogels were systematically investigated under variant conditions. The swelling curves (viz. swelling ratio versus time) for all dry hydrogels (with and without ca-BNNS) were steep in the beginning and tended to tail off and flatten (Fig. 5 and S5, ESI†). The equilibrium swelling ratio in pure water at 25 ºC was determined to be 10.4 for PAAm hydrogel and it slightly increased with the increment of ca-BNNS loading for the co-crosslinked hydrogels (BNNS0.11.5/PAAm). Once the hydrogel was solely cross-linked by caBNNS (i.e., BNNS2.5/PAAm), the equilibrium swelling ratio dramatically increased to 74, which was more than four times larger than that of BNNS1.5/PAAm hydrogel. The equilibrium swelling ratio was temperature and pH dependent for both PAAm and BNNS2.5/PAAm hydrogels. For example, the equilibrium swelling ratio of BNNS2.5/PAAm hydrogel increased with the raise of swelling temperature and it reached 120 after swelling in pure water for ~30 h at 60 ºC (Fig. 5). Moreover, the equilibrium swelling ratios of BNNS2.5/PAAm and PAAm hydrogels measured in NaOH solution (pH = 12.1) were nearly one order of magnitude larger than those measured in HCl (pH = 1.3) and NaCl (0.9 wt%) solutions. It was worth pointing out that the equilibrium swelling ratios of BNNS2.5/PAAm hydrogels were consistently larger than those of PAAm samples in all the tested systems. Most importantly, the water retentivity of BNNS2.5/PAAm hydrogel is superior to those of previously reported 41-45 hydrogels. Specifically, after equilibrium swelling in pure water at 25 ºC, all the hydrogles (PAAm and BNNS0.1-2.5/PAAm) were stored in air to evaluate the water retentivity under ambient conditions. The BNNS2.5/PAAm hydrogel remained swollen and its water content was estimated to be still as high as 94 wt% after one-year storage (Fig. 5 and Fig. S6, ESI†). The water retentivities of co-crosslinked hydrogels (BNNS0.11.5/PAAm) were better than that of PAAm as observed in the deswelling measurements, but they were still considerably lower than that of BNNS2.5PAAm hydrogel. The significantly enhanced water retentivity and swelling performance were probably ascribed to the hierarchical pore structures of BNNS2.5/PAAm hydrogels. Supposedly, the hierarchical pores enabled BNNS2.5/PAAm hydrogel to expand and thus generated plenty of hydrophilic and capacious cavies for water suction. Meanwhile, the hydrophilic environment inside hierarchical pores minimized the escape of water from BNNS2.5/PAAm hydrogels.
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sheets and amide groups attached to the polyacrylamide 40 mainchains.
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Swelling Ratio (g/g)
90 60
25 °C 45 °C 60 °C
30
0
15
30 45 Time (h)
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3 mm
(b)16
120
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DOI: 10.1039/C5NR07578E
12 8
25 °C 45 °C 60 °C
4
0
75
(c)
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30 45 Time (h)
60
75
(d)
400
100
in HCl in NaCl in NaOH
100
0
(e) PAAm
10
20 30 Time (h)
BNNS0.1
40
BNNS0.5
(b)
60 in HCl in NaCl in NaOH
40 20 0
BNNS1.0
10
20 30 Time (h)
BNNS1.5
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BNNS2.5
Fig. 5 Swelling ratio–time curves of BNNS2.5/PAAm (left panel) and PAAm (right panel) dry hydrogels in water at different temperatures (a and b) and in aqueous solution with different pH values (c and d); (e) photograph of BNNS2.5/PAAm hydrogel showing its extraordinary water retentivity (water content is as high as 94 wt%) after storage under ambient conditions for more than a year. Photographs of the hydrogels with different ca-BNNS contents were shown for comparison.
The swollen BNNS2.5/PAAm hydrogel showed excellent flexibilities as well, which might be due to its hierarchical pore structure again. The BNNS2.5/PAAm hydrogel was able to withstand compression, tensile, knotting and torsion even under high degree of deformations (Fig. 6). The elongation and compressive strength of a BNNS2.5/PAAm hydrogel (water 46 content ~95%) exceeded 10,000% and 8 MPa at 97% strain, respectively (Fig. 7). Importantly, the hydrogels quickly recovered without any obvious damage once the external forces were removed. The elastic modulus of BNNS2.5/PAAm hydrogel (water content ~98%) was measured to be 1.4 kPa. In sharp contrast, the PAAm hydrogel containing ~92 wt% water is fragile in nature. It ruptured when the compression strength reached 0.04 MPa or fractured when a tensile stress was loaded. Similarly, all the BNNS0.1-1.5/PAAm hydrogels containing 92-93 wt% water, where the ca-BNNS was partially replaced with covalent cross-linker (BIS), ruptured during compression tests. Although the compression strength of BNNS0.1-1.5/PAAm hydrogels gradually improved with the increase of ca-BNNS content and finally reached 0.21 MPa for the BNNS1.5/PAAm hydrogel, these values were still substantially lower than that of BNNS2.5/PAAm hydrogel (Fig. S7, ESI†).
(e)
(d)
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Fig. 6 Flexibility of BNNS2.5/PAAm and PAAm hydrogels. (a) complete recovery of BNNS2.5/PAAm sample from compression, in comparison with (b) fracture of PAAm sample under compression; BNNS 2.5/PAAm hydrogels (c) in a elongation measurement and (d, e and f) withstand various deformations. 8
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Fig. 7 Compression stress–strain curve of BNNS2.5/PAAm hydrogel .
Conclusions In summary, h-BN was treated with molten citric acid to produce aqueous dispersible boron nitride sheets (ca-BNNSs). Characterization data indicated that the treatment likely generated hydrophilic –NH and –OH groups at the edges or defect sites of boron nitride sheets instead of linking massive amounts of citric acid molecules onto sheet surfaces. The resultant ca-BNNSs exhibited desirable aqueous compatibility and enabled the fabrication of high-performance hydrogels, targeting high water retentivity and flexibility. The
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BNNS2.5/PAAm hydrogel (initially swelling in water) largely remained swollen (water content ~94 wt%) even after oneyear storage under ambient conditions. Importantly, the swollen BNNS2.5/PAAm hydrogel (water content ~95 wt%) was highly flexible. Its elongation and compressive strength exceeded 10,000% and 8 MPa at 97% strain, respectively. Moreover, the aforementioned hydrogel recovered upon the removal of compression force, without obvious damage. The substantially improved water retentivity and flexibility revealed that ca-BNNSs served as a highly promising new platform in the development of high-performance nanocompoiste hydrogels for many technological applications.
Acknowledgements This work was supported by the NSFC (51272152) and Guangdong Natural Science Foundation (S2013010014171). Additional supports from Guangdong Province (2012KJCX0053, 2014KCXTD012 and 2013A061401016) and SRF for ROCS (SEM) were also acknowledged. Jijie Chen, Weixiong Liang and Zhou Lu participated in this project through an undergraduate research program.
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