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Bio-Based Artificial Nacre with Excellent Mechanical and Barrier Properties Realized by a Facile in-situ Reduction and Cross-Linking Reaction Kiran Shahzadi, Imran Mohsin, Lin Wu, Xuesong Ge, Yijun Jiang, Hui Li, and Xindong Mu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b05780 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 3, 2016
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Bio-Based Artificial Nacre with Excellent Mechanical and Barrier Properties Realized by a Facile in Situ Reduction and Cross-Linking Reaction Kiran Shahzadi,a Imran Mohsin,b Lin Wu,c Xuesong Ge,a Yijun Jiang,a* Hui Li,a Xindong Mu,a* a. Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101, China. E-mail:
[email protected];
[email protected]. Fax: +86-532-80662724; Tel: +86-532-80662725. b. Shenzhen Institute of Advanced Technology, University of Chinese Academy of Sciences, Shenzhen, China. c. Qingdao Technical College, Qingdao, Shandong Province 266000. ABSTRACT: Demands for the high strength integrated materials have substantially increased across various kinds of industries. Inspired from the relationship of excellent integration of mechanical properties and hierarchical nano/microscale structure of the natural nacre. A simple and facile method strategy to fabricate the high strength integrated artificial nacre based on sodium carboxy methyl cellulose (CMC) and borate cross-linked graphene oxide (GO) sheets has been developed. The tensile strength and toughness of cellulose based hybrid material reached to 480.5 ± 13.1 MPa and 11.8 ± 0.4 MJm-3 by a facile in situ reduction and cross-linking reaction between CMC and GO (0.7%), which are 3.55 and 6.55 times that of nature nacre. This hybrid film exhibit better thermal stability and flame retardness. More interestingly, the hybrid material showed good water stability than original water soluble CMC. This type of hybrid has great potential applications in aerospace, artificial muscle, and tissue engineering. KEYWORDS: bio-based, artificial nacre, in situ reduction and cross-linking, graphene oxide, high strength and toughness inorganic additives, including glass flake,7 alumina flake,8 graphene oxide,9 layered double hydroxides,1 nanoclay,3 and flattened double-walled carbon nanotubes.10 Recently, 2D graphene has attracted much research interest owing to its outstanding electrical, thermal and mechanical properties11 and many graphene-based devices have been fabricated12 such as bulk composites13 onedimensional fibers14 supercapacitors.11 As the water-soluble derivative of graphene, graphene oxide(GO), with many functional groups on the surface, is one of the best candidates for fabricating artificial nacre, because functional surface groups allow for chemical cross-linking to improve the interfacial strength of the adjacent GO layers. Until
Natural composites of seashell nacre and bones with hierarchical micro- and nano-structures have been studied for at least half-century due to their extraordinary mechanical properties, complemented by rare biological functionalities1-3 Consequently, several strategies (e.g. layer-by-layer (LBL) deposition, freezing-drying, and filtration) were proposed to mimic them4-6 especially nacre that possessed ordered brick-and-mortar (B&M) arrangement of inorganic nanotablets (95vol%) and small amount of bio macromolecules.5 Inspired by the intrinsic relationship between the structures and the mechanical properties lying in the natural nacre, different types of nacre-like layered nanocomposites have been fabricated with two-dimensional (2D)
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now, several methods have been developed to functionalize individual GO sheets and enhance the resultant mechanical properties, including divalent ion (Mg+2, Ca+2) modification,15 polyallylamine16 or alkyl amine, functionalization,17 glutaraldehyde (GA) treatment18 π-π interaction19 and hydrogen 20 bounding. Although the obtained strength and stiffness are significantly enhanced but graphene introduction in their presence in very little quantities (∼0.001–0) to polymer to get desirable results for strength require more studies. In nature, self-supporting macroscopic organisms especially the higher plants rely on the combined microscopic stiffness and strength of plant cell walls. These cellular building blocks depend heavily on borate chemistry to form reinforced intercellular networks.3 For the healthy growth of many plants borate is required21-22 (where 0.01 dry wt. %) significantly increases the mechanical properties of plant tissue.5 Borate ions have ability to form covalent bonds with oxygen-containing functional groups in counter ion environments and with different pH that makes it the cross-linker of choice in plants and a vital nutrient for nearly all plant species that require mechanically sound structures.23 In this paper, inspired by borate chemistry in nature plant and strength enhancement in GO sheets,24 a simple in situ reduction and
crosslinking reaction was employed to fabricated hybrid film composed of GO and sodium car boxy methyl cellulose(CMC). This hybrid bio-based film can exhibit an artificial nacre structure, although the amount of GO is very little (0.7%). Importantly, this hybrid system show great improvement in mechanical properties only by in situ reduction and crosslinking. The strength and toughness can reach to 480.5 ± 13.1 MPa and 11.8 ± 0.4 MJm-3 respectively, which is about 3.55 and 6.55 times that of natural nacre. At the same time, the barrier property can be dramatically improved by this method. RESULT AND DISCUSSION In a typical preparation, GO was synthesized by hummer’s method.25 Well dispersed CMC–GO borate linked nanocomposites were fabricated by simple solution casting technique followed by cross-linking phenomena to get desirable film with high strength, thermally stable, water stable and good barrier properties. Possible mechanism for the hybrid is shown in Figure 1. In which graphene layers after crosslinking with borate strongly interact with CMC through hydrogen bonding. This dual binding effect of boron covalent bonding with hydroxyl moieties of GO and hydrogen bonding interactions between GO and CMC play important role in film properties.
Figure 1. Possible mechanism for hybrid film.
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While Figure 2 showing the SEM images for the cross sections of hybrid film. Compared with the smooth surface of pure CMC in Figure S1 (Supporting Information) a clear layered structure appeared when the GO concentration reached 0.7%, which suggests that a self-assembly of GO in the film happens. The nanostructure of the hybrid films clearly indicating the graphene layers. These layers are compactly arranged in all hybrid films containing different concentration of graphene oxide with borate (0.09%) as shown in Figure 2a and film without borate Figure 2b. Furthermore, when concentration of GO increased
to 4.9%, the SEM (Figure 2c) shows that the layered structure disappears, which indicated the GO could not be dispersed very well when increased the concentration of GO. SEM-EDS mapping and EDS spectra were also employed to confirm the presence and distribution of B element in the hybrid film as shown in Figure 2d and 2e. It should be noticed that the inter sheet spacing in cross-linked hybrid film seems to be less than the film without cross linking. These closely packed layers may have played a role in enhancing film strength. XRD and FTIR were also employed to characterize the structure of hybrid film and CMC.
Figure 2. SEM images. (a) 0.7%GO/CMC, (b) 0.7%GO/CMC+0.09%Borate, (c) 4.9%GO/CMC+0.09% Borate, (d) showing SEM-EDS mapping for 0.7% GO/CMC+0.09%Borate, and (e) EDS spectra for 0.7% GO/CMC+0.09%Borate film. 1600 cm-1 corresponding to C-O stretching, asymmetrical and symmetrical modes of carboxylate ions respectively26 in Figure 3a (5). While for pure GO with characteristic peaks appearing at 1724 cm-1, 1040 cm-1, 1630 cm-1, 3370 cm-1 were attributed to C=O, C-O, C=C and OH stretching respectively27 Figure 3b(3). Interestingly, it is found when the reaction system was heated above 90 oC, GO peak at 1724 cm-1 attributed to C=O was not presents in the hybrid films indicating partial reduction as some of oxygen containing groups removed from hybrid film.28 Additionally, it is found a little colour change took place in hybrid film when the temperatures for preparation were higher than 90 oC Figure S3 (Supporting Information), which also indirectly give the proof for the reaction between the GO and CMC.
Figure S2 (Supporting Information) shows XRD diffractions of graphite, GO, CMC and hybrid films. The XRD peak of GO was at approximate 2θ=10.66 proving successful oxidation of graphite. CMC showed a broad peak at 2θ=22.39 indicating semicrystalline structure.26 It should be noticed that these hybrid films with low concentrations of GO (
