High-strength and morphology-controlled aerogel based on carboxymethyl cellulose and graphene oxide
Introduction
Aerogels, usually made of colloidal particles or polymers, have a three-dimensional network structure and are filled with gas (Du, Zhou, Zhang, & Shen, 2013). Aerogels are widely used in fields of special garments (Shaid, Wang, & Padhye, 2015), aviation (Schulz et al., 2016), capacitor (Ouyang, Sun, Memon, Wang, Geng, & Huang, 2013; Ye & Feng, 2014), drug delivery (Ulker & Erkey, 2014) and energy saving building (Cuce, Cuce, Wood, & Riffat, 2014) due to its low density, high specific surface area, high porosity and low thermal conductivity. Judging from raw materials, the aerogels are classified into inorganic aerogels and organic aerogels. Inorganic aerogels, such as silica (SiO2) aerogel, have a very low thermal conductivity (Xie, He, & Hu, 2013), which are commonly used as a thermal insulation material, but they are brittle and difficult to be prepared in large sizes. Traditional organic aerogels, such as resorcinol-formaldehyde (RF) aerogel (Schwan & Ratke, 2013), overcoming the brittleness of inorganic aerogels, have good flexibility. But most of them have toxicity and are not environmentally friendly. So, aerogels made of bio-based materials, as the new generation of aerogels, have attracted extensive attention for their environmental friendliness and renewability (García-González, Jin, Gerth, Alvarez-Lorenzo, & Smirnova, 2015; Wang, Sánchez-Soto, & Abt, 2016). Carboxymethyl cellulose (CMC), derived from carboxymethylation of the hydroxyl groups on the cellulose backbone, is one of the best-known derivatives of cellulose. CMC aerogels combine the advantages of cellulose and traditional aerogels (Huang, Liu, Wu, Li, & Wang, 2017). CMC-based aerogels can be fabricated by an economic and environmentally friendly freeze-drying method (Javadi et al., 2013). However, pure CMC aerogels have poor mechanical strength because of their own structure (Li et al., 2017).
To date, several methods were reported to improve mechanical properties of cellulose aerogels, including cross-linking with organic compounds (Jiang & Hsieh, 2017) or combining with inorganic materials such as aluminum (Yuan et al., 2016). Although the mechanical properties of cellulose aerogels were improved by those methods, the environmentally friendly properties of cellulose aerogels are destroyed. The efficient synthesis of environmentally friendly cellulose aerogels with improved mechanical properties is still challenging.
Recent work has shown that two-dimensional materials such as clays and graphene could provide organic polymer-based nanocomposites with excellent mechanical properties (Donius, Liu, Berglund, & Wegst, 2014; Jiao et al., 2016). Graphene oxide, as a derivative of graphene, has good dispersion in aqueous solution due to the oxygenated functional groups on its surface, making it an ideal nanofiller to prepare bio-based composites (Qiao et al., 2015; Xu et al., 2015). Besides, it has been found that boric acid could crosslink with hydroxyl groups on the GO sheets to form boron ester bonds, increasing the strength of GO films by 255% (An, Compton, Putz, Brinson, & Nguyen, 2011; Shahzadi et al., 2017).
Herein, inspired by toughening effect of GO nanosheets and borate crosslinking, we are first time to report environmentally friendly GO-CMC aerogels crosslinked by borate with controllable morphologies and improved mechanical properties. Interestingly, by controlling the heat transfer rate, composite aerogels with isotropy and anisotropy structure were prepared, and the mechanical properties and heat insulating properties were studied. When GO content was up to 5 wt%, the compressive strength and Young’s modulus of composite aerogels reached 349 kPa and 1029 kPa, which were 1.6 and 4.5 times that of CMC aerogels, respectively. Additionally, only 0.1 wt% amount of GO could also greatly improved mechanical properties of the composite aerogels.
Section snippets
Materials
CMC (1000 ∼ 1400 mpa.s, USP grade, Aladdin Ind. Co), graphite powder (<20 μm, Sigma Aldrich Chemistry), Na2B4O7 (Tianjin Hengxing Chemical Reagent Co.). All the chemicals used in the experimental work were reagents grade and were used without further purification. The distilled water was used for the whole experiments.
Preparation of GO
Graphene oxide was prepared from graphite powder by a modified Hummer’s method (Hummers & Offeman, 1958; Marcano et al., 2010). The obtained GO had good dispersion stability in
XRD and FTIR of CMC/GO aerogels
The XRD patterns of GO, CMC and GO/CMC composite aerogels with 5 wt% GO were shown in Fig. 2a. The XRD pattern of GO was used to illustrate the degree of oxidation. As a comparison, the XRD pattern of the pristine graphite was shown in the supporting information (Fig. S2). The XRD pattern of the graphite showed a diffraction peak at 2θ = 26.5° and this peak nearly disappeared in GO with a new diffraction peak appearing at 2θ = 11.2°, proving successful oxidation of graphite (Krishnamoorthy,
Conclusion
In this paper, GO/CMC composite aerogels with isotropic and anisotropic structures were prepared by controlling the heat transfer rate of the system. The compressive strength of isotropic aerogel reached 5 times and 14 times of axial and radial anisotropic aerogel, respectively. And the effect of GO content in isotropy composite aerogels on their properties was also discussed. The results showed that the mechanical properties of composite aerogels increased with the increase of GO content. When
Acknowledgements
This work was supported by K. C. Wong Education Foundation, Youth Innovation Promotion Association of CAS, the Young Taishan Scholars Program of Shandong Province (tsqn20161052) and the key basic research project of Shandong Natural Science Foundation (ZR201708240147).
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These authors contribute equally to this work.