Formation of enhanced gelatum using ethanol/water binary medium for fabricating chitosan aerogels with high specific surface area
Graphical abstract
Introduction
Aerogels are a family of solid materials with low density, high porosity and large internal surface area [1], [2], which consist of three-dimensional (3D) network skeletons of randomly interconnected nanoparticles with diameters of 3–10 nm, describing a system of well-reachable pores[3]. Porous materials with exceptional physical structures usually display a series of attracting properties such as thermal, acoustic, electrical insulators, catalyst supports, drug carriers, cosmic dust collectors, and nuclear waste storage materials [4], [5], [6], [7], [8], [9]. Hence high porosity owned will be a very essential factor to aerogels with versatile functional applications, suggesting the unrivalled importance to specific surface area (SSA) [10], [11].
However, the poor mechanical performance of aerogels is a big disadvantageous to practical applications due to their skeletal structures [12]. In natural constructions, it is found that a continuous fibrous structures introduced can broaden material utilization and the resulting properties [13], e.g., spider network has a relative density comparable to that of aerogels but it is undoubtedly structurally strong [14]. Inspired by this, numerous attempts to reinforce mechanical properties of aerogels have been proposed, e.g., chemical cross-linking with reactive polymers [15], and impregnating individual secondary phases [16], [17], [18]. Wherein the hybridization of aerogels with organic polymers is a potential route for reinforcement, unfortunately increasing organic contents does not only inevitably boost structural inhomogeneity at nanoscale, but unexpectedly come at the cost of a noticeable decrease in porosity and SSA [19], [20], [21], [22]. Accordingly, aerogels with these essential properties are yet challenging.
Biomaterials from sustainable resources such as polysaccharides, proteins, and plant oils, can reduce largely the overall carbon footprint of the products and relieve the environmental burden [23], [24], [25]. In contrast to conventional aerogels composed of spherical nanoparticle skeletons, biomass aerogels are anticipated to be a better choice for flexibility derived from the nature of entangled nanofibers polymers, or focusing upon the structural homogeneity in nanoscale to gain an optical property. Isogai’s group recently employed a breakthrough, who developed a kind of translucent cellulose-based aerogel with ordered nanofibrous skeletons [26]. More recently, a great progress was made by Takeshita’s group, who prepared translucent chitosan-based aerogels using the cross-linking approach [27]. However, the vital SSA advance is hardly under way in available information, especially, SSA of aerogels possessing traditional organic-nature always shows the tendency to a great reduction in accessible surface area, compared with inorganic ones because of longer structural chains and bigger diameters in size [28]. Therefore, it is of paramount importance to search for novel ways on synthesis of biomass aerogels with large SSA, which is also robustly required to explore in organic aerogel growth field [29].
In view of a few reports on synthesis of cross-linked silica aerogels [30], [31], [32], which motivate us to utilize reasonable sol solvents in gel preparation. Herein, we report a new strategy to accomplish the fabrication of chitosan aerogels (CAs) with improved SSA in allowing for moderate chemical gelation, structural homogeneity and production cost. An extremely fascinating phenomenon has been discovered that implementing chemically cross-linked course for chitosan sols could be greatly promoted in ethanol/water system. The critical role of gelation under lower substance concentration conditions in engineering aerogels, which is often neglected in most accessible gelling processes of sols, is analyzed. Furthermore, this work provides aging and mechanistic explanations dependent upon the collected evidence, and demonstrates that CAs synthesized exhibit the integrated properties of well-distributed cellular morphological texture, high SSA and superior thermal resistance.
Section snippets
Materials
Chitosan (deacetylation rate: >80%; viscosity: 20–200 mPa·s at 5 g L−1; RT), acetic acid (AR), formaldehyde solution (36–38 wt.%), and ethanol (AR) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Deionized and doubly distilled water was employed in all experiments. All reagents in this work were used as received without further purification.
Formation of Chitosan Gels
Chitosan of 1 g was added in the mixed solvents containing 2 wt.% acetic acid and ethanol to obtain the chitosan solution of 10 g L−1. In
Formation of CGs in ethanol/water system
We attempted to design well-structural CAs, particularly the aerogels formed under ultralow substance concentration of chitosan, so that to reap many potential properties such as high BET area, pore size required, and moderate thermal stability. Thus a key prerequisite in preparing aerogels is how to form proper CGs in an exceeding low concentration. Considering it, experiments to yield CGs were employed to understand the gel course. Interestingly, when we adopted ethanol as the partial
Conclusions
CAs with high SSA, fine pore size, and excellent thermal property were prepared successfully using a facile cross-linking gelation method in ethanol/water binary medium. An attractive phenomenon was found that wet chitosan gel could be still formed even in a fairly low substance concentration under ethanol/water system, signifying the key role of ethanol for improving gelatum. Because the production of micro-dispersed active phases in gelling process enable to exert the construction of
Competing financial interests
The authors declare no competing financial interests.
Acknowledgments
The work presented here was supported by the National Natural Science Foundation of China (Grant no. 51302317), Natural Science Foundation of Hunan Province (Grant no. 14JJ3008), and Hunan Provincial Innovation Foundation for Postgraduate (Grant no. CX2016B002). The authors gratefully acknowledged the generous support from the Program for Science and Technology Innovative Team in Colleges of Hunan Province, and the Program for Technology Innovative Group of National University of Defense
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