Oscillatory shear induced gelation of graphene–poly(vinyl alcohol) composite hydrogels and rheological premonitor of ultra-light aerogels
Graphical abstract
Introduction
There is fast growing interest in graphene oxide (GO) hydrogels [1], [2], [3], [4], [5], [6] and graphene aerogels [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. This may be due to that GO is the “super gelator” in aqueous solution with a critical gelation concentration <0.5 wt.% [5], [6]. Since the first report on graphene aerogels in 2009 [7], many researchers have developed graphene aerogels based on the hydrogels composed of GO by freeze-drying [7], [8], [9], [10], super critical CO2 drying [11], [12], [13], [14], [15], [16], [17], and microwave-irradiation [18]. Direct synthesis of graphene aerogels by chemical vapour deposition (CVD) has also been reported [19], [20]. The researches on graphene aerogels point to various applications such as fiber [9], sensor [12], oil absorber [14], supercapacitor [20], energy storage [15], [21], electromagnetic interference shielding [22], microbial fuel cell [23], and water purification [24]. The density of graphene aerogels reported in the literature is about 3–96 mg/cm3 [8], [15], [16], [17], which are comparable values to those of silica aerogels. Synergistically assembled GO aerogels with organic molecules [14], polymers [25], [26], nanoparticles [27], and carbon nanotubes (CNT) [28], [29], [30], [31] have been reported. Especially, GO/CNT composites aerogels show ultra-flyweight density as low as 0.16 mg/cm3 [29], and superelastic and resistant to fatigue [31].
The understanding on the evolution of 3D structures in GO hydrogels is one of the core sciences for the aerogels, too, since many technologies to fabricate graphene aerogels are based on GO hydrogels [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. The evolution of microstructure in dispersions can be monitored by small amplitude oscillatory shear (SAOS) measurements, if stress relaxation is much faster than the time required for structural change [32], [33]. The storage modulus (G′) reflects the number of network points in the gels, and increases by gelation proceeding. Therefore, many researchers employ SAOS measurements to monitor the degree of gelation of GOs [1], [2], [3], [4], [5], [6] and polymers [34], [35], [36], [37]. If SAOS affected the structural evolution of gels in measurements, the results could be distorted. In fact, oscillatory-shear induced ordering has often been observed in concentrated colloidal suspensions [38], [39]. In this work, we explored the gelation of aqueous dispersions composed of GO and poly(vinyl alcohol) (PVA) through rheological study. Strikingly, our studies showed that SAOS can induce the gelation of graphene dispersions. “Shake gels” has also been reported in polymer dispersions with nano-clay [34], [35] and nano-silica [36]. The “shake gels” behavior is the shear induced gelation which is a class of shear thickening phenomena [36]. After cessation of flow, the “shake gels” return to solution [34], [35], [36]. While, the SAOS-induced GO hydrogels were stable in this study. It is worth to note that SAOS-induced gelation is rarely observed phenomena in condensed matter physics. The rheological properties measured after the completion of SAOS-induced gelation was highly related with the density and the electrical conductivity of graphene aerogels in this study. We anticipate that the SAOS induced-gelation and rheological premonitory of ultra-light aerogels, reported in this work, do a great impact on the further studies on GO hydrogels and aerogels.
Section snippets
Materials
Poly(vinyl alcohol) (PVA) was purchased from Sigma Aldrich (99 mol% hydrolyzed, = 89,000–98,000 g). Flake graphite powder (19 μm nominal size) was supplied from Asbury Carbon. GO was synthesized from the purified flake graphite by modified Hummers method [40], following the procedures reported in our previous work [41]. To prepare GO hydrogels, 4.8 ml GO dispersion (5 mg/ml) was added into 1.2 ml PVA solution (4.8–72.0 mg PVA). Aqueous PVA solution was added to the dispersions with various
Oscillatory shear induced re-structuring of GO–PVA composite hydrogels
Aqueous graphene-oxide (GO) solution was prepared from purified flake graphite by the modified Hummers method. GO/PVA dispersions were prepared with various PVA weight ratios to GO (rP/G). GO concentration kept as 0.4 wt.% in the various dispersions prepared with rP/G = 3:1, 2:1, 1:1, 1:2, 1:3, 1:4, 1:5, 1:10, and 1:15. The rP/G 3:1, 2:1, 1:1, 1:2, 1:3, 1:4, 1:5, 1:10, and 1:15 corresponds to 1.20, 0.80, 0.40, 0.20, 0.13, 0.10, 0.08, 0.04, 0.026 wt.% PVA in the dispersions, respectively.
Conclusions
In conclusion, the GO hydrogels are thickened by small amplitude oscillatory shear (SAOS) stress. The thickening is more pronounced, when the GO hydrogels are agitated with lower frequencies. The thickening is observed in small amplitude oscillatory shear flow if and only if γ ≤ 20%. The GI-GO hydrogels (thickened hydrogel by SAOS) are stable and exhibit plateau modulus . The depends on the PVA contents in GO hydrogels. Ultra-light aerogels are obtained by freeze-drying of the GO
Acknowledgments
This work is supported by the research fund from Dong-A University.
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