Elsevier

Polymer

Volume 55, Issue 1, 14 January 2014, Pages 287-294
Polymer

Oscillatory shear induced gelation of graphene–poly(vinyl alcohol) composite hydrogels and rheological premonitor of ultra-light aerogels

https://doi.org/10.1016/j.polymer.2013.11.011Get rights and content

Abstract

Time evolution of 3-D structure of GO hydrogels was explored by rheological measurements. We showed that the GO hydrogels were thickened drastically by oscillatory shear flow if and only if both the angular frequencies and shear strain are small. We also showed that the plateau modulus (GN) of GO hydrogels is rheological premonitory of ultra-light aerogels. The ultra-light aerogel fabricated in this work exhibited low density of 4.0 mg/cm3, low surface resistivity of 6.6 Ω/sq, high specific area of 1069 m2/g, and high recoverable strain of 94% in compression. The glass transition temperature of poly(vinyl alcohol) (PVA) in the aerogel was 49 K higher than that of pure PVA. The thermal stability of the GO/PVA aerogel in air environment was superior to that of dried GO in itself in thermogravimetric analysis (TGA).

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, M¯w = 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 (GN). The GN 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.

References (45)

  • O.C. Compton et al.

    Carbon

    (2012)
  • Z.Y. Sui et al.

    Carbon

    (2011)
  • Z. Fan et al.

    Chem Phys Lett

    (2013)
  • J. Li et al.

    J Colloid Interface Sci

    (2012)
  • S.T. Nguyen et al.

    Colloids Surf A

    (2012)
  • D.C. Pozzo et al.

    Colloids Surf A Physicochem Eng Asp

    (2004)
  • Y.D. Yan et al.

    Physica A

    (1994)
  • H.M. Kim et al.

    Thin Solid Films

    (2011)
  • H. Bai et al.

    J Phys Chem C

    (2011)
  • Y. Xu et al.

    ACS Nano

    (2010)
  • H. Bai et al.

    Chem Commun

    (2010)
  • S.I. Jun et al.

    App Phys Lett

    (2012)
  • J. Wang et al.

    ECS Trans

    (2009)
  • V.H. Luan et al.

    J Mater Chem A

    (2013)
  • Z. Xu et al.

    ACS Nano

    (2012)
  • W.F. Chen et al.

    Nanoscale

    (2011)
  • L. Chen et al.

    J Mater Chem

    (2012)
  • X.Z. Wu et al.

    J Mater Chem

    (2012)
  • X.T. Zhang et al.

    J Mater Chem

    (2011)
  • M.A. Worsley et al.

    J Am Chem Soc

    (2010)
  • H. Hu et al.

    Adv Mater

    (2013)
  • Z.P. Chen et al.

    Nat Mater

    (2011)
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