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Colloidal and rheological behavior of aqueous graphene oxide dispersions in the presence of poly(ethylene glycol)
Accepted Manuscript Title: Colloidal and rheological behavior of aqueous graphene oxide dispersions in the presence of poly(ethylene glycol) Author: Ruiwen Shu Qing Yin Honglong Xing Dexin Tan Ying Gan Guocai Xu PII: DOI: Reference:
S0927-7757(15)30267-3 http://dx.doi.org/doi:10.1016/j.colsurfa.2015.10.006 COLSUA 20214
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
22-8-2015 6-10-2015 7-10-2015
Please cite this article as: Ruiwen Shu, Qing Yin, Honglong Xing, Dexin Tan, Ying Gan, Guocai Xu, Colloidal and rheological behavior of aqueous graphene oxide dispersions in the presence of poly(ethylene glycol), Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2015.10.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Colloidal and rheological behavior of aqueous graphene oxide dispersions in the presence of poly(ethylene glycol)
Ruiwen Shu*, Qing Yin, Honglong Xing*, Dexin Tan, Ying Gan and Guocai Xu
School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, China
*Corresponding authors E-mail: [email protected] (R. Shu), [email protected] (H. Xing) Tel: (86)-554-6668497
Graphical abstract:
GO/PEG
3
GO
10
25 s -1
= 0.1 rad/s
100 50
GO/PEG
G',G'' (Pa)
0
0.15
(Pa.s)
G p ' (Pa)
150
0.20
0.10
0
5 10 15 cp (mg/mL)
20
cg = 15 mg/mL
2
10
cp / (mg/mL)
1
10 0.05
0 5 10 20
-1
100 s
100 0.00 0
5
10
15
20
10
cg (mg/mL)
1
-1
10
0
10
(rad/s)
1
2
10
The effects of the concentration of graphene oxide (GO) and poly(ethylene glycol) (PEG) on the colloidal and rheological behavior of GO aqueous dispersions were investigated.
The
critical concentration of isotropic-nematicphase transition of GO aqueous dispersions was about 6 mg/mL. Significantly, the aqueous GO/PEG dispersions presented a lowest viscosity and minimum storage modulus G′(ω
= 0.1 rad/s)
at the saturation adsorbing concentrationcp,s.
Therefore, the PEG concentration had a significant effect on the linear and non-linear rheological behavior of GO/PEG dispersions.
Highlights:
The critical concentration of isotropic-nematic phase transition for aqueous GO dispersions was determined by shear rheology.
Effect of PEG concentration on the linear and non-linear rheological behavior of aqueous GO/PEG dispersions was firstly studied.
PEG chains adsorbing effects were studied by Atomic force microscopy images and zeta-potential measurements.
2
Abstract In this work, the effects of the concentration of graphene oxide (GO) and poly(ethylene glycol) (PEG) on the colloidal and rheological behavior of aqueous GO dispersions were investigated. mg/mL, respectively.
The concentration of GO and PEG was varied from 0 to 20
Atomic force microscopy (AFM) images and zeta-potential
measurements demonstrated that PEG chains were adsorbed on the colloidal GO sheets. The red-shift of D and G bands in the Raman spectra of GO/PEG compared with pure GO suggested that there was hydrogen bond interaction between GO and PEG.
Steady state
shear results indicated that GO aqueous dispersions changed from Newton fluids to pseudoplastic fluids as the concentration of GO increased.
The critical concentration of
isotropic–nematic phase transition of aqueous GO dispersions was about 6 mg/mL. Significantly, the aqueous GO/PEG dispersions presented a lowest viscosity and minimum storage modulus G′(ω
= 0.1 rad/s)
at the saturation adsorbing concentration cp,s, which were
determined by steady state shear and small amplitude oscillatory frequency sweep, respectively. Therefore, the PEG concentration had a significant effect on the linear and nonlinear rheological behavior of GO/PEG dispersions.
In addition, the concentrated aqueous
GO dispersions showed typical yield flow behavior and the yield stress σy firstly decreased and then increased with the increasing of GO concentration. These results could contribute to understand the interplay between microstructure and mechanical behavior of GO/polymer in aqueous dispersions, and also provide some guidance for the processing of GO-based polymer nanocomposites.
Keywords graphene oxide; poly(ethylene glycol); isotropic–nematic phase transition; small amplitude oscillatory shear; yield stress
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1. Introduction Graphene is a two-dimension sheet of single atomic thick layer of hexagonally arranged carbon atom [1,2].
Graphene has attracted significant interests due to its extraordinary
physical properties which make it an ideal candidate for a wide range of applications, such as actuators [3,4], supercapacitors [5], hydrogen storage [6], nanocomposites [7]. In particular, graphene/polymer nanocomposites have become a hot topic in the field of polymer-based nanocomposites due to their excellent electrical, mechanical properties and thermal stability [8-11].
However, graphene/polymer nanocomposites still face several
challenges during processing, such as homogenous dispersion of graphene in polymer matrix, interfacial interactions and the effect of graphene fillers on the viscoelasticity of nanocomposites [9,11]. It is well-known that rheological characterization can provide much valuable information on the processing property of polymer-based nanocomposites. Therefore, it is necessary to explore the rheological performance of graphene/polymer nanocomposites.
Up to now, there have been many reports on the rheological behavior of
graphene/polymer nanocomposites, while which mainly focused their attention on graphene/polymer melts [12-15].
However, the graphene are usually processed from
graphene oxide (GO) dispersed in some common solvents, like water, N,N’-di methyl for mami de (DMF), and N-methyl pyrrolidone (NMP) [16].
Moreover, aqueous GO
dispersions have been widely used as precursor to produce chemically converted graphene in large-scale [17,18].
Thus, it is fundamentally important to investigate the rheological
behavior of aqueous dispersions of GO/polymer.
However, to the best of our knowledge,
there are few reports on the rheological behavior of aqueous dispersions of GO/polymer so far [19]. Poly(ethylene glycol) (PEG) is a common water-soluble non-ionic polymer which has been widely used as a kind of model polymers to tailor the interaction between colloidal
4
particles in aqueous dispersions [20,21]. The colloidal and rheological behavior of aqueous dispersions of inorganic nanoparticles, such as silica sphere [22], montmorillonoid [23], and Laponite® [24-26] in the presence of PEG have been widely reported. Therefore, PEG could be an ideal polymeric additive to explore the colloidal and rheological behavior of aqueous dispersions of GO/polymer. In the present study, we investigated the effects of the concentration of GO and PEG on the colloidal and rheological behavior of GO aqueous dispersions through atomic force microscopy (AFM), zeta-potential and rheological measurements. It was found that the GO sheets with a single atomic layer thickness were successfully prepared by ultrasonic exfoliation with an aspect ratio of ~ 570.
The concentrated aqueous dispersions of GO
showed characteristic shear-thinning behavior, and thus could be classified as a kind of yield stress fluids.
The adsorbing behavior of PEG on GO sheets in aqueous dispersions were
probed by AFM and zeta-potential measurements.
The critical concentration of isotropic–
nematic liquid crystal transition in aqueous GO dispersions was approximate 6 mg/mL. Furthermore, the effect of the concentration of PEG on the rheological properties of aqueous dispersions of GO was probed by small amplitude oscillatory frequency sweep and steady state shear tests. Significantly, a lowest viscosity and minimum storage modulus G′(ω = 0.1 rad/s) at the saturation adsorbing concentration cp,s were observed in the aqueous GO/PEG dispersions with the increasing of PEG concentration. Therefore, the study of colloidal and rheological behavior of aqueous dispersions of GO/PEG could contribute to understand the phase transition of a class of two-dimensional anisotropic colloid sheets with large aspect ratio and also maybe provide some guidance for the processing of GO/polymer nanocomposites.
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2. Experimental section 2.1 Materials Natural graphite flakes (purity degree > 99%, Qingdao Huatai Lubricant Sealing S&T Co. Ltd., Qingdao, China) were used after dried at 60 oC in vacuum for 12 h.
Poly(ethylene
glycol) (PEG, analytical grade) with molecular weight Mw of 20000 g/mol was used after dried at 50 oC in vacuum for 12 h.
All other reagents were analytical grade and used as
received without further purification. Water was purified by deionization and filtration with a Millipore purification apparatus (18.2 MΩ·cm).
2.2 Sample preparation Graphite oxide was prepared by the improved Hummers′ method [27]. Typically, a mixture of concentrated H2SO4/H3PO4 (360:40 mL) was first added to a mixture of graphite flakes (3.0 g) and KMnO4 (18.0 g). Then, the reaction mixture was heated to 50 °C and stirred for 12 h.
Next, the reaction was cooled to room temperature and poured onto ice (400 mL).
Lastly, H2O2 (30%) was added drop-wise into the reaction mixture until the color of the solution became bright yellow.
In order to remove the excess metal salts, the mixture was
filtered and washed with 1:10 HCl aqueous solution.
The product was firstly purified by
centrifugation at 12000 rpm for 20 min, decantation and re-dispersed in de-ionized water repeatedly for several times until the pH of the dispersion became neutral (pH ≈ 7).
Then,
the product was subjected to dialysis of two weeks to completely remove residual salts and acids. The as-prepared graphite oxide was freeze dried under a vacuum for 48 h. The aqueous stock dispersion of GO (20 mg/mL) was achieved by dispersing 0.2 g of graphite oxide into 10 mL of de-ionized water using an ultrasonicator (180 W) for 1.5 h at room temperature. A series of aqueous dispersions of GO with desired concentration were acquired by diluting the GO stock dispersion with de-ionized water.
6
An aqueous stock solution of PEG (50 mg/mL) was prepared by dissolving dried PEG in de-ionized water and occasional shaking until a homogeneous solution was obtained (ca. 24 h). The aqueous dispersions of GO/PEG were prepared by mixing stock solutions of GO and PEG according to the required volume ratio. ultrasonicated for 15 min.
The mixed dispersions were further
The concentration of GO (cg, in mg/mL) in all GO/PEG
dispersions was fixed at 15 mg/mL, while the concentration of PEG (cp, in mg/mL) was changed from 0 to 20 mg/mL.
2.3 Characterization Fourier transform infrared (FT-IR) spectrum of the graphite oxide was recorded in the wavenumber range of 400 ~ 4000 cm-1 using a Nicolet 380 spectrometer (Thermoscientific, USA). X-ray diffraction (XRD) measurements were performed on a LabX XRD-6000 (Shimadzu, Japan) using Cu-Kα radiation (λ = 0.154 nm) in the scattering range (2θ) of 10 ~ 60o with a scan rate of 2 o/min.
The Raman spectrum of the graphite oxide was acquired at room
temperature using LabRAM-HR (Horiba Jobin Yvon, France) over the range of 300 ~ 3000 cm-1.
The sheet dimensions and thickness of GO were characterized by AFM images
recorded in tapping mode (SPA-300HV & SPI3800N, Seiko, Japan).
Sample for AFM
imaging was prepared by depositing diluted aqueous dispersion of GO (0.1 mg/mL) on freshly cleaved mica surface and dried in air at 25 oC.
The zeta-potential of aqueous
dispersions of GO containing PEG was measured with a Zetasizer Nano-ZS90 (Malvern, Britain) at 25 oC. The aqueous dispersions of GO/PEG (cg = 15 mg/mL) with different PEG concentration were injected into the sample cell. The zeta-potential value was the average of at least five successive measurements.
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2.4 Rheological measurements Rheological measurements were carried out at 25 oC with a stress-controlled rheometer ARG2 (TA, USA) using a cone and plate geometry with diameter of 25 mm and cone angle of 2o. Silicone oil was laid on the edge of the fixture plates to prevent solvent evaporation. First, the dynamic strain sweep was carried out at angular frequency of 6.28 rad/s to determine the linear viscoelasticity region.
Then, the dynamic frequency sweep was performed over the
range of 0.1~100 rad/s within the linear viscoelasticity region.
In the steady state shear
measurements, viscosity was monitored as a function of shear rate ranging from 0.001 to 1000 s-1. The shear rate was increased continuously within the 60 s integration time. All samples were allowed to equilibrate for 15 min after loading into the rheometer prior to each measurement.
3. Results and Discussion 3.1 Structure characterization of graphite oxide Graphite oxide prepared by the improved Hummers method was characterized by FT-IR, XRD and Raman spectrum, respectively. The FT-IR spectrum of graphite oxide is shown in Fig. 1 (a). The bands at 1095 and 1735 cm−1, corresponding to the C-OH and C=O stretching vibration of the -COOH group, respectively.
The band at 1620 cm−1 is attributed to the
bending vibration of adsorbed water molecules and the contribution of C=C stretching vibration.
The bands at 3420 and 1390 cm−1 are assigned to -OH stretching and bending
vibrations, respectively.
Thus, the FT-IR spectrum of graphite oxide is well accorded with
the report by Tour et al [27].
Fig. 1 (b) shows the XRD patterns of primitive graphite and
graphite oxide, respectively. The (001) peak of graphite oxide is at 2θ = 9.7o, corresponding to an interlayer spacing of 0.91 nm and the (001) peak of graphite is at 2θ = 26.6o, corresponding to an interlayer spacing of 0.34 nm.
8
It is well-known that Raman
spectroscopy is a powerful and non-destructive technique for the characterization of graphitic materials [28].
Fig. 1 (c) depicts the Raman spectrum of graphite oxide.
Two prominent
peaks in the range of 1300~1650 cm-1 can be observed, which are attributed to the D and G bands of graphite oxide. Therefore, combining the results of FT-IR spectrum, characteristic XRD patterns with Raman spectrum indicate that the graphite oxide is successfully prepared.
3.2 Morphology characterization of GO The AFM topographic image and height section profile display typically single layered GO sheet of 1.1 nm thickness, as shown in Fig. 2 (a). The lateral dimension of the GO sheets was estimated as the statistical average from the histogram of GO sheet sizes, as shown in the Fig. 2 (b). The GO sheets mainly have lateral dimensions in the range of 600 ± 200 nm and the number-average lateral dimension is 570 nm.
Therefore, the graphite oxide is fully
exfoliated into single layer GO by ultrasonication [29].
3.3 Isotropic-nematic transition in aqueous dispersions of GO The effect of GO concentration on the flow behavior of aqueous dispersions of GO was investigated by steady state shear rheology.
Fig. 3 (a) shows steady state shear viscosity
curves of aqueous dispersions of GO with different GO concentration (cg).
For cg ≤ 5
mg/mL, the viscosity η of GO dispersions increases slightly compared with pure water (cg = 0 mg/mL).
As cg further increases, the aqueous GO dispersions changes from Newtonian
fluids to pseudoplastic fluids.
For cg ≥ 6 mg/mL, η decreases with the increasing of shear
rate over the tested region, exhibiting a characteristic shear-thinning behavior due to GO sheets in the dispersions align under the influence of shear flow field. In order to determine the critical concentration cg,c at which the aqueous GO dispersions change from isotropic fluids to nematic liquid crystal, the shear viscosity η is plotted against
9
.
cg at shear rate = 25 and 100 s-1, as depicted in Fig.3 (b). We note that the viscosity η of GO dispersions exhibits non-monotonic behavior with increasing of the cg.
At low GO
concentration (cg ≤ 5 mg/mL), aqueous GO dispersion is in isotropic phase and η slightly increases with increasing of the cg.
Afterwards, η increases up to a critical weight
concentration (cg ~ 6 mg/mL) and get a maximum.
With the further increases in cg, η goes
down and exhibits a minimum before increasing again.
This viscosity reduction above a
critical weight concentration could be attributed to the formation of a nematic liquid crystalline phase. The viscosity minima approximately correspond to the GO concentration for entire nematic phase formation.
The transition from isotropic to nematic phase can be
observed for the GO concentration range between the maxima and minima in shear viscosity [30].
In the present study, cg ~ 6 mg/mL is considered as the critical concentration for the
isotropic-nematic phase transition. If the density of GO (ρg) takes 1.8 g/mL [31], the critical volume fraction c in our study is equal to 0.33%. The c value is well according with the results reported by Kim et al. [30] and close to the theoretical predicated value (0.25%) for polydispersed infinitely thin platelets according to the Onsager’s theory [32]. Small amplitude oscillatory shear (SAOS) rheology can detect the dynamic mechanical response of complex fluids close to equilibrium state [33].
Typical curves of the angular
frequency (ω) dependence of dynamic storage (G′) and loss (G″) moduli for GO dispersions at different GO concentration are shown in Fig.3 (c). For cg = 5 mg/mL, G′ < G″ at all tested frequencies, and the G′ and G″ show apparent frequency dependence, suggesting that GO dispersions behave as viscoelastic fluids. The G′, G′′ ~ curve of aqueous graphene oxide dispersion is fitted with single element Maxwell model and find the G′ ~ 1.8, G′′ ~ 1.3, respectively.
We note that the power index slightly deviate from the value of 2 and 1 for
ideal viscoelastic fluids. This phenomenon maybe due to the aqueous GO dispersion is a kind of complex fluids, its flow behavior could not obey the classic Maxwell model. For cg = 10 10
and 20 mg/mL, GO dispersions show almost frequency-independent G′ and a shallow minimum in G″ at a certain frequency, indicating a solid-like behavior. The above isotropic-nematic phase transition phenomenon could be explained as follows.
Aqueous dispersions of GO constitute a class of 2D-anisotropic colloids with
competing interactions, i.e. long-range electrostatic repulsion, originating from ionized carboxylic acid groups located on the rim of the sheets, and weak attractive interactions originating from the unoxidized graphitic domains [29].
Moreover, according to the
Onsager’s theory, a most essential interaction namely the short-range excluded volume interactions [30,32] should be noted due to the large aspect ratio of GO nanosheets. At very low GO concentration, the GO sheets are individually dispersed in water with weak interaction between GO sheets, thus, the rheological characteristic of GO dispersions is Newtonian flow behavior and in isotropic phase.
While the concentration of GO exceed a
critical concentration cg,c, the GO dispersions could transformed into a nematic liquid crystal [30,32], exhibiting a characteristic shear-thinning behavior under steady shear flow. .
Fig. 4 shows the shear stress (σ) as a function of shear rate ( ) for GO aqueous dispersions with different GO concentration (cg). .
For cg ≤ 5 mg/mL, a linear relationship
.
between σ and (σ ~ 1), indicating a Newtonian flow behavior. For cg ≥ 6 mg/mL, the yield .
stress (σy) can be obtained by fitting the σ~ curves with Herschel-Bulkley equation (σ = σy + .
n
) [34].
The σy is plotted against cg, as shown in the inset of Fig. 5.
It can be clearly
observed that σy firstly decreases and then increases with the increasing of the cg. Therefore, the concentrated aqueous dispersions of GO are typical yield stress fluids.
3.4. Effect of PEG adsorption on the rheology of aqueous GO/PEG dispersions The adsorption of PEG on GO sheets can be intuitively characterized by AFM image. Fig. 5
11
(a) depicts the typical AFM topographic image of GO/PEG aqueous dispersion with cg = 0.1 mg/mL and cp = 0.1 mg/mL. Apparently, the GO sheets are partially covered by PEG chains, which are identified as the pink dots.
However, the lateral dimensions of GO sheets in
GO/PEG dispersion change a little compared with GO in pure water. In order to further explore the interaction between GO and PEG, we perform Raman spectra characterization of freeze-dried samples GO/PEG and pure GO, as shown in Fig. 5 (b).
Two prominent peaks in the range of 1300~1650 cm-1 can be observed, which are
attributed to the D and G bands of GO. As for the sample GO/PEG, the D-band shifts from 1354 cm-1 to 1348 cm-1, while the G-band shifts from 1599 cm-1 to 1595 cm-1 compared with sample GO, respectively.
This red-shift phenomenon indicates that there is a certain
interaction between GO and PEG.
Because the GO carries many oxygen containing
functional groups (hydroxyl group, carbonyl group, carboxyl group and epoxy group, etc.) [29] and PEG chains carry the terminal hydroxyl group and ether bond, so the interaction between GO and PEG could be attributed to hydrogen bond interaction.
Therefore,
combining the results of AFM with Raman spectra, we believe that the PEG chains are successfully adsorbed on the GO sheets in aqueous dispersion. Since zeta-potential (ζ) measurement is an effective way to test the stability of a colloid, we adopted it to evaluate the adsorption effect of PEG on the GO sheets. Fig. 6 shows the zeta-potential of GO dispersions (cg = 15 mg/mL) as a function of PEG concentration. The viscosity of corresponding GO/PEG dispersions was used in calculating the zeta-potential instead of water viscosity. The GO sheets are negatively charge at pH ≈ 7 on being dispersed in water due to ionized carboxylic acid groups. The zeta-potential of pure GO sheets is about -50 mV, which makes them completely dispersed and stable in aqueous dispersion because of the strong electrostatic repulsion. With the addition of PEG, the absolute zeta-potential starts to decrease.
When the PEG concentration (cp) equal to 5 mg/mL, the zeta-potential of the 12
dispersion becomes ca. -22 mV.
This reduction in absolute zeta-potential induced by PEG
addition is due to the adsorption of PEG chains on the GO sheets.
Similar results can be
found in Laponite®/PEG aqueous dispersions reported by Tong et al [35].
As cp further
increases, the zeta-potential changes a little, suggesting that the saturation adsorbing concentration of PEG (cp,s) is about 5 mg/mL. In the present study, the critical overlap concentration (c*) of PEG in aqueous solution is estimated as 2.93 wt%, according to the results reported by Bhatia et al. [36] , which is much larger than the saturation adsorbing concentration cp,s of 5 mg/ml (0.5 wt%) in this study. Obviously, the PEG concentration (cp = 0 ~ 20 mg/mL) in aqueous solution locates at the diluted concentration region rather than the semi-diluted region.
Therefore, continuous
polymer network couldn’t be formed in aqueous PEG solutions. In order to investigate the effect of PEG concentration (cp) on the non-linear flow behavior of GO/PEG aqueous dispersions, we perform steady state shear tests. Fig.7 shows steady state shear viscosity curves of GO/PEG dispersions with different cp at cg = 15 mg/mL. For all PEG concentration, the viscosity η of GO/PEG dispersions decreases with the increasing of shear rate over the tested region, exhibiting a characteristic shear-thinning behavior. However, as cp increases, the viscosity η firstly decreases and then increases, i.e., at cp = 5 mg/mL, the GO/PEG dispersions has the lowest viscosity. The effect of PEG concentration (cp) on the linear viscoelastic behavior of GO/PEG aqueous dispersions was monitored by SAOS frequency sweep.
Fig. 8 depicts the angular
frequency (ω) dependence of dynamic storage (G′) and loss (G″) moduli of aqueous dispersions of GO/PEG at different cp.
For all cp, aqueous dispersions of GO/PEG show
almost frequency-independent G’ and a shallow minimum in G” at a certain frequency, indicating a solid-like behavior. The inset in Fig. 8 shows the plateau moludus (G′p, defined as the storage modulus G′ at ω = 0.1 rad/s) as a function of cp. It is clearly demonstrated that
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G′p decreases firstly and then increases with the increasing of cp. The effect of PEG concentration (cp) on the steady state shear viscosity η and linear storage modulus G′ of aqueous dispersions of GO/PEG could be explained as follows. At cp = 0 mg/mL, i.e. pure concentrated GO dispersion (cg = 15 mg/mL) is in liquid crystal phase. When adding water-soluble PEG into aqueous GO dispersion, the PEG chains are adsorbed on the GO sheets, which are verified by the AFM image and zeta-potential measurements. Significantly, the aqueous GO/PEG dispersions present a lowest viscosity and minimum storage modulus G′(ω
= 0.1 rad/s)
at the saturation adsorbing concentration cp,s.
These
phenomena suggest that there is attractive interaction in GO/PEG dispersion. Specifically, it may be attributed to the bridging effect or depletion interaction.
The mean end-to-end
distance of PEG (20 kDa) is estimated as 17 nm according to the reports by Bhatia et al. [36] and the interlayer spacing of graphene oxide (15 mg/mL) is about 50 nm according to the small-angle X-ray scattering (SAXS) results reported by Gao et al [37]. Therefore, the PEG bridging interaction could be existed in GO/PEG dispersion.
However, the depletion
attractive interaction [38] caused by free PEG chains in aqueous dispersions when the PEG concentration above the value of saturation adsorbing concentration cp,s cannot be excluded. Therefore, we don't confirm the type of the interaction and which kind of interaction actually dominates in aqueous GO/PEG dispersion in the present study. This weak attraction interaction can induce the GO sheets to aggregate and thus clusters of GO would be formed, which decreases the effective volume fraction of GO in aqueous dispersions. As a result, the nematic liquid crystal phase should partially melt, so the linear storage modulus G′ and viscosity η decrease. However, we cannot observe the solid-liquid transition like Laponite®/PEG systems [25] in aqueous GO/PEG dispersions. This could be due to the large aspect ratio of GO sheet (~ 570) compared with Laponite® sheet (~ 25) [39]. Further increases the concentration of PEG
14
enhances the strength of attraction interaction, and thus the linear elastic modulus G′ (elasticity) again increases. Therefore, the PEG concentration has a significant effect on the linear and non-linear viscoelastic behavior of aqueous GO dispersions.
4. Conclusions To summarize, we have investigated the effects of the concentration of GO and PEG on the colloidal and rheological behavior of aqueous dispersions of graphene oxide.
The GO
dispersions changed from Newton fluids to pseudoplastic fluids with the increasing of the concentration of GO.
The concentrated GO dispersions were liquid crystal phase and the
critical concentration of isotropic-nematic transition was about 6 mg/mL. Moreover, the redshift of D and G bands in the Raman spectra of GO/PEG compared with pure GO suggested that there was hydrogen bond interaction between GO and PEG, and the results of AFM images and zeta-potential measurements clearly demonstrated that PEG chains were effectively adsorbed on the GO sheets.
The results of SAOS frequency sweep and steady
state shear tests demonstrated that the PEG concentration had a significant effect on the linear and non-linear rheological behavior of GO/PEG dispersions. These results are an important step towards understanding the interplay between microstructure and mechanical response of aqueous dispersions of GO/PEG or GO/other polymers, and also provide some guidance for the processing of GO/polymer nanocomposites.
Acknowledgements This work was financially supported by the Natural Science Foundation of China (Grant No. 51507003; 51477002) and the Doctor’s Start-up Research Foundation of Anhui University of Science and Technology.
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stabilized particles, Langmuir 21 (2005) 9964-9969. [21] D. Qiu, T. Cosgrove, A.M. Howe, Steric interactions between physically adsorbed polymer-coated colloidal particles: Soft or hard?, Langmuir 23 (2007) 475-481. [22] A.A. Zaman, N. Delorme, Effect of polymer bridging on rheological properties of dispersions of charged silica particles in the presence of low-molecular-weight physically adsorbed poly (ethylene oxide), Rheol. Acta 41 (2002) 408-417. [23] A. Alemdar, N. Güngör, O. Ece, O. Atici, The rheological properties and characterization of bentonite dispersions in the presence of non-ionic polymer PEG, J. Mater. Sci. 40 (2005) 171-177. [24] W. Sun, Y. Yang, T. Wang, H. Huang, X. Liu, Z. Tong, Effect of adsorbed poly (ethylene glycol) on the gelation evolution of Laponite suspensions: Aging time-polymer concentration superposition, J. Colloid Interf. Sci. 376 (2012) 76-82. [25] A.K. Atmuri, G.A. Peklaris, S. Kishore, S.R. Bhatia, A re-entrant glass transition in colloidal disks with adsorbing polymer, Soft Matter 8 (2012) 8965-8971. [26] L. Zulian, F.A. de Melo Marques, E. Emilitri, G. Ruocco, B. Ruzicka, Dual aging behaviour in a clay–polymer dispersion, Soft Matter10 (2014) 4513-4521. [27] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, ACS Nano 4 (2010) 4806-4814. [28] K.N. Kudin, B. Ozbas, H.C. Schniepp, R.K. Prud'Homme, I.A. Aksay, R. Car, Raman spectra of graphite oxide and functionalized graphene sheets, Nano Lett. 8 (2008) 36-41. [29] B. Konkena, S. Vasudevan, Glass, Gel, and Liquid Crystals: Arrested States of Graphene Oxide Aqueous Dispersions, J. Phys. Chem. C 118 (2014) 21706-21713. [30] P. Kumar, U.N. Maiti, K.E. Lee, S.O. Kim, Rheological properties of graphene oxide liquid crystal, Carbon 80 (2014) 453-461.
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[31] J.E. Kim, T.H. Han, S.H. Lee, J.Y. Kim, C.W. Ahn, J.M. Yun, S.O. Kim, Graphene oxide liquid crystals, Angew. Chem. Int. Edit. 50 (2011) 3043-3047. [32] M.A. Bates, D. Frenkel, Nematic–isotropic transition in polydisperse systems of infinitely thin hard platelets, J. Chem. Phys. 110 (1999) 6553-6559. [33] J. Mewis, N.J. Wagner, Colloidal suspension rheology, Cambridge University Press 2012. [34] N. Koumakis, A. Pamvouxoglou, A. Poulos, G. Petekidis, Direct comparison of the rheology of model hard and soft particle glasses, Soft Matter 8 (2012) 4271-4284. [35] X. Hu, T. Wang, L. Xiong, C. Wang, X. Liu, Z. Tong, Preferential adsorption of poly (ethylene glycol) on hectorite clay and effects on poly (N-isopropylacrylamide)/hectorite nanocomposite hydrogels, Langmuir 26 (2009) 4233-4238. [36] S. Kishore, Y. Chen, P. Ravindra, S.R. Bhatia, The effect of particle-scale dynamics on the macroscopic properties of disk-shaped colloid–polymer systems, Colloid. Surface. A 482 (2015) 585-595. [37] Z. Xu, C. Gao, Graphene chiral liquid crystals and macroscopic assembled fibres, Nat. Commun. 2 (2011) 571. [38] W. Poon, The physics of a model colloid-polymer mixture, J. Phys-Condens. Mat. 14 (2002) R859-R880. [39] B. Ruzicka, E. Zaccarelli, A fresh look at the Laponite phase diagram, Soft Matter 7 (2011) 1268-1286.
19
Figure captions
20
(a)
1095
3420
1735 1620 1390
Transmittance (%)
graphite oxide
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
(001)
Intensity (a.u.)
(b)
graphite oxide (001)
primitive graphite 10
20
30
40
50
2(degree)
(c) Intensity (a.u.)
G D
400
800
1200
1600 2000 2400 -1
2800
Raman shfit (cm )
Fig.1.(a) FT-IR spectrum of graphite oxide; (b) XRD patterns of primitive graphite and graphite oxide; (c) Raman spectrum of graphite oxide.
21
(a)
(b) Percentage (%)
40
30
20
10
0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Width (m)
Fig.2. Tapping-mode AFM topographic images of GO aqueous dispersion: (a) The height profile for a single GO sheet; (b) The size and size distribution of GO sheets.
22
23
(a)
cg / (mg/mL)
GO
20 15 10 8 6 5 4 2 1 0
3
10
(Pa.s)
102 101 100 10-1 10-2 10-3 10-3
10-2
10-1
100
101
102
103
-1
(s ) (b) 0.20 GO
25 s-1
(Pa.s)
0.15
0.10
0.05
100 s-1
0.00 0
5
10
15
20
cg (mg/mL) (c)
103
G' G''
GO
20 mg/mL
G',G'' (Pa)
102 101
10 mg/mL
100 10-1
5 mg/mL G'' ~ 1.3
10-2
G' ~ 1.8 -3
10
10-1
100
101
(rad/s)
Fig.3. (a) Steady state shear viscosity curves of aqueous GO dispersions with different .
GO concentration (cg); (b) Shear viscosityη of GO dispersions as a function of cg at = 25 and 100 s-1; (c) Angular frequency dependence of the storage modulus G′ (filled symbols) and loss modulus G″ (open symbols) of GO dispersions with differentcg. Dashed lined indicates the Maxwell model fitting result.
24
10
y (Pa)
3
2
GO
2
cg / (mg/mL)
1 0 4 6 8 10 12 14 16 18 20
(Pa)
1
10
cg (mg/mL)
100 10-1
~ 1 10-2 10-3
10-2
10-1
100
-1
101
102
20 15 10 8 6 5 4 2 1 0
103
(s ) .
Fig. 4.Shear stress (σ) versus shear rate ( ) curves of aqueous GO dispersions with different GO concentration (cg). Inset: yield stress (σy) of aqueous GOdispersions as a function of cg.
25
-20
GO/PEG
(mV)
-25 -30 -35
cg = 15 mg/mL
-40 -45 -50 0
5
10
15
20
cp (mg/mL) Fig.6.Zeta-potential (ζ) of aqueousGO/PEG dispersions as a function of PEG concentration (cp) with GO concentration cg = 15 mg/mL.
GO/PEG
(Pa.s)
103
cg = 15 mg/mL
102
cp / (mg/mL) 0 5 10 20
101 100 10-1 10-2 10-3
10-2
10-1
100
101
102
103
(s-1) Fig.7.Steady state shear viscosity curves of aqueousGO/PEG dispersions with different cpat GO concentration cg = 15 mg/mL.
26
103
Gp' (Pa)
150
= 0.1 rad/s
100 50
GO/PEG
G',G'' (Pa)
0 0
5 10 15 cp (mg/mL)
20
cg = 15 mg/mL
2
10
cp / (mg/mL)
1
10
0 5 10 20
100 10-1
100
101
(rad/s)
102
Fig.8.Angular frequency dependence of the storage modulus G′ (filled symbols) and loss modulus G′′ (open symbols) of aqueousGO/PEG dispersions with differentcpat GO concentration cg = 15 mg/mL.Inset: plateau modulus (G′p) versuscp curve.
27
S0927-7757(15)30267-3 http://dx.doi.org/doi:10.1016/j.colsurfa.2015.10.006 COLSUA 20214
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
22-8-2015 6-10-2015 7-10-2015
Please cite this article as: Ruiwen Shu, Qing Yin, Honglong Xing, Dexin Tan, Ying Gan, Guocai Xu, Colloidal and rheological behavior of aqueous graphene oxide dispersions in the presence of poly(ethylene glycol), Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2015.10.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Colloidal and rheological behavior of aqueous graphene oxide dispersions in the presence of poly(ethylene glycol)
Ruiwen Shu*, Qing Yin, Honglong Xing*, Dexin Tan, Ying Gan and Guocai Xu
School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, China
*Corresponding authors E-mail: [email protected] (R. Shu), [email protected] (H. Xing) Tel: (86)-554-6668497
Graphical abstract:
GO/PEG
3
GO
10
25 s -1
= 0.1 rad/s
100 50
GO/PEG
G',G'' (Pa)
0
0.15
(Pa.s)
G p ' (Pa)
150
0.20
0.10
0
5 10 15 cp (mg/mL)
20
cg = 15 mg/mL
2
10
cp / (mg/mL)
1
10 0.05
0 5 10 20
-1
100 s
100 0.00 0
5
10
15
20
10
cg (mg/mL)
1
-1
10
0
10
(rad/s)
1
2
10
The effects of the concentration of graphene oxide (GO) and poly(ethylene glycol) (PEG) on the colloidal and rheological behavior of GO aqueous dispersions were investigated.
The
critical concentration of isotropic-nematicphase transition of GO aqueous dispersions was about 6 mg/mL. Significantly, the aqueous GO/PEG dispersions presented a lowest viscosity and minimum storage modulus G′(ω
= 0.1 rad/s)
at the saturation adsorbing concentrationcp,s.
Therefore, the PEG concentration had a significant effect on the linear and non-linear rheological behavior of GO/PEG dispersions.
Highlights:
The critical concentration of isotropic-nematic phase transition for aqueous GO dispersions was determined by shear rheology.
Effect of PEG concentration on the linear and non-linear rheological behavior of aqueous GO/PEG dispersions was firstly studied.
PEG chains adsorbing effects were studied by Atomic force microscopy images and zeta-potential measurements.
2
Abstract In this work, the effects of the concentration of graphene oxide (GO) and poly(ethylene glycol) (PEG) on the colloidal and rheological behavior of aqueous GO dispersions were investigated. mg/mL, respectively.
The concentration of GO and PEG was varied from 0 to 20
Atomic force microscopy (AFM) images and zeta-potential
measurements demonstrated that PEG chains were adsorbed on the colloidal GO sheets. The red-shift of D and G bands in the Raman spectra of GO/PEG compared with pure GO suggested that there was hydrogen bond interaction between GO and PEG.
Steady state
shear results indicated that GO aqueous dispersions changed from Newton fluids to pseudoplastic fluids as the concentration of GO increased.
The critical concentration of
isotropic–nematic phase transition of aqueous GO dispersions was about 6 mg/mL. Significantly, the aqueous GO/PEG dispersions presented a lowest viscosity and minimum storage modulus G′(ω
= 0.1 rad/s)
at the saturation adsorbing concentration cp,s, which were
determined by steady state shear and small amplitude oscillatory frequency sweep, respectively. Therefore, the PEG concentration had a significant effect on the linear and nonlinear rheological behavior of GO/PEG dispersions.
In addition, the concentrated aqueous
GO dispersions showed typical yield flow behavior and the yield stress σy firstly decreased and then increased with the increasing of GO concentration. These results could contribute to understand the interplay between microstructure and mechanical behavior of GO/polymer in aqueous dispersions, and also provide some guidance for the processing of GO-based polymer nanocomposites.
Keywords graphene oxide; poly(ethylene glycol); isotropic–nematic phase transition; small amplitude oscillatory shear; yield stress
3
1. Introduction Graphene is a two-dimension sheet of single atomic thick layer of hexagonally arranged carbon atom [1,2].
Graphene has attracted significant interests due to its extraordinary
physical properties which make it an ideal candidate for a wide range of applications, such as actuators [3,4], supercapacitors [5], hydrogen storage [6], nanocomposites [7]. In particular, graphene/polymer nanocomposites have become a hot topic in the field of polymer-based nanocomposites due to their excellent electrical, mechanical properties and thermal stability [8-11].
However, graphene/polymer nanocomposites still face several
challenges during processing, such as homogenous dispersion of graphene in polymer matrix, interfacial interactions and the effect of graphene fillers on the viscoelasticity of nanocomposites [9,11]. It is well-known that rheological characterization can provide much valuable information on the processing property of polymer-based nanocomposites. Therefore, it is necessary to explore the rheological performance of graphene/polymer nanocomposites.
Up to now, there have been many reports on the rheological behavior of
graphene/polymer nanocomposites, while which mainly focused their attention on graphene/polymer melts [12-15].
However, the graphene are usually processed from
graphene oxide (GO) dispersed in some common solvents, like water, N,N’-di methyl for mami de (DMF), and N-methyl pyrrolidone (NMP) [16].
Moreover, aqueous GO
dispersions have been widely used as precursor to produce chemically converted graphene in large-scale [17,18].
Thus, it is fundamentally important to investigate the rheological
behavior of aqueous dispersions of GO/polymer.
However, to the best of our knowledge,
there are few reports on the rheological behavior of aqueous dispersions of GO/polymer so far [19]. Poly(ethylene glycol) (PEG) is a common water-soluble non-ionic polymer which has been widely used as a kind of model polymers to tailor the interaction between colloidal
4
particles in aqueous dispersions [20,21]. The colloidal and rheological behavior of aqueous dispersions of inorganic nanoparticles, such as silica sphere [22], montmorillonoid [23], and Laponite® [24-26] in the presence of PEG have been widely reported. Therefore, PEG could be an ideal polymeric additive to explore the colloidal and rheological behavior of aqueous dispersions of GO/polymer. In the present study, we investigated the effects of the concentration of GO and PEG on the colloidal and rheological behavior of GO aqueous dispersions through atomic force microscopy (AFM), zeta-potential and rheological measurements. It was found that the GO sheets with a single atomic layer thickness were successfully prepared by ultrasonic exfoliation with an aspect ratio of ~ 570.
The concentrated aqueous dispersions of GO
showed characteristic shear-thinning behavior, and thus could be classified as a kind of yield stress fluids.
The adsorbing behavior of PEG on GO sheets in aqueous dispersions were
probed by AFM and zeta-potential measurements.
The critical concentration of isotropic–
nematic liquid crystal transition in aqueous GO dispersions was approximate 6 mg/mL. Furthermore, the effect of the concentration of PEG on the rheological properties of aqueous dispersions of GO was probed by small amplitude oscillatory frequency sweep and steady state shear tests. Significantly, a lowest viscosity and minimum storage modulus G′(ω = 0.1 rad/s) at the saturation adsorbing concentration cp,s were observed in the aqueous GO/PEG dispersions with the increasing of PEG concentration. Therefore, the study of colloidal and rheological behavior of aqueous dispersions of GO/PEG could contribute to understand the phase transition of a class of two-dimensional anisotropic colloid sheets with large aspect ratio and also maybe provide some guidance for the processing of GO/polymer nanocomposites.
5
2. Experimental section 2.1 Materials Natural graphite flakes (purity degree > 99%, Qingdao Huatai Lubricant Sealing S&T Co. Ltd., Qingdao, China) were used after dried at 60 oC in vacuum for 12 h.
Poly(ethylene
glycol) (PEG, analytical grade) with molecular weight Mw of 20000 g/mol was used after dried at 50 oC in vacuum for 12 h.
All other reagents were analytical grade and used as
received without further purification. Water was purified by deionization and filtration with a Millipore purification apparatus (18.2 MΩ·cm).
2.2 Sample preparation Graphite oxide was prepared by the improved Hummers′ method [27]. Typically, a mixture of concentrated H2SO4/H3PO4 (360:40 mL) was first added to a mixture of graphite flakes (3.0 g) and KMnO4 (18.0 g). Then, the reaction mixture was heated to 50 °C and stirred for 12 h.
Next, the reaction was cooled to room temperature and poured onto ice (400 mL).
Lastly, H2O2 (30%) was added drop-wise into the reaction mixture until the color of the solution became bright yellow.
In order to remove the excess metal salts, the mixture was
filtered and washed with 1:10 HCl aqueous solution.
The product was firstly purified by
centrifugation at 12000 rpm for 20 min, decantation and re-dispersed in de-ionized water repeatedly for several times until the pH of the dispersion became neutral (pH ≈ 7).
Then,
the product was subjected to dialysis of two weeks to completely remove residual salts and acids. The as-prepared graphite oxide was freeze dried under a vacuum for 48 h. The aqueous stock dispersion of GO (20 mg/mL) was achieved by dispersing 0.2 g of graphite oxide into 10 mL of de-ionized water using an ultrasonicator (180 W) for 1.5 h at room temperature. A series of aqueous dispersions of GO with desired concentration were acquired by diluting the GO stock dispersion with de-ionized water.
6
An aqueous stock solution of PEG (50 mg/mL) was prepared by dissolving dried PEG in de-ionized water and occasional shaking until a homogeneous solution was obtained (ca. 24 h). The aqueous dispersions of GO/PEG were prepared by mixing stock solutions of GO and PEG according to the required volume ratio. ultrasonicated for 15 min.
The mixed dispersions were further
The concentration of GO (cg, in mg/mL) in all GO/PEG
dispersions was fixed at 15 mg/mL, while the concentration of PEG (cp, in mg/mL) was changed from 0 to 20 mg/mL.
2.3 Characterization Fourier transform infrared (FT-IR) spectrum of the graphite oxide was recorded in the wavenumber range of 400 ~ 4000 cm-1 using a Nicolet 380 spectrometer (Thermoscientific, USA). X-ray diffraction (XRD) measurements were performed on a LabX XRD-6000 (Shimadzu, Japan) using Cu-Kα radiation (λ = 0.154 nm) in the scattering range (2θ) of 10 ~ 60o with a scan rate of 2 o/min.
The Raman spectrum of the graphite oxide was acquired at room
temperature using LabRAM-HR (Horiba Jobin Yvon, France) over the range of 300 ~ 3000 cm-1.
The sheet dimensions and thickness of GO were characterized by AFM images
recorded in tapping mode (SPA-300HV & SPI3800N, Seiko, Japan).
Sample for AFM
imaging was prepared by depositing diluted aqueous dispersion of GO (0.1 mg/mL) on freshly cleaved mica surface and dried in air at 25 oC.
The zeta-potential of aqueous
dispersions of GO containing PEG was measured with a Zetasizer Nano-ZS90 (Malvern, Britain) at 25 oC. The aqueous dispersions of GO/PEG (cg = 15 mg/mL) with different PEG concentration were injected into the sample cell. The zeta-potential value was the average of at least five successive measurements.
7
2.4 Rheological measurements Rheological measurements were carried out at 25 oC with a stress-controlled rheometer ARG2 (TA, USA) using a cone and plate geometry with diameter of 25 mm and cone angle of 2o. Silicone oil was laid on the edge of the fixture plates to prevent solvent evaporation. First, the dynamic strain sweep was carried out at angular frequency of 6.28 rad/s to determine the linear viscoelasticity region.
Then, the dynamic frequency sweep was performed over the
range of 0.1~100 rad/s within the linear viscoelasticity region.
In the steady state shear
measurements, viscosity was monitored as a function of shear rate ranging from 0.001 to 1000 s-1. The shear rate was increased continuously within the 60 s integration time. All samples were allowed to equilibrate for 15 min after loading into the rheometer prior to each measurement.
3. Results and Discussion 3.1 Structure characterization of graphite oxide Graphite oxide prepared by the improved Hummers method was characterized by FT-IR, XRD and Raman spectrum, respectively. The FT-IR spectrum of graphite oxide is shown in Fig. 1 (a). The bands at 1095 and 1735 cm−1, corresponding to the C-OH and C=O stretching vibration of the -COOH group, respectively.
The band at 1620 cm−1 is attributed to the
bending vibration of adsorbed water molecules and the contribution of C=C stretching vibration.
The bands at 3420 and 1390 cm−1 are assigned to -OH stretching and bending
vibrations, respectively.
Thus, the FT-IR spectrum of graphite oxide is well accorded with
the report by Tour et al [27].
Fig. 1 (b) shows the XRD patterns of primitive graphite and
graphite oxide, respectively. The (001) peak of graphite oxide is at 2θ = 9.7o, corresponding to an interlayer spacing of 0.91 nm and the (001) peak of graphite is at 2θ = 26.6o, corresponding to an interlayer spacing of 0.34 nm.
8
It is well-known that Raman
spectroscopy is a powerful and non-destructive technique for the characterization of graphitic materials [28].
Fig. 1 (c) depicts the Raman spectrum of graphite oxide.
Two prominent
peaks in the range of 1300~1650 cm-1 can be observed, which are attributed to the D and G bands of graphite oxide. Therefore, combining the results of FT-IR spectrum, characteristic XRD patterns with Raman spectrum indicate that the graphite oxide is successfully prepared.
3.2 Morphology characterization of GO The AFM topographic image and height section profile display typically single layered GO sheet of 1.1 nm thickness, as shown in Fig. 2 (a). The lateral dimension of the GO sheets was estimated as the statistical average from the histogram of GO sheet sizes, as shown in the Fig. 2 (b). The GO sheets mainly have lateral dimensions in the range of 600 ± 200 nm and the number-average lateral dimension is 570 nm.
Therefore, the graphite oxide is fully
exfoliated into single layer GO by ultrasonication [29].
3.3 Isotropic-nematic transition in aqueous dispersions of GO The effect of GO concentration on the flow behavior of aqueous dispersions of GO was investigated by steady state shear rheology.
Fig. 3 (a) shows steady state shear viscosity
curves of aqueous dispersions of GO with different GO concentration (cg).
For cg ≤ 5
mg/mL, the viscosity η of GO dispersions increases slightly compared with pure water (cg = 0 mg/mL).
As cg further increases, the aqueous GO dispersions changes from Newtonian
fluids to pseudoplastic fluids.
For cg ≥ 6 mg/mL, η decreases with the increasing of shear
rate over the tested region, exhibiting a characteristic shear-thinning behavior due to GO sheets in the dispersions align under the influence of shear flow field. In order to determine the critical concentration cg,c at which the aqueous GO dispersions change from isotropic fluids to nematic liquid crystal, the shear viscosity η is plotted against
9
.
cg at shear rate = 25 and 100 s-1, as depicted in Fig.3 (b). We note that the viscosity η of GO dispersions exhibits non-monotonic behavior with increasing of the cg.
At low GO
concentration (cg ≤ 5 mg/mL), aqueous GO dispersion is in isotropic phase and η slightly increases with increasing of the cg.
Afterwards, η increases up to a critical weight
concentration (cg ~ 6 mg/mL) and get a maximum.
With the further increases in cg, η goes
down and exhibits a minimum before increasing again.
This viscosity reduction above a
critical weight concentration could be attributed to the formation of a nematic liquid crystalline phase. The viscosity minima approximately correspond to the GO concentration for entire nematic phase formation.
The transition from isotropic to nematic phase can be
observed for the GO concentration range between the maxima and minima in shear viscosity [30].
In the present study, cg ~ 6 mg/mL is considered as the critical concentration for the
isotropic-nematic phase transition. If the density of GO (ρg) takes 1.8 g/mL [31], the critical volume fraction c in our study is equal to 0.33%. The c value is well according with the results reported by Kim et al. [30] and close to the theoretical predicated value (0.25%) for polydispersed infinitely thin platelets according to the Onsager’s theory [32]. Small amplitude oscillatory shear (SAOS) rheology can detect the dynamic mechanical response of complex fluids close to equilibrium state [33].
Typical curves of the angular
frequency (ω) dependence of dynamic storage (G′) and loss (G″) moduli for GO dispersions at different GO concentration are shown in Fig.3 (c). For cg = 5 mg/mL, G′ < G″ at all tested frequencies, and the G′ and G″ show apparent frequency dependence, suggesting that GO dispersions behave as viscoelastic fluids. The G′, G′′ ~ curve of aqueous graphene oxide dispersion is fitted with single element Maxwell model and find the G′ ~ 1.8, G′′ ~ 1.3, respectively.
We note that the power index slightly deviate from the value of 2 and 1 for
ideal viscoelastic fluids. This phenomenon maybe due to the aqueous GO dispersion is a kind of complex fluids, its flow behavior could not obey the classic Maxwell model. For cg = 10 10
and 20 mg/mL, GO dispersions show almost frequency-independent G′ and a shallow minimum in G″ at a certain frequency, indicating a solid-like behavior. The above isotropic-nematic phase transition phenomenon could be explained as follows.
Aqueous dispersions of GO constitute a class of 2D-anisotropic colloids with
competing interactions, i.e. long-range electrostatic repulsion, originating from ionized carboxylic acid groups located on the rim of the sheets, and weak attractive interactions originating from the unoxidized graphitic domains [29].
Moreover, according to the
Onsager’s theory, a most essential interaction namely the short-range excluded volume interactions [30,32] should be noted due to the large aspect ratio of GO nanosheets. At very low GO concentration, the GO sheets are individually dispersed in water with weak interaction between GO sheets, thus, the rheological characteristic of GO dispersions is Newtonian flow behavior and in isotropic phase.
While the concentration of GO exceed a
critical concentration cg,c, the GO dispersions could transformed into a nematic liquid crystal [30,32], exhibiting a characteristic shear-thinning behavior under steady shear flow. .
Fig. 4 shows the shear stress (σ) as a function of shear rate ( ) for GO aqueous dispersions with different GO concentration (cg). .
For cg ≤ 5 mg/mL, a linear relationship
.
between σ and (σ ~ 1), indicating a Newtonian flow behavior. For cg ≥ 6 mg/mL, the yield .
stress (σy) can be obtained by fitting the σ~ curves with Herschel-Bulkley equation (σ = σy + .
n
) [34].
The σy is plotted against cg, as shown in the inset of Fig. 5.
It can be clearly
observed that σy firstly decreases and then increases with the increasing of the cg. Therefore, the concentrated aqueous dispersions of GO are typical yield stress fluids.
3.4. Effect of PEG adsorption on the rheology of aqueous GO/PEG dispersions The adsorption of PEG on GO sheets can be intuitively characterized by AFM image. Fig. 5
11
(a) depicts the typical AFM topographic image of GO/PEG aqueous dispersion with cg = 0.1 mg/mL and cp = 0.1 mg/mL. Apparently, the GO sheets are partially covered by PEG chains, which are identified as the pink dots.
However, the lateral dimensions of GO sheets in
GO/PEG dispersion change a little compared with GO in pure water. In order to further explore the interaction between GO and PEG, we perform Raman spectra characterization of freeze-dried samples GO/PEG and pure GO, as shown in Fig. 5 (b).
Two prominent peaks in the range of 1300~1650 cm-1 can be observed, which are
attributed to the D and G bands of GO. As for the sample GO/PEG, the D-band shifts from 1354 cm-1 to 1348 cm-1, while the G-band shifts from 1599 cm-1 to 1595 cm-1 compared with sample GO, respectively.
This red-shift phenomenon indicates that there is a certain
interaction between GO and PEG.
Because the GO carries many oxygen containing
functional groups (hydroxyl group, carbonyl group, carboxyl group and epoxy group, etc.) [29] and PEG chains carry the terminal hydroxyl group and ether bond, so the interaction between GO and PEG could be attributed to hydrogen bond interaction.
Therefore,
combining the results of AFM with Raman spectra, we believe that the PEG chains are successfully adsorbed on the GO sheets in aqueous dispersion. Since zeta-potential (ζ) measurement is an effective way to test the stability of a colloid, we adopted it to evaluate the adsorption effect of PEG on the GO sheets. Fig. 6 shows the zeta-potential of GO dispersions (cg = 15 mg/mL) as a function of PEG concentration. The viscosity of corresponding GO/PEG dispersions was used in calculating the zeta-potential instead of water viscosity. The GO sheets are negatively charge at pH ≈ 7 on being dispersed in water due to ionized carboxylic acid groups. The zeta-potential of pure GO sheets is about -50 mV, which makes them completely dispersed and stable in aqueous dispersion because of the strong electrostatic repulsion. With the addition of PEG, the absolute zeta-potential starts to decrease.
When the PEG concentration (cp) equal to 5 mg/mL, the zeta-potential of the 12
dispersion becomes ca. -22 mV.
This reduction in absolute zeta-potential induced by PEG
addition is due to the adsorption of PEG chains on the GO sheets.
Similar results can be
found in Laponite®/PEG aqueous dispersions reported by Tong et al [35].
As cp further
increases, the zeta-potential changes a little, suggesting that the saturation adsorbing concentration of PEG (cp,s) is about 5 mg/mL. In the present study, the critical overlap concentration (c*) of PEG in aqueous solution is estimated as 2.93 wt%, according to the results reported by Bhatia et al. [36] , which is much larger than the saturation adsorbing concentration cp,s of 5 mg/ml (0.5 wt%) in this study. Obviously, the PEG concentration (cp = 0 ~ 20 mg/mL) in aqueous solution locates at the diluted concentration region rather than the semi-diluted region.
Therefore, continuous
polymer network couldn’t be formed in aqueous PEG solutions. In order to investigate the effect of PEG concentration (cp) on the non-linear flow behavior of GO/PEG aqueous dispersions, we perform steady state shear tests. Fig.7 shows steady state shear viscosity curves of GO/PEG dispersions with different cp at cg = 15 mg/mL. For all PEG concentration, the viscosity η of GO/PEG dispersions decreases with the increasing of shear rate over the tested region, exhibiting a characteristic shear-thinning behavior. However, as cp increases, the viscosity η firstly decreases and then increases, i.e., at cp = 5 mg/mL, the GO/PEG dispersions has the lowest viscosity. The effect of PEG concentration (cp) on the linear viscoelastic behavior of GO/PEG aqueous dispersions was monitored by SAOS frequency sweep.
Fig. 8 depicts the angular
frequency (ω) dependence of dynamic storage (G′) and loss (G″) moduli of aqueous dispersions of GO/PEG at different cp.
For all cp, aqueous dispersions of GO/PEG show
almost frequency-independent G’ and a shallow minimum in G” at a certain frequency, indicating a solid-like behavior. The inset in Fig. 8 shows the plateau moludus (G′p, defined as the storage modulus G′ at ω = 0.1 rad/s) as a function of cp. It is clearly demonstrated that
13
G′p decreases firstly and then increases with the increasing of cp. The effect of PEG concentration (cp) on the steady state shear viscosity η and linear storage modulus G′ of aqueous dispersions of GO/PEG could be explained as follows. At cp = 0 mg/mL, i.e. pure concentrated GO dispersion (cg = 15 mg/mL) is in liquid crystal phase. When adding water-soluble PEG into aqueous GO dispersion, the PEG chains are adsorbed on the GO sheets, which are verified by the AFM image and zeta-potential measurements. Significantly, the aqueous GO/PEG dispersions present a lowest viscosity and minimum storage modulus G′(ω
= 0.1 rad/s)
at the saturation adsorbing concentration cp,s.
These
phenomena suggest that there is attractive interaction in GO/PEG dispersion. Specifically, it may be attributed to the bridging effect or depletion interaction.
The mean end-to-end
distance of PEG (20 kDa) is estimated as 17 nm according to the reports by Bhatia et al. [36] and the interlayer spacing of graphene oxide (15 mg/mL) is about 50 nm according to the small-angle X-ray scattering (SAXS) results reported by Gao et al [37]. Therefore, the PEG bridging interaction could be existed in GO/PEG dispersion.
However, the depletion
attractive interaction [38] caused by free PEG chains in aqueous dispersions when the PEG concentration above the value of saturation adsorbing concentration cp,s cannot be excluded. Therefore, we don't confirm the type of the interaction and which kind of interaction actually dominates in aqueous GO/PEG dispersion in the present study. This weak attraction interaction can induce the GO sheets to aggregate and thus clusters of GO would be formed, which decreases the effective volume fraction of GO in aqueous dispersions. As a result, the nematic liquid crystal phase should partially melt, so the linear storage modulus G′ and viscosity η decrease. However, we cannot observe the solid-liquid transition like Laponite®/PEG systems [25] in aqueous GO/PEG dispersions. This could be due to the large aspect ratio of GO sheet (~ 570) compared with Laponite® sheet (~ 25) [39]. Further increases the concentration of PEG
14
enhances the strength of attraction interaction, and thus the linear elastic modulus G′ (elasticity) again increases. Therefore, the PEG concentration has a significant effect on the linear and non-linear viscoelastic behavior of aqueous GO dispersions.
4. Conclusions To summarize, we have investigated the effects of the concentration of GO and PEG on the colloidal and rheological behavior of aqueous dispersions of graphene oxide.
The GO
dispersions changed from Newton fluids to pseudoplastic fluids with the increasing of the concentration of GO.
The concentrated GO dispersions were liquid crystal phase and the
critical concentration of isotropic-nematic transition was about 6 mg/mL. Moreover, the redshift of D and G bands in the Raman spectra of GO/PEG compared with pure GO suggested that there was hydrogen bond interaction between GO and PEG, and the results of AFM images and zeta-potential measurements clearly demonstrated that PEG chains were effectively adsorbed on the GO sheets.
The results of SAOS frequency sweep and steady
state shear tests demonstrated that the PEG concentration had a significant effect on the linear and non-linear rheological behavior of GO/PEG dispersions. These results are an important step towards understanding the interplay between microstructure and mechanical response of aqueous dispersions of GO/PEG or GO/other polymers, and also provide some guidance for the processing of GO/polymer nanocomposites.
Acknowledgements This work was financially supported by the Natural Science Foundation of China (Grant No. 51507003; 51477002) and the Doctor’s Start-up Research Foundation of Anhui University of Science and Technology.
15
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Figure captions
20
(a)
1095
3420
1735 1620 1390
Transmittance (%)
graphite oxide
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
(001)
Intensity (a.u.)
(b)
graphite oxide (001)
primitive graphite 10
20
30
40
50
2(degree)
(c) Intensity (a.u.)
G D
400
800
1200
1600 2000 2400 -1
2800
Raman shfit (cm )
Fig.1.(a) FT-IR spectrum of graphite oxide; (b) XRD patterns of primitive graphite and graphite oxide; (c) Raman spectrum of graphite oxide.
21
(a)
(b) Percentage (%)
40
30
20
10
0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Width (m)
Fig.2. Tapping-mode AFM topographic images of GO aqueous dispersion: (a) The height profile for a single GO sheet; (b) The size and size distribution of GO sheets.
22
23
(a)
cg / (mg/mL)
GO
20 15 10 8 6 5 4 2 1 0
3
10
(Pa.s)
102 101 100 10-1 10-2 10-3 10-3
10-2
10-1
100
101
102
103
-1
(s ) (b) 0.20 GO
25 s-1
(Pa.s)
0.15
0.10
0.05
100 s-1
0.00 0
5
10
15
20
cg (mg/mL) (c)
103
G' G''
GO
20 mg/mL
G',G'' (Pa)
102 101
10 mg/mL
100 10-1
5 mg/mL G'' ~ 1.3
10-2
G' ~ 1.8 -3
10
10-1
100
101
(rad/s)
Fig.3. (a) Steady state shear viscosity curves of aqueous GO dispersions with different .
GO concentration (cg); (b) Shear viscosityη of GO dispersions as a function of cg at = 25 and 100 s-1; (c) Angular frequency dependence of the storage modulus G′ (filled symbols) and loss modulus G″ (open symbols) of GO dispersions with differentcg. Dashed lined indicates the Maxwell model fitting result.
24
10
y (Pa)
3
2
GO
2
cg / (mg/mL)
1 0 4 6 8 10 12 14 16 18 20
(Pa)
1
10
cg (mg/mL)
100 10-1
~ 1 10-2 10-3
10-2
10-1
100
-1
101
102
20 15 10 8 6 5 4 2 1 0
103
(s ) .
Fig. 4.Shear stress (σ) versus shear rate ( ) curves of aqueous GO dispersions with different GO concentration (cg). Inset: yield stress (σy) of aqueous GOdispersions as a function of cg.
25
-20
GO/PEG
(mV)
-25 -30 -35
cg = 15 mg/mL
-40 -45 -50 0
5
10
15
20
cp (mg/mL) Fig.6.Zeta-potential (ζ) of aqueousGO/PEG dispersions as a function of PEG concentration (cp) with GO concentration cg = 15 mg/mL.
GO/PEG
(Pa.s)
103
cg = 15 mg/mL
102
cp / (mg/mL) 0 5 10 20
101 100 10-1 10-2 10-3
10-2
10-1
100
101
102
103
(s-1) Fig.7.Steady state shear viscosity curves of aqueousGO/PEG dispersions with different cpat GO concentration cg = 15 mg/mL.
26
103
Gp' (Pa)
150
= 0.1 rad/s
100 50
GO/PEG
G',G'' (Pa)
0 0
5 10 15 cp (mg/mL)
20
cg = 15 mg/mL
2
10
cp / (mg/mL)
1
10
0 5 10 20
100 10-1
100
101
(rad/s)
102
Fig.8.Angular frequency dependence of the storage modulus G′ (filled symbols) and loss modulus G′′ (open symbols) of aqueousGO/PEG dispersions with differentcpat GO concentration cg = 15 mg/mL.Inset: plateau modulus (G′p) versuscp curve.
27