Effect of Functional Groups on the Agglomeration of Graphene in Nanocomposites

Zheling Lia, Jingwen Chua, Cheng Yangb, Sijia Haob, Mark A. Bissetta,

Ian A. Kinlocha, Robert J. Younga

aNational Graphene Institute/School of Materials, University of Manchester, Oxford

Road, Manchester M13 9PL, UK

bResearch Center of Graphene Applications, Beijing Institute of Aeronautical

Materials, Beijing, 100095, China

Corresponding Author: Robert J. Young

E-mail: robert.young@manchester.ac.uk Tel: +44-161-306-3550

Abstract

A base wash procedure was used to partially remove the oxygen

functional groups from the graphene oxide (GO) flakes to prepare the

base-washed GO (BwGO). It is found that the base wash treatment

does not alter the physical properties (size, thickness etc.) of the GO

significantly but the chemical composition is changed.

Nanocomposites were prepared by incorporating the BwGO flakes

obtained in a poly(vinyl alcohol) (PVA) matrix. It was found the storage

modulus of the nanocomposites is enhanced from 4.4 GPa to 6.5 GPa

with 5 wt% of BwGO. This is in agreement with the micromechanical

estimation obtained by using Raman spectroscopy that follows the

interfacial stress transfer from the matrix to the BwGO fillers. The

lower effective modulus of BwGO than GO as calculated using the

classical ‘rule of mixtures’ is due to the loss of functional groups on

GO that serve as a surfactant to prevent the flakes from re-

1

agglomerating. An agglomeration factor ηa is therefore proposed and a

concept of ‘effective volume fraction’ is introduced to quantify and

evaluate the level of agglomeration of fillers in nanocomposites,

which can be otherwise difficult to visualize optically. It is found that

the removal of the functional groups causes the flakes to

re-agglomerate, and reduces the ‘effective volume fraction’ by about

10~20%.

Keywords: Graphene, Agglomeration, B - Mechanical properties, D-Raman

spectroscopy

1. Introduction

Since its isolation and identification in 2004, graphene has attracted enormous

attention for both fundamental studies and practical applications [1]. At present, more

focus is aimed towards applications, among which graphene-reinforced nanocomposites

is the one that is progressing most rapidly [2, 3]. Its high stiffness, strength and

stretchability all endow the composites with extraordinary mechanical properties even at

low filler loadings [4, 5]. Graphene is chemically processible in that it can be

functionalized with, for example, epoxide, hydroxyl and carbonyl groups on its basal

plane and edges [6, 7]. Unlike graphene, the formation of hydrogen bonds between GO

and the media facilitates its dispersion in water and moreover, significantly enhances

interface interaction with polymers such as PVA [8-10]. More important for

nanomaterials, the bonds can prevent the GO flakes from re-agglomerating, which

retains the large surface area of GO to enable an effective interaction with the matrix.

On the other hand, due to the functionalization and the accompanied breaking of the sp2

2

bonds, the stiffness and strength of GO drop to only about 25% of those of graphene [4,

11]. The diverse preparation methods, however, lead to a variation in surface chemistry

and hence the interfacial interaction with the matrix being inevitably unpredictable [12].

Many groups have therefore tried to understand the mechanisms of the oxidation [12,

13] and the effects, both positive and negative, of the functional groups on the

interfacial interaction with the matrix in composites. By using the Hansen and

Hildebrand parameters, Konios et al.[14] found that functionalization has a strong effect

on the solubility and stability of GO and reduced GO. Using a base-wash method,

Rourke et al.[13] demonstrated that GO consists of graphitic plane and oxidative debris

non-covalently complexed rather than the conventionally-accepted model where all the

oxygen groups are covalently bonded to the graphitic plane. It has also been found that

the functional groups of GO not only improves the interfacial stress transfer between

matrix and fillers [8, 15], but also between the individual GO layers [16]. Another

positive effect of the functional groups is that they prevent the GO from re-

agglomerating thus a high surface area is retained to interact with the matrix, which

enhances the mechanical properties of composites significantly [10]. Many of the key

parameters in micromechanical models have already been discussed for nano-fillers,

such as volume fraction [9], lateral dimensions (aspect ratio) [17, 18] and the spatial

orientation [19-21]. However, the effect of agglomeration, which is less of an issue for

micro-fillers such as fibers but becomes more crucial for nano-fillers because of their

large surface area, is missing in many micromechanical models [9, 18]. Effort has been

made to predict the mechanical impact of agglomerations by defining a ‘filler-rich’

region and a ‘resin-rich’ area [22], to evaluate the level of agglomeration [23] and to

reveal its influence on mechanical properties of nanocomposites [24]. Nevertheless, a

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simple way of quantifying the level of agglomeration and evaluating its impact still has

not been reported.

Raman spectroscopy has been used widely to follow the deformation mechanics of

monolayer graphene-based materials [16, 25-27] and enhanced bulk nanocomposites [8,

28, 29] by correlating the stress/strain sensitive Raman band position to the strain in the

graphene. In this work, the functional groups of GO have been removed [13] and the

BwGO obtained is mixed with PVA to prepare a nanocomposite. The advantages of this

base-wash approach is that it does not alter the lateral dimension and thickness of the

flakes noticeably [18, 25] or generate massive defects/holes [30, 31] all of which have

been demonstrated to have significant impact on properties. It only removes the

oxidative functional groups so ensures the amount of functional groups is the only

variable. The spatial orientation of fillers and the interfacial stress transfer to the matrix

was investigated by employing Raman spectroscopy. Through the concept of ‘effective

volume fraction’, the effect of functional groups on the re-agglomeration of fillers thus

on the mechanical properties of the nanocomposites is quantified and discussed.

2. Experimental

2.1 Materials and sample preparation

PVA (Mw~89000-98000, 99+%hydrolysed, Sigma Aldrich) was used as received.

The graphite (Grade 2369) was supplied from Graphexel Ltd. Other reagents were of

analytical grade and used without further purification.

GO was prepared using the modified Hummers’ method [32, 33], basically by

attacking the graphite using strong acid. Details of the preparation procedures can be

found elsewhere [8]. The base wash procedure was applied according to the previous

report [13] and details can be found in S1 in Supplementary Materials. PVA powder

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was dissolved in H2O with a PVA concentration of 10 wt% at 90 oC. The PVA solution

and the BwGO suspension were then mixed with BwGO loadings of 0 wt% (neat PVA

for comparison), 1wt%, 2wt%, 3wt% and 5wt%, as detailed in S2 in Supplementary

Materials.

2.2 Characterization

Atomic force microscopy (AFM) image was obtained using a Dimension 3100 AFM

(Bruker). Optical image was obtained using a Nikon Eclipse LV100ND microscope

with the BwGO flakes deposited on a SiO2/Si substrate. Scanning electron microscopy

(SEM) images were obtained by using an EVO60 VPSEM (Zeiss). Fourier-transform

infrared (FTIR) spectra were obtained in the transmission mode using a Nicolet 5700

spectrometer (ThermoFisher Scientific Inc.), and the GO and BwGO samples were dried

and mixed with KBr for the analysis. X-ray diffraction patterns were obtained using a

PANalytical X’Pert X-Ray diffractometer (Philips) with a Cu Kα radiation source

(λ=1.542Å). The tensile properties of the neat PVA and PVA/BwGO nanocomposites

were determined using an Instron-1122 universal testing machine, with dumbbell shape

specimens of width ~4 mm and gauge length ~15 mm. Before testing the samples were

left in the laboratory for ~ 24h at a temperature of 23±0.1oC and a humidity of around

50±5%. The loading rate of the tensile test was 1.0 mm/min. The dynamic mechanical

properties were measured with a DMA Q800 (TA Instruments). Specimens were heated

from -10 oC to 120 oC at 3 oC/min. A frequency of 1 Hz and a static force of 0.005 N

were used.

The correlation of the mass fraction wf (wt%) with the volume fraction Vf (vol%) of

the BwGO in the nanocomposites were determined using Eq.(1) [9]:

5

V f=w f ρ p

wf ρp+(1−w f )ρg Eq. (1)

where ρp ~1.3 g/cm-3 and ρg ~2.2 g/cm-3 represent the density of PVA and BwGO,

respectively [9]. The filler loadings of 1 wt%, 2 wt%, 3 wt% and 5 wt% can be

converted to 0.6 vol%, 1.2 vol%, 1.8 vol% and 3.0 vol%, respectively. The dried

nanocomposite films were aligned as shown in Fig.1(a) to characterize the spatial

orientation of BwGO by polarized Raman spectroscopy [20]. The laser was parallel to

the X axis and the polarization of the incident and scattered radiation were parallel to

each other. The specimen was rotated around the X axis at different angles Φ. For the

in-situ Raman deformation tests, the BwGO/PVA films were dried on top of a

poly(methyl methacrylate) (PMMA) beams and were placed into a four-point bending

rig on the Raman microscope stage (Fig.1(b)). The specimen beams were then deformed

stepwise and at each strain level Raman spectra were collected. The strain was

measured using a resistance strain gauge bonded close to the BwGO/PVA films. The

laser spot size was around 2 μm in diameter [34] and the polarization of the laser beam

was set parallel to the tensile direction, with no analyzer used.

3. Results and Discussion

3.1 Characterization of GO and BwGO

It is shown in the AFM image (Fig. 2(a)) that the BwGO flakes typically have a

lateral dimension of ~10 µm, almost the same as the GO, demonstrating the bash wash

procedure used in this work does not alter the flake size noticeably [8]. Although the

AFM height profiles (inset in Fig. 2(a)) show the typical thickness of monolayer in the

order of 2 nm, folding and restacking of the BwGO flakes can still be observed,

different from what has been observed in GO [8]. This is confirmed in the optical

6

micrograph (Fig.S1) which shows that heavy restacking of the flakes, demonstrating

that the removal of the oxygen functional groups leads to the flake re-agglomeration.

The FTIR transmission spectra of both GO and BwGO are generally similar, but the

O-H stretching peak at around 3400 cm-1 is much weaker in BwGO than that of GO

(Fig. 2(b)). Additionally the GO spectrum has a peak ~1720 cm-1 indicating the presence

of C=O stretching which is absent in the spectrum of BwGO. The peak ~1380 cm-1 for

the C-OH bending and the peak ~1090 cm-1 corresponding to the C-O-C bond [15] are

weaker in the spectrum of BwGO, suggesting fewer of these groups present.

The structure of the raw materials and the nanocomposites has also been studied

through XRD (Fig. S2), which shows a large expansion of interlayer space during the

oxidation of graphite, but almost no change when the oxidative debris is removed from

the GO during the base wash procedure. The spatial orientation of the GO in the

nanocomposites was quantified by using polarized Raman spectroscopy. It has been

demonstrated by Li et al.[8] that the Raman D band intensity (ID) is a maximum when

the polarization of the Raman laser is parallel to the in-plane vibration of GO while a

minimum when they are perpendicular to each other. The spatial orientation can be

quantified by fitting ID as the function of Φ (Fig.1(a)) using (Eq.(2)) and more

importantly, they are further correlated to the modulus of the composite (Ec) through the

Krenchel orientation factor ηo for 2D flakes (Eq.(3)) [20]. This has been demonstrated

recently to be ‘1’ for perfectly-oriented flakes, and ‘8/15’ if the flakes are oriented

randomly [20]. Three different locations of each sample were tested and one

representative result from each sample is shown in Fig. 3. It can be seen that ηo

increases as the BwGO loading increases, suggesting an increasing spatial orientation of

BwGO. This agrees with the result for the reduced GO [21], but not with the

7

observation in the GO reinforced PVA nanocomposites where ηo decreases as the GO

loading increases [20]. This is because, for BwGO or rGO, as the loading increases, the

flakes become settled and self-oriented in the plane of the nanocomposite film due to

the constraint in space [21]. This is also the mechanism for the larger graphene flake

having better spatial orientation [35]. However, for GO, the presence of functional

groups on the basal plane actually act as weak points where wrinkling, crumpling and

fracture can potentially occur as the filler loading increases [36], which increases the

degree of disorder so a decreased spatial orientation can be observed. It can also be seen

from the SEM images that the BwGO flakes are generally aligned in the plane of

nanocomposites films (Fig. S3).

I sample(Φ )=I o⋅{ 815

+⟨P2(cosθ )⟩(−1621

+ 87

cos2Φ)+⟨ P4(cosθ )⟩( 835

− 87

cos2Φ+cos4Φ)} Eq.(2)

ηo=8

15+ 8

21⟨P2(cosθ )⟩+ 3

35⟨ P4 (cosθ )⟩

Eq. (3)

3.2 Mechanical properties of the nanocomposites

With well dispersed and spatially aligned BwGO in the PVA matrix, the mechanical

properties of the nanocomposites have been studied using tensile testing and DMTA.

The stress-strain curves of tensile test are shown in Fig. 4(a), where it can be seen that

the Young’s modulus (Ec) and ultimate tensile strength increases as the BwGO loading,

but the strain to failure tends to decrease. A higher loading of BwGO stiffens but also

embrittles the nanocomposites.

The thickness of films (~30 μm) was however too thin for an accurate measurement

of the modulus of nanocomposites especially at the initial stage, so the storage modulus

8

of the nanocomposites was also measured. The storage modulus as the function of

temperature is shown in Fig. 4(b), and the values at 20 oC are summarized in Table 1.

As the BwGO loading increases from 0 wt% to 5 wt%, the room temperature storage

modulus increases from to 4.4 GPa to 6.5 GPa. Based on this, the effective modulus of

fillers (Eeff) i.e. the actual modulus rather the theoretical value, can be estimated by

using the classical rule of mixtures:

Ec=Eeff V f+(1−V f ) Em Eq. (4)

where Em is the matrix modulus. The estimated Eeff is also shown in Table 1, and it can

be seen that Eeff drops as the BwGO loading increases, probably due to the aggregation

at higher BwGO loading, similar to the observation in GO/PVA nanocomposites [8].

3.3 In-situ deformation tests by Raman spectroscopy

In order to estimate Eeff in micromechanics, the BwGO/PVA nanocomposites were

deformed and characterized in-situ by Raman spectroscopy, which has been

demonstrated to be able to measure Eeff of GO in PVA nanocomposites [8]. Typical

Raman spectra of GO, BwGO, PVA and 1wt% BwGO/PVA nanocomposites are shown

in Fig. S4(a). The nanocomposites with different BwGO loadings were then deformed

using a four-point bending rig as described in the Experimental section. The Raman D

band position (ωD) has been demonstrated to downshift with ε, and the shift rate

(dωD/dε) reflects the stress transfer efficiency from the matrix to the BwGO filler, with

the higher the value of dωD/dε, the better the stress transfer [8, 16]. The reason for this is

that the value of dωD/dε is proportional to the change of the volume of the crystal

(strain) from the knowledge of the Grüneisen parameter, and a higher dωD/dε

corresponds to a larger strain of BwGO that requires a larger stress. The D band of a 3

wt% BwGO/PVA nanocomposite before and after deformation is shown in Fig. S4(b),

9

and a strain-induced shift of ωD can be clearly seen, similar to that of GO [8]. In detail,

ωD as the function of ε is shown in Fig. 5 for all the BwGO loadings. Generally the

values of dωD/dε are about -7 cm-1/% strain, but the average values tend to decrease as

the BwGO loading increases, indicating that interfacial stress transfer is less efficient.

Thus it appears that there is a poorer interface at higher BwGO loading.

A calibration has been established for GO/PVA nanocomposites [8] to convert the

dωD/dε to Eeff of fillers using the knowledge of the Grüneisen parameter [26, 37]. A

generic form is given as:

Eeff=−

dωD /dε

(dωD /dε )ref

×tgra

tBwGO× Egra

Eq.(5)

where (dωD/dε )ref = -30 cm-1/% is the reference value of dωD/dε as calculated

theoretically [38]. The parameters tgra and tBwGO are the thickness of graphene and

BwGO, taken as 0.34 nm and 0.85 nm, respectively. It is noted that the thickness of

BwGO is the same as that of GO as revealed by AFM and XRD (Fig. 2, Fig. S2 and

[8]). The ratio of tgra and tBwGO in Eq.(5) corresponds to the reduction of Eeff due to the

expansion in the flake thickness. Egra is the modulus of graphene ~ 1050 GPa. This

equation yields the values of Eeff as shown as the blue points in Fig. 6(a), in good

agreement with the Eeff obtained using DMTA (Table 1 and black points in Fig. 6(a)),

confirming the validity of this micromechanical estimation. The estimated results are

also compared with the values obtained in GO/PVA nanocomposites that were prepared

in the same manner (Fig. 6(b)) [8]. It can be seen the Eeff from both GO and BwGO

follow similar behavior and decrease as the filler loading increases. However, values of

Eeff for BwGO are about 10~20% lower than those of GO for all the filler loadings (Fig.

6(b)).

10

It has been shown that the spatial orientation [20] and lateral dimension [25, 31] of

fillers also play significant roles in determining Eeff. Re-arranging Eq. (4) by taking into

account the Krenchel orientation factor ηo and the length factor ηl, gives:

Ec=ηl ηo EBwGO⏟Eeff

V f +(1−V f ) Em

Eq. (6)

where EBwGO denotes the theoretical modulus of BwGO. The values of ηo as calculated

above are summarized in Table 2. The length factor ηl of BwGO flakes are calculated

according to the ‘shear-lag’ theory [18]:

ηl=1−tanh (ns /2 )ns /2

Eq. (7)

where n=[Gm/EBwGO·Vf/(1-Vf)]1/2 and Gm=Em/ 2(1+v) is the shear modulus of the PVA

matrix [17] which can be calculated by using Em≈4.4 GPa (Table 1) and the Poisson’s

ratio of the PVA matrix v ≈0.4 [39]. s is the aspect ratio of BwGO, and can be taken as

about 12000 according to Fig. 2. Consequently, the values of ηl are calculated in Table

2, for 1 wt%, 2 wt%, 3 wt% and 5 wt% BwGO/PVA nanocomposites, respectively. As

the base wash procedure does not alter the lateral dimensions, it is assumed GO and

BwGO fillers have similar values of s thus ηl.

Direct measurement revealed a modulus of GO ~250 GPa [11] and it is assumed to

be the same for EBwGO. This is lower, however, than the theoretical value ~ 420 GPa (Eq.

(5)) due to the presence of defects. However, substituting the values of Eeff in Fig. 6, ηo

and ηl in Table 2 into EBwGO=Eeff/ηoηl as shown in Eq.(6), EBwGO is calculated to have an

average value of ~150 GPa, still somewhat lower than even for the direct measurement

[11]. It is speculated that this reduction of Eeff is a result of different levels of dispersion

and agglomeration of GO and BwGO in the nanocomposites [13]. As agglomeration is

11

dependent on the lateral dimensions and spatial orientation of the nanofillers, an

agglomeration factor ηa can be added into Eq.(6) as:

Ec=ηl ηo EBwGO⏟Eeff

⋅ηa v

⏟veff

+(1−v ) Em

Eq. (8)

where ηa is defined between 0 and 1. When ηa=0, all the BwGO flakes are agglomerated

hence they are no longer ‘nano-flakes’ but a bulk graphite instead, thus nearly no stress

transfer takes place [40]. When ηa=1, there is no agglomeration and all the BwGO

flakes are in the matrix and have good interaction with the matrix to enable efficient

stress transfer. Therefore, veff= ηav can be defined as ‘effective volume fraction’ that

reflects the ‘true’ volume fraction of which all the BwGO flakes being individually

dispersed without any agglomeration. The vlues of ηa were calculated for both the GO

(based on the results in Ref. [8] and [20]) and BwGO with different filler loadings, as

shown in Fig. 7.

In both GO and BwGO nanocomposites, ηa decreases (i.e. the degree of

agglomeration increases) as the filler loading increases, however, in different ways. In

GO nanocomposites, the values of ηa do not vary dramatically until 5 wt%, indicating a

fairly good dispersion of GO at low filler loading. The increased agglomeration at 5 wt

% can be primarily from the restacking of the flakes (no stacking order as it cannot be

observed by XRD (Fig. S2)), but also some ‘edge-edge’ or ‘edge-plane’ interaction

[41]. In contrast, the BwGO fillers start with an even better dispersion than GO, but ηa

drops dramatically as the loading increases even at low filler loading. This demonstrates

the tendency of BwGO to agglomerate as the loading increases because of the lack of

functional groups [15, 42], further confirming the results using molecular dynamics

simulations [43]. In practice, this is crucial because achieving a higher loading of

12

graphene filler is quite a hot topic in the fabrication of graphene nanocomposites. The

significance of this is that it characterizes the agglomeration of the graphene flakes

without functional groups which can be difficult to observe by using other

techniques after being mixed with the matrix (Fig. S3). The agglomeration level of

BwGO being better than that of GO at 1 wt% is probably due to the poorer spatial

orientation of BwGO than that of GO (Table 2) hence there is less chance for the basal

planes to attach and restack. It is noted that the different Eeff in GO and BwGO may also

result from the different interfacial stress transfer due to the functional groups [15].

However, the GO flakes have been found to have a good stress transfer efficiency so

that even the strain drop predicted by ‘shear-lag’ theory at the edges can be neglectable

[16]. Accordingly, for flakes with such large size, the effect of interface on Eeff only has

a marginal effect.

4. Conclusions

Using a base wash procedure, oxygen functional groups have been

removed from GO flakes, and the BwGO flakes obtained have been

incorporated into a PVA matrix to make a nanocomposite. The spatial

orientation and the interfacial stress transfer of the BwGO/PVA

nanocomposites have been investigated and compared with those of

the nanocomposites prepared with GO. It has been demonstrated that

generally the micro-structure and mechanical properties of both

nanocomposites are very similar, but the effective moduli of the

BwGO fillers are ~10% lower than that of the GO fillers. An

agglomeration factor ηa has been proposed, along with the ‘effective

volume fraction’ veff to quantify and evaluate the level of

13

agglomeration of both fillers in nanocomposites, which would

otherwise be difficult using other techniques. It has been found that

the removal of functional groups causes the flakes to re-agglomerate,

and reduces the ‘effective volume fraction’ of the BwGO by about

10~20%.

Acknowledgements

This work was supported by EPSRC (award no. EP/I023879/1) and the Beijing

Institute of Aeronautical Materials (BIAM).

Conflict of Interest

The authors declare no competing financial interest.

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X

Y

F

Laser Propagation

Laser Polarization

(a) (b)Z

Graphene Flake

Nanocomposite

Force

Laser Propagation

Fig.1 Experimental set-up of (a) the polarized Raman spatial orientation test and (b) the

in-situ Raman deformation test.

19

500 1000 1500 2000 2500 3000 3500 4000

O-H

C-O-C C-OH C=O

BwGO

GO

Tran

smitta

nce

Wavenumber (cm-1)

10 μm 20 nm(a)

0 5 10 15 20 25

-2024

0 5 10 15 20 25147

Position (m)

Hei

ght (

nm)

(b)

Fig. 2 (a) AFM image of the BwGO flake. The inset is the height profiles along the lines

with the corresponding colors. (b) FTIR spectra of GO and BwGO.

-20-10

0

10

20

3040

5060

708090100110

0.00.10.20.30.40.50.60.70.80.91.01.1

o = 0.59

P2(cos) = 0.20 P4(cos) = -0.26

1 wt% BwGO/PVA

D B

and

Inte

nsity

-20-10

0

1020

3040

5060

708090100110

0.00.10.20.30.40.50.60.70.80.91.01.1

o = 0.63

P2(cos) = 0.20 P4(cos) = 0.29

2 wt% BwGO/PVA

D B

and

Inte

nsity

-20-10

0

1020

3040

5060

708090100110

0.00.10.20.30.40.50.60.70.80.91.01.1

o = 0.71

P2(cos) = 0.32 P4(cos) = 0.67

3 wt% BwGO/PVA

D B

and

Inte

nsity

-20-10

0

1020

3040

5060

708090100110

0.00.10.20.30.40.50.60.70.80.91.01.1

o = 0.80

P2(cos) = 0.58 P4(cos) = 0.48

5 wt% BwGO/PVA

D B

and

Inte

nsity

(a) (b)

(c) (d)

Fig. 3 ID as the function of the angle Φ for (a) 1wt%, (b) 2 wt%, (c) 3 wt% and (d) 5 wt

% BwGO/PVA nanocomposites.

20

0 50 100 150 200 250 300

0

10

20

30

40

50

60 2wt% bwGO/PVA 3wt% bwGO/PVA 5wt% bwGO/PVA

Stre

ss (M

Pa)

Strain (%)

PVA 1wt% bwGO/PVA

(a) (b)-20 0 20 40 60 80 100 120

0

1

2

3

4

5

6

7

8

Sto

rage

Mod

ulus

(GP

a)

Temperature (o)

PVA 1wt% bwGO/PVA 2wt% bwGO/PVA 3wt% bwGO/PVA 5wt% bwGO/PVA

Fig. 4 (a) Stress-strain curves and (b) storage modulus as the function of temperature of

neat PVA and BwGO/PVA nanocomposites with different BwGO loadings.

0.0 1.0 2.0 3.0 4.0 5.0 6.07-6-5-4-3-2-1-

0123

3.6 - =epolS 1.2 mc 1-%/

AVP/OGwB %tw 3

D

mc)

1-

(

(%) niartS0.0 1.0 2.0 3.0 4.0 5.0 6.0

7-6-5-4-3-2-1-

0123

6.6 - =epolS 6.1 mc 1-%/

AVP/OGwB %tw 5

D

mc)

1-

(

(%) niartS

(a) (b)

(c) (d)

0.0 1.0 2.0 3.0 4.0 5.0 6.07-6-5-4-3-2-1-

0123

0.8 - =epolS 0 mc 1-%/

AVP/OGwB %tw 2

D

mc)

1-

(

(%) niartS0.0 1.0 2.0 3.0 4.0 5.0

6-

5-

4-

3-

2-

1-

0

1

2

3

8.7 - =epolS 4.3 mc 1-%/

AVP/OGwB %tw 1

D

mc)

1-

(

(%) niartS

Fig. 5 ωD as a function of strain ε of (a) 1 wt%, (b) 2 wt%, (c) 3 wt% and (d) 5 wt%

BwGO/PVA nanocomposites.

21

1 2 3 4 5

60

80

100

120

140

160

BwGO Loading (wt%)

E eff -

DMTA

(GPa

)

60

80

100

120

140

160

Eeff - Ram

an (GPa)

(a) (b)

1 2 3 4 5

60

80

100

120

140

160

GO BwGO

E eff -

Ram

an (G

Pa)

BwGO Loading (wt%)

Fig. 6 (a) The calculated values of Eeff based on DMTA data (black points) and dωD/dε

in Raman spectroscopy (blue points). (b) Eeff as a function of the filler loadings, of

BwGO and GO in PVA nanocomposites (after Ref. [8]).

1 2 3 4 50.20

0.25

0.30

0.35

0.40

0.45 GO BwGO

a

Filler Loading (wt%)

Fig. 7 The values of ηa for GO (black, after Ref. [8] and [20]) and BwGO (red) with

different filler loadings.

22

Table 1 Storage modulus at 20 oC and the corresponding Eeff of BwGO in

nanocomposites.

Samples Storage modulus at 20 oC (GPa) Eeff (GPa)PVA 4.4 ± 0.7 -

1wt% BwGO/PVA 5.1 ± 1.0 1092wt% BwGO/PVA 5.6 ± 0.8 1023wt% BwGO/PVA 5.8 ± 0.3 795wt% BwGO/PVA 6.5 ± 0.6 72

Table 2 Values of ηo and ηl calculated for the 1 wt%, 2 wt%, 3 wt% and 5 wt%

BwGO/PVA nanocomposites.

Samples ηo ηl

1wt% BwGO/PVA 0.66 ± 0.14 0.962wt% BwGO/PVA 0.74 ± 0.09 0.983wt% BwGO/PVA 0.69 ± 0.04 0.985wt% BwGO/PVA 0.80 ± 0.09 0.98

23

24