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Accepted Manuscript

The physical properties of sulfonated graphene / poly(vinyl alcohol) composites

Rama K. Layek, Sanjoy Samanta, Arun K. Nandi

PII: S0008-6223(11)00780-9

DOI: 10.1016/j.carbon.2011.09.039

Reference: CARBON 6845

To appear in: Carbon

Received Date: 10 May 2011

Accepted Date: 15 September 2011

Please cite this article as: Layek, R.K., Samanta, S., Nandi, A.K., The physical properties of sulfonated graphene /

poly(vinyl alcohol) composites, Carbon (2011), doi: 10.1016/j.carbon.2011.09.039

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http://dx.doi.org/10.1016/j.carbon.2011.09.039http://dx.doi.org/10.1016/j.carbon.2011.09.039http://dx.doi.org/10.1016/j.carbon.2011.09.039http://dx.doi.org/10.1016/j.carbon.2011.09.039

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The physical properties of sulfonated graphene / poly(vinyl alcohol) composites

Rama K. Layek, Sanjoy Samanta , Arun K. Nandi*

Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur,

Kolkata 700 032, India

*Corresponding author: Tel: 913324734971 (extn. 561), Fax: 913324732805,

email address: psuakn@mahendra.iacs.res.in (A. K. Nandi).

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Abstract: Composites of poly(vinyl alcohol) (PVA) with sulphonated graphene (SG)

show fibrillar, dendritic & rod like structures for SG1, SG3 and SG5 samples,

respectively, where the number indicates weight percent of SG. Differential scanning

calorimetry shows a new peak in addition to that of PVA arising from the supramolecular

organization of the components in SG1 and SG3. 17 and 36% increases of PVA

crystalline thickness and 77 and 79% increases in amorphous overlayer thickness for SG1

and SG3 over PVA are evident from small angle X-ray scattering results but SG5 does

not show any change. Atomic force microscopy results of SG suggest aggregation at

higher concentration and the composites exhibit composition dependent mechanical

properties with the highest increase of stress (177%), strain at break (45%) and toughness

(657%) in SG3 over PVA. Youngs modulus increases with increasing SG concentration

with a maximum 180% increase in the SG5 sample. The storage modulus of SG3 shows

the highest increase (1005%) over PVA. A ten orders of magnitude increase of dcconductivity over PVA and a 10 fold increase in the dendritic SG3 to that of other

composites are observed. SG1 is semiconducting, SG3 shows an electronic memory and

SG5 exhibits a rectification property.

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1. Introduction:

Graphene/polymer composites are interesting materials for their good electrical and

mechanical properties. A major difficulty arises during its preparation because of

scarcity of a soluble / dispersible graphene in a common solvent with the polymer.

Graphene oxide (GO) provides important routes for its modification [1-13] e.g. (i)

amidation of -COOH group [14, 15] (ii) nucleophilic substitution of epoxy group [16]

(iii) diazonium salt coupling, [17,18] (iv) polymer grafting via atom transfer radical

polymerization (ATRP) [9,10,19] etc. A particular type of modification has its own

advantages for making the composites as interfacial interaction between the graphene and

polymer varies yielding composites of different properties. The interfacial interaction can

also be tuned with the composition of the hybrid.

So far GO is mostly used to make the polymer composites and in GO/ poly(vinyl

alcohol)(PVA) composite the tensile strength has increased by 35% for 3 wt% GO [20].

Also it has increased by 76% with a 60% decrease of strain at break for 0.7 wt % filler

[21]. In a PVA / reduced graphene oxide (RGO) composite a 200% increase of tensile

strength with a 75% decrease of elongation at break is reported at 3 vol% graphene

concentration [22]. In an electrochemically modified graphene (ECG) / PVA composite

the tensile strength has increased by 50% with only 4% increase of elongation at break

for the 7 wt % filler but above it the elongation at break decreases [23]. Thus in the

graphene/PVA composites an increase in tensile strength is mostly accompanied by a

decrease in strain at break, so it would be of great importance if both the above properties

can be enhanced simultaneously.

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Also in the most graphene / PVA composites conductivity is not reported and only in

ECG / PVA composite the reported conductivity is 10-7 S/cm for 6 wt% graphene [23].

This value is rather low compared to the graphene/ polystyrene composite (0.15 S/cm for

1.2wt% graphene) [24]; graphene/ polycarbonate composites (0.5 S/cm for 2.2vol %

graphene) [25]and RGO / poly (vinylidene fluoride) (PVDF) composites(10-4 S/cm for 4

wt% RGO) [26]. Here we are interested to increase both the conductivity and mechanical

properties of PVA, significantly. For this purpose RGO is obviously a good choice, but it

has the difficulty of dispersion in aqueous medium required to mixing with PVA. In order

to alleviate the problem the anchoring -SO 3H group on the graphene surface followed by

reduction (with hydrazine) may be a promising method to yield a highly conducting and

dispersible graphene in aqueous medium [18]. The anchored SO3H group in the

sulphonated graphene (SG) is a stronger H-bonding group compared to -COOH / -OH

groups present in RGO and it may strongly supramolecularly interact with PVA through

H-bond formation with the OH group of PVA. The strong and directional nature of H-

bonding interaction may yield new supramolecular structure of the composite. Recently,

oriented structure of GO is reported at a high GO concentration in the GO/PVA

composite showing significant mechanical property enhancement in the plane of

alignment [27]. Actually at lower GO concentration anisotropic oriented structures are

not produced but here we can expect an oriented structure even at the low filler

concentration due to the anchoring of a strongly interacting SO3H group on the

graphene surface.

Sulphonation is performed by oxidizing graphite with HNO3 and then reacting with

sulphanilic diazonium salt [18]. PVA is then blended with 1, 3 and 5 wt % of SG by

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solvent casting method to observe a variation in physical properties. PVA is a

biocompatible polymer and it is used in tissue engineering, drug delivery and in other

biotechnological devices [28, 29]. So any enhancement in mechanical and electronic

properties of PVA with SG would be helpful for fabricating new biotechnological

devices. Here we report that both stress (177%) and strain at break (45%) increase till the

addition of 3% SG concentration contributing to a 235% increase of toughness but above

3% SG the stress increases with a decrease of strain at break. A new type of morphology

(dendritic) is observed at 3% (w/w) SG concentration and 10 orders hike in the

magnitude of dc conductivity over PVA is achieved in these new composites. Probable

explanation from the supramolecular organization of SG / PVA complex and its variation

with composition are discussed.

2. Experimental:

2.1 Materials. Graphite, sodium borohydride, sulfanilic acid (Aldrich, USA) and sodium

nitrate, potassium permanganate, 35 % hydrocholoric acid, hydrazine hydrate solution

(99%, synthetic grade), (Merck, Mumbai) are used as received. PVA (Aldrich, USA;

99+% hydrolyzed ) has vM = 130,000 which is measured from the intrinsic viscosity in

water at 30 oC (Mark- Houwink constants, K=42.8 X10 -5 dl/g & = 0.64) . The PVA is

used as received and water is doubly distilled before use.

2.2 Modification of graphene:

At first, the starting material graphite oxide(GO) is prepared from graphite powder by

oxidizing with KMnO4/ NaNO3 mixturein concentrated H2SO4 using Hummers method

[30]. Then, 75 mg GO is dispersed in 75ml water (0.1% w/v) and is sonicated for 1 h in

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an ultrasonic bath (60W, frequency 28KHz, Model AVIOC, Eyela). A clear brownish

dispersion of GO is formed indicating the exfoliation of GO in the medium. The process

of synthesizing SG from GO consists of three steps : (a) pre-reduction of GO with

sodium borohydride; (b) sulfonation with the aryl diazonium salt of sulfanilic acid; and

(c) post-reduction with hydrazine to remove epoxy (>O) functionality completely (Fig.1).

The pre-reduction of GO is essential: (i) to enable the sulphonation reaction by increasing

the domain size of sp2 carbon for reaction with aryl diazonium salt and (ii) it also helps to

achieve complete reduction during hydrazine treatment after sulphonation. The later

reduction is necessary to get more sp

2

carbon atoms in the graphene and hence to get

extended conjugation in the graphene [18]. The pH of GO dispersion is adjusted between

pH 9-10 with the addition of 5% (w/v) sodium carbonate solution. Then 15 ml sodium

borohydride solution (4%, w/v) in water is mixed with the GO dispersion and is kept at

80 oC for 1 h under constant stirring. The partially reduced product (reduced graphene

oxide, RGO) is washed with water until its pH becomes 7 and it is re-dispersed in water

for diazonium coupling. For this purpose 46 mg sulphanillic acid and 10mg sodium

nitrate are dissolved in 10 ml water with the addition of 1.15 ml 12 N HCl at ice cooled

condition. The mixture is then added to the RGO dispersion at 0oC and is kept for two

hrs with stirring. It is centrifuged, washed repeatedly with water until pH becomes 7. The

product is then re-dispersed in 100 ml water for final reduction with 2ml hydrazine

hydrate solution under refluxed condition for 24 h at 100oC. Finally it is washed with

water thoroughly and dried in vacuum at 60 oC. The mechanism of sulfonation of RGO

by diazonium salt of sulfanilic acid involves the homolytic fission of dinitrogen from the

diazonium salt leading to the generation of aryl radical which binds to the graphene

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surface via carbon-carbon covalent bond [31, 32]. This is also evident from the absence

of nitrogen in the PSG sample (SI Fig.1).

In order to obtain an experimental support of the scheme (Fig.1) elemental analysis data

of sulfonated graphene (SG) has been performed from the EDXS experiment (SI Fig. 1).

The sulphur contents before and after hydrazine hydrate treatment are represented by the

S:C atomic ratio showing the values 1:41 and 1:43 for pre-reduced sulfonated graphene

(PSG) and SG samples, respectively. This indicates a small (0.28 wt%) loss of sulphur

with respect to carbon during hydrazine treatment. These sulphur contents are very close

to that reported by Si and Samulski [18] in the SG sample.

Graphene oxide after pre-reduction with NaBH4 (i.e.RGO) shows an average thickness

of 1.800.07 nm measured from AFM height profile (SI-Fig.2). It increases after

sulphonation by diazonium coupling (i.e. PSG) to 2.000.16 which on reduction with

N2H4 remains almost unchanged with an average value of 1.99 0.2 nm for SG. Thus due

to sulphonation an increase of graphene thickness is observed. The degree of surface

modification has also been evaluated from TGA thermograms (SI Fig.3) of the above

three samples taking the TGA thermogram of pure graphite as a reference. From the

figure it is evident that there is 33% weight loss in RGOindicating the presence of 33%

oxygenous functional material. After sulphonation by diazonium treatment there is 10%

more loss suggesting 10 wt % sulfanilic acid group is introduced on the RGO surface. On

the final reduction with N2H4, H2O the weight loss from the TGA reference is 27.5%

indicating a decrease of (43-27,5 =) 15.5% weight loss after reduction by hydrazine

hydrate (i.e.SG). This result implies a significant increase of SP2 character of graphene

after hydrazine reduction due to removal of oxygenous materials. From the AFM results

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Figure 1. A schematic presentation of SG preparation from graphite powder (GO = graphene oxide; RGO

reduced graphene oxide, PSG = pre-reduced sulphonated graphene and SG= sulphonated graphene after

reduction b h drazine.

0-5 oC

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we have observed the average thickness of the SG sheets =1.990.20nm, average length

=35293 nm and average breadth = 21971.nm (from ~20 measurements) for 1% (w/w)

SG concentration.

2.3 Composite preparation: 250mg PVA is dissolved in 5ml water by heating at 90oC

and is cooled to room temperature (30 oC). SG solution is prepared in 5ml water by

sonication for 30 minutes and is then mixed with the PVA solution in different

proportions. The mixture is sonicated for another 30 minutes for homogeneous mixing.

Finally slow evaporation of the solvent in a petri-dish yields films of the composites

which are dried at 60

o

C for three days in vacuum. The composites are designated as

SG0.5, SG1, SG3, and SG5, respectively, the numbers indicate the weight percent of SG

in the composites. In a similar fashion the composites of PSG are prepared for

comparison in properties and are designated as PSG3 and PSG5 for 3 and 5 % (w/w) PSG

samples.

2.4 Characterization:

2.4.1 Microscopy: The field emission scanning electron microscopy (FESEM) is

conducted by casting the films from the composite solution in a fresh silicon wafer via

slow evaporation of the solvent on a hot plate. They are dried at 60oC in a vacuum for

three days and the morphology is studied using a FESEM instrument (Jeol GSM-5800).

The transmission electron microscopy (TEM) is conducted from a thin film on carbon

coated copper grid produced by dispersing SG in water and by taking a drop of it on the

grid. It is dried slowly at 30 oC for a week. The atomic force microscopy (AFM) is

conducted in the non contact mode at a resonance frequency of250 KHz of the tip using

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an AFM instrument (Veeco, model AP 0100). The films are cast on mica surface from a

drop of the SG dispersion (without PVA) in water.

2.4.2 Spectral characterization: Raman spectra are studied by casting films of the

composite solutions on quartz plate and slow evaporation of water on a hot plate at ~50

oC followed by vacuum drying at 60 oC for three days. Raman studies were performed

using a micro Raman spectrometer (Agitron) with spot size of 1m2 using 785 nm laser.

FTIR study of the samples are performed using a FTIR instrument (model 8400S

Shimadzu). The films are cast from aqueous solutions (2% w/v) by spreading over the

silicon wafer surface. The films are dried on a hot plate (60

o

C) in air and finally in

vacuum at 60 oC for three days. The FTIR peaks at 1042 and 1250 cm-1 of PSG and those

at 1042, 1125 and 1170 cm-1 in the SG (SI Fig.4) correspond to the presence of SO3H

group [18].

2.4.3 Thermal study: A Perkin Elmer differential scanning calorimeter (DSC 7,

Diamond) working under nitrogen atmosphere is used to measure the thermal properties.

It is calibrated with indium before use. 4-5 mg samples are taken in aluminum pans and

are crimped using a universal crimper. They are scanned from -20 oC at the heating rate

of 20oC / min to 242

oC. The crystallinity (c) values of PVA and of the composites are

calculated from the equation:

uc

H

H0

=

where H is the measured enthalpy of fusion (from DSC) and uH0

is the enthalpy

of fusion of pure PVA crystal (138.6 J g-1) [33].

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2.4.4 X-ray scattering: Wide angle X-ray scattering (WAXS) and small angle X-ray

scattering (SAXS) patterns are obtained by fixing the composite films on an aluminum

holder and using a Bruker AXS diffractometer (model D8) fitted with a Lynx Eye

detector. The instrument is operated at 40 kV and 40 mA current. For WAXS, the

samples are scanned from 2=5 to 35 o at the scan rate 0.5 sec per step and for SAXS, the

instrument is scanned from 2=0.2 to 5o at the scan rate of 2 sec per step with a step size

of 0.02 o. The crystalline thickness is calculated by multiplying crystallinity with the long

distance and the interlamellar amorphous overlayer thickness is measured from the

difference between long distance and crystalline thickness.

2.4.5 Mechanical properties: The storage modulus (G'), loss modulus (G'') and tan

values of the composites are measured using a dynamic mechanical analyzer (DMA)

(model Q-800, TA instruments). Sample films (25mm 5mm 0.15mm) are prepared by

pouring the aqueous solution on a die, slow evaporation of the solvent on a hot plate and

are finally dried in vacuum at 60oC for three days. The films are installed at the tension

clamp of a calibrated instrument. The samples are heated from -50 oC to 100 oC at the

heating rate of 10 oC/min. The G', G'' and tan values are measured at a constant

frequency of 1 Hz with a static force of 0.02 N.

Tensile tests are carried out from water cast films of uniform thickness using a universal

testing machine (Zwick Roell, model Z005) at a strain rate of 1 mm/min at 30oC. Each

experiment is repeated for three times to observe reproducibility.

2.4.6 Conductivity measurement: The dc-conductivity of the films is measured by fixing

them between two gold electrodes. The electrical connection is made through copper

wires using silver paste on the gold electrodes. The area and thickness of the samples are

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measured by a screw gauge. The conductivity () of the above films are measured by two

probe technique with an electrometer (Keithley, model 617) at 30 oC using the equation:

=R

1

a

l(1)

where l is the thickness and a is the area and R is the resistance of the sample.

The current-voltage (I-V) studies are performed using the same samples by applying

voltage from -5 to +5 V and the current is measured at each applied voltage.

3. Results and discussion:

3.1 Morphology and structure:

The FESEM micrographs (Fig.2) of SG3 and SG5 show dendritic and rod like

morphology, respectively. SG1 has patches of closely spaced fibrillar morphology (SI

Fig-5), but none of these morphologies are present in pure SG and PVA (SI Fig-6). The

enlarged FESEM picture of SG1 (SI Fig.5) show closely spaced fibrils in patches widely

Fig. 2 FESEM micrographs of (a) SG3 and (b) SG5 composites

b

2m2m

a

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separated from each other, that of SG3 shows densely branched fibers producing the

dendrites and in SG5 the folded SG sheets produce a rod shaped structure. The evolution

of different morphologies with composition is interesting and it may arise due to the

variation of interfacial interaction between SG and PVA.

The TEM micrographs (Fig.3) indicate sheet morphology of SG and dendritic

morphology of SG3. It is interesting to note that the sheet morphology of SG is totally

absent in SG3, instead the fibrils constitute the dendrites. The new morphology may arise

from the self organization of the supramolecular complex between SG and PVA

producing fibrils and hence dendrites. The evidence of supramolecular interaction (H

bonding) comes from the absence of infrared absorption bands of OH and SO3H

groups (2647, 2890 & 3060 cm-1) [34] of SG in any of the composites (SI Fig.7) and a

schematic model of the SG - PVA supramolecular interaction is presented in Fig.4. Both

the nature of supramolecular complex and its self-organization may depend on

a b

Fig. 3 TEM images of (a) SG and (b) SG3 sample

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the variation of interfacial interaction with composition and it would be evident from the

following discussions.

.

In order to elucidate the cause of different morphologies we have made the DSC ( Fig.5),

WAXS( Fig.6), SAXS (Fig.7, SI Fig.8) and Raman spectra (SI Fig.9) and the data are

summarized in Table-1. In the DSC thermograms (Fig.5), pure PVA and SG5 show only

one endothermic peak while SG1 and SG3 have two endothermic peaks corresponding to

the two different species present in SG1 and SG3. The higher temperature peak is for the

melting of PVA crystal entrapped between the SG nanostructures and the lower

temperature broad peak may arise from the supramolecular complex of PVA and SG. At

lower SG concentration (1% w/w) a majority of PVA chains supramolecularly organize

with SG and finally crystallize to produce fibers. As the SG concentration is increased to

Fig. 4 H-bonding interaction between PVA and SG

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5 1 0 1 5 2 0 2 5 3 0 3 5

1 9 .8 0

1 9 .8 0S G 1

1 9 .5 8

S G 5

S G 3

2 5 .2 4

S G 0 .5

1 9 .4 8

1 9 .4 0

8 .0 7

P V A

S G

2 ( d e g r e e )

Intensity(a.u

)

Fig. 6 WAXS patterns of SG and different PVA-SG composites.

Fig. 5 DSC melting endotherms (scan rate 200C/min) of different PVA -SG composites

1 0 0 1 5 0 2 0 0

2 2 8 .8

HeatFlow(Endo

Up)

P V A

T e m p e r a tu r e (0C )

1 3 4 .3

2 3 2 .8

S G 1

S G 3

1 4 1 .6

2 3 4 . 6

2 2 9 .7

S G 5

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3% the supramolecular complex organizes in dendritic form while for SG5 rods are

produced. The fibril formation of SG-PVA supramolecular complex may be understood

in the following way. DSC and WAXS results (Fig.5 and Fig.6) indicate that PVA is a

semi-crystalline polymer and the SAXS result (Fig.7 & SI Fig.8) indicates the presence

of a lamellar peak. The long distance (13.2 nm) of PVA have increased by 56 and 64%

reflecting 17 and 36% increase in crystalline thickness and 77 and 79% increase in

amorphous overlayer thickness for SG1 and SG3 systems, respectively. A probable

reason is that SG sheets enter into the inter-lamellar amorphous zone causing an increase

of amorphous overlayer thickness of PVA. The small increase in crystalline thickness of

PVA may be attributed to the disposition of SG sheet near the crystal amorphous

interface enhancing irregular chain folding which increase the crystal / amorphous

interface

1 2 3 4 5

Intensity

0.670

SG 3

PVA

2 (degree)

0.41

0

Fig. 7 SAXS patterns of PVA and SG3 composite.

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Table-1. Melting temperature (Tm), enthalphy of fusion (Hf), long distance (L),

crystallinity (1-)H, crystalline thickness (lc) and amorphous thickness (la) of PVA-SG

composites.

reflecting an increase in crystalline thickness. Also the specific interaction between SG

and PVA at the interface might also contribute to some extent to the increase of

crystalline thickness. This PVA lamella and the amorphous layer of SG-PVA complex

supramolecularly organize to produce fibrillar morphology and it has been clarified in a

schematic model (Fig.8). The longitudinal growth of fibers may be assisted by the

supramolecular interaction of SG sheets present at the end of fibrils facilitating the nuclei

to grow longitudinally. The presence of SG sheets is confirmed from the Raman spectra

(SI fig.9) of the composites where the D and G bands of SG is clearly observed in all the

composites, reflecting its concentration is responsible for the morphology change. At the

SampleTm (

oC) Hf

(J/g)

L

(nm)

(1-)H (%) lc (nm) la (nm)

PVA 228.8 49.6 13.17 36 4.74 8.43

SG1

232.8

134.3

37.7

30.2

20.52 27 5.58 14.94

SG3

234.6

141.6

41.1

49.4

21.52 30 6.46 15.06

SG5 229.7 54.3 13.57 40 5.39 8.14

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inter-fibrillar region some SG and PVA chains (with defect structure e.g entangles, knots

etc.) may exist and the population of SG sheets in the inter fibrillar region increases as

Fig. 8 Suppramolecular organization of PVA and SG for (a) SG1 (b) SG3 and (c) SG5 samples

producing different morphology.

= PVA lamella

= SG sheet

PVA:SG = 95:5

Grow

th

PVA:SG = 99:1

PVA:SG = 97:3

Stacked fiber

Dendritic structure

Aggregation induced bended

SG sheets coated with PVA

Adhesive force

Cohesive force

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the SG concentration increases. In SG3 sample the interfibrillar SG concentration is large

and the lateral dimensions of SG (length & width ~352 & 219 nm) are also large making

a difficulty of diffusion of impurities from the growth font yielding proturbances causing

the fibrils to splay in the whole space. The high diffusion constant / growth rate (D/G)

ratio causes the branching of fibrils producing dendrites (Fig.8). The dendrites consist of

radiating fibrils which share a common crystallographic plane with that of the mother

[35, 36].

A decrease of fusion enthalpy (H) of PVA (higher melting peak) in SG1 (Table-1) is

probably due to the complexation of a fraction of PVA chains with SG and it is also true

for SG3 sample. In both cases a broad peak for melting of SG-PVA supramolecular

complex is observed at ~140oC and it is absent in SG5 sample, where the SG sheets

become folded producing a rod like structure. It is evidenced from the almost unchanged

Tm and invariant Hf of SG5with that of pure PVA (Table-1) and a slight increase over

PVA may be attributed to the confinement of the chains between the nanorods. At

increased SG concentration (5 % w/w) intramolecular attraction between SG sheets

causes self-assembly between the sheets which experience an unbalanced force causing

bending. The interfacial interaction is of two types: (i) force of adhesion of SG with PVA

and (ii) force of cohesion among the SG sheets. Though it is difficult to quantify the

magnitude of the above forces, it is likely that the force of adhesion is lower than that of

cohesion which increases with SG concentration because at higher concentration SGs are

not dispersed rather they are aggregated. In the aggregated SG sheets the upper sheet is in

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an unbalanced state of tension for the inequality of the two forces causing it to bend

giving the shape of rod above which PVA wraps.

In the composites studied here the SG concentration is varied from 0.5 to 5 wt%. At the

lower concentration SG remain exfoliated within the PVA matrix and with the gradual

increase of SG aggregated structure may result. Interfacial interaction is dependent on the

surface area, so in aggregated state a decrease in interfacial interaction is expected. Here

PVA concentration is large and in excess, so the variation of interfacial interaction only

results from the variation of SG concentration; as a consequence, the self organization of

the system changes. The aggregation of SG sheets with increasing SG concentration may

be evidenced from a blank experiment where the SG samples (designated as BSG1,

BSG3, and BSG5 analogous to those of the composites) are dispersed in water under

similar condition of making the composite without adding any PVA. The absence of PVA

avoids any coating of the polymer on SG in the dried state. The thicknesses of the films

are measured from AFM (Fig.9). In BSG1 the graphene sheets are widely separated from

each other and from the height profiles of 25 particles the histogram of thickness of SG

sheets are made. The thickness varies from 1.1 to 2.4 nm having an average value of 1.99

nm. In BSG3 the number density of SG sheets is high but they are homogenously

dispersed. On the other hand, in BSG5 the graphene sheets are aggregated and density is

lower than SG3. The average thickness of BSG3 and BSG5 are 2.89 and 21.67 nm,

respectively. These results suggest that SG thickness increases due to aggregation at

higher concentration. In the PVA matrix the environment of SG sheets may not be

exactly identical and the state of aggregation may decrease to some extent. PVA chains

have pendent OH group similar to water molecules, so the chemical force on SG would

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be of same nature when it is dispersed either in aqueous medium or in PVA. But the high

viscosity of high molecular weight PVA may decrease the aggregation of SG sheets to

some extent. The highly dense but homogenously dispersed state of SG in SG3 is

interesting and it supports the cause of dendrite formation discussed above.

From WAXS patterns (Fig.6) it is evident that PVA exhibits a sharp diffraction peak at

2=19.80 (dhkl=4.48 A

0) which progressively shifts to lower 2 value and finally reaches

at 19.40 (dhkl= 4.57A0) for SG5. Though unusual, the specific interaction between SG and

PVA may expand the unit cell dimensions of PVA to some extent (2%) in the

composites. In SG5 the increase of d-spacing is somewhat lower than that of SG1 & SG3

(cf; relative decrease of 2 values) and the increase of dhkl indicates an interaction also

exists between PVA and the SO3H groups at the outer surface of SG rods. In literature

[20-23, 37] there are some reports on WAXS of PVA/GO or PVA/graphene composites.

GO has a diffraction peak at 110 corresponding to its interlayer spacing (7.8 A0). This

diffraction peak disappears when mixed with PVA indicating exfoliation of GO sheets

[37]. The PVA peak intensity at 2 = 200 decreases in all the composites with increase in

graphene / GO concentration indicating a decrease of PVA crystallinity, however, the

peak position remains unchanged. But in the present system the decrease in 2 value is

unique due to the presence of strongly interacting -SO3H group.

The fillers in the blends / composites may present in the intercrystalline region which

includes interlameller, interfibriliar and interspherulitic zones [38]. The distribution of the

filler in the three zones depends on the dimension of the filler, concentration of the filler

and its interaction with the polymer. If the filler is within the interlamellar amorphous

region as shown in Fig.7, the interlamellar amorphous overlayer thickness would increase

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and it is reflected in the present SAXS data (Table-1) for SG1 and SG3 due to their lower

thickness value. An enlarged long spacing in the two is observed over PVA, but in SG5

the long spacing value is almost the same. The large increase of interlamellar amorphous

overlayer thickness than

1.2 1.4 1.6 1.8 2.0 2.2 2.40

12

3

4

5

6

7

Noofparticle

Thickness (nm)

a

Fig. 9 AFM image of SG at (a) 0.01 (w/v) (b) 0.03 (w/v) and (c) 0.05 (w/v)

2.1 2.4 2.7 3.0 3.3 3.60

12

3

4

5

6

7

8

N

oofparticle

Thickness (nm)

b

5 10 15 20 25 30 35 400

2

4

6

8

10

N

oofparticle

Thickness (nm)

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crystalline thickness indicates that the supramolecular complex is produced both in the

crystal amorphous interface and more preferably within the amorphous overlayer region

of PVA crystals. Considering the average thickness of SG sheets (1.99 & 2.89 nm in

BSG1 and BSG3 from AFM analysis) it is evident that a maximum of 3 SG sheets can

enter into the interlamellar amorphous layer of PVA to increase the amorphous overlayer

thickness by ~8 nm. In SG5 the aggregated SG sheet (av. thickness. 21.7nm) can not

enter into the amorphous overlayer region making an almost unchanged value in the

crystal and amorphous overlayer thickness with that in PVA. Hence it may be concluded

that in SG5 the SGs are not inter-lamellar whereas in SG1 and in SG3 samples SGs are

not only inter-lamellar but also inter fibrillar.

3.2 Mechanical properties:

0 50 100 150 200 250 3000

20

40

60

80

100

120

140

160

180

200

SG5

SG3

SG1

SG0.5PVA

TensileStress(M

Pa)

Strain (%)

0 1 2 3 4 52

4

6

8

Young'sModulu

s(GPa)

SG % (w/w)

Fig. 10 Mechanical property of PVA and its composites with

sulphonated graphene at indicated compositions.

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The mechanical properties of different SG composites (Fig.10 and Table-2) are

interesting. Both stress and strain at break gradually increase with increasing SG

concentration except for SG5 where an increase of stress (157%) occurs though strain at

break decreases abruptly (even lower than that of pure PVA). Among the three

composites the highest increase of both stress (177%) and strain at break (45%) is

observed in the dendritic SG3. Consequently, the toughness of the material has the value

of 657 Nmm, with a 235 % increase over PVA. To our knowledge, it is the highest

increase of toughness yet reported in the literature for the PVA-graphene composite.

Table-2. Stress at break, Strain at break, Youngs modulus, Toughness of PVA and PVA-

SG composites at 30 oC.

Due to nano-confinement the PVA chains experience lesser flexibility for the large

cohesive force of the SG nanosheets. On application of stress, the alignment of SG sheets

Sample Strain %

increase

Stress (MPa) %

increase

Modulus

(GPa)

%

Increase

Toughness

(Nmm)

%

Increase

PVA 187 - 48 - 2.9 - 196 -

SG1 266 42 97 102 3.9 36 526 169

SG3 272 45 133 177 5.3 86 657 235

SG5 84 -55 101 111 8.0 180 222 13.5

PSG3 256 36.8 130 170 5.5 89.7 645 229

PSG5 101 -45 104 116.7 7.8 169 227 15.8

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changes relaxing the PVA chains, hence causing an increase of strain at break. In SG5 as

the rods are randomly oriented having lower surface area than that of sheets the strain at

break has decreased and the stress at break has increased to 111% over PVA. The

Youngs modulus increases progressively with increasing SG concentration in the

composite and the highest value (8GPa) is achieved for SG5 (Table-2). The reported

enhancements of Youngs modulus are 62% for 0.7 wt% GO [9], 100% for 1.8 vol%

RGO [10] and 128% for 3wt% GO [8]. However, in the present system 180% increase of

Youngs modulus, is a maximum yet reported in the literature for the PVA/ graphene

composites. It is to be noted here that GO/ PVA composites causes a decrease of percent

strain with increase of stress on addition of GO [9,10, 20], but in the present system both

strain and stress increase till 3 wt % addition of SG. So the SO3H group of SG is

influencing the mechanical property of PVA much more effectively than that of COOH

group of GO. The mechanical properties of SG composites are compared by making

composites with PSG samples at two compositions identical to SG3 and SG5 composites.

Youngs modulus, strain at break and toughness are comparable to those for SG

coposites (Table- 2) suggesting that hydrazine treatment used for reduction of PSG does

not affect the mechanical property almost unchanged.

The storage modulus (G) vs temperature plot (Fig.11), show a significantly large

increase in the composites and it is very large in SG3 (Table-3). The highest (1005%)

increase of SG3 over PVA is observed at 30 oC enabling it to behave as a promising

reinforced plastic at ambient condition. The loss modulus (G) vs temperature plot has

also similar trend and SG3 exhibits the highest value compared to the other composites

(SI Fig.10). This indicates that the cause of increase G and G is the same and the

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reinforcement of dendrites is the highest due to the very large surface area of the

dendrites. Recently Brinson & coworkers have reported 650% and 1019 % increases of

G for 44% and 60% (w/w) GO, respectively, in GO / PVA composites [27]. We have

found 1005% increase in storage modulus for 3% (w/w) SG content only. So in the

present system

addition of a very small amount of SG is very much effective in increasing G and it is

due to the formation of supramolecular organized structure. The peak temperature of

Tan vs temperature plot (SI Fig-10) indicates glass transition temperature (Tg ) of PVA

in the composites. It has decreased in the composites and the lowest Tg in SG3 (Table-3)

compared to those of the others is for its dendritic morphology. The densely spaced and

widely distributed fibrils decrease the interaction amongst the PVA segments. This

-40 -20 0 20 40 60 80 1000

4000

8000

12000

16000

20000

1. PVA

2. SG 0.5

3. SG 1

4. SG 3

5. SG 5

5

4

3

21

Stortagemod

ulus

Temperature (0

C)

Fig.11 Storage modulus -temperature plots of different PVA-SG composites.

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causes an increase of free volume of PVA in the system facilitating the onset of

segmental motion at lower temperature [39,40]. So in the present system addition of a

very small amount of SG is very much effective in increasing the segmental motion of

PVA chains hence decreasing the Tg .

Table-3. Storage modulus (G') and Glass transition temperature (Tg) from Tan plots

and values of PVA-SG composites.

3.3 Conductivity:

Graphene is a good conductor of electricity [41, 42] and PVA is an insulator, ( =5.310-

14S/cm), but SG0.5, SG1, SG3 and SG5 have conductivity 6.110

-10, 1.2 10

-5, 0.9 10

-4,

and 1.5 10-5

S/cm at 30o

C. Thus compared to pure PVA there is 10 orders increase in

the magnitude of dc-conductivity in SG3 which also shows a 10 fold higher conductivity

than that of other composites. This is due to the easier hopping of charge carriers through

the closely spaced fibrils of SG3 dendrites. Also the planar structure of dendrites causes

Sample Tg (oC)G'

(MPa)

at-30 oC

%

increase

G' (MPa)

at 0 oC

%

increase

G'

(MPa)

at 10oC

%

increase

G'(MPa)

at 30 oC

%

increase

PVA 32.2 11265 - 3507 - 3496 - 749 -

SG1 24.8 13995 24.2 10885 210 10874 211 6946 827

SG3 21.8 17463 55 14426 311 14341 310 8274 1005

SG5 29.2 16946 50 7417 112 7332 109 1708 128

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an easier charge movement contributing to the highest conductivity. We have plotted

conductivity vs. SG concentration (Fig.12) and it shows a percolation threshold at a SG

concentration of 0.37 wt%. This low percolation threshold is

indicative of good dispersion of SG in the composite at lower concentration and at a 1

wt% SG the system shows a saturation in the conductivity value.

It is true that in some graphene polymer composites the conductivity is very high e.g in

graphene/ polystyrene composite it has the highest conductivity (0.15 S/cm) for 1.2 wt%

graphene [24]; 0.5 S/cm for 2.2 vol % graphene in graphene / polycarbonate composites

[25]; 0.01 S/cm for 2.5 vol% graphene in PS / graphene composites [43] etc. But there

are also reports where modified graphene composites has much lower conductivity e.g.

10-5 S/cm for PVDF/grahene-PMMA [10] for 5 wt% filler, 10-4 S/cm for RGO /PVDF

composite at 4wt% filler [26]. In a surfactant wrapped graphene / PVC composites the

0 1 2 3 4 5

-1 4

-1 2

-1 0

-8

-6

-4

0.37

log()

SG % (w/w)

Fig. 12 Conductivity log() vs SG% (w/w) plot

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conductivity is 10-4 S/cm at 5.5 vol % filler [44] and in ECG / PVA composite the

conductivity is 10-7 S/cm for 6 wt% filler [23].So different conductivity is reported and

particularly for composites with the modified graphene conductivity is lower. In the

present system the conductivity is in the lower group and no definite reason can be

afforded for this lower conductivity. One probable reason is that the nonconducting PVA

becomes supramolecularly linked with SG from all sides of its surface decreasing its

conducting paths. This supramolecularly interacted SG / PVA complex organizes into

fibrils, dendrites or rods. So SG is not dispersed as sheets as graphene in the first group of

composites but its supramolecularly organized structure with decreased conducting path

become dispersed throughout the matrix yielding lower conductivity. Also during

sulphonation some sp 2 hybridized bonds being converted into sp 3 hybridized bonds,

which are not totally recovered to sp2

hybridized bonds during reduction by hydrazine,

and destroys the conjugation to some extent. Due to the above reasons conductivity of

SG/PVA composites is lower than other graphene/polymer (polystyrene or

polycarbonate) composites

We have compared the conductivity of PSG/PVA composites at two identical

compositions with those of SG/PVA composites. The conductivity of PSG3 is 1.110-7

S/cm which is three orders lower over SG3 composite (0.910 -4 S/cm). In PSG5

composite the conductivity is found to be 1.2 x 10-7 S/cm which is also 2 orders lower

over SG5 sample (1.5 x10-5

S/cm). Thus a 2- 3 orders increase in magnitude of

conductivity is observed in the hydrazine reduced SG3 samples. As discussed in the

mechanical property section the percentage increases of stress, Youngs modulus and

toughness of PSG composites are comparable with SG composites (Table- 2) suggesting

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that hydrazine treatment increases the conductivity keeping the mechanical property

almost unchanged.

The current-voltage (I-V) characteristic curves of SG3 and SG5 composites are

presented in Fig.13, clearly indicating a significant variation of I-V properties with

composition. SG1 shows a typical semi-conducting behavior, SG3 shows an electronic

memory and SG5 shows a rectification behavior. The increased graphene concentration

Fig. 13 Current-voltage (I-V) characteristic curves of different SG-PVA composite at indicted compositions.

-6 -4 -2 0 2 4 6

-70

-35

0

35

70

-40

0

40

80

-300

-150

0

150

300

SG1

Curre

nt(amp)

Voltage (V)

SG5

SG3

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has generated different morphology which may be attributed to the different I-V

behavior. The semiconducting nature of SG1 may be attributed to the patches of fibrils

with a gap between them (SI Fig.5) causing a hindrance to the charge transport. The

memory effect in SG3 is due to its dendritic morphology. In the forward bias it shows an

inflection voltage at 3.01 V where the charge carrier reaches the conduction band but on

decreasing voltage in the backward bias the charges are retained at the junction points of

fibrils in the dendrites. At a much lower voltage (threshold voltage= 0.6 V) the charge

becomes annihilated. The SG5 sample has rod like morphology showing a rectification

property (rectification ratio=3.5). Like carbon nanotubes the graphene ring is a good

acceptor of electrons and here electrons become stabilized by resonance between the

graphene rings [45] and can act as a p-type semiconductor. PVA acts as a n-type

semiconductor due to the presence of lone pair of electrons on the oxygen atom of OH

group. Hence the uniform distribution of rods makes the system an effective p-n junction

suitable for rectification. It is to be noted that in the positive bias it also exhibits memory

effect (inflection voltage 3.4 V) due to the charge trapping by the graphene rods and it

requires a lower voltage for annihilation (threshold voltage 2 V).

4. Conclusions: PVA/SG composite exhibits different morphology at different

compositions of the hybrids due to supramolecular organization. SG1 has fibrillar

morphology, SG3 shows dendritic morphology and SG5 exhibits rod like morphology.

The supramolecular interaction (H-bonding) is evident from Fourier transform infrared

spectroscopy (FTIR). DSC study shows a new peak in addition to that of PVA arising

from supramolecular organization of the components in SG1 and SG3 samples but this is

absent in SG5 sample. SAXS results indicate 17 and 36% expansion of PVA crystalline

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thickness and 77 and 79% increase in amorphous overlayer thickness for SG1 and SG3

systems but SG5 do not show any appreciable change from that of PVA. AFM results

indicate aggregation of SG sheets with increasing concentration and a maximum of three

SG sheets can enter into the interlamellar amorphous zone of PVA crystal during

composite formation. The composites exhibit a dramatic change in mechanical property

which is also dependent on composition, hence morphology of the system. The highest

increase of stress (177%), strain at break (45%) and toughness (235%) over PVA is

observed in the dendritic SG3. Youngs modulus increases progressively with increasing

SG concentration in the composites showing the highest increase (180%) over PVA for

SG5 system. The storage modulus of SG3 shows the highest (1005%) increase over PVA

at 30 oC. Also a 10 orders increase in the magnitude of dc conductivity over PVA and 10

fold increase in dc conductivity in the dendritic SG3 than any other morphology are

achieved. In the I-V characteristic curves, SG1 exhibit typical semiconducting nature,

SG3 shows an electronic memory and SG5 exhibit a rectification behavior. Similar

dendritic type morphology may be achieved in other systems also by proper modification

of graphene, suitable for supramolecular interaction with the polymer of interest.

Acknowledgment: We gratefully acknowledge DST, New Delhi (grant No.SR/SI/PC-

26/2009) and DST funded Unit of Nano Science at IACS for financial assistance. RKL

acknowledges CSIR new Delhi for granting fellowship. We also acknowledge Dr. Nikhil

Jana of IACS for helping in micro Raman spectra.

Supporting Information Available: EDXS, AFM, TGA, Raman spectra, FESEM,

SAXS, FTIR, Loss Modulus and Tan plots are available at the web free of charge.

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