poly(vinyl alcohol) composites

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Available at www.sciencedirect.com

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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

A R T I C L E I N F O

A B S T R A C T

Article history:

Composites of poly(vinyl alcohol) (PVA) with sulfonated graphene (SG) show fibrillar, den-

Received 10 May 2011

dritic and rod like structures for SG1, SG3 and SG5 samples, respectively, where the number

Accepted 15 September 2011

indicates weight percent of SG. Differential scanning calorimetry shows a new peak in

Available online 24 September 2011

addition to that of PVA arising from the supramolecular organization of the components in SG1 and SG3. Seventeen percent 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. Young’s 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 10 orders of magnitude increase of dc conductivity 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.  2011 Elsevier Ltd. All rights reserved.

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.

* Corresponding author: Fax: +91 3324732805. E-mail address: [email protected] (A.K. Nandi). 0008-6223/$ - see front matter  2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.09.039

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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 107 S/cm for 6 wt% graphene [23]. This value is rather low compared to the graphene/polystyrene composite (0.15 S/cm for 1.2 wt% graphene) [24]; graphene/polycarbonate composites (0.5 S/cm for 2.2 vol% graphene) [25] and RGO/poly (vinylidene fluoride) (PVDF) composites (104 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 –SO3H 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 sulfonated 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. Sulfonation is performed by oxidizing graphite with HNO3 and then reacting with sulfanilic diazonium salt [18]. PVA is then blended with 0.5,1, 3 and 5 wt% of SG by 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% hydrochloric acid, hydrazine hydrate solution (99%, synthetic grade), (Merck, Mumbai) are used as received. PVA (Aldrich, USA; 99+% hydrolyzed) has Mv ¼ 130; 000 which is measured from the intrinsic viscosity in water at 30 C (Mark- Houwink

constants, K = 42.8 · 105 dl/g and a = 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 mixture in concentrated H2SO4 using Hummers method [30]. Then, 75 mg GO is dispersed in 75 ml water (0.1% w/v) and is sonicated for 1 h in an ultrasonic bath (60 W, frequency 28 kHz, 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 sulfonation 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 sulfonation. The later reduction is necessary to get more sp2 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 C 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 sulfanilic acid and 10 mg 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 0 C 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 2 ml hydrazine hydrate solution under refluxed condition for 24 h at 100 C. Finally it is washed with water thoroughly and dried in vacuum at 60 C. 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 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 sulfur 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 sulfur with respect to carbon during hydrazine treatment. These sulfur 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 1.80 ± 0.07 nm measured from AFM height profile (SI Fig. 2). It increases after sulfonation by diazonium coupling (i.e. PSG) to 2.00 ± 0.16 which on

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817

Fig. 1 – A schematic presentation of SG preparation from graphite powder (GO = graphene oxide, RGO = reduced graphene oxide, PSG = pre-reduced sulfonated graphene and SG = sulfonated graphene after reduction by hydrazine).

reduction with N2H4 remains almost unchanged with an average value of 1.99 ± 0.2 nm for SG. Thus due to sulfonation 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 RGO indicating the presence of 33% oxygenous functional material. After sulfonation 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 we have observed the average thickness of the SG sheets = 1.99 ± 0.20 nm, average length = 352 ± 93 nm and average breadth = 219 ± 71.nm (from 20 measurements) for 1% (w/w) SG concentration.

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2.3.

5 0 ( 2 0 1 2 ) 8 1 5 –8 2 7

Composite preparation

Two hundred and fifty milligrams of PVA is dissolved in 5 ml water by heating at 90 C and is cooled to room temperature (30 C). SG solution is prepared in 5 ml water by sonication for 30 min and is then mixed with the PVA solution in different proportions. The mixture is sonicated for another 30 min for homogeneous mixing. Finally slow evaporation of the solvent in a petri-dish yields films of the composites which are dried at 60 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 60 C 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 C for a week. The atomic force microscopy (AFM) is conducted in the non contact mode at a resonance frequency of 250 kHz of the tip using 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 C followed by vacuum drying at 60 C for three days. Raman studies were performed using a micro Raman spectrometer (Agitron) with spot size of 1 lm2 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 C) in air and finally in vacuum at 60 C for three days. The FTIR peaks at 1042 and 1250 cm1 of PSG and those at 1042, 1125 and 1170 cm1 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 C at the heating rate of 20 C/min to 242 C. The crystallinity (kc) values of PVA and of the composites are calculated from the equation: kc ¼

DH DH0u

where DH is the measured enthalpy of fusion (from DSC) and DH0u is the enthalpy of fusion of pure PVA crystal (138.6 J g1) [33].

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 2h = 5 to 35 at the scan rate 0.5 s per step and for SAXS, the instrument is scanned from 2h = 0.2 to 5 at the scan rate of 2 s per step with a step size of 0.02. 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 0 ), loss modulus (G00 ) and tan d values of the composites are measured using a dynamic mechanical analyzer (DMA) (model Q-800, TA instruments). Sample films (25 mm · 5 mm · 0.15 mm) 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 60 C for three days. The films are installed at the tension clamp of a calibrated instrument. The samples are heated from 50 C to 100 C at the heating rate of 10 C/min. The G 0 , G00 and tan d 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 30 C. 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 measured by a screw gauge. The conductivity (r) of the above films are measured by two probe technique with an electrometer (Keithley, model 617) at 30 C using the equation: r¼

1 l  R a

ð1Þ

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

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

enlarged FESEM picture of SG1 (SI Fig. 5) show closely spaced fibrils in patches widely 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 and 3060 cm1) [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 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 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 (Figs. 5 and 6) indicate that PVA is a semi-crystalline polymer and the SAXS result (Fig. 7 and 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

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

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Fig. 4 – H-bonding interaction between PVA and SG.

19.40 229.7

SG5

SG5

141.6 SG3 232.8

134.3

SG3

19.58

234.6

Intensity (a.u)

Heat Flow(Endo Up)

19.48

SG1

19.80

SG0.5

19.80

SG1 228.8

PVA 25.24 PVA

SG 8.07 100

150

200 0

Temperature( C) Fig. 5 – DSC melting endotherms (scan rate 20 C/min) of different PVA–SG composites.

be attributed to the disposition of SG sheet near the crystal amorphous interface enhancing irregular chain folding which increase the crystal/amorphous interface 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

5

10

15

20

25

30

35

2 θ (degree) Fig. 6 – WAXS patterns of SG and different PVA–SG composites.

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 it’s concentration is responsible for the morphology change. At the inter-fibrillar region some SG and PVA chains (with defect structure, e.g.

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Intensity

0.41

SG3

0.67

0

PVA 1

2

3

4

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5

2 θ (degree) Fig. 7 – SAXS patterns of PVA and SG3 composite.

entangles, knots, etc.) may exist and the population of SG sheets in the inter fibrillar region increases as the SG concentration increases. In SG3 sample the interfibrillar SG concentration is large and the lateral dimensions of SG (length and width 352 and 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 (DH) 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 140 C 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 DHf of SG5 with 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 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 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 ˚ ) which proa sharp diffraction peak at 2h = 19.8 (dhkl = 4.48 A gressively shifts to lower 2h value and finally reaches at 19.4 ˚ ) for SG5. Though unusual, the specific interac(dhkl = 4.57 A

Table 1 – Melting temperature (Tm), enthalphy of fusion (DHf), long distance (L), crystallinity (1  k)DH, crystalline thickness (lc) and amorphous thickness (la) of PVA–SG composites. Sample PVA SG1 SG3 SG5

Tm (C)

DHf (J/g)

L (nm)

(1  k)DH (%)

lc (nm)

la (nm)

228.8 232.8 134.03 234.6 141.6 229.7

49.6 37.7 30.2 41.1 49.4 54.3

13.17 20.52

36 27

4.74 5.58

8.43 14.94

21.52

30

6.46

15.06

13.57

40

5.39

8.14

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a

b

c

Fig. 8 – Supramolecular organization of PVA and SG for (a) SG1 (b) SG3 and (c) SG5 samples producing different morphology.

tion between SG and PVA may expand the unit cell dimensions of PVA to some extent (62%) in the composites. In SG5 the increase of d-spacing is somewhat lower than that of SG1 and SG3 (cf; relative decrease of 2h 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 ˚ ). This peak at 11 corresponding to its interlayer spacing (7.8 A diffraction peak disappears when mixed with PVA indicating exfoliation of GO sheets [37]. The PVA peak intensity at 2h = 20 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 2h 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 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 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 and 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. 21.7 nm) 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

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

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7

No of particle

6 5 4 3 2 1 0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

No of particle

Thickness (nm)

8 7 6 5 4 3 2 1 0 2.1 2.4 2.7 3.0 3.3 3.6

Thickness (nm)

No of particle

10 8 6 4 2 0

5 10 15 20 25 30 35 40

Thickness (nm)

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

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. 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 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 Young’s modulus increases progressively with increasing SG concentration in the composite and the highest value (8 GPa) is achieved for SG5 (Table 2). The reported enhancements of Young’s modulus are 62% for 0.7 wt% GO [9], 100% for 1.8 vol% RGO [10] and 128% for 3 wt% GO [8]. However, in the present system 180% increase

of Young’s 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. Young’s modulus, strain at break and toughness are comparable to those for SG composites (Table 2) suggesting that hydrazine treatment used for reduction of PSG does not affect the mechanical property. The storage modulus (G 0 ) vs temperature plot (Fig. 11), show a significantly large increase in the composites and

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180

Tensile Stress (M Pa)

160 140 120 100

20000 8

1. PVA 2. SG 0.5 3. SG 1 4. SG 3 5. SG 5

4 6

16000

SG3

Stortage modulus

Young's Modulus (GPa)

200

5 0 ( 2 0 1 2 ) 8 1 5 –8 2 7

4 2

0

1 2 3 SG % (w/w)

4

5

SG5

80

PVA

2

1

8000

40 20

5

4000

SG1

60

12000

3

0 -40

SG0.5

-20

0

20

40

60

80

100

0

Temperature ( C) 0 0

50

100

150

200

250

300

Strain (%) Fig. 10 – Mechanical property of PVA and its composites with sulfonated graphene at indicated compositions.

it is very large in SG3 (Table 3). The highest (1005%) increase of SG3 over PVA is observed at 30 C enabling it to behave as a promising reinforced plastic at ambient condition. The loss modulus (G00 ) 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 0 and G00 is the same and the 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 0 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 0 and it is due to the formation of supramolecular organized structure. The peak temperature of tan d 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 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

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

in increasing the segmental motion of PVA chains hence decreasing the Tg.

3.3.

Conductivity

Graphene is a good conductor of electricity [41,42] and PVA is an insulator, (r = 5.3 · 1014 S/cm), but SG0.5, SG1, SG3 and SG5 have conductivity 6.1 · 1010, 1.2 · 105, 0.9 · 104, and 1.5 · 105 S/cm at 30 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 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

Table 2 – Stress at break, strain at break, Youngs modulus, toughness of PVA and PVA–SG composites at 30 C. Sample PVA SG1 SG3 SG5 PSG3 PSG5

Strain

% Increase

Stress (MPa)

187 266 272 84 256 101

– 42 45 55 36.8 45

48 97 133 101 130 104

% Increase – 102 177 111 170 116.7

Modulus (GPa) 2.9 3.9 5.3 8.0 5.5 7.8

% Increase – 36 86 180 89.7 169

Toughness (N mm) 196 526 657 222 645 227

% Increase – 169 235 13.5 229 15.8

CARBON

825

5 0 (2 0 1 2) 8 1 5–82 7

Table 3 – Storage modulus (G 0 ) and Glass transition temperature (Tg) from tan d plots and values of PVA–SG composites.

PVA SG1 SG3 SG5

Tg (C)

G 0 (MPa) at 30 C

32.2 24.8 21.8 29.2

11,265 13,995 17,463 16,946

% Increase – 24.2 55 50

G 0 (MPa) at 0 C 3507 10,885 14,426 7417

-4

log(σ)

-6 -8 -10 -12 -14 0 0.37

1

2

3

4

5

SG % (w/w)

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

conductivity, e.g. 105 S/cm for PVDF/grahene-PMMA [10] for 5 wt% filler, 104 S/cm for RGO /PVDF composite at 4wt% filler [26]. In a surfactant wrapped graphene/PVC composites the conductivity is 104 S/cm at 5.5 vol% filler [44] and in ECG/PVA composite the conductivity is 107 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 sulfonation some sp2 hybridized bonds being converted into sp3 hybridized bonds, which are not totally recovered to sp2 hybridized bonds during reduction by hydrazine, and destroys the g 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 1.1 · 107 S/cm which is three orders lower over SG3 composite (0.9 · 104 S/cm). In PSG5 composite the conductivity is found to be 1.2 · 107 S/cm which is also 2 orders lower over SG5 sample (1.5 · 105 S/cm). Thus a 2–3 orders increase in magnitude of conductivity is observed in the hydrazine reduced SG3

% Increase – 210 311 112

G 0 (MPa) at 10 C

G 0 (MPa) at 30 C

% increase

3496 10,874 14,341 7332

– 211 310 109

% Increase

749 6946 8274 1708

– 827 1005 128

samples. As discussed in the mechanical property section the percentage increases of stress, Young’s modulus and toughness of PSG composites are comparable with SG composites (Table 2) suggesting 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 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

80 40

SG5

0 -40 300

Current (μ amp)

Sample

150

SG3

0 -150 -300 70 35 0

SG1

-35 -70 -6

-4

-2

0

2

4

6

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

826

CARBON

5 0 ( 2 0 1 2 ) 8 1 5 –8 2 7

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 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. Young’s 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 C. 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.

Acknowledgments 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. R.K.L. acknowledges CSIR New Delhi for granting fellowship. We also acknowledge Dr. Nikhil Jana of IACS for helping in micro Raman spectra.

Appendix A. Supplementary data EDXS, AFM, TGA, Raman spectra, FESEM, SAXS, FTIR, loss modulus and tan d plots are available at the web free of charge. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon. 2011.09.039.

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