Role of Enhanced Hydrogen Bonding of Selectively Reduced Graphite

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Role of Enhanced Hydrogen Bonding of Selectively Reduced Graphite Oxide in Fabrication of Poly(vinyl alcohol) Nanocomposites in Water as EMI Shielding Material Kunal Manna, Suneel Kumar Srivastava, and Vikas Mittal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03356 • Publication Date (Web): 07 Jul 2016 Downloaded from http://pubs.acs.org on July 11, 2016

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The Journal of Physical Chemistry

Kunal Manna†, Suneel Kumar Srivastava†,* and Vikas Mittal‡ †

Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, United Arab Emirates

‡

ABSTRACT: According to available literature, hydrophobic nature of graphene restricts the fabrication of its polymer nanocomposites in aqueous medium without its modification or through in situ synthesis from graphite oxide. In view of this, present work reports on fabrication of nanocomposites of poly(vinyl alcohol) filled with selectively reduced graphite oxide (SRGO) in aqueous medium. FTIR study establishes the formation of nanocomposites through enhanced intermolecular hydrogen bonded network between –OH (PVA) and surplus –OH functionalities of graphene sheet in SRGO in comparison to that derived from PVA/graphite oxide (GO). PVA/SRGO-0.2 nanocomposite showed maximum improvements in tensile strength (127%), elongation at break (25 %) and thermal stability (45°C) compared to neat PVA. This is ascribed to better dispersion and presence of extensive intermolecular hydrogen bonding between PVA and SRGO. All PVA/SRGO nanocompopsites exhibited relatively higher glass transition temperature (Tg), melting temperature (Tm) and crystallization temperature (Tc). The formation of conducting network also accounts for good electromagnetic shielding efficiency (EMI SE) of PVA/SRGO nanocomposites in 2-8 GHz frequency range with respect to PVA. It is anticipated, this novel approach could be further extended in synthesizing many other SRGO filled polar polymer nanocomposites in aqueous medium for their diverse applications.

1. Introduction Recent progress in modern technology and high speed communication requires the advancement of wireless technology. In view of this, every stage of sophisticated lifestyle electronic devices are equipped with radiation devices to make it a suitable wireless instrument, e.g. satellite communication, mobile phones, disaster and accident surveillance etc. have been proved to be an inherent way to solve increasing complexities in our growing life. All these devices are equipped with very high frequency radiator depending on their way of use. As a consequence, the use of reckless electromagnetic (EM) radiation creates a new kind of pollution, known as electromagnetic interference (EMI) or radio frequency (RF) noise. The electromagnetic interference (EMI) results in the malfunctioning of the devices and slows down telecom and remote data transport.1 Therefore, it is mandatory to take effective steps to prevent such radiation disaster by developing suitable shielding materials. According to the basics of EMI, absorption and reflection of EM wave by these shielding materials are considered to be most important factors. Absorption depends on the thickness and bulk of the material. In contrast, reflection of EM wave occurs from the surface of the shielding material and does not depend on its thickness. According to earlier reports concern ferromagnetic material like Fe, Co, Ni, γ-Fe2O3, Fe3O4 and ceramic, like TiO2, SiO2, ZnO, BaTiO3 and carbonyl iron has been investigated in EMI shielding1-5 as EM wave absorber. But the problem of heavy weight, film processing difficulties, etc. made them unsuitable for this purpose. On the other hand, dielectric material composed of transition metal oxides, like BaTiO3, TiO2, SiO2, ZnO suffer from lack of permittivity at GHz. frequency range, processing difficulties, poor dispersion, as well as agglomeration during film processing.6 In addition, poor dispersion and agglomeration of these materials restrict their use as fillers in polymer matrices due to seepage of EM

wave. As a result, lightweight carbon based materials, such as carbon black, graphite, expanded graphite, reduced graphite oxide, graphene, single and multi walled carbon nanotube (CNT) and carbon nanofiber (CNF) has been investigated as other alternative EMI shielding materials. However, micrometer sized graphite and carbon black exhibited poor dispersibility and high percolation threshold. CNT and CNF are costly enough to draw considerable attention for large scale preparation. Additionally, they suffer from the difficulties in purification and prolonged functionalization process. The compatibility between graphene and polymer remain another vital issue during the formation of their nanocomposites. Simultaneously, poor dispersion of graphene in the polymer matrix causes insignificant improvements in shielding efficiency performance, thermal and mechanical property and needs its modification. 7,8 Graphene consists of low availability of hydroxyl (–OH) and carboxylic acid (–COOH) functionality in contrast to graphite oxide (GO).3,9 In our previous work10 we reported selective reduction of –COOH groups of graphite oxide (product referred as selectively reduced graphite oxide, SRGO) to achieve considerable –OH functionality in graphite oxide and its properties similar to graphene.11,12 Poly (vinyl alcohol) (PVA) is a polar polymer exhibiting high biocompatibility, good transparency, high gas barrier properties, nontoxicity and film forming properties accounting for its multifaceted applications.13,14 Further, water accounts for the high solubility of PVA as well as good dispersion medium of SRGO. Therefore, it is anticipated that good compatibility, better exfoliation and uniform dispersion of SRGO in water and its strong intermolecular hydrogen bonding with alternating –OH groups of PVA could lead to the formation of mechanically and thermally improved PVA/SRGO nanocomposites in comparison to nanocomposites derived from GO and PVA.15,16

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Scheme 1: Flow diagram of preparation of SRGO According to available reports, varieties of carbonacaeous nanostructures based PVA nanocomposites, such as SWCNT,17 MWCNT,18 CNF19 and graphene nanoribbon20 composites have been reported as EMI shielding materials. In addition, transition metal oxides (Fe2O3, ZnO, SiO2, ZrO2, and TiO2),6 CdS,21 and Ag/Fe2O322 composites have also been investigated. However, typical modification procedure many times created problems due to their prolonged steps, complex apparatus, costly modifying agent, waste of materials during multistep etc. Further, undesirable bulky functional groups in modifiers limit its interaction with PVA. But in our case, selective reduction of –COOH could enhance –OH functionality of GO and overcome these problems. The considerable improvement in conductivity is also anticipated at low percolation threshold involving conducting network formation through extensive intermolecular hydrogen bonding between PVA and SRGO. All this, is likely to enhance EMI shielding efficiency, thermal and mechanical properties of PVA/SRGO nanocomposites simultaneously. No such method has been reported till now to prepare PVA/SRGO nanocomposites using such enhanced hydrogen bonding interaction. In view of this, we focused our work fabrication of PVA/SRGO nanocomposites through green method and characterization by XRD, FT-IR, AFM, UV-Vis, SEM and TEM. We have also investigated mechanical, thermal properties, conductivity and performance as EMI shielding material at 2-8 GHz. frequency range of our prepared PVA/SRGO nanocomposites. 2. Experimental Section Materials: Graphite Micro-850 was received from The Asbury Graphite Mills, INC, Asbury Warren County, NJ. Sodium nitrate was supplied from S. D. Fine Chemicals, India. Potassium permanganate, Hydrogen peroxide and concentrated Sulphuric acid were obtained from Merck, India. Metallic sodium was purchased from Sisco Research Laborat ory, Mumbai, India. Methanol, Ethanol and Acetone were

procured from SRL Pvt., Mumbai, India. PVA (Degree of Polymerization, Dn : 1700-1800, number average Mol. Wt., Mn : 74800-79200) was purchased from Loba chemicals Mumbai. Preparation of Graphite Oxide: Graphite oxide (GO) was prepared by Hummers’ method.23 According to this method, 100 mg graphite was mixed with 50 mg NaNO3 and 5 mL of conc. H2SO4, and cooled down resulting mixture to 0°C followed by stirring for 10 min. Thereafter, 300 mg KMnO 4 was slowly added while maintaining the temperature below 5 °C under constant stirring for another 30 min. After that, 10 mL water was added and the temperature was raised to 90 °C. The mixture was further diluted with 20 mL water forming a dark brown mixture and 3-4 mL H2O2 was added and left overnight. The product was washed with 10% HCl followed by warm water for several times in a centrifuge under 4000 rpm and dried at 600 C. Preparation of Selectively reduced Graphite Oxide: According to earlier reported procedure,10 0.1 g of GO was dispersed in 50 mL of methanol in a 100 cc round bottom flask fitted with a condenser. Subsequently, carboxylic group in GO were esterified to form esterified GO by adding catalytic amount of conc. H2SO4 to the above mixture maintained at 95 °C for 10 h followed by continuous stirring. It is followed by careful addition of ~5 g of metallic sodium and heating continued for 24 h. Finally, the resulting product was centrifuged several times and dried in vacuum for 24 h. The transformation of GO to SRGO is displayed in Scheme 1. Preparation of PVA/SRGO nanocomposite films: PVA/SRGO nanocomposites were prepared by simple cost effective solution mixing method. Accordingly, 1 g PVA was dissolved in 20 mL Millipore water at ~90°C for 2 h in an oil bath and the solution was consequently allowed to cool to room temperature. In another round bottom flask, required amount of as prepared SRGO was dispersed in minimum amount of millipore water under ultrasonic treatment for 1 h. Following this, it was added to aqueous PVA solution at room temperature and magnetically stirred for 30 min. Finally, the


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solution was cast on a glass petridis and kept at 60oC for film formation. In this manner, samples consisting of 0, 0.2, 0.5, 1.0, 1.5 and 2 phr of SRGO was mixed with respect to aqueous PVA solution at room temperature and respective films were cast in a petridis. Finally, dried films were peeled off from this for further characterizations and application. Characterization Techniques: X-ray diffraction (XRD) measurements were employed at room temperature to characterize GO, SRGO and PVA nanocomposites with a Phillip, Holland instrument with CuKα radiation (0.1541 nm) in the range 5° to 70°, with a scanning rate of 3° / min. GO and SRGO were analysed as powder and PVA nanocomposites were analysed as thin films. FTIR analysis of powder compressed samples (GO and SRGO dispersed in KBr) was recorded in the wavenumber range of 400–4000 cm−1 on Perkin-Elmer FTIR Spectrometer RXI. PVA nanocomposites were analysed as thin film on the same machine. Spectrometer RXI using Ultraviolet-visible (UV-vis) absorption spectrum was recorded on Varian Cary 5000 UVVisible spectrophotometer in the range of 200-800 nm. Atomic force microscopy (AFM) study was performed by Scanning Probe Microscope (SPM), Model: Multiview - 1000. The morphological study of the samples was done by scanning electron microscope (SEM) and field emission scanning electron microscope (FESEM) on ZEISS, JSM-5800 and BRUKER scanning electron microscope instrument operating at 20 kV respectively. Transmission electron micrographs were taken on TECNAI G2, SEI (Netherland) instrument operating at 200kV was used to obtain digitally acquired images on a Gatan multipole charge-coupled device (CCD) camera for the samples placed on carbon-coated Cu grid from dispersion in ethanol. For, Tensile strength (TS) and elongation at break (EB) of the neat polymer and the nanocomposites, dumbbell like tensile specimens were punched out from cast film using ASTM Die-C. The tests were carried out as per the ASTM D 412-98 method in a universal testing machine (Hounsfield H10KS) at a crosshead speed of 200 mm min−1 at 25 ± 2 °C. Three parallel runs were done in case of each sample to get the average. In order to investigate the thermal properties of the prepared PVA/SRGO nanocomposites thermogravimetric analysis (TGA) was performed by taking ca 5-10 mg of each sample using a Redcroft 870 thermal analyzer, Perkin Elmer, with a linear heating rate of 10◦C / min. over the temperature range 35– 1000°C under nitrogen atmosphere. A Discovery DSC apparatus was used for thermal characterisation of the materials studied. The DSC measurements were performed in a nitrogen atmosphere over the temperature range from -80 oC to 250 oC. The samples were first heated at a linear heating rate of 20 oC min-1 to 250 0C and held there for 1 min; subsequent cooling to -80 °C was performed at V = 20 °C min1 under normalized heat flow. Then samples were reheated at 10 0C min-1 from -80 oC to 250 °C, and recooled to -80 °C at a cooling rate of 20 °C min-1. Only, the scans recorded in the second cycle were taken into consideration. Room temperature DC conductivity was measured by using HIOKI 3532 impedance measurement machine. EMI shielding efficiency of the PVA/SRGO nanocomposites was measured on the thin films of thickness 0.1- 0.3 mm and diameter 1.5 cm in the frequency range of 100 KHz – 8 GHz by using ENA Series Network Analyzer, E5071C, Agilent Technology. The S 11 (or S22) i.e. reflection coefficient (R) and S12 (or S21) i.e.

transmission coefficient (T) parameter measurements were carried out with a wave guide of diameter 2 cm. Therefore, the circular films of PVA/SRGO nanocomposites of 1.5 cm diameter were punched and placed between the respective holders. After that, the sample holder was connected between two coaxial wave guide adaptors and tightened and finally connected to the network analyzer to measure the scattering parameters S21 and S11 to calculate the shielding efficiency (SE) parameters. 3. Results and Discussion: 3.1 X-ray Diffraction Figure 1 shows XRD patterns of graphite, GO and SRGO. It is noted that (001) diffraction peak of flake graphite is located at 2θ = 26.5° (d001=0.34 nm) completely disappeared and typical diffraction peak of GO appeared at 2θ = 10.9° (d001=0.82 nm). In selectively reduced graphite oxide, (referred as SRGO), XRD showed appearance of a peak at 2θ ~ 24.50° (d002=0.36 nm) analogous to that noticed during preparation of reduced graphite oxide by using normal reducing agents (NaBH4, N2H4, hydroquinone heat treatments etc.).24,25

Figure 1: XRD of graphite, graphite oxide (GO) and selectively reduced Graphite Oxide (SRGO)

Figure 2: XRD of (a) neat PVA, (b) PVA/ SRGO-0.2, (c) PVA/ SRGO-0.5, (d) PVA/ SRGO-1, (e) PVA/ SRGO-1.5 and (f) PVA/ SRGO-2



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XRD of PVA/SRGO nanocomposites in Figure 2 showed presence of a diffraction peak at ~2θ = 19.2° (d101=0.46 nm) of pure PVA.26-28 Further, it is noted that intensity of this peak is more intense in nanocomposites compared to neat PVA. Such improved crystallinity of PVA could influence properties of its nanocomposites due to super mechanical strength of SRGO and restrict PVA chain arrangement termed as “molecular movement restriction” effect.29 Additionally, disappearance of SRGO diffraction peak in XRD of PVA nanocomposites suggested exfoliation of SRGO in PVA matrix. However, overshadowing of SRGO peak by semi-crystalline PVA also cannot be ruled out. Therefore, PVA/SRGO nanocomposites need to be further analysed by TEM before drawing any conclusive inference on exfoliation of SRGO in PVA. 3.2 FT-IR Analysis Figure 3 shows the FT-IR spectra of GO, esterified GO and selectively reduced graphite oxide (SRGO). The characteristic bands in GO at 3400, 1715, 1050, 1268 and 1564 cm-1 correspond to respective stretching vibrations of O–H, C=O, C–O, C–OH bending and skeletal vibrations from unoxidized graphitic domains respectively10,29 confirmed presence of –OH and –COOH groups. The shifting of 1720 cm-1 (>C=O) peak of GO to 1735 cm-1 in SRGO suggested conversion of – COOH to –COOCH3. Further, peaks corresponding to C=O (1715 cm-1) and C–O (1050 cm-1) stretching vibration of – COOH group almost disappeared in SRGO and peak intensity of –OH stretching (3400 cm-1) increased considerably. These findings clearly demonstrated the selective reduction of – COOH groups of GO to –OH (SRGO). FT-IR spectra of PVA and its SRGO nanocomposites in Figure 4 shows that –OH stretching frequency (~3365 cm-1) is shifted to lower frequency range with SRGO loadings in PVA compared to neat PVA. This is ascribed to the presence of intermolecular hydrogen bonding between –OH groups of PVA and SRGO. It is anticipated that incorporation of the SRGO in PVA matrix weakens the hydrogen bonding of the –OH groups of PVA leading to “hydrogen bond barrier” effect. Further, new hydrogen bond formation takes place between –OH functionality of PVA and surplus –OH functionality in SRGO as inferred from deconvoluted FTIR spectra (Figure S1).

Figure 3: FTIR spectra of (a) GO, (b) SRGO, (c) Esterified GO

Figure 5: Schematic Representation of Formation of Intermolecular Hydrogen bonded network between PVA and SRGO nanosheet

Figure 4: FTIR spectra of (a) neat PVA, (b) PVA/SRGO-0.2, (c) PVA/SRGO-0.5, (d) PVA/SRGO-1, (e) PVA/SRGO-1.5 and (f) PVA/SRGO-2

Figure 6: Plot of % H-bonding vs. SRGO loading.


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In addition, increase of SRGO loading in PVA increases the extent of hydrogen bonding due to the presence of more number of –OH functionalities. Figure 5 shows schematic presentation of formation of such intermolecular hydrogen bonded network between PVA and SRGO nanosheet. It is noted that presence of such inter molecular hydrogen bonding causes the blue shift of the –OH stretching peak from ~3365 cm-1 to ~3306 cm-1.29 Figure 6 shows variation of % hydrogen bonding versus SRGO loading in PVA nanocomposites. It is evident intra molecular hydrogen bonding in neat PVA (43 %) has been reduced to lowest (29 %) in maximum SRGO loaded (2 phr) PVA nanocomposite. The data (Table S1) of Figure 6 has been generated from the deconvoluted FTIR spectra (Figure S1) of PVA/SRGO nanocomposites. Further, it is also seen from Figure 6, that % of intermolecular hydrogen bonding has been considerably increased from 26 % (PVA) to 36 % (PVA/SRGO-2). No significant change has been observed in % of free (non hydrogen bonded) –OH groups. Such improvements in hydrogen bonding could be a key factor in improvement of various physicomechanical properties along with electromagnetic attenuates in the corresponding PVA/SRGO nanocomposites.30

Figure 7: TEM images of (a) GO and (b) SRGO

3.4 UV-Vis Spectra Figure 9 shows the UV-Vis transmission spectra of pure PVA and PVA/SRGO nanocomposites. It is clearly evident that neat PVA films exhibit appreciable transparency (7580%) in the range of 250-800 cm-1. In contrast, incorporation of the SRGO in PVA introduced strong scattering/absorption leading to significantly low transmittance in PVA/SRGO composites.37

3.3 TEM and AFM analysis of SRGO TEM image of SRGO in Figure 7 shows the appearance of ultrathin transparent multilayer folded sheets unlike stacked layers of several folds and distortions observed in GO. 29 Figure 8 represent AFM of GO and SRGO to provide idea about their thickness of individual layers. It is noted that height of layers in GO is ~3-5 nm due to presence of different oxygen functionalities (>C=O,–COOH, –OH, –O–). Further, presence of distortions and folds in GO could be ascribed to the disruption of original conjugation of graphite and introduction of lattice defects. In contrast, lower height of SRGO layer (~2.53 nm) could be ascribed to the absence of –COOH functionalities. Additionally, smaller thickness of layers in SRGO is an indicator for its larger specific surface area.29,31-36 Figure 9: UV-Vis Spectra of PVA/SRGO nanocomposites

(b)

(d)

Figure 8: (a) AFM image, (b) height profile of GO, (c) AFM image and (d) height profile of SRGO



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Figure 10: SEM images of fracture surface of (a) neat PVA, (b) PVA/SRGO-0.2 and FESEM image of (c) PVA/SRGO-0.2 nanocomposites

3.5 Morphological Analysis of PVA/SRGO nanocomposites Scanning electron microscopy (SEM) has been carried out to investigate the degree of exfoliation/dispersion and fracture surface morphology of the SRGO in PVA matrix in PVA/SRGO nanocomposites and findings are displayed in Figure 10 (a) and (b). The rough fractured surface morphology is clearly observed in 0.2 phr SRGO loaded PVA nanocomposite in contrast to smooth fractured surface in pure PVA. Further, a well dispersed densely packed distribution of SRGO nanosheets is also observed in PVA matrix consisting of little wrinkles.38 A close up inspection from high magnification FESEM image in Figure 10 (c) shows that SRGO sheets at the fracture surface of the PVA/SRGO-0.2 nanocomposites are not easily distinguishable. Therefore, it may reasonably be concluded that SRGO sheets embedded in the PVA matrix are crumpled, wrinkled or even folded.39 The reinforcing effect of such homogeneously dispersed filler could be reflected in the remarkably improved mechanical strength. 3.6 Mechanical Properties The variation in mechanical properties of PVA and its SRGO nanocomposites has been investigated and displayed in Figure 11. It shows maximum enhancement (~127 %) in tensile strength (TS) in 0.2 phr SRGO filled PVA. This is found to be much superior than reported earlier involving various fillers, such as graphene oxide,29,39 graphene,41-44 SWCNT, 45,46 MWCNT,47-48 LDH,49 graphene-MWCNT39,50 etc (Table S2). Further, elongation at break (EB) is maximum enhanced (~50 %) in PVA/SRGO-1 nanocomposites.42 It may be stated that existence of effective intermolecular hydrogen bonding between –OH groups (PVA and the excess –OH functionalities of SRGO) could account for such enhancement in mechanical properties of PVA/SRGO nanocomposites. As a consequence, it leads to the reduction in the intra-molecular hydrogen bonding of neat PVA due to “hydrogen bond barrier” effect.29 Alternatively, possibility of decrease in crystallinity of PVA due to incorporation of SRGO leading to “molecular movement restriction effect” also cannot be ruled out. 29 XRD of SRGO showed structural integrity similar to reduced graphene oxide (RGO).10 Therefore, it is anticipated highly improved mechanical properties of PVA involve efficient load transfer through hydrogen bonding between PVA and SRGO.29 Further, higher SRGO loadings in PVA is accompanied by regular fall in EB possibly due to enhanced rigidity of PVA chains.28,37 Our findings as displayed in Figure 11 also established that TS and EB of PVA/SRGO

nanocomposites are much superior compared to that of PVA/GO nanocomposites. This could be attributed to restricted mobility of PVA chains while undergoing hydrogen bonding with SRGO nanosheets having large aspect ratio. Young’s modulus (YM) of PVA/SRGO and PVA/GO nanocomposites were calculated from stress-strain plots (Figure S2) and the corresponding data is provided in Table S3. These findings show improvement in YM by 170 and 95 % in 0.2 phr filled GO and SRGO respectively. Table S3 also shows that YM of PVA/SRGO nanocomposites at any other higher loadings were always superior compared to either neat PVA or PVA/GO nanocomposites possibly due to strong polymer-filler interaction. Further, it is noted that 5 phr loaded PVA nanocmposites exhibited maximum improvement (251%) in YM.18,26,27,29,38,39

Figure 11: Plot of TS and EB vs filler content of GO and SRGO in PVA nanocomposites.


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Figure 12: Thermograms of PVA/SRGO nanocomposites

3.7 Thermogravimetric Analysis

3.8 Differential Scanning Calorimetry (DSC) Analysis

Thermal stability behaviour of PVA and its SRGO nanocomposites was studied in temperature range of 30 °C to 1000 °C under nitrogen atmosphere and corresponding thermograms are displayed in Figure 12. It is noted that neat PVA as well as its nanocomposites undergo degradation through several steps. The first step (250-300°C) corresponds the exclusion of side groups due to the degradation of PVA. 39 It is also noted that the first onset degradation of neat PVA (~48°C) is significantly improved by 93°C in 0.2 phr loaded SRGO. The second degradation takes place in the temperature range of ~ 350 to 500°C due to the breakdown of polymer backbone.51 The final degradation step (500-1000°C) corresponds to residue formation. Table 1 records thermal data of pure PVA and its SRGO nanocomposites corresponding to temperature at which the weight loss is 15% (T15%). It is noted that thermal stability of PVA/SRGO nanocomposites is always enhanced compared to neat PVA as well as to that of PVA/GO nanocomposites (Figure S3). PVA/SRGO-1.5 showed maximum improvement in thermal stability (35 0C) due to better dispersion of SRGO in PVA matrix. Further extensive hydrogen bonding between –OH functionality in PVA and SRGO could inhibit the mobility of polymer chains and heat flux and account for maximum enhancement in thermal stability of PVA/SRGO nanocmposites. The decrease in T15% in PVA nanocomposites filled with 2 phr of SRGO is in all probality due to aggregation of SRGO.41

As inferred from FTIR study, intermolecular hydrogen bonding between the excess –OH groups present in SRGO and PVA chains accounts for the effective load transfer in PVA/SRGO nanocomposites. Accordingly, mobility of chains in PVA in its SRGO composite is also likely to be restricted. In order to verify this fact, DSC of PVA/SRGO nanocomposites were carried out and findings are displayed in Figure 13. This provided an idea about the change in glasstransition temperature (Tg) of PVA/SRGO nanocomposites, if any, with respect to neat PVA counterpart. Further, Tg at the half-height of a heat capacity step, temperatures of onset transition (Tg’) and completion (Tg”), characteristic of relaxation strength in the glass transition (ΔC p), melting point (Tm) at the endothermic melting peak maximum and melting enthalpy (ΔHm ) has also been analyzed and corresponding all these findings are summarized in Table 2. It is clearly evident that incorporation of SRGO is accompanied by increase in Tg, Tm and Tc followed by reduction in ΔCp and % crystallinity (χcr). This could be ascribed to the enhanced hydrogen bonding interaction between alternating –OH (PVA) and surplus –OH (SRGO) functionality. As a result, restricted mobility of PVA chains in PVA/SRGO nanocomposites accounts for its increment in Tg of PVA/SRGO nanocomposites compared to neat PVA.52 It is also noted from Figure 13 (a) and (b) that Tg (and Tm) is maximum increased in PVA/SRGO-1.0 composite by ~5 °C (and ~4.18 °C) compared to neat PVA. However, aggregation of SRGO at higher filler loadings resulted in lowering of Tg as well as Tm of PVA.41 In contrast, Tc increased continuously with filler loading as evident from Figure 13 (c) due to the ease of crystallization with increase in temperature in PVA/SRGO nanocomposites. The crystallinity of PVA/SRGO nanocomposites is likely to play a key role on mechanical property performance due to semi-crystalline behaviour of PVA Therefore, % crystallinity (χcr) of neat PVA and its SRGO nanocomposites were calculated according to the following equation (1): 27,52

Table 1: Degradation temperature at 15 % wt. loss of PVA/SRGO and PVA/GO nanocomposites Sample

T15% (◦C)

Sample

T15% (◦C)

PVA

266

PVA

266

PVA/SRGO-0.2

285

PVA/GO-0.2

278

PVA/SRGO-0.5

280

PVA/GO-0.5

280

PVA/SRGO-1 PVA/SRGO-1.5

297 301

PVA/GO-1 PVA/GO-1.5

277 281

PVA/SRGO-2

279

PVA/GO-2

288

 H 

m   100%  cr   0   H m  


(1)


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Figure 13: DSC plots of PVA/SRGO nanocomposites (a) Glass Transition temperature, (b) Melting temperature and (c) Crystallization temperature

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where, enthalpy characteristics ΔHm values were calculated from the area of corresponding endothermic peaks in DSC melting curves of neat PVA and PVA/SRGO nanocomposites based on ‘‘triangle method”.52 It is also to be noted that melting enthalpy for a perfect 100% PVA crystal (ΔH m°) has been considered as 138.6 J g-1 .27,52 Table 2 records corresponding % crystallinity data of PVA and its SRGO nanocomposites. It is evident that loading of 0.2 phr SRGO in PVA decrease its crystallinity to some extent. Further, no significant changes in crystallinity have been observed at higher SRGO loadings in PVA. In view of this, it is anticipated that crystallinity alone cannot account for significant increase in TS of the PVA/SRGO nanocomposites. Therefore, it is anticipated momentous improvements in mechanical properties of PVA composites could be ascribed to load transfer through formation of effectual intermolecular hydrogen bonding network between the surfeit –OH groups of SRGO and enfolded PVA matrix. Additionally, excellent exfoliation and large aspect ratio of SRGO also favor the stress transport across the interface of PVA-SRGO. As a consequence of this, even 0.2 phr loading of SRGO in PVA matrix is escorted by significant “molecular movement restriction effect” 27,29 as well, results in the enhancement in mechanical properties of PVA/SRGO nanocomposites. 3.9 Room Temperature DC Conductivity Analysis Several polymer carbonaceous nanostructures based nanocomposites exhibit reasonable conductivity compared to insulating polymer, e.g. polystyrene/graphene, 53 54 55 PMMA/graphite and PVA/CNT. In most of these cases, modification of the filler is mandatory in order to make it compatible with polymer matrix. However, such modification also introduced structural defect leading to poor electronic properties with respect to the unmodified filler resulting in higher percolation threshold.56,57 As a consequence, charge conducting ability of graphite is disrupted in graphite oxide due to the absence of diffused π electronic cloud, disordered conjugation and lattice defects.58 Room temperature dc conductivity of GO and SRGO are found to be ~4.6 × 10-4 and ~0.02 S cm-1 respectively. These findings clearly demonstrated inferior electrical conductivity of PVA/graphite oxide nanocomposites.59 Measurement of room temperature DC electrical conductivity (σRT) of neat PVA and SRGO nanocomposites has been carried out and displayed in Figure 14. Accordingly, σRT values of PVA, PVA/SRGO-0.2, PVA/SRGO-0.5, PVA/SRGO-1.0, PVA/SRGO-1.5 and PVA/SRGO-2 correspond to 1.25×10-6, 9.5×10-6, 3.4×10-5, 6.8×10-5, 9.0×10-5 and 1.7×10-4 S cm-1 respectively. These findings clearly demonstrated that conductivity of PVA/SRGO nanocomposites is always higher (σRT maximum: PVA/SRGO2) than PVA alone. It is anticipated that SRGO sheets in PVA/SRGO nanocomposites are aligned in a regular fashion with proper interconnection due to the H-bonding with the

Table 2: DSC crystallization and melting parameters of PVA and PVA/SRGO nanocomposites Samples

Amount of PVA (g)

SRGO (phr)

Tg’ (oC)

Tg (oC)

Tg’’ (oC)

ΔCp (Jg-1K-1)

Tc (oC)

Tm (oC)

ΔHm (Jg-1)

χcr (%)

PVA

1

0

70

75.5

81

0.63

193.55

221.92

53.32

38.47

PVA/SRGO-0.2 PVA/SRGO-0.5

1 1

0.2 0.5

71 71

77 78

83 85

0.56 0.57

195.30 195.10

223.57 222.84

48.99 50.29

35.35 36.28

PVA/SRGO-1.0

1

1.0

73

80

87

0.54

196.63

226.10

51.98

37.50

PVA/SRGO-1.5

1

1.5

73

77

81

0.46

196.47

224.79

50.57

36.49

PVA/SRGO-2.0

1

2.0

73

76.5

80

0.46

198.34

225.24

50.48

36.43


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The Journal of Physical Chemistry It may be mentioned that the reflection occurs due to the difference in impedance value between the air and the corresponding materials. In contrast, absorption occurs due to the dissipation of electromagnetic radiation energy converting it into heat energy. In case of internal multiple reflections, successive reflection takes place in between internal planes of two walls of the materials. The total shielding efficiency, SET (dB) is related to SER, SEA and SEM as (2)

SET (dB)  SER  SE A  SEM

It may be noted that the contribution of SEM is negligible corresponding to 12-15 dB. Therefore, above equation is transformed as

Figure 14: Plot of Conductivity vs. SRGO loading for PVA/SRGO nanocomposites

SET (dB) ~ SER  SEA

(3)

Mathematically SET (dB) can be expressed as: alternate –OH groups of PVA and surplus –OH functionalities of graphene sheet in SRGO. Further, de-oxygenation by selective reduction of –COOH groups to – OH efficiently restores π-conjugation for facile electron transport. As a result, π electron of SRGO tends to move in a continuous pathway consisting of interconnected conducting network.58 It is also noted that conductivity of PVA (1.25×10-6 Scm-1) increases nearly 10 times on adding 0.5 phr SRGO. Such significant improvement in conductivity of PVA/SRGO nanocomposites at such low percolation threshold could be attributed to the formation of conducting network through highly dispersed SRGO sheets in PVA matrix. 4.0 Electromagnetic Interference Shielding Effectiveness SRGO nanosheets being highly conducting (~0.02 Scm-1) carry diffused π electrons resulting in the formation of more number of conducting electronic pathways. The above reason causes the successive increment of DC conductivity with increasing SRGO loading, which is also reflected in the EMI shielding effectiveness (EMI SE). When electromagnetic radiation interacts with any material, reflection (SER), absorption (SEA), multiple internal reflections (SEM) accounts for partial shielding, whereas remaining fraction is transmitted through it.

Figure 15: Plot of Reflectance vs. frequency (Hz.) of PVA/SRGO nanocomposites

P SET (dB)  10 log 10  T  PI

 E   20 log 10  T   EI

  

(4)

where PI / EI and PT /ET are the power of incident and transmitted electric field intensity respectively. The absorptivity (A), reflectivity (R) and transmissivity (T) coefficients can be expressed in terms of scattering parameters (S11 / S22 and S12 / S21),

E T   T  EI

2

 2 2   S12  S 21 

(5)

2

E  2 2 R   R   S11  S 22  EI 

(6)

These are helpful in calculating A on the basis of R and T as below:

A  1 R T

(7)

Figure 15 shows the variation of reflectance versus frequency of neat PVA and its SRGO nanocomposites. It shows that the reflectance of neat PVA decreases continuously from -5 dB to -15 dB from 2 to 8 GHz respectively. It also indicated that reflectance increases with increasing SRGO loading in PVA over the entire frequency range. Accordingly, neat PVA, PVA/SRGO-0.2, PVA/SRGO-0.5, PVA/SRGO-1.0, PVA/SRGO-1.5 and PVA/SRGO-2 exhibited reflectance in the range of -3.5 to -11.9 dB, -3.5 to 10.8 dB, -2.5 to -7.5 dB, 1.1 to -3.7 dB and -0.5 to -1.4 dB respectively. In all probability, such enhancement in reflectance could be attributed to the successive increase in the conductivity of the respective PVA/SRGO nanocomposites. 60 Figure 16 shows the variation of the EMI SE of PVA and its SRGO loaded nanocomposites in the frequency range of 2-8 GHz. These findings clearly demonstrated the dependence of the EMI SE of PVA/SRGO nanocomposites on SRGO loadings as well as on frequency under consideration.



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parameters. It may be mentioned that the real part of permittivity is related as

'

C Q Q0  Q'   C0 Q0 Q0

(8)

where Q’, C, Q (and Co, Qo) correspond to induced charge, specific capacitance, electric charge of materials under experiment (and in vacuum) respectively.71

Figure 16: Plot of EMI Shielding Efficiency (EMI SE) vs. frequency (Hz.) of PVA/SRGO nanocomposites

EMI SE of neat PVA, PVA/SRGO-0.2, PVA/SRGO-0.5, PVA/SRGO-1.0, PVA/SRGO-1.5 and PVA/SRGO-2 corresponds to 1.5-4.5, 12.2-4.7, 22.7-9.0, 34.1-13.2 and 38.23-17.7 dB respectively over the entire frequency range. It is concluded that the EMI SE of PVA/SRGO nanocomposites is always improved compared to neat PVA. The results may be endorsed to the interplay of the reflection in the upshot of the increased conductivity of PVA/SRGO nanocomposites. The reflectance making a major contribution towards total EMI SE is also inevitable from the increased conductivity accounting for the formation of an interconnected conducting network throughout the insulating PVA matrix.21,61-64 According to equation (2) earlier, we showed that EMI shielding efficiency (EMI SE) depends on reflection as well as absorption. The reflection depends on the conductivity of the material under consideration.1,65,66 It is anticipated that loading of 0.2 phr SRGO is not sufficient to provide adequate conducting network in PVA. In contrast, higher SRGO loading in PVA matrix overcome this barrier due to available higher reflection area and dense interconnected conducting network accounting for enhanced EMI SE of corresponding nanocomposites. 18,67 It may also be noted that the target value of EMI shielding effectiveness needed for commercial application is ~ 20 dB (equal to or less than 1% transmittance of electromagnetic wave). Therefore, it is anticipated that our thermally and mechanically improved PVA/SRGO nanocomposites could find better commercial applications.68 In order to set up the correlation between the observed shielding response and electromagnetic attributes, the complex permittivity (ɛ*= ɛ’-iɛ’’) study of PVA and its SRGO nanocomposites have been executed using the experimental scattering parameters (S11 and S21) obtained from VNA instrument followed by applying standard Nicholson–Ross and Weir algorithm.69 Accordingly, It may be mentioned that the real part of ɛ * (ɛ’) is directly linked to the extent of polarization occurring in the material. This ɛ’ represents the storage capability of the electric energy. When polarization is increased, the storage capacity of the electrical energy is also enhanced.70 In contrary, the dissipation of electrical energy can be explained through the complex part (ɛ”) of the electromagnetic

Figure 17: Plots of (a) Real permittivity, (b) complex permittivity and (c) Tan delta vs Frequency (Hz.) for PVA/SRGO nanocomposites


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When reverse electric field is applied, a decrease in Q’ induces retardation in electronic oscillations at high frequency. Figure 17 (a) and (b) show variation of room temperature frequency dependence of ɛ’ and ɛ’’ of neat PVA and PVA/SRGO nanocomposites, respectively. For PVA composites i.e. PVA/SRGO-0.2, PVA/SRGO-0.5, PVA/SRGO-1.0, PVA/SRGO-1.5 and PVA/SRGO-2 the values of ɛ’ are in the range of 35.8-20.6, 46.7-25.8, 52.5-32.8, 52.8-36.4 and 43.5- 38.5 respectively, which are higher than neat PVA (37.8-17.8) in the frequency range of 2-8 GHz. In the meantime, the values of ɛ’’ for PVA/SRGO-0.2, PVA/SRGO-0.5, PVA/SRGO-1.0, PVA/SRGO-1.5 and PVA/SRGO-2 are in the range of 12.55-4.50, 20.55-8.2, 26.04-11.90, 24.30-13.20 and 24.3-13.2 respectively which are also higher than pure PVA (13.14-3.60). The conductivity of SRGO nanosheets increases the conductivity of the corresponding PVA/SRGO nanocomposites resulting in the increments of the values of ɛ’ and ɛ”. Actually, in presence of microwave the dipolar and electrical polarization in a material are induced by its polarizability. The polarizability again is influenced by the permittivity which depends on conductivity and electric polarization of a material. So, for that reason, it can be said that with increasing conductivity the values of ɛ’ and ɛ” increases. Figure 17 (c) shows frequency dependent plots of dielectric tangent loss factor (tan δ) of neat PVA and its’ SRGO nanocomposites in the frequency range of 2-8 GHz. The observed values of the corresponding nanocomposites in the range of 2-8 GHz are 0.25 to 0.2, 0.35 to 0.2, 0.44 to 0.31, 0.5 to 0.36, 0.46 to 0.35 and 0.55 to 0.37 for PVA, PVA/SRGO0.2, PVA/SRGO-0.5, PVA/SRGO-1.0, PVA/SRGO-1.5 and PVA/SRGO-2 respectively. It is anticipated that highly dispersed SRGO into the PVA matrix plays a key role to induce the enhanced polarization under microwave irradiation. As a result, the electronic flux increases in the composites with SRGO loading, a fact already established in permittivity discussion. All these findings also support the highest values of ɛ’, ɛ” and then δ in PVA/SRGO nanocomposites. In all probability, the increased random collision between the free electron at higher loading of SRGO is likely to be hindered, leading to the dissipation of electromagnetic energy as heat. Also, it leads to dielectric loss in presence of a continuous conducting network through highly dispersed SRGO into the PVA matrix enhanced by intermolecular hydrogen bonding due to accumulation of charges on the interfacial region to form large dipoles.71

Nanocomposite films derived from PVA and SRGO has been fabricated through a green approach in aqueous medium by simple solution mixing method. FTIR investigations indicated the formation of intermolecular hydrogen bonded network between PVA and SRGO. The dispersion as well as hydrogen bonding accounts for significantly enhanced tensile strength (~127%), elongation at break (~25 %) and thermal properties (45°C) at very low (0.2 phr) loading of SRGO in PVA. It has also been established that considerable improvement in conductivity at low percolation threshold (0.5 phr) can be achieved involving the formation of conducting network through extensive intermolecular hydrogen bonding between PVA and SRGO. As a consequence of this, significant enhancement in EMI shielding efficiency (~25 dB)

of the prepared thermally and mechanically improved PVA/SRGO composite films has been observed in the frequency region of 2-8 GHz following reflection as a dominant primary mechanism.

Supporting Information Deconvoluted FTIR Spectra of PVA/SRGO nanocomposites, stress-strain plots of PVA/GO and PVA/SRGO nanocomposites, thermograms of PVA/GO nanocomposites and a comparative table of tensile strength with other reported works. (PDF)

*E-mail: [email protected] Tel: +91 03222-283334 †The work has been done by Mr. K. Manna under the supervision of Prof. S.K.Srivastava and equal contribution has been made in writing and review of this manuscript. ‡Prof. V. Mittal has performed the DSC measurements of the samples. All authors have given approval to the final version of the manuscript.

S. K. Srivastava is grateful to DRDO, India, for providing grant for ENA Network Analyzer and IIT Kharagpur for providing other necessary facilities in this work. One of the authors (KM) gratefully acknowledges IIT Kharagpur for providing financial support.

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TOC Graphic

14 ACS Paragon Plus Environment

Role of Enhanced Hydrogen Bonding of Selectively Reduced Graphite

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