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aNational Physical Laboratory, Council of S

Krishnan Road, New Delhi, 110012, India.

45609310; Tel: +91-11-45609308bDepartment of Physics, University of Allaha

Cite this: RSC Adv., 2014, 4, 10123

Received 21st November 2013Accepted 12th December 2013

DOI: 10.1039/c3ra46898d

www.rsc.org/advances

This journal is © The Royal Society of C

CuO nanoellipsoids for superior physicochemicalresponse of biodegradable PVA

Kajal Kumar Dey,ab Prabhat Kumar,a Raja Ram Yadav,b Ajay Dhara

and Avanish Kumar Srivastava*a

PVA nanocomposites comprising of nanoparticles of CuOwere prepared and their various physicochemical

properties were evaluated. CuO with its impressive array of mechanical, optical, dielectric and hydrophilic

properties as well as environmental friendliness is expected to augment the properties of the PVA matrix.

The strong interaction between the polymer matrix and the nanofillers was confirmed via XRD and FT-IR

investigations. The thermal stability and the mechanical strength of the polymer membranes indeed

showed notable improvements in the composites. Thermal conductivity also increased to an extent

beyond the theoretical prediction. The dielectric constant of the nanocomposite film was about 2 times

higher than that of the pure PVA membrane. The high optical transparency of the PVA membrane also

was not compromised with the inclusion of nanosized CuO particles.

1. Introduction

Given the rapid expansion of the polymer industry, greenpolymers or biodegradable polymers with balanced propertiesfor industrial applications are a matter of much interestamongst material scientists and engineers.1–3 Although most ofthese biodegradable polymers have excellent propertiescomparable to many petroleum-based plastics and are poten-tially excellent alternatives for commercial commodity plastics,low mechanical strength, thermal stability etc. continue to bebottlenecks towards their commercialization. To counter thiscritical challenge researchers have been contemplating the ideaof designing composite materials of polymers incorporatingnanoscale objects to realize the transfer of some of the excep-tional properties of the nanoparticles to the bulk polymermatrix.4,5

Poly vinyl alcohol (PVA) is a semi-crystalline polymer that iswater soluble and completely biodegradable, and has attractivetraits such as hydrophilicity, chemical resistance, emulsifying,adhesivity and excellent lm forming capability.6,7 It is,currently the largest volume synthetic resin produced in theworld. The number of existing and potential applications of PVAin elds such as biomedical applications,8 drug delivery,9 bio-separation, carriers for cell immobilization,10 barrier lms forfood and biomedical packaging11 explain why the properties ofPVA are being widely examined across the research fraternity allover the world.

cientic and Industrial Research, Dr K. S.

E-mail: aks@nplindia.org; Fax: +91-11-

bad, Allahabad, 211002, India

hemistry 2014

Besides, the high optical transparency of PVA enables it to beused in optically clear so contact lenses.12 To further improvethe performance level of PVA regarding these applications, itsproperties such as thermal stability, moisture resistance andmechanical strength are being tuned by introducing nanosizedllers within the polymer matrix. Carbon derivative materialssuch as graphene, carbon nanotubes (CNTs) and diamond7,13–19,minerals like clay and macromolecules such as cellulose20,21

have all had varying degrees of success in composite materials.The carbon derivatives, especially the nanotubes, have atendency to agglomerate which can impede their potentialinuence on the polymer’s properties to some extent.

But given how good a host material PVA can be for metal22,23 orsemiconductor materials,24,25 it is a great surprise that so fewliterature reports exist on their exploitation as ller nano-materials. Especially, semiconductormetal oxides with their widerange of excellent properties have the potential to be suitablenanollers if explored.26–31 Besides, the oxide materials havethe possibility to strike a strong host–ller relationship with thehydroxy-functionalized PVA chains via hydrogen bonding. Thebasic aim is to improve some of the properties withoutcompromising the polymers’ other advantageous properties.

In this article, we report the utilization of CuO nanoparticlesto fabricate PVA nanocomposite materials with augmentedthermal and mechanical stability and modied thermalconductivity and optical property. The reason behind selectingCuO as a ller material lies in its inexpensive and simplesynthesis procedures, environmental benignity and impressiveoptical and mechanical properties.32,33 Additionally, the hydro-philic nature of CuO should enable a facile dispersion withinthe hydrophilic PVA matrix in an aqueous medium leading tostrong interaction between them.34

RSC Adv., 2014, 4, 10123–10132 | 10123

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Recently, scientists have shown keen interest in studyingthe electrical properties of PVA with its potential suitability asa dielectric material in microchips, suspension insulators,components of transformers, switch boards etc. and a goodnumber of articles have been published focussing on thedielectric behaviour of PVA and its composites.28,35,36 On theother hand CuO nanoparticles have been reported to behave asa giant dielectric material.37,38 Taking the above facts intoconsideration we have also explored the inuence of CuOnanoparticles on the dielectric properties of PVA.

2. Experimental2.1 Materials

Cu(NO3)2$3H2O (cupric nitrate trihydrate, 99.5%, Alfa Aesar)and NaOH (sodium hydroxide) were of analytical grade andwere used without any further purication. Poly vinyl alcohol(PVA, Mw z 125 000) was purchased from CDH Ltd.

2.2 Synthesis of CuO nanoparticles

CuO nanoparticles were prepared via a simple wet chemicalsolution method. In this method 1 wt% aqueous solution ofcopper nitrate was treated with 1 M NaOH solution until thepH of the solution reached 10.0 and a dense blue-greenprecipitate of Cu(OH)2 appeared. This precipitate was calcinedat a temperature of 130 �C for the duration of 10 hours toobtain black CuO powder. Details of the synthesis procedureand related growth mechanism, and the effect of pH have beendiscussed in a previous publication.31

2.3 Casting of the nanocomposite lms

The CuO–PVA nanocomposite lms were prepared via asimple solution casting method. 5 g of PVA was dissolved in 50ml of distilled water under constant stirring at roomtemperature.

The requisite amount of CuO nanoparticles was added to10 ml of distilled water followed by ultrasonication for 10minutes to prepare uniform aqueous dispersions. Theamount of CuO was chosen according to the aimed weightratio of the nanocomposite components. Next, the aqueousdispersion was added dropwise to the PVA solution undervigorous stirring to obtain a uniform distribution of thenanoparticles in the polymer matrix. This solution wasspread uniformly on glass plates and kept inside a micro-ovenfor 12 hours maintaining a temperature of 55 �C. The resultantlms were peeled off and used for further property evaluations.In the current work, we prepared 3 different nanocompositelms with one being pure PVA lm (S0) and the other twomaintaining a PVA to CuO weight ratio of 500 : 1 (S1) and50 : 1 (S2).

2.4 Characterization methods

The crystallographic characteristics of the samples were iden-tied through X-ray diffraction (XRD) via a Rigaku benchtopX-ray diffractometer using monochromatized Cu-Ka radiation(l ¼ 1.54059 A). The morphological investigations of the

10124 | RSC Adv., 2014, 4, 10123–10132

sample were carried out through scanning electron microscopy(SEM) images recorded on a Zeiss EVO MA-10 scanning elec-tron microscope. Microstructural characterization at highmagnications was performed using STEM (Scanning Trans-mission Electron Microscopy) attached to a Field EmissionScanning Electron Microscope (FESEM, model-Zeiss Supra40VP) and high resolution transmission electron microscopy(HR-TEM: FEI Tecnai G2 F30 STWIN at 300 keV). Thermaltransitions and stability were probed applying by differentialscanning calorimetry (DSC) and thermogravimetric analysis(TGA) through a Mettler Toledo TGA/DSC 1 high temperature(HT) Star system. The thermal conductivity of the samples wasrecorded using a hot disc thermal constant analyzer (TPS-500).The corresponding experimental error associated is �2%. TheUV-Vis spectra of the samples were recorded by a JASCOUV-VIS/NIR Spectrophotometer (model V-670) and the lumi-nescence characteristics were investigated by photo-luminescence spectroscopy using a Perkin-Elmer LS-55luminescence spectrophotometer (Xe source). Dielectric prop-erty data of the samples were investigated using an iImpedanceanalyzer Wayne Kerr (6540A), UK within the frequency range20–100 MHz at room temperature.

The mechanical properties of the lms were evaluated usingan Instron 4411 Universal testing machine under tensilemeasurements. The grip distance and the cross head speedwere maintained respectively at 15 mm and 50 mm min�1. Thesamples were designed as rectangular stripes (60 mm� 10 mm)and the thickness of the three samples was measured to be0.36 mm (S0), 0.50 mm (S1) and 0.68 mm (S2).

3. Results and discussions3.1 Enhancement of polymer crystallinity

Fig. 1 exhibits the X-ray diffraction patterns of the pure PVA andthe nanocomposite samples. The inset provides the XRDpattern for the synthesized CuO nanoparticles. The position ofthe diffraction peaks and the relative intensity of the peaksagree perfectly with the standard JCPDF card no. 05-0661,corresponding to the crystal system of monoclinic CuO(space group: C2/c; a: 0.4684 nm, b: 0.3425 nm, c: 0.5129 nm;b ¼ 99.47�).

The X-ray diffraction proles of pure and CuO nanoparticleincorporated PVA lms, are all characterized by an intense peakat �19.5� which is typical of crystalline PVA. The small peaksobserved at�28� and�41� for S0 and S1 are similar to the halosobserved in the diffraction prole of pure water.6 Diffractionpatterns for the composite lms (S1 and S2) consist of peakscorresponding to CuO and these peaks gain in intensity from S1to S2, which is in sync with the respective CuO nanollerconcentration of the two samples. More signicantly, the cor-responding diffraction peak for PVA also becomes more intensein the XRD proles of the composites compared to the pure PVAsample, suggesting an increase in the crystallinity of the poly-mer. The calculated percent crystallinity of PVA from the XRDpattern was 32.3% for S0 and 35.2% and 41.5% for S1 and S2respectively.

This journal is © The Royal Society of Chemistry 2014

Fig. 1 XRD patterns of the pure PVA membrane (S0) and theCuO–PVA nanocomposite membranes (S1 and S2). Increased intensityof the PVA crystalline peak in the nanocomposites is visibly apparent.Inset provides the XRD pattern for the CuO nanoparticles.

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3.2 Strong interaction between the nanoparticles and thepolymer chain

The increase in crystallinity of the polymer suggests strongchemical interactions between the CuO nanollers and the PVAfunctional groups. This phenomenon can be corroborated withthe FT-IR spectroscopic analysis of the samples.

Fig. 2 FT-IR spectra of pure PVA and the nanocomposites. (a) IRspectra across the entire region with the dotted lines indicatingdifferent vibrational modes of various chemical bonds. (b) IR spectrawithin the region 3000–3700 cm�1 to signify the effect of the CuOnanoparticles on the –OH functional group of PVA.

This journal is © The Royal Society of Chemistry 2014

Fig. 2a shows the vibrational spectra of the PVA membraneand its CuO incorporated composites in the IR region. Thebands observed between 3000 and 3600 cm�1 are characteristicof the –OH functional groups present in PVA. The band at2915 cm�1 corresponds to the stretching of –CH2– present inthe PVA chain. The bands at 1380 and 1265 cm�1 occur due tothe vibrational motion of the C–H groups, whereas, the band at842 cm�1 arises due to the C–C stretching motion. Two otheroxygen containing functional groups were detected at1715 cm�1 (C]O functional groups from aliphatic acids oraldehydes) and at 1094 cm�1 due to the stretching mode of C–Obonds. The band at the right corner of the spectra at 530 cm�1

observed for S1 and S2, can be assigned to the Cu–O vibrationalmotion.31 The FT-IR troughs appear to be particularly intense inthe composite S2 indicating that the insertion of CuO nano-particles had signicant effect on the bonding interactionswithin the PVA structural framework.

One particular mode of interaction between the metal oxideparticles and the polymer can be the hydrogen bonding betweenthe –OH functional groups of PVA and the oxygen in CuO.Fig. 2b focuses on the –OH functional group region of the FT-IRspectra (3000–3600 cm�1). The modication of the spectra frompure PVA membrane (S0) to the composites (S1 and S2) isapparent with the –OH dening positions becoming lessdelineated. Additionally, the positions appear to be blue shiedowing to the slight elongation of the O–H bond due to itshydrogen bond formation with the oxygen of CuO.

Further evidence of the strong interaction between thecomponents of the CuO–PVA composites can be obtained fromthe photoluminescence (PL) spectra of the membranes (Fig. 3).

Fig. 3 Photoluminescence (PL) spectra of pure PVA and the nano-composites. The emission intensity for the CuO inclusive polymermembranes appears to be highly diminished. Inset provides themagnified PL spectra of the two CuO–PVA nanocomposites.

RSC Adv., 2014, 4, 10123–10132 | 10125

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PVA molecules exhibit photoluminescence emission in thevisible region (within 400–500 nm region) due to a p* / nelectronic transition in free –OH groups depending on theirspatial distribution within oriented PVAmolecules.39 In the caseof our samples, two close prominent peaks observed for thepure PVA membrane at 420 nm and 434 nm can be attributed totransitions in syndiotactic (s) PVA and isotactic (i) PVA respec-tively. But, in the case of the two composite membranes theluminescence is almost quenched (shown as the inset of Fig. 3with amagnied region for enhanced clarity) indicating that thepresence of the CuO nanoparticles comprehensively reordersthe delocalized n-electron system of the residual –OH groups ofthe PVA backbone.

3.3 Microstructural features

Fig. 4 shows the microstructural features of the CuO nano-particles, pure polymeric membranes and the composite lms.The CuO particles were uniformly characterized by an elongatedellipsoidal morphology accompanied by a 100–200 nm averagewidth and 600–800 nm average length (Fig. 4a and b). The edgesof the particles appeared to be rather smooth as was suggestedby the STEM image (Fig. 4b). The cross-sectional image of theparticles revealed a papaya-like frame indicating the non-uniform accumulation of smaller CuO nanocrystals across thecross section (Fig. 4c). The pure PVA membrane consisted ofbeehive or mesh like structures dispersed within the PVAmatrix(Fig. 4d).

The microstructural images of the composites showed notonly the presence but also the relative concentration of nano-particles in the composite matrices. For the composite S1, a fewCuO nanoellipsoids were found to be dispersed on the PVAmatrix as revealed by the SEM micrograph of the sample(Fig. 4e). The corresponding SEM micrograph for S2 shows ahigher density of CuO ellipsoids within the compositemembranes (Fig. 4f), indicative of the higher CuO concentra-tion. Fig. 4g shows the STEM image of S2 revealing the disper-sion of CuO nanoellipsoids over the PVA mesh-like structure.CuO particles distributed over the PVA membrane. Fig. 4hshows the TEM image of the composite S2 revealing the randomorientation of CuO nanoellipsoids while forming the compositemembrane. The CuO–polymer interface was seemingly compactwith no visible porosity or distortions observed. The lattice scaleimage of a dispersed CuO nanoellipsoid consisting of clear 002lattice planes was provided in the inset. The microscopy imagesalso refute the formation of CuO nanoparticle agglomerationswithin the polymer nanocomposites, thus denying the possi-bility of any signicant physical or direct chemical interactionbetween the nanoparticles themselves within the polymermatrix.

The incorporation of CuO nanoellipsoids within the PVAmatrix and the accompanying crosslinking through H bondinginteractions between the oxygen atoms of CuO and the –OHfunctional groups from PVA have been depicted in Scheme 1.

So, now that we know that the synthesized CuO nano-ellipsoids were well dispersed within the PVA matrix andblended in via substantial interaction with the residual

10126 | RSC Adv., 2014, 4, 10123–10132

functional groups of the polymer, we shall focus on how theymanaged to improve the various important physicochemicalproperties of PVA.

3.4 Enhancement in thermal stability and conductivity

Thermal stability of the polymer nanocomposites was studiedvia analytical tools DSC and TGA. Both experiments were carriedout under an argon atmosphere maintaining a heating rate of10 �C min�1. Fig. 5a shows the DSC thermogram of the pristinepolymer membrane and its composites emphasizing theirmelting and dissociating behaviors. The melting point (Tm) ofthe composites did not seem to have any shi in positionrelative to the pure polymer membrane, but the enthalpy ofmelting (DHm) attained a notable, even if little, improvement.This increase in melting enthalpy has a constructive effect onthe mechanical properties of PVA due to its semi-crystallinenature. The enhanced crystallinity of the composites relative tothe pure polymer as detected in the XRD spectra, possibly leadsto enthalpy augmentation. Additionally the strong surfaceinteraction between the nanoparticles and the polymer mighthave reduced the mobility of the PVA chain to some extent.

However, the melting point of the polymer being unaffectedindicates that the CuO nanoparticles had exerted their inter-action mostly through the amorphous region of the polymerand the crystalline portion remained immune. Fig. 5b depictsthe TGA thermogram of the samples which provides insightregarding the thermal decomposition behavior of the samples.Here, the initial weight loss (up to 260 �C) for S0 can beattributed to the vaporization of the adsorbedmoisture and CO2

on the polymer membrane. Interestingly, the composites S1 andS2 barely show any initial weight loss within this temperatureregion indicating that, the addition of CuO nanoellipsoids hasrendered the polymer membrane surface more resistanttowards the adsorption of the aerial contaminants.

The beginning of the actual decomposition or thermalpyrolysis of the membranes is clearly observed in both DSC andTGA thermograms. The temperature at which this decomposi-tion commences was determined to increase in the compositesthan in the pure PVA membrane (Table 1) and seems to movehigher with the increment in CuO concentration. The degra-dation that is observed due to the chain stripping or conden-sation of –OH functional groups apparently becomes slower aswe move from the pristine PVA to the composites as the slopesappear to be shiing towards higher temperature. The retar-dation of the decomposition can be further corroborated withthe corresponding enthalpy (DHd) obtained from the DSC(Table 1), which clearly indicates that the degradation processesbecome increasingly more heat absorbing in the compositescompared to the pristine PVA.

Another interesting observation via TGA was the presence ofresidual mass for the PVA–CuO composites, which was esti-mated to be 12.8% for S1 and 21.3% for S2. But for the pure PVAmembrane no such mass was observed. The residual masswhich could be a combination of organic olens, alkenes andother aliphatic compounds and non-degraded polymer chain40

may be indicative of the fact that CuO could be acting as a

This journal is © The Royal Society of Chemistry 2014

Fig. 4 Micrographs of the CuO nanoparticles, PVA membranes and CuO–PVA composite membranes. (a) SEM image of the ellipsoidal shapedCuO nanoparticles, (b) STEM image of a few CuO nanoparticles, (c) STEM micrograph presenting the cross sectional image of a single CuOnanoellipsoid, (d) SEMmicrograph of a pure PVAmembrane showingmesh like microstructural characteristics, (e) SEMmicrograph of CuO–PVAnanocomposite (S1) showing the presence of a few CuO nanoparticles on the polymer matrix, (f) SEM micrograph of CuO–PVA nanocomposite(S2) revealing a dense dispersion of CuO nanoparticles. (g) STEM image showing CuO nanoellipsoids attached to the mesh of PVA, (h) HRTEMimage of S2 showing CuO nanoparticles dispersed without any agglomeration within the PVA matrix. Inset provides the lattice scale image of adispersed CuO nanoparticle consisting of 002 lattice planes.

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catalyst surface for some of the side reactions leading to theorganic chars or it could be that the interaction of CuO with thepolymer chain simply made it partially prohibitive to conden-sation–degradation. At this moment, we cannot discard acumulative effect of the two phenomena either.

Overall, the TG/DSC analysis suggests that the introductionof CuO nanollers has signicantly improved the polymer's

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thermal properties. The stability with respect to both meltingand degradation in the composites seemed to be positivelyaffected due to the strong interaction between the nanoparticlesand PVA.

In fact, not only the thermal stability, but the thermalconductivity of the polymer too resulted in substantialenhancement as shown in Fig. 6. The thermal conductivity of

RSC Adv., 2014, 4, 10123–10132 | 10127

Scheme 1 Transformation from pristine PVA to PVA–CuO compos-ites, represented through optical and microscopic images and aschematic reaction diagram.

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the membranes was measured within the temperature range of0–60 �C and it was found that the conductivity increased50–60% over the entire temperature range in the nano-composites compared to the pure polymeric membrane. Previ-ously, an improvement of �30% in the thermal conductivity ofnanodiamond incorporated PVA was reported.17 But sphericalshape diamond nanoparticles, having a high thermal conduc-tivity of their own (�2000Wm�1 K�1), were found to be effective

Fig. 5 (a) DSC plot of pure PVA (S0) and the CuO–PVA nanocompositecomposites (S1 & S2). The residual weight after the completion of the de

10128 | RSC Adv., 2014, 4, 10123–10132

within the realm of theoretical predictions based on classicaltheories such asMaxwell's. In our case, however, the presence ofellipsoidal shaped CuO nanoparticles required a more specictheoretical model to predict the extent of enhancement. Lewisand Nielsen proposed such a theoretical model41 modifyingHalpin–Tsai equations to incorporate various particle shapes.According to their model;

Kc ¼ Km

1þ Abq

1� bqj(1)

where, Kc ¼ thermal conductivity of the composite, Km ¼thermal conductivity of the polymer matrix, A ¼ 2.8 (for aspectratio 6),33,43 q ¼ volume fraction of the nanollers

b ¼ Kf=Km � 1

Kf=Km þ A(2)

Kf ¼ thermal conductivity of CuO ¼ 77 W m�1 K�1 (ref. 42)

j ¼ 1þ�1� fm

fm2

�q (3)

fm ¼ packing fraction; assumed value 0.82 (for rod shapedrandomly oriented particles, the closest approximationavailable).43

The calculated thermal conductivity values for composite S2using the above model (Fig. 6; C) accounted only for 5% of theobserved enhancement. In fact any comprehensive explanationat the moment needs thorough experimental investigationcoupled with theoretical base work. But here wemust mention acouple of factors that might have augmented the thermalconductivity value beyond the theoretical prediction. Theincreased crystallinity is one such factor which would strongly

s (S1 & S2). (b) TGA curves of pure PVA (S0) and the CuO–PVA nano-gradation/pyrolysis reaction is indicated by the thin arrows.

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Table 1 Thermal parameters of pure PVA (S0) and CuO–PVA (S1 and S2) samples

SampleMelting point(Tm/�C)

Melting enthalpy(DHm/J g

�1)Beginning of pyrolysis(Td/�C)

Enthalpy of degradation(DHd/J g

�1)Residual weight(%)

S0 201 22.1 264.2 219 —S1 201 24.3 274.7 242 12.8S2 201 29.7 286.7 309 21.3

Fig. 6 Thermal conductivity measurements of the pure PVAmembrane (S0) and the CuO–PVA nanocomposites (S1 and S2) acrossa temperature range of 0–60 �C. Theoretically predicted valuesaccording to the Lewis–Nielsen equation for S2 have also beenincorporated.

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enhance the phonon mode of vibration and thus increase theheat conduction. Besides, the effective medium theory forparticles with high aspect ratios predicts very low interfacialresistance and thus high thermal conductivity for thenanocomposites.44

Not only the thermal properties, but also the introduced CuOnanoparticles with their inherent unique set of physical andchemical attributes and based on their strong interfacialinteraction with PVA, could be assumed to signicantly alter thepolymer’s mechanical and electrical properties. First, we shalldiscuss the tensile properties.

Fig. 7 Stress–strain curves for pure PVA membrane (S0), CuO/PVA ¼1/500 (S1), CuO/PVA ¼ 1/50 (S2).

3.5 Improved tensile characteristics

Fig. 7 presents the stress (s)–strain (3) curves for a PVAmembrane and the CuO incorporated composites with differentller concentrations. The corresponding curve for PVA resem-bles a typical s–3 curve for a polymer membrane with charac-teristic yielding behaviour. The curves for the compositemembranes represent similar behaviour albeit with differentmagnitudes for the stress–strain values.

Table 2 lists these values for each of the samples. Followingthe addition of 0.2 wt% CuO nanoparticles (S1), the Young’smodulus for the polymer increased (from 4.28 GPa to 4.99 GPa)and the tensile strength increased 38% (from 47.56 MPa to

This journal is © The Royal Society of Chemistry 2014

65.37 MPa). The elongation at break also increased indicatingthat the composite membrane is somewhat less brittlecompared to pure PVA. This is conrmed by the calculatedtoughness values which suggest S1 to be slightly less brittlecompared to S0.

The increase in elastic modulus can be predicted by applyingthe simple rule of mixing and utilizing the following equation asa rst approximation

Ec ¼ ECuOVCuO + EPVA(1 � VCuO) (4)

where, EC, ECuO, and EPVA are the elastic modulus of thecomposite, CuO and PVA respectively. VCuO is the volume frac-tion of CuO nanoparticles present within the polymer matrix.

AssumingECuO¼81.6GPa,45EPVA¼4.28GPaand that 0.2wt%CuO corresponds to VCuO¼ 0.0106, the calculated EC¼ 5.09 GPa,which is comparable to the value that we obtained (¼ 4.99).

From a more abstract perspective, the improvementobserved can be largely attributed to the nanosize of the llersresulting in a sufficiently high interfacial contact area with thepolymers and thus providing strong interaction to prevent thestretching of amorphous chains or sheer yielding of crystallites.The improved crystallinity in the composite materials also canbe one of the causes for the increased elastic modulus.However, such improvements do not get reciprocated for S2which, apart from its tensile strength (56.09 MPa), shows

RSC Adv., 2014, 4, 10123–10132 | 10129

Table 2 Tensile parameters of pure PVA (S0), and CuO–PVA nano-composites (S1 and S2)a

SampleElongation atbreak (%)

Tensile strength(MPa)

Young's modulus(GPa)

Toughness(J g�1)

S0 1.81 47.56 4.28 196.6S1 2.55 65.37 4.99 201.1S2 1.82 56.09 4.79 88.2

a The toughness value was calculated as the area enclosed by the stress–strain curve divided by density (g m�3) of the corresponding material.

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properties which are comparable to PVA. The toughness valueshows that S2 has become much more brittle compared to purePVA. The reason behind this may be that the excess of CuOnanoparticles provides a number of entanglements within thepolymer matrix. Besides, the large number of hydrogen bondsformed outweighs the interfacial interaction and makes thepolymer susceptible towards deformation induced by weak-ening of the bonds within the polymer chain and thusrendering it easier to be stretched. So, the admixture of excessCuO nanoparticles is unfavourable for the promotion of thepolymer’s tensile properties.

3.6 Modication of electrical properties

The dielectric properties of PVA and the CuO compositemembranes were studied through capacitance measurement ofrectangular strips cut off from the original membranes. Fig. 8apresents the plot depicting the variation of the real part of thedielectric constant (3r) with frequency of the alternating electriceld (u) applied. Notable observations from the plot are (i) CuOnanollers enhanced the dielectric constant of PVA, theenhancement was around two fold in the lower frequencyregion but dropped off as we increased the frequency; (ii) thecomposites were more responsive towards the changingfrequency than the pure polymeric membrane and (iii) therelative concentration of the CuO nanoparticles in the two

Fig. 8 Representative plots for the dielectric properties of pure PVAmem(a) real part of the dielectric permittivity (3r) against logarithm of the fremodulus (M0 0) against log(u).

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composites did not induce a lot of difference in their respectivedielectric constants. To rationalize these observations we haveto deliberate over the various components of dielectric polari-zation (i) atomic, (ii) electronic, (iii) orientational and forheterogeneous systems such as our composite material there isthe feasibility of another component (iv) interfacial polariza-tion. The presence of the polymeric matrix causes some of thecharge carriers of CuO to get trapped which in turn induces theinterfacial polarization within the composite systems. In thelow frequency region, this component, induced by the nano-ellipsoids of CuO, elevates the dielectric constant of PVA. Butthe interfacial polarization and orientational polarization alsohappen to be the two slowest components in terms of attainingequilibrium with the alternating frequency of the current. Andso, as we increase the frequency of the current the contributionfrom these two components becomes increasingly smaller andthe 3r value goes down rapidly. In the MHz region the corre-sponding 3r values of the pure polymer and the CuO compositesare almost comparable. The interfacial polarization effect wasless pronounced in the dissipation factor or tan d spectra of thesamples (Fig. 8b). Here too, the dissipation factor decreaseswith increasing frequency, illustrating the relaxation process.The low dielectric loss of the polymeric membrane whichremained almost the same even aer the inclusion of the CuOnanoparticles bodes well for the composites’ application as aninsulating material. The relatively higher value of dielectric lossat a lower frequency could be due to the mobile charges in thepolymer backbone.

The presence of the bulk relaxation phenomenon for boththe polymer and its composites was evident by the peakformation in the loss modulus spectra (Fig. 8c). Interestingly,the maxima of the peak decrease substantially in the compos-ites and also shi towards higher frequency indicating dimin-ished relaxation time. The calculated relaxation time was of theorder of 10�3 in the pure PVA membranes and the order of 10�4

in the composites. The broadening of the peaks in thecomposites compared to the pure polymeric sample can beattributed to the interaction between the nanollers of CuO.

brane (S0) and the PVA–CuO nanocomposites membranes (S1 and S2);quency (u), (b) loss tangent (tan d) against log(u) and (c) electric loss

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3.7 Optical transparency

Fig. 9 displays the UV-Vis spectra of the pure PVA membraneand the two CuO incorporated nanocomposites membranes.The PVA membrane was measured to have transparency wellabove 90% across the visible region. Such high transparencyenables PVA lms to be utilized as polarizer lms in electricaldevices. The observed trough at around 265 nm in the spectraarises due to a p/ p* transition in the PVA molecule. With theintroduction of CuO nanoellipsoids the transparency valuebecame slightly lower in the two composites, but never the lessmaintained a very high value of more than 80%. The loweringcan be attributed to the scattering of light caused by thenanosized CuO particles present within the matrix.

4. Conclusions

We have prepared CuO nanoellipsoid incorporated nano-composites of PVA with improved physicochemical properties.A simple solution casting method was applied during thesynthesis. The nanocomposites possessed enhanced crystal-linity compared to the pure PVA membrane. Strong host–llerinteractions and the presence of H-bonding were establishedthrough the vibrational spectroscopy and photoluminescencestudies. The thermal stability in terms of the energy required tomelt the polymer membrane or the pyrolysis of the polymerchain was observed to increase in the composites. The degra-dation temperature was also enhanced slightly. Thermalconductivity of the composite membranes increased 50–60%across a temperature range of 0–60 �C. The increment wasbeyond the theoretical prediction of modied Halpin–Tsaiequation, indicating strong interactions between the hostpolymer and the ller nanoparticles. The tensile strength andelastic modulus were both superior for the composites

Fig. 9 UV-Vis spectra of PVA and the composites.

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compared to the pure PVA and the toughness was comparable.The Young's modulus value was comparable to that of 0.2 wt%loading and thus ideal reinforcement was revealed. The nano-particles also improved the dielectric constant nearly two-foldwithout compromising its dielectric loss properties. Opticaltransparency of the PVA lms remained quite high even aerthe incorporation of nanoparticles. Overall, we have been ableto prepare high quality PVA nanocomposite lms by usingeconomic and easily processable CuO nanoparticles and theimproved properties indicated that CuO can be a suitablealternative to the conventional carbon based nano-llers.

Acknowledgements

We thank the Director, NPL, New Delhi, India for providing thenecessary experimental facilities. Dr A. Dhar, Dr S. N. Sharma,Mr K. N. Sood and Mr P. C. Mandal are gratefully acknowledgedfor providing the necessary instrumentation facilities for XRD,FT-IR, SEM and UV-Vis spectroscopy respectively. We thank DrB. P. Singh for letting us carry out the experiments regardingmechanical properties. We also express our gratitude towardsDr A. M. Biradar and Dr J. P. Rana for the dielectric propertymeasurements. Kajal Kumar Dey acknowledges the nancialsupport from Council of Scientic and industrial research,India (Grant no. 31/001(0325/2009-EMR-I)).

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