electrical properties and thermal degradation of poly(vinyl chloride)/polyvinylidene fluoride/zno...

Research ArticleReceived: 15 December 2015 Revised: 4 May 2016 Accepted article published: 26 May 2016 Published online in Wiley Online Library:

(wileyonlinelibrary.com) DOI 10.1002/pi.5161

Electrical properties and thermal degradationof poly(vinyl chloride)/polyvinylidenefluoride/ZnO polymer nanocompositesMayank Pandey,a Girish M Joshi,a* Amitava Mukherjeeb and P Thomasc

Abstract

In this work, polymer nanocomposites consisting of a poly(vinyl chloride) (PVC) and polyvinylidene fluoride (PVDF) polymernetwork with ZnO nanoparticles as a dopant were prepared by solution casting. An XRD study of the PVC/PVDF/ZnO polymernanocomposites shows predominantly sharp and high intensity peaks. However, the intensity and sharpness of the XRD peaksdecreases with further increment in loading of ZnO (wt%), which reveals a proper intercalation of ZnO nanoparticles withinthe PVC/PVDF polymer system. Fourier transform infrared spectroscopy has been used to verify the chemical compositionalchange as a function of ZnO nanoparticle loading. TGA analysis clearly describes the thermal degradation of the pure polymerand polymer nanocomposites. The complex dielectric function, AC electrical conductivity and impedance spectra of thesenanocomposites were investigated over the frequency range from 10 Hz to 35 MHz. These spectra were studied with respect tothe Wagner−Maxwell− Sillars phenomenon in the low frequency region. Nyquist plots of the PVC/PVDF/ZnO nanocompositeswere established from impedance measurements. The temperature-dependent DC ionic conductivity obtained from the Nyquistplots follows Arrhenius behaviour.© 2016 Society of Chemical Industry

Keywords: polymer nanocomposites; optical absorption spectra; dielectric properties; ionic conductivity; AC conductivity

INTRODUCTIONIn recent trends the entire production of commercially availableproducts has been based on the incorporation of nanoparticles.Nanoparticles with a size smaller than 100 nm are substantiallydependent on their size scale. This nanoparticle field is definedfrom a combinational study of traditional fields such as chem-istry and solid state physics.1 In this regard, the incorporation ofnanoparticles in a polymer backbone plays a very important rolein physical properties. Embedded nanoparticles in the polymermatrix have been used in the past few years because of theirunique applications in the form of nanocomposites such as opti-cal, magnetic, sensors and biosensors.2,3 They are also attractivebecause the same method is applicable to change the basic prop-erties of pure polymers.

Nanocomposites are a special class of materials with uniquephysical properties and they are used for many applications invarious areas.4 – 7 The dispersion of inorganic nanofillers in anorganic polymer results in polymer nanocomposites. These poly-mer nanocomposites play an essential role in applications just bycontrolling the nanostructure, composition and morphology. Theattractive mechanical, thermal, optical and electrical properties ofnanoparticle composites differ from those of both the pure poly-mer and the inorganic fillers.8,9

In the present study polymer nanocomposites were prepared byadding ZnO nanoparticles to a poly(vinyl chloride)/polyvinylidenefluoride (PVC/PVDF) homogeneous polymer blend. ZnO is awide band gap semiconductor used for a variety of commercialapplications. It has drawn the attention of researchers due toits properties which have been used for transparent electrodes

in liquid crystal displays and electronic applications of ZnO asthin film transistors and light emitting diodes.10 Recently highlysensitive room temperature sensors based on the UV light emit-ting diode activation of ZnO have been reported.11 However,polymers such as PVDF and PVC were selected for the preparationof polymeric nanocomposites. PVDF belongs to the class of ferro-electric polymers which exhibit efficient piezoelectric/pyroelectricand dielectric properties.12 PVDF is a technologically importantsemicrystalline polymer and is expected to have high anodicstability due to strong electron withdrawing functional groups.13

PVDF also has a high dielectric constant (𝜀= 8.4), a relatively lowdissipation factor and high permittivity. PVC is a commerciallyavailable polymer with good dielectric constant (𝜀= 3) and is agood mechanical stiffener.14,15 The mechanical properties of PVCare enhanced with increasing molecular weight, but decreasewith temperature. PVC also has good insulation properties, butdue to its highly polar nature its electrical insulating property isinferior to non-polar polymers.

∗ Correspondence to: GM Joshi, Department of Physics, Polymer NanocompositeLaboratory, School of Advanced Sciences, VIT University, Vellore-632014, TN,India. E-mail: [email protected]

a Department of Physics, Polymer Nanocomposite Laboratory, School ofAdvanced Sciences, VIT University, Vellore-, 632014, TN, India

b Centre for Nanobiotechnology, VIT University, Vellore, 632014, India

c Dielectric Materials Division, Central Power Research Institute, Bangalore, 560080, India

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www.soci.org M Pandey et al.

Figure 1. Flowchart of prepared films of pure PVDF, PVC and PVC/PVDF/ZnO samples.

In the present study, polymer nanocomposites were preparedby adding ZnO to a PVC/PVDF homogeneous polymer blend. Themain objective of this study was to identify the effect of ZnOnano-powder on the electrical, thermal and morphological prop-erties of the PVC/PVDF blend. Hence, to fulfil the objective of thisstudy structure and chemical composition studies of the polymericnanocomposites were carried out using XRD and Fourier transformIR spectroscopy (FTIR). The morphological investigation was car-ried out using SEM. The UV− visible characteristics demonstrateabsorbance peaks with respect to different wavelengths. The ther-mal stability and weight loss due to temperature were identified bythe DSC and TGA characterization methods. The electrical proper-ties of the prepared nanocomposites were analysed by the Nyquistplot and the temperature-dependent conductivity was evaluated.

EXPERIMENTALMaterials and methodsPVDF of molecular weight 5.34× 105 was obtained from PragatiPlastics Pvt Ltd, New Delhi, India. The polymer PVC (molecularweight 233 000 g mol−1, molar density 1.4 g mol−1 at 25 oC)was supplied by Sigma Aldrich, Mumbai, India. ZnO nanoparti-cles were procured from Sigma Aldrich (average particle size about50 nm with specific surface area 80.425 m2 g−1). PVC/PVDF/ZnOnanocomposites were prepared by solution casting. The desiredamount (in wt%) of PVC and PVDF was dissolved separately in 30mL of tetrahydrofuran procured from Sisco Research Laboratory,Mumbai, India. The mixtures were stirred (300 rpm) continuouslyfor 12 h at room temperature (30 oC) to obtain homogeneous solu-tions. Simultaneously different loadings (5− 15 wt% in steps of 5wt%) of ZnO nanoparticles were dissolved in acetone procuredfrom Sisco Research Laboratory and given an ultrasonication treat-ment initially for 30 min at room temperature (30 oC). Impurities

were removed by the ultrasonication treatment, and the mixturewas stirred (300 rpm) for 12 h at room temperature (30 oC). Oncethe proper dispersion of nanoparticles had occurred in the solvent,the three separate solutions were mixed together and stirred (300rpm) for 8 h at room temperature (30 oC). The prepared homo-geneous solution was poured into a Petri dish and dried in anoven for 12 h under controlled temperature (60 oC) conditions.The prepared PVC/PVDF/ZnO nanocomposite films, thickness 130μm, were peeled off and kept in a desiccator to remove furthertraces of solvent. The change in the surface appearance of thenanocomposite films for different loadings of ZnO nanoparticles inthe PVC/PVDF polymer matrix is shown in Fig. 1. The change fromopaque to translucent properties of the nanocomposite films for5− 10 wt% loading of ZnO, as shown in Fig. 1, describes the goodoptical absorbance properties.

CharacterizationThe structural characterization of PVC/PVDF/ZnO nanocompos-ites was performed using XRD. The structural changes were anal-ysed using Cu K𝛼 radiation of wavelength 𝜆= 1.5406 Å usinga Bruker AXSD8 focus Advance X-ray diffraction meter (Rigaku,Japan, Tokyo) Ni-filtered. The scans were taken in the 2𝜃 rangefrom 10∘ to 80∘ with a scanning speed of 1∘ min−1 and step sizeof 0.01∘. The complex chemical composition and bonding of poly-mer with nanoparticles were recorded using FTIR spectroscopy(Shimadzu-IR Affinity-1 spectrometer) in the wavenumber range500–4000 cm−1 operated in transmittance mode. The surface mor-phology of the PVC/PVDF/ZnO nanocomposites was examined bySEM (Carl Zeiss EVO/185H, UK) at 20 μm resolution. Before scan-ning, the samples were sputtered with gold for 1 h in a vacuumevaporator. All the images were taken at 10 kV accelerating volt-age. The UV–visible spectrum of the polymer nanocomposites was

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obtained using a Shimadzu UV-2401PC UV–visible spectropho-tometer in the range 200–600 nm. The optical absorbance of theprepared PVC/PVDF/ZnO nanocomposites was recorded and thedirect/indirect energy band gap was evaluated from the UV spec-trum. Thermal characterization of the polymer nanocompositeswas by DSC (Mettler Toledo model DSC 821e) using a heating rateof 10 oC min−1 under a nitrogen atmosphere at a flow rate of60 mL min−1, with an aluminium pan. TGA was done using theTA Instruments (UK) model TGA Q500, with alumina as the ref-erence material. The experiments were carried out at a heatingrate of 10 ∘C min−1. The electrical properties of the PVC/PVDF/ZnOnanocomposites were investigated by demonstrating the dielec-tric parameters, impedance spectrum and AC/DC conductivity. Asample of small size (10 mm diameter, silver pasted both sides) wasplaced between fixture electrodes and kept inside a dry tempera-ture calibrator. The electrical parameters of the polymer nanocom-posites were examined by using a PSM-1735 impedance analyserunder a varying range of temperature (30–150 ∘C) and a broadrange of frequency (10 Hz to 35 MHz). All the samples were testedthree times and the average was used for calculations.

RESULTS AND DISCUSSIONXRD analysisThe XRD patterns of pure PVDF, PVC, ZnO nanoparticles, PVDF-PVCblend and PVC/PVDF/ZnO nanocomposites over the 2𝜃 range10∘ − 70∘ are shown in Figs 2(i) and 2(ii). The XRD pattern of purePVDF in Fig. 2(i) shows sharp peaks at 2𝜃 = 18.60∘ and 39.56∘indicating the semicrystalline nature of PVDF. These XRD resultsfor pure PVDF are in agreement with previously reported literaturevalues.16,17 The crystalline structure of pure PVDF can be classifiedinto 𝛼, 𝛽 , 𝛾 and 𝛿 phase on the basis of the shape of crystallization.The maximum intensity peak is observed at 19.9∘ which belongs tothe 𝛼-phase for pure PVDF. However, for pure PVC an amorphousnature was observed in the XRD patterns as shown in Fig. 2(i).Several intense peaks are observed at 2𝜃 = 31.77∘, 36.23∘, 47.56∘and 56.61∘ in Fig. 2(i) revealing the highly crystalline nature ofthe ZnO nanoparticles. The XRD pattern of the ZnO nanoparticlesrepresents some reflections corresponding to (100), (002), (101),(102) and (110) which are matched to the standard diffractionpattern of ZnO.18 The particle size of ZnO calculated from theXRD peaks at 31.77∘ (100), 34.41∘ (002) and 36.23∘ (101) usingDebye− Scherer formulae are 45, 44 and 55 nm respectively.

D = 0.9𝜆𝛽 cos 𝜃

(1)

where 𝜆 is the X-ray wavelength (1.54 Å), 𝜃 is the Bragg diffractionangle and 𝛽 is the peak width at half maximum. The reducedintensities of the peaks of the PVC/PVDF blend in comparison topure polymers as shown in Fig. 2(ii), curve (a), reveals that theblending of PVC with PVDF forms a miscible blend in which somePVDF also exists in the crystalline phase.

The characteristic peaks of ZnO are present in the XRD patternsof the ZnO-doped polymer nanocomposites, which confirms theintercalation of ZnO particles in the PVC/PVDF polymer blend(Fig. 2(ii), (b)− (d)). Further, the low intensity peaks comparedto other peaks of PVDF in the XRD pattern of PVC/PVDF/ZnOnanocomposites (Fig. 2(ii), (b) and (c)) infer that the formationof cation complexes with the functional group of the PVC/PVDFblend has reduced the crystallinity of the nanocomposites.19,20

This confirms the intercalation of nanoparticles within thePVC/PVDF polymer system.

Figure 2. XRD spectra of (i) pure PVDF, PVC, ZnO and (ii) (a) PVDF/PVC::50:50 and PVC/PVDF/ZnO for (b) 45:50:5, (c) 40:50:10 and (d) 35:50:15loading wt%.

FTIR analysisIR transmittance spectra of pure PVC, PVDF and ZnO-doped poly-mer nanocomposite films (Figs 3(i) and 3(ii)) were recorded at roomtemperature in the region 500–4000 cm−1. The spectra exhibitthe stretching and bending vibration characteristics and chemi-cal composition of the ZnO-doped polymer nanocomposite films.The FTIR spectrum of PVC shows peaks at 2972 cm−1 and 2910cm−1 consisting of the CH2 asymmetric stretching vibration mode(Fig. 3(i), curve (a)). The peak at higher wavenumber shows theasymmetric stretching bond of C−H and the lower peak is forthe symmetrical stretching bond of C−H. The peaks around 1400cm−1 are assigned to the C−H aliphatic bending bond. The peakat 1250 cm−1 is attributed to the bending bond of C−H near Cl.The C−C stretching bond of the PVC backbone chain occurs in therange 1000− 1100 cm−1. Finally, peaks in the range of 600− 650cm−1 correspond to the C−Cl gauche bond. However, PVDF showsa peak at 1396 cm−1 for C−H deformation (Fig. 3(i), curve (b)).As far as the IR spectra of the PVC/PVDF blend are concerned,the chemical composition of both the polymers initialized equally.However, on adding 5 wt% of ZnO as shown in Fig. 3(ii), curve (a),peaks are obtained at 1382 cm−1, assigned to C−H deformation,and at 1165 cm−1, assigned to C−O−C stretching, which alsoshows the presence of PVDF in the composite.

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Figure 3. FTIR spectra (i) (a) pure PVC, (b) pure PVDF, (c) PVC/PVDF::50:50and (ii) is for PVC/PVDF/ZnO for (a) 45:50:5, (b) 40:50:10 and (c) 35:50:15loading wt%. Inset represents the major changes in FTIR peaks.

For higher loading of ZnO (10 wt%) (Fig. 3(ii), curve (b)), the IRpeak obtained at 1427 cm−1 assigned to the CH2 band in phaserepresents PVC and the peak at 1396 cm−1 assigned to C−Hdeformation represents the PVDF polymer. The major changesobtained in the FTIR spectra of the nanocomposites are shownin the inset to Fig. 3(ii). With change in the loading of ZnO inthe polymers, a change in the transmittance peak is obtained.This is explained by the chemical reaction between ZnO andthe polymers. A better compatibility in chemical composition isobtained.

SEM micrograph analysisSEM is an efficient and versatile instrument to analyse themicrostructure and surface morphology of polymer nanocom-posites. SEM micrographs of the PVC/PVDF blend and thePVC/PVDF/ZnO polymer nanocomposites are shown in Figs4(a)− 4(d). The SEM micrograph of PVC/PVDF shown in Fig. 4(a)represents a highly disordered polymer chain with a combina-tion of small and large pores. Proper crosslinking between PVCand PVDF polymer is observed with a tortuous and circuitous

path (Fig. 4(a)). Furthermore on adding ZnO nanoparticles to thePVC/PVDF polymer system a drastic change is observed in SEMmicrostructure, as shown in Figs 4(b)− 4(d). It is observed fromthe SEM microstructure of PVC/PVDF/ZnO 45:50:5 (wt%) thatthe polymer nanocomposite has numerous almost sphericallyshaped rings with a limited number of pores of very small size. Theformation of the outer rings is due to the proper intercalation andcombinational effect between the PVC and PVDF polymers. How-ever, the number of small pores inside the polymer rings revealsthe presence of ZnO nanoparticles in the polymer nanocompos-ite. As the loading of ZnO nanoparticles increases the number ofcircular rings increases as shown in Fig. 4(c).

It is observed that, as the number of nanoparticles increases inthe polymer nanocomposites, the number of small pores presentinside the circular rings start combining and become a part ofthe polymer rings. Initially, for 5 wt% loading of ZnO nanopar-ticles in the PVC/PVDF polymer blend, the diameter of the par-ticular polymer rings is 71.52 μm which is obtained from instru-ment. However, the diameter of the polymer rings is reduced to11.27 μm for 10 wt% loading of ZnO nanoparticles with a largenumber of small pores inside them for the same scale. A smoothand orderly arrangement of small and large ring structure withthe absence of small pores in SEM morphology is obtained for15 wt% loading of ZnO nanoparticles. The increased porositywith uniformly dispersed pores obtained for 5 wt% loading ofZnO nanoparticles leads to the formation of better connectivitybetween the nanoparticles and polymer, giving rise to higher ionicconductivity.21

UV−visible analysisOptical studies were carried out in order to understand betterthe effect of the nano-powder. The optical absorbance spectrarecorded for the polymer nanocomposites in the range 200− 400nm are shown in Fig. 5. In UV region bands were obtained between200 and 350 nm for the PVC/PVDF blend and the ZnO-dopedpolymer nanocomposites with different absorption intensities andwavelengths. The absorption band obtained at almost 250 nm maybe assigned to 𝜋 −𝜋* bonding.

This is caused by attractive polarization forces between thesolvent and absorber, which lowers the energy of both excitedand unexcited states. The optical band gap (Eg) of the sampleswas evaluated using the following equation and the Tauc plotmethod.22,23

(𝛼h𝜈)n = B(

h𝜈-Eg

)(2)

where 𝛼 is the absorption coefficient, h𝜈 is the photon energy andn depends on the type of transition: n may be equal to 1/2, 2,3/2 and 3 corresponding to the allowed indirect, allowed direct,forbidden direct and forbidden indirect transition respectively.24

Figures 6(i) and 6(ii) represent the variation of (𝛼h𝜈)1/2 and the(𝛼h𝜈)2 intercept with the h𝜈 axis. The energy gap is obtained byfitting the linear part of the curve and finding the intersection ofthe straight line with the h𝜈 axis. The variation in optical bandgap was obtained for different loadings of ZnO nanoparticles inthe PVC/PVDF polymer blend. Values of 4.99 and 5.03 eV werecalculated for the indirect and direct optical band gaps for thePVC/PVDF blend. However, on adding ZnO the direct opticalband gap varies from 4.98 to 5.02 eV and the indirect opticalband gap varies from 4.94 to 5.09 eV. For 5− 10 wt% loadingof ZnO in the polymer nanocomposite the optical band gapdecreases. This change is caused by a loading effect of ZnO, namelya compositional change in the polymer blend.25 However, an



Figure 4. SEM micrographs of (a) PVC/PVDF:: 50:50 and PVC/PVDF/ZnO for (b) 45:50:5, (c) 40:50:10 and (d) 35:50:15 loading wt%.

Figure 5. UV-visible spectra of (a) PVC/PVDF:: 50:50 and (b) 45:50:5, (c)40:50:10 and (d) 35:50:15 for PVC/PVDF/ZnO loading wt%.

increase in optical band gap was observed for 10 wt% loading ofZnO in the polymer nanocomposite. This variation describes theinversely proportional characteristic between optical band gapand degree of disorder in the film. Hence this characterizationassists in finding properties that relate to the band gap of themodified blend with ZnO.

TGA and DSC analysisThermal analysis of PVC/PVDF/ZnO nanocomposites was eval-uated with the TGA and DSC characterization techniques. TheTGA technique is used to determine polymer degradation tem-peratures in polymer composite materials.26 Figures 7(i) and 7(ii)

represent the TGA of pure and ZnO-doped polymer nanocompos-ites. As seen in the figures, TGA for all samples are characterizedby four weight loss regions. The weight loss obtained between 30and 300 oC in the TGA for pure and ZnO-doped polymer nanocom-posites can be due to strong and weak bonds of solvent moleculeswith polymers and nanoparticles. A significant weight loss isobtained at 300− 350 oC for all samples. This loss is particularlyimportant for pure polymer in comparison to ZnO-doped polymernanocomposites. It can also be associated with the loss of func-tional groups. A third gradual weight loss is obtained at 400− 500oC for almost all samples, as shown in Fig. 7(ii), (a)− (c). This maybe attributed to degradation of polymer side chains. The abovementioned loss is again less important for nanocomposite sam-ples. Finally the weight changes at temperatures higher than 500oC may be caused by further thermodegradation of the polymer.It is noteworthy that for ZnO-doped polymer nanocomposites aslight change can be seen in the weight loss in comparison topure polymer. Hence with TGA we can clearly identify the thermaldegradation of pure polymer and the polymer nanocomposites.

The DSC measurements indicate that the PVC/PVDF/ZnOnanocomposites have only one endothermal peak. The DSCthermogram of pure PVDF is shown in Fig. 8(i). The single peakobtained for pure PVDF at nearly 150 oC represents the meltingpeak. However, for the PVC/PVDF blend a shift in the peak from150 to 159 oC was obtained. The shift and decrease in peak widthand heat flow represents proper entanglement between the poly-mers. The DSC thermogram of PVC/PVDF/ZnO nanocomposites isshown in Fig. 8(ii), (a)− (c). The observed melting peak T m shifts tolower values with the addition of ZnO. The shift was obtained dueto the effect of ZnO in the host polymer structure. As the loadingof ZnO was increased in the polymer nanocomposites, the T m

value also changed which reveals disruption of the crystallinity ofthe host polymer. The interaction between the polymer networkand ZnO at the interface of the nanoparticles is expected to be



Figure 6. Plots for (i) Indirect optical band gap and (ii) Direct optical bandgap for (a) PVC/PVDF:: 50:50 and (b) 45:50:5, (c) 40:50:10 and (d) 35:50:15for PVC/PVDF/ZnO loading wt% by using Tauc plot method.

weaker than a covalent bond which is the major reason for thechange in the crystalline nature of the polymer.

The effect of the nanoparticles on T g can be explained by theenthalpic interaction between the polymer and the nanoparticles.Either an increase or decrease in T g can be induced depending onthe specific interaction.27,28 This clearly indicates that the move-ment of polymer chains becomes easier on the addition of ZnO tothe PVC/PVDF polymer blend.

Dielectric and conductance spectral analysisThe temperature-dependent complex permittivity (real part 𝜀′

and dielectric loss 𝜀′′) spectrum of the PVC/PVDF/ZnO polymernanocomposites for various loadings is shown in Figs 9(a) and9(b). These spectra describe the dielectric related features suchas dielectric polarization processes and the polymer dynamics.The dielectric constant 𝜀′ was evaluated by using the capacitance,thickness and area of the sample and the vacuum permittivity.The dielectric loss is directly related to the energy dissipatedand comprises the contribution from ionic transport as well asfrom the polarization of a charge or a dipole.29 A large increasein 𝜀′ and 𝜀′′ values was obtained with a decrease in frequencybelow 100 Hz, which is due to the significant contribution of theWagner−Maxwell− Sillars effect on the low frequency dielectricpolarization of the material. This polarization effect occurs dueto accumulation of ions near the electrode surfaces. Anotherimportant aspect is that the 𝜀′ spectra for 5 wt% loading of ZnOhave a point of inflection in the range 103 − 104 Hz.

Figure 7. TGA of (i) pure PVDF and PVC/PVDF::50:50 (ii) PVC/PVDF/ZnO for(a) 45:50:5, (b) 40:50:10 and (c) 35:50:15 loading wt%.

The 𝜀′ and 𝜀′′ spectra of the PVC/PVDF/ZnO composite for 5 wt%loading of ZnO increase gradually with increasing temperature.These changes are also attributed to an increase in charge densityas an additional contribution from interfacial polarization. The 𝜀′′

spectrum for the PVC/PVDF/ZnO composite for 5 wt% loading ofZnO has no peak, which shows the cooperative segmental motionbetween the polymer and ZnO nanoparticles. However, the 𝜀′

and 𝜀′′ values increase for higher loading of nanoparticles in thePVC/PVDF polymer system, as shown in Fig. 9(b).

The increment in dielectric loss values at lower frequencies isdue to free charge motion within the material.30 For low fre-quencies there is a certain time for charges to build up at theinterfaces before the field changes direction which contributesto very large values of 𝜀′′. This phenomenon is also called theconductivity relaxation process.31 The increase in 𝜀′ value withincreasing ZnO concentration indicates that the amount of dipoledensity contributing to the polarization is raised. These results indi-cate another aspect of the relaxation process or mechanism. Tounderstand the mechanism of the electrode potential process orthe polymer segmental relaxation process, the imaginary part ofthe AC conductivity 𝜎′′ was plotted along with 𝜀′′ as a functionof frequency, as shown in Figs 9(a) and 9(b). From the figures it isfound that with respect to temperature there is no change in the



Figure 8. DSC thermograms of (i) pure PVDF and PVC/PVDF:: 50:50 (ii)PVC/PVDF/ZnO for (a) 45:50:5, (b) 40:50:10 and (c) 35:50:15 loading wt%.

value of 𝜎′′. But the increase in 𝜎′′ with respect to frequency ismainly due to an increase in the Wagner−Maxwell− Sillars effect.No peak of 𝜀′′ and 𝜎′′ occurs. Hence these findings could not con-firm the charging/discharging process or the polymer segmentalprocess.

The real part of the AC conductivity 𝜎′ and the loss tangenttan 𝛿 spectra of the PVC/PVDF/ZnO nanocomposites at differenttemperatures are depicted in Figs 10(a) and 10(b). The 𝜎′ valuesof these nanocomposites increase nonlinearly with increase infrequency on a logarithmic scale, which shows ion transportationon a different time scale under the influence of an AC electric field.Previous investigations of polymer composites and electrolyteshave established that the jumps of ions between different ionsites is a consequence of AC conductivity dispersion.32,33 It isexpected that the 𝜎′ value depends on the 𝜀′′ values (𝜎′ =𝜔𝜀0𝜀

′′).Further it is found that the 𝜎′ values for different compositionsof polymer nanocomposites increase with decrease of frequencyin the Wagner−Maxwell− Sillars region. Therefore the highestfrequency region of the 𝜎′ values is fitted by Joncher’s power law,𝜎′(𝜔)= 𝜎DC +A𝜔n, where A is a pre-exponential factor and n is afractional exponent ranging from 0 to 1. The fitted region of 𝜎′ isshown in the inset of Figs 10(a) and 10(b).

Figure 9. Frequency dependant spectra of real part 𝜀′ and loss 𝜀′′ of thecomplex dielectric function of PVC/PVDF/ZnO films for (a) 45:50:5 and (b)35:50:15 loading wt% at different temperatures.

The temperature-dependent conductivity 𝜎′ gradually increaseswith increasing temperature. This spectrum (Figs 10(a) and 10(b))has two implications: first, the ion mobility increases due to theincreased polymer chain segmental motion and, second, the ionconcentration increases with increase of temperature. But XRDshows that the ZnO particles have been dissociated in the compos-ite and hence the increase in temperature does not have any addi-tional effect on ion concentration. The 𝜀′′ maximum with respectto the Wagner−Maxwell− Sillars region is directly correlated withconductivity. It is possible to transform the𝜀′ and𝜀′′ equations intotan 𝛿 = 𝜀′′/𝜀′. This procedure is mainly used to fit the tan 𝛿 spectra.The tan 𝛿 peaks appear almost at the frequency region where theDC plateau occurs in their 𝜎′ spectra. Therefore the frequency atwhich tan 𝛿 peaks are obtained can be assigned to the polymersegmental dynamic relaxation process. The tan 𝛿 value shows agradual decrease with increase in loading of ZnO (Fig. 10) whichconfirms the polymer segmental dynamic increase. The relaxationtime is determined by the relation 𝜏 tan𝛿 = 1/2𝜋f p(tan𝛿), where f p(tan𝛿)

is the frequency corresponding to the tan 𝛿 peak.34 The 𝜏 tan𝛿 val-ues are in the range 3.10− 4.21 μs for 5 wt% loading of ZnO inthe PVC/PVDF polymer system. However, a 𝜏 tan𝛿 value of 0.55 μsis obtained at 60 oC and 8.2 μs at 70− 100 oC for 15 wt% loadingof ZnO in the PVC/PVDF polymer system. It is found that the 𝜏 tan𝛿



Figure 10. Frequency dependant real part of ac conductivity 𝜎′ andloss tangent (tan 𝛿) spectra of PVC/PVDF/ZnO films for (a) 45:50:5 and(b) 35:50:15 loading wt% at different temperatures. Inset in (a) and (b)represents the fitted region by power law.

value decreases below 60 oC with the addition of ZnO nanoparti-cles. This suggests a decrease in ion−dipolar interaction strengthand simultaneously enhanced polymer segmental motion.

AC impedance spectroscopy and ionic conductivity analysisNyquist impedance plots (Z′′ versus Z′) of PVC/PVDF with differ-ent loadings of ZnO nanoparticles at different temperatures aredepicted in Fig. 11. The complex impedance plot is most suit-able for deriving the ionic conductivity of polymer composites.35,36

The spectra consist of a slight semicircular region with a lineardissipation region at higher frequency. Figure 11(a) represents thelinear region which is due to the electrode polarization effect.

However, Fig. 11(b) represents the combination of a semicircu-lar region with a linear region, which demonstrates the combi-national effect of the electrode polarization effect and polymersegmental motion. From Fig. 11(b) it is noted that the semicircleincreases with loading of ZnO nanoparticles. This result suggeststhat only the capacitive component of the polymer composite pre-vails when the concentration is increased. The impedance spec-trum also reveals a shift of the semicircle towards the higher

Figure 11. Complex impedance plane plots (Z′′ vs Z′) of PVC/PVDF/ZnOfilms for (a) 45:50:5 and (b) 35:50:15 loading wt% at different temperatures.

resistance side on the real axis with increase in temperature.The lower frequency region impedance of an ideal capacitiveelement (𝜒 c =−j/2𝜋fC) should be parallel to the imaginary axiswhich we analysed for PVC/PVDF/ZnO nanocomposites. The Z′′

versus Z′ complex impedance plot is fitted by using Z-view soft-ware for obtaining bulk electrical resistance. The common inter-cept of the semicircle on the real axis gives the bulk resis-tance Rb of the composite film. The frequency value correspond-ing to this intercept point separates the bulk properties of thematerial from the Wagner−Maxwell− Sillars effect frequencyregion.37

In the present study, 𝜎DC values of the investigated nanocom-posites were determined by using the relation 𝜎DC = ts/RbA, wherets is the thickness of the sample and A is the surface area ofthe film, as shown in Fig. 12. The 𝜎DC values of PVC/PVDF/ZnOare 1.29× 10−6 S cm−1 and 2.53× 10−7 S cm−1 for 5 wt% and15 wt% loading of ZnO nanoparticles at 30 oC. The linear varia-tion of 𝜎DC versus 1000/T plots shows Arrhenius behaviour andalso suggests a thermally activated process. The decrease in relax-ation which is obtained from tan 𝛿 confirms the strong correla-tion with ionic conductivity. The value of the activation energyEa of PVC/PVDF/ZnO nanocomposites is determined using therelation

𝜎DC = 𝜎0 ∗ exp

(Ea

kT

)(3)

where k is the Boltzmann constant and T is the absolute tempera-ture. The Ea values are 0.084 and 0.022 eV for 5 and 10 wt% load-ing of ZnO nanoparticles. The low activation energy suggests thatthere is a fast hopping mechanism for ion transportation in the



Figure 12. Reciprocal temperature dependence of DC ionic conductivity𝜎DC and linear fit of PVC/PVDF/ZnO for (a) 45:50:5 wt% and (b) 35:50:15wt% at different temperatures.

nanocomposites, which may be due to transient type couplingbehaviour and polymer segmental dynamics.38,39 Therefore thisresult suggests that both segmental motion of polymer chains andthe hopping mechanism contribute equally in coupled form for iontransportation in PVC/PVDF/ZnO nanocomposites.

CONCLUSIONSZnO nanoparticle based homogeneous polymer nanocompositeswere prepared by solution casting. XRD demonstrates the forma-tion of cation complexes with the functional groups and structuralvariation of the PVC/PVDF system after adding ZnO nanoparti-cles. FTIR spectroscopy reveals the change in transmittance peakwith the change in loading of ZnO in the polymer nanocom-posites. This explains the better compatibility in chemical com-position between polymer and nanoparticles. SEM studies showa well arranged ring pattern and well dispersed pores in thenanocomposites resulting in better connectivity which increasesthe ionic conductivity. DC conductivity occurs in the plateauregion from the 𝜎′ spectra. The 𝜎DC value is 1.29× 10−6 S cm−1 forthe PVC/PVDF/ZnO polymer nanocomposite with 5 wt% loadingof ZnO nanoparticles at 30 oC. The linear variation of log 𝜎DC ver-sus 1000/T plots shows Arrhenius behaviour and also suggests athermally activated process. This ionic conductivity study revealsthe segmental motion of polymer chains and a hopping mecha-nism which contribute equally in coupled form for ion transporta-tion in the PVC/PVDF/ZnO nanocomposites. Hence the overall con-clusion made from this study is that the doping of ZnO particlesalone enhances the electrical, thermal and structural propertiesof PVC/PVDF polymer blends, which can be further developed forelectrochemical sensor applications.

ACKNOWLEDGEMENTSThe authors would like to thank the Naval Research Board, DefenseResearch and Development Organization (NRBDRDO), New Delhi,for financial support under Project No. 259/Mat./ 11–12, providing

the instrumentation facility for electrical characterization. Theauthors also thank the management of VIT University for providingSEM under the DST-FIST project.

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