Characterization of SiO 2 nanoparticles dispersed (PVA PEO) blend based nanocomposites as the polymeric nanodielectric materials

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1 Indian Journal of Engineering & Materials Sciences Vol. 23, December 2016, pp Characterization of SiO 2 nanoparticles dispersed (PVA PEO) blend based nanocomposites as the polymeric nanodielectric materials Shobhna Choudhary* Dielectric Research Laboratory, Department of Physics, Jai Narain Vyas University, Jodhpur , India Received 16 May 2016; accepted 15 November 2016 Polymer nanocomposite (PNC) films consisting of poly(vinyl alcohol) (PVA) and poly(ethylene oxide) (PEO) blend matrix dispersed with nanosize particles of silicon dioxide (SiO 2 ) [i.e., (PVA-PEO) x wt% SiO 2, where x is 0, 1, 3 or 5] have been prepared by aqueous solution-cast method. The structural properties of the PNCs are characterized by X-ray diffraction measurements, which confirm the semi-crystalline structures of these materials. The complex dielectric function, alternating current electrical conductivity, electric modulus and impedance spectra of the PNC films have been investigated in the frequency range 20 Hz to 1 MHz by employing the dielectric relaxation spectroscopy. Effects of SiO 2 concentrations and temperatures on the dielectric permittivity, electrical conductivity and structural dynamics of these PNC materials have been explored. The dielectric relaxation mechanism of the PVA PEO blend based these materials is mainly governed by the PEO chain segmental dynamics. The relaxation times and conductivity activation energies of the PNC films are determined from the Arrhenius plots. Dielectric and electrical parameters values reveal the suitability of these materials as lowpermittivity nanodielectrics for the radio frequency electric field. Keywords: Nanodielectric, Dielectric properties, Electrical conductivity, Dielectric relaxation time, Polymer blend Among the synthetic hydrophilic polymers, poly(vinyl alcohol) (PVA) and poly(ethylene oxide) (PEO) have excellent flexible-type film forming ability when the films are prepared by solution-casting method. In these polymeric films, the macro molecules chains have intra- and inter-molecular hydrogen bonded connectivity through their functional groups. The aqueous solution cast PVA film exhibits high degree of optical transparency, and therefore, PVA is frequently used as binder in preparation of various composite materials suitable for optoelectronic and microelectronic devices 1-3. The high degree of solvating power of PEO for alkali metal salts makes it the most suitable matrix in preparation of flexible-type solid polymer electrolytes (SPEs) 4-8. Further, inherent non-toxicity, non-carcinogenicity, high biocompatibility and high degree of swelling in aqueous solutions established the PVA and PEO as potential materials in pharmaceutical industries In search of advanced polymeric materials, polymers blending became the most popular technique for designing of materials having a wide variety of useful properties over the pristine *Corresponding author ( shobhnachoudhary@rediffmail.com) polymers Therefore, the polymer blends have emerged as a vibrant field of research and development in the branches of chemistry, physics, materials science, pharmaceutical, engineering and technology. An important factor in the development of new materials based on polymeric blend is the degree of miscibility between the different polymers in the mixture which directly governs the final properties of the blend material. To obtain a one phase system in solid polymer blend prepared by solution-casting method, it is usually necessary the existence of favourable specific intermolecular interactions between the base components in the solution and such interactions must retain in the solid state after evaporation of the solvent. Most of the studies have revealed that the blends of PEO and PVA are essentially incompatible due to formation of weak inter-polymer chains interactions Although, PEO and PVA in aqueous solution form the hydrogen bonded blend structures in presence of water molecules, but strength of such hetero-molecular interactions gradually diminishes during slow evaporation of water molecules from the solution when it is casted to form the film. Due to this fact, finally polymers phase separated PVA PEO blend films are formed.

2 400 INDIAN J. ENG. MATER. SCI., DECEMBER 2016 In the past one decade, intensive research is in progress on the inorganic ceramic nanofiller dispersed polymeric nanocomposite (PNC) materials for improvement in their structural and dielectric properties Such materials may have combination of the useful properties of inorganic nanofiller (e.g., mechanical, thermal and chemical stability) and the organic polymer (e.g., flexibility, processability, ductility and dielectric permittivity). The interfacial electrostatic interactions between the nanoparticles of filler and functional groups of polymer chains established during the preparation of PNCs are the most decisive factor affecting the various useful properties of the resulting new composite materials. In principle, proper selection of nanofiller and its homogenous dispersion in polymer matrix can alter and control the desired properties both in increasing and decreasing way, but it is not always easy to obtain as what would be expected from the use of mixture rule. Therefore, it is required to characterize the properties of each composite material by employing suitable experimental technique in order to explore their technological applications. The use of silicon dioxide (silica) (SiO 2 ) nanoparticles as inorganic filler in the polymers matrices has attracted intense academic and industrial interest for a variety of technological applications 33. The electrostatic interactions of SiO 2 nanoparticles with PVA and PEO structures in PVA SiO 2 nanocomposites and PEO SiO 2 nanocomposites have been deeply investigated using various experimental techniques and the results established the formation of nanocomposite materials with improved thermomechanical properties. Survey of literature reveals that the PNC materials consisting of PVA PEO blend matrix dispersed with nanosize particles of SiO 2 have not been investigated yet, specially from their dielectric application point of view. Keeping in mind the properties of PVA and PEO based nanocomposites, in the present study, the nanocomposites of PVA PEO blend dispersed with silica nanoparticles were prepared and the same were characterized by employing X-ray diffraction (XRD) and dielectric relaxation spectroscopy (DRS). DRS is a powerful technique for characterization of dielectric and electrical properties of the polymeric nanocomposites and also their structural dynamics 28-32, Therefore, DRS investigations on (PVA PEO) SiO 2 nanocomposites are reliable results for recognizing the promising electrical and electronic applications of such materials. The increasing industrial demands of polymeric nanocomposites for electrical insulations and also as flexible-type dielectric substrate in fabrication of microelectronic devices result in a new area of materials sciences called nanodielectrics Commonly, the term nanodielectric is used for the inorganic nanoparticles dispersed polymer films when they are considered as electrical insulators 30,32. The aim of this manuscript is to explore the suitability of (PVA PEO) SiO 2 nanocomposite films as flexibletype polymeric nanodielectric materials by detailed study of their dielectric and electrical properties over the frequency range 20 Hz 1 MHz. Experimental Procedure PVA (M w = g mol -1 ) was obtained from Loba Chemie, India. PEO (M w = g mol 1 ) and SiO 2 nanopowder (product no , 99.5 trace metal basis and 5-15 nm spherical and porous particles sizes determined from the TEM images) were obtained from Sigma-Aldrich, USA. The nanocomposites of PVA PEO dispersed with x wt% SiO 2 (x = 0, 1, 3 or 5 wt% amounts of SiO 2 to the weight of PVA PEO blend) were prepared by solution casting method. For each sample, initially, equal amounts of PVA and PEO (50/50 wt%) were dissolved in double distilled deionized water in separate glass bottles, and then these were mixed to obtain PVA PEO blend solution. The required amount of SiO 2 for each sample was firstly dispersed in deionized water. After that this aqueous dispersed SiO 2 was mixed slowly in polymer blend solution, under continuously magnetic stirring for 1 h to obtain a homogenously nanoparticles dispersed (PVA PEO) SiO 2 solution. This solution was cast on to a poly propylene dish and was kept to dry at room temperature for few days to obtain the polymer nanocomposite (PNC) film. PNC films of various SiO 2 concentrations were prepared by following the same procedure. Finally, the (PVA PEO) x wt% SiO 2 films were dried in vacuum oven at 40 C for 24 h, and after that these films were used for their structural and dielectric characterizations by employing XRD and DRS techniques, respectively. XRD diffractograms of the SiO 2 nanopowder, and PVA, PEO and (PVA PEO) x wt% SiO 2 films were recorded in reflection mode at a scan rate of 0.05 /s using a PANalytical X pert Pro multipurpose diffractometer of Cu-Kα radiation (λ = nm). DRS measurements of the PNC films were carried out using an Agilent technologies 4284A precision LCR meter and 16451B solid dielectric test fixture over the

3 CHOUDHARY: SiO 2 NANOPARTICLES DISPERSED (PVA PEO) BLEND BASED NANOCOMPOSITES 401 frequency range from 20 Hz to 1 MHz and temperatures from C. The spectra of complex dielectric function *(ω) = j, alternating current (AC) electrical conductivity σ*(ω) = σ + jσ, electric modulus M*(ω) = M + jm and complex impedance Z*(ω) = Z jz of the PNC films were determined using the procedure of measurements and equations described in detail elsewhere 32,52. Results and Discussion XRD patterns and structural analysis The XRD patterns of SiO 2 nanopowder, and the pristine PVA film, PEO film and (PVA PEO) x wt% SiO 2 nanocomposite films are shown in Fig. 1. The SiO 2 exhibits broad and diffuse peak which confirms its amorphous structure 37. The PVA and PEO traces have diffraction peaks, which are corresponding to concurrent 101 and 101 crystal reflection plane for PVA 32, and 120 and concerted 112,032 reflection planes for PEO 4. These results confirm the semicrystalline structure of these polymers, but the sharp and high intensity peaks of PEO reveal its relatively high crystallinity. The XRD pattern of PVA PEO blend also has crystalline peaks of PEO but these are of low intensities as compared to pristine PEO, which reveals that the crystallinity of PEO decreases when it is blended with PVA. These results of the blend infer the formation of some amount of PVA PEO in miscible phases. Table 1 shows that the PEO diffraction peaks intensities (I) and their angular positions (2θ) vary anomalously with the increase of SiO 2 concentration in the PVA PEO blend. These results infer the formation of nanocomposites in which linking of polymers chains occurs through the electrostatic interactions with SiO 2 nanoparticles. It also seems that SiO 2 acts as complex builder for the PVA and PEO chains, and its behaviour may be identical to the complexation of PVA and PEO chains through cations of the alkali metal salts when the PVA PEO blend matrix is used in preparation of solid polymer electrolytes 53,54. The values of d-spacing and crystallite size L corresponding to 120 and concerted 112,032 reflection planes for PEO of these nanocomposite materials are determined by Bragg s relation λ = 2dsinθ and the Scherrer s equation L = 0.94λ/βcosθ, respectively, where β is the full width at half-maximum FWHM (i.e., the broadening of the peak at half height). The observed values of d and L for these polymeric nanocomposites are given in Table 1. Due to accuracy of X-ray wavelength λ up to nm, the evaluated d and L values have the accuracy of nm and 0.01 nm, respectively. Table 1 shows that d and L values vary anomalously with increase of SiO 2 concentration in the (PVA PEO) x wt% SiO 2 nanocomposites, which reveals that the structures of these polymeric nanocomposites change with the amount of SiO 2 nanoparticles dispersed in the PVA PEO blend matrix. SiO 2 concentration dependent dielectric behaviour Dielectric spectra Figure 2 shows the frequency dependent real part and loss part of complex dielectric function Table 1 Values of Bragg s angle 2, basal spacing d, full width at half maximum FWHM, crystallite size L and crystalline peak intensity I of (PVA PEO) x wt% SiO 2 nanocomposite films. Fig. 1 XRD patterns of SiO 2 nanopowder, and pristine PEO film, PVA film and (PVA PEO) x wt% SiO 2 nanocomposite films x wt% SiO 2 2 ( o ) d (nm) FWHM 10 3 (rad) L (nm) I (counts) 120 reflection peak parameters of PEO ,032 reflection peak parameters of PEO

4 402 INDIAN J. ENG. MATER. SCI., DECEMBER 2016 (permittivity) *(ω) and also the dielectric loss tangent (tanδ = / ) spectra of (PVA PEO) x wt% SiO 2 nanocomposites at 30 C. For dielectric materials, the is a measure of electrical energy storing ability of the material whereas represents the energy loss through heating effect. For the polymeric nanocomposites, their values are function of frequency, amount of nanofiller, and the electrostatic interactions between the polymer and nanoparticles of the ceramic filler. Further, the interfacial polarization also contributes in the values, which occurs due to accumulation of charges at the interfaces of different conductivity constituents of the nanocomposites. For these polymeric nanocomposites, it is found that the values non-linearly decrease from ~ 6 to 2.5 with the increase of frequency from 20 Hz and finally approach the steady state limiting value of permittivity near 1 MHz. As compared to the spectra of PVA SiO 2 nanocomposite 37 and PEO SiO 2 nanocomposite 55, the shape of spectra of (PVA PEO) x wt% SiO 2 nanocomposites are found significantly different. The decrease of with increase in frequency is due to the fact that at low frequency the electric dipoles (or polymer chains) can follow the electric field but as the frequency of exciting field increases they can no longer follow the fast varying field, and as a result of which the values of these materials decrease. The variation of values with SiO 2 concentration of the nanocomposites at various frequencies is shown in Fig. 3. This figure infers that there is a small but anomalous variation in values with increase of SiO 2 concentrations at audio frequencies (e.g., 100 Hz, 1 khz and 10 khz), whereas at radio frequencies (e.g., 100 khz and 1 MHz) these values are found almost independent of SiO 2 concentration. The anomalous variation in values with increase of SiO 2 concentration at audio frequencies is attributed to the significant changes in the interactions between polymer and fillers, and also contribution of Maxwell-Wagner-Sillars polarization effect. The low values of at radio frequencies reveal the suitability of these nanocomposites as low-permittivity value flexible type nanodielectrics for radio frequencies electric field operated microelectronic devices. The and tanδ spectra of the investigated (PVA PEO) x wt% SiO 2 nanocomposites exhibit the dielectric relaxation peak in the intermediate frequency region corresponding to the structural dynamics of these polymeric nanocomposites (Fig. 2). The tanδ peak is found at high frequency as compared to the peak frequency of the same film which is a common characteristics of polymeric nanocomposite materials 28,31,55. Basically, the tanδ is ratio of to the values at same frequency which results in shift of tanδ peak slightly high frequency side as compared to the peak frequency. The structural dynamics is concerned to the local chain motion of the PEO and Fig. 2 Frequency dependent real part and loss part of the complex dielectric function, and loss tangent tanδ of (PVA PEO) x wt% SiO 2 nanocomposite films at 30 C Fig. 3 Plots of SiO 2 concentration dependent values of (PVA PEO) x wt% SiO 2 nanocomposite films at various fixed frequencies and fixed temperature 30 C (lines are a guide to the eye)

5 CHOUDHARY: SiO 2 NANOPARTICLES DISPERSED (PVA PEO) BLEND BASED NANOCOMPOSITES 403 PVA PEO blend structures in the nanocomposites. In the previous studies, relaxation peak was not found for the PVA SiO 2 nanocomposites 37 over the same experimental frequency range, but PEO SiO 2 nanocomposites exhibited such relaxation peaks in the intermediate frequency range 55. Therefore, these comparative results infer that for the (PVA PEO) blend based nanocomposites there is dominant contribution of PEO which governs the structural dynamics of the polymer blend composite system. It is expected because of the high flexibility of PEO matrix as compared to that of the PVA. Further, the relaxation peaks of (PVA PEO) x wt% SiO 2 nanocomposites are found at higher frequencies as compared to that of the PEO x wt% SiO 2 nanocomposites 55, which infers that the structural dynamics enhances in the PVA PEO blend based nanocomposites as compared to that of the PEO based nanocomposites dispersed with SiO 2 nanofiller. It is further observed that the magnitude and frequency values of these relaxation peaks vary anomalously with the increase of SiO 2 concentration in these polymeric nanocomposites. This behaviour reveals that there are uneven variations in the strength of electrostatic interactions occurring between the polymers chain and the nanoparticles of the filler in these nanocomposites with the increase of filler concentration. The values of dielectric relaxation times and tanδ are determined using the relation = (2πf p( ) ) 1 and tanδ = (2πf p(tanδ) ) 1, respectively, where f p( ) and f p(tanδ) are the frequency values corresponding to the peaks of and tanδ spectra, respectively. The determined values of these relaxations times are given in Table 2. The evaluation of both the and tanδ values is interesting in regards to validation of the structural dynamics by representing the dielectric properties in two different forms of the dielectric loss spectra of these polymeric nanocomposite materials. Further, the increase of and tanδ with decrease of frequency in low frequency region seems the presence of Maxwell- Wagner-Sillars (MWS), interfacial polarization relaxation process below 20 Hz. Electric modulus spectra The complex electric modulus spectra M*(ω) of the composite materials are frequently derived by using the complex permittivity *(ω) spectra from the relation M*(ω) = 1/ *(ω) in order to identify the bulk response after nullifying the contribution of electrode polarization (EP) effect 28,29,31, The spectra of real part M and loss part M of electric modulus for (PVA PEO) x wt% SiO 2 nanocomposites, at 30 C, are depicted in Fig. 4. It is found that the M values non-linearly increase with the increase of frequency, whereas M spectra exhibit a modulus relaxation process in the intermediate frequency region. The non-zero values of M at low frequencies confirm that the observed response of the spectra in the same low frequency range (Fig. 2) is representing the materials bulk properties and there is no contribution of EP effect. The values of modulus relaxation time M (also called conductivity relaxation time σ ) are determined by using the relation M = (2πf p(m) ) 1, where f p(m) is the frequency value corresponding to the peak of M spectra. The observed M values of the polymeric Table 2 Values of dielectric relaxation time, loss tangent relaxation time tanδ, electric modulus relaxation time M, DC ionic conductivities σ dc(i) and σ dc(ii), and fractional exponents n (I) and n (II) of the PVA PEO blend and its nanocomposite films. x wt% SiO 2 ε (μs) tanδ (μs) M (μs) σ dc(i) (S/cm) n (I) σ dc(ii) 10 9 (S/cm) (PVA PEO) x wt% SiO 2 films T ( C) (PVA PEO) blend film T ( C) (PVA PEO) 3 wt% SiO 2 films n (II)

6 404 INDIAN J. ENG. MATER. SCI., DECEMBER 2016 nanocomposites at various concentrations of SiO 2 are given in Table 2. These M values are of the order of microsecond and slightly lower than the corresponding tanδ values of the studied nanocomposites. AC conductivity and impedance spectra The spectra of real part σ and loss part σ of the complex AC electrical conductivity σ*(ω), and the resistive part Z and reactive part Z of complex impedance Z*(ω) of (PVA PEO) x wt% SiO 2 nanocomposites, at 30 C, are shown in Fig. 5. On loglog scale, it is found that the σ and Z values of these materials have some deviation from linearity in the intermediate frequency region, whereas σ and Z values change linearly over the entire frequency range. Further, the frequency dependent σ and Z values are found about one order of magnitude lower than that of the corresponding σ and Z values, respectively, of these materials. Furthermore, the σ values of the PNC films are about S/cm at 20 Hz, and with the increase of frequency these attain to 10 7 S/cm at 1 MHz. To understand the dispersion behaviour of σ spectra, these are separately plotted in Fig. 6. This figure clearly reveals the presence of two different dispersion regions (low frequency region (I) and the high frequency region (II)) in the σ spectra which are mainly due to semicrystalline nature of the investigated materials. Similar type behaviour of σ spectra of PVA PEO blend were also reported 22. These dispersion regions were fitted to the Jonscher s power law 59 relation Fig. 5 Frequency dependent real part σ and loss part σ of the complex AC electrical conductivity, and real part Z and reactive part Z of complex impedance of (PVA PEO) x wt% SiO 2 nanocomposite films at 30 C Fig. 4 Frequency dependent real part M and loss part M of complex electric modulus of (PVA PEO) x wt% SiO 2 nanocomposite films at 30 C Fig. 6 Frequency dependent real part σ of the complex AC electrical conductivity of (PVA PEO) x wt% SiO 2 nanocomposite films at 30 C. Solid lines show the Jonscher s power law fit of experimental data at low frequencies (Region-I) and high frequencies (Region-II)

7 CHOUDHARY: SiO 2 NANOPARTICLES DISPERSED (PVA PEO) BLEND BASED NANOCOMPOSITES 405 σ (ω) = σ dc + Aω n, separately, as shown by solid line in Fig. 6. In the power law relation, σ dc is the direct current (DC) electrical conductivity, A is the pre-exponential factor and n is the fractional exponent ranging between 0 and 1. The σ dc(i) and σ dc(ii) values obtained by correspondingly low (audio) and high (radio) frequencies σ fit to the power law fit are recorded in Table 2 along with the respective frequency region n (I) and n (II) values. The n values of these materials are found lower than unity for both the regions confirming the hopping-type charge conduction mechanism which is mostly observed in disordered materials 60,61. The behaviour of the σ dc values with SiO 2 concentration has been discussed in later section. The high Z values as compared to the Z values at same frequency (Fig. 5) suggest the dominant capacitive behaviour of these materials. Further, the low frequency impedance values of these PNC materials are about ~10 6 Ω which confirms their high electrical insulation behaviour at audio frequencies, and therefore these nanodielectrics confirm their suitability as insulating materials for the electrical and electronic devices working under low frequency electrical field. nanocomposites, at 30 C. Both the parameters vary anomalously with increase of SiO 2 concentration. The monotonous trend of variation in various relaxation times values was observed for these materials. Further, it is found that the values of these relaxation times are in the order > tanδ > M, which is a common characteristics of the polymeric nanocomposite materials 28,31,55. The behaviour of σ dc(i) and σ dc(ii) values with SiO 2 concentration is not identical, and these values differ by about three orders of magnitude. Further, the comparison in variation of and σ dc with SiO 2 concentration reveals that there is inverse correlation between and σ dc(ii) values, i.e., increase of value decreases the σ dc(ii) value and vice versa, whereas no such correlation is observed between and σ dc(i) values of these nanocomposites. Temperature dependent dielectric behaviour (PVA PEO) blend Figure 8 presents the, and tanδ spectra of PVA PEO blend film at different temperatures. It is observed that as the temperature of the film increases the values of all these dielectric spectra also increase confirming their Relaxation and conductivity plots Figure 7 shows the SiO 2 concentration dependent relaxation times (, tanδ and M ) and DC conductivity (σ dc(i) and σ dc(ii) ) values of (PVA PEO) x wt% SiO 2 Fig. 7 SiO 2 concentration dependent relaxation times and conductivity plots of (PVA PEO) x wt% SiO 2 nanocomposite films at 30 C (lines are a guide to the eye) Fig. 8 Frequency dependent real part and loss of the complex dielectric function, and loss tangent tanδ of PVA PEO blend film at different temperatures. Insets show the enlarged view of the relaxation peaks

8 406 INDIAN J. ENG. MATER. SCI., DECEMBER 2016 thermally activated behaviour. To understand the trend of the increase of values with temperature, the values are plotted against temperature, at various frequencies, in Fig. 9. It is observed that, at radio frequencies the value increases linearly with increase of temperature but at audio frequencies the increase is relatively stronger than linearity. These results show that the MWS interfacial polarization and DC conductivity contribution at low frequencies are the reasons behind the non-linear behaviour of values with the increase of temperature of the PVA PEO blend film. The and tanδ spectra of PVA PEO blend exhibit relaxation peaks and the magnitudes of these peaks increase with increase of temperature of the blend film (see inset of Fig. 8). Further, the peak positions have gradual shift towards higher frequency region which confirms the thermally activated behaviour of the structural dynamics of the blend film. In polymeric dielectric materials, it is expected that the increase of temperature reduces the strength of heterogeneous molecular interactions and also increases the free volume in the polymeric matrix which results in enhancement of the structural dynamics as it is revealed from the shift of relaxation peak towards high frequency side of the and tanδ spectra. The values of relaxation times and tanδ of the PVA PEO blend are also determined using the values of respective peak frequencies, and the observed values are given in Table 2. The temperature dependent M and M spectra of PVA PEO blend film are shown in Fig. 10. It is observed that the M values decrease with the increase of temperature of the blend film whereas M values increase. Identical to the temperature dependent behaviour of and tanδ peaks, the M peak also shifts towards high frequency side and its height increases with the increase of temperature of the film. The temperature dependent values of relaxation time M is also determined using the frequency value corresponding to the M peak and the evaluated M values are given in Table 2. Figure 11 shows the σ spectra of PVA PEO blend film at various temperatures. It is found that the σ values increase with increase of Fig. 10 Frequency dependent real part M and loss part M of complex electric modulus of PVA PEO blend film at different temperatures Fig. 9 Temperature dependent values of PVA PEO blend film at various fixed frequencies (lines are a guide to the eye) Fig. 11 Frequency dependent real part σ of the complex AC electrical conductivity of (PVA PEO) blend film at different temperatures (Solid lines show the power law fit of experimental data at low frequencies (Region-I) and high frequencies (Region-II))

9 CHOUDHARY: SiO 2 NANOPARTICLES DISPERSED (PVA PEO) BLEND BASED NANOCOMPOSITES 407 temperature and these spectra also have two separate power law regions which are exhibited at all the temperatures. The temperature dependent σ dc(i) and σ dc(ii) values of the PVA PEO blend film are determined by the Jonscher s power law fit of σ spectra in the low and high frequency regions, respectively, and these values are given in Table 2. The spectra of σ, Z and Z of the PVA PEO blend film are found less influenced by the temperature over the range C. Figure 12 shows the Arrhenius plots of the relaxation times, tanδ and M, and DC conductivities σ dc(i) and σ dc(ii) of the PVA PEO blend film. The behaviour of and σ dc plots confirms that these parameters have inverse correlation with the increase of temperature, which implies that the charge mobility in the investigated polymer blend film is governed by the structural dynamics. The values of relaxation time activation energy E and conductivity activation energy E σ of the blend film are determined using the Arrhenius relations = 0 exp(e /kt) and σ dc = σ 0 exp( E σ /kt), respectively, where 0 and σ 0 are the pre-exponent factors, k is the Boltzman s constant and T is temperature in absolute scale. The observed E and E σ values of the PVA PEO blend film are marked on the respective plots in the figure. The E values obtained from various relaxation times of the same material have a small variation. Further, it is found that the E values are significantly low as compared to the E σ values of the blend film, which infers that there are different heights of the potential barrier for the charges when they are relaxing and conducting. (PVA PEO) 3 wt% SiO 2 nanocomposite film Figure 13 presents the, and tanδ spectra of (PVA PEO) 3 wt% SiO 2 film at different temperatures. It is observed that as the temperature of the film increases its values also increase which confirm the enhancement in dielectric polarization. The behaviour of the increase of with temperature, at various frequencies, is shown in Fig. 14. It is observed that, at higher frequencies, the value Fig. 13 Frequency dependent real part and loss of the complex dielectric function and loss tangent tanδ of (PVA PEO) 3 wt% SiO 2 nanocomposite film at different temperatures Fig. 12 Arrhenius plot of the relaxation times and DC electrical conductivity σ dc of PVA PEO blend film (lines are linear fit of data points) Fig. 14 Temperature dependent values of (PVA PEO) 3 wt% SiO 2 nanocomposite film at various frequencies (lines are a guide to the eye)

10 408 INDIAN J. ENG. MATER. SCI., DECEMBER 2016 increases linearly with increase of temperature (increment factor is very small), but at lower frequencies the increase is high and have a non-linear behaviour. This temperature dependent behaviour of values of the nanocomposite film is found identical to the PVA PEO blends film. Figure 13 shows that the magnitudes of and tanδ relaxation peaks increase with increase of temperature and the peak positions have gradual shift towards higher frequency region confirming the increase in structural dynamics of the nanocomposite material. The values of relaxation times and tanδ of the (PVA PEO) 3 wt% SiO 2 film at various temperatures are also determined from the values of peak frequencies of the respective spectra, and the obtained values of relaxation times are reported in Table 2. The temperature dependent M and M spectra of (PVA PEO) 3 wt% SiO 2 film are shown in Fig. 15. These spectra have dispersion similar to that of the PVA PEO blend at various temperatures as described earlier in this manuscript. The values of modulus relaxation time M at various temperatures of the PNC film are also determined using the frequency values corresponding to the M peaks and these values are listed in Table 2. Figure 16 presents the σ spectra of (PVA PEO) 3 wt% SiO 2 film at various temperatures. The values of σ dc(i) and σ dc(ii) of the nanocomposite film are determined by Jonscher s power law fit in the low and high frequency regions, respectively, and the observed values are given in Table 2. It is found that the temperature dependent σ dc values of 3 wt% SiO 2 dispersed PNC film are low as compared to that of the pristine PVA PEO blend film, which suggests that the dispersed SiO 2 nanoparticles in the polymer blend matrix reduce the charge conductive paths. Figure 17 depicts the Arrhenius behaviour of the relaxation times (, tanδ and M ) and conductivity Fig. 16 Frequency dependent real part σ of the complex AC electrical conductivity of (PVA PEO) 3 wt% SiO 2 nanocomposite film at different temperatures (Solid lines show the Jonscher s power law fit of experimental data at low frequencies (Region-I) and high frequencies (Region-II)) Fig. 15 Frequency dependent real part M and loss part M of complex electric modulus of (PVA PEO) 3 wt% SiO 2 nanocomposite film at different temperatures Fig. 17 Arrhenius plots of the relaxation times and DC electrical conductivity σ dc of (PVA PEO) 3 wt% SiO 2 nanocomposite film (lines are a guide to the eye)

11 CHOUDHARY: SiO 2 NANOPARTICLES DISPERSED (PVA PEO) BLEND BASED NANOCOMPOSITES 409 (σ dc(i) and σ dc(ii) ) values of the (PVA PEO) 3 wt% SiO 2 film. The values of E and E σ are determined from these Arrhenius plots and the same are marked in the figure. Further, it is found that E values are significantly low as compared to that of the E σ values of the PNC film, which reveal that the barrier heights facing by the charges during their relaxation is low and for conduction is high. Conclusions The detailed dielectric and electrical properties of the (PVA PEO) x wt% SiO 2 nanocomposites are analyzed. The complex dielectric permittivity, electric modulus, AC conductivity and the impedance spectra were reported and discussed in view of the suitability of these composite materials as flexible-type nanodielectrics over audio and radio frequencies operated electrical and electronic devices. Results revealed that the dispersed SiO 2 nanoparticles in the PVA PEO blend matrix influences the dielectric relaxation and the electrical conductivity of the materials. The DC conductivity varies anomalously with increase of SiO 2 concentration in the nanocomposites. The dielectric relaxation peak corresponding to structural dynamics and electrical conduction in the tanδ and M spectra of (PVA PEO) SiO 2 films were observed. The temperature dependent relaxation processes and DC conductivity values of the PNC film obey the Arrhenius behaviour. The activation energies of relaxation processes and the conductivity of these nanodielectric materials are reported. The real part of permittivity increases anomalously with increase of SiO 2 concentration in the PNC materials, whereas at fixed concentration of SiO 2, it increases non-linearly at audio frequencies and linearly at radio frequencies with the increase of temperature. These polymeric nanocomposite materials confirm their suitability as low-permittivity nanodielectric for the radio frequency electric field, and have high electrical insulation and poor conductivity for the audio frequency electric field. Acknowledgements Author is thankful to Prof. R J Sengwa of JNV University, Jodhpur, for providing the experimental facilities and the help in interpretation of the results. References 1 Fernandes D M, Winkler Hechenleitner A A, Lima S M, Andrade L H C, Caires A R L & Gómez Pineda E A, Mater Chem Phys, 128 (2011) Van Etten E A, Ximenes E S, Tarasconi L T, Garcia I T S, Forte M M C & Boudinov H, Thin Solid Films, 568 (2014) Rao J K, Raizada A, Ganguly D, Mankad M M, Satayanarayana S V & Madhu G M, J Mater Sci, 50 (2015) Choudhary S & Sengwa R J, Mater Chem Phys, 142 (2013) Choudhary S, Bald A, Sengwa R J, Chęcińska-Majak D & Klimaszewski K, J Appl Polym Sci, 132 (2015) Choudhary S, Indian J Pure Appl Phys, 53 (2015) Das S & Ghosh A, J Appl Phys, 117 (2015) Chaurasia S K, Saroj A L, Shalu, Singh V K, Tripathi A K, Gupta A K, Verma Y L & Singh R K, AIP Adv, 5 (2015) Rajewski R A & Stella V J, J Pharm Sci, 85 (1996) Hassan C M & Peppas N A, Adv Polym Sci, 153 (2000) Khutoryanskiy V V, Int J Pharm, 334 (2007) Edsman K & Hägerström H, J Pharm Pharmacol, 57 (2005) Bernal A, Kuritka I & Saha P, J Appl Polym Sci, 127 (2013) Cheng J, Wang S, Chen S, Zhang J & Wang X, Polym Int 61 (2012) El-Houssiny A S, Ward A A M, Mansour S H & Abd-El- Messieh S L, J Appl Polym Sci, 124 (2012) Li L, Chen N & Wang Q, J Polym Sci B, 48 (2010) Lian Z & Ye L, J Thermoplast Compos Mater, 26 (2013) Paladhi R & Singh R P, J Appl Polym Sci, 51 (1994) Sawatari C & Kondo T, Macromolecules, 32 (1999) Ferdous S, Mustafa A I & Khan M A, J Macromol Sci A, 40 (2003) Lai W C & Liau W B, J Appl Polym Sci, 92 (2004) Mishra R & Rao K J, Solid State Ionics, 106 (1998) Mishra R & Rao K J, Eur Polym J, 35 (1999) Sengwa R J, Choudhary S & Sankhla S, Indian J Eng Mater Sci, 16 (2009) Abd El-Kader F H, Hakeem N A, Elashmawi I S & Ismail A M, Indian J Phys, 87 (2013) Hassounah I, Shehata N, Hudson A, Orler B & Meehan K, J Appl Polym Sci, 131 (2014) Abdullah O Gh, Saber D R & Hamasalih L O, Univ J Mater Sci, 3 (2015) Sengwa R J, Choudhary S & Sankhla S, Compos Sci Tech, 70 (2010) Khutia M & Joshi G M, J Mater Sci: Mater Electron, 26 (2015) Zhou Y, He J, Hu J & Dang B, J Appl Polym Sci, 133 (2016) Sengwa R J & Choudhary S, Macromol Symp, 362 (2016) Choudhary S & Sengwa R J, J Appl Polym Sci, 133 (2016) Zou H, Wu S & Shen J, Chem Rev 108 (2008) Peng Z & Kong L X, Polym Degrad Stab, 92 (2007) Wang C, Wei J, Xia B, Chen X & He B, J Appl Polym Sci, 128 (2013) Lue S J & Shieh S J, Polymer, 50 (2009) Choudhary S & Sengwa R J, AIP Conf Proc, 1728 (2016) Madathingal R R & Wunder S L, Macromolecules, 44 (2011) 2873.

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