Dielectric dispersion and relaxations in (PVA-PEO)-ZnO polymer nanocomposites

Author’s Accepted Manuscript Dielectric dispersion and relaxations in (PVA– PEO)–ZnO polymer nanocomposites Shobhna Choudhary

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S0921-4526(17)30474-X http://dx.doi.org/10.1016/j.physb.2017.07.066 PHYSB310133

To appear in: Physica B: Physics of Condensed Matter Received date: 19 March 2017 Revised date: 24 July 2017 Accepted date: 27 July 2017 Cite this article as: Shobhna Choudhary, Dielectric dispersion and relaxations in (PVA–PEO)–ZnO polymer nanocomposites, Physica B: Physics of Condensed Matter, http://dx.doi.org/10.1016/j.physb.2017.07.066 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Dielectric dispersion and relaxations in (PVA–PEO)–ZnO polymer nanocomposites Shobhna Choudhary a,b,* a

Dielectric Research Laboratory, Department of Physics, Jai Narain Vyas University, Jodhpur – 342 005, India b CSIR-National Institute of Science Communication and Information Resources, New Delhi – 110 012, India

Abstract The organic-inorganic nanocomposite materials consisted of poly(vinyl alcohol) (PVA) and poly(ethylene oxide) (PEO) blend matrix (50/50 wt%) dispersed with zinc oxide (ZnO) nanoparticles have been prepared by the aqueous solution-cast method. The dielectric dispersion and relaxation processes in these polymer nanocomposite (PNC) films (i.e., (PVA–PEO)–x wt% ZnO; x = 0, 1, 3 or 5) have been investigated over the frequency range from 20 Hz to 1 MHz by employing the dielectric relaxation spectroscopy (DRS). Influence of ZnO contents on the complex dielectric permittivity, electrical conductivity, electric modulus and impedance properties of these PNC materials have been explored. The dielectric permittivity and the relaxation time values corresponding to polymers cooperative chain segmental motion significantly change with the variation of ZnO contents in the PVA–PEO blend matrix at ambient temperature. The temperature dependent relaxation times and dc conductivity values of (PVA–PEO)–3 wt% ZnO film have been investigated which obey the Arrhenius behaviour. The dielectric permittivity of the film as a function of temperature exhibits linear behaviour at radio frequencies and non-linear variation at lower audio frequencies. X-ray diffraction

* Correspondence to: (E-mail: [email protected])

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measurements confirm a huge decrease in crystalline phase of the polymer blend matrix on the addition of 1 wt% ZnO nanoparticles. These PNC materials have low values of dielectric permittivity and electrical conductivity which confirm their suitability as novel flexible-type polymer nanodielectric for the insulation in microelectronic devices, whereas the fast chain segmental dynamics and high amorphous phase reveal these materials as a potential candidate for the preparation of nanocomposite solid polymer electrolytes.

Keywords: Polymer nanocomposites, Dielectric properties, Electrical conductivity, Relaxation times, Activation energy, X-ray diffraction

1. Introduction Synthetic polymers have been established as the most suitable base matrices for the preparation and designing of flexible-type solid materials starting from simple composites and extending up to advanced functional nanomaterials for their enormous technological importance [1–4]. Among these polymers, poly(vinyl alcohol) (PVA) and poly(ethylene oxide) (PEO) are biodegradable, inherently non-toxic and hydrophilic in character. They have excellent flexible-type film forming ability when their films are prepared by solution casting method [5–8] and also by melt-pressing technique for the low melting temperature PEO [9]. The presence of hydroxyl groups in the carbon chain backbone of PVA macromolecules acts as a functional group for intra- and intermolecular hydrogen bonding and plays a major role in the formation of various kinds of interactions with inorganic nanofillers [5,7,10–16]. On the other hand, the presence of

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ether oxygen in the PEO chain backbone behaves as a functional group which realizes this polymer as a highly suitable solid solvent for the alkali metal salts in the preparation of solid polymer electrolytes (SPEs) for their applications in the various ion conducting devices [17–20]. In the past decade, intensive research is in progress on the inorganic nanoparticles dispersed polymeric nanocomposite materials [21–24]. These organic-inorganic materials have a combination of several useful properties of inorganic nanofiller (e.g. rigidity, thermal and chemical stability) and also of the organic polymer (e.g. flexibility, processability, ductility, dielectric and electrical). The interfacial electrostatic interactions formed between the nanofiller and the functional groups of the polymer during the preparation of polymeric nanocomposites are the most decisive factor governing the various useful physical and structural properties of the final product. Zinc oxide (ZnO) is one of the most important multifunctional and high crystallinity compound semiconductor of the wide band gap (3.37 eV), which has enormous electronic and optical properties. The use of ZnO nanoparticles as an inorganic filler in the PVA and PEO matrices has received substantial academic and technological interest. The interactions of ZnO nanoparticles with PVA and PEO structures in the PVA–ZnO nanocomposites [10,12,25–29] and the PEO–ZnO nanocomposites [30–35], respectively, have been investigated by using various experimental techniques and their results have established the formation of nanocomposite materials by solution-cast method which exhibits several improved technological properties. In order to fulfill the exponentially growing industrial requirement of novel properties polymers, there is a continuous demand to synthesize new polymers of

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required properties. Instead of synthesizing new polymers of such required properties, the blending of polymers has become the most economic, effective and green chemistry technique to achieve the polymer matrix of tailored properties over the pristine polymers [1,2,4,36–42]. Miscibility of polymers in the blend is one of the most important factors governing the various physico-chemical properties of the final material; however, the process used to combine two polymers can also influence the properties of the final product of the polymer blend. In regards to polymer blends, PVA and PEO blend films were mostly prepared by the solution-casting method and their properties were characterized by employing various spectroscopic and other related techniques [43–49]. These studies revealed that the blends of PEO and PVA are essentially immiscible due to insignificant interpolymer chains hydrogen bonding interactions. The immiscible polymer blends are further described as compatible or incompatible. The studies on PVA–PEO blend doped with inorganic nanofillers revealed that the interfacial interactions between polymers and nanofiller particles help in the enhancement of polymers miscibility which can turn the material into compatible type PNC blend [39,50– 53]. The structural, thermal, mechanical and optical properties of zinc oxide (ZnO) doped PVA–PEO blend nanocomposites were explored earlier [52,53], but the dielectric and electrical properties of these nanocomposites are yet to be investigated for confirming their suitability as solid flexible-type polymeric nanodielectric, e.g., design of novel flexible-type polymer based dielectric materials using nanotechnology [9,32,40,54– 64]. Therefore, in the present work, the PNC films of PVA–PEO blend dispersed with ZnO nanoparticles were prepared and the same were characterized by employing X-ray

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diffraction (XRD) and dielectric relaxation spectroscopy (DRS). The aim of this study is to explore the suitability of (PVA–PEO)–ZnO films as the flexible-type polymer nanodielectric in view of their technological and industrial applications.

2. Experimental 2.1 Material PVA (Mw = 77×103 g mol-1) of the Loba Chemie, India, and PEO (Mw = 6×105 g mol–1) and ZnO nanopowder (particle size < 100 nm) of the Sigma-Aldrich, USA, were used for the preparation of PNC materials. The PNC films of PVA–PEO blend dispersed with x wt% amount of ZnO (x = 0, 1, 3 and 5) were prepared by solution-casting method. For each sample, firstly the equal amounts of PVA and PEO (i.e., 50/50 wt/wt%) were dissolved in deionized water separately. PVA powder was dissolved in deionized water at 80 °C whereas the PEO powder was dissolved in deionized water at 40 °C under continuous magnetic stirring. Subsequently, these PVA and PEO aqueous solutions were mixed to obtain PVA–PEO blend solution. The required amount of ZnO for each PNC film was initially dispersed in the deionized water. After that, aqueous dispersed ZnO was added slowly into the polymer blend solution, under continuous magnetic stirring for 1 h to obtain a homogenously ZnO nanoparticles dispersed (PVA–PEO)–x wt% ZnO solution. Finally, this solution was cast on to a poly propylene dish and was kept to dry at room temperature to obtain the PNC film. The PNC films of various ZnO concentrations were prepared by the same procedure. The thicknesses of (PVA–PEO)–x wt% ZnO films were 0.22, 0.19, 0.20 and 0.18 mm corresponding to x = 0, 1, 3 and 5 wt %, respectively.

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These PNC films were dried at 40 °C for 24 h in a vacuum oven prior to their characterizations. 2.2 Measurements The XRD patterns of the ZnO nanopowder and (PVA–PEO)–x wt% ZnO films were recorded in reflection mode at the scan rate 0.05° s−1 using a PANalytical Xpert Pro MPD diffractometer of Cu-Kα radiation operating at 1800 W (45 kV and 40 mA). The DRS measurements of the PNC films were carried out using an Agilent technologies 4284A precision LCR meter equipped with 16451B solid dielectric test fixture over the frequency range from 20 Hz to 1 MHz. Frequency dependent values of capacitance Cp, resistance Rp and dielectric loss tangent (tanδ = /) of the dielectric test fixture loaded with the SPE film were measured in parallel circuit operation for the evaluation of dielectric and electrical spectra of the film. Prior the sample measurement, the open circuit calibration of the cell was performed to eliminate the effect of stray capacitance of the cell leads. During the DRS measurements, the temperature of the PNC film loaded dielectric test fixture was controlled by mounting it into the microprocessor controlled oven. The spectra of complex dielectric permittivity *(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 these PNC films were determined from the measured frequency dependent values of Cp, Rp and tanδ using the following equations [39]; ε 

t g Cp ε0 A

and ε  ε tanδ

(1)

σ * (ω)  σ  j σ  j ω ε 0 ε * (ω)  ω ε 0 ε  j ω ε 0 ε

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(2)

M * (ω) 

Z * (ω) 

1 ε ε   M   j M   2 j 2 2 ε * (ω) ε   ε  ε   ε  2

Rp ω Cp Rp2 1  Z   j Z    j Y * (ω) 1 (ω Cp Rp ) 2 1 (ω Cp Rp ) 2

(3)

(4)

In the above equations, tg and A are the thickness and area of cross-section (equal to the surface area of active electrode of the dielectric test fixture) of the film,  = 2f is the angular frequency of the electric field and 0 = 8.85 pF/m is the permittivity of vacuum.

3. Results and discussion 3.1 Structural analysis The XRD patterns of ZnO nanopowder, PVA, PEO and (PVA–PEO)–x wt% ZnO films are shown in Fig 1. The ZnO powder exhibits various characteristic diffraction peaks which are in consistent with its well crystallized hexagonal wurtzite structure [12,25,31]. The observed XRD peaks of the pristine PVA and PEO films are found in good agreement with the earlier studies [15,65,66] which confirms their semicrystalline structures. The XRD pattern of PVA–PEO blend film (x = 0) exhibits diffraction peaks at 2θ = 19.54 and 23.66° which attribute to (120) and concerted (112),(032) reflection planes of the PEO and these peaks also confirm the presence of PEO crystalline phases in the blend [65]. The values of basal d-spacing and crystallite size L of the PEO in these PNC materials are determined using the Bragg’s relation  = 2dsin and the Scherrer’s equation L = 0.94λ/βcosθ, respectively. The calculated values of d and L along with the values of peaks position 2θ and their intensity values I are given in Table 1. It is found that the 2θ values of both the crystalline peaks of PVA–PEO blend increase anomalously 7

with the increase of ZnO contents in the blend matrix, and due to which the d-spacing values also decrease. A large decrease is observed in the values of L corresponding to (120) reflection plane of PEO at 1 and 3 wt% ZnO contents in the PVA–PEO blend (Fig. 2) which confirms the formation of large electrostatic interactions between the ZnO nanoparticles and the functional groups of the polymers chains. Further, the L value of PNC film containing 5 wt% ZnO is found nearly equal to that of the pristine PVA–PEO blend, which may be owing to some agglomeration of ZnO nanoparticles at higher concentrations in the PNC films [52]. Fig. 2 shows that the relative intensity RI values of both the PEO diffraction peaks for the PNC film containing 1 wt% ZnO greatly reduce confirming that the film acquires optimum nanocomposite properties at this concentration, which is inconsistent with the earlier reported results [31,32,52].

3.2 Nanofiller concentration dependent dielectric behaviour 3.2.1 Dielectric spectra Figure 3 shows the frequency dependent values of the real part  and loss part  of complex dielectric permittivity *(ω) and also the dielectric loss tangent (tanδ = /) for the (PVA–PEO)–x wt% ZnO nanocomposites at 30 C. For a dielectric material, the  value is a measure of its electrical energy storing ability (i.e., the strength of dielectric polarization) under the influence of time varying electric field, whereas the  value represents the electric energy loss per cycle through Joule heating effect [67]. From Fig. 3, it can be noted that both the  and  spectra of the studied nanocomposite materials exhibit three clearly distinguishable characteristics regions over the frequency range from 20 Hz to 1 MHz which are marked in the figure. Firstly, the  and  values increase 8

strongly with the decrease of frequency below 1 kHz which confirms the dominant contribution of interfacial polarization (IP) effect (also called Maxwell-Wagner-Sillars (MWS) effect) in the complex permittivity values of PNC material at low frequencies [67–69]. Secondly, the dielectric dispersion in the intermediate frequency region which starts around 1 kHz and extends up to 100 kHz is attributed to the dipolar polarization of polymers chain segments. In this region the  values non-linearly decrease with the increase of frequency, whereas the  spectra exhibit the α–relaxation peak of the PEO local chain dynamics [31,66,70–72]. Thirdly, in the frequency region above 100 kHz, the  values tend to approach steady state with the increase of frequency which can be assigned to the high frequency limiting permittivity ∞ values of the PNC films. In the high frequency region, the  values initially have a small decrease with the increase of frequency and finally become very low for these PNC materials. The tanδ spectra of the (PVA–PEO)–x wt% ZnO nanocomposites also have three distinguishable regions (Fig. 3), which are identical to the different regions as observed in the  spectra. These spectra exhibit relatively intense dielectric relaxation peaks in the intermediate frequency region corresponding to the polymer main chain segmental dynamics (-relaxation). The single relaxation peak in both the  and tanδ spectra over the experimental frequency range at ambient temperature suggests the cooperative local chain motion of the PVA and PEO polymers in the blend matrix with a dominant contribution of the PEO chain segmental dynamics which is due to its highly flexible and linear chain structure. This fact is also favoured by the α–relaxation peak of pristine PEO which is exhibited in the same intermediate frequency region [9,31,66,73], whereas such relaxation peak was not observed for the pristine PVA film over the frequency range 9

from 20 Hz to 1 MHz, at room temperature [6,7]. Further, it is found that both the  and tanδ values sharply enhance with the decrease of frequency below 1 kHz i.e., in the IP effect dominated frequency range. The tanδ values of these PNC materials at 1 MHz are found significantly low (tanδ < 0.03 at 1 MHz for 1 wt% ZnO) which suggest the suitability of these PNC films as potential candidate for nanodielectric substrate having low dielectric permittivity and low loss for the fabrication of radio frequency operated flexible type microelectronic devices [6,54,56,74,75]. The values of frequency fp corresponding to the relaxation peaks of the  and tanδ spectra were used to determine the relaxation times  and tanδ, respectively, from the relation  = (2πfp)–1. The observed  and tanδ values for these PNC films which are recognized as polymer nanodielectric materials are recorded in Table 2. It is found that both  and tanδ values are of the order of few microsecond duration and these vary anomalously with the increase of ZnO contents in the PVA–PEO blend matrix. This finding confirms the significant changes in the filler-polymer electrostatic interactions as a function of filler contents in these PNC materials. The variation of  values with the increase of ZnO concentration in these PNCs at fixed frequency can be noted from Fig. 4. This figure confirms that initially, there is a significant decrease of  values when 1 wt% ZnO was loaded in the PVA–PEO blend matrix, and after that, these values show an increase with the further increase of the ZnO contents up to 5 wt%. For 5 wt% ZnO containing PNC film, the ε values are found higher than that of the pristine PVA–PEO blend. Earlier studies on some polymeric nanodielectric have also reported the lowering of dielectric permittivity values as compared to that of the pristine polymer values which are mainly due to 10

nanoconfinement phenomenon [31,56,66,76–78]. According to this phenomenon, the electrostatic interaction force acting between the doped nanoparticles and the functional groups of polymer chain constrains the molecular motion, due to this fact the effective dielectric polarization of the PNC material reduces even though the added inorganic nanofillers have higher dielectric permittivity value as compared to that of the polymer matrix, provided that the dispersed nanoparticles in the polymer matrix are well separated. The  value of ZnO nanopowder is 10.26 at 1 MHz and 27 °C [79], besides this, the decease of  value of the PNC film containing 1 wt% ZnO is mainly due to the nanoconfinement effect. Further, Fig. 4 shows that the  values of 5 wt% ZnO loaded nanocomposite film are nearly equal to that of the pristine PVA–PEO blend matrix at various frequencies except for the IP effect dominated frequency, i.e., 100 Hz. From these comparative results, it can be concluded that the 1 wt% ZnO dispersed PNC film has optimum tunable nanodielectric properties. Therefore, the (PVA–PEO)–1 wt% ZnO film can serve as frequency tunable polymeric nanodielectric material of low value dielectric permittivity over the complete audio frequency range and also at lower radio frequencies 3.2.2 Electric modulus spectra The complex electric modulus spectra M*(ω) of a composite dielectric material can be derived from the complex permittivity *(ω) data by using the relation M*(ω) = 1/*(ω). These spectra are the most appropriate in order to explore the bulk response after nullifying the contribution of electrode polarization (EP), electrode-dielectric contact and adsorbed impurity effects [67]. The spectra of real M and imaginary M parts of electric modulus for (PVA–PEO)–x wt% ZnO nanocomposites, at 30 C, are derived from their 11

dielectric permittivity spectra and these are depicted in Fig 5. It is found that the M values non-linearly increase with the increase of frequency and finally reach to a steady state near 1 MHz. The non-zero values of M at low frequencies confirm that the lower frequency  spectra (Fig. 3) of these nanocomposites are free from electrode polarization (EP) effect and the observed  values represent the materials bulk properties. From Fig. 5, it can be noted that the M spectra of the investigated PNC films exhibit two relaxation peaks in the lower and intermediate frequency regions, whereas only the intermediate frequency region relaxation peak was found for the pristine PVA– PEO blend film. The low frequency peak is attributed to the MWS relaxation process as observed in the other polymeric nanocomposites [68,69], whereas the intermediate frequency region peak represents the PVA and PEO cooperative chain segmental motion (α–relaxation) of the studied polymeric nanocomposites. Further, the intensities of MWS peaks are found relatively much higher than that of the α-relaxation peaks intensities, and this result is in good agreement with the earlier work on the polymeric nanocomposites [68,69]. The values of relaxation times M and MWS corresponding to the α-relaxation and the MWS-relaxation processes of these PNC materials were determined from their respective peak frequency values and these relaxation time values are recorded in Table 2. From this table, it can be seen that the trend of variation in M and MWS values with the increase of ZnO concentrations in the PVA–PEO film is found identical to that of the  and tanδ values i.e.,  > tanδ > M which infers that these relaxation processes are mutually correlated. From Fig. 5, it is clear that the PVA–PEO blend without nanofiller does not exhibit the MWS relaxation process and there is only the α-relaxation process in 12

the same experimental frequency range. These comparative results on the relaxation processes reveal that the IP effect appears in the dominant mode only when ZnO nanoparticles are added in the PVA–PEO blend matrix, and it is due to the formation of more heterogeneous interfaces of different conductivity constituents in the investigated polymer nanodielectric films. 3.2.3 Electrical conductivity and impedance spectra The spectra of real σ and imaginary σ parts of the complex ac electrical conductivity σ*(ω), and the resistive Z and capacitive reactance Z parts of the complex impedance Z*(ω) for the (PVA–PEO)–x wt% ZnO nanocomposites, at 30 C, are shown in Fig. 6. On a log-log scale, the σ and Z values of these materials show non-linear function of frequency, whereas σ and Z values vary linearly with the increase of frequency from 20 Hz to 1 MHz. Further, the real parts of both these complex quantities are found lower than that of corresponding imaginary parts values for the (PVA–PEO)–x wt% ZnO materials. The σ and σ values of these materials are very low confirming their suitability as prominent polymeric nanodielectric over the audio and radio frequency ranges. The observed values of Z > Z confirms the dominant capacitive behaviour of these polymer nanodielectric materials. Further, in the lower audio frequency range, the impedance values of the PNC films have been found of the order of some mega ohms which also suggest the suitability of these materials as an electrical insulator for the conventional microelectronic devices [74]. Fig. 7 depicts the enlarged view of σ spectra for the (PVA–PEO)–x wt% ZnO films which exhibits two different non-linear dispersion regions, e.g., the low frequency range part of the spectra labeled as Region–I and the high frequency range part of the 13

spectra labeled as Region–II. These σ regions are separately governed by the power law relation σ(ω) = σdc + Aωn, and their fit values are shown by the solid lines in the figure. Such power law fit conductivity behaviour in the separate frequency regions for these nanodielectric materials is exhibited due to their semicrystalline structures as reported earlier for the PVA–PEO blend [47,51] and also for the PEO based nanocomposites [31,66]. The dc electrical conductivity values σdc(I) and σdc(II) obtained by the power law fit over the low frequency and high frequency regions, respectively, are recorded in Table 2 along with their exponent n(I) and n(II) values. The n values are found lower than unity confirming the hopping mechanism of charge transportation in these nanodielectric materials. Further, Table 2 shows that the σdc(I) and σdc(II) values of these PNC films vary anomalously with the increase of ZnO concentration in PVA–PEO blend matrix. For these PNC materials, the σdc(II) values are found around 3 to 4 orders of magnitude higher than that of the σdc(I) values at 30 °C. 3.2.4 Correlation between relaxation times and dc conductivity Fig. 8 shows the ZnO concentration dependent relaxation times  (e.g. , tanδ, M and MWS) and the dc conductivity (σdc(I) and σdc(II)) values of (PVA–PEO)–x wt% ZnO nanocomposites, at 30 C. It is found that the  values corresponding to polymer chain segmental motion, i.e., , tanδ and M which were determined from different formalism, vary monotonously with the increase of ZnO concentration in the PVA–PEO blend matrix. The variation of MWS values with ZnO concentration also obeys the same trend of variation as that of the  values of polymer segmental motion. Further, the magnitude of  values are found in the order  > tanδ > M for these PNC films which is a common characteristic of the PNC material as observed in several PEO based polymeric 14

nanocomposites [8,9,31,39,40,66,73]. Fig. 8 shows that the σdc(I) and σdc(II) values vary inversely to each other with the increase of ZnO concentration for these PNC films. Further, a close look at the comparative changes in the various relaxation times and the conductivity values with the variation of ZnO concentration in the PVA–PEO blend matrix reveals that there is a correlation between the cooperative polymer chain segmental motion relaxation times  and the σdc(II) values, i.e., the increase of  decreases the σdc(II) values and vice versa. This relation between  and σ values is very interesting in regards to the use of these nanocomposites for the preparation of solid polymer electrolytes (SPEs) because in such SPE materials the transportation of ions occurs in coupled mode of polymer chain segmental motion and in this process the increase of chain segmental motion enhances the ions mobility and hence the ionic conductivity of the SPE material [17,18].

3.3 Temperature dependent dielectric behaviour The ,  and tanδ spectra of (PVA–PEO)–3 wt% ZnO film at temperatures 30, 40, 50 and 60 °C are depicted in Fig. 9. From the figure, it can be seen that as the temperature of the PNC film increases, its  values also increase confirming the thermally activated dielectric polarization behaviour of such polymeric nanodielectric materials. The increase of temperature increases the free volume in the composite polymer matrix providing more favourable space with less hindrance to the dipolar reorientation which results in the enhancement of effective dielectric polarization (i.e., dielectric permittivity value) of the polymeric dielectric materials [6,31,40,66,69,80–82]. Fig. 9 shows that both the  and tanδ values monotonically increase with the increase of

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temperature of the PNC film, and the increase is relatively very high in the MWS effect dominated frequency range (i.e., below 1 kHz) whereas it is almost insignificant near 1 MHz. Insets of the figure (enlarged view of the intermediate frequency range spectra) show that the position of  and tanδ relaxation peaks have a gradual shift towards higher frequency side as the temperature of PNC film increases which confirm the increase of polymer chain segmental dynamics mainly due to enhancement in free volume of such flexible-type nanodielectric material. The temperature dependent values of relaxation times  and tanδ of the (PVA–PEO)–3 wt% ZnO film are also determined using the frequency values corresponding to their relaxation peaks, and the obtained  values are reported in Table 2. It is found that these relaxation times values decrease with the increase of temperature of the PNC film, and their Arrhenius behaviour is discussed in the later section. The temperature dependent  values of the (PVA–PEO)–3 wt% ZnO film, at selective frequencies viz., 100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz, are plotted in Fig. 10. It is observed that the  value increases linearly with the increase of temperature at all the selective frequencies, except at 100 Hz where it exhibits non-linear behaviour due to dominant contribution of the IP effect. Further, it is found that as the frequency value increases the rate of increment in  value (i.e., slope) with the increase of temperature decreases, and it is relatively very low at the radio frequencies. These results confirm that this PNC material is thermally stable as far as its dielectric permittivity is concerned at radio frequencies and over the temperature range 30–60 °C. Further, over the MWS effect dominated frequency range (i.e., f < 1 kHz), this PNC film behaves as non-linear nanodielectric with the temperature variation. These results reveal that the 16

interfacial polarization of such PNC material is highly sensitive to the temperature in comparison to their temperature dependent dipolar polarization. Fig. 11 shows the temperature dependent M and M spectra of the (PVA–PEO)–3 wt% ZnO film. This figure reveals that at a fixed frequency, the M values of this polymer nanodielectric film have a decrease with the increase of temperature which is expected as per the relation M*(ω) = 1/*(ω). The M spectra reveal that the peak intensity corresponding to the -relaxation process increases with the increase of temperature of the PNC film, whereas the MWS-relaxation process peak intensity has an initial increase from 30 to 50 °C and above it this process saturates. Both these relaxation processes peaks shift gradually towards high frequency side with the increase of temperature of the PNC film which is a common characteristic of the polymeric nanodielectrics [31,40,66,69]. The values of relaxation times M and MWS of the PNC film at various temperatures are determined using the frequency values corresponding to their peaks in the M spectra and the observed relaxation times values are recorded in Table 2. These values were also used for the confirmation of the Arrhenius behaviour of the PNC film in the next section. Table 2 reveals that these  values decrease with the increase of temperature confirming the enhancement in the flexibility of their polymeric matrix. Fig. 12 presents the σ spectra of (PVA–PEO)–3 wt% ZnO film at various temperatures. The values of σdc(I) and σdc(II) of this PNC film are determined by power law fit in the low and high frequency regions, separately, as shown by the solid lines and the observed σdc values from these fits are recorded in Table 2. From the Table, it is noted that the dc electrical conductivity values of both the regions increase with the increase of

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temperature of the PNC film which may be owing to the hopping of charge polarons in the complex material. Fig. 13 depicts the Arrhenius behaviour of the relaxation times (, tanδ, M and MWS) and also of the dc conductivity (σdc(I) and σdc(II)) values of the (PVA–PEO)–3 wt% ZnO film. The values of these relaxation times activation energy E and also the conductivity activation energy Eσ were determined from the slopes of these Arrhenius plots using the relations  = 0 exp(E/kBT) and σdc = σ0 exp(–Eσ/kBT), respectively, and the observed activation energy values are marked in the figure. It is found that the E values determined from the polymer chain segmental motion relaxation times which were obtained from the spectra of various complex quantities are found same, i.e., 0.24 eV. Further, the E value of MWS-relaxation time is 0.51 eV which is found two times higher in comparison to the E value of polymer chain segmental relaxation time activation energy for the PNC film. The Eσ values are found 0.24 eV and 0.15 eV corresponding to the σdc values obtained from low frequency region (σdc(I)) and the high frequency region (σdc(II)), respectively. The observed E and Eσ values are significantly low which confirms the suitability of the studied polymer nanocomposite film matrix for the preparation of solid polymer nanocomposite electrolytes.

4. Conclusions The detailed dielectric properties and electrical conduction behaviour of the (PVA–PEO)–x wt% ZnO films over the frequency range 20 Hz to 1 MHz were reported in this work. The complex dielectric permittivity, electric modulus, ac electrical conductivity and the impedance behaviour of these PNC films with the variation of ZnO 18

contents were analyzed in detail and the effect of added ZnO nanoparticles on these properties of the materials was confirmed. Results revealed that the dispersion of 1 wt% ZnO nanoparticles in the PVA–PEO blend matrix significantly disturbs the polymers dipolar ordering and reduces the value of dielectric permittivity, whereas loading of 5 wt% ZnO content has relatively less influence on the dielectric polarization but greatly enhances the polymer cooperative chain segmental dynamics (α-relaxation). These PNC materials exhibit MWS- and -relaxation processes in the low and intermediate frequency regions of their 20 Hz to 1 MHz range spectra. The α-relaxation time and the dc electrical conductivity values of the PNC film obey the Arrhenius behaviour and the values of their activation energies (E and Eσ) are found significantly low i.e., 0.24 eV. It has also been revealed that the dc conductivity values of these PNC materials are governed by the cooperative polymer chain segmental relaxation process. The results of this work confirm that the 1 wt% ZnO containing PNC film can be used as low value dielectric permittivity substrate material for the development of radio frequency operated microelectronic devices especially as a gate insulator for the field effect transistor. The temperature dependent dielectric permittivity behaviour of the PNC film is non-linear at lower audio frequencies electric field, whereas it is linear for the radio frequencies and has a little increase with the increase of temperature confirming it as a stable permittivity polymeric nanodielectric material over the temperature range from 30 to 60 °C. The PVA and PEO crystalline phases exist in the PVA–PEO blend, but their amount significantly reduces on the addition of 1 wt% ZnO content in the blend matrix. The relatively fast polymer chain segmental dynamics and the enhanced amorphous phase recognize these PNC materials as a potential candidate for the

19

preparation of nanocomposite solid polymer electrolytes (NSPEs), and therefore, the work may be extended on the NSPEs using these PNC matrices with the doping of lithium salt for their use in the development of rechargeable lithium ion batteries.

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24

Table 1. Values of Bragg’s angle 2, basal spacing d, full width at half maximum FWHM, crystallite size L, crystalline peak intensity I and relative intensity RI of PEO peaks with respect to PVA–PEO blend peaks (2θ = 19.54 and 23.66) of the (PVA– PEO)–x wt% ZnO polymer nanocomposite films x d 2 FWHM103 o (wt%) (nm) () (rad) (120) reflection peak parameters of PEO 0 19.54 0.454 5.95 1 19.62 0.452 10.16 3 19.63 0.452 9.35 5 19.66 0.451 6.37 (112),(032) reflection peak parameters of PEO 0 23.66 0.376 14.12 1 23.81 0.373 14.80 3 23.81 0.373 13.98 5 23.86 0.373 14.24

L (nm)

I (counts)

RI (%)

24.69 14.47 15.71 23.07

3601 1737 2309 2473

100.0 48.2 64.1 68.7

10.48 10.00 10.59 10.39

3358 2672 2509 2336

100.0 79.6 74.7 69.6

Table 2. Values of MWS relaxation time MWS, dielectric relaxation time , loss tangent relaxation time tanδ, electric modulus relaxation time M, dc ionic conductivity σdc(I) and σdc(II), and fractional exponent n(I) and n(II) of the (PVA–PEO)–x wt% ZnO polymer nanocomposite films

PNC films x (wt%) 0 1 3 5 T (C) 30 40 50 60

MWS (ms)

ε (μs)

tanδ (μs)

– 3.1 4.6 3.3

18.4 16.9 17.6 7.0

11.0 10.8 12.8 4.0

4.6 2.9 1.6 0.8

17.6 13.3 9.8 7.2

12.8 9.9 6.9 5.6

σdc(I) σdc(II) M n(I) –12 (10–9 S/cm) (μs) (10 S/cm) (PVA–PEO)–x wt% ZnO films 8.3 13.3 0.80 6.9 8.0 21.3 0.55 5.5 9.4 36.3 0.69 5.2 2.9 15.9 0.57 13.8 (PVA–PEO)–3 wt% ZnO film 9.4 36.3 0.69 5.2 6.7 37.5 0.54 6.4 5.2 50.1 0.5 7.2 4.1 82.1 0.5 8.9

25

n(II) 0.93 0.75 0.89 0.85 0.89 0.83 0.79 0.76

Intensity (a.u.)

ZnO

PEO PVA

PVA-PEO

x=1 x=3 x=5

10

20

30

40

50

60

70

2 (degree)

Figure 1. XRD patterns of ZnO nanopowder, pristine PEO film, PVA film and (PVA– PEO)–x wt% ZnO polymer nanocomposite films (x = 0, 1, 3 and 5) at room temperature

26

L (nm)

30

120 peak 112,032 peak

25 20 15 10 5

RI (%)

100 80 60 40 0

1

2

3

4

5

x wt% ZnO

Figure 2. ZnO concentration dependent PEO crystallite size L and relative intensity RI of the PEO peaks in the (PVA–PEO)–x wt% ZnO polymer nanocomposite films (x = 0, 1, 3 and 5)

27

' "

8 7 6 5 4 3 2

x 0 1 3 5



-relaxation

IP-relaxation

1.0 0.5 0.0

tan

0.25 0.20 0.15 0.10 0.05 10

1

10

2

10

3

10

4

10

5

10

6

f (Hz)

Figure 3. Frequency dependent real part  and loss part  of complex dielectric permittivity, and loss tangent tanδ of (PVA–PEO)–x wt% ZnO polymer nanocomposite films (x = 0, 1, 3 and 5) at 30 C

28

100 Hz 1 kHz 10 kHz 100 kHz 1 MHz

7

'

6 5 4 3 2 0

1

2

3

4

5

x wt% ZnO Figure 4. ZnO concentration dependent real part  values of complex dielectric permittivity of (PVA–PEO)–x wt% ZnO polymer nanocomposite films (x = 0, 1, 3 and 5) at 30 C

29

0.5

M'

0.4 0.3 x 0 1 3 5

0.2 0.1 0.08

MWS-relaxation

M"

0.07 -relaxation

0.06 0.05 0.04 0.03 0.02 0.01 10

1

10

2

10

3

10

4

10

5

10

6

f (Hz)

Figure 5. Frequency dependent real part M and loss part M of complex electric modulus of (PVA–PEO)–x wt% ZnO polymer nanocomposite films (x = 0, 1, 3 and 5) at 30 C

30

', " (S/cm)

10

-6

10

-7

10

-8

10

-9

-10

10

-11

Z', Z" ()

10

10

7

10

6

10

5

10

4

10

3

10

2

x 0 1 3 5

" '

Z"

10

Z'

1

10

2

10

3

10

4

10

5

10

6

f (Hz)

Figure 6. Frequency dependent real part σ and loss part σ of complex ac electrical conductivity, and also the real part Z and reactive part Z of complex impedance of (PVA–PEO)–x wt% ZnO polymer nanocomposite films (x = 0, 1, 3 and 5) at 30 C

31

10

10

-7

x 0 1 3 5

-8

dc(II)

' (S/cm)

Region-II

10

-9

-10

10

dc(I)

-11

10

Region-I

10

1

2

10

10

3

4

10

5

10

6

10

f (Hz) Figure 7. Frequency dependent real part σ of complex ac electrical conductivity of (PVA–PEO)–x wt% ZnO polymer nanocomposite films (x = 0, 1, 3 and 5) at 30 C. Solid lines show the power law σ(ω) = σdc + Aωn fit of experimental data over the audio frequency range (Region-I) and also the radio frequency range (Region-II)

32

-4

10

10

-2

10

-3

 (s)

MWS -5

10

 tan M

10

-11

dc(II)

10

-8

10

-9

dc (S/cm)

10

-10

dc(I)

0

1

2

3

4

5

6

x wt% ZnO

Figure 8. ZnO concentration dependent relaxation times and dc conductivity plots of (PVA–PEO)–x wt% ZnO polymer nanocomposite films (x = 0, 1, 3 and 5) at 30 C

33

30 C 40 C 50 C 60 C

'

8 6 4

"

12 9 6 3

tan

0 1.2 1.0 0.8 0.6 0.4 0.2 0.0 10

1

10

2

10

3

10

4

10

5

10

6

f (Hz) Figure 9. Frequency dependent real part  and loss part  of complex dielectric permittivity, and dielectric loss tangent tanδ of (PVA–PEO)–3 wt% ZnO polymer nanocomposite film at different temperatures. Insets show the enlarged view of the spectra over the frequency range 1 kHz–100 kHz

34

100 Hz 1 kHz 10 kHz 100 kHz 1 MHz

7

'

6 5 4 3

30

35

40

45

50

55

60

T (C) Figure 10. Temperature dependent real part  values of complex dielectric permittivity of (PVA–PEO)–3 wt% ZnO polymer nanocomposite film at various frequencies

35

0.4

M'

0.3 0.2 30 C 40 C 50 C 60 C

0.1

M"

0.08 0.06 0.04 0.02 10

1

10

2

10

3

10

4

10

5

10

6

f (Hz)

Figure 11. Frequency dependent real part M and loss part M of complex electric modulus of (PVA–PEO)–3 wt% ZnO polymer nanocomposite film at different temperatures

36

-7

10

30 C 40 C 50 C 60 C

dc(II)

-8

' (S/cm)

10

Region-II -9

10

dc(I) -10

10

Region-I 1

10

2

10

3

10

4

10

5

10

6

10

f (Hz) Figure 12. Frequency dependent real part σ of complex ac electrical conductivity of (PVA–PEO)–3 wt% ZnO polymer nanocomposite film at different temperatures. Solid lines show the power law fit of experimental data over the audio frequency range (Region-I) and also the radio frequency range (Region-II)

37

 (s)

10

-2

10

-3

10

-4

MWS

 tan

E = 0.24 eV

M

10

dc (S/cm)

E = 0.51 eV

E = 0.24 eV

-5

10

-7

10

-8

10

-9

E = 0.24 eV

dc(II) E = 0.15 eV

10

-10

10

-11

dc(I) E = 0.24 eV

3.0

3.1

3.2 3.3 -1 1000/T (K )

3.4

3.5

Figure 13. Arrhenius behaviour of various relaxation times  and dc electrical conductivity σdc of (PVA–PEO)–3 wt% ZnO polymer nanocomposite film

38