poly(ethylene oxide) blend nanocomposites

Composites Science and Technology 70 (2010) 1621–1627

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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech

Dielectric properties of montmorillonite clay filled poly(vinyl alcohol)/poly(ethylene oxide) blend nanocomposites R.J. Sengwa *, Shobhna Choudhary, Sonu Sankhla Dielectric Research Laboratory, Department of Physics, JNV University, Jodhpur 342 005, India

a r t i c l e

i n f o

Article history: Received 22 December 2009 Received in revised form 28 May 2010 Accepted 4 June 2010 Available online 9 June 2010 Keywords: A. Nanocomposites A. Nanoclays A. Polymers B. Electrical properties Dielectric spectroscopy

a b s t r a c t Organic–inorganic nanocomposites of poly(vinyl alcohol) (PVA)–poly(ethylene oxide) (PEO) blend filled with montmorillonite (MMT) nanoclay up to 10 wt.% concentration were synthesized by aqueous solution-cast technique. The complex dielectric function, electrical conductivity, electric modulus and impedance spectra of the nanocomposites were measured in the frequency range 20 Hz–1 MHz at ambient temperature. A direct correlation was observed between the real part of dielectric function and the mean relaxation time of the polymer chain segmental dynamics, with the exfoliated and intercalated MMT clay structures, and the extent of miscibility between PVA and PEO due to hydrogen bonded bridging through exfoliated MMT clay nanosheets. The large increase of dielectric relaxation time revealed that the dispersed exfoliated nanoscale MMT clay in the polymers blend matrix produces a large hindrance to the polymer chain dynamics. Results confirm that the real part of dielectric function of the nanocomposites can be tailored by varying amount of MMT clay filler for their use as nanodielectric materials in the microelectronic technology. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction In last two decades studies and investigations on polymer–clay nanocomposites (PCNs) have drawn substantial attention owing to improvements in their mechanical, thermal, gas permeability, electrical, optical and pharmaceutical properties at nanoscaled level as compared with pristine polymer [1–10]. The PCNs are known as organic–inorganic nanocomposites and their useful properties can be controlled by preparation route. The hydrogen bonding (H-bond) interactions between polymer (organic material) and the nanoscale intercalated/exfoliated clay (inorganic material) in the polymer matrix results three different phases in PCN materials; (a) microphase separated composites, where polymer matrix and clay remain immiscible, (b) intercalated clay structures, where polymer molecules enter between the clay galleries, and (c) exfoliated structures, where individual clay nanoplatelets are dispersed in the polymer matrix. Poly(vinyl alcohol) (PVA) and poly(ethylene oxide) (PEO) are water soluble and biodegradable polymers and can form a variety of H-bonded complexes with additives/dopants, which makes them industrially important polymers and their blends can be of significant practical utility. PVA is a polymer with a carbon chain backbone with hydroxyl groups attached to methane carbons while PEO is a simple chain polymer with etheric linkage. Due to a large range

* Corresponding author. Tel.: +91 291 2720857; fax: +91 291 2649465. E-mail address: [email protected] (R.J. Sengwa). 0266-3538/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2010.06.003

H-bond formation ability of hydroxyl group of PVA monomer units and ether oxygen of PEO monomer units, these polymer matrices are extensively used with the montmorillonite (MMT) clay for the synthesis of organic–inorganic nanocomposite materials. MMT clay is composed of joined silica and alumina sheets of about 1 nm thickness and 100–1000 nm range aspect ratio (length/width). Its chemical composition is described by general formula: M þ y ðAl2y Mgy Þ ðSi4 ÞO10 ðOHÞ2  nH2 O. It contains negatively charged layered silicates ionically bonded with metal cations such as Na+, Ca2+, K+, or Mg2+. However, the cation amount ratio can vary dependently on the mineral source and its cation exchange treatment to render hydrophilic MMT more organophilic and to increase its interlayer spacing so that organic compounds (polar solvents and polymers) may easily enter the galleries of the interlayers, which results the intercalation and exfoliation of the polymer-layered silicate structures. Synthesis of PVA–MMT clay nanocomposites and their thermal, mechanical, optical and electrical characterization for technological applications has been a subject of several investigations [11–15]. These studies confirmed that PVA–MMT clay nanocomposites have large enhancement of Young’s modulus and stress at break is relatively insensitive to the clay concentration. There is moderate decrease of toughness with MMT clay loading up to 10 wt.% of the PVA weight, and water permeability decreases to about 40% of the pure PVA films for clay loading of only 4–6 wt.%. In addition to having high melting point, thermal degradation properties of PVA–MMT clay nanocomposites also show significant improvement. The

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enhancement in useful properties of PVA–MMT clay nanocomposites are owing to the formation of direct hydrogen bonds between hydroxyl groups (–OH) of PVA monomer units and siloxane (Si–O) groups of MMT platelets, and also the bridging through H2O in their hydrocolloids [11–13]. Among the linear chain polymer, PEO–clay nanocomposites have also been extensively investigated using various spectroscopic techniques [16–21]. In fact the Na+–MMT clay miscibility in PEO matrix is due to the large number of Na+ ions in the MMT clay galleries that are coordinated by the ether oxygen atoms of the PEO monomer units. Further, it is understood that PEO interacts not only with the interlayer cations but with the free –OH groups of clay layers via hydrogen bonding [16–19]. PEO has high degree intercalation in clay galleries and also shows adsorption on exfoliated clay sheets, which improves their thermal, mechanical and permeability properties. The salt added PEOMMT clay nanocomposites have established their use as solid-state electrolyte ionic conductors for rechargeable batteries [6,19]. Polymer blends are widely used in design of suitable materials of new physical and structural properties [22–29]. Aqueous blending of PVA and PEO forms H-bonded heteropolymer network in presence of water molecules but their aqueous solution-cast films have separate phases of the polymers. Earlier, Mishra and Rao [25] prepared the PVA–PEO blend films by enhancing their miscibility in mixed solvent of water and n-propanol for their dielectric characterization. Recently, we have investigated the dielectric properties of PVA–PEO blend–MMT clay hydrocolloids for their use in coating technology [29]. It is interesting to increase the extend of miscibility between the PVA and PEO by bridging through nanoscale exfoliated and intercalated MMT clay in aqueous solution-cast films. Survey of literature exhibits that dielectric properties and polymer chain dynamics of PVA–PEO blend–MMT clay nanocomposite films have not been studied yet to explore their technological applications as low dielectric constant materials of improved thermal and mechanical properties. Such PCN dielectric materials are also known as ‘nanodielectrics’ that means dielectrics with nanotechnology, which are replacing conventional insulating materials providing tailored performance, by simply controlling the concentration of nanoinclusion. Further, characterization of polymer chain dynamics in PCN materials is an interesting issue in condensed matter physics from both fundamental and technological point of view. From fundamental point of view it is challenging to investigate how the polymer chain mobility is modified by the presence of and interaction with the dispersed nanoscale MMT clay structures in the polymer blend matrix. At the same time, synthesis of PCN material aims at modification of chain dynamics by the nanocomposite filler that improves several properties, such as thermal stability, mechanical properties and barrier properties. In this manuscript, an attempt is made to investigate the complex dielectric function, alternating current (ac) electrical conductivity, electric modulus, and impedance properties of aqueous solutioncast PVA–PEO blend–MMT clay nanocomposite films of varying MMT clay concentration over the frequency range 20 Hz–1 MHz. The aim of this study is to establish a correlation between dielectric properties and the structures of polymers blend–MMT clay nanodielectrics using dielectric spectroscopy, which proved its potential in detection of intercalated and exfoliated nanoscale MMT structures in a variety of PCN materials [2,3,7–10,19,22,28–36].

montmorillonite (MMT) clay (Nanoclay PGV, a product of NanocorÒ) was purchased from Sigma–Aldrich, USA. The MMT clay is white in colour, and have 145 meq/100 g cation exchange capacity (CEC), 150–200 aspect ratio, 2.6 g/cc specific gravity, and 9–10 pH value on 5% dispersion. The negative charge imbalance on the flat surfaces of the MMT platelets was neutralized by adsorption of most notably sodium cations, so it is also represented by Na+– MMT nanoclay.

2. Experimental details

e0 ¼

tg C p e0 A

ð1Þ

2.1. Materials

e00 ¼ e0 tan d

ð2Þ

1

The PVA of average molecular weight 77,000 g mol of the laboratory grade was obtained from Loba Chemie, India and the PEO of viscosity average molecular weight 6  105 g mol1 was obtained from Sigma–Aldrich, USA. Polymer grade hydrophilic

2.2. Preparation of PVA–PEO blend–MMT clay nanocomposite films by solution-cast technique For the 0, 0.5, 1, 2, 3, 4, 5 and 10 wt.% MMT clay concentration (weight fraction concentration for a total 1.5 g of PVA + PEO + MMT clay), 0.00, 0.0075, 0.015, 0.03, 0.045, 0.06, 0.075, and 0.15 g amounts of MMT clay powder were added each in 5 ml double distilled deionized water in separate glass bottles with air tight caps. The vigorous stirring of these samples with Teflon coated magnetic stir bar on a magnetic stir plate for 24 h results the MMT clay hydrocolloids. The balance weight 1.125, 1.1194, 1.1138, 1.1025, 1.0913, 1.08, 1.0688 and 1.0125 g of PVA and 0.375, 0.3731, 0.3713, 0.3675, 0.3638, 0.36, 0.3563 and 0.3375 g of PEO for the 3:1 weight ratio of PVA to PEO for the preparation of PVA–PEO blend corresponding to the 0–10 wt.% MMT clay concentration were dissolved in water following two step procedure. Firstly each amount of PVA was dissolved in 22 ml double distilled deionized water at 90 °C in separate glass bottles and then the corresponding amounts of PEO were added in the respective PVA concentration bottles. After that, the respective concentration of MMT clay hydrocolloids were mixed in the aqueous PVA–PEO blend. To get intercalated and exfoliated MMT clay colloidally stable in suspension in the PVA–PEO blend matrix, these mixtures were magnetically stirred vigorously for another 24 h. The PVA–PEO blend–MMT clay nanocomposite films were cast by pouring the prepared hydrocolloids of varying clay concentration in 60 mm diameter stainless steel rings directly onto optically smooth glass plates and were allowed to dry at room temperature for a week. Room dried films of thickness 0.25 mm were further dried under vacuum at room temperature for 24 h before their measurements. 2.3. Dielectric measurements Agilent 4284A precision LCR meter and Agilent 16451B solid dielectric test fixture having a four terminals nickel-plated cobal (an alloy of 17% cobalt + 29% nickel + 54% iron) electrodes of diameter 38 mm, were used for the dielectric measurements in the frequency range 20 Hz–1 MHz. Frequency dependent values of parallel capacitance CP, parallel resistance Rp and loss tangent tan d (dissipation factor D) with sample, were measured for the determination of dielectric/electrical function of the PVA–PEO blend–MMT clay nanocomposite films at ambient temperature (30 °C). Prior to the sample measurements, the open circuit calibration of the cell was performed to eliminate the effect of stray capacitance.The real part (permittivity) e0 and imaginary part (dielectric loss) e00 of the complex relative dielectric function e ðxÞ ¼ e0  je00 of the nanocomposite films were determined from the Eqs. (1) and (2), respectively [37]:

where tg is the thickness of the PVA–PEO blendMMT clay films, A is the area of the cell electrode and e0 (8.854  10–12 F/m) is free space dielectric constant. Fig. 1 shows the frequency dependent spectra of e0 and e00 of the solution intercalated PVA–PEO blendMMT clay

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0 wt.% MMT 0.5 wt.% MMT 1 wt.% MMT 2 wt.% MMT 3 wt.% MMT 4 wt.% MMT 5 wt.% MMT 10 wt.% MMT

5

ε'

4

3

2

e0

00

e xÞ

¼ M0 þ jM ¼

e þe 02

002

þj

e00

ð3Þ

e þ e002 02

The M0 and M00 spectra of the PVA–PEO blend–MMT clay films of varying clay concentration are shown in Fig. 3. The values of mean relaxation time corresponding to polymer chain dynamics were evaluated from the relaxation peak frequencies of dielectric and electric modulus loss spectra and these values are plotted against MMT clay concentration in Fig. 4. The real part r0 of ac complex conductivity, r ðxÞ ¼ r0 þ jr00 of the PVA–PEO blend–MMT clay nanocomposites were obtained from the following equation:

ð4Þ

where x = 2pf is the angular frequency of the applied alternating electric field. Fig. 5 shows the frequency dependent r0 values of the PVA– PEO blendMMT clay films of varying MMT clay concentration. In Fig. 6, the r0 values of PVA–PEO blendMMT clay films at 1 MHz were plotted against MMT clay concentration for the comparison. The dc electrical conductivity rdc of these films were obtained from the intercept of straight line fit of the low frequencies r0 values. The rdc values are also plotted against MMT clay concentration in Fig. 6. The frequency dependent real part Z0 and imaginary part Z00 of complex impedance Z*(x) of the PCNs material were evaluated by the following equation:

0.4 0.3 0.2 0.1

Z  ð xÞ ¼ 1

2

10

3

10

4

10

10

5

10

6

10

Frequency (Hz) Fig. 1. Frequency dependent real part e0 and imaginary part e00 of the complex dielectric function of PVA–PEO blend–MMT clay nanocomposite films at varying MMT clay concentration (wt.%). The solid lines are smooth joining of the experimental data points, as guides for the eyes.

nanocomposites with varying MMT clay concentration. In Fig. 2, the e0 values of PVA–PEO blendMMT clay nanocomposites at fixed frequencies of 1 kHz and 1 MHz were plotted against MMT clay concentration (wt.% MMT). The real part M0 and imaginary part M00 of the complex electric modulus M*(x) of the PVA–PEO blend–MMT clay nanocomposites were determined by the following equation

xC p R2p 1 Rp  j ¼ Z 0  j Z 00 ¼ Y ð xÞ 1 þ ðxC p Rp Þ2 1 þ ð xC p Rp Þ 2

ð5Þ



Fig. 7 shows the Z00 vs. Z0 complex plane plots of the PVA–PEO blendMMT clay nanocomposites of varying MMT clay concentration.

0.6

M'

0.0

1 ð

r0 ¼ xe0 e00

0.5

ε''

M ðxÞ ¼

0.4

0.2 4.5

0 wt.% MMT 0.5 wt.% MMT 1 wt.% MMT 2 wt.% MMT 3 wt.% MMT 4 wt.% MMT 5 wt.% MMT 10 wt.% MMT

1 kHz 1 MHz

4.0

0.03

M''

ε'

3.5 3.0 2.5

0.02

2.0 1.5 0

2

4

6

8

10

wt.% MMT Fig. 2. MMT clay concentration (wt.%) dependent real part e0 of the complex dielectric function of PVA–PEO blend–MMT clay nanocomposite films at fixed frequencies, 1 kHz and 1 MHz. The solid lines are smooth joining of the experimental data points, as guides for the eyes.

1

10

2

10

3

10

4

10

5

10

6

10

Frequency (Hz) Fig. 3. Frequency dependent real part M0 and imaginary part M00 of the complex electric modulus of PVA–PEO blend–MMT clay nanocomposite films at varying MMT clay concentration (wt.%). The solid lines are smooth joining of the experimental data points, as guides for the eyes.

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τε

-3

10

8

τM

7

Z" (MΩ )

τε (s), τM (s)

6

-4

10

5

0 wt.% MMT 0.5 wt.% MMT 1 wt.% MMT 2 wt.% MMT 3 wt.% MMT 4 wt.% MMT 5 wt.% MMT 10 wt.% MMT

4 0.3

3

0.2

2

0.1

1 -5

10

0

2

4

6

8

0.0

0

10

0.00

0.0

wt.% MMT

0.2

0.4

0.01

0.6

0.02

0.8

Z' (MΩ )

Fig. 4. MMT clay concentration (wt.%) dependent polymer chain segmental relaxation time se and ac ionic conduction relaxation time sM of the PVA–PEO blend–MMT clay nanocomposite films. The solid lines are smooth joining of the experimental data points, as guides for the eyes.

Fig. 7. Complex impedance plane plots (Z00 vs. Z0 ) of the PVA–PEO blend–MMT clay nanocomposite films at varying MMT clay concentration (wt.%). Inset shows the enlarged view of the dispersion in upper experimental frequency range. The solid lines are smooth joining of the experimental data points, as guides for the eyes.

-5

10

3. Results and discussion

-6

σ ' (S/m)

10

-7

10

0 wt.% MMT 0.5 wt.% MMT 1 wt.% MMT 2 wt.% MMT 3 wt.% MMT 4 wt.% MMT 5 wt.% MMT 10 wt.% MMT

-8

10

-9

10

-10

10

1

2

10

10

3

10

4

5

10

6

10

10

Frequency (Hz) Fig. 5. Frequency dependent real part r0 of the complex ac conductivity of PVA–PEO blend–MMT clay nanocomposite films at varying MMT clay concentration (wt.%). The solid lines are smooth joining of the experimental data points, as guides for the eyes.

-5

10

σ dc

-10

10

σ ' (S/m)

σ dc (S/m)

σ'

-11

10

The high resolution optical microscopic study reveals the separate clusters of PVA and PEO in the aqueous solution-cast PVA–PEO blend film, which confirmed the poor miscibility of these polymers blend in their film form. Mishra and Rao [38] confirmed that the aqueous solution-cast blends from PEO and PVA system are essentially incompatible, although they found evidence of weak interpolymer chain interactions in the blend by their IR and NMR studied. The low H-bond connectivity between PVA and PEO is due to the oxygen in PEO, which is etheric in nature. But in the present study, it is observed that the loading of only 0.5 wt.% MMT clay in aqueous PVA–PEO blend results in high extent of miscibility between PVA and PEO in their solution-cast nanocomposite films, which is mainly due to the formation of H-bond bridging between the polymers through the nanoscale dispersed exfoliated MMT clay. The miscibility i.e., compatibility of PVA and PEO in presence of exfoliated MMT sheets can be explained by the schematic illustration depicted in Fig. 8. There may be various schemes to explain the miscibility in these nanocomposites, but the proposed model in Fig. 8 is more realistic. In this scheme, the –OH groups from PVA units and the siloxane (Si–O) groups from the MMT sheets form the PVA–MMT hydrogen bonds, and simultaneously the formation of ion–dipole type interactions between the etheric oxygen atoms of PEO and the MMT interlayer sodium cations, which bridges the PVA and PEO through MMT nanosheets in their nanocomposites. Further, there is the large probability of the formation of hydrogen bonds between the surface hydroxyl groups of the MMT clay and oxygen atoms of the PEO chain, because the distances between PEO oxygen atoms are roughly comparable to that of the hydroxyl groups in MMT clay sheets. In the similar way, Lim et al. [27] also confirmed the extent of miscibility between the blend of PEO and PMMA in presence of exfoliated nanosheets of MMT clay, which is governed by the polymersclay interactions.

-6

0

2

4

6

8

10

10

3.1. Complex dielectric function spectra

wt.% MMT Fig. 6. MMT clay concentration (wt.%) dependent dc conductivity rdc and real part r0 of the complex ac conductivity at 1 MHz of PVA–PEO blend–MMT clay nanocomposite films. The solid lines are smooth joining of the experimental data points, as guides for the eyes.

The e0 dispersion spectra of PVAPEO blend–MMT clay nanocomposite films decrease with the increase of frequency from 20 Hz to 1 MHz (Fig. 1), which is an evidence of the dielectric relaxation processes in these materials. Fig. 2 shows that the e0 values of

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PVA chain PVA –OH groups

MMT siloxane groups

O

O

O

O

H

H

H

H

O

O

Surface of exfoliated MMT sheet O –

Interlayer cations (Na+) Ion-dipole interactions

O

O

O

Si

Si

Si

O –

H O

PVA–MMT interactions

O

PEO chain

1 nm

Si

Clay surface hydroxyl groups

H O

Dipole-dipole interactions Etheric oxygen of PEO

Fig. 8. Schematic illustration model of PVA and PEO interactions with exfoliated MMT sheet (H-bond bridging effect), which explains the miscibility of PVA and PEO through MMT in the PVA–PEO blend–MMT clay nanocomposites.

the nanocomposite films vary anomalously with the increase of MMT clay concentration at fixed frequency. It has been established that the e0 values have strong correlation with the structures of polymer and the dispersed MMT clay in the PCNs materials [2,3,6–10,21,28–36,39]. In PCNs, a large amount of exfoliated MMT clay in the polymer matrix decreases the e0 values, whereas due to predominance of intercalated clay these values increases. The intercalation and exfoliation of clay in the polymer matrix is hybrid process and the dominate state of these structures governs the variation in e0 values. X–ray diffraction (XRD), fourier transform infra-red (FTIR) spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies confirmed that due to the hydrophilic behaviour of PVA and PEO molecules there is intercalation of polymer chains in MMT clay galleries and also the large adsorption of polymers on the exfoliated MMT clay surfaces [11,13,17,18]. In the present study a significant increase in e0 value (nearly by 1) on loading of only 0.5 wt.% MMT clay in PVAPEO blend at fixed frequencies i.e., 1 kHz and 1 MHz (Fig. 2) confirms predominance of intercalated MMT clay structures, which promote the miscibility between PVA and PEO by bridging them through H-bonding as schematically illustrated in Fig. 8. At the 1–2 wt.% MMT clay concentration, the e0 values are found to decrease as compared to that of the 0.5 wt.% MMT clay concentration, which suggests the comparative decrease in the amount of intercalated structures and simultaneously the increase of exfoliated structures at these MMT clay concentrations in the PVA–PEO blend matrix. Above 2 wt.% MMT clay concentration the e0 values of these nanocomposites again increases with the maximum at the 5 wt.% MMT clay (Fig. 2). The comparative e0 values at fixed frequency suggest that the loading of the 5 wt.% MMT clay in PVAPEO blend enhances its e0 by nearly 1.5, whereas there is a small increase in e0 value at the 2 wt.% MMT clay loading in these nanocomposites as compared to that of the PVA–PEO blend film of without MMT clay. Further, it is also found that the e0 value of PVAPEO blend is 1.63, which is significantly small as compared to the e0 values; 3.15 and 2.21 of pure PVA and PEO films, respectively, at 1 MHz. The low e0 value of PVAPEO blend at 2 wt.% MMT clay loading confirms its suitability as low dielectric constant nanocomposite material of enhanced thermal and mechanical property. The frequency dependence e00 dispersion spectra of PVAPEO blend– MMT clay nanocomposites show broader loss peak corresponding to the dielectric relaxation process of the polymer chain segmental motion (Fig. 1). Further, two loss peaks are appeared in the e00

spectra of the 10 wt.% MMT clay concentration nanocomposite film. The broadness in e00 peaks may be either due to the contribution of ohmic conductivity or the Maxwell–Wagner (MW) interfacial polarization that occurs at the interfacing boundaries of different conductivity components in the composite materials [40]. The exact values of loss peak frequency fp(e00 ) of the investigated nanocomposites were obtained from the derivative of the e0 spectra (denoted as e00deriv ) to eliminate conductivity effects [40], which is derived from the following equation:

e00deriv ¼ 

p @ e0 ðxÞ  e00 2 @ lnðxÞ

ð6Þ

The values of polymer local chain motion relaxation time se of the nanocomposites were evaluated from the dielectric loss peak frequency fp(e00 ) value using the relation se ¼ ð2pfp ðe00 ÞÞ1 . The evaluated se values of the investigated nanocomposites are plotted against wt.% MMT clay concentration in Fig. 4. 3.2. Complex electric modulus spectra Considering the charges as independent variable, polymer chain segmental motion relaxations for these nanocomposite materials were also analysed by the modulus formalism in terms of a dimensionless quantity. The interpretation of relaxation phenomena via the electric modulus formalism offers some advantages upon dielectric function treatments since large variation in the real and imaginary parts of the dielectric function at low frequencies are minimized. Further, difficulties occurring from the electrode nature, the electrode/dielectric specimen contact and the injection of space charges and adsorbed impurities can be neglected. The M0 spectra of PVAPEO blend–MMT clay nanocomposite gradually increases with the increase of frequency, whereas sharp peaks in M00 spectra were observed (Fig. 3). These peaks are also attributed to the polymer local chain motion relaxation process in the electric modulus spectra. The values of electric modulus relaxation time sM is determined from the loss peak frequency fp(M00 ) using the relation sM ¼ ð2pfp ðM 00 ÞÞ1 . The evaluated sM values are plotted against wt.% MMT clay concentration in Fig. 4. For the 10 wt.% MMT clay concentration nanocomposite film, two relaxation peaks in the M00 spectra are observed in the experimental frequency range. The higher frequency peak may be attributed to the re-orientation dynamics of polar groups of the polymer chain.

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3.3. Polymer chain dynamics Fig. 4 shows that the se and sM values of PVAPEO blend–MMT clay nanocomposites increases almost linearly up to the 2 wt.% MMT clay loading and higher than this concentration there is an anomalous variation. These values confirm that the polymer local chain dynamics in these PCNs is influenced by the comparative predominance of exfoliated and intercalated MMT clay structures. The increase in the relaxation time is an evidence of the increase in hindrance to the polymer chain dynamics, which is mainly governed by the H-bond interactions between polymer chain and the exfoliated/intercalated structures of the MMT clay [3,4,7,29– 31,35,36,39]. These heterogeneous interactions are responsible for the extent of miscibility between the PVA and PEO macromolecules, which bridges them through the exfoliated/intercalated MMT clay structures [29,36]. The almost linear increase of the se and sM values up to the 2 wt.% MMT clay loading in PVAPEO blend reveals the increase of polymers blend miscibility, whereas above 2 wt.% MMT clay concentration the anomalous change in se values may be due to the increase of clay tactoids in the nanocomposites. The aqueous suspension of MMT clay has intercalated and exfoliated structures with some agglomerates of staged silicate layers so total exfoliation/intercalation of the MMT clay cannot be achieved through aqueous solution suspension by vigorous magnetically stirred. On mixing the aqueous suspended MMT clay with aqueous PVA–PEO blend, the formation of H-bond interactions between MMT clay and polymer chain give rise to cover the dispersed MMT clay surfaces i.e., PVA/PEO molecules are partially adsorbed on the clay particles and help to link the clay aggregates (tactoids) together when the hydrocolloids are casted in film form [13,21]. On increasing the MMT concentration above the limiting value i.e., 2 wt.% for these nanocomposites, the amount of MMT clay tactoids may vary anomalously and hence there is an anomalous variation in the observed relaxation times. The effect of the change in angular oriented states of dispersed MMT clay in the PVA–PEO blend at higher MMT clay concentrations cannot be ruled out for the anomalous variation in the relaxation times, because the dielectric relaxation times of the nanocomposites are governed by both the amount of exfoliated clay and its ordered structures [36,39,41]. Further, the sM values of these nanocomposites were found less than that of the corresponding se values, which confirms that the effect of electrode/dielectric contact, the electrode nature and the absorbed impurities contributed to the dielectric formalism relaxation processes also increases hindrance to the polymer chain dynamics. The appearance of additional loss peak in both the dielectric and electric modulus loss spectra at higher frequencies in the 10 wt.% MMT clay loaded PVA–PEO blend–MMT clay nanocomposite reveals the formation of large range interactions of the polymers chain polar groups with the dispersed MMT clay exfoliated/intercalated structures. These large heterogeneous interactions reduces the polymer chain dynamics and allowed to distribute the dielectric dispersion into multiple relaxation processes, which are corresponding to the polymer chain segmental motion and the dipolar re-orientation motion, as observed in the 10 wt.% MMT clay loaded PVA–PEO blend–MMT clay nanocomposite film. 3.4. Electrical conductivity Fig. 5 shows that the real part r0 of ac conductivity of PVAPEO blend–MMT clay increases almost linearly in two steps with the change in slopes around the frequency region from 1 kHz to 10 kHz. The dc conductivity rdc of these materials were estimated from the linear fit of r0 spectra in the frequency range from 20 Hz to 1 kHz. Fig. 6 shows that the rdc and the 1 MHz frequency r0 val-

ues of these nanocomposites vary anomalously within one order of magnitude with the increase of MMT clay concentration. The electrical conductivity of the PCNs materials is governed by the polymer chain dynamics, clay interlayer cations mobility, and the parallel and perpendicular oriented structures of the exfoliated/ intercalated MMT clay in the polymer matrix [2,32,36,39]. The significant increase of the rdc at only 0.5 wt.% MMT clay filler confirms the contribution of clay interlayer cations in the electrical conductivity, although the polymer chain dynamics is reduced as evidenced from the increase of their relaxation times (Fig. 4). up to the 10 wt.%. Further, a close resemblance is found between the rdc and r0 (1 MHz) at various MMT clay concentration of the PVAPEO blend–MMT clay nanocomposites. 3.5. Impedance spectra Fig. 7 shows that the complex impedance plane plots (Z00 vs. Z0 ) of the PVAPEO blend–MMT clay nanocomposites have semicircle arcs starting from the origin and inclined at different angles to the real axis. The shape of impedance spectra of a dielectric material gives the information regarding the contribution of electrode polarization phenomena (EP) (formation of electric double layer (EDLs) between electrodes surfaces and the dielectric material interface) and about the current carriers whether they are electrons or ions. Generally ion conducting dielectric materials shows two characteristic arcs in the complex impedance plane plots corresponding to electrode polarization effect in the low frequencies regions and the bulk material properties in the high frequency region [8–10]. For these PCNs materials, observed single arc represents the bulk material property, which implies the formation of good electrical contact between the dielectric material and the nickel-plated cobal electrodes over the experimental frequency range [36,39]. The large values of imaginary part Z00 as compared to the real part Z0 of the complex impedance confirm a high capacitive behaviour of these films owing to their low conductivity [6,21,36,39]. 4. Conclusions The manuscript presents dielectric properties of PVAPEO blend–MMT clay nanocomposites using various formalisms and their detail analysis in view of polymer dynamics and structural conformations. The values of real part of dielectric function e0 and the polymer chain segmental dynamics of the investigated nanocomposites show a direct correlation with the exfoliated and intercalated MMT clay structures in the PVA–PEO blend matrix. This finding confirms the use of dielectric relaxation spectroscopy as diagnostic sensor in the development of testing and monitoring technique in the area of nanocomposite formation specially the degree of dispersion of nanoscale MMT clay in the polymer matrix and the hindrance to the polymer local chain dynamics. The dielectric study revealed that the e0 value can be tailored in the range of nearly 1 by loading the 0.5–2 wt.% MMT clay in the PVAPEO blend matrix for their use as low dielectric constant nanodielectrics of improved mechanical and thermal properties in low frequency microelectronic technology. The increase in polymer chain segmental motion relaxation time with the increase of MMT clay concentration up to the 2 wt.% promote the miscibility between PVA and PEO, which is mainly due to bridging of polymers chains through H-bonds with intercalated and exfoliated nanostructures of MMT clay sheets. The dc conductivity of these nanocomposites vary anomalously within the one order of magnitude with the increase in MMT clay concentration up to the 10 wt.%. The investigated nanodielectrics form good electrical contact with nickel-plated cobal electrodes in the low frequency alternating current electric field.

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