Influence of ZrO2 filler on physico-chemical properties of PVA/NaClO4 polymer composite electrolytes

  • Jagadish Naik
  • R. F. Bhajantri
  • Vidyashree Hebbar
  • Sunil G. Rathod
Original Research
  • 58 Downloads

Abstract

The ZrO2-filled PVA/NaClO4 polymer nanocomposite is a freestanding electrolyte film and is prepared using the solution casting method in an aqueous medium. These prepared samples were characterized for structural, morphological, optical, thermal, and electrical properties. FT Raman studies confirmed the interaction between PVA and NaClO4, and dispersion of ZrO2 fillers in the PVA/NaClO4 polymer electrolyte. The surface roughness was observed from AFM images. Fitting the values of UV absorption to Tauc’s equation, the optical energy band gaps have been evaluated and correlated to the electrical conductivity. The maximum electrical conductivity of 4.3 × 10−3 (± 0.0002) S/cm was obtained for 3 wt% ZrO2-filled PVA/NaClO4 polymer nanocomposite. The thermal degradation kinetic parameter was calculated by fitting thermo gravimetric analysis values in Broid’s model.

Graphical abstract

Possible interaction of ZrO2 filled PVA/NaClO4 reflected in Raman peaks in FT-Raman spectra

Keywords

Thermal properties Polymer nanocomposite Optical band gap Kinetics ZrO2 

1 Introduction

There were many large volumes of articles published about polymer nanocomposite; despite that, the understanding of changes in their basic physical properties due to insertion of nanofillers remains unanswered. This is because of the complexity of polymer nanocomposite and lack of sufficient experimental data. The size and shape of the nanoparticles as well as state of dispersion in the polymer matrix have a large impact on the properties of a polymer nanocomposite. In some cases, it is difficult to obtain uniform dispersions, which results in variations in properties for systems of the same composition prepared using different techniques [1]. To generalize, understanding the variation in the basic properties of polymer nanocomposites requires investigation of various compositions under various conditions. The field of polymer nanocomposites has been one of the most popular current research areas for the last several decades because of their wide applicability in microelectronics [2], biomaterials [3], drug delivery [4], miniemulsion particles [5], fuel cell electrode polymer bound catalysts [6, 7], layer-by-layer self-assembled polymer films [8], nanofibers [9], imprint lithography [10], polymer blends [11], sensors [12], microelectromechanical systems (MEMS) [13], and photovoltaics [14]. Incorporation of inorganic nanoparticles in a polymer matrix has attracted attention due to their new physical–chemical properties, differing from the polymer matrix and the nanoparticles [15]. This kind of behavior of nanocomposites has opened their applications in the various areas listed above.

Among the different nanofillers, metal oxides have become important candidates due to their potentially unique optical, electronic, mechanical, and structural characteristics [16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26]. Zirconia (ZrO2) is one of the important metal oxides for this purpose; it has three crystal forms monoclinic, tetragonal, and cubic, and good mechanical and thermal resistance [16], high melting point, chemical stability, low electrical conductivity, high refractive index and band gap, hardness, and biocompatibility [17, 18, 19, 20, 21]. ZrO2 has been used in many applications including transparent optical devices [22], fuel cells [23], catalysts, sensors [24], microelectronics [25], ceramics, and biomaterials [18, 19, 26, 27].

Poly (vinyl alcohol) (PVA) is a water-soluble, semi-crystalline, synthetic polymer with the chemical formula [CH2CH(OH)]n. PVA is mainly used for emulsion polymerization aids, colloidal protection, textile sizing agents, architectural applications, fibers, paper coatings, and adhesives. It has gained wide application due to its excellent film casting, emulsifying, and adhesive properties; relatively high tensile strength; and good flexibility. Its thermal characteristics vary with the degree of crystallinity, which is heavily dependent on the degree of hydrolysis and the average molecular weight of the polymer. The molecular weight and tacticity of the PVA and the nature of a doping element when used play a critical role in tuning the physico-chemical properties as per application requirements [28, 29, 30].

In comparison with traditional batteries, all the solid-state metal ion batteries offer high energy densities which are very important contribution to next-generation energy storage technology. In that precept, the solid electrolytes offer new possibilities in a large-scale energy storage system for application such as electric vehicle and grid energy storage, but the research regarding solid-state electrolytes requires in-depth approach in order to enhance energy density, ionic conductivity, electrochemical stability, and mechanical property. To achieve all above properties in single electrolyte, thorough understanding of salt–polymer interaction, polymer–filler interaction, salt–filler interaction, theoretical modeling to predict the experimental results, details of the ion transport mechanism, and a concern towards the lower cost for large-scale applications is required [31]. Ideal solid polymer electrolyte, i.e., the salt/polymer combination, requires high viscosity and high ionic conductivity with other physico-chemical properties. The higher viscosity leads to lower ionic conductivity and poor cyclability at ambient temperature. One more limitation is the breakdown in electrode/electrolyte interphase due to the formation of unwanted side products during the salt–polymer interaction. It may affect the ion transport, ion polarization, and charge carrier concentration [32]. The high segmental polymer chain mobility and high number of efficient ion salt dissociation are required to get high ionic conductivity [31].

The host dry polymer matrix with sodium salt acts as a solid solvent. It is a dry system, which suffers low ionic conductivity at ambient temperature, but the addition of fillers may enhance the conductivity by decreasing the glass transition temperature. For high-capacity large-scale storage and severe safety concerned applications, sodium-based batteries are ideal, but a lack of sodium solid-state electrolytes with high ionic conductivity at ambient temperature is the demerit. Incorporation of ZrO2-like inorganic species provides new structural arrangement to the polymer matrix, leading to enhanced conductivity and mechanical properties along with a wide operating temperature range due to increase in thermal resistance of the polymer matrix [33]. The overall information regarding sodium solid polymer electrolytes encourages researchers to work in this field [31].

Even though lithium ion batteries have been successfully implemented in power electronics, sodium ions have been considered as a most appealing alternative to lithium ion due to their lower cost, most abundant availability, richer intercalation chemistry, and ease of recycling. There are many reports showing successful implementations of sodium ions in high-temperature Na/S cells for megawatt electrical storage and for Na/NiCl2 Zebra-type systems for electric vehicles, both of which take advantage of the highly conducting solid beta-alumina ceramics containing Na+ at temperatures, i.e., 300 °C [34, 35]. However, sodium ion technology needs to be focused to achieve the properties shown by lithium ions at room temperature. If it is achieved, the future electronics will be based on these low-cost, sustainable materials.

The ionic conductivity is also dependent on the kind of anion present in the salt. The conductivity is dependent on polarizability of the anion. In case of sodium salts, conductivity increment follows PF6 > ClO4 > CF3SO3. Looking at these results, NaPF6 may be the best choice as an electrolyte, but solubility of NaPF6 is very less in comparison with NaClO4. This will reduce ion density in the composites. At higher concentrations, NaPF6 forms insoluble impurities in the electrolyte. Along with this, the higher reactivity of the NaPF6 reduces the battery performances. The NaClO4 has a relatively low anion polarization and stable cycle performance with negligible fading over 100 cycles, compared to all other sodium salts. NaClO4 has very high melting temperature of about 468 °C, which introduces higher thermal stability to the electrolyte. Looking at the wide operating temperature range, relatively high conductivity, higher thermal stability, and chemical stability of NaClO4, make one of the best choices to use it as an electrolyte for sodium ion batteries [36, 37, 38]. In that perspective, this paper is mainly focused on the preparation of polymer nanocomposites composed of PVA, NaClO4 salt, and ZrO2 filler and the study of its optical, thermal, and surface morphology and room temperature electrical conductivity properties.

2 Experimental methods

2.1 Materials used

The polymer PVA, with molecular weight 1,40,000 MW, and zirconium dioxide (ZrO2), with molecular weight 123.22 g/mol, used in this work were procured from Himedia Laboratories Pvt, Ltd., India. Sodium per chlorate (NaClO4, with molecular weight 122.44 g/mol) was purchased from Sigma-Aldrich Co. India.

2.2 Preparation of polymer nanocomposite

The 20% NaClO4-doped PVA, using 0 to 5 wt% ZrO2 nanoparticle-filled PVA composites, were prepared by the solution casting method [39, 40]. The preparation of various concentrations of ZrO2-filled PVA/NaClO4 composite films is shown in Fig. 1. A known quantity of PVA powder was dissolved in double distilled water. On the other hand, the calculated 20 wt% of NaClO4 was dissolved separately in double distilled water. Both the solutions were mixed with continuous stirring. Later, different weight% (wt%) of ZrO2 nanoparticles were dispersed in these prepared polymer solutions. The above prepared solutions were kept aside to get a suitable viscosity. So, the obtained viscous solution was ultrasonicated to get better dispersion and poured onto cleaned Petri glass plates and dried at room temperature. The dry films were peeled off and kept in vacuum desiccators [29, 40].
Fig. 1

Preparation of ZrO2-filled PVA/NaClO4 composite films

Figure 1 represents schematically the preparation of polymer nanocomposite. The photographic images of various concentrations of ZrO2-filled PVA/NaClO4 composite films were shown in Fig. 2. The flexibility of the film is shown in the folded film photographs as shown in Fig. 2. The Mitutoyo-7327 dial thickness gauge with accuracy 0.001 mm is used to measure the thickness of the prepared film. The obtained freestanding films were having an average of 100 μm thickness.
Fig. 2

Photographs of different concentrations of ZrO2-filled PVA/NaClO4 composite films

2.3 Characterization techniques

Fourier transform (FT) Raman spectra were measured using a Thermo Fisher Scientific Inc. (USA) model (NXR) FT Raman module spectrophotometer using Nd:YAG 1064-nm laser as an exciting source, which has 4000–40 cm−1 range of wave number and a resolution of 8 cm−1. The microstructural and optical properties were studied using a Shimadzu Co. Japan model Shimadzu UV-1800 UV–visible spectrophotometer to record the absorbance and transmittance behavior of the PVA/NaClO4 at various ZrO2 wt% in the wavelength range of 190–800 nm and the scan rate of 10 nm/min. The films for the UV–visible absorbance measurements were formed by placing 1 cm × 2 cm piece solid polymer films in a sample holder. Atomic force microscopy (AFM) was performed using nanosurf EZ2-FlexAFM in the air at an ambient condition using the dynamic force mode with a scan rate of 156 kHz and the resolution of 256 samples per line with 10 μm × 10 μm image size.

Thermo gravimetric analysis (TGA) and differential thermal analysis (DTGA) were performed on a TA Instrument Co., Inc (USA) Model Q600. The balance was calibrated with the given standards and the TGA temperature was calibrated with Curie temperature standards. To understand the kinetics of the thermal decomposition, the heating was applied at a ramp of 10 °C/min to 600 °C from 25 °C, with samplings every 0.5 s. The TGA pan was filled with a sample variance with a weight from 5 to 9 mg. Nitrogen (100 ml/min) was used as a sample purge gas for the pyrolysis experiments in the TGA unit and air was used for forced cooling after completion of degradation.

The DC conductivity studies were carried out using a Keithley instrument, USA, model Keithley 236, I-V programmable source meter, using a two-probe method at room temperature. Here, the polymer films were sandwiched between two steel electrodes and the measurements were carried out, in resistance mode, by applying voltage in the range 0–5 V; in successive steps of 0.1 V, the current was measured. Generally, the measured resistance consists of three components, i.e., Rmeasured = Rsample + Rcontact + Rwires, where Rmeasured is the resistance measured, Rsample is the sample resistance, Rcontact is the contact resistance, and Rwires is the connected wire resistance [41]. Short-length and high conducting copper wires are connected for resistance measurement so that the magnitudes of the component, Rwires, are much smaller than other terms and can be neglected. High conducting silver-coated steel electrodes and a rough surface of the sample reduce the contact resistance between electrode and electrolyte. The calculated contact resistance of the steel electrode was of the order of 10−6 Ωm, negligible compared to other terms. Considering the volume resistivity and geometry of the samples, the actual electrical resistance was calculated using the equation Rsample = (resistivity × thickness)/cross-sectional area. Before measurement, the instrument was calibrated with an open circuit and closed circuit using standard methods to reduce errors, and the experiments were repeated to check repeatability on the values, with the average error of ± 0.05.

3 Results and discussion

3.1 FT Raman studies

Raman spectroscopy is applied to understand one or more of the following factors: structural composition, crystallinity, chemical reactions, and molecular orientations. Figure 3a–f represents the FT Raman plots of various concentrations of ZrO2-filled PVA/NaClO4 polymer composites. Strong band appeared at 2918 cm−1 due to the presence of C–H vibration. Appearance of 936 cm−1 band could be assigned to the presence of spectroscopically free ClO4 anion [42, 43]. In literature, the broadband appears due to vibration, primarily involving sodium ion, and is at around 360 cm−1. Position of peaks related to Na+ and ClO4 ions is at lower and higher wave numbers. This indicates that NaClO4 dissolved completely; thus, formation of Na+ and ClO4 contact ion pair is impossible, which confirms the absence of interaction between unsolvated ions and NaClO4 completely interacted with long-chain PVA molecules [43]. The peak appeared at 1093 cm−1 is shifted towards 1128 cm−1 assigned to C–O stretching modes. The peak appeared at 1730 cm−1 is corresponding to the C=O stretch. The band observed at 630 cm−1 is due to the presence of C=O out of plane bending. The band due to CH3 twisting is observed at 930 cm−1. The band at 1367 cm−1 presents CH3 symmetric deformation in the composite. The band at 1433 cm−1 is attributed to CH2 asymmetric deformation in the polymer composites. The bands at 1726 and 2918 cm−1 represent C=O stretching and C–H asymmetric deformation in CH3 in the polymer nanocomposites respectively.
Fig. 3

af FT Raman spectra of different concentrations of ZrO2-filled PVA/NaClO4 composites

The band due to shear mode at 1432 cm−1 was shifted towards 1436 cm−1 after filling ZrO2 nanoparticle in the PVA/NaClO4 matrix. The observed shift or increase in intensity is not because of the interaction between ZrO2 and PVA/NaClO4 polymer matrix. This may be possible for incorporation of ZrO2 produces defects in the polymer matrix. These defects influence the average bond length of the chain sequence and may lead to a shift in Raman frequency or variation in intensity of the peak [44, 45]. Figure 4 represents possible interaction between PVA, NaClO4, and ZrO2.
Fig. 4

Possible interaction of ZrO2-filled NaClO4/PVA polymer composites

3.2 Atomic force microscopic studies

Figure 5a, b represents the AFM image of 3 wt% ZrO2-filled NaClO4/PVA polymer composites and Fig. 5c, d represents the AFM image of 0 wt% ZrO2-filled NaClO4/PVA polymer composites. An AFM image evidences for composites, possessing a heterogeneous medium. The surface morphology of nanofilled film (Fig. 5a) shows lumps. These lumps which occurred may be due to nanofiller association with the polymer matrix. Cluster-like lumps are also observed due to the agglomeration of fillers. From the figure, it is observed that various sizes around 80 to 300 nm of ZrO2 particles were dispersed in the polymer matrix. The 630-nm- to 1.8-μm-size clusters were observed in the polymer matrix. The RMS value of roughness within the scanned area is 15 nm for 3 wt% ZrO2-filled NaClO4/PVA and 3 nm for 0 wt% ZrO2-filled NaClO4/PVA. Incorporation of ZrO2 fillers enhanced surface roughness of the matrix. The dark background observed in the AFM topography image corresponds to the presence of an amorphous phase in the prepared polymer nanocomposite. This enhances the segmental motion of the polymer chain as well as Na+ ion tunneling in the prepared electrolyte, which makes electrolyte flexible and conductive [29, 46].
Fig. 5

AFM image of 3 wt% ZrO2-filled PVA/NaClO4 composites (a, b) and PVA/NaClO4 composites (c, d)

3.3 UV–visible studies

Figure 6 represents the UV–visible transmittance spectra of NaClO4/PVA and different concentrations of ZrO2-doped PVA/ NaClO4 composites. From the figure, it is observed that the transmittance decreases with an increase in ZrO2 wt%. As the concentration of ZrO2 increases, the average roughness increases; this causes a higher light scattering, leading to an increase in opacity. The transmittance depends on the film thickness and thicknesses of the prepared films are around 100 to 120 μm. This is also one of the reasons for the lower transmittance of the polymer composites and the transmittance increases with an increase in wavelength and decrease in thickness of the composite film. The reduction in transmittance in the region of 200–300 nm is due to the presence of UV absorbing double bonds and oxygen-containing functionalities. Further details about transmittance are explained in terms of absorbance.
Fig. 6

UV–visible transmittance spectra of different concentrations of ZrO2-filled PVA/NaClO4 composites

Absorbance process plays an important role in the optical properties of polymer. The absorption spectra of freestanding PVA/NaClO4 films filled with ZrO2 wt% are shown in Fig. 7a. From Fig. 7a, it is observed that the absorption peak observed in the UV region is at 273 nm. This band may be due to n → π* transitions of the carbonyl groups. The broadness of the bands was increased as an increase in ZrO2 wt% and intensity increases linearly with increasing wt% of ZrO2. The additive ZrO2 is a direct band gap insulator and photon-absorbing metal oxide. It is not showing any characteristic of d-d transitions in the visible region (400–800 nm) due to do configuration of Zr+4 ions. The absorption peaks observed in UV region are because of the O2−(2p) - Zr4+(A1g) charge transfer transitions, corresponding to the excitation of electrons from the valence band to the conduction band. By combining the effects of O2− and PVA/NaClO4, the broadness of the band increased. Increased light extinction (absorbance) by incorporation of ZrO2 nanoparticles may be due to the light scattering and nanofiller UV absorption [47].
Fig. 7

a UV–visible absorbance with wavelength and b ln(α) versus for different concentrations of ZrO2-filled PVA/NaClO4 composites

The intercept of interpolation to zero absorption with the photon energy axis is taken as the value of absorption edge and is tabulated in Table 1. The absorption edge values for polymer composites increase with ZrO2 wt%, i.e., absorption edge for the PVA/NaClO4 polymer matrix is 241 nm and it is shifted to 283 nm for 5 wt% ZrO2-filled PVA/NaClO4 polymer matrix. This increment of absorption edge is attributed to the change in crystalline nature, induced by ZrO2 nanoparticle. From this, the changes induced in the number of available final states may be related to composition ratio.
Table 1

Optical parameters of ZrO2-filled PVA/NaClO4 films

Sample name

Absorption edge (nm)

E u

(eV)

E g (direct)

(eV)

E g (indirect)

(eV)

0 wt% ZrO2

241

0.240

5.457

4.978

1 wt% ZrO2

245

0.432

5.197

4.777

2 wt% ZrO2

251

0.657

5.041

4.422

3 wt% ZrO2

262

0.754

4.936

4.132

4 wt% ZrO2

275

0.793

4.738

4.168

5 wt% ZrO2

283

0.870

4.642

4.268

Absorbance process plays an important role in the optical properties of polymer. The absorbance values were fitted to Tauc’s equation to understand the dependence of absorbance on photon energy. Tauc’s equation is given by
$$ a(v)=B{\left( hv-{E}_g\right)}^x/ hv $$
(1)
where E g is an optical band gap of the substance, h is Planck’s constant, ν is the corresponding frequency, and x is the parameter that gives the type of electron transition [48]. Two distinct linear relations for x = ½ direct transition and for x = 2 indirect transition were found corresponding to different inter band absorption processes [29, 49]. Factor B is constant within the optical frequency range and depends on the transition probability; α(υ) is the absorption coefficient, calculated using α(v) = A/d, where A is absorbance and d is film thickness. Figure 7b represents ln(α) versus plot. The activation energy (E u ) is calculated by taking the reciprocal of the slope of the linear portion of the ln(α) versus plot. The obtained value for E u is tabulated in Table 1. The E u for the PVA/NaClO4 polymer matrix is 0.24 eV and it is increased to 0.87 eV for 5 wt% ZrO2-filled PVA/NaClO4 polymer composite.
On the basis of Tauc’s equation, the values of the direct band gap and the indirect band gap were obtained from the plot of (αhν)2 versus and (αhν)1/2 versus as shown in Fig. 8a, b respectively. Both the direct and indirect transitions have been calculated by the intercept of the interpolation to zero in graphs (αhν)2 and (αhν)1/2 with photon energy axis and corresponding values were tabulated in Table 1.
Fig. 8

Variation of a (αhν)1/2 versus and b E g indirect versus ZrO2 wt% of different concentrations of ZrO2-filled PVA/NaClO4 composites

The E g (direct) for the PVA/NaClO4 polymer matrix is 5.457 eV and it is decreased to 4.642 eV for 5 wt% ZrO2-filled PVA/NaClO4 polymer composite. The E g (indirect) for the PVA/NaClO4 polymer matrix is 4.978 eV and it is decreased to 4.132 eV for 3 wt% ZrO2-filled PVA/NaClO4 polymer composite and increased to 4.268 eV for 5 wt% ZrO2-filled PVA/NaClO4 polymer composite. In both the cases, the E g is maximum for PVA/NaClO4 matrix and decreases with increase in ZrO2 wt%, which indicates the formation of additional energy levels (defects). These additional energy levels produce localized states in the optical band gap that increases with increasing concentration of the defects. And also, the addition of ZrO2 forms the crystallites by local cross-linking within the amorphous phase of the polymer, leading to enhancement in crystallinity. This clearly indicates the changes in electronic structure and microstructure of the polymer due to the addition of ZrO2. But in case of indirect transition, the 3 wt% ZrO2-filled composite has shown minimum Eg and later it increased but not more than the PVA/NaClO4 polymer matrix.

3.4 Thermal analysis

Figure 9 shows the TGA thermograms of the PVA/NaClO4 polymer matrix filled with different concentration of ZrO2. The derivative peak temperature (T p ), percent weight loss, and final char content are given in Table 2. The TGA thermograms show four degradation steps, suggesting the existence of more than one degradation process. The first transition region is at around 30 to 160 °C (step I). In this step, the weight loss of the composite is about 10%. The T p has been increased with an increase in ZrO2 content, attributed to the presence of thermal process due to evaporation of physically weak and chemically strong bond of H2O and OH moieties, explaining the existence of physical transitions.
Fig. 9

TGA thermograms of different concentrations of ZrO2-filled PVA/NaClO4 composites

Table 2

Thermal degradation steps and char content

Degradation

Step I

Step II

Step III

Step IV

Char content [%] ± 01

Sample name

T p —1

% weight loss

T p —2

% weight loss

T p —3

% weight loss

T p —4

% weight loss

PVA/NaClO4(P/N)

65.62

12.89

297.81

38.78

439.78

16.54

549.00

23.06

8.13

P/N + 1%ZrO2

86.36

10.00

283.12

40.84

430.84

12.89

537.66

20.29

8.29

P/N + 2%ZrO2

91.89

08.75

280.68

37.44

439.02

11.83

541.29

21.46

10.58

P/N + 3%ZrO2

91.44

09.81

275.23

36.82

429.62

15.26

536.59

17.83

11.59

P/N + 4%ZrO2

93.53

10.30

280.83

38.32

425.38

16.88

526.97

20.13

12.16

P/N + 5%ZrO2

94.77

12.46

297.58

39.85

436.21

14.63

538.56

17.57

13.54

The second transitional region covers a wider temperature range 210 to 360 °C (step II). It includes melting point, physical transition, and the degradation temperature. The total weight loss of 36 to 41% occurred. The higher value of weight loss indicates the existence of a chemical degradation process. This degradation process occurs due to partial dehydration and the formation of polyene. The third degradation process occurs in the range 380 to 400 °C (step III). This degradation occurs due to intermolecular cyclization and condensation, i.e., scission of C–C bonds in polymeric backbone. Which means polyene decomposes into low-mass oxygen-containing products. During this carbonation process, 16% weight loss occurs. The fourth transition region at 490 to 600 °C (step IV) suggests that there may be a lack of saturation of end groups related to the combination and termination process (complex formation) between PVA and NaClO4 which leads to random scission in this temperature range. It leads to thermo-oxidation of carbonized residue [50, 51, 52].

The DTA curves of polymer composite films are shown in Fig. 10. The PVA/NaClO4 samples exhibit good thermal resistance to thermal decomposition up to 258 °C and increase up to 308 °C as ZrO2 wt% increases. And also, the intensity of the peak decreases with an increase in ZrO2 density and reveals that ZrO2 enhanced the thermal stability of polymer composite. The analysis of TGA and DTA results was strictly corroborated with each other [51]. As ZrO2 wt% increased, the char content values increased. The char content values from TGA confirmed the homogeneous dispersion of ZrO2 nanofiller at lower wt% within the PVA/NaClO4 polymer matrix. The weight loss gradually decreases as it is loaded with various ZrO2 wt%. The thermal stability of pristine polymer matrix is related to the weak bonds in the PVA/NaClO4 principal initiation sites of degradation. The degradation and release of volatile products occurred at higher temperatures in the presence of ZrO2 and an increase in ZrO2 content showed influence on the thermal stability of polymer nanocomposite. The improved thermal stability is attributed to the additive effect of ZrO2 fillers and the chemical crosslink reaction between PVA and NaClO4.
Fig. 10

DTA thermograms of different concentrations of ZrO2-filled PVA/NaClO4 composites

From the TGA curves, it can be considered that degradation of polymer composites starts from step II, because step I was due to absorption of humidity. Onset temperatures were tabulated in Table 3 by considering starting temperature of degradation of step II. Onset temperature decreased with an increase in ZrO2 concentration which reveals decrement in the thermal stability. Step II represents the major decomposition of polymer composite. At this step, using Broido’s method, kinetic and thermodynamic parameters were calculated by developing plots (Fig. 11) of ln(ln(1/Y)) versus 1/T for this decomposition segment and tabulated in Table 3, where Y is the fraction of the compound which did not get decomposed. Thermo kinetic data related to decomposition reaction could be calculated from TGA results [53]. The thermodynamic parameters are useful to understand the decomposition reaction in the solid state. The enhancement in the decomposition product formation is well understood by the value of Gibbs free energy (ΔG) in polymer composite. This ΔG depends on two other parameters called entropy (ΔS) and enthalpy (ΔH) by the standard relation
$$ \varDelta G=\varDelta H- T\varDelta S $$
Table 3

Kinetic parameters

Sample name

Ea

(kJ/mol)

ΔH

(kJ/mol)

ln A

ΔS

(kJ/mol K)

ΔG

(kJ/mol)

Onset temp.

oC ± 05

PVA/NaClO4(P/N)

48.0

− 2.4745

0.00148

− 67.4077

17.5999

237

P/N + 1%ZrO2

47.5

− 2.3523

0.00192

− 67.2137

16.6769

232

P/N + 2%ZrO2

80.4

− 2.3324

0.00107

− 67.0755

16.4944

223

P/N + 3%ZrO2

123.1

− 2.2870

0.00042

− 67.0117

16.1566

220

P/N + 4%ZrO2

56.5

− 2.3334

0.00106

− 67.0801

16.5045

220

P/N + 5%ZrO2

36.7

− 2.4729

0.00179

− 67.3090

17.5570

217

Fig. 11

Plots of Ln(Ln(1/Y)) versus (1/T) × 103 in decomposition range 200 to 400 °C

The value of ΔH shows the energy difference between decomposed product and initial composite structure. This explains the formation of new decomposed composite due to the potential energy barrier. It is calculated using a standard formula
$$ \varDelta H={E}_a-R{T}_d $$
The value of ΔS shows the rate of disorderness of the system, which means the situation of the system in its own thermodynamic equilibrium and it is calculated using the formula
$$ \varDelta S=\frac{\varDelta H}{T}-4.57\log \left(\frac{T}{K_1}\right)-47.22 $$

The negative value of ΔS suggests that the activated state of polymer complex is rigid and lower values of ΔS reveal that interaction between ZrO2 and PVA/NaClO4 is very less; too long time is required to form ZrO2/PVA/NaClO4 complex. It clearly suggests that ZrO2 particles were well dispersed in the PVA/NaClO4 polymer matrix [42]. The ΔG and ΔS collectively reveal that fillers are immiscible in the polymer matrix.

3.5 Conductivity studies

Linear relationship between current and voltage can be observed in Fig. 12 for the PVA/NaClO4 polymer matrix, filled with different concentrations of ZrO2. The graph shows ohmic behavior. Figure 13 illustrates the variation in conductivity of the PVA/NaClO4 polymer matrix filled with different concentrations of ZrO2. The dispersion of ZrO2 changes the crystallinity of the sample, which subsequently affects the conductivity. The variation in crystallinity is clearly visible from FT Raman and UV–visible studies. Therefore, electrical conductivity measurements were carried out for the prepared composites. The plot shows that as ZrO2 wt% increases, the conductivity increases up to 3 wt% and later it decreases. The increase in conductivity is due to ZrO2 particles acting as a nucleation center for the formation of minute crystallites; these crystallites aid in the formation of amorphous phases in the polymer electrolyte and they form a new kinetic path through polymer ceramic boundaries. These less ordered regions become more flexible, which increases the segmental motion of the polymer chains which enhances conductivity. But conductivity increases up to an optimum concentration of ZrO2 density and later decreases.
Fig. 12

Current–voltage graph of different concentrations of ZrO2-filled PVA/NaClO4 composites

Fig. 13

Variation of conductivity versus ZrO2 wt% for different concentrations of ZrO2-filled PVA/NaClO4 composites

ZrO2 fills the gap between adjacent conducting phases and forms an insulating phase between them. At lower percentage, ZrO2 is wrapped by conductive polymer matrix so it will support conductivity. Further increase in ZrO2 fillers beyond 3 wt% increases the density of the insulating phase. These insulating fillers form clusters of insulating phases. These phases oppose the ionic movement or polymer chain segmental motion as a mere insulator. Hence, conductivity decreases [54, 55]. Gustav et al. reported that PVA mixed with lithium bis(triflouromethane) sulfonamide salt shows conductivity of about 10−4 Scm−1 at 60 °C [56]. F. Kingslin Mary Genova et al. prepared electrolytes based on lithium triflate-doped PVA-PAN polymer electrolyte and reported conductivity of about 10−5 Scm−1 [57]. YF Huang et al. prepared polyethylene glycol-borate ester/lithium fluoride in graphene oxide/poly(vinyl alcohol)-based electrolyte membrane with a conductivity of about 10−4 Scm−1 and predicted that such electrolytes are promising candidates for a new battery separator [58]. Looking at the literature, in case of 3 wt%, ZrO2 film shows conductivity around 10−4 Scm−1; therefore, the prepared composite films will be a promising candidate for a battery separator in sodium-based battery applications.

4 Conclusion

The freestanding films of PVA and NaClO4 polymer nanocomposites with different concentrations of ZrO2 were prepared using the solution casting method. FT Raman studies confirmed the interaction between PVA and NaClO4 and well-dispersed ZrO2 particles within a polymer matrix. The AFM image of nanofilled film shows lumps which occurred due to the nanofiller association with the polymer matrix. The thermal stability of the composites was observed around 230 °C. Broid’s method was used to calculate thermodynamic parameters. Thermal kinetic parameters also support the results obtained from FT Raman studies. Optical energy band gap and conductivity values are in correlation with each other. Increase in conductivity is attributed to the formation of less ordered region and decrease in conductivity at higher wt% is attributed to the aggregation of the insulating phase in between conducting sites. Three weight percent ZrO2-filled PVA/NaClO4 composite has shown good properties that can be used as a polymer electrolyte.

Notes

Acknowledgements

The authors are thankful to USIC, Karnatak University, Dharwad, for FT Raman, TGA, and AFM facilities.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflicts of interest.

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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  • Jagadish Naik
    • 1
  • R. F. Bhajantri
    • 2
  • Vidyashree Hebbar
    • 2
  • Sunil G. Rathod
    • 1
  1. 1.Department of PhysicsMangalore UniversityMangalagangotriIndia
  2. 2.Department of PhysicsKarnatak UniversityDharwadIndia

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