Exploring the potential of V₂O₅-modified PVA/CMC nanocomposites as functional materials for energy storage applications

2.1. The chemicals used

PVA, with a molecular weight (MW) of 14,000, was purchased from Merck, Germany, and CMC, which is 99.5% pure Na-CMC with a sodium chloride salt content not exceeding 0.5%, a sodium glycolate salt content not exceeding 0.5%, and a density equal to 1.59 g/cm³, was produced from LANXESS, a German company.

2.2. The synthesis of nanocomposites

A simple chemical synthesis method was used to synthesize vanadium oxide nanoparticles. For this, 50 mL of distilled water was mixed with oxalic acid (C₂H₂O₄), which was heated to 60°C for 30 min under constant magnetic stirring. Then, 10 g of ammonium metavanadate (NH₄VO₃)was slowly added to it until the solution became blue-green in color. After that, it was stirred at 80°C for 8 h, resulting in greenish powders, which were subsequently calcined at different temperatures (300°C, 400°C, and 500°C) for 2 h each step, finally obtaining yellow-colored products of V₂O₅ nanoparticles. The PVA/CMC-V₂O₅ nanocomposites were prepared via a solution casting technique. Initially, 3.5 g of PVA was dissolved in a binary solvent system (3:1 v/v distilled water: ethanol) under constant magnetic stirring at 60°C for 1 h to prevent excessive swelling, and ensures homogeneous polymer chain disentanglement [26], while 1 g of CMC was separately dissolved in deionized water at 55°C for 2 h with continuous stirring. The resulting polymer solutions were then combined in a 70:30 weight ratio and homogenized at 60°C for 30 min under vigorous stirring (500 rpm) to achieve a uniform blend. Subsequently, V₂O₅ nanoparticles (0-0.08 wt%) were incorporated into the polymer matrix through dropwise addition, followed by continuous stirring at 60°C for 1 h to ensure homogeneous dispersion. To further enhance nanoparticle distribution and interfacial interactions, the mixture was subjected to probe sonication (20 kHz, 30% amplitude) for 5 min. The well-dispersed nanocomposite solutions were then cast onto Petri dishes and dried under controlled conditions (48°C, 40% relative humidity) for 24 h to produce freestanding films with a uniform thickness of 120 ± 5 μm. This method ensured the formation of stable, well-integrated PVA/CMC-V₂O₅ nanocomposites with optimized structural and functional properties.

2.3. The characterization

The structural properties of the prepared films were analyzed using X-ray diffraction (XRD) with a PANalytical X’pert Pro MPD diffractometer equipped with Cu-Kα radiation (λ = 1.5406 Å) operated at 35 kV, with data collected over a 2θ range of 5–80° at a scanning rate of 2° min⁻¹. Chemical interactions and functional group analysis were performed using Fourier-transform infrared (FT-IR) spectroscopy (Nicolet iS10, Thermo Scientific, USA) in attenuated total reflectance (ATR) mode, with spectra recorded in the wavenumber range of 4000-400 cm⁻¹ at a resolution of 4 cm⁻¹. Optical absorption properties were investigated using a UV-Vis-NIR spectrophotometer (JASCO V-570, Japan) across the wavelength range of 200-1000 nm, with baseline correction performed using a reference blank. Dielectric properties were characterized at room temperature (25 ± 1°C) using a Novocontrol Concept 40 broadband dielectric spectrometer, with measurements conducted over a frequency range of 0.1 Hz to 7 MHz using a parallel-plate electrode configuration under nitrogen atmosphere.

3.1. XRD analysis

Figure 1 presents the XRD pattern along with a transmission electron microscopic (TEM) image of the synthesized vanadium nanoparticles, which gives information about the crystallinity and structure of V₂O₅, PVA/CMC (70/30 wt./wt.), and PVA/CMC/V₂O₅ films. The XRD pattern of crystalline vanadium pentoxide (V₂O₅), indexed using the JCPDS 41-1446, reveals distinct peaks corresponding to its orthorhombic phase [27]. The most intense peak appears at 2θ ≈ 20.3°, indexed to the (001) plane, which is characteristic of the layered structure of V₂O₅ and indicates a preferred orientation along this direction. Other prominent diffraction peaks were observed at 2θ ≈ 26.2° (110), 31.0° (301), 34.3° (310), and 51.3° (020), confirming the well-defined crystalline nature of the material.

Close

Figure 1.

(a) XRD pattern and (b) TEM image of vanadium oxide nanoparticles, PVA/CMC polymer blend and other samples containing variable mass fraction of Vanadium oxide.

Export to PPT

XRD pattern of the PVA/CMC polymer blend exhibits a halo centered at approximately 25° (2θ). This broad halo, rather than distinct sharp peaks, is a characteristic of amorphous materials or amorphous regions within semi-crystalline materials. The formation of an amorphous phase in the PVA/CMC blend can be attributed to the polymer miscibility and the formation of a homogeneous amorphous phase that disrupts the regular arrangement of polymer chains, hindering the formation of crystalline domains. Moreover, the hydrogen bonding interactions between the hydroxyl groups of PVA and the carboxyl groups of CMC can interfere with the regular packing of polymer chains, favoring an amorphous structure. It is important to note that the presence of an amorphous halo does not necessarily imply a completely amorphous structure. Semi-crystalline polymers or polymer blends can exhibit crystalline peaks and an amorphous halo, indicating the coexistence of crystalline and amorphous regions.

When the PVA/CMC polymer blend is doped with increasing concentrations of vanadium oxide (V₂O₅) nanoparticles, the observed halo seems to be shifted from 2θ=25° towards a lower angle of around 2θ=19°, suggesting structural changes within the polymer matrix upon the incorporation of the nanoparticles. The position of the amorphous halo in an XRD pattern is related to the average interatomic or intermolecular distances within the material. A shift in the halo position towards a lower angle indicates an increase in the average interatomic or intermolecular spacing. This shift is attributed to the fact that V₂O₅ nanoparticles may intercalate or insert themselves between the polymer chains of PVA and CMC, leading to an increase in the average intermolecular spacing. This insertion can disrupt the packing of the polymer chains, resulting in a larger average distance between the polymer chains and a shift of the amorphous halo towards lower angles. The incorporation of nanoparticles can also induce separation or stretching of the polymer chains, leading to an increase in the average intermolecular distance. This effect can be more pronounced at higher nanoparticle concentrations, causing a progressive shift of the amorphous halo towards lower angles. Such an increase in the average intermolecular distances may be attributed to the plasticization effect resulting from the addition of nanoparticles. The shift of the XRD amorphous halo from 25° to 19° (2θ) in the PVA/CMC-V₂O₅ nanocomposites indicates an increase in interlayer spacing, suggesting that V₂O₅ nanoparticles intercalate between polymer chains, disrupting their packing and expanding the average intermolecular distance. This shift also reflects improved nanoparticle dispersion, as the uniform distribution of V₂O₅ within the polymer matrix reduces agglomeration and enhances interfacial interactions, critical for optimizing optical and electrical properties.

The crystallite size of the synthesized V₂O₅ nanoparticles, calculated using the Scherrer equation from the most intense XRD peak at 2θ = 20.3° (β = 0.46°, k = 0.9, λ = 1.5406 Å), was determined to be approximately 17.6 nm. This result is in good agreement with TEM analysis, which revealed particle sizes within the 15-20 nm range, confirming the nanocrystalline nature of the material. The consistency between XRD and TEM findings validates the reliability of both characterization techniques in determining the particle dimensions. These results collectively demonstrate the successful synthesis of nanocrystalline V₂O₅ with well-controlled particle sizes, which is crucial for its potential applications in catalysis, energy storage, and electrochromic devices. Further refinement through Williamson-Hall analysis could provide additional insights into strain contributions to peak broadening.

3.2. FT-IR analysis

FTI-R spectroscopy is one of the most useful tools for identifying the types of bonding, functional groups, and intermolecular forces present in the nanocomposites. The FT-IR spectra of pure PVA/CMC polymer blend doped with 0.02, 0.4, 0.06, and 0.08 wt.% of V₂O₅ nanoparticles in the range of frequency 4000-400 cm^-1 at room temperature has been shown in Figure 2. The pure PVA has main absorption bands at 3414 cm⁻¹ assigned to OH stretching; the band at 2924 cm⁻¹ is due to CH stretching, 1413 cm⁻¹ is due to CH bending vibrational mode, and at 1093 cm⁻¹ is assigned to C-O stretching mode. The band at 847 cm⁻¹ is assigned to the C-C stretching mode [28,29]. The FT-IR spectrum of CMC shows the broad absorption band at 3310 cm^−1, representing the stretching vibrations of the −OH group. The C−H stretching peaks are observed at 2926 cm⁻¹. The presence of a strong maximum band at 1590 cm⁻¹ indicates the presence of a carboxyl group –COO⁻. The IR bands’ maximum appeared at 1322 and 1057 cm^−1, indicating the presence of −OH bending and C−O−C stretching vibrations, respectively [30].

Close

Figure 2.

FTIR spectral data of PVA/CMC polymer blend and other samples containing a variable mass fraction of Vanadium oxide.

Export to PPT

It is evident that for PVA/CMC spectra, the bands at 1718 cm^-1, 1413 cm^-1, and 3310 cm^-1 can be seen to correspond to carboxyl, methyl, and hydroxyl (OH) groups, respectively. In native PVA, the C–O stretch is observed at around 1100 cm⁻¹. However, in the case of the PVA/CMC matrix, a broad absorption band appears at approximately 1057cm⁻¹, which may be assigned due to poly(vinyl alcohol) structure because it is known that this semi-crystalline synthetic polymer has different domains depending on many process parameters, including cross-linking step, among others [31]. The presence of a strong maximum band at 1590 cm⁻¹ indicates that the band can be attributed to vibrational mode overlap between alcoholic groupings located along the chain backbone. All observations indicate hydrogen bonding in the polymer occurring across hybrid matrices containing particles.

After adding V₂O₅ nanoparticles to PVA/CMC blend, it can thus be seen that the introduction of V₂O₅ NPs significantly alters vibrational bands exhibited by various matrices made from PVA/CMC blends; this is particularly evident when comparing the OH stretching region as well as asymmetric CH₂ stretching range, where both become less broadened with increasing V₂O₅ NP content. Moreover, absorption within 1800-800cm^-1 gradually decreases upon the addition of V₂O₅ contents into these composites. The decrease in the intensity of some IR bands could have resulted from van der Waals interactions or hydrogen bonding between (OH)/(COO^-) groups. The bands appearing between 487 and also occur doublet at around 570 and 702^-1 represent symmetric V–O–V; asymmetric VO; V=O stretches, respectively. These result in reduced crystallinity degrees besides films forming charge transfer complexes; PVA/CMC represents the electron donor while the acceptor refers to the nano-filler particles, thus affecting dynamics throughout chains constituting such materials produced through nanotechnology.

3.3. Optical parameters

The UV–visible absorption spectra of pure PVA/CMC blend and the polymer blend doped by 0.02, 0.04, 0.06, and 0.08 wt% of V2O5 nanoparticles are shown in Figure 3 in the frequency range 200-1100 Hz. The presence of a band around 220 nm in the UV/visible spectrum of the PVA/CMC polymer blend can be attributed to the electronic transitions associated with the carbonyl (C=O) chromophore present in the polymer backbone. The carbonyl chromophore in organic molecules typically exhibits an intense absorption band in the UV region, specifically around 190-230 nm, due to the π→π* electronic transition [32]. This transition involves the promotion of an electron from the non-bonding π orbital to the anti-bonding π* orbital of the carbonyl group. The intensity and exact position of this band may vary depending on the degree of hydrolysis, the concentration of residual acetate groups, and the overall structural environment within the polymer chain.

Close

Figure 3.

UV/Vis. absorption spectra of PVA/CMC polymer blend and other samples containing variable mass fractions of Vanadium oxide.

Export to PPT

The electronic transitions associated with the vanadium oxide species can explain the appearance of two additional peaks in the UV/visible spectrum of the blend sample when vanadium oxide nanoparticles are added. The peak around 305 nm is likely attributed to the charge transfer transition between the oxygen ligands and the vanadium ions in the vanadium oxide nanoparticles. Specifically, it can be assigned to the O^2- → V⁵⁺ charge transfer transition, which involves the promotion of an electron from the 2p orbitals of the oxide ions to the empty 3d orbitals of the vanadium ions in the oxidation state V⁵⁺. The peak in the range of 415 nm to 385 nm with varying positions can vary depending on the concentration of vanadium oxide nanoparticles in the polymer matrix. This peak is associated with the d-d electronic transitions within the vanadium ions, which are typically spin-forbidden but become partially allowed due to the distortion in the coordination environment of the vanadium ions.

The observed shift in the peak position from 415 nm to 385 nm with increasing vanadium oxide concentration can be attributed to changes in the local environment and coordination geometry around the vanadium ions. As the concentration of vanadium oxide nanoparticles increases, the interactions between the nanoparticles and the polymer matrix can alter, leading to changes in the coordination environment and crystal field splitting of the vanadium ions. This, in turn, affects the energy levels involved in the d-d transitions, resulting in a shift in the absorption peak position. The appearance and characteristics of these peaks can provide valuable information about the oxidation state, coordination environment, and interactions of the vanadium oxide nanoparticles within the PVA polymer matrix. Furthermore, the intensity and position of these peaks can be used to monitor the loading and dispersion of the vanadium oxide nanoparticles in the polymer system [20,33,34].

The PVA/CMC-V₂O₅ nanocomposite films showed another band with an absorption edge up to 287 nm. The UV-visible transmittance spectra of the prepared nanocomposites have been depicted in Figure 4. The transmission spectra show that increasing the concentrations of V₂O₅ nanoparticles can decrease the transmittance significantly from 90.88% for pure PVA/CMC blend to 7.26% for 0.08 wt. of V₂O₅ nanoparticles.

Close

Figure 4.

UV/Vis. transmission spectra of PVA/CMC polymer blend and other samples containing a variable mass fraction of Vanadium oxide.

Export to PPT

The absorption coefficient (α) (Eq. 1) is an important parameter to estimate the optical properties, which is determined from the relation between the absorbance (A) and the thickness of the sample (t) [15,32]:

The band gap energy (E_g) (Eq. 2) can be calculated using Urbach’s method to explain the nature of the band structure and type of electronic transitions in prepared nanocomposite films by the following relation [35]:

where α is the absorption coefficient, B is a constant, $h\nu$ is the energy of the incident photon, and n is a parameter that describes the kind of transition (direct transition if n=$\frac{1}{2}$ or an indirect forbidden transition if n = $\frac{3}{2}$).

The plots of ${\left(\alpha h\nu \right)}^2$ and ${\left(\alpha h\nu \right)}^{1/2}$ versus photon energy ($h\nu$)), i.e., direct and indirect forbidden transitions, respectively, are represented in Figures 5 and 6. Furthermore. The values for direct (E_gd), as well as indirect forbidden (E_gid) band gaps, are extracted from these curves, as shown in Table 1. It can be seen from Table 1 that an increase in V₂O₅ nanoparticle concentration within the PVA/CMC polymer matrix lowers both direct and indirect forbidden transition energies (shifts them towards lower energies). This is attributed to PVA/CMC disorder, which is caused by different localized states created within the band gap between valence and conduction bands as a result of defects formed in the PVA/CMC polymeric matrices. These additional states trap electrons between bands; hence, the optical band gap decreases.

Close

Figure 5.

Relation between photon energy and (ahn)² for the studied samples.

Export to PPT

Close

Figure 6.

Relation between photon energy and (ahn)^3/2 for the studied samples.

Export to PPT

The extinction coefficient characterizes the exclusive attribute of a substance’s composition, indicating its capacity to absorb light at a specific wavelength. It is determined by evaluating the proportion of light that is scattered or absorbed within each unit of average distance. Furthermore, the extinction coefficient (k), can be determined using the relation $k=\frac{\alpha \lambda }{4\pi }$, measures the fraction of light that is lost as a result of scattering and absorption over a given unit of distance.

Figure 7 shows the relation between the extinction coefficient ($k$) as a function of incident light wavelength for the nanocomposite films. The decrease of k with increasing the wavelength within the range of 200-250 nm (in the UV region) implies that higher energy photons can be absorbed, causing faster transfer rates between different states. Thus, the lower energy will scatter or reflect into the film, giving rise to a lower k value. However, higher values of k were observed at longer wavelengths because they were not able to excite this amount of electron movement through the material as their shorter counterparts did above them, instead scattering more frequently off atoms along their respective paths before coming out the other end. So, the larger k values are recorded at those regions. Additionally, it is noticed from the figure that as V₂O₅ concentration increases in the PVA/CMC matrix, so do k values go up accordingly; this is mainly because the absorption coefficient for these types of polymers tends to become higher with an increase in concentration.

Close

Figure 7.

The relation between the extinction coefficient ($k$) as a function of incident light wavelength for the nanocomposite film.

Export to PPT

Transmittance data also provides another way of calculating refractive index (n) (Eq. 3) from the following equation [36,37]:

The refractive index (n) is plotted against wavelength for PVA/CMC-V₂O₅ nanocomposite films as shown in Figure 8. At low frequency, the refractive index (n) drops rapidly before leveling off across longer visible wavelengths (λ > 400 nm) – this trend holds regardless of film composition or increasing amounts of dopant V₂O₅ concentration. The values for refractive index increase stepwise up until about 1.52 for pure PVA/CMC blend, then they rise more sharply around 2.67 for the higher content of V₂O₅ nanoparticles, attributed to enhanced packing density associated with increased levels of V₂O₅ incorporation into PVA/CMC polymer matrices.

Close

Figure 8.

Relation between the extinction coefficient ($k$) as a function of incident light wavelength for the nanocomposite film.

Export to PPT

The optical dielectric properties, such as real (${\epsilon }_r$) and imaginary (${\epsilon }_i$) parts (Eqs. 4,5) can be estimated from the equations [26,38]:

Figures 9 and 10 show the relation between ${\epsilon }_r$ and ${\epsilon }_i$ as a function of the photon energy (hn). It illustrates that the values of ${\epsilon }_r$ and ${\epsilon }_i$ increase with the concentration of V₂O₅ in the polymeric matrices. This is consistent with their dependence on k and n values. Furthermore, more defects and structure disordering in polymer films lead to a higher optical dielectric constant. For example, starting from 0.07 for pure PVA/CMC blend up to 8.21 for PVA/CMC- V₂O₅ nanocomposite films are observed values of ${\epsilon }_r$. Similarly, from 0.0063 for pure PVACMC to 0.064 for PVA/CMC- V₂O₅ nanocomposite films are found as values of ${\epsilon }_i$. The real part relates to the refractive index and describes how much the electric field is slowed down in the material. Therefore, virgin polymer blend (PVA/CMC) shows a gradual decrease with increasing photon energy, and sometimes small peaks or shoulders are reported due to electronic transitions in the polymers. Higher values compared to the pure polymer blend were expected for the PVA/CMC blend containing a variable mass fraction of the V₂O₅ blend due to the higher refractive index of V₂O₅, which may show additional features or peaks corresponding to electronic transitions in V₂O_5, and increasing V₂O₅ content should lead to an overall increase in ε₁ values. The higher ε_r values indicate stronger interaction between the electric field and the material, while the gradual increase in ε_r with V₂O₅ content suggests an increase in the material’s polarizability.

Close

Figures 9.

Relation between ${\epsilon }_r$ and the photon energy (hn).

Export to PPT

Close

Figures 10.

Relation between ${\epsilon }_i$ and the photon energy (hn).

Export to PPT

The imaginary part relates to absorption and describes energy loss in the material. Therefore, virgin polymer blend (PVA/CMC) usually shows low values in the visible range, indicating transparency and exhibiting peaks in the UV region due to electronic transitions in the polymers. Higher values compared to the pure polymer blend were expected for the PVA/CMC blend containing a variable mass fraction of the V₂O₅ blend, especially in the visible and UV regions. ε_i may show characteristic absorption peaks of V₂O₅, which could become more pronounced with increasing V₂O₅ content, and the overall magnitude of ε_i was observed to increase with higher V₂O₅ mass fraction content, indicating enhanced light absorption and potential color changes. The overall optical dielectric may be affected by the anisotropy of vanadium oxide nanoparticles having a preferred orientation in the polymer matrix that shows directional dependence and particle size effects exhibiting quantum confinement effects, potentially altering the dielectric function compared to bulk V₂O₅. Additionally, the interface between V₂O₅ and the polymer matrix could contribute to the overall dielectric response.

The energy dissipation characteristics of high-energy electrons can be effectively described by two prominent parameters: the volume energy loss function ($VELF$) (Eq. 6) and the surface energy loss function ($SELF$)(Eq. 7). These parameters can be computed using the following mathematical equations [36]:

Figures 11 and 12 show the relationship between VELF and SELF values ​​as a function of photon energy.

Close

Figures 11.

Relationship between VELF and photon energy.

Export to PPT

Close

Figures 12.

Relationship between SELF and photon energy.

Export to PPT

At the molecular level, V₂O₅ nanoparticles interact with the PVA/CMC polymer blend through hydrogen bonding between the surface oxygen atoms of V₂O₅ and the hydroxyl groups of PVA/carboxylate groups of CMC, as well as potential coordination complexes formed between vanadium ions (V⁵⁺) and electron-rich oxygen sites in the polymers. These interactions create a crosslinked network that improves nanoparticle dispersion and interfacial adhesion within the matrix. The hydrogen bonding and charge-transfer complexes significantly enhance charge transport by providing efficient hopping pathways for electrons and ions, as evidenced by the 40% increase in AC conductivity at optimal V₂O₅ loading. Additionally, the polar groups in the polymers align under electric fields, while V₂O₅’s high permittivity amplifies interfacial polarization, leading to an 11-fold increase in dielectric constant (from 0.07 to 8.21). The intercalation of ions into V₂O₅’s layered structure, combined with the polymer’s expanded interchain spacing (indicated by the XRD halo shift from 25° to 19° 2θ), further enhances charge storage capacity. These synergistic effects collectively improve the nanocomposite’s performance in energy storage applications by optimizing both charge transport and dielectric properties.

3.4. Electrical characterization

3.4.1. AC conductivity

Figure 13 shows the plot between log(σ) versus log (f) at room temperature for the PVA/CMC blend and PVA/PCMC blend doped with different amounts of V₂O₅ nanoparticles. The conductivity behavior of all the prepared nanocomposites increases with frequency (Log f) and V₂O₅ concentration. For all filler concentrations in V₂O₅-(PVA/CMC), conductivity increases as frequency increases.

Close

Figure 13.

A plot between log(σ) versus log (f) at room temperature for the PVA/CMC blend and PVA/PCMC blend doped with different amounts of V₂O₅ nanoparticles.

Export to PPT

The conductivity spectra of all samples exhibit two distinct linear dispersion regions. The nanocomposites demonstrate higher electrical conductivity compared to the pristine PVA/CMC blend, confirming that the incorporation of V₂O₅ nanoparticles enhances charge transport. This improvement arises from increased interfacial polarization upon nanofiller addition, facilitating ion hopping along the polymer chains. The conductivity enhancement is attributed to (i) greater amorphous regions promoting charge carrier mobility and (ii) the formation of percolation pathways through interconnected V₂O₅ networks. These networks enable efficient charge transfer by reducing structural constraints at phase interfaces, allowing unrestricted carrier movement between adjacent particles and polymer phases. Consequently, the system achieves rapid electron transfer through optimized conduction pathways, leading to the observed rise in room-temperature conductivity.

For all samples, it is noticed that the electrical conductivity improved as the frequency increased. At lower frequencies, the different V₂O₅ concentrations affect conductance in nanocomposites. Relaxation is due to the migration of electrons, which can be explained by looking at how conductivity varies with frequency. Two forms of movement are postulated; one involves hopping, while the other involves tunneling between equilibrium points. The dependency on the frequency of conductivity is given by Juncher’s power law, and total conductivities are expressed as sums of AC and direct conductivity (DC) conductivities (Eq. 8):

(8)

${\sigma }_{total}={\sigma }_{DC}+{\sigma }_{AC}$

3.4.2. Dielectric analysis

Dielectric properties reveal the amount of charge materials can store, and they indicate how much ions or free charge carriers affect a material’s electrical conductivity. The complex dielectric constant, ${\epsilon }^{*}$ (Eq. 9) is calculated using the Debye model as [39,40]:

(9)

${\epsilon }^{\text{*}}={\epsilon }^{\prime }+i{\epsilon }^{″}$

where ${\epsilon }^{\prime }$is the dielectric constant and ${\epsilon }^{\prime }$ is the dielectric loss. Figures 14 and 15 represent the frequency (f) dependence of both ${\epsilon }^{\prime }\text{and} {\epsilon }^{″}$ for PVA/CMC-V₂O₅ nanocomposite films. Both figures exhibit the same trend, which is gradually decreasing with the frequency (f) and stabilizing at a fixed value in higher frequency regions. At lower-frequency regions, doped polymer nanocomposite materials and values of ${\epsilon }^{\prime }\text{and} {\epsilon }^{″}$ because of interfaces between the PVA/CMC blend itself, as well as a metal electrode, or even longer time taken by dipoles to interact with the applied field before reaching a steady state. The observed dielectric behavior can be explained by the Maxwell-Wagner two-layer model for heterogeneous systems. At low frequencies, the significant dispersion arises primarily from space charge polarization effects at the electrodes. As frequency increases, both the real (ε’) and imaginary (ε’’) components of the complex permittivity decrease due to the reduced response time available for dipole alignment and interfacial polarization. This frequency-dependent response eventually saturates in the high-frequency region, where ε’ and ε’’ become nearly frequency-independent as the polarization mechanisms can no longer follow the rapid field oscillations. The transition between these regimes reflects the characteristic relaxation processes in the material, with the low-frequency response dominated by interfacial effects and the high-frequency behavior governed by intrinsic material properties.

Close

Figure 14.

Dependence of both ${\epsilon }^{\prime }$on frequency f.

Export to PPT

Close

Figure 15.

Dependence of both ${\epsilon }^{″}$on frequency f.

Export to PPT

The AC conductivity data of the PVA/CMC-V₂O₅ nanocomposites exhibit frequency-dependent behavior consistent with Jonscher’s power law (σ_AC=A ω^s σ_AC​= A ω^s), where σ_total=σ_DC+A ω^s σ_total​= σ_DC​ + A ω^s. The observed increase in conductivity with frequency (Figure 13) and the linear dispersion regions confirm this trend, attributed to hopping/tunneling mechanisms. Fitting parameters (e.g., exponent s and prefactor A) would typically reflect the degree of ionic/electronic interactions which would quantify the role of V₂O₅ in enhancing charge transport.

3.4.3. Impedance spectroscopy

Figure 16 depicts the real part of impedance ($z^{\prime }$) versus frequency (Log f) for PVA/CMC-V₂O₅ nanocomposites. The enhancement in the $z^{\prime }$ with the enrichment of V₂O₅ concentration in PVA/CMC blend films at the lower frequency region is observed. As the frequency increases, the values of $z^{\prime }$ decreases, which means that AC conductivity increases with the increasing frequency range of all samples. The integration of $z^{\prime }$at higher frequency for all the samples indicates the release of space charge as a consequent decrease of barrier potential in the nanocomposite films. The increase in V₂O₅ concentration in the polymer blend enhances resistivity at a lower frequency.

Close

Figure 16.

The real part of impedance ($z^{\prime }$) versus frequency (Log f) for PVA/CMC-V₂O₅ nanocomposites.

Export to PPT

Figure 17 depicts the imaginary part of impedance ($z^{″}$) versus Log f for the prepared nanocomposites. The behavior of $z^{\prime }$ is not uniformly distributed. The values of $z^{″}$ increase initially and then decrease with a higher frequency range. The merging of all $z^{″}$ values at higher frequency and lower frequency range signifies the process of build-up of space charge polarization effect in all the samples. Both real ($z^{\prime }$) and imaginary ($z^{″}$) parts of impedance decrease as frequency increases, which means there exist relaxation processes within nanocomposite films.

Close

Figure 17.

The imaginary part of impedance ($z{\text{'}}^{\text{'}}$) versus frequency (Log f) for PVA/CMC-V₂O₅ nanocomposites.

Export to PPT

Figure 18 shows the Cole-Cole plot [41] for a V₂O₅-doped PVA/CMC blend. The electrical resistance of PVA/CMC samples increases as the concentration of V₂O₅ in them increases; this is opposed to the applied frequency. The impedance characteristics of the material are governed by both bulk resistance (R_b) and grain boundary resistance (R_g), with their relative contributions being strongly influenced by V₂O₅ concentration. Increased V₂O₅ loading leads to enhanced bulkiness, which elevates both R_b and R_g, as evidenced by the expansion of the impedance arc area in Nyquist plots. This behavior indicates two concurrent processes: (1) intensified bulk polarization effects due to higher vanadium content, and (2) impeded ionic migration through the material matrix. The observed resistance enhancement follows trends seen in analogous heterogeneous systems, where filler incorporation modifies both intra-grain and inter-grain charge transport pathways. The correlation between V₂O₅ concentration and impedance response suggests that charge carrier mobility is increasingly constrained by structural modifications at higher filler loadings, consistent with Maxwell-Wagner polarization mechanisms in composite materials.

Close

Figure 18.

Cole-Cole plots for a V₂O₅-doped PVA/CMC blend.

Export to PPT

While previous studies have explored PVA/CMC blends [19,20] or V₂O₅ composites [20,21] independently, this work introduces three critical advancements including dual-functionality design: Unlike conventional PVA/CMC systems limited to structural applications or V₂O₅ composites optimized solely for conductivity, our nanocomposite simultaneously achieves tunable optical bandgap (4.1→3.2 eV) and enhanced dielectric properties (εᵣ = 0.07→8.21), addressing the longstanding trade-off between transparency and charge storage. Interfacial engineering: Whereas prior V₂O₅ composites [22, 24] suffered from nanoparticle agglomeration, our solution-casting protocol enables homogeneous dispersion up to 0.08 wt% V₂O₅, evidenced by XRD’s shifted amorphous halo (25°→19° 2θ), a structural modification unreported in PVA/CMC literature [19,30] and, mechanistic unification: We establish the first quantitative relationship between V₂O₅-induced polymer chain separation (XRD), charge transfer complexes (FTIR), and macroscopic dielectric enhancement (40% conductivity increase), resolving inconsistencies in earlier studies [18,33] that treated these properties independently. This systematic approach unlocks new opportunities for multifunctional energy materials.