3.1 Structural Properties
Figure 1 shows the XRD patterns of the MgO-PVA nanocomposite films at different weight percentages, as well as pristine PVA film and MgO nanoparticles in powder form. The XRD pattern of pristine PVA film (Fig. 1(b)) describes the semi-crystalline nature of PVA polymer. PVA exhibits a distinct peak at 19.68 Å, which matches to (101) crystalline phase of the polymer. The d-spacing was found to be 4.59 Å. Another low-intensity peak was seen at 40.9 Å, confirming the semi-crystalline nature of pristine PVA. This might be owing to significant intra-molecular –OH bonding between the different PVA monomer segments[35]. Gh. Mohammed et al.[21] also reported two diffraction peaks for PVA around 2θ = 19.6 Å and 40.9 Å.
The XRD pattern of MgO nanoparticles confirms its crystalline nature. The diffractogram of MgO NP shows peaks at 2θ = 37.13◦, 43.14◦, 62.66◦, 75.14◦, and 79.11◦ which corresponds to the reflection planes of the MgO cubic phase with space group Fm-3m (Periclase, JCPDS No. 96-900-6787). The observed characteristic diffraction peaks of MgO NP are disappearing from the XRD spectrum of MgO-PVA PNC films. (Figure.1). This result suggests that the nanoparticles lose their crystallinity when reinforced into the polymer matrix [8]. Additionally, this implies that when MgO NPs are integrated into PVA, they may fill the interstitial space between the polymer chains, causing the powder MgO NP to lose its intrinsic crystallinity[33]. Moreover, it was observed that the intensity of all the (101) peaks of PVA is reduced suggesting that the addition of MgO nanofiller deteriorates the crystallinity of the PVA matrix[5]. Also, it depicts that the H-bond of the -OH group in the PVA chains has been broken, and the molecular chains are all now free to revolve. The macroscopic characteristics of a polymer are determined by its degree of crystallinity (XC). X-ray diffraction (XRD) data is utilized to determine the crystallinity of the samples, and these results are presented in Table 1. The electrical characteristics of polymer composites are extensively documented in the literature to be proportional to their crystalline structure. A higher amorphous region enhances the electrical conductivity in the samples[36].
Table 1
Calculated crystallinity values of prepared samples using obtained XRD Data
| Name of the sample | Crystallinity XC |
| Pure PVA | 61.24 |
| PM1 | 71.27 |
| PM2 | 59.38 |
| PM3 | 57.33 |
| PM4 | 56.29 |
3.2 Chemical properties
To investigate the chemical interaction of PVA with MgO nanofiller, FTIR transmission spectra for the PNC films were analyzed (Fig. 2). A broad and strong absorption peak in the FTIR spectrum of PVA, spanning 3000 cm-1 to 3700 cm-1, peaking at 3281cm-1 can be attributable to –O-H symmetrical stretching vibrations. Such a vibrational peak indicates the existence of significant intermolecular hydrogen bonding between the nanofiller and polymer matrix. With the incorporation of MgO nanofiller in the PNC, the –OH stretching vibration peak was discovered to be shifted towards to the lower wavenumber. The augmentation of MgO nanofiller into the PVA matrix dissociates the hydrogen bonding (–OH) in the hydroxyl group of PVA, which in turn results in the formation of hydrogen bonds between the MgO nanofiller and –OH chain of PVA. The band detected at 2924 cm− 1 identified the C − H stretching vibrations of the − CH2 − alkyl group. The absorption band between 1300 cm− 1 to 1500 cm− 1 peaking at 1422 cm− 1 suggests the presence of –CH/CH2 deformation vibrations. The bands at 1705 cm− 1 and 1527 cm− 1 are attributed to C = C stretching and C = O stretching mode, respectively. The moderate absorption peak detected at 1242 cm− 1 is responsible for a C–H group. The intensity of the absorption band at 1242 cm− 1 in pure PVA was shifted to 1254 cm− 1 with the addition of MgO nanofiller, whereas it decreased completely as the MgO content increased. The sharp absorption peak at 1066 cm-1 was identified as an ether stretching band (C–O) in the C − O−H groups. Thus, all of the PVA characteristic peaks are consistent with earlier studies. Furthermore, most of the characteristic peaks of PVA are also detected in all of the PVA-MgO PNC films.
Finally, deviations in the peak positions of FTIR transmittance spectra after the incorporation of MgO nanofiller confirm the interaction of MgO nanofiller with the PVA matrix, while decreased intensity in some of the peaks revealed the alteration in the crystalline regions of PNC films[37]. These results are consistent with PXRD measurements. All findings point to the creation of a chemical interaction between PVA molecules and MgO nanofiller. This type of interaction is predicted to change the mobility of the PVA chain, and hence altering the electrical and dielectric characteristics of the PNCs [8]which have been investigated in the next section.
3.3 Surface Properties and Elemental Analysis
The surface morphology of prepared PNC films is studied using FESEM analysis. Figure 3 shows FESEM images of MgO-PVA PNC films (PM2 sample) in different magnifications. Due to its improved crystallinity and favourable dielectric properties, the PM2 sample is employed to study surface morphology. The white spots with irregular shapes are observed in the images representing the MgO NPs. It is evident from the images that MgO NPs are evenly distributed in the PVA polymer matrix leading to the formation of localized domains and phase segregations in the sample. The abundance of oxygen atoms present in the MgO nanofiller, facilitates the interaction between PVA and MgO through hydrogen bonding[5]. The FESEM images of pure PVA demonstrate a clean morphology devoid of any impurities.
The EDAX analysis of nanocomposites is often performed to check the chemical compositions and uniform dispersion of filler material in the polymer matrix. Figure 3 illustrates the combined EDAX spectra of MgO-PVA PNC film (PM2 sample). The EDAX spectrum of the PM2 sample displays prominent peaks of Carbon (C), Oxygen (O) and Magnesium (Mg), as predicted from the stoichiometry of the film, without any signature of other elements, hence confirming the purity of the sample.
3.4 Dielectric Properties
Figure 4 (a) exemplifies the frequency-dependent behavior of the real part of dielectric permittivity (Ɛ’) as a function of frequency for different weight percentages of MgO NPs in the PVA polymer matrix at ambient temperature. It is observed that all the PNC films exhibit the same dielectric frequency dispersion characteristic, with the dielectric constant being high at lower frequencies and low at higher frequencies. With rising frequency, the dielectric constant falls progressively before remaining constant at higher frequencies. These results are well explained using electrode polarization and space charge effects Similar observations are reported in graphene-doped PVA films[38]. The electrode effect, involves charge carriers being inhibited at the electrodes and interfacial effects in samples.
In polymer composites, the alignment of dipoles with the applied electric field during field reversal is more pronounced in the low-frequency region compared to the high-frequency range. This causes a decrease in the dielectric constant (Ɛ') as the frequency increases, as a result of polarization effects and reduced dipole orientation. This behavior aligns with the Maxwell-Wagner Sillar polarization phenomenon.
Experimental measurements reveal that as the concentration of MgO nanoparticles (NP) in the PVA matrix increases, the dielectric constant (Ɛ') also rises across all measured frequencies. This behavior can be attributed to the formation of interface regions between the PVA chains due to the addition of MgO nanoparticles. When conductive fillers are incorporated into an insulating polymer matrix, these interface regions play a crucial role in determining the dielectric constant of the composite films.
The observed high dielectric constant in the composite films is also influenced by the space charge polarization effect. This phenomenon occurs due to the movement of charge carriers that are confined by barriers in the composite structure. According to the theoretical model proposed by Vo and Shi[39], the concentration of conductive fillers in the samples results in a greater accumulation of free charge carriers at the interface areas. This allows for faster polarization, leading to an increase in the dielectric constant (Ɛ').
The results obtained in the present study are in accordance with the model since MgO NP is having larger aspect ratio leading to the formation of bigger interface regions within the polymer matrix[39]. Thus, the value of Ɛ’ increases with increasing the concentration of MgO in the PNC films [38]. However, the increase in Ɛ’ value is not uniform with an increase in MgO concentration in the PNC films, out of which the PM2 sample shows a high value for dielectric constant. Figure 4(b) depicts the frequency dependence of the imaginary part of dielectric permittivity (Ɛ’’) for all prepared PNC films at constant temperature. It follows the same pattern as that of the real part of dielectric permittivity (Ɛ’). It is noted the Ɛ’’ decreases exponentially as a function of increasing applied frequencies till it reaches a frequency of 104 Hz, after which it remains almost constant throughout the frequency range which explains the polar nature of PVA polymer[36]. This plateau region of the graph is due to lower values of Ɛ’’ at higher frequencies, pointing to them being non-lossy dielectric materials[11]. Further, it is the prerequisite behavior of the materials for using them as dielectric materials in practical applications. The high value of dielectric loss in the lower frequency can be attributed to the interfacial polarization, where dipoles will get adequate time to orient themselves in the field direction [40]. Conversely, at a high-frequency range, the polymer dipoles fail to polarize along with the field because of the rapid reversal of the polarity, resulting in a decrease in the dielectric loss value[36]. Also, it can be seen that dielectric loss (Ɛ’’) increases monotonically with the loading concentration of MgO NP. The steep increase in Ɛ’’ can be ascribed to the formation of mobile charges within the polymer chains[41]. Moreover, it can be comprehended in terms of electrical conductivity, which is linked to dielectric loss.
The above-mentioned dielectric loss is commonly quantified in terms of the loss tangent (tanδ). Figure 4(c) illustrates the tanδ of the synthesized polymer nanocomposites at room temperature on frequency variation. The correlation between tanδ and frequency for prepared samples reveals some noteworthy behavior. The graph can be divided into three parts, in the lower frequency region tanδ rises with frequency owing to an increase in Ohmic property; in the higher frequency band, a reduction is attributed to the growing nature of the reactive component over Ohmic component[42]. In addition to that the curve achieves a maximum value at a particular frequency that corresponds to the maximal energy transfer. All of the graphs show the same trend, with a peak at a certain frequency, whilst peaks shift towards a higher frequency with a loading concentration of MgO. Shifting of peaks towards the high-frequency side indicates a drop in relaxation time, and thereby quicker segmental relaxation[43]. On the other hand, there is a massive shift in the peak even on less doping concentration (that is PM1) compared to pure PVA film. Though, there is a shift in all peaks PM2 sample is showing a subtle change in the measured tanδ values compared to other PNC films. Similar results are observed for dielectric constant as well as dielectric loss measurements. The increase in loading concentration of MgO filler correlates to an increase in matrix amorphicity, which is clear from XRD results. In turn, the amorphous nature results in an increase in the mobility of charge carriers in the polymer matrix. Finally, all the results strongly suggest the efficient dispersion of nanofiller material in the matrix.
The conduction mechanism in PNCs provides useful information regarding the behavior of electric charge carriers in the system, which can be well understood by measuring its conductivity( σac )[44]. In this respect, the dependence of σac on frequency was studied. Figure 4 (d) depicts the correlation between log (σac) and frequency for pure PVA and all prepared PNC films. The poor ac conductivity value in the low-frequency area may be attributed to a lack of mobile charge carriers available for hopping from one polarized state to another caused by the substantial polarization effect happening at the electrode and interfaces[36]. On the other hand, at a higher frequency range, notable nonlinearity in the curves can be seen with increased slope values depicting a considerable rise in AC conductivity. Similar dispersion behavior was seen in all the samples. This might be linked to an increase in both mobility and the number of charge carriers due to an increase in filler concentration[40]. Among other PNC films, the PM2 sample exhibits enhanced conductivity of about 1.175x10^-5 S/cm-1, whereas the pure PVA film had a conductivity of 1.6474x10^-7 S/cm-1. At higher concentrations, values of all the dielectric parameters are less compared to the PM2 sample, which may be due to the agglomeration of nanofiller.[43]
3.5 Impedance Spectroscopic Analysis
The diagram, Fig. 5(a-b) depicts the impedance plots of the real part Z′ and imaginary part Z″ with respect to the applied frequency for prepared samples at ambient temperature, respectively. The real impedance of the pure PVA film exhibited nonlinear behavior at lower frequencies up to 100 kHz, after which it remained steady at higher frequencies. In MgO-incorporated PNC films, Z′ was observed to be less dispersive compared to pure PVA film. However, on reinforcement of MgO NP, the value of Z′ decreased tremendously. The value of real impedance (Z′) drops with increasing frequency for all samples and tends to merge at a higher frequency range due to the likely release of space charges[45].
The Cole-Cole plot is a graphical representation of the complex impedance of a material as a function of frequency[46]. The plot consists of a semicircle portion and a tail or straight-line portion, which provide information about the bulk and interfacial properties of the material, respectively. The Cole-Cole plot can be used to study the electrical properties of materials such as pure and doped MgO PVA films. Figure 5 (c-d) of the article includes a collection of Cole-Cole graphs that show the frequency-dependent electrical response of PVA and PVA/MgO nanocomposites. The results for four distinct MgO nanoparticle compositions in the PVA matrix are displayed in the graphs, illustrating the impact of nanoparticle concentration on the electrical characteristics of the composite material.
When a small amount of MgO nanoparticles is added to the PVA matrix, the resulting composite exhibits a Cole-Cole plot with a depressed semicircle and an inclined straight line. The intercept of a depressed semicircle with the Z′ (real) axis in the impedance plots shows the bulk resistance (Rb) of each PNC film [40]. The position of the semicircle depends on the concentration of MgO nanoparticles. As the concentration of MgO nanoparticles in the PVA matrix is increased, the semicircle portion of the Cole-Cole plot shifts towards lower frequencies and the tail portion becomes more pronounced. This is due to the increase in interfacial polarization and the corresponding increase in the contribution of the interfacial properties to the overall electrical response of the composite[47]. As a result, the electrical response of the composite is influenced by both the bulk and interfacial properties of the material.
The comparison of the all-dielectric and impedance data demonstrated that the addition of MgO nanoparticles to the PVA matrix improved the electrical conductivity of all MgO/PVA nanocomposites. The data also indicated that the optimal concentration for enhancing charge transport within the nanocomposite was at a particular composition of PM2. However, in some cases, low conductivity could be due to the agglomeration of MgO nanoparticles, which increased the intergranular distance between the grains and thus caused higher resistance due to the nanoparticles' high surface area.
The study analyzed the electrical characteristics of the nanocomposites and discovered that the material's ability to store electrical charge (measured by the dielectric constant) and the energy dissipated as heat during charge storage (measured by the dielectric loss) increased with the rise in MgO nanoparticle concentration. Furthermore, the addition of MgO nanoparticles resulted in a reduction in electrical resistance and an increase in electrical capacitance. This outcome implies that incorporating MgO nanoparticles in PVA enhances the material's electric charge transport.
Overall, the current study shows that the addition of MgO nanoparticles in PVA can improve the electrical conductivity of the nanocomposite material, but the ideal concentration needs to be established. Additionally, the research emphasizes the significance of nanoparticle dispersion in the polymer matrix for achieving the highest conductivity.