A Comprehensive Study of Structural, Thermal, and Dielectric Properties of Melt-Processed Polypropylene/Ni_0.9Zn_0.1Fe₂O₄ Nanocomposites

Abstract

This article explores the processing of structural, thermal, and dielectric properties of polypropylene (PP) polymer nanocomposites modified with Ni_0.9Zn_0.1Fe₂O₄. The PP/Ni_0.9Zn_0.1Fe₂O₄ nanocomposites are manufactured by the melt-processing method using a Brabender Polyspeed B. The XRD and FTIR structural investigations assure good incorporation of Ni_0.9Zn_0.1Fe₂O₄ into the PP matrix. It should be noted that adding Ni_0.9Zn_0.1Fe₂O₄ NPs to the PP polymer matrix enhances the polymer’s thermal stability. Utilizing the Coats–Redfern model, kinetic thermodynamic parameters such as activation energy (Ea), enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG*) are deduced from TGA data. The dielectric results showed an increase in ε′ with the introduction of nanoparticles into the PP matrix. As the content of Ni_0.9Zn_0.1Fe₂O₄ NPs in these nanocomposite films increases, the loss tangent values decrease at higher frequencies while increasing at lower frequencies. The estimated ε_s and ε_∞ of PP nanocomposites using Cole–Cole plots reveal an improvement when NPs are added to PP. We believe that the proposed work suggests a relevant step towards the practical application of PP/Ni_0.9Zn_0.1Fe₂O₄ nanocomposites.

1. Introduction

The thermal, mechanical, and electrical properties of polymer nanocomposites have generated great interest in academic research [1,2,3,4]. Polymer nanocomposites can have enhanced thermal stability, improved heat resistance, and higher thermal conductivity. The thermal conductivity of polymer nanocomposites can be increased by the addition of nanoparticles with high thermal conductivity, such as graphene, carbon nanotubes, or metal particles. Surprisingly, even small amounts of nanofillers can revolutionize the properties of polymer composites compared to pure polymer [5,6,7,8,9]. Therefore, polymer nanocomposites have found wide application in many fields, including electromagnetic shielding, charge storage capacitors, microwave absorbers [10,11,12], EM detectors [13], optical integrated circuits, sensors, medical devices, aerospace, packaging materials, consumer goods, and so on [14]. Various scientific methodologies can be used to adapt the physical properties of polymeric nanomaterials for a specific application. The physical and chemical properties of polymer nanocomposites are related to the shape, size, and content of the nanoparticles [15,16,17,18,19,20,21,22,23,24]. Since it offers amazing properties that can be used in catalysis, biomedicine, high-density information storage devices, transformer cores, electromagnetic interference (EMI) suppressors, rod antennas, gas detecting, and environmental remediation, nano-ferrite is subject to substantial research [25,26,27,28,29].

Polypropylene (PP) is readily available and cost-effective from refineries across the world. Its low density makes it easy to process using melt-processing methods [30,31,32,33]. Furthermore, its high chemical resistance makes PP ideal for packaging, automobiles, and other applications [34,35,36]. Therefore, scientists have been actively studying polypropylene nanocomposites to enhance the physical properties of pure PP [35,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54]. There are research works related to the preparation and investigation of thermal and dielectric properties of polypropylene nanocomposites. This is an active area of research with great potential for developing new materials with improved properties. Siddiqui et al. [40] successfully manufactured new magnetic bionanocomposites of graphene bonded to magnetically modified polypropylene (as the polymer matrix). The overall conductivity of the composites was significantly improved. Rheological experiments demonstrated that increasing the nanofiller concentration led to an enhancement of viscoelastic characteristics, exhibiting more solid-like behavior. The dispersion of the nanofillers within the PP matrix created conductive connections, resulting in innovative PP/MCNC composites with an 800% increase in shielding efficiency compared to pure PP. Bendaoued et al. [43] demonstrated that nanofillers were synthesized using the sol-gel process and incorporated as fillers in polypropylene composites to enhance the thermal, rheological, and interfacial compatibility of the final material. The addition of nano-TiO₂ improved the thermal degradation stability of polypropylene. The results revealed that incorporating nano-TiO₂ into the polypropylene matrix increased the degree of crystallinity. Due to its superior thermal and viscoelastic properties, including high storage and loss moduli and degree of crystallinity, the nanocomposite PP@5% TiO₂ is a suitable candidate for application in the manufacture of recycled wind turbine blades. Rishaban et al. [42] referred to the significantly improved mechanical and thermal stability of the created polymer/multi-layered graphene nanocomposites. P. Tambe et al. [32] demonstrated that the NaOH-treated halloysite nanotubes (H.N.Ts) are superior fillers for reinforcing the PP matrix. This modification is cost-effective for the industry because NaOH is available in large quantities at a lower cost than other synthetic organic modifiers. In comparison to clean PP and H.N.Ts-filled PP nanocomposites, the mechanical characteristics enhancement is greatest for the 3 wt.% NaOH-treated H.N.Ts-filled PP nanocomposites. Uo Uyor et al. [39] studied the mechanical, thermal, and dielectric properties of sandwich-structured BN-BaTiO₃-BN/PP nanocomposites. Hydrothermal and assembly processes were used to create the BN-BaTiO₃-BN sandwich nanoparticles. A rheometer prepared the nanocomposites using the melt compounding process. The prepared PP nanocomposites exhibited improved thermal properties above 20 °C. Additionally, the dielectric constant increased by nearly 132%, from 2.02 at 100 Hz for pure PP to 4.68 for PP/5BN-15BT nanocomposite. Moreover, the nanocomposite maintained a low loss of approximately 0.05 at 100 Hz. L.G. Furlan et al. [38] discussed the role of processing conditions on the thermal and mechanical properties of PP nanocomposites. Using a co-rotating twin-screw extruder, polypropylene montmorillonite (PP-MMT) nanocomposites were fabricated. Their findings demonstrated that a medium shear intensity profile, as opposed to a high one, produced a greater improvement in mechanical properties. The dispersion and interaction of MMT particles within the PP matrix influenced the reinforcement effect. Patil et al. [35] studied the mechanical and thermal properties of polypropylene (PP)/multiwall carbon nanotubes (MWCNTs) nanocomposites. They fabricated the nanocomposites using compression molding equipment. The mechanical and thermal behavior of the nanocomposites was then evaluated using ASTM standards. The results revealed that adding MWCNTs to the PP matrix enhanced both mechanical and thermal properties. The highest tensile strength (62.80%) was measured at 1.2 wt.% MWCNT loading. At 1.5 wt.%, impact strength and hardness increased by 82.14% and 12.44%, respectively. However, low weight percent increases were accompanied by subsequent drops in glass transition temperature.

The addition of Ni_0.9Zn_0.1Fe₂O₄ nanoparticles as a nanofiller to a polypropylene (PP) polymer matrix can have significant impacts on the structure, thermal, and dielectric properties of the resulting composite material. Ni_0.9Zn_0.1Fe₂O₄ nanoparticles exhibit ferrimagnetic behavior [55], possess high thermal stability [56], and have higher dielectric constants [57] compared to the polymer matrix. Therefore, the introduction of Ni_0.9Zn_0.1Fe₂O₄ nanoparticles can improve the thermal stability and dielectric constant of the composite.

The mechanisms by which nanoparticles influence the thermal and dielectric properties of polymers are still not fully understood. Further research is needed to elucidate these mechanisms, which will enable the rational design of nanocomposites with desired properties. Therefore, in current research, PP/Ni_0.9Zn_0.1Fe₂O₄ nanocomposites were synthesized using the melt-processing method. The aim of this research is to investigate the effect of adding Ni_0.9Zn_0.1Fe₂O₄ nanoparticles (NPs) to the PP matrix on the structural, thermal, and dielectric properties. XRD and FTIR analysis to confirm the dispersion and interaction between the NPs and the polymer. The addition of NPs has improved the thermal resistance of the polymer, potentially leading to applications in high-temperature environments. Using TGA data and the Coats–Redfern model, the activation energy, enthalpy, entropy, and Gibbs free energy associated with the thermal degradation process were determined. The introduction of NPs influences the dielectric constant (ε’) and loss tangent of the material. By analyzing the Cole–Cole plots, the static and high-frequency permittivities (ε_s and ε_∞) of the nanocomposites were estimated, potentially leading to advancements in electrical insulation or dielectric materials.

2. Materials and Methods

2.1. Materials

Ferric nitrate AR (Fe(NO₃)₃·9H₂O, M.W 404, ALPHA CHEMIKA, Maharashtra, India 99%), nickel nitrate AR (Ni(NO₃)₂·6H₂O, M.W 290.8, Nile Company, Cairo, Egypt, 99%), zinc nitrate AR (Zn(NO₃)₂·₆H₂O, M.W 297.49, Cairo, Egypt, 99%), commercial bovine gelatin (Foodkum, Cairo, Egypt), and polypropylene raw material PPR-MS16 (GM1600E, SINOPEC, Beijing, China) were used for the preparation of PP/Ni_0.9Zn_0.1Fe₂O₄ nanocomposites.

2.2. Sample Preparation and Characterization Techniques

According to Reference [58], Ni_0.9Zn_0.1Fe₂O₄ nanoparticles were synthesized using the green sol-gel autocombustion process. Nanocomposites composed of PP and Ni_0.9Zn_0.1Fe₂O₄ nanoparticles were then successfully fabricated via the melt-processing technique. The ratios of Ni_0.9Zn_0.1Fe₂O₄ were 0.0, 5.0, 10, and 15 wt%, as shown in

Table 1. Composition of polypropylene nanocomposites.

Sample ID	Polypropylene (wt%)	Ni0.9Zn0.1Fe2O4 (wt%)
PP	100	0.0
PP-NZF5	95	5.0
PP-NZF10	90	10
PP-NZF15	85	15

. The PP and nanoparticles were pre-dried to remove any moisture that could affect the processing and final properties of the nanocomposite. A Brabender Polyspeed B was the specific type of internal mixer used to mix the molten polymer with the nanoparticles under controlled temperature and pressure. After mixing, the molten mixture was shaped into disk shapes using a compression molding technique. The nanocomposites were stored in a desiccator to minimize moisture absorption before characterization.

Structural analyses of the prepared nanocomposites were conducted using a Bruker AXS D8 Advance diffractometer (Berlin, Germany) equipped with CuKα radiation (λ = 0.154060 nm) and a collimator size of 480 µm. A high-performance FE-SEM Quanta FEG 250 provided high-resolution imaging and analysis of the polymer nanocomposite morphology. High-resolution imaging of materials and particle size at the nanoscale was achieved using a JEM-2100 instrument (JEOL Ltd., Tokyo, Japan). Functional group identification, molecular composition determination, and investigation of chemical bonding within the polymer nanocomposites were performed using a JASCO FT/IR-6100 (JASCO, Tokyo, Japan) spectrometer. Thermogravimetric analysis (TGA) of a polymer nanocomposite was conducted using an SDT Q600 TGA/DSC (TA Instruments, New Castle, DE, USA) analyzer with a heating rate of 10 °C/min. The dielectric properties of the polymer nanocomposites were recorded over a wide frequency range (20 Hz–2.0 MHz) using a Keysight E4980 LCR meter (Octopart, New York, NY, USA). The thickness of the composite samples was 2.0 mm. The electrodes were coated with gold and had an across-section area of 3.14 cm². The samples were pre-dried to remove moisture prior to dielectric properties measurements.

3. Results

3.1. XRD Analysis

Figure 1 demonstrates the XRD spectra of polypropylene doped with Ni_0.9Zn_0.1Fe₂O₄ nanoparticles at different weight ratios: 0.0, 5.0, 10, and 15 wt%. The diffraction pattern of PP exhibits characteristic peaks at 14.11°, 16.78°, 18.47°, 19.84°, 21.10°, 21.84°, and 25.21°, corresponding to the lattice planes of the monoclinic crystals of (110), (040), (130), (111), (131), (060), and (220), respectively [44,59,60,61,62]. The XRD patterns of PP/Ni_0.9Zn_0.1Fe₂O₄ nanocomposites reveal the predominant peaks of PP along with additional peaks at 36°, 43°, 57.26°, and 62.90°, indicating the presence of Ni_0.9Zn_0.1Fe₂O₄ nanoparticles within the nanocomposites and confirming the formation of PP/Ni_0.9Zn_0.1Fe₂O₄ nanocomposite films [63,64]. Furthermore, the shift of major Ni_0.9Zn_0.1Fe₂O₄ nanoparticle peaks in the nanocomposite diffraction patterns towards lower angles due to crystal lattice changes confirms the successful formation of PP nanocomposites. Ni_0.9Zn_0.1Fe₂O₄ nanoparticles in polypropylene matrix induce strain in the lattice due to a mismatch in thermal expansion or processing stresses. This strain disrupts atomic arrangement, increasing interplanar spacing and shifting diffraction peaks to lower angles. Moreover, as the concentration of nanoparticles increases in the nanocomposite, the probability of interaction and stress transfer between the nanoparticles and the polymer matrix also increases. This can lead to a more pronounced lattice strain in the nanoparticles, resulting in a larger peak shift at higher weight percentages [65]. Some of the major peaks are indexed with Miller indices, as shown in the figure.

XRD peak broadening in PP/Ni_0.9Zn_0.1Fe₂O₄ polymer nanocomposites can be attributed to several factors related to the size and strain of the crystalline phases, as well as the presence of multiple phases. According to the Scherrer equation, smaller crystallites diffract X-rays less efficiently, leading to broader peaks in the XRD pattern. This broadening effect becomes more pronounced as the crystallite size in both the PP matrix and the Ni_0.9Zn_0.1Fe₂O₄ nanofillers decreases. Strain within the crystal lattice caused by imperfections, dislocations, or interfacial stresses between the PP and Ni_0.9Zn_0.1Fe₂O₄ phases can also broaden the XRD peaks. The presence of non-uniform phases or mixed phases within the nanocomposite can contribute to peak broadening. This can arise from incomplete mixing of the PP and Ni_0.9Zn_0.1Fe₂O₄ components [66].

3.2. FTIR Analysis

One of the suitable techniques for studying such changes in these nanocomposite films is FTIR spectroscopy. Figure 2 illustrates the corresponding FTIR spectra within the range of 4000–400 cm⁻¹ wavenumber for PP and its nanocomposites modified with different ratios of 5, 10, and 15 wt% Ni_0.9Zn_0.1Fe₂O₄ NPs. All spectra were quite similar, with each displaying the characteristic peaks of PP [44,67,68,69]. In the spectrum of PP, the vibrations of asymmetric CH₃, asymmetric CH₂, stretching CH₃, symmetric bending CH₃, rocking CH₃ or wagging CH, rocking CH₃ or stretching C-C, rocking CH₃ or rocking CH, and stretching C-C were attributed to the peak locations detected at wavenumbers of 2949, 2916, 2867, 1454–1375, 1165, 997, 972, 840, and 807 cm⁻¹, respectively [67].

In the spectra of PP nanocomposites, additional peaks appear at wavenumbers of 1645 and 460 cm⁻¹, which were assigned to the N-H band and octahedral complexes of the spinel cubic structure, respectively [58,63]. These extra peaks, observed in the nanocomposites, confirm the incorporation of Ni_0.9Zn_0.1Fe₂O₄ NPs in the PP matrix, in agreement with the XRD results. The peak at 1645 cm⁻¹ is assigned to the N-H stretching vibration of adsorbed amine groups on the polypropylene matrix interacting with the Ni sites in the spinel. This suggests that there is a chemical bonding interaction between the amine groups on the polypropylene and the Ni atoms in the NPs. This type of interaction can lead to improved compatibility and dispersion of the NPs within the matrix, enhancing the mechanical and physical properties of the composite material. The peak at 460 cm⁻¹ is assigned to the Fe-O vibrations in the octahedral sites of the spinel structure. The presence of Zn dopant ions can potentially affect these vibrations, leading to a shift in the peak position or intensity. This suggests that the Zn dopant ions may be influencing the local bonding environment around the Fe atoms in the NPs. This could, in turn, affect the interaction between the NPs and the polypropylene matrix.

3.3. TEM Microscopy

TEM image and electron diffraction of a selected area of Ni_0.9Zn_0.1Fe₂O₄ NPs are shown in Figure 3. As shown in the figure, nanoparticles have a spherical shape with an average crystal size of 29 nm, and some appear as dark regions due to the agglomeration of nanoparticles. Agglomeration is caused by the magneto-static interaction between particles. Furthermore, the agglomeration phenomenon can be explained as nanoparticles having a permanent magnetic moment, which is directly proportional to their volume [58,70].

We compared the information obtained from electron diffraction (Figure 3b) with the bulk crystallographic data obtained from XRD. This showed a confirmation of the crystal structure obtained from XRD.

3.4. FESEM Microscopy

Figure 4 illustrates the FESEM images of PP/Ni_0.9Zn_0.1Fe₂O₄ nanocomposite films to investigate the morphology of the nanocomposites. The FESEM micrographs show a good distribution of Ni_0.9Zn_0.1Fe₂O₄ nanoparticles within the PP matrix, although some agglomeration is present. The appearance of agglomeration in the FE-SEM results (yellow circles) agrees well with the TEM results shown in Figure 3a. This suggests a good interaction between the Ni_0.9Zn_0.1Fe₂O₄ and the PP polymer matrix, confirming the formation of nanocomposites. This finding is consistent with the XRD and FTIR results [63,64].

3.5. Thermogravimetric Analysis

Figure 5 presents TGA/DTG plots of PP/Ni_0.9Zn_0.1Fe₂O₄ nanocomposite films. It is clear that each curve exhibits a single degradation stage with a peak temperature within the range of 467–499 °C. The peak temperature is slightly affected by the addition of Ni_0.9Zn_0.1Fe₂O₄ NPs to the PP matrix. As the Ni_0.9Zn_0.1Fe₂O₄ NPs concentration increases, the mass losses of PP/Ni_0.9Zn_0.1Fe₂O₄ nanocomposite films decrease, which is consistent with previous research [63,71]. As the nanoparticle loading increases (10 wt% and 15 wt%), the interfacial interaction between the nanoparticles and the polypropylene matrix becomes more significant compared to low loadings (5 wt%). This interaction can act as a barrier to the diffusion of volatile decomposition products, hindering the degradation process and leading to an increase in peak temperature. In comparison to pure PP, the addition of nanoparticles in the polymer matrix improved the thermal characteristics of polymer nanocomposite, as listed in

Table 2. TGA data of polypropylene loaded with Ni_0.9Zn_0.1Fe₂O₄.

Ni0.9Zn0.1Fe2O4 NPs Percentage (wt%)	Degradation Stage			Mass Loss (%)
	∆T (°C)	Tpeak (°C)	Mass Loss (%)	∆ m250	∆ m350	∆ m450	∆ m550
0	353–594	490	99.64	0.17	0.44	22.05	99.58
5	311–594	467	97.00	0.66	2.36	44.27	97.04
10	342–596	498	94.89	0.69	1.94	8.3	94.83
15	322–596	499	92.38	0.96	2.65	9.07	92.27

. The high crosslinking density and secondary connections conferred upon Ni_0.9Zn_0.1Fe₂O₄ NPs with the polymer chains can be attributed to the enhancement of the thermal stability of the nanocomposites. Also, the incorporation of nanoparticles into the polymer matrix restricted polymer chain movement, reduced free radical exchange and hence decreased heat degradation [63,72].

The degree of conversion (α) can be used to express the reaction rate using the following Equation (1) [73]:

$$\alpha =\frac{m_i-m_t}{m_i-m_f}$$

(1)

where m_i, m_f, and m_t are the initial, final, and current sample mass at the instant t.

The Coats–Redfern approximation method is a powerful tool for analyzing thermogravimetric analysis (TGA) data, particularly in the context of studying thermal decomposition processes [6,74,75,76,77,78,79]. Therefore, we use this method to determine the kinetics parameters of the TGA thermal decomposition stage shown in Figure 5.

$$Ln\left[-\frac{Ln\left(1-\alpha \right)}{T^2}\right]=-\frac{E_a}{RT}+Ln[\frac{AR}{\phi E_a}]$$

(2)

where E_a, R, A, and $\phi$ are the activation energy, the real gas constant, the reaction rate constant, and the heating rate, respectively. Equation (2) states that straight-line results from plotting $Ln\left[-\frac{Ln\left(1-\alpha \right)}{T^2}\right]$ versus $\frac{1000}{T}$ with a slope and intercept directly proportional to the $E_a$ and A, respectively as depicted in Figure 6. Estimated activation energies of the nanocomposites are reduced as Ni_0.9Zn_0.1Fe₂O₄ NPs are added, indicating that Ni_0.9Zn_0.1Fe₂O₄ NPs have a significant impact on the PP. Additionally, the following well-known formulae [78,80,81] were used to compute the changes in entropy (ΔS*), enthalpy (ΔH*), and Gibbs free energy (ΔG*) of the activation:

$${\mathsf{\Delta }\mathrm{S}}^{*}=R\left[Ln\left(\frac{Ah}{K_B{T}_p}\right)-1\right]$$

(3)

$${\mathsf{\Delta }\mathrm{H}}^{*}=E_a-R{T}_p$$

(4)

$${\mathsf{\Delta }\mathrm{G}}^{*}={\mathsf{\Delta }\mathrm{H}}^{*}-{\mathsf{\Delta }\mathrm{S}}^{*}{T}_p$$

(5)

where T_p is the peak temperature of the DTG curve, k_B is the Boltzmann constant, and h is the Plank constant. Entropy (${\mathsf{\Delta }\mathrm{S}}^{*}$) and enthalpy (${\mathsf{\Delta }\mathrm{H}}^{*}$) provide information on the system’s level of order, total thermal motion, and Gibbs or free energy (${\mathsf{\Delta }\mathrm{G}}^{*}$) provides information on the stability of the system.

The values of the computed thermodynamic parameters ($E_a$, ${\mathsf{\Delta }\mathrm{S}}^{*}$, ${\mathsf{\Delta }\mathrm{H}}^{*}$, and ${\mathsf{\Delta }\mathrm{G}}^{*}$) are listed in

Table 3. Thermal kinetic parameters for PP/Ni_0.9Zn_0.1Fe₂O₄ nanocomposites.

Ni0.9Zn0.1Fe2O4 NPs Percentage (wt%)	Ea (KJ/mol)	A (S−1)	ΔS* (J/mol.K)	ΔH* (KJ/mol)	ΔG* (KJ/mol)
0	393.83	2.64 × 1011	−38.70	389.75	408.71
5	315.83	2.55 × 109	−76.90	311.95	347.89
10	264.67	1.04 × 107	−123.18	260.53	321.90
15	381.55	1.41 × 1011	−44.08	377.40	399.39

in accordance with the Coats–Redfern method. It is evident that as the Ni_0.9Zn_0.1Fe₂O₄ NP concentrations rise, the values of all thermodynamic parameters fall. This can be explained by noting that when NPs rise, random macromolecule chain scission prevails in polymeric matrices, and activation energy decreases. Additionally, each sample has negative entropy, indicating ordered systems and potentially more ordered activated states that could result from the chemisorption of other light sources. The values of $E_a$, ${\mathsf{\Delta }\mathrm{S}}^{*}$, ${\mathsf{\Delta }\mathrm{H}}^{*}$, and ${\mathsf{\Delta }\mathrm{G}}^{*}$ in our work are less than in [74].

3.6. Dielectric Measurements

Figure 7 illustrates the frequency dependence of the real dielectric constant (ε′) of PP and its nanocomposites at room temperature in the frequency range $20 \mathrm{H}\mathrm{z}\le f\le 2 \mathrm{M}\mathrm{H}\mathrm{z}$. Over most of the frequency range studied, the dielectric constants of the polymer nanocomposites were almost frequency-independent. Polarizations relate to the PP matrix, Ni_0.9Zn_0.1Fe₂O₄ NPs, and PP-NPs interfaces in this study, although the nonpolar character of PP may have a major effect on the tiny dielectric response at the applied frequency range. Some polymer nanocomposites have previously shown the same characteristic [39,82,83,84]. The dependence of ε′ on Ni_0.9Zn_0.1Fe₂O₄ NPs concentration exhibited an enhancement with rising NP concentrations for weight ratios ranging from 0 to 15 wt%. This strong dependence of ε′ on nanofiller concentration has already been found for nanocomposites [63,64,85,86,87,88,89,90,91]. The good interaction between NPs and the PP matrix is confirmed via such behavior. Basically, it can be attributed to the formation of microcapacitor networks in the PP matrix as the content of the nanofiller rises. The accumulation of charge carriers in the interior surface of the PP matrix may also be the cause of this increase, as shown by the Maxwell–Wagner–Sillars effects [64,92,93]. The 15 wt% modified film has a lower dielectric constant than other nanocomposite films, which could be because of cluster formation [6].

Figure 8 shows the variation in dielectric loss (log(ε″)) with angular frequency log(ω) for PP/Ni_0.9Zn_0.1Fe₂O₄ nanocomposites films at room temperature. Generally, every specimen behaves in the same way as depicted, decreasing as frequency rises and then increasing at higher frequencies. The minimum dielectric loss shifts to higher frequencies as Ni_0.9Zn_0.1Fe₂O₄ Nps content increases. Equation (6) [94] gives the frequency-dependence relationship of ε′′:

$${\epsilon }^{″}\left(\omega \right)=\left({\epsilon }_s-{\epsilon }_{\infty }\right)2{\pi }^{2}N{(\frac{n{e}^2}{{\epsilon }_s})}^3{K}_{B}T{\tau }_o^m{W}_m^{-4}{\omega }^m$$

(6)

where

$$m=-\frac{K_{B}T}{{\mathrm{W}}_m}$$

where ε_s, ε_∞, n, N, e, k_β, T, τ_o, and W_m, are static permittivity, infinity permittivity, number of electrons, the concentration of localized sites, elementary charge, Boltzmann’s constant, absolute temperature, relaxation time, the energy required to move an electron from one site to the infinite, and angular frequency. By applying this model to our results and fitting a portion of the data in the frequency range of 20 Hz to 84 kHz to a linear equation, we may find the values of W_m presented in Table 3. Generally, the values of W_m tabulated in Table 3 are greater than those of PVC/NiO (0.025–0.027 eV) [92] but lower than those of Polyester/Ni_0.5Zn_0.5Fe₂O₄ (0.16–0.43 eV) [64], and their values grew as the Ni_0.9Zn_0.1Fe₂O₄ Nps concentration increased.

The variation in the tangent loss (tan(δ)) of PP loaded with different weight ratios of Ni_0.9Zn_0.1Fe₂O₄ NPs at room temperature in the frequency range 20 Hz to 2 MHz is displayed in Figure 9. The measured values of tan(δ) of PP nanocomposites are frequency-dependent at all frequency ranges, as can be seen. For PP, the tangent loss initially has high values at low frequencies and rapidly decreases as the frequency rises to 4226 Hz; thereafter, the tangent loss increases. Tan(δ) for the PP is consistent with the strong conductivity at high frequencies associated with charge carrier hopping or tunneling [63,64]. The trendlines of the nanocomposites feature two regions: the first region shows a decrease in tan (δ) with frequency due to a reduction in dielectric dispersion or because the nanocomposite films’ crystallinity has improved [95], while the second region exhibits an increase in loss tangent with frequency. The nanocomposite films’ minimal loss tangents are, respectively, at 0.6325, 1.416, and 1.5 MHz and 5, 10 and 15 wt%.

The Cole–Cole curves of PP/Ni_0.9Z_0.1Fe₂O₄ nanocomposite films at room temperature are revealed in Figure 10. The semicircle on each Cole–Cole graph represented relaxation processes. As a result, the values of static permittivity (ε_s), infinity permittivity (ε_∞), and dielectric intensity (Δε) that are given in Table 3 are obtained by fitting these semicircles. According to Table 3, the values of ε_∞ and ε_s for films made of PP/Ni_0.9Z_0.1Fe₂O₄ nanocomposites are greater than those of PP, confirming that the addition of NPs increased the dielectric constant of PP.

According to the relation [96], the relaxation time (τ_o) can be estimated from the Cole–Cole analysis:

$$\ln (\frac{v}{u})=(1-\alpha )\left[\ln (\omega )+\ln ({\tau }_o)\right]$$

(7)

where u is the distance between the experimental point and ε_∞, and v is the distance between ε_s and the experimental point on the Cole–Cole plot. According to Figure 11, the relationship between Ln(ω) and Ln(v/u) is fitted with a linear line. By knowing the slope and intercept of the fitting line, τ_o can be calculated, and by applying this equation, ${\tau }_{\mathrm{o}}=1/2\pi {\mathrm{f}}_{\mathrm{o}}$, we may determine f_o which they listed in Table 3. The obtained values of ${\tau }_{\mathrm{o}}$ are higher than those obtained in PVC/CuO [97] and lower than Polyester/Ni_0.5Zn_0.5Fe₂O₄ [61]. As listed in

Table 4. The deduced ε_s, ε_∞, Δε, τ_o, f_o, and W_m of PP/Ni_0.9Zn_0.1Fe₂O₄ nanocomposites.

Ni0.9Zn0.1Fe2O4 NPs Percentage (wt.%)	εs	ε∞	Δε	τo (µs)	fo (kHz)	Wm (eV)
0	1.52	1.26	0.26	24.2	6.58	0.106
5	2.13	2.09	0.04	5.42	29.36	0.068
10	2.58	2.49	0.09	0.99	160.76	0.119
15	1.84	1.76	0.08	1.05	151.58	0.126

, the relaxation time (${\tau }_{\mathrm{o}}$) decreases as the Ni_0.9Zn_0.1Fe₂O₄ NP content increases; this behavior is shown in other works [97,98].

Figure 12 depicts the variation in the AC impedance (Z’) of the PP/Ni_0.9Zn_0.1Fe₂O₄ nanocomposite within the frequency range of $20 \mathrm{H}\mathrm{z}\le f\le 2 \mathrm{M}\mathrm{H}\mathrm{z}$ at room temperature. The values of Z’ fall with frequency and attain nearly minimal constant values at high frequencies, which can be explained in part by charge carrier hopping or tunneling. The polymer nanocomposite films’ improved crystallinity, which results in less hopping interchain and intrachain ion movements, caused the inclusion of nanoparticles to also provide an improvement in Z’ values [99].

Ni_0.9Z_0.1Fe₂O₄ is a spinel ferrite with good electrical conductivity due to the presence of mixed valence states (Fe²⁺ and Fe³⁺) and the hopping mechanism of charge carriers. When incorporated into the PP matrix, it creates conductive pathways, reducing the overall impedance of the composite and thereby increasing Z’. Meanwhile, the interface between the Ni_0.9Z_0.1Fe₂O₄ nanoparticles and the PP matrix creates an additional layer with unique electrical properties. This interfacial region can exhibit polarization under an applied electric field, contributing to the overall impedance response and potentially increasing Z’. Moreover, the presence of Ni_0.9Z_0.1Fe₂O₄ nanoparticles can trap mobile ions and electrons within the PP matrix, preventing them from accumulating at the electrode interfaces. This reduces space charge buildup, which can contribute to leakage current and lower Z’ values [100].

4. Conclusions

The successful synthesis of PP/Ni_0.9Zn_0.1Fe₂O₄ nanocomposites was achieved and confirmed via structural techniques involving XRD, FTIR, and SEM. Further, TEM, XRD, and TEM findings revealed an average crystal size of approximately 29 nm for the Ni_0.9Zn_0.1Fe₂O₄ nanoparticles. TEM images also confirmed the spherical morphology and lack of aggregation of the nanoparticles. FESEM micrographs demonstrated the successful incorporation of the Ni_0.9Zn_0.1Fe₂O₄ NPs into the polymer matrix. TGA/DTG analysis showed improved thermal characteristics and reduced weight loss. However, the peak temperature range shifted between 468 and 499 °C upon introducing Ni_0.9Zn_0.1Fe₂O₄ NPs. Kinetic and thermodynamic parameters, including activation energy, enthalpy, entropy, and Gibbs free energy, were determined from TGA/DTG data using the Coats–Redfern equation. Notably, the values of Ea, ΔS, ΔH, and ΔG* decreased with increasing Ni_0.9Zn_0.1Fe₂O₄ content. Dielectric loss and AC impedance exhibited dependence on both frequency and composition. The real part of the complex permittivity (e’) increased with the addition of Ni_0.9Zn_0.1Fe₂O₄ NPs. Cole–Cole analysis revealed a single relaxation semicircle and provided values for ε_s, ε_∞, ∆ε_, τ₀, and f_o. The combination of dielectric and magnetic properties of PP/Ni_0.9Zn_0.1Fe₂O₄ nanocomposites suggests potential use in electromagnetic interference (EMI) shielding materials.