Study of the spectroscopic, magnetic, and electrical behavior of PVDF/PEO blend incorporated with nickel ferrite (NiFe2O4) nanoparticles

Pure nickel ferrite nanoparticles (NiFe2O4 NPs) were prepared by the solvothermal method. Different concentrations of NiFe2O4 were added to PVDF/PEO blend to prepare the PVDF/PEO-NiFe2O4 nanocomposite films. The films were characterized using different techniques in detail. Pure NiFe2O4 NPs images have a semispherical shape and roundness of the edges, with average particle size ~ 4.3–8.8 nm, and polycrystalline structure. Pure NiFe2O4 NPs show that micrometrical agglomeration suggests the presence of pore-free crystallites on the surface. The spectroscopic techniques such as XRD, FTIR, and UV–visible have confirmed the interaction between PVDF/PEO and NiFe2O4 NPs. Still, the nanocomposites exhibit a smooth surface with typical spherulitic clusters revealing the semi-crystalline structure of the PVDF/PEO-NiFe2O4 nanocomposites. The values of ε′ and ε″ were increased as an increase of NiFe2O4 due to the high value of dielectric permittivity of NiFe2O4. The appearance of semi-circles in the plot of M″ with Log confirms the single phase of the samples. The dielectric measurements show that M′ is inversely proportional to ε′. At higher temperatures, M′ levels off at frequencies higher than those at lower temperatures because the relaxation processes were spread over a range of frequencies. The values of saturation magnetization of PVDF/PEO-NiFe2O4 films were linearly increased as an increase of NiFe2O4 indicates improvement in the magnetic vector arrangement. Due to the enhancement of the magnetic properties, they can further exploit the films for magnetic applications.


Introduction
In the past years, the science of polymeric nanocomposites has demonstrated its potential in many applications in the electronics and technology fields. Therefore, ferrites and magnetic nanomaterials with appropriate physical and chemical properties that contain transition elements (XFe 2 O 4 ) (where X = Mn, Mg, Co, Fe, Ni or Zn). The magnetic ferrites are used for diagnostic purposes, computer memory storage and drug guidance. This is due to its impressive size and shape-dependent magnetic properties. Nickel ferrite (NiFe 2 O 4 ) has many applications for its high magnetic anisotropy, high saturation magnetization, and supermagnetic structure. NiFe 2 O 4 has a wide range of magnetic properties based on particle size and morphology, such as super magnetic behavior [1][2][3][4][5][6][7][8][9][10][11].
Researchers have recently been interested in nanotechnology, especially nanocrystalline materials [12,13], which play a general role in many applications due to their unique electrical and magnetic properties, such as storing magnetic information with a high density. Nickel ferrite (NiFe 2 O 4 ) is a versatile soft ferrite material that is technological of great importance for its ideal magnetic properties such as gas sensor, magnetic storage systems, magneto-optical, magneto resistive, magnetic resonance imaging and microwave devices. NiFe 2 O 4 has a cubic structure with an inverse spinel structure and shows suitable ferromagnetism arising from magnetic moment of antiparallel rotation [14,15]. Spinel nickel ferrite (NiFe 2 O 4 ) [16], among the most significant ferrite materials, has been extensively researched due to its wide range of applications, including catalysts, magnetic materials, microwave devices, gas sensors [17]. NiFe 2 O 4 has an inverse spinel structure and has a cubic structure [18].
Polymer nanocomposites are one of new nanocomposites in which the polymer acts as the matrices with other nanoscale components such as the nanofiller [19,20]. Various inorganic fillers have been added to the polymeric matrices to improve their properties [21]. Poly ethylene oxide (PEO) has become the most investigated as a promising polymer. PEO is a soluble in water, highly crystalline thermoplastic with a moderate tensile modulus, good mechanical and electrical characteristics [22]. The high crystallinity of PEO is detrimental to their ionic conductivity. The PEO-based electrolytes have been found to have low ionic conductivity in the range of temperatures [23]. The suggestion for mixing PEO could solve the brittleness problem while also improving the system's ionic conductivity [24]. Polyvinylidene fluoride (PVDF) is a highly concentrated polymeric material. Pyroelectric and piezoelectric capabilities have been observed. PVDF and its mixes (with one or more other polymers) have gotten a lot of press in recent years for advanced techniques [22]. PVDF with fillers or nanofillers is widely used for improved performance and extended uses [25].
In our paper, NiFe 2 O 4 nanoparticles are prepared using the solvothermal method and are characterized using XRD, TEM, FESEM and ATR-FTIR techniques. Then, studying for the first one, their addition on (85/15 wt/wt%) PVDF/ PEO using casting technique methods to be used in industrial application and characterized using XRD, FESEM, ATR-FTIR, magnetic properties and also the dielectric properties are studied.

Preparation of PVDF/PEO-NiFe 2 O 4 nanocomposites
Amount (85/15 wt/wt%) of PVDF/PEO was dissolved in dimethyl sulfoxide (DMSO) as a filler with regularly stirred until uniform viscous liquid. Drop by drop, the produced NiFe 2 O 4 NPs in mass fractions of 2, 4, 6, 8 and 10 wt% was added to the PVDF/PEO solution. The resultant solution was sonicated to achieve homogeneity and a good dispersed solution. The solution was then cast onto Petri dishes and stored for 12 h in a vacuum oven at 70 °C until the solvent had completely evaporated.

Measurement techniques
Determined the crystallinity of the prepared films from X-ray diffraction (XRD) spectra, which were captured using a PANalyticalX'Pert Pro diffractometer with Cu-K radiation in the 2θ range from 5° to 60°. Transmission electron microscope (TEM) (JEM-2100F electron microscope) at 200 kV was used to determine the size of NiFe 2 O 4 NPs. A field emission scanning electron microscope (FESEM) (Quanta 250 FEG) was used to examine the morphology of the samples (EDS). Using Gwyddion software, 3D micrographs and roughness were characteristics from FESEM picture. The comparative examination of the films was analyzed using Vertex 80 Bruker in attenuated total reflectance (ATR) mode in the wavenumber from 2000 to 400 cm −1 . UV-Vis spectra were obtained at room temperature with a Jasco V-630 spectrophotometer about 200-1000 nm. The magnetic properties of the samples were carried out at room temperature in the range from − 20,000 to + 20,000 Oe using a vibrating sample magnetometer (VSM) (AGM & VSM Magnetometer, MicroMag 3900 series from former Princeton Measurement Co., currently Lake Shore Cryotronics, Inc., Westerville, OH, USA). A high-resolution Alpha-A Analyzer from Novocontrol was used to make broadband dielectric spectroscopic measurements in the frequency range 0.1 Hz to 7 MHz.  10-032. There are not any impurity peaks found, which confirmed the high purity of the prepared NiFe 2 O 4 . The average crystal size (S) of pure NiFe 2 O 4 is estimated using Scherrer equation [26]:

Results and discussion
where is is the X-ray wavelength (1.5408 Å), μ is defined as the full width at half maximum peak and θ (rad) is the Bragg angle. The calculated average crystal size of pure NiFe 2 O 4 nanoparticles is found to be 5.3 nm.
As stated in the formula below, values of lattice constant (a) were obtained using information from the 'Miller indices (h k l)' and inter-planar spacing (d) values [27]: The inter-planar spacing d of the most intense peak (311) is given from the below equation [28]: The calculated lattice constant of the most intense peak (311) is 8.29 Å. The following relation was used to calculate the unit cell volume of NiFe 2 O 4 NPs which is proportional to the lattice constant values [29]: The unit cell volume is 569.72 Å 3 . The dislocation density (δ) was calculated using the equation below using the average crystallite size values and it is found to be equal 3.5 × 10 16 lines/m 2 : The following equations were used to calculate the hopping length (L A and L B ) values [30,31]: Values of L A and L B are 7.17 Å and 5.86 Å. The TEM of pure NiFe 2 O 4 NPs has a semispherical shape with the roundness of some edges with an average particle size of nearly 4.3-8.8 nm calculated from the histogram. The result agrees with XRD data computed using Scherrer's formula. Let us get the concept of a lattice and how atoms are arranged basically. So advanced specific area electron diffraction (SAED) spectrum is a two-dimensional spectrum obtained in a reciprocal lattice. SAED gives us a more accurate idea of the crystal structure. The SAED study shows a superimposed bright ring-shaped spot around the center point with varying diameters confirming the polycrystalline structure of pure NiFe 2 O 4 material. Figure 2 shows XRD diffraction of (85/15 wt/wt%) PVDF/PEO and PVDF/PEO doped with the different weight percentages (2, 4, 6, 8 and 10 wt) of NiFe 2 O 4 NPs. As seen for undoped PVDF/PEO, there are diffraction patterns at 2θ = 18.7°, 20.2°, 36.2° and 39.4° [32,33]. For PVDF/PEO modified with NiFe 2 O 4 NPs, the diffraction pattern at 2θ = 20.2° shifted toward a higher theta degree and their broadening increased with NiFe 2 O 4 NPs. Also, there is a noticeable decrease in their intensity compared to the undoped blend. Besides, the diffraction peaks at 2θ = 18.7° and 39.4° are disappeared with the increase of NiFe 2 O 4 NPs, confirming the interaction between PVDF/PEO and the prepared NiFe 2 O 4 NPs. Scheme 1 represents the possible chemical reaction between PVDF/PEO and NiFe 2 O 4 NPs.
The morphological study and energy-dispersive X-ray (EDX) analysis of the prepared NiFe 2 O 4 NPs is shown in Fig. 3   As shown in Fig. 4b, there is no visible interface between the 2 polymers, indicating that PVDF and PEO are compatible. Figure 4b, c depicts a homogenous multiferroic sphere fabrication with good dispersion and spherical form. The sizes of the spheres ranged from 3 to 5 μm, which means that NiFe 2 O 4 NPs magnetic fillers are inserted into the PVDF/PEO polymer chains. Figure 5 shows 3D images of (85/15 wt/wt%) PVDF/PEO and PVDF/PEO incorporated with 4 wt % NiFe 2 O 4 NPs and 10 wt% NiFe 2 O 4 NPs. The 3D image reveals that polymer nanocomposite's surface becomes rougher (see Table 1) compared to pure polymer blend which means that the PVDF/PEO-NiFe 2 O 4 NPs improve the polymer blend surface to be suitable to apply in many industrial applications.  [18,35]. For (85/15 wt/wt%) PVDF/PEO, according to previous literature [18,36,37], band at 1400 cm −1 corresponds to wagging modes of CH 2 , 1231 cm −1 attributed to CH 2 twisting mode of PEO. The band at 1170 cm −1 is due to the stretching mode of CF 2 of PVDF. The band at 1071 cm −1 corresponded to the stretching mode of C-O of PEO.   Figure 7 represents UV-Vis spectra of 85/15 wt/wt%) of PVDF/PEO and PVDF/ PEO incorporated with (2, 4, 6, 8 and 10 wt%) NiFe 2 O 4 NPs. As seen for pure PVDF/PEO blend, an absorption peak at 192 nm is assigned to π-π* (chromophoric group of PVDF/PEO) [37]. An increase of NiFe 2 O 4 , this peak shifts toward a higher wavelength with the broadening of the peak. Another peak appeared at 348 nm [38], which was assigned to NiFe 2 O 4 NPs. The intensity of this peak is gradually increased with increases of NiFe 2 O 4 NPs. The data suggested that the interaction between NiFe 2 O 4 NPs and PVDF/PEO occurs.
The dielectric behavior of the material has been studied over a wide range of frequencies from 0.1 Hz to 7 MHz and temperatures from 30 to 120 °C. The complex impedance spectroscopy technique enables us to separate the real (ε′) and imaginary (ε″) components of the dielectric parameters and hence provides a true picture of the properties of materials. The dielectric permittivity (ε * ) is expressed as [39,40]: The variation between ε′ and ε″ with Log (f) for PVDF/PEO blend doped by 0, 2, 4, 6, 8 and 10 wt% of nickel ferrite (NiFe 2 O 4 ) in the range of frequency about 0.1 Hz to 7 GHz at room temperature is shown in Fig. 8a, b. The figures display that the dielectric constant (ε′) dramatically decreases with an increase in frequency. At low frequencies, dipoles follow the field. The values of dielectric constant (ε′) are very high due to 2 sources, one of them associated with mobile carriers and the other because the studied material is polarized in structure. As the frequency increases, dipoles begin to lag behind the field, so the dielectric constant decreases. When frequency reaches the characteristic frequency, the maximum due to the effect of moving ions that cause higher ionic conductivity [42,43]. The complex modulus formula is a very important and convenient measurement tool for identifying, analyzing, and interpreting the dynamic aspects of the electrical transport process in a material. The electric modulus (M * ) is useful in rationalizing the conduction mechanisms and identifying the microscopic processes responsible for the local dielectric relaxation and polarization processes. The modulus (M * ) is defined as reciprocal dielectric permittivity (ε * ) [29][30][31][32]: where M′ is the real part and M″ is the imaginary part for the electric modulus. Figure 9a, b depicts the relation between both M′ and M″ with Log (f) for the present system. The real part (M′) values about minimum values at the lower frequencies due to the negligible contribution of electrode polarization indicate that the electrode effect has a marginal electrode polarization. Further, the minimum values of the capacitive nature of the polymeric material are found at low frequency. Then, an increase of M′ to reach the plateau at the intermediate region of the frequency and a linear increase at high frequencies. As we see in Fig. 9b, the appearance of semicircles or arc in the spectrum of the relation between M″ with Log (f) confirms the single phase of the materials. Figure 10 depicts the graph between M′ as a function and Log (f) of the frequency of the PVDF/PEO with 10% wt of NiFe 2 O 4 in the range of temperatures from 30 to 120 °C. At the lower frequency range, the values of M′ are nearly zero because the restoring force givers the mobility of charge carriers. This suggests the long-range mobility of charge carriers due to electrode polarization. Also, an increase of M′ with increasing frequency has been founded, supporting the conduction phenomena. At the higher range of the frequency, the values of M′ reach the maximum value. This behavior of M′ is inversely proportional to ε′. At higher temperatures, M′ levels off at frequencies higher than those at lower temperatures because the relaxation processes are spread over a range of frequencies. Figure 11 depicts the graph between the imaginary part of (M″) and Log (f) of the frequency of the prepared samples. As we have seen, the given spectrum is asymmetric concerning the maximum peak. We saw broadening at each peak on either side of the peak maximum indicated by Debye's exemplary behavior. It was interpreted depending on the relaxation time distributions attributed to the nature of the material other than Debye, and the charge carriers appeared traveling in the range over long distances with low frequencies located to the left of the peaks. While in the area to the right of the peak, the peak area indicates the transition from longrange to short-term mobility. The extreme shifts of the spectral modulus M″ toward higher frequencies with an increasing indication that the dielectric relaxation is thermally activated also.
In general, the measurement of tan δ as a function of frequency often a loss is attributed to polarization and conduction inside the composite samples [44]. Figure 12 depicts the variation dissipation loss factor (tan δ) as a function of frequency at different temperatures. The tan δ values increase with the frequency values, pass through a maximum value of tan δ, and then decrease. The maximum values of tan δ, which get shifted to the higher frequency, confirm that the samples can be represented by a parallel combination of resistance (R) and capacitance (C). The highest value (peak) found at intermediate of the frequency confirms the relaxation phenomenon. The minimum values of tangent loss (tan δ) for the pure polymer blend may be due to the amorphous nature, which increased after the addition of NiFe 2 O 4 due to an increase in the crystallinity. Magnetization data allow determining the magnetic parameters such as saturation magnetization (M s ), saturation retentively or remanence magnetization (M r ), and coercive force (H ci ), where H ci is the field needed to bring the magnetization back to zero. Figure 13 shows the magnetization (emu/g) as a function of magnetic field (Oe) of pure nickel ferrite (NiFe 2 O 4 ) nanoparticles in the range from − 20 to + 20 kOe. The magnetization parameters for pure NiFe 2 O 4 are measured  Figure 14 shows the magnetization against the magnetic field of PVDF/PEO blend doped with 0, 0.4, 0.8 and 1.2 wt% of NiFe 2 O 4 nanoparticles. As expected for the polymeric materials, the magnetization of the PVDF/PEO blend is close to zero due to the absence of NiFe 2 O 4 nanoparticles embedded in nonmagnetic blend matrices. The magnetic nanoparticles are concentrated into a chain-like shape due to the magnetic induction, thus enhancing magnetic interactions between dipole and dipole particles. Structure, the projection of the magnetization vector along the direction of the field will be higher than the random projection distribution of NiFe 2 O 4 nanoparticles in the PVDF/PEO matrix with a large-shaped contrast effect.
The results indicate that the magnetic behavior observed in the nanocomposites originated from the magnetic NiFe 2 O 4 nanoparticles attributed to the increasing tendency of the nanoparticles to aggregate on increasing their content. Whereby arise increase in the NiFe 2 O 4 content increases the magnetic moment per unit volume of the prepared sample, which leads to a rising increase in its magnetization. It was also certain that the M S values of the composites were much lower than the values of the bulk magnetic particles. The magnetic values of the compounds indicate a magnetic behavior caused by the addition of NiFe 2 O 4 .The magnetization parameters of NiFe 2 O 4 -PVDF/PEO to be M s , M r and H ci are tabulated in Table 2. Due to the magnetic properties of the samples, they can further exploit the magnetic composite films for magnetic applications.

Conflict of interest
The authors declare that they have no conflict of interest.
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