Bismuth oxide (Bi₂O₃ 99.99% purity), polystyrene (PS) granules purchased from Sigma Aldrich, polyvinylidene fluoride (PVDF 99.99%) & solvents like dichloromethane, NMP and benzene were purchased from Alfa Aesar respectively.

The BO/PS and BO/PVDF composites with varying concentration of BO = 1 to 5 wt% were synthesized by solution casting method. In this method, filler and polymers were individually dispersed in crystallizable benzene using ultrasonicator for one hour. Then both the solutions were mixed and further sonicated till homogeneously dispersion. This mixed solution was poured in the Petri dish floating on mercury surface to get composites of uniform thickness of 100 µm.

X-ray diffractometer (Bruker AXS, ApexII) in the angle (2θ) range 20˚ to 60˚ with step resolution 0.2˚ was used to investigate the structure of the composites. FT-IR spectra of all synthesized composites were collected by using Alpha Bruker FT-IR Spectrometer (ECO-ATR) in wave number range 4000 cm⁻¹ to 500 cm⁻¹ at room temperature. Scanning electron microscope JSM-7610F Plus, JEOL Ltd Tokyo, was used for surface morphology and energy dispersive x-ray (EDX) spectra for elemental analysis of the fabricated composites. Impedance analyzer WAYNE KERR Electronics, London, UK (6500B series) was used for dielectric response measurements in the frequency range 1 kHz to 3.16 MHz.

Figure 1 show XRD spectra of pristine PS, PVDF, BO, BO_1%PS_99%, BO_5%PS_95%, BO_1%PVDF_99% and BO_5%PS_95% composites. The average crystallite size of the order of 50 nm for Bi₂O₃ has been found by Scherer equation.

The diffraction spectrum of bismuth oxide shows peaks at angles 27.3˚, 33.1˚, 46.4˚ and 54.8˚ corresponds to planes (120), (121), (311) and (241) respectively

matches well with JCPDS file (76 - 1730) belongs to monoclinic phase of Bi₂O₃ [21] [22]. XRD spectrum of pristine PS showed only a hump reflecting amorphous nature of polystyrene. [23] However diffraction peaks at 2θ = 26.4˚, 34˚ and 46˚ are obtained for PVDF, [24] showing the crystalline nature of the polymer. The position of diffraction peaks for composites like BO_1%PS_99%, BO_5%PS_95%, BO_1%PVDF_99% and BO_5%PS_95% composites remains the same as those of individual only intensities at higher concentration of filler found to be increased indicating the phases of dispersed bismuth oxide.

Figures 2(a)-(i) show SEM images of pristine PS, PVDF, Bi₂O₃, BO/PS and BO/PVDF composites at different magnifications. The surface morphology of pristine PS as shown in Figure 2(a) looks like colloidal structure, however SEM images of PVDF Figure 2(b) and Figure 2(c) appearing like spherical ball shape connected to each other. The SEM micrographs as shown in Figure 2(d) and Figure 2(e) appearing like cylindrical needle shape of bismuth oxide (Bi₂O₃) match well with previous reports. [25] [26] The porous structure formation in BO/PS composites as shown in Figure 2(f) and Figure 2(g), may help to trap

charge carriers during charge discharge process. The rods of BO surrounded by PVDF as shown in Figure 2(h) and Figure 2(i) represents a significant interaction between filler and polymer which has been also obtained by bonding characteristics as confirmed by FT-IR measurements.

Figure 3(a) and Figure 3(b) represents energy dispersive x-ray spectra of BO/PS & BO/PVDF composites, confirms presence of bismuth, oxygen, and carbon in the fabricated composites of PS and PVDF. [27]

Figure 4(a) and Figure 4(b) show FT-IR spectra for pristine BO_, PS, BO/PVDF composites and BO, PS, BO/PS composites. As shown in Figure 4, peaks for BO are observed at 563 cm⁻¹, 604 cm⁻¹, 1535 cm⁻¹, 1685 cm⁻¹ and around 3600 cm⁻¹ corresponds to Bi-O stretch, Bi-O-Bi (metal-oxygen-metal) stretching vibrations, NO 3 − group and O-H (hydroxyl group) stretching respectively. These peaks are reference of monoclinic structure of Bi₂O₃, and matches well with earlier reports [28]. As shown in Figure 4(a), absorption peaks at 604 cm⁻¹, 760 cm⁻¹ (CF₂ bending), 871 cm⁻¹ (C-C-C), 1173 cm⁻¹ (CF₂ stretch), 1395 cm⁻¹ (CH₂ wagging) and 1685 cm⁻¹ (C = C stretching vibration) are characteristic peaks of α-PVDF. Appearance of absorption peaks at the same wave number in composites and individual is evidence of well orientation and bonding of filler with PVDF. FT-IR spectrum of pristine PS as shown in Figure 4(b) showing various peaks at wavenumbers like 684 cm⁻¹, 754 cm⁻¹, 1494 cm⁻¹, and 3026 cm⁻¹ related to C-H bending, aromatic C = C stretching and aromatic C-H stretching vibrations and characteristic peak of styrene [29].

Figure 5(a) and Figure 5(b) show frequency vs dielectric constant of pristine PVDF as well as BO/PVDF composites and PS as well as BO/PS composites. The dielectric constant of pristine PS is obtained 3.687 at freq. of 1 kHz and found to be increased for BO_5%PS_95%. up to 5.246, similarly, dielectric constant of PVDF is obtained 9.754 and found to be increased for BO_5%PVDF_95% upto 17.449 attributes

the accumulation of charge at grain boundaries leading to the formation of intrinsic dipoles in composites of BO/PS and BO/PVDF results dielectric constant approx. equivalent to the BO. Thus, on increasing BO concentration within the polymers, there might be maximum possibility of increase in intrinsic dipoles and polarization area of the composites, which results as enhancement in dielectric constant. However, values of dielectric constant decreases with increase of frequency attributes all type of polarization (interfacial, dipolar, vibrational and electronic polarization) contributes in permittivity, but dipoles are unable to flip with field when frequency is increased causing the polarization and dielectric constant to decrease with increasing frequency. [30]

Figure 6(a) and Figure 6(b) show dissipation factor (tanδ) vs frequency response of pristine PVDF, BO/PVDF composites as well as PS, and BO/PS composites respectively. It indicates that tanδ increases with increasing BO concentration in both the polymers PS and PVDF. Dissipation factor attains large values for BO_5%PS_95% and BO_5%PVDF_95% composites. As shown in Figure 6(b), PS and BO/PS composites have large tanδ values at smaller frequencies and decreases with frequency up to 500 kHz, after this it increases again with increasing frequency.

However, for PVDF and BO/PVDF composites tanδ decreases with frequency up to 100 kHz and then increases rapidly with increasing frequency, as shown in Figure 6(a). The dielectric constant of BO_5%PS_95% and BO_5%PVDF_95% composites increased 1.4 and 1.8 times as compared to pristine PS and PVDF which is very close to the reports available in the literature [31]. Thus, filler BO improves dielectric performance of composites but dissipation factor limits its concentration. Table 1 showed values of dielectric constant with log f for pristine PS & PVDF as well as their various composites.