3.1. Morphology and structural analysis of the nanomaterials
Figure 1 shows the SEM and TEM morphologies of the nanomaterials FeO, ZnO and FeO/ZnO and their elemental analyses. Hydrothermal reaction generates ZnO as flowers according to the Ostwald ripening mechanism [27]. MEA forms an intermediate complex with the Zinc precursor and it forms rod like structures. The rod like structures further grow in to the form of flower like structures with typical sizes ≤ 1 µm. For the pure ZnO, the average diameter of the flower shows 400 nm whereas for the FeO, plate-like sheets are observed with an average size of 220 nm. For the FeO/ZnO hybrid material, the average diameter of the nanoparticle became 280 nm, with a slightly deformed flower like appearance. This is because during the hybrid particle synthesis, the ZnO flowers are formed on the FeO particles, since the hydrothermal reaction of the former was performed in the presence of the latter [28].
The EDX spectra shown in Fig. 1g to 1i give information about the elements present in the synthesized filler particles. It is clear that the elements Zn, and Fe are present in the ZnO and FeO samples, respectively, whereas both elements are observed in the hybrid. Absence of any other elements confirm the purity of the samples. In addition, the elemental composition of the samples are given in the insets that also confirms the structural integrity of the samples.
3.2. Morphology and structural analysis of the electrospun nanocomposites
Electrospun fibers of the polymer nanocomposites are investigated for their morphology and average fiber diameter. Figure 2 shows the SEM images of defect free fibers without the formation of beads, attributed to the optimum concentration of the PVDF solution used for electrospinning. Nanoparticles are embedded within the polymer chains and thus are not visible in the SEM images. The average fiber diameters are calculated from the diameter distribution curves, provided in the insets of the SEM images. The average diameter values observed are 249 ± 109 nm for the neat PVDF, 324 ± 148 nm for the PVDF-FeO, 442 ± 268 nm for the PVDF-ZnO, 412 ± 167 nm for PVDF-FeO/ZnO at 1 wt.% and 417 ± 188 nm for the PVDF-FeO/ZnO at 3 wt.% nanocomposites. It’s clear that the average fiber diameter enhances for the nanocomposites due to enhanced viscosity and networking effect attributed to the good distribution of filler particles [29]. For the hybrid composites, much variation in fiber diameters are not observed due to the similar levels of filler dispersion within the PVDF polymer.
Structural information of the PVDF composite fibers are explored from the FTIR and XRD studies as evidenced from the Fig. 3. The FTIR spectra of the PVDF nanocomposites in Fig. 3a shows the absorbance peak at 842 cm− 1, which is due to the presence of crystalline β-phase [30]. Absence of peaks at 976 and 766 cm− 1 corresponds to the absence of α-phase. While the peak at 1176 cm− 1 corresponds to the β-phase, the peak at 1398 cm− 1 is due to the γ-phase [31, 32]. The crystalline β-phase proportions for the PVDF fibers can be calculated from the absorbance values at 860 cm− 1 (Aβ) and 760 cm− 1 (Aα) as per the following equation [21].
The value enhances from 21.36 for the PVDF to 22.39 for the PVDF composites containing hybrid filler materials. This shows the increase in β-phase with the introduction of metal oxide nanomaterial (crystallinity value given in Table 3).
Crystallization behavior of the PVDF composites can also be explored from these spectral analyses. XRD pattern for the fibers given in Fig. 3b demonstrates the semicrystalline nature of the PVDF with peak positions at 18.6º and 20.4º respectively attributed to the (020) and (021)/(201) crystal planes of α-phase of the PVDF [33]. The peak at 20.4º also corresponds to the (110) and (200) crystal planes of the β-phase PVDF. A prominent peak at 36.4º also appears with the introduction of nanoparticles, which is due to the (020) and (101) crystal planes of the β-phase. Peaks at 40.8º and 56.8º can be the secondary peaks from overtone bands. Crystalline fractions within the composite fibers can be analyzed from the variation in peak intensities with the introduction of filler particles. It’s clear from the figure that the electroactive phase enhances considerably with the introduction of metal oxides, and β-phase transformation also exist [34, 35]. This is because of the influence of semiconducting fillers in determining the diploe alignments within the PVDF chains [20].
3.3. Thermal and mechanical stability of the electrospun nanocomposites
Thermogravimmetric and derivative thermogravimmetric curves obtained for the PVDF nanocomposite fibers are provided in Fig. 4. It is observed that the PVDF composites are stable up to 430 ºC and starts degradation after this point. The decomposition temperature for the neat PVDF is at 462 ºC, whereas this value enhances with the introduction of ZnO and FeO filler particles [36]. With the addition of FeO and ZnO, the decomposition temperature becomes respectively 470.58 ºC and 469.92 ºC, indicating the similar effect of both nanofillers. This is because of the high distribution of the filler materials within the polymer medium and the restricted movement of PVDF chains by the nanomaterials. The well distributed nanoparticles cause for the higher crosslink density, stronger adhesion with the polymer chains and improved crystallinity, leading to the high thermal stability [37]. However the hybrid composite enhanced thermal stability to much higher value reaching up to 482.6 ºC for the composite containing FeO/ZnO at 1 wt.%.
Table 1 shows the tensile behavior of the fiber samples in terms of tensile strength, Young’s modulus and stiffness values. All values show a regular increase with the addition of nanoparticles and also with increased concentration. When the hybrid filler is compared with the FeO and ZnO individual composites, higher tensile values are achieved due to the filler synergy. Both semiconducting metal oxide nanoparticles are oriented in all directions within the PVDF polymeric chain, and forms interconnected networks [38]. The stress transfers from the polymer to filler networks during the tensile measurements. High stiffness is also observed for the PVDF composite containing 3 wt% FeO/ZnO combination.
Table 1
Tensile properties of the PVDF nanocomposite fibers.
Samples
|
Young's Modulus
(MPa)
|
Tensile Strength
|
Stiffness N/mm
|
PF
|
6.95 ± 1.5
|
0.156 ± 0.12
|
0.52 ± 0.05
|
PF-FeO
|
8.87 ± 0.52
|
0.192 ± 0.15
|
0.63 ± 0.10
|
PF-ZnO
|
9.71 ± 3.5
|
0.313 ± 0.09
|
0.69 ± 0.08
|
PF-FZ1
|
12.54 ± 2.1
|
0.370 ± 0.10
|
0.81 ± 0.08
|
PF-FZ3
|
17.52 ± 2.5
|
0.475 ± 0.11
|
1.03 ± 0.12
|
3.4. Piezoelectric properties and correlation with crystallinity
Piezoelectric properties of the composite fibers are tested by preparing the nanogenerators with the help of aluminium foil, copper wires and PET substrates [11]. Rectangular fibers of 2.5 cm X 2.5 cm dimension were sandwiched between the aluminium foils and copper wires were connected using conducting tape on both sides. Double side adhesive tapes were used to wrap PET substrates on both sides of the sample. This PENG is tested for the piezoelectric output voltage as per the established protocol [21], explained in the Sect. 2. Figure 5 represents the piezoelectric output voltage obtained for all the PVDF fibers when placed under a load of 2.5 N and at 1 MΩ resistance. With increased vibrational frequency, the output voltage increases and reaches a point at which the voltage levels off.
The PENG made from the 3 wt.% of the hybrid nanofiller shows the highest output compared to all the other composite fibers, attributed to the reinforcing effect achieved by the filler synergy. The metal oxides form interconnected networks throughout the sample and allows the mechanical vibration transport and the electrical energy transfer [39]. This is happening as the FeO and ZnO filler materials have unique dipole alignment and ferroelectric characteristics helping in generating the piezoelectricity [21, 22]. Electrospinning allows the dipole alignment, which is responsible for the piezoelectricity [11]. The dipoles in the ZnO and FeO also contribute to the enhanced piezoelectric response. Moreover, the nanoparticles act as stress connecting points, thus separating the electrospun PVDF fibers to many segments. This causes increased local deformations and thus again results in the high piezoelectric output voltage.
Figure 6a compares the maximum output voltages obtained for all the samples, and shows the flexibility of the PENG. The voltage variation of the PVDF/FeO/ZnO at 3 wt.% composite fiber for 2000 cycles is also represented in the Fig. 6b. In all cases, the hybrid composite is identified to have good response, which is attributed to the following factors. When the ZnO and FeO hybrid particles are present in the composites, the β-phase crystallinity was more as evident from the FTIR and XRD results. This is further explained with the help of DSC studies and it is believed that the hybrid fillers cause β-phase nucleation [40]. It is also established that the interfacial effects and electroactive phase nucleation happen in the presence of ferrite nanoparticles with magnetic characteristics such as FeO [41]. The FeO has surface charges and its crystalline nature contribute to good piezoelectric property enhancement. Higher mechanical stability is achieved for the hybrid filler reinforced samples, due to the networking effect and this enhances the stress of fibers and the PVDF local stress. Moreover, the semiconducting nanoparticles can form connections or conducting paths throughout for the induced charge transfer within the polymer nanocomposites. The significance of current research in comparison with the published studies is illustrated in the following table (Table 2). It is clear that the advantage of the current research lays in the less concentration of nanomaterial used, absence of complicated nanomaterial processing and in the high voltage obtained.
Table 2
Comparison of the current piezoelectric voltage achieved with the published values.
Nanocomposite
|
Fabrication
|
Concentration
|
Output Voltage
|
Reference
|
PVDF/graphene/BaTiO3
|
Electrospinning
|
0.15 wt% (graphene)/15 wt% BaTiO3
|
11 V
|
[11]
|
PVDF/Fe–ZnO
|
Gamma irradiation of casted films
|
2 wt%
|
2.4 V
|
[23]
|
PVDF/Cloisite 30B
|
Electrospinning
|
15 wt%
|
70 V
|
[33]
|
PVDF/TiO2–Fe3O4-MWCNT
|
Electrospinning
|
2 wt%
|
51.42 mV/N
|
[39]
|
Current Study
|
Electrospinning
|
3 wt%
|
5.9 V
|
|
Figure 7 illustrates the heating and cooling curves of the PVDF composite fibers, investigated by the DSC analysis. The melting and crystallization temperatures of the composites and the crystallinity are illustrated in the Table 3. The melting temperature of neat PVDF increases from 158.92 ºC to 161.07 ºC, when the FeO/ZnO are added to the polymer. This is attributed to the increased interaction between the polymer and the metal oxide nanomaterials, coming from the good dispersion of metal oxides and its reinforcing effect. Crystallization temperature of the PVDF nanocomposites also follow similar trend of variation indicating the change in crystallization behavior by the introduction of metal oxides. The typical reasons for the improved crystallization behavior of the nanocomposites are the high viscosity and the increased entanglement [42].
Table 3 shows the melting and crystallization temperatures of the PVDF composites and the crystallinity values of the samples. Crystallinity (Xc) of the fibers are calculated from the melting enthalpy (ΔHm) value and weight fraction of filler particles (ϕ) in PVDF composites according to the following equation [43].
ΔHmo is the melting enthalpy value for a typical α- PVDF crystal and is fixed as 104.6 J/g.
The crystallinity values of the PVDF composite fibers corresponds to the crystalline regions of both α and β phase [44]. From the values given in Table 3, a small but gradual increase of PVDF crystallinity with increase of filler concentration is observed, which can be due to the filler network formed within the polymer chains as a result of good dispersion. There is a small decrease in crystallinity when the higher concentration of FeO/ZnO is considered, because of the hindered PVDF chain diffusion by the highly concentrated nanomaterials, preventing the crystallization processes [44].
Table 3
Crystallization and melting parameters of PVDF nanocomposite fibers
Samples
|
Tm (ºC)
|
Tc (ºC)
|
Crystallinity from DSC Xc (%)
|
Crystallinity from FTIR
|
PF
|
158.92
|
129.94
|
33.31
|
21.36
|
PF-FeO
|
160.34
|
131.02
|
33.96
|
21.95
|
PF-ZnO
|
160.20
|
130.14
|
33.15
|
21. 54
|
PF-FZ1
|
160.89
|
131.64
|
34.43
|
22.39
|
PF-FZ3
|
161.07
|
131.96
|
34.03
|
22.17
|
3.5. Dielectric properties of the nanocomposites
The frequency dependent dielectric properties of the nanocomposites are demonstrated in the Fig. 8. The dielectric constant for the PVDF-FeO/ZnO sample at 3 wt.% nanoparticle concentration shows a value of 21, which is around 7 times higher than that of the neat polymer fibers. This enhancement in dielectric constant value for the nanocomposite is due to the Maxwell-Wagner-Sillars (MWS) effect by which the interfacial polarization occurs with the charge accumulation and short-range dipole dipole interactions at the PVDf/nanoparticle interface with the electrical field. With frequency, the number of aligned dipoles decreases and thus the dielectric constant. At higher frequencies, the dipoles lag with the applied electric filed as well. In addition to the MWS effect, the total interfacial area per unit volume of the nanoparticle also influences the dielectric constant value enhancement. Higher interfacial area reduces the interparticle distance in unit volume, leading to inter particle coupling and enhanced average polarization [5]. Moreover, the beta phase of the PVDF is improved by the addition of FeO and ZnO nanomaterials, which also contribute towards higher dielectric constant. Figure 8b shows the variation in loss tangent (increasing trend with the nanoparticle loading) with frequency. In this figure, a relaxation region of 102 to 105 Hz frequency is observed which can be attributed to the α-C type of relaxation [40].