Introduction

The polymer nanostructured materials or polymer nanocomposites represent the alternative of conventional polymers and polymer blends. Nanocomposites are formed when discrete constituents on the order of a few nanometers are incorporated in the polymer matrix [1, 2]. In past few decades, the utility of inorganic nanoparticles in polymer matrix as additives to enhance polymer performance has been established and now provide numerous industrial applications [3]. The piezoelectric polyvinylidene fluoride (PVDF) and its copolymers are widely applied materials in both actuation and sensing mechanisms. In these nanocomposites, the final properties depend essentially on parameters such as grain size of particle, method of preparation of composites, and on the dispersion of the ceramic particles into the polymer matrix [4–7]. Piezoelectric ceramic materials have been under investigation for their active responses and their effective use in sensor and actuator applications [8]. Nanocomposites of electroactive ceramics and a ferroelectric polymer are very attractive for many applications as their properties can be easily tailored to suit particular performance requirements [9–14]. The piezoelectric and pyroelectric properties of PVDF have led for important technological applications. Piezoelectric polymers have advantage over piezoelectric ceramics because of light weight and excellent polarization properties [15–17]. Therefore, piezoelectric polymer (PVDF) is more useful for industrial as well as medical application. Despite advantageous mechanical properties such as flexibility and reasonable strength, it has excellent piezoelectric response [18]. Although, there are several techniques to understand structural dynamics in polymer nanocomposites, however, X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical microscopy techniques are very useful tool to investigate the structural morphology of polymer nanocomposites material [19–21], whereas differential thermal analysis (DTA) and differential scanning calorimetry (DSC) provide the thermal behavior of polymer nanocomposites.

Bucci and Fieschi et al. proposed the method of thermally stimulated depolarization/discharge current (TSDC) for the insulating polymers [22]. TSDC was become a widely used experimental technique for the investigation of various insulating material parameters such as charge storage, charge decay process, thermal stability, activation energy, and relaxation process [23]. TSDC studies on polymer has been shown that the total charge storage in polymeric material and various mechanism, which contribute the charge storage and transport process during the charging and discharging of polymers [24–29].

DTA can be used to study the melting point, polymorphism, thermal stability of the compound and comparison of the structure of the compounds can be studied. The DTA signal is the temperature difference between sample and reference, typically plotted against temperature. DSC is very similar to DTA, but due to a more sophisticated construction not only temperature itself, but additionally the heat flow into (or from) the sample can be measured. This allows, in addition to temperature, also measuring the heat energy (e.g., heat of fusion) that is consumed by the sample. TSDC is important to study the thermo-electrical properties of material under polarization and depolarization mode.

Although there have been several reports [30, 31] on PVDF/BaTiO3 nanocomposites with high loading of BaTiO3 (up to 50 mass% by volume), however, low loading concentration of BaTiO3 is not much reported in the literature. The β-phase PVDF shows the good piezoelectric coefficient due to net dipole moment present in material, therefore, β-phase is the most important in so many applications [32]. There are so many crystal phases are present in polymeric samples; the crystal phase morphology of PVDF films is verified by various methods like Fourier transform infrared spectroscopy (FTIR) and XRD to ensure the β-phase [33]. In the present work, we will study the effect of low loading concentration (i.e. 2, 4, 6 and 8 mass%) of BaTiO3 on β-phase PVDF. Structural morphology and thermo-electrical properties are investigated by using XRD, SEM, FTIR, DSC, DTA, and TSDC techniques. These experiments will provide the valuable information about the molecular interactions in PVDF and PVDF–BaTiO3 nanocomposite samples. The main objective of this work was to understand the polarization–depolarization mechanism as well as change in structural morphology of PVDF/PVDF–BaTiO3 samples. The outcome of TSDC will be correlated with nature of d33 coefficient. This is an attempt to develop the correlation of TSDC as a main experiment with other experimental techniques.

Experimental

Material

β-phase PVDF and BaTiO3 nanoparticle (cubic crystalline phase) of size less than 100 nm and with purity ≥99 % trace metal basis were procured by Sigma Aldrich. The dielectric constant of BaTiO3 is 5,379 at 100 Hz. All the samples of Pristine (PVDF) and BaTiO3 filled PVDF nanocomposite samples were prepared by using the solution grown technique. The analytical grade N–N Dimethylformamide (DMF) used as a solvent was procured from Merck India Ltd., New Delhi, India.

Sample preparation

The thin sample of PVDF and PVDF + BaTiO3 was prepared. The 10-g amount of PVDF powder dissolved in a glass beaker containing 100 mL of N,N-dimethylformamide (DMF) solvent, then the solution of PVDF/DMF was kept onto magnetic stirrer for 5 h at 333 K. The BaTiO3 nanocomposite powders 5 mass% of PVDF were dissolved in 20 mL DMF in separate glass beaker, then the solutions of BaTiO3 nanoparticles were kept onto magnetic stirred at simultaneous conditions of PVDF/DMF solution. Then the solution of BaTiO3 nanoparticles mixed drop by drop in the solution of PVDF/DMF. Then the solution of PVDF/BaTiO3 nanocomposite was kept onto stirred for 24 h. Solutions were seen at regular intervals to check the viscosity. After that the solution was sonicated for 15 min at ultrasonic frequency for the homogenous and better dispersion of BaTiO3 nanoparticles into PVDF with the help of sonicator. Now, final solution was poured onto glass substrates. The solvent was then allowed to evaporate inside an oven at room temperature for 24 h to yield the desired samples. The nanocomposite samples in the form of film were easily pealed off from glass substrates. The dried films of nanocomposites were subjected to room temperature to outgas at a vacuum of 10−5 Torr for a further period of 24 h to remove the residual solvent. The films were aluminized over the central circular area of diameter 3.5 cm using high vacuum coating unit.

Characterization

The XRD patterns of β-phase polyvinylidene fluoride (PVDF)/BaTiO3 nanocomposite have been recorded at room temperature using diffractometer with Cu Kα (λ = 1.5405 Å) radiation in θ–θ locked couple mode from the Brukar AXS D8 diffractometer with scan speed of 1° min−1. The diffraction angle (2θ) has been varied from 5 to 80° with a step size of 0.02. The measurements were done under ambient pressure conditions at room temperature. Each experiment was repeated at least twice and with both faces of the samples alternatively exposed to the X-rays to check the reproducibility.

The surface morphology of the β-phase (PVDF)/BaTiO3 nanocomposite was obtained using the SEM (LEO 435 VP) and inverted contrast microscopy (ICM). The film surfaces were gold coated prior to being scanned under high resolution scanning microscope.

DSC and DTA were performed on a Perkin Elmer (Pyris Diamond) instrument in nitrogen environment (200 mL min−1) to record the thermal stability of β-phase PVDF nanocomposite samples. The samples were heated with the heating rate of 10 °C min−1 from 30 to 300 °C. The weight loss and thermal decomposition temperature were measured and reported as a function of temperature.

The samples were thermally poled at room temperature with different polarizing field (i.e., 10, 20, 30, 40 kV cm−1). The aluminum electrode was vacuum aluminized by vacuum coating unit. The total time of polarization was taken to be 2.5 h including cooling of samples under electrical stress. The TSDC was measured by means of Keithley electrometer (Model no. 6514) connected in external circuit with constant heating rate of 3 °C min−1. In order to understand the surface morphology of polymers and polymer nanocomposite samples, the micrograph of PVDF and PVDF–BaTiO3 samples was recorded by optical micrograph (Nikon ECILTSE, Model No. PE2000). The d33 coefficients of PVDF and PVDF–BaTiO3 samples were measured by piezo tech (PM 100) piezometer at 110 Hz frequency.

Results and discussion

XRD analysis

XRD studies have been carried out in order to monitor the formation of the nanocomposites. XRD analysis of β-phase PVDF and nanocomposites (i.e., β-phase PVDF + BaTiO3) have been taken with different mass% of BaTiO3. XRD analysis used to study the structural change in crystalline behavior of PVDF nanocomposites with the incorporation of BaTiO3 in PVDF matrix. XRD spectra of pristine β-phase PVDF and nanocomposites have been shown in Fig. 1. Pristine PVDF spectra are showing the one sharp peak at 20.72° and one more peak at 40° as shown in XRD spectra of pristine PVDF, a sharp peak around 20.7, which corresponding to β-phase as reported in the literature [34], whereas β-phase PVDF nanocomposites spectra are showing some more sharp peaks at different value of 2θ. XRD result showing the full width half maxima (FWHM) increases with the decreasing the crystallite size along with the increasing concentration of nanofillers till 4 mass%, after 4 mass% crystallite size increases. The observed increase in peak intensity and FWHM is generally associated with decrease in crystallite size of the polymer nanocomposite. The crystallite size and width of the peak have been related by Scherer formula [35], the average crystallite size L has been calculated by Scherer formula

Fig. 1
figure 1

X-ray Diffraction spectra of β phase PVDF and PVDF + BaTiO3 nanocomposite

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$$ L = \frac{k\lambda }{\beta \cos \theta } $$
(1)

where β is the FWHM of the peak (in radian) and k is the shape factor whose value is equal to 0.9. The k = 0.9 in the above equation, the crystallite size of β-phase pristine PVDF and BaTiO3 filled β-phase PVDF nanocomposites samples were calculated. The crystallite size is decreasing after loading of BaTiO3 nanoparticles with different concentration in β-phase PVDF. Scherer has also showed that crystallite size L is inversely proportional to FWHM (β). The degree of % crystallinity of polymer nanocomposites samples has been calculated using the following relation [36]:

$$ K = \frac{A}{{A^{'} }} \times 100\,\% $$
(2)

where A is the total area of the peaks (area of crystalline and amorphous peaks) and \( A^{'} \) is the total area under the diffraction pattern. We found that as we increase the concentration of BaTiO3 nanofillers in β-phase PVDF, the percentage crystallinity of the nanocomposites has been decreases with decreasing intensity of the main peak up to 4 mass% loading concentration, then % crystallinity increasing with intensity, and again it decreases at 8 mass% of BaTiO3. Similar decreasing behaviors of the percentage crystallinity have been reported in our earlier work [20].

SEM and optical micrograph

Figure 2a–e show the SEM of Pristine β-phase PVDF and β-phase PVDF–BaTiO3 with different mass% (2, 4, 6 and 8 mass%) of nanoparticles. Scanning electron micrographs show uniform dispersion of BaTiO3 within β-phase PVDF. BaTiO3 nanofillers have been imbedded in the pores available in the polymer matrix as shown in SEM images. Whiteness of the images increases with increase in loading concentration. Figure 3 shows the optical micrographs of pristine PVDF and PVDF/BaTiO3 nanocomposite. The crystalline morphology for PVDF observed spherulites (i.e., Fig. 3a, b). The similar patterns of spherulites in PVDF have been reported in the literature [37, 38]. The size of spherulites can be controlled by nucleation, cooling rate, and can vary from micrometer to centimeter [39].

Fig. 2
figure 2

Scanning electron micrograph of β phase PVDF and PVDF + BaTiO3 nanocomposites

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Fig. 3
figure 3

Optical micrographs (a), PVDF (b). PVDF + 4 mass% BaTiO3 nanocomposite

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In Fig. 3a the size and shape of the spherulites are almost homogeneous and arrange in regular manner due to semi crystalline nature of PVDF. Also spherulites represent the crystalline portion of the polymer. However, in Fig. 3b the size and shape of the spherulites are heterogeneous, somewhere small and somewhere large and arrange in irregular manner. This change in size of spherulites may be due to decreased in crystallinity or increased in amorphousity of PVDF. Therefore, BaTiO3 play a nucleation role into nanocomposite of PVDF/BaTiO3 by decreasing the crystallinity of PVDF. These results are supported by XRD data.

FTIR spectroscopy

Figure 4 shows the FTIR spectra of pristine and nanocomposites samples at various contents of BaTiO3. The vibration modes of chemical bonds are characterized by the absorption bands of FTIR spectra. The various absorption bands in pristine and nanocomposite samples are assigned and summarized in Table 1. The FTIR spectra of the nanocomposite samples do not show any major change in bands, except some broadening and an increase/decrease in the intensity of bands have observed. Some weak bands present in pristine PVDF disappear at higher contents of BaTiO3 but at wave number of 1664.16 and 1748.43 cm−1, bonds are become strong with content of BaTiO3 as compare to pristine sample. Further β-phase of PVDF is confirmed by the bond position at wave number 836.45 cm−1 by FTIR which is originating due to the CH2 group [34].

Fig. 4
figure 4

FTIR spectroscopy spectra of phase PVDF and PVDF + BaTiO3 nanocomposites

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Table 1 FTIR analysis of β phase PVDF and PVDF + BaTiO3 nanocomposites
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Thermal analysis

DSC

DSC measurement is the advancement of DTA analysis in which we have investigate the thermal degradation temperature of polymeric samples, whereas the DSC analysis shows the increasing or decreasing nature of polymeric samples. Figure 5 shows the DSC of β-phase PVDF and nanocomposites with different mass% of BaTiO3. DSC spectra are not showing the glass transition temperature (T g) in β-phase PVDF and in nanocomposites samples because β-phase PVDF nanocomposites result shows the glass transition temperature in the negative range of temperature as reported in the literature [40], but DSC spectra is showing the small peaks at 166.89, 166.89, 165.19, 166.01, 166.01 °C at pristine, 2, 4, 6, and 8 mass% of BaTiO3 in PVDF matrix. These peaks are melting temperature peaks, which are verifying with DTA result of β-phase PVDF–BaTiO3 nanocomposites, XRD and DTA result as shown in Fig. 6 are also showing almost the same behavior of polymer nanocomposites with increasing concentration of nanofillers. DSC spectra show that melting point is not much more affected by loading of BaTiO3 nanofillers in β-phase PVDF matrix.

Fig. 5
figure 5

DSC spectra of β phase PVDF and PVDF + BaTiO3 nanocomposites

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Fig. 6
figure 6

DTA spectra of β phase PVDF and PVDF + BaTiO3 nanocomposites

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Thermally stimulated discharge current (TSDC)

There are considerable interests in the TSDC of PVDF and PVDF–BaTiO3 nanocomposites because of its use in piezoelectric and pyroelectric polymers. The dependence of the piezoelectric properties of PVDF and PVDF nanocomposites on the poling conditions, thus, one has to optimize the poling condition. TSDC might help to achieve this goal.

In semicrystalline polymers having a friction of the amorphous phase partially confined within the crystalline lamellae, as it is clear from microstructure of PVDF and PVDF–BaTiO3 nanocomposites (Fig. 3a, b). Therefore, chain will have the constrained mobility and we may expect the serious changes in glass transition dynamics. Adam–Gibbs develops a theory by introducing the concept of cooperatively rearranging region that defines the smallest region around a relaxing unit [41]. Due to this unit, the transition will undergo around this small region. This region is order of nanometer, therefore, if chain in amorphous regions is constrained within geometric below such sizes, they are incapable to relax with the same characteristic times as the unconstrained chain, and will exhibit higher T g values. In our earlier [40] publication, this situation is achieved due to incorporation of ZnO nanoparticle in PVDF. The confinements imposed nanoparticles by varying the size of crystalline lamellae are enough to affect the segmental dynamics in polymers. The local motion of charge carrier will also be affected by size of lamellae. Hence, the confinement is imposed by the nanofillers of definite size is very important to understand relaxation mechanism in semicrystalline polymers like PVDF. The existence of nanofillers in matrix of semicrystalline polymers may introduce new relaxation process appears at high temperature quite above the glass transition temperature. In PVDF, the conformational mobility of the chains of the amorphous phase is significantly limited near the crystallites. This means that the relaxation process in PVDF and PVDF–BaTiO3 nanocomposites depend on the crystalline fraction and also on microstructure.

The existence of interfacial regions with properties lying those of crystalline and undisturbed amorphous regions has been revealed by percentage crystallinity. XRD result shows that % crystallinity of PVDF decreases with increase in BaTiO3 nanoparticle contents. This evidence was demonstrated by SEM. The SEM images clearly demonstrate the existence of interface between amorphous and crystalline region. SEM also represents that the BaTiO3 nanoparticles are encapsulated between amorphous and crystalline boundaries.

In general, the TSDC of PVDF is characterized by two peaks one at −42 °C corresponding to glass rubber transition attributed by dipolar relaxation in amorphous phase. However, second peak at 140–180 °C is probably due to dipolar relaxation in the crystalline phase or in the region intermediate between crystalline and amorphous phases. Since this peak appeared well above glass transition temperature, therefore, destruction of charges accumulated near the amorphous–crystalline boundaries (Maxwell–Wagner effect) is also responsible for this peak. Figures 7, 8, 9 and 10 show a peak and, therefore, it involves the undistributed polarization. The PVDF nanocomposites are heterogeneous system consisting of different components or phases. These components have different dielectric constants and conductivities, electric charges are accumulated near the interface, when sample heated and subjected to a field. These charges are frozen in the interface if the field is maintained during cooling. During TSDC run, the interfacial charges are destroyed, because local field in the dissimilar parts of the sample is reversed so that neutralizing charges of opposite sign are conveyed to the interface. This process of giving rise to a peak is known as interfacial relaxation peak caused by Maxwell–Wagner effect. This peak is only expected in semicrystalline or micro and nanocomposite material as observed in present study. The activation energy of this peak is increasing with increase of polarizing field (i.e., Table 2). The volume friction of nanoparticles increases the current TSD as shown in Figs. 7, 8, 9 and 10. This is similar to increase of piezoelectric coefficient. The similar behavior of increase of piezoelectric coefficient is demonstrated in the literature [42]. The piezoelectric coefficient is observed to be increased with BaTiO3 contents; for PVDF pristine, its value was 37.8, but for PVDF/BaTiO3 its values are lying between 48.3 and 53.1. This increase of current TSD demonstrates the higher polarizability of PVDF. The relaxation time, charge storage, and activation energy are significantly affected by volume friction of BaTiO3 in PVDF matrix.

Fig. 7
figure 7

TSDC spectra of β phase PVDF and PVDF + BaTiO3 nanocomposites at polarizing field of 10 kV/cm

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Fig. 8
figure 8

TSDC spectra of β phase PVDF and PVDF + BaTiO3 nanocomposites at polarizing field of 20 kV/cm

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Fig. 9
figure 9

TSDC spectra of β phase PVDF and PVDF + BaTiO3 nanocomposites at polarizing field of 30 kV/cm

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Fig. 10
figure 10

TSDC spectra of β phase PVDF and PVDF + BaTiO3 nanocomposites at polarizing field of 40 kV/cm

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Table 2 TSDC parameters of PVDF–BaTiO3 thermo-nanoelectrets
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Another fact supporting to this peak is the activation energy. The activation energy value lies between 0.40 and 1.67 eV, is evidence for traps are energetically distributed in the bulk of polymer. The higher value of activation energy supporting the view that TSDC peak arises from space charge motion [40]. High temperature peak in PVDF is also altered drastically by impurities, fillers, absorbed water, and swelling agents. The fillers clearly increase the charge storage as shown in Table 2. This may be probable due to their dissociation or promote dissociation. It is observed that from Figs. 7, 8, 9 and 10 the position of peak is almost same and its height being affected because of low temperature polarization of all samples. The structure of polymer nanocomposite is complicated, therefore, it is very difficult to distinguish several relaxation processes and no any theory is being developed for TSDC of nanocomposites. However, appearance of single peak in TSDC experiment shows that the single relaxation process is the possible mechanism of these results. Activation energy corresponding to TSDC peak refers that the dipolar and interfacial polarizations are merged together and produce the single peak.

Conclusions

β-phase PVDF and BaTiO3 filled polymer nanocomposites film have been prepared successfully. SEM images supported the uniform dispersion of nanoparticles in the bulk of polymer using very simple technique of sample preparation. The XRD spectra analyzed that BaTiO3 reduces the crystallinity of polymer. The effect of BaTiO3 nanoparticle on thermal degradation is significantly affected, however, melting point of β-phase PVDF is remains almost same. It is concluded from FTIR interferrometry that the BaTiO3 is chemically non-interacting nature with PVDF molecules; therefore, the interaction of BaTiO3 is van der waals type of weak interaction with β-phase PVDF. It is further concluded that the weak interaction of BaTiO3 is the origin of significant variation of crystallinity and thermal degradation.

It could be understand by DSC, DTA, and TSDC techniques that TSDC peak (i.e., depolarization peak) of nanocompomsite material is similar to decomposition peak as observed by DSC and DTA. Therefore, TSDC measurement is not only limited to study the charge storage, charge transport, and charge relaxation phenomenon in material but also applied to study thermal properties by means of glass transition temperature, melting temperature, and polymorphism. Further, increase in depolarization current according to increase in concentration of BaTiO3 in PVDF matrix is an agreement with increasing value of d33 coefficients.