Synthesis and dielectric investigations of bismuth sulfide particles filled PVA: Polypyrrole core-shell nanocomposites

Materials Science & Engineering B 224 (2017) 171–180

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Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb

Synthesis and dielectric investigations of bismuth sulfide particles filled PVA: Polypyrrole core-shell nanocomposites Vidyashree Hebbar, R.F. Bhajantri

MARK

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Department of Physics, Karnatak University, Dharwad 580 003, Karnataka, India

A R T I C L E I N F O

A B S T R A C T

Keywords: Bismuth sulfide Nanocomposite PVA:Polypyrrole Blend Dielectric SEM

High dielectric composites of bismuth sulfide (Bi2S3) filled in poly (vinyl alcohol) (PVA)/polypyrrole (PPy) blend were prepared. Fourier Transform Infrared Spectroscopic analysis infers the encapsulation of Bi2S3 particles by PVA: PPy matrix. The thermal activation is estimated using Coats and Redfern equation from thermogravimetric analysis results. SEM micrographs revealed that Bi2S3 filled PVA:PPy blend composite films possess smooth surface and homogeneous dispersion of particles. The frequency dependence of dielectric constant, loss tangent and electric modulus were analyzed. The characteristic relaxation times for dielectric loss within the composites are calculated. The AC conductivity is maximum for 8 wt% of bismuth sulfide particles. The dielectric parameters and AC conductivity are temperature dependent. The dielectric response parameter s follows the correlated barrier hopping model and accordingly effective barrier height (Wm) of the composite for 8 wt% Bi2S3 filled PVA: PPy are determined.

1. Introduction Electrically conducting polymer matrices are of enormous and effective interest for their exploitation in the field of polymer research and molecular engineering, since they own high conductivity as compared to other polymers and are used as a replacement for metals [1]. Polypyrrole (PPy) is one of the capable conducting polymers which have got increased attention because of its practical applications based on the ease of synthesis, low cost, excellent environmental and thermal stability, relatively low density and high conductivity [2]. It has a potential application in the field of electronic devices such as an actuator, supercapacitor etc. [3]. PPy can act as a potential energy storage material, because of its intrinsic electrical conductivity and redox properties [4]. However, PPy is limited with insolubility, poor processability and lack of essential mechanical strength, which hinder its capacity for huge production. The limitations of PPy can be conquered by introducing it into the other mechanically stable polymers while producing the composite or by blending with a suitable polymer matrix. PVA is one of the most preferable semi-crystalline polymers for the fabrication of environmental friendly, biodegradable, and water-soluble polymer composite as it owns excellent adhesion, and renowned chemical and physical properties [5,6]. The inorganic materials covered with conducting polymers are of much interest, as they possess environmental stability and easy processability and also expose the specific properties of the inorganic core

⁎

Corresponding author. E-mail address: [email protected] (R.F. Bhajantri).

http://dx.doi.org/10.1016/j.mseb.2017.08.001 Received 10 May 2017; Received in revised form 11 July 2017; Accepted 1 August 2017 0921-5107/ © 2017 Elsevier B.V. All rights reserved.

[7]. The one-dimensional filler particles gained extensive attention because of their large aspect ratio. Bi2S3 is a V–VI group compound, ntype crystalline semiconductor. The earlier reported band gap of bulk Bi2S3 is 1.3 eV-1.7 eV that lies in the visible solar energy spectrum and in turns it is a promising material for the photovoltaic applications [8,9]. Several studies have been reported on the morphologies of Bi2S3 in the form of particles, nanorods, nanotubes, nanowires, nanoflakes and nanoflowers [10]. It has been proposed to be useful as a good solidstate thermoelectric material, due to its significant thermoelectric properties [11]. The electrical properties of polymer nanocomposites are different from those of bulk materials due to an increased number of interfacial atoms and defects. The AC impedance approach is used for characterizing the electrical properties of polymeric materials. The dielectric property of the composites depends on the volume fraction, size, and also shape of conducting fillers, as well as other factors such as preparation method, the interfacial interaction between the fillers and the polymer. Various new technological applications require energy storage capacitors (i.e., batteries) in order to be functional. In other words, the high dielectric permittivity is needed for the extensive application. High dielectric constants have been reported for a variety of materials in recent years, including ceramics, ceramic/polymer and polymer/ polymer (or organic/polymer) composites [12]. On the other hand, the polymer nanocomposites with one-dimensional filler provided with a drawback of high dielectric loss. Moreover, polymer-coated inorganic

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was carried out using Ferric chloride as the oxidizing agent. It is notable that PPy-based composites produced using FeCl3 present a higher level of conductivity than those prepared using other oxidizing agents [16]. 2.187 g of FeCl3 is dissolved in 5 ml of distilled water and is added dropwise to the PVA: pyrrole mixture, along with continuous stirring. The black homogeneous mixture formed designates the polymerization. The composite film is obtained by casting this solution into the Petri dishes kept in hot air oven at a constant temperature of 30 °C, for the evaporation of the water content. Black colored composite films were peeled off after complete drying.

materials endow a new kind of material with improved electrical, optical, magnetic, and dielectric properties and enhanced stability that has substantial applications in the photovoltaics, thermoelectrics, infrared spectroscopy and also field emission techniques [7]. Numerous works have been reported on PVA: PPy composite, however only a few works have been reported on metal sulfides filled PVA: PPy composites. Harun et al. reported the preparation of PVA: PPy composite polymer films by chemical oxidative polymerization by using FeCl3 as dopant and oxidant. The variation of dielectric properties in the frequency range from 20 Hz to 1 MHz at room temperature with the concentration of FeCl3 was studied [13]. Goutam Chakraborty et al. studied the electrical transport properties of CNT-PVA: PPy system. They reported the semiconducting nature and also DC magnetoconductivity of the composites [3]. M. T. Ramesan synthesized CuS filled PVA: PPy composites using ammonium persulfate as oxidant and reported the AC and DC conductivity study of the samples [14]. J. Liu et al. studied the dielectric properties of Bi2S3/poly(vinylidene fluoride) composites and reported the dielectric permittivity for 10% filler doped sample is about 100 at 103 Hz [15]. The objective of the present work is to build up a multifunctional composite with convenient conductivities and superior physical properties. With this orientation, pure PVA: PPy blend and different amount of bismuth sulfide particles coated with PVA: PPy blend composites were synthesized by in-situ chemical oxidative polymerization. The acquired polymer nanocomposite films were characterized for structural (FT-IR), thermal (TGA) and morphological (SEM) variations. The dielectric properties and AC conductivity with frequency at room temperature were measured. The composite exhibiting highest conductivity, considered for the further study with temperature and process of conduction is discussed.

2.4. Synthesis of Bi2S3 filled PVA: PPy blend composite The aforementioned procedure for the polymerization of pyrrole in PVA medium is followed but in presence of different wt% of Bi2S3 particles (1%, 3%, 5%, 8%). Here, bismuth sulfide particles are added prior to the addition of monomer. All the films were stored in a vacuum desiccator for further investigation. The thickness of the prepared composite films was measured using Mitutoyo-7327 dial thickness gauge, which is obtained in the range 320–340 μm ( ± 1 μm).

2.5. Characterizations The FT-IR spectroscopy analysis of polymer blend nanocomposite films was carried out on ThermoFisher NICOLET FT-IR-6700 by KBr pelleting method in the wave number range of 400–4000 cm−1 with the resolution 4 cm−1. The polymer films were mixed with KBr salt in the ratio 1:100 and crushed with an agate mortar and pestle to make the pellets. The TGA thermograms of the prepared samples were obtained on TA instruments model SDT Q600 V20.9 Build 20 from 25 °C to 600 °C at a rate of 10 °C min−1 in the platinum crucible under flowing Nitrogen at a rate of 60 ml/min. The weight of the dried composite film utilized in the analysis is about 5 mg. The Scanning Electron Microscopy (SEM) of gold sputtered 1 wt% and 8 wt% Bi2S3 filled PVA: PPy polymer film samples was recorded on JEOL Model JSM - 6390LV in SEI mode with the resolution of 10 μm and the acceleration voltage of 15 kV. The morphological images of pure PVA: PPy blend), 3 wt% and 5 wt% Bi2S3 filled PVA: PPy films were recorded using CARL ZEISS instrument with an acceleration voltage of 5 kV and the scanning was done with WD about 4.7 mm in FEI mode. The prepared polymer nanocomposite films were cut into a small disk of diameter 1.5 cm and were placed between the stainless-steel electrodes of diameter 1.3 cm to measure the dielectric parameters. The dielectric properties of the composites were studied on HIOKI-IM 3570 high precision Impedance Analyzer in the frequency range 4 Hz to 5 MHz in LCR mode. The variation of dielectric properties with temperature is measured with the help of dry temperature calibrator (Model DPI-1100) set at the temperatures 303 K, 313 K and 323 K ( ± 1 K) on the same impedance analyzer. The measurement temperature is restricted to this range because of the hardening of the composite films beyond this temperature. Frequency dependent values of series capacitance (C) and loss tangent (tan δ) were taken for the different samples. Prior to the sample measurements, the open circuit and closed circuit calibration were performed for the instrument to eliminate the effect of stray capacitance. The real part (permittivity) and imaginary part (dielectric loss) of the complex relative dielectric function

2. Experimental 2.1. Materials Pyrrole monomer (C4H5N) with the molecular weight 67.09 g mol−1 purchased from Spectrochem Pvt. Ltd. Mumbai, India was stored under refrigeration before use for the synthesis. The other materials such as FeCl3 (M. W.= 169.21 g mol−1), polymer polyvinyl alcohol (PVA) ([(-C2H4O-)n], M. W. ∼1,25,000 g mol−1) were obtained from M/s. s. d. fine chemical Limited, Mumbai, India. Bismuth nitrate pentahydrate (98% (Bi(NO3)3·5H2O, M. W. = 485.07 g mol−1, Alfa Aesar, England) and Sodium sulfide (Na2S·9H2O) were used for the preparation of the samples without further purification. 2.2. Synthesis of Bi2S3 particles The weighed amount of Bi(NO3)3·5H2O was added to the distilled water to get 0.01 M concentration. The mixture is allowed for continuous stirring up to three hours and the clear white solution was obtained. To this solution, 0.03 M of Na2S·9H2O was added drop by drop until the solution turned into dark brown. The resultant mixture was left for 12 h to stabilize [13]. The precipitate obtained is washed with absolute ethanol and distilled water several times to remove unreacted contents, and dried at 60 °C to get dark brown colored Bi2S3 particles. 2.3. Synthesis of PVA: PPy blend composite

ε ∗ (ω) = ε′−iε″ The pre-weighed quantity (1.5 g) of the polymer PVA is dissolved completely in distilled water (30 ml) by heating to a temperature of 40 °C, with continuous stirring. The solution is left to cool to room temperature. 0.5 ml of cooled pyrrole monomer diluted in 15 ml of distilled water is added to the viscous PVA solution and stir for 30 min to get the homogeneous mixture. The above mixture in kept in an ice bath to maintain the temperature 0-5 °C. The polymerization of pyrrole

(1)

are calculated with the relation ε' = Cd/εoA where, C is capacitance value, d is the thickness of the sample, A is the area of the electrode and εo(=8.85 × 10−12 F m−1) is permittivity of free space and ε″ is the dielectric loss (ε″ = ε' tan δ) [17]. The complex electric modulus composed of the real part (M') and imaginary part (M″) is determined by the following equation [18] 172

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M ∗ (ω) =

ε′ ε″ 1 = M′ + iM″ = 2 + 2 ε ∗ (ω) ε′ + ε″ 2 ε′ + ε″ 2

cm−1 is resulted by the overlapping of –NH and –OH stretching vibration of Polypyrrole and PVA, respectively which specifies the proper blending of the polymers [14]. The band is shifted to higher wavenumber with the increase in the concentration of Bismuth sulfide particles within the polymer blend matrix. The bands at 2930 and 2858 cm−1due to the asymmetric and symmetric stretching vibrations of –CH2 group are also observed [19]. The characteristic IR peaks for C]C stretching of the aromatic ring of Polypyrrole (1553 cm−1), C–N stretching vibration (1402 cm−1) along with the vibration peak correspond to –CH in-plane motion (1328 cm−1) are revealed in the acquired FT-IR spectra [20]. The peak at 1199 cm−1 attributed to the breathing vibration of pyrrole ring [5]. The prominent band of the blend at 1085 cm−1 ascribed to the in-plane deformation vibration of –CH and –NH group, slightly shifted to higher wavenumber with the decrease in intensity as the content of Bismuth sulfide particles increases, suggest the successful incorporation of Bi2S3 particles within the PVA: PPy polymer blend matrix [19,6]. The IR bands at 924 cm−1 and 840 cm−1 are due to –CH in-plane and out of plane bending deformation [14]. A small peak near 678 cm−1attributed to C–C out of plane ring deformation or C–H rocking which is slightly shifted to higher wavenumber [20]. In the FT-IR spectrum of pure Bi2S3 the band at about 700 cm−1 assigned to the metal (Bi) and Sulfur (S) interaction. The band at 994 cm−1 may attribute to the resonance interactions between the modes of vibration correspond to Sulfur ion [21]. Intriguingly, in the FT-IR spectra of Bi2S3 filled PVA: PPy polymer blend composites, the characteristic peaks of pure Bi2S3 are not present and also no new bands are appeared, suggest that the Bi2S3 particles have been enclosed during polymerization [7]. However, the shift and change in intensity of some bands of the composites impute that the Bi2S3 particles were well incorporated within PVA: PPy blend matrix.

(2)

The real part of complex AC conductivity at different frequencies for the polymer composites at room temperature was obtained using the relation,

σac = 2πfεo ε′tanδ = εo ε″ω

(3)

where f is the frequency, ω = 2πf is the angular frequency, ε' is the dielectric constant and tan δ is the loss factor. The total conductivity of the material is measured as

σtotal (ω) = σdc + Aω s

(4)

The AC conductivity of the materials follows the empirical formula of the frequency dependence given by the Jonscher’s universal power law

σac = Aω s

(5)

where both A and s depends on the composition and temperature. A has the dimension of electrical conductivity and s is the power law exponent such that 0 < s < 1, which is dimensionless. 3. Results and discussions 3.1. FT-IR spectroscopic analysis The effect of the introduction of bismuth sulfide particles on the microstructure of polymer blend matrix PVA: PPy is established by Fourier Transform Infrared (FT-IR) spectroscopic studies. The FT-IR spectra recorded for the pure PVA: PPy blend, bismuth sulfide filled composites and pure Bi2S3 particles are represented in Fig. 1. As observed in Fig. 1, the strong broad IR band nearly about 3300

3.2. Thermal analysis The TGA thermograms and the corresponding derivative thermogravimetry (DTG) data curves of pure PVA: PPy blend and Bi2S3 incorporated blend are depicted in Fig. 2 (a) and (b), respectively. It is observed that all of them exhibit the weight loss in four stages. In the first stage (30 °C to 140 °C), the weight loss of the composites is about 15% due to evaporation of physically absorbed water content. The range of temperature from 160 °C to 220 °C termed as the stage II, where the maximum degradation occurs and the loss of weight from 230 °C to 280 °C (stage III) are attributed to total decomposition of filler and polymer chains by the evaporation of gases like CO, CO2, NH3 etc. [22,23]. The rate of loss in weight increases with the increase in Bi2S3 content within the polymer matrix is depicted in DTG curves (Fig. 2(b)). The exothermic peaks in the range 280 °C–480 °C (stage IV) can be assigned to the oxidation of carbonaceous reside from the decomposed polymer blend composites. There was no significant thermal effect observed in DTG curves and the corresponding TG curves above 500 °C implying that the carbon compounds get completely volatilized from the polymer blend composites [23]. The residual mass content of the composites decreased from 46% (for pure PVA: PPy) to 33.25% (8 wt% Bi2S3/PVA: PPy) with increase in wt.% of Bi2S3 particles in the blend. The initiation of the main degradation step for the pure PVA: PPy blend is at the temperature 161 °C and this gets shifted to lower temperature up to 3 wt% Bi2S3 concentration and then shifted to higher temperature with the increase in incorporation of Bi2S3 into the polymer matrix as shown in Fig. 2 (a) and (b), suggest the enhancement in thermal stability after 5 wt% Bi2S3 addition, which can also be attributed to the increase in orderliness of polymer chains around the Bi2S3 particles after 5 wt% Bi2S3 addition [24,25]. The increase in the intensity of the derivative curves at the main degradation step, specify that the weight loss is increasing with increase in Bi2S3 content in the polymer blend and is most apparent for 8 wt% Bi2S3/PVA: PPy composite. The maximum of the derivative of weight loss data gives the temperature at which weight loss is maximum for a particular composite (inflection

Fig. 1. FT-IR spectra of pure PVA: PPy blend, bismuth sulfide filled composites and pure Bi2S3 particles.

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Fig. 2. (a) The TGA thermograms and the corresponding (b) DTG curves of pure PVA: PPy blend and Bi2S3 particles incorporated blend.

temperature ‘TI’) [26]. The thermal activation energy of the prepared polymer blend nanocomposites were calculated with TGA thermal decompositions using Coats and Redfern equation [27], which is related to the residual weight of the samples. The first order Coats and Redfern equation used is given as:

log ⎡ ⎢ ⎣

−log(1−α ) ⎤ R ⎡ 2RT ⎤ 1 Ea = log 1− − ⎢ ⎥ ⎥ T2 E E RT Δ 2.303 a⎣ a ⎦ ⎦

(6)

where T is the absolute temperature, Ea is the activation energy in J mol−1, R (=8.3136 J mol−1 K−1) is the universal gas constant and α is the fractional weight loss at a particular temperature (T) which is given by

α=

wi−wt wi−wf

(7)

where wi and wf are the initial and final weight of the sample, respectively, wt is the weight of the sample at temperature T. The activation energy of the composites are determined by plotting the graph of –log [–log (1–α)/T2] against 1/T as the Eq. (6) which is in the form of the equation of straight line. The activation energy of the samples were estimated as

Ea = 2.303 × R × Slope

Fig. 3. Plot of −log [−log (1 − α)/T2] against 1/T to determine the activation energy for pure PVA: PPy blend and Bi2S3 particles incorporated blend.

Table 1 The inflection temperature, calculated apparent activation energies, intercept values with errors for pure PVA: PPy blend and Bi2S3 particles incorporated blend composites.

(8)

The plots of –log [–log(1–α)/T ] against 1/T for the determination of activation energy are given in Fig. 3. The values of inflection temperature, calculated apparent activation energies, intercepts with fitting errors are given in Table 1. From this table, it is clear that the activation energy values lie in the range of 39.45–51.8 kJ mol−1 with a statistical regression coefficient of 0.99 and the values are decreases with the increase in Bi2S3 content within the polymer matrix up to 5 wt%, indicates that, the incorporation of particles causes the decrement in thermal stability of the composites at lower concentration (up to 3 wt %) and increment in the thermal stability for higher concentration (after 5 wt% of Bi2S3). 2

3.3. X-Ray diffraction studies

Sample

Inflection Temperature ‘TI’ (K)

Activation energy ‘Ea’ (kJ mol−1)

Intercept

Pure PVA: PPy 1% Bi2S3/ PVA: PPy 3% Bi2S3/ PVA: PPy 5% Bi2S3/ PVA: PPy 8% Bi2S3/ PVA: PPy

185.92

51.80 ± 0.10

−1.47 ± 0.01

183.93

48.06 ± 0.17

−1.05 ± 0.02

184.08

44.26 ± 0.09

−0.36 ± 0.01

189.17

39.45 ± 0.12

0.64 ± 0.02

192.16

40.94 ± 0.09

0.60 ± 0.01

Bi2S3 (JCPDS: 17-0320) [28].

The XRD pattern of synthesized Bi2S3 particles are presented in Fig. 4. The XRD pattern of synthesized Bi2S3 exhibits reflection peaks at 2θ values of 15.68°, 17.75°, 22.29°, 24.91°, 28.89°, 31.65°, 35.52°, 39°, 39.92°, 42.52°, 49.09°, 52.49°, 57.17° assigned to (0 2 0), (1 2 0), (2 2 0), (1 3 0), (2 1 1), (2 2 1), (2 4 0), (0 4 1), (1 4 1), (4 2 1), (1 6 0), (3 5 1), (4 5 1) crystalline planes, which specifies the orthorhombic structure. All diffraction peaks exhibited by the pure Bi2S3 and Bi2S3 particles filled composites were consistent with the literature data of

3.4. Scanning electron microscopic analysis The morphology of the prepared composites was studied by SEM and the SEM surface micrographs of the pure PVA: PPy blend, composite films and synthesized Bi2S3 particles are represented in Fig. 5. The cross-sectional images of 1 wt% and 8 wt% Bi2S3 particles filled PVA: PPy blend composite films are given in Fig. 5 (g) and (h). The 174

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Dielectric polarization in a material is contributed from the mechanisms such as electronic, ionic, dipolar and interfacial polarization. The high value of dielectric constant at low frequency is mainly due to polarization at the interfaces. According to Maxwell-Wagner-Sillars (MWS) effect, in the low-frequency range, the free charge carriers generated by surface polarization are blocked at the interfaces between the filler and polymer matrix. In the present composites, this process might result in the formation of dipoles on the Bi2S3 particles. At low frequency, the dipoles and molecules have enough time to orient in the direction of applied field and thus result in the high value of dielectric constant. However, at high frequency, the dipoles and molecules fail to catch up the change in electrical field frequency, which tends to weaken the dependence of dielectric constant on the amount of filler. This phenomenon is inherent in all polymer composites with conducting or semiconducting fillers. Out of all the prepared polymer blend composites, 8 wt% Bi2S3/PVA: PPy carries the highest dielectric constant at a lower frequency (4 Hz, 8600) and (100 Hz, 1150) which are unusually higher than the reported values [14,15]. The dielectric loss (tan δ) also increased with increase in Bi2S3 content in the polymer blend composites because of increase in a number of mobile charge carriers in the system [31]. With the increase in frequency, the tangent loss initially decreased and at an intermediate frequency, the dielectric loss tangent attains the highest value due to relaxation phenomenon, in all the composites. The composites show the dielectric loss tangent peak in the range 1 kHz to 3.16 MHz. The lowest tangent loss of pure PVA: PPy might be due to the amorphousness of the surface, and the orderliness get raised with the addition of Bi2S3 particles. The dielectric loss tangent is maximum for 5 wt% Bi2S3/PVA: PPy composite and the decrement is observed for 8 wt% Bi2S3/PVA: PPy composite due to the development of heterogeneity in the system. The maximum value of tan δ of 5 wt% Bi2S3/PVA: PPy at 50 kHz is 6.87. The measured dielectric loss tangent at a given frequency can be roughly ascribed to the loss due to polarization and conduction. The relaxation occurs in the polymer blend composites is connected with the MWS polarization, which takes place at the interface between amorphous and crystalline phases [32]. The relaxation peak is the one where the jumping frequency of the localized electric charge carrier is approximately equal to that of the externally applied electric field [33]. In the polymer composites, the relaxation process depends on the factors such as the fraction of ion pairs, the concentration of additive and temperature. The measurement of relaxation frequency and thus relaxation time are the probe to predict the flexibility of the polymer composite. In the plots, it is noted that tan δ increases with frequency and reaches a maximum value and thereafter decreases. The relaxation time τo is estimated using the relation ωτo = 1 and is mentioned in Table 2. In the present composites, the relaxation peak shift towards high frequency with the increase in filler content suggest the decrement in the strength of relaxation which in turn impute the enhanced flexibility of the polymer chain [34]. The enhancement in the flexibility causes the increment in conductivity of the composite. The relaxation time is highly reduced for 8 wt% Bi2S3/PVA: PPy which allows the easy orientation of the dipoles and hence contribute to dielectric permittivity. In general, the AC conductivity becomes predominant at a higher frequency; however, the conductivity is almost independent of frequency at lower frequency range [35]. Similarly, in the present case, the AC conductivity of the composites are constant at low frequency and increases from about 100 Hz and almost constant after 10 kHz except for 8 wt% Bi2S3/PVA: PPy composite. This certifies that the prepared polymer blend composites obey Jonscher’s power law. The behavior of conductivity at high frequency represents a bulk relaxation phenomenon. The AC conductivity increased with the increase in the concentration of filler and shows its maximum for 8 wt% Bi2S3/PVA: PPy composite. In this sample, the conductivity attains maximum value at 5.4 MHz and then again decreases. The maximum conductivity

Fig. 4. XRD pattern of synthesized Bi2S3 particles.

surface morphology of pure PVA: PPy in Fig. 5 (a) appears to be rough by the presence of voids which might be due to the air bubbles formed while preparing the film. The polymer blend composites filled with Bi2S3 particles possess highly dense matrix as noticed in the surface micrographs. A careful inspection of the SEM images of the polymer blend nanocomposites reveals that the Bi2S3 particles are dispersed homogeneously within the polymer blend matrix. The surfaces of the composite films are highly smooth, although few particles get aggregated. The smoothening of the surface after the addition of the filler matched with SEM result reported by Shaoqiang Cao et al. [29]. It is clearly visible in the SEM morphology of composites that few larger aggregates were present and also no phase separation in the matrix, caused by a good adhesion of Bi2S3 particles with PVA: PPy blend matrix [30]. The SEM surface micrograph of 5 wt% Bi2S3/PVA: PPy with 1 μm resolution in Fig. 5 (d) demonstrate that the Bi2S3 particles are nearly spherical shape and the particles wrapped by the polymer (PVA: PPy) network [14], which supports the FT-IR data. The diameter of the core-shell structured spheroids calculated with PIXEL Ruler ranges between 172 nm to 815 nm. To further illustrate the dispersion of the Bi2S3 particles within the polymer blend medium, the crosssectional view of 1 wt% and 8 wt% Bi2S3/PVA: PPy also recorded. The size of the spheroids in the cross-sectional SEM view of the 8 wt% Bi2S3/PVA: PPy composite is calculated with PIXEL Ruler, which is in the range 626 nm to 1.56 µm. The increment in the number of particles and their systematic distribution within the polymer blend matrix can be justified with the cross-sectional SEM images. 3.5. Variation of dielectric properties with composition The measurement of dielectric parameters is helpful in understanding the capacitive behavior and the conduction mechanism of dielectric materials. The variation of dielectric constant, tangent loss and AC conductivity with frequency for the different wt.% of Bi2S3 particles filled PVA: PPy blend composites are represented in Fig. 6. All the composites exhibit similar nature of response with the frequency. The dielectric response is highest at the lower frequency and as the frequency increases the dielectric constant tends to decrease and becomes constant, while the conductivity is increased. The magnitude of dielectric constant at low frequency is enhanced with the concentration of Bi2S3 particles, might be due to the filling of voids. With the increasing Bi2S3 content, the conductive phase of the polymer matrix began to connect and form the conductive network where the dielectric constant increases abruptly. 175

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Fig. 5. SEM morphological images of (a) pure PVA: PPy blend (10 μm), (b) 3 wt.%Bi2S3/PVA: PPy composite, (c, d) 5 wt.%Bi2S3/PVA: PPy composite (10 μm, 1 μm), (e) 1 wt.%Bi2S3/ PVA: PPy composite, (f) 8 wt.%Bi2S3/PVA: PPy composite. Cross-sectional SEM images of (g) 1 wt% and (h) 8 wt.% Bi2S3 particles filled PVA: PPy composite films.

composition by plotting log σac versus log ω in the region 3.8 ≤ log ω ≤ 4.2 (the frequency dependent region immediately after the frequency independent region), which represents straight lines with the slope equal to the exponent s and the intercept equal to log A on the vertical axis at log ω = 0 and the plot is given in Fig. 6 (d). Generally, s takes values between 0 and 1. When s = 0, the electrical conduction is the DC conduction, but when s > 0, the conduction is the AC conduction [33]. It is observed that all the composites possess the

obtained for 8 wt% Bi2S3/PVA: PPy composite at room temperature is 0.0897 S cm−1. The increase in the filler content leads to the increment in the compactness and hence, connecting the conductive phases by filler network formation. These conductive links and increased probability of hopping of charge carriers result in enhanced conductivity [36]. The slowdown in the enhancement of conductivity in the higher frequency region might be due to the shorter conductive paths [37]. The power law exponent s in Eq. (5) was calculated for all the 176

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Fig. 6. The variation of (a) Dielectric constant, (b) Dielectric loss tangent, (c) AC conductivity and (d) Plot of log of AC conductivity versus log of angular frequency to determine s for the different wt.% of Bi2S3 particles filled PVA: PPy blend composites.

3.6. Variation of dielectric properties with temperature

frequency exponent in the range 0.5–1, which is the condition for ionic conductors [38]. The obtained values of relaxation time, conductivities and frequency exponent s for different wt.% Bismuth sulfide filled PVA: PPy composites are recorded in Table 2. The polymer chain segmental motion relaxations for these polymers blend nanocomposite materials were also analyzed by the modulus formalism in terms of a dimensionless quantity. The presence of long tail is the resultant of the large capacitance associated with the electrode. Both M′ and M″ attain high values at the high-frequency end and approach zero at low frequency suggests the elimination of electrode polarization effect [17]. The variation of M′ and M″ with frequency at room temperature is given in Fig. 7 (a) and (b), respectively. In the M″ plot, no conductivity peak is observed as it may lie outside the higher frequency of the measurement range.

The dielectric properties such as dielectric constant, dissipation factor and AC conductivity are studied as a function of frequency at the temperatures 303 K, 313 K, 323 K and are represented in Fig. 8 (a), (b) & (c), respectively. The study at high temperature is obstructed because of the hardening of polymer composite films. It is noted that the temperature has a profound effect on the dielectric properties of the polymer blend composites. With the increase in the temperature, the dipoles within the composites are free to orient in the direction of applied field. So that the dielectric constant of the 8 wt% Bi2S3/PVA: PPy composite raises up to 313 K and after that, the decrement is observed, might be due to the disorderliness in the system which prohibits the alignment of dipoles. It is noted that the maximum of tan δ shifts towards the high-frequency side with increasing temperature up to 313 K and again shifted to lower frequency side, which support that the temperature dependence of dielectric relaxation process [39]. At low

Table 2 The values of relaxation time, conductivities and frequency exponent s for different wt.% Bismuth sulfide filled PVA: PPy composites. Sample Pure PVA: PPy 1% Bi2S3/PVA: 3% Bi2S3/PVA: 5% Bi2S3/PVA: 8% Bi2S3/PVA:

PPy PPy PPy PPy

Relaxation time τo (μs)

Maximum AC conductivity σac (S cm−1)

Frequency exponent s

3.95 4.55 5.71 3.84 1.73

1.17 × 10−3 2.15 × 10−3 1.19 × 10−2 1.48 × 10−2 8.90 × 10−2

1.086 0.797 0.812 0.668 0.899

177

± ± ± ± ±

0.001 0.002 0.0004 0.0003 0.004

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Fig. 7. Variation in (a) Real and (b) Imaginary part of the complex electrical modulus with frequency for different wt.% Bi2S3 particles incorporated PVA: PPy blend.

Fig. 8. Variation of the (a) Dielectric constant, (b) Dissipation factor and (c) AC conductivity (d) Plot of log of AC conductivity versus log of angular frequency to determine s for 8 wt% Bi2S3/PVA: PPy composite as a function of frequency at different temperatures.

relaxation time, as there arises the constraints to mobility of dipoles. Therefore, the measurement temperature is restricted to 323 K. The loss tangent get decrease with the temperature at high-frequency region might be due to the inability of the dipoles to orient in the field direction [41].

frequency, dielectric loss tangent corresponds to 8 wt% Bi2S3/PVA: PPy composite increases with increase in experimental temperature due to the thermally activated relaxation of freely rotating dipoles and possible electron-phonon interaction [40]. The prepared samples become highly rigid at temperature 323 K, which may cause the increase in 178

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increase in the temperature causes the inflexibility imputed in the increased relaxation time. When the temperature is increased beyond this range, the composite film is hardened so that get break. The effective barrier height can be considered as the band gap in case of semiconducting materials. It is inferred from the Table 3 that the barrier height is decreasing with increase in temperature which in turn controls the conductivity. The 8 wt% Bi2S3/PVA: PPy composite exhibit the maximum conductivity of 0.0357 S cm−1 at 303 K, nearly about the ambient temperature. The study of temperature effect on the real and imaginary part of the electrical modulus with frequency for the 8 wt% Bi2S3/PVA: PPy blend composite is shown in Fig. 9. The M′ and M″ approach zero with a long tail feature at low frequencies, indicating that the material is capacitive in nature. As the temperature increase, the orientation of dipoles becomes easier which in turn contribute to an increment in electric modulus, apart from the other dielectric parameters. With the increase in temperature, the credible moduli peak shift towards the low-frequency region. At higher frequencies, both M′ and M″ reach the maximum and the shapes of the modulus curves are asymmetric and the peak may lie outside the measured range.

The conductivity of the materials depends on numerous parameters such as a number of charge carriers, charge migration between the coordinate sites of the polymer host, polymeric chain segmental motions and the morphological structure of the polymer. The increase in AC conductivity with frequency is the result of enhanced charge migration facilitated by the conducting pathways formed due to the filling of Bismuth sulfide particles and increased thermal movement of polymer chain segments [42]. The values of the frequency exponent yield the clear picture of conduction mechanism in the polymer composite. In order to estimate the variation of frequency exponent with temperature for 8 wt% Bi2S3/ PVA: PPy composite, log σac versus log ω in the region 3.5 ≤ log ω ≤ 4.6 (the initial of relaxation peak) is considered and it gives the straight lines with slope equal to the exponent s and the intercept equal to log A at log ω = 0 and the plot in presented in Fig. 8 (d). The calculated power law exponent s decreases with the rise in temperature signify that the mechanism of AC conduction arisen due to the hopping [40,35]. The nature of the conduction process in semiconducting polymer composites is explained mainly by quantum mechanical tunneling (QMT) (which includes: Overlapping large polaron tunneling, small polaron tunneling, electron tunneling) model and correlated barrier hopping (CBH) model. Overlapping large polaron tunneling (OLPT) conductivity mechanism requires that s should depend both on frequency and temperature and its value starting from ‘1’ at room temperature then must decrease first and again increase after passing a minimum. In small polaron tunneling, s must increase with the temperature. The electron tunneling model proposes that s should be independent of temperature and should depend on frequency. It is clear that the present samples do not follow any of these behaviors. However, according to CBH model, s decreases with temperature and the charge carriers hop between the sites over the potential barrier separating them [43]. Generally, CBH model is applicable only for the amorphous materials and it is valid for temperatures above 100 K, which can be relevant to the present sample. The behavior of s in CBH model ought to be just the behavior that we obtained in the experimental results for the sample 8 wt% Bi2S3/PVA: PPy for varying temperature [44, 35]. In the CBH model, the frequency exponent s is evaluated as

s = 1−

6kB T Wm + kB T ln(ωτo)

4. Conclusions Bi2S3 filled PVA: PPy blend polymer blend electrolyte films have been prepared by solution cast technique. The FT-IR study infers that the Bi2S3 particles were successfully embedded in the polymer matrix. TGA analysis reveals the photothermal stability of the high wt.% Bi2S3 filled composite and the thermal activation energy were estimated with Coats and Redfern equation. The SEM micrograph images and crosssectional SEM images suggest the homogeneous dispersion of Bi2S3 particles in the PVA: PPy blend matrix and viable wrapping of particles by the matrix. The dielectric behavior of the films has been analyzed in terms of the frequency response of dielectric permittivity (ε') and dissipation factor (tan δ). The high and temperature dependent dielectric constant was observed at lower frequencies followed by the anomalous dispersion at higher frequencies confirm the existence of Maxwell–Wagner–Sillars polarization. Furthermore, the dielectric permittivity increases with the filler content and 8% Bi2S3/PVA: PPy composite possess the maximum permittivity at 313 K. The dielectric loss tangent also increases in proportion to the content of particles within PVA: PPy matrix and it decreases with the temperature. The AC conductivity spectra of the materials obey Jonscher’s power law at a higher frequency. The AC conductivity of 8% Bi2S3/PVA: PPy increases with increase in frequency and is maximum at 303 K and then decreases, while the frequency exponent s decreases with the increase in temperature. These results are in good agreement with the correlated barrier hopping (CBH) model. The values of effective barrier height had been estimated. These high and variable dielectric permittivity values make the Bi2S3/PVA: PPy excellent material for technologically important dielectric applications such as insulators in capacitors and electrolytes in batteries. With the high dielectric constant and low dielectric tangent loss, 8% Bi2S3/PVA: PPy will be a prominent dielectric material among all the prepared polymer composites. The modulus studies depicted the excellent capacitive nature of the Bi2S3/PVA: PPy composites.

(9)

where kB, Wm, T, ω, and τo are Boltzmann constant, effective barrier height, absolute temperature, angular frequency and characteristic relaxation time, respectively. Since ωτo=1 in Eq. (9), the maximum barrier height Wm can be estimated as

Wm =

6kB T 1−s

(10)

The values of maximum AC conductivity of the composite 8 wt% Bi2S3/PVA: PPy, estimated power law exponent, the effective barrier height and relaxation time calculated for different temperatures are tabulated as Table 3. The time of the thermally activated conduction process decreases initially up to 313 K, i. e, the composite becomes flexible. The further Table 3 Temperature dependent maximum AC conductivity, estimated power law exponent the effective barrier height and relaxation time of the sample 8 wt% Bi2S3/PVA: PPy. Temperature (K)

Maximum AC conductivity ‘σac’ (S cm−1)

Frequency exponent ‘s’

Effective barrier height ‘Wm’ (eV)

Relaxation time ‘τo’ (μs)

303 313 323

0.0357 0.0149 0.0042

0.844 ± 0.0004 0.550 ± 0.0003 0.424 ± 0.0001

1.008 0.360 0.290

1.58 0.88 5.30

Acknowledgements One of the authors, Vidyashree Hebbar is thankful to Karnatak University, Dharwad for awarding UGC-UPE fellowship. The authors are thankful to UGC, New Delhi for the SAP-CAS Phase-II (F.530/9/ CAS-II/2015(SAP-I) for providing research grants, and Science and Engineering Research Board (SERB), Department of Science and Technology (DST), Government of India for the research project(SB/ EMEQ-089/2013). The authors are thankful to USIC, Karnatak University, Dharwad for FT-IR and TGA facilities. The authors would 179

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