Structural, Electrical and Dielectric Properties of Dodecylbenzene Sulphonic Acid Doped Polypyrrole/Nano-Y2O3 Composites

We report a facile strategy to synthesized the PPy-DBSA-Y2O3 composites with increasing amount of Y2O3 (2%, 4% and 8%) by in situ polymerization method. The samples were characterized by XRD, FTIR, SEM and dielectric spectroscopy. Structural and morphological analysis reveals the conformation of composite formation, increase in crystallinity and particle size also increased by doping Y2O3 into PPy-DBSA matrix. The frequency dependent AC conductivity (σac), dielectric constant (ε′) and loss factor (ε″) were measured in the frequency range from 20 Hz to 1 MHz. All measured quantities; σac, ε′ and ε″ are enhanced with increase in Y2O3 content in the composites at all frequencies. Dielectric constant value for all samples is very high at low frequency however decreases with increase in frequency. Synthesis of PPy-DBSA-Y2O3 composite materials with a large dielectric constant is very important for the improvement of electronic devices applications.


Introduction
Intrinsically conducting polymers (ICPs) have significant applications in electromagnetic shielding, electrical displays, rechargeable batteries, optical computers and molecular electronics, several types of sensors and actuators, smart structures in electronic devices, drug release systems, synthetic nerves and inducting exchange membranes [1][2][3][4]. One of the outstanding features of ICPs is that it is promising to control conductivity of these polymers over extensive range from insulating to metallic by proper doping, addition of fillers and by intercalating these polymers into different inorganic host materials such as TiO 2 [5], TaS 2 [6], CdPS 3 [7], V 2 O 5 [8] and Y 2 O 3 [9]. Intercalation of conducting polymers in the interlameller galleries of nano-Y 2 O 3 clay has been explored by several researchers [10]. Clay based polymer composites have been focus of a large number of investigations during the past two decades because of their numerous engineering applications. These composites show better mechanical and thermal properties, cheap permeability for gases and flame retardant and fire dowsing characteristics [4,11,12]. Intercalation of polymer into the clay galleries and exfoliation of nanostructured clay platelets in the polymer matrix provide exclusive combination of organic and inorganic components at molecular level which communicates synergistic properties to conducting polymer clay composites which cannot be achieved from individual components, such as enhanced and well-disciplined electrical conductivity [13], improved thermal stability and outstanding mechanical, barrier and anticorrosive properties [14,15]. Composites of conducting polymers and nanostructured clays were used as conductive fillers in insulating polymer exhibits the increase in electrical conductivity and low percolation threshold [16] as compare with pure conducting polymer. Among several conducting polymers, polypyrrole (PPy) is one of the most studied conducting polymer, due to environmental stability, low cost, ease of synthesis and controlling their electrical properties [17]. PPy and its derivatives have been successfully employed in electronic devices as hole and electron transport semiconducting materials in transistors and LEDs [18,19]. In the existing work, to understand the mechanism of conduction and dielectric properties may be valuable in developing the device performance and designing the electronic devices. The survey of literature reveals that the detailed conductivity/dielectric studies on PPy-DBSA-Y 2 O 3 composites are scarce. Herein, we report the PPy-DBSA-Y 2 O 3 composites prepared by in situ polymerization method and characterized these samples by XRD, FTIR, SEM and dielectric spectroscopy.

Experimental Details
The Pyrrole (Fluka) was refined under reduce pressure also stored at low temperature before to use. Ammonium persulphate (APS), dodecylbenzene sulfonic acid (DBSA) and yttrium oxide (Y 2 O 3 ) were achieved from Sigma-Aldrich and used as obtained. The all materials were used as presented without any more purification.

Preparation of Polypyrrole (PPy)
The required amount of APS was isolated in 100 mL of deionized water and stirred for 1 h and then pyrrole was dissolved regularly within the suspension throughout the stirring at 5 °C. The monomer to oxidant molar ratio was kept 1:1. The suspension was left for 1 day for polymerization. Finally, the suspension was filtered and cleaned with deionized water until the filtrate turns to colorless. The greenish black paste of PPy was achieved which was desiccated at 60 °C in vacuum oven for 1 day.

Preparation of PPy-DBSA-Y 2 O 3 Composite
The 0.15 mol of DBSA was dissolved in 100 mL of deionized water, 0.3 mol of Pyrrole was put in gradually in the mixture solution which was retained on magnetic stirring. 30% hydrochloric acid (HCl) was introduced into the reaction mixture to keep pH value 0 and 1 with strong magnetic stirring. To this solution, Y 2 O 3 powder was dissolved into the polymerization mixture under stirring. After 1 h, the proper amount of APS was dissolved in 100 mL of de-ionized water drop wise under strong stirring for half hour period. The monomer/oxidant/dopant molar ratio was taken 1:1:1/4. Then, 1 L methanol was also incorporated into the mixture and kept it at room temperature for 2 days to attain complete polymerization. After all, the suspension was filtered and cleaned with de-ionized water up to filtrate changes to colorless. The greenish black paste of PPy-DBSA-Y 2 O 3 was conquered which was desiccated at 60 °C in vacuum oven for 1 day.

Measurements
X-ray powder diffraction analysis was carried out employing automated diffractometer, Panalytical X'Pert PRO equipped with CuKα radiations (λ = 1.54 Å). The Fourier transform infrared spectra was recorded on KBr pellet samples in the range of 4000-500 cm −1 by using a Perkin-Elmer Fourier transforms infra-red spectrometer. Scanning electron microscopy was carried out on an EVO50 ZEISS instrument. Dielectric and Conductivity were measured of pressed pellets by applying 10 ton pressure in stainless steel die of 4-mm in diameter and 1-mm thickness in press machine by using a two point technique in the frequency range 20 Hz to 1 MHz. The samples were connected to a Keithley 2400 electrometers and a current source electrometer.

Structural Analysis
The X-ray diffraction pattern for polypyrrole (PPy), PPy-DBSA and their composite of PPy-DBSA-Y 2 O 3 are revealed in Fig. 1. Figure 1d shows the main crystalline peaks of Y 2 O 3 nanoparticles observed at 2θ = 29.46°, 34.12°, 48.73° and 57.90° corresponds to periodicity d = 0.16, 0.14, 0.10 and 0.91 nm in the direction of (222), (400), (440) and (622) planes is in good agreement with that on the JCPD card S25-1200, which describes that the sample has cubic crystal system taking space group I213. Adopting Scherer's formula [20,21], the calculated crystalline size of Y 2 O 3 was found to be 95 nm. From X-ray diffraction intensity versus 2θ, it is explored that the XRD pattern of pristine PPy indicates no sharp peak except a broad peak at 2θ = 22°-28° (Fig. 1a) which is typical peak of PPy and suggests that the nature of PPy is amorphous has been reported [22]. The XRD pattern for PPy-DBSA also amorphous exposed in the range of 20°-30° in Fig. 1b. As in the XRD pattern of PPy-DBSA-Y 2 O 3 composite (Fig. 1c) was shifted towards higher angle due to the intercalation of PPy and Y 2 O 3 nanoparticles during the process of synthesis. Consequently, d-values are calculated experimentally via employing Bragg's relation [23]. In Fig. 1c

FTIR Spectra
FTIR spectra in absorption mode of pure polypyrrole (PPy), PPy-DBSA and composites of PPy-DBSA-Y 2 O 3 are publicized in Fig. 2 (curve a-e). The FTIR spectra of PPy and PPy-DBSA ( Fig. 2a-b) peak at 1090 cm −1 relates to symmetric C-H in the plane mode. The peak at 1200 cm −1 is N-H in the plane mode and it was typical absorption peak for PPy. The stretching modes of PPy are established in the region 1600-1400 cm −1 . The peak at 1550 cm −1 can be assigned for C-C stretching mode of PPy chains [25]. The C=C stretching vibrations at 1560 and 1480 cm −1 , C-N and C-C vibrations at 1394 cm −1 and C-H out of plane twisted vibration of PPy of polymer chain [26,27]. Two other peaks of SO 3 near 1032 and 1007 cm −1 are identified in doped PPy which are characteristic peaks of sulphonic group [28,29].

Scanning Electron Microscopy
The surface morphology of the samples was investigated by means of Scanning electron microscopy analysis. The SEM micrograph of pure Y 2 O 3 nanoparticles and PPy-DBSA-Y 2 O 3 composite can be attained and are shown in Fig. 3a and b. In order to substantiate the crystallinity of conducting polypyrrole (PPy) doped with dodecylbenzene sulfonic acid (DBSA) along with yttrium oxide (Y 2 O 3 ) nanoparticles. Micrograph of pristine polypyrrole show bright granular agglomerates and platelet structure of pure yttrium oxide, however, the particle size of doped polypyrrole enhanced moreover the structure is flakier. From this, it may be concluded that the composite of PPy-DBSA-Y 2 O 3 is exhibiting enlargement in particle size moreover a dual structure of platelet in addition to flakier.

Dielectric Constant (ε′)
The variation of ε′ versus frequency (20 Hz to 1 MHz) for PPy, PPy-DBSA along with PPy-DBSA-Y 2 O 3 composites with different weight percentage of Y 2 O 3 (2%, 4% and 8%) at room temperature is plotted in Fig. 4. The ε′ for all samples varies nonlinearly as function of frequency up to 1 MHz. A decrease in ε′ was observed by increasing the frequency and it is nearly constant at higher frequencies. At low frequency region dielectric distribution is observed but it becomes nearly independent at high frequency region. Further, we have observed that the values of ε′ for these composites are reduces by increasing the weight percentage of Y 2 O 3 . This can be attributed that the dipoles moving smaller distance within the polymeric chains in the direction of applied field. Conversely, a high value of ε′ was observed experimentally at low frequency region due to polarization as well as hopping of charge carriers (polarons and bipolarons) between the localized sites in the system.

Dielectric Loss (ε")
The variation of ε″ versus frequency (20 Hz to 1 MHz) for PPy, PPy-DBSA and PPy-DBSA-Y 2 O 3 composites with increasing weight percentage of Y 2 O 3 at room temperature is exposed in Fig. 5. The ε″ for all samples reduces at low frequency region but it remains constant at higher frequencies. The ε″ was found to be decreases by increasing the frequency as well as it shows that the amount of energy disintegrates in dielectric with applied field. At low frequency region, the presence of insulating grain boundaries and large value of resistivity so required more energy for the hopping of charge carriers between the localized sites. Hence, we observed low value of ε″ at high frequency region because conducting grains present minimum resistance by increasing the weight percentage of Y 2 O 3 .

Frequency Dependence of AC Conductivity
The variation in AC conductivity (σ ac ) versus frequency ranging from 20 Hz to 1 MHz for PPy, PPy-DBSA and PPy-DBSA-Y 2 O 3 composites at room temperature is presented in Fig. 6. Pristine PPy and all the composites show frequency independent AC conductivity at low frequency region and followed high frequency dispersive region where AC conductivity depends upon the frequency and follow a fractional power law. This universal behavior was observed in a present study of synthesized PPy-DBSA-Y 2 O 3 composites. The real part of the electrical conductivity σ (ω) can be expressed as: where σ 0 stands for dc conductivity, ''A" pre-exponential factor with units of electrical conductivity, "ω" angular frequency of applied field and "n" is fractional exponent a dimensionless factor. DC conductivity becomes independent of frequency as n approaches to zero, when n ≤ 1, electrical conductivity depends on frequency [30]. Frequency dependent fractional power law of AC conductivity has been explored variety of disordered materials, for example polymers, amorphous semiconductors along with conducting glasses [31]. Fractional power law (Eq. 1) was employing to observe the hopping of charge carriers (polarons and bipolarons) and also illustrate the AC conduction in conducting polymers [32][33][34]. AC conductivity enhances as frequency of applied field enhances in disorder polymers due to the hopping of charge carriers (polarons and bipolarons) moving small distance within the polymer chain. From Fig. 6, it is clear that σ ac (ω) increases with increase in frequency which can be due to restricted charges in high frequency region. Thus, fractional power law of AC conductivity was valuable tool to describe the hopping of charge carriers in conducting polymers [35].

Conclusions
The different weight percentage of Y 2 O 3 nanoparticles based composites of PPy doped DBSA were successfully prepared by in situ polymerization technique. XRD and FTIR results exhibit the intercalation of conducting PPy-DBSA between nano-Y 2 O 3 layers and thus, confirm the formation of PPy-DBSA-Y 2 O 3 composites. The SEM also confirms the formation of dual phase of platelet as well as flakier structure in PPy-DBSA-Y 2 O 3 . AC conductivity (σ ac ), dielectric constant (ε′) and loss factor (ε″) were enhanced with increased in Y 2 O 3 content in PPy-DBSA composites may be suitable candidates for electronic devices applications.
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