Effect of Polyvinyl Alcohol on the Growth, Structure, Morphology, and Electrical Conductivity of Polypyrrole Nanoparticles Synthesized via Microemulsion Polymerization

Polypyrrole (PPy) nanoparticles were synthesized via microemulsion polymerization technique using sodium dodecyl sulfate as surfactant. Polyvinyl alcohol (PVA) was added as soft template during polymerization to modify the structure and properties of PPy nanoparticles. The synthesized materials namely, PVA-free and PVA added were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and DC electrical conductivity measurements. The sample synthesized in the presence of PVA has longer conjugation length as estimated from FT-IR investigation. Temperature dependence (4.2–300 K) of DC electrical conductivity measurement reveals that the PVA has a strong effect on the polymerization mechanism of PPy giving evidence of H-bonded assistance during polymerization leading to the synthesis of better ordered polymer. A growth mechanism has been proposed which explains the H-bonded assistance of PPy polymerization leading to enhanced structural ordering.


Introduction
Conjugated polymers have attracted considerable attention in the past few decades because of their potential application in electronic devices [1].Their ease of processing and chemically tunable properties makes them useful for electronic, optoelectronic, electromechanical, and sensing device application [2,3].During the recent years conducting polymer nanostructures have received increasing attention from both fundamental research as well as application point of view.Conducting polymer nanostructures show high electrical conductivity, large surface area, short path lengths for the transport of ions, and high electrochemical activity as compared to its macrogranular structure or self-supporting films [4][5][6][7].It can be synthesized by several approaches such as well-controlled solution synthesis [8,9], softtemplate methods [10], hard-template methods [11], and electrospinning technology [12].Recently some conducting polymer nanowires and nanorods have been synthesized via hydrogen bonding [13].
Polypyrrole (PPy) is one of the most studied conducting polymers because of its good environmental stability, facile synthesis, ion exchange capacity, biocompatibility, and higher conductivity [14,15].It can be used in drug delivery, rechargeable batteries, supercapacitors, anhydrous electrorheological fluids, microwave shielding, and corrosion protection [16][17][18].Soft template uses microemulsion polymerization which allows particles to transfer into spherical aggregates through the surfactant template for the production of PPy nanostructures.Conducting polymer nanoparticles (particle size) ∼50-200 nm [19,20] and ∼20-50 nm [21] have been synthesized.It has been observed that the microemulsion polymerization has increased the yield of the PPy nanoparticles, the extent of the π-conjugation along the polymer backbone, and the ordered arrangement of the macromolecular chains.Song et al. 2004 have reported the synthesis of PPy nanoparticles doped with a variety of alkylbenzenesulfonic acid (ABSA) [22].PPy doped with short alkyl chain showed higher conductivity than long alkyl chain.These achievements indicate that the microemulsion polymerization is powerful technique for the fabrication of polymer nanostructures.
In the present paper, we report the synthesis of PPy nanoparticles by microemulsion polymerization.The temperature-dependent (4.2 to 300 K) dc conductivity measurement along with structural investigations indicates that the addition of PVA during the polymerization affects the polymerization mechanism.

Experimental
PPy nanoparticles were synthesized using doubly distilled 0.1 M pyrrole monomer (Fluka chemie) with 0.5 M ferric chloride (FeCl 3 ) (Sigma Aldrich).Sodium dodecyl sulfate (SDS) (Fluka) (0.03 M) was used as surfactant.The solution of SDS was prepared in distilled water (18 MΩ cm).Continuous stirring was carried out for 6 hrs at 275 K in inert atmosphere during polymerization.The black precipitate of PPy from the reaction mixture was filtered via vacuum filtration.The filtered precipitate was washed repeatedly with methanol and distilled water till clear and colorless filtrate was obtained.Finally, the PPy nanoparticles were dried for 6 hrs in vacuum oven at 333 K and was designated as sample A. PPy nanoparticles were also synthesized with the addition of polyvinyl alcohol (PVA) (5% by weight of pyrrole) in the abovementioned reaction mixture, and similar procedure was followed for polymerization and filtration and was designated as sample B.
Fourier transform infrared (FT-IR) spectra of both samples A and B were recorded using Smart orbit ATR-single reflection accessory of the Thermo Scientific Nicolet 5700 spectrometer, with a diamond crystal, taking four scans at a resolution of 4 cm −1 , and details of different vibration peaks are given in Table 1.X-ray diffraction of samples A and B was taken by Rigaku-make powder X-ray diffractometer for Cu-Kα (λ = 1.5404Å) radiation.The scanning electron micrographs (SEMs) of samples A and B were taken using SEM model Zeiss EVO MA-10 microscope to investigate the surface morphology of the samples.A thin layer of gold was sputtered on the samples before loading in the microscope probe to nullify any charging effect.For dc electrical conductivity (σ dc ) measurement, circular pellets of both samples A and B having dia ∼5 mm were made by using hydraulic press at ∼5 × 10 8 Pa.Conductivity measurements were performed by using four-probe technique.Keithley 238 high current source measuring unit was used for applying constant current and Keithley 6517A Electrometer/High Resistance meter was employed to measure the voltage.The thickness of the samples was accurately measured to calculate the bulk conductivity.Temperature-dependent (4.2-320 K) dc conductivity measurement was performed in liquid helium Dewar using Keithley's 224 Programmable Constant Current Source and 195 A Digital Multimeter.

Results and Discussion
A key property of a conducting polymer is the presence of conjugated double bond which gives rise to the electrical conductivity, as it allows the efficient transfer of electrons or positive charges along the polymer backbone.Increasing conjugation length and improving order throughout polymer chain will allow charge to migrate along a longer distance and hence enhances the conductivity.In the present work, the conjugation length and structural ordering of PPy nanoparticles have been influenced by the addition of polyvinyl alcohol (PVA) during the microemulsion polymerization.
Figure 1 shows the FT-IR spectra of samples A and B confirming the synthesis of polymers.Sample A shows the peaks at 1553 and 1481 cm −1 which correspond to C=Cstretching and C-C-stretching vibrations, respectively [23].The peaks at 1280 and 1211 cm −1 are attributed to the C-N in plane deformation and vibrations of the pyrrole ring.The peak at 1040 cm −1 is attributed to N-H in plane deformation vibration [23,24].The peaks at 787 and 925 cm −1 correspond C-H wagging and C-H out of plane vibrations, respectively.The peak at 679 cm −1 is attributed to N-H out of plane vibration [24].
Sample B shows the peaks at 1538 and 1453 cm −1 which correspond to the stretching vibration of C=C and C-C, respectively [23].The peaks at 1289 cm −1 correspond to C-N in plane deformation.The peak at 1156 cm −1 is attributed to the vibration of the pyrrole ring.The band of N-H in plane deformation is located at 1025 cm −1 while C-H out of vibration is found at 960 cm −1 [23,24].The peaks at 764 and 690 cm −1 corresponds to C-H wagging vibration and N-H out of plane vibration, respectively [24].The characteristic peaks of PVA could not be observed in the spectrum, indicating that alcohol has been washed out from the final PPy nanostructures.The FT-IR spectra for both samples A and B have all the characteristic peak of PPy, which suggest that it has been successfully synthesized.However, there are few differences between the spectra of samples A and B. The band of C=C stretching vibration located at 1553 cm −1 in the FT-IR spectrum of sample A synthesized in the absence of PVA shifts to 1538 cm −1 for sample B synthesized in the presence of PVA.Due to this shift the room temperature (∼300 K) conductivity of sample B (∼7.5 × 10 −2 S/cm) is higher than sample A (∼1.6 × 10 −2 S/cm).The intensity of the C=C and C-C band in the two samples is different which can be linked with the conjugation length of PPy.The conjugation length is estimated from the ratio of peak intensity of C=C and C-C stretching vibrations [25].The higher ratio represents longer effective π conjugation along the PPy chains and hence higher conductivity.PPy synthesized using PVA shows higher conjugation (∼0.98) than the one synthesized without it (∼0.91).This shows that the PVA considerably influenced the molecular structure of PPy.The hydrogen in the O-H group of PVA is capable of forming bond with the hydrogen in the N-H group of the pyrrole ring.These hydrogen bonds are known to be weak, and PVA is soluble in water, and quantity of PVA added was very small; therefore, these bonds get broken, and PVA is removed during the washing of the sample.FTIR confirms that this as no characteristic peak for PVA is observed.
Figure 2 shows the XRD pattern of the synthesized PPy nanoparticles with and without PVA.PPy nanoparticles synthesized in the absence of PVA (sample A) exhibit broad scattering peaks at 2θ value around 25.6 It can be said here that sample synthesized in the presence of PVA is partially crystalline and has better ordering than sample synthesized in the absence of PVA.Higher ordering in PPy prepared in the presence of PVA may be attributed to the alignment given to the PPy chains via hydrogen bonding between N-H group of pyrrole ring and oxygen atom of PVA.The d spacing (interlayer spacing) value for the sample A is ∼3.46 Å under whereas for sample B the values of d 1 and d 2 are ∼6.28Å and 3.79 Å, respectively.
The SEM micrograph in Figure 3(a) represents the general morphological features of the PPy nanoparticles obtained without using PVA (sample A) during the experiment process.The SEM image reveals the presence of globular particles with diameter ranging from 50 to 100 nm.These globular particles conglutinate together to form bigger aggregates.The size measured by particle size analyzer was found to around 80 to 100 nm (result not shown).Figure 3(b) shows the typical SEM images of PPy nanoparticles obtained when PVA was introduced to the experiment process (sample B).The SEM image reveals the existence of lamella disc-like structures with diameter ranging from 150 to 200 nm.The size measured by particle size analyzer was found to around 200 to 300 nm (result not shown).On careful observation it can be said that the sample looks well-ordered and the level of aggregation in this sample is very low as compared to the sample prepared in the absence of PVA.This suggests that the presence of the PVA in polymerization mixture has strongly influenced the morphology of synthesized PPy and is also responsible for better order and reduction in the aggregations.The inset in Figures 3(a) and 3(b) shows their respective transmission electron micrographs which further support the variation in particle size as well as a core-shell structure, the material of the shell being amorphous, and the core being formed by the more closely packed polymer macromolecules.
The variation of dc conductivity (σ dc ) for both the samples A and B is shown in Figure 4 as functions of (a) 1000/T, (b) T −1/2 in the temperature range 4.2-300 K.The temperature-dependent variation of dc conductivity data indicates that the charge transport seems to occur by phonon-assisted hopping or thermally activated jumps between localized states [26][27][28].According to Mott and Davis, the dc conductivity (σ dc ) can be written [27] as where σ 0 is the infinite temperature conductivity, σ dc is the conductivity at temperature T, and T 0 is the Mott's characteristic temperature that determines the thermally activated hopping among localized states at different energies and is also considered as a measure of disorder.We have shown).The linearity factor of the PPy prepared in PVA is 0.99935 while for PPy prepared in absence of PVA is 0.96196.These linearity factors illustrate that the Mott's 1D-VRH mechanism of T −1/2 type seems to be more appropriate [27,29] for explaining the mechanism of dc conduction in both PPy samples.The temperature region where (1) is valid should gives [26] the activation energy as where k B is the Boltzmann's constant and E A is the activation energy.Equation ( 2) can be correlated to the parameters of (1) and can be written as It is evident from (3) that a plot of log E A versus log T should yield a straight line of slope = (-(γ−1)).To verify this, the activation energy (E A ) evaluated at different temperatures from Figure 4(a) has been plotted as log E A versus log T in the temperature range 8-290 K and is shown in Figure 5.It can be clearly seen that the behavior of activation energy with temperature matches with a straight line corresponding to γ = 1/2 which is shown as a solid line in Figure 5.This gives a clear indication that the Mott's VRH mechanism of T −1/2 type dominates the mechanism of charge transport in both PPy samples.Equations ( 1) and (3) give the slope T 0 from log σ dc versus T −1/2 plots (Figure 4) [27,30] as where T 0 is the characteristic temperature, B 0 is dimensionless constant ∼1.66 [27,30], k B is Boltzmann's constant, N (E F ) is the density of states at the Fermi level, and α (=1/r P ) is the coefficient of exponential decay of the localized states involved in the hopping process.
Based on the experimental observations, a mechanism explaining the role of PVA in improving the order and conjugation length has been proposed in Figure 6.The hydrogen in the O-H group of PVA is capable of forming bond with the hydrogen in the N-H group of the pyrrole ring.Thus hydrogen bonding takes place between the N-H group of the Pyrrole ring and O-H group of the PVA molecule.Now, PVA acts as a bridging agent between the PPy chains and forms a temporary soft template which controls the growth of the PPy chains as shown in Figure 6.PVA forms linkage between the PPy chains via hydrogen bonding.These hydrogen bonds act as temporary soft template which is responsible for increasing the order of the PPy nanoparticles and hence increase in conjugation length and conductivity.

Conclusion
PPy nanoparticles synthesized by microemulsion polymerization in the presence of PVA show better ordering and conjugation length than those synthesized in absence of PVA.This study shows the importance of H-bonding assistance during polymerization and gives rise to a promising method to the controllable synthesis of other conducting polymers with high conductivity.

Figure 2 :
Figure 2: X-ray diffraction pattern of samples synthesized in the absence (sample A) and presence (sample B) of PVA.

Figure 3 :
Figure 3: (a) SEM of PPy nanoparticles (sample A) and (b) SEM of PPy (sample B).Inset shows their corresponding TEM images.

Figure 5 :
Figure 5: Plot of log E A (E A is the activation energy derived from Figure 4(a)) versus log T in the temperature range 8-290 K.

Table 1 :
Peaks observed in FT-IR spectra of Sample A and B.
Figure 1: FT-IR spectra of PPy nanoparticles synthesized in the absence (sample A) and presence (sample B) of PVA.
• , which suggest that PPy is virtually amorphous.PPy nanoparticles synthesized in the presence of PVA (sample B) exhibit two peaks at 2θ values around 14.1 • and 23.45 • with almost same intensity which suggests that this sample B is less amorphous than sample A.
1), where d is the dimensionality.The values of d = 1, 2, 3 signify the 1-dimensional, 2-dimensional, and 3-dimensional Mott's VRH.A plot of log σ dc versus (1/T) γ should give a straight line for proper value of γ.It is evident from this figure that for both the samples that is, prepared in presence (sample A) and absence of PVA (sample B) exhibits the linear dependence of log σ dc versus T −1/2 (Figure4(b)) and is better than that of log σ dc versus T −1/4 (result not Figure 6: Mechanism of growth of PPy in presence of PVA showing the H-bonding assisted growth.