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Various methods have been developed for the synthesis of intrinsically electronic conducting polymers. Among them, chemical and electrochemical processes have been extensively studied and used. Although photopolymerization is considered one of the less developed methods, recent studies have shown that this novel technology has the potential to offer several advantages for the development of a number of applications in the aerospace, telecommunications, and electronic industries [2-5]. Some of these advantages include faster rates of polymerization, low energy consumption, and reduced VOC. Besides, one of the major advantages of photopolymerization is that it can be utilized to prepare films and coatings on both conducting and non-conducting substrates where electrochemical and chemical processes have limitations and drawbacks. However, the availability of photoinduced electron acceptors for this technique is limited and the conductivities of polymers obtained via this process are very low as compared to the traditional chemical and electrochemical processes. Despite these disadvantages, this method can be often used in the applications where low conductivities and faster production are needed [6-7].
There have been a few instances in the literature where simultaneous synthesis and incorporation of metal nanoparticles into polypyrrole matrix have been achieved via photopolymerization in the presence of salts of copper, gold, and silver [8-9]. Among these metal salts, silver salts such as silver nitrate (AgNO3), silver perchlorate (AgClO4), silver tosylate (AgTs), and silver nitrite have been most commonly used. However, it was found that AgNO3 is far better electron acceptor than other salts in terms of rate of polymerization, conductivity, and yield [10-11]. In most of these reports, the solvent used for the photopolymerization of pyrrole in the presence of AgNO3 was the mixture of methanol and water. It can be explained from the fact that pyrrole is slightly soluble in water and therefore leads to phase separation in aqueous AgNO3 solutions. Thus, in order to prevent this phase separation, the mixture of methanol and water would help in dissolving both pyrrole and AgNO3 to get homogeneous solution. However, it is well known that the presence of water as solvent causes the defects in polypyrrole such as carbonyl and hydroxyl groups. Moreover, it was surmised that a fast evaporation of volatile solvents such as methanol, ethanol, and isopropanol used in our previous work might be one of the crucial reasons in forming non-uniform surface texture.
In this study, ethylene glycol has been introduced as a common solvent to dissolve both pyrrole and AgNO3. AgNO3 is found to be considerably soluble in ethylene glycol (80g of AgNO3 is soluble in 100g of ethylene glycol). Moreover, ethylene glycol as a solvent has many advantages such as higher boiling point, lower evaporation rate, and considered to be low or no volatile organic content (VOC). In Chapters 3 and 4, a mixture of water and methanol was used as a solvent for the photopolymerization of pyrrole. In this chapter, a comparative study has been performed between the properties of polypyrrole/Ag composites obtained in the water - methanol mixture and ethylene glycol as two different solvents. In addition, the effect of concentration of AgNO3 on the properties such as morphology, chemical structure, composition, and conductivity of polypyrrole/Ag composite has also been investigated. This work was assumed to serve as a bridge between the previous work in Chapters 3 and 4, and the work presented in Chapter 6.
Synthesis of polypyrrole/Ag composites
Pyrrole purchased from Sigma Aldrich was vacuum distilled and stored in refrigerator prior to use. AgNO3, ethylene glycol, and methanol were also purchased from Sigma Aldrich. In order to determine the effect of AgNO3 concentration, four different molar ratios of pyrrole to AgNO3 (4:1, 8:1, 16:1, and 32:1) were employed for both solvents. For synthesis of polypyrrole/Ag composite, the mixture of pyrrole and AgNO3 solution was stirred for 15 min and applied on glass. The substrates were degreased with hexane and dried for few minutes before application of each formulation. Dymax 5000-EC flood curing equipment with mercury vapor lamp as the light source having the UV-light intensity of 225mW/cm2 was used to apply UV light. The time of UV exposure for each formulation was 2 minutes.
Table 5.. Reactions for the synthesis of polypyrrole/Ag composites
Time of UV exposure
50% Water + 50% Methanol
50% Water + 50% Methanol
50% Water + 50% Methanol
50% Water + 50% Methanol
Characterization of polypyrrole/Ag composites
For fourier transform-infrared spectroscopy (FTIR), the samples were prepared by making pellets using 95% of KBr and 5% of the respective sample. Nicolet FTIR spectrometer was used for the FTIR characterization.
A JEOL JSM-6300 scanning electron microscope was used to obtain the images for morphology of samples. The samples for SEM were prepared by sprinkling the ground powder onto carbon tape, which was attached to aluminum mounts. The magnification (x 25,000), accelerating voltage (2 kV), and the scale (1 μm) were specified on each image.
Thermogravimetric analysis (TGA) was carried out at a heating rate of 10°C/ minute up to 800°C using a TA instruments Q500. The analysis of TGA results was performed using the software, Universal Analysis 2000.
Veeco Dimension 3100 atomic force microscope with contact mode and current sensing probe was used as C-AFM to characterize the coatings prepared on aluminum substrate for surface morphology and current density. The platinum-iridium (Pt/Ir) coated cantilevers (Model: SCM-PIC, 0.01-0.025 ohm-cm Antimony (n) doped Si, spring constant 0.25 N/m) were purchased from Veeco Instruments. The bias voltage between the substrate and the coatings was varied from 100 mV to 3 V depending on the conductivity of sample.
Results and discussion
Scanning electron microscopy (SEM)
Figure 5.1 shows the SEM images of the samples PPyAg_EG1, PPyAg_EG2, PPyAg_EG3, and PPyAg_EG4; Figure 5.2 shows the SEM images of the samples PPyAg_MW1, PPyAg_MW2, PPyAg_MW3, and PPyAg_MW4.
Figure 5.. SEM images of PPyAg_EG1, PPyAg_EG2, PPyAg_EG3, and PPyAg_EG4
Figure 5.. SEM images of PPyAg_MW1, PPyAg_MW2, PPyAg_MW3, and PPyAg_MW4
In all samples, hexagonal shaped silver metal (Ag) particles were identified. Interestingly, the Ag particles were found to be well dispersed in the samples PPyAg_EG1, PPyAg_EG2, PPyAg_EG3, and PPyAg_EG4. In contrast, the Ag particles were rarely seen on the surface of the samples PPyAg_MW1, PPyAg_MW2, PPyAg_MW3, and PPyAg_MW4. The morphology of polypyrrole in all the composites (PPyAg_EG1, PPyAg_EG2, PPyAg_EG3, and PPyAg_EG4) prepared in the presence of ethylene glycol as solvent was observed to be regular spherical or cauliflower shaped particles. It indicates that the concentration of AgNO3 in the presence of ethylene glycol might have minor effect on the nucleation and propagation of photopolymerization. However, the polypyrrole in the composites prepared in the presence of methanol-water mixture was found to contain highly packed and agglomerated larger particles. In addition, the morphology of polypyrrole appeared to be affected by the concentration of AgNO3 in the presence of methanol-water mixture.
Fourier transform infrared spectroscopy (FTIR)
The FTIR spectra of samples PPyAg_EG1, PPyAg_EG2, PPyAg_EG3, and PPyAg_EG4 are shown in Figure 5.3 and the FTIR spectra of samples PPyAg_MW1, PPyAg_MW2, PPyAg_MW3, and PPyAg_MW4 are shown in Figure 5.4. The characteristic peaks of polypyrrole were identified in all spectra. When compared the spectra of the samples PPyAg_EG1, PPyAg_EG2, PPyAg_EG3, and PPyAg_EG4 with each other, no significant differences were observed. Similar result was observed in the case of PPyAg_MW1, PPyAg_MW2, PPyAg_MW3, and PPyAg_MW4. It indicates that the chemical structure of polypyrrole was not affected by the AgNO3 concentration in either of these two solvents.
Figure 5.. FTIR spectra of PPyAg_EG1, and PPyAg_EG2, PPyAg_EG3, and PPyAg_EG4
Figure 5.. FTIR spectra of PPyAg_MW1, and PPyAg_MW2, PPyAg_MW3, and PPyAg_MW4
When the peak shifts between the polypyrroles prepared in both solvents were compared, no significant differences were found indicating the absence of any effect of the presence of solvent on the chemical structure of polypyrrole. Even though the dopant NO3- vibration at 1383 cm-1 was noticed in all of them, no considerable trend was observed with respect to the AgNO3 concentration.
X-ray photoelectron spectroscopy (XPS)
The doping levels in samples PPyAg_EG1, and PPyAg_EG2, PPyAg_EG3, and PPyAg_EG4 were determined using X-ray Photoelectron Spectroscopy (XPS). High resolution spectra of nitrogen for these samples are shown in Figure 5.4. In these spectra, the peaks with binding energies of 406.6 eV and 408.0 eV represent the NO3- that was present as dopant in polypyrrole. The binding energies at 398.6 eV and 400.3 eV are attributed to -N= and -NH- respectively in polypyrrole. In addition, two binding energies at 401.98 eV and 403.20 eV are assigned to the positively charged nitrogen species (N+). As shown in Table 5.2, the comparison between the values of N+ with the corresponding -NH- concentration in respective samples indicated a slight increase in the doping level from PPyAg_EG4 to PPyAg_EG1.
Table 5.. The composition (%) of nitrogen species and doping levels in the samples
Doping level (N+/ -NH-)
Figure 5.. XPS spectra of PPyAg_EG1, PPyAg_EG2, PPyAg_EG3, and PPyAg_EG4
Thermogravimetric analysis (TGA)
Figure 5.6 shows the TGA derivative weight loss curves of PPyAg_EG1, PPyAg_EG2, PPyAg_EG3, and PPyAg_EG4. Figure 5.7 shows the TGA derivative weight loss curves of PPyAg_MW1, PPyAg_MW2, PPyAg_MW3, and PPyAg_MW4. In all curves, two peaks at around 200°C and 400 - 600°C corresponding to NO3- and polypyrrole respectively were identified. When the peaks of polypyrrole were compared between the samples prepared in ethylene glycol and that prepared in methanol-water mixture, the decomposition of this peak occurred at higher temperatures in the case of the samples prepared in methanol-water mixture. Moreover, this peak gradually shifted to lower temperatures from PPyAg_MW1 to PPyAg_MW4.
In congruent with XPS results, the ratio of NO3- to polypyrrole peaks, which also represents the doping level in polypyrrole, decreased from PPyAG_EG1 to PPyAg_EG4 and PPyAG_MW1 to PPyAg_MW4. However, the reduction in peak height of polypyrrole from PPyAg_EG4 to PPyAg_EG1 can be attributed to either the increase in silver metal concentration or the increase in NO3 doping level in polypyrrole. A clear trend of peak height of polypyrrole in the samples PPyAG_EG1 to PPyAg_EG4 was observed. However, this was not observed in the case of PPyAg_MW4 to PPyAg_MW1. Further, while a clear broadening trend of polypyrrole peak occurred in the case of PPyAg_EG4 to PPyAg_EG1, all three samples PPyAg_MW1, PPyAg_MW2, and PPyAg_MW3 apparently showed equally broad peak of polypyrrole. When this peak was compared in between the samples prepared in ethylene glycol and that prepared in methanol-water mixture, the peaks in the case of samples PPyAg_MW1 to PPyAg_MW4 were relatively broader than those in the case of PPyAg_EG1 to PPyAg_EG4.
Figure 5.. TGA derivative weight loss curves of PPyAg_EG1, PPyAg_EG2, PPyAg_EG3, and PPyAg_EG4
Figure 5.. TGA derivative weight loss curves of PPyAg_MW1, PPyAg_MW2, PPyAg_MW3, and PPyAg_MW4
The conductivity and current density in all samples were determined using conductive AFM. Figure 5.8 shows the current density images of the samples PPyAg_EG2, PPyAg_EG3, PPyAg_MW2, and PPyAg_MW3. All the samples exhibited high surface current densities. As it can be seen in these figures, the difference in current densities in these samples was apparently not significant. Although the current density in the sample PPyAg_EG2 seemed to be slightly higher than that in PPyAg_EG3, it could be due to either higher concentration of silver metal or higher conductivity of polypyrrole in PPyAg_EG2. In general, the samples prepared in ethylene glycol showed higher conductivities than the samples prepared in methanol-water mixture.
Figure 5.. The current density images of PPyAg_EG2, PPyAg_EG3, PPyAg_MW2, and PPyAg_MW3
The effect of solvent and AgNO3 concentration in solution on the properties of polypyrrole/Ag composite was investigated in terms of morphology, chemical structure, chemical composition, and conductivity measurements. The yield of polypyrrole/Ag composite was found to increase as the concentration of AgNO3 increased. It was found from the TGA results that the increase in the concentration of AgNO3 caused to increase in Ag silver metal concentration from PPyAg_EG4 to PPyAg_EG1 and also led to higher molecular weight polypyrrole. SEM results showed that the increase in AgNO3 concentration had no effect on the morphology of polypyrrole/Ag composite in PPyAg_EG4 to PPyAg_EG1. But, the morphology of polypyrrole/Ag composite was significantly affected by the concentration of AgNO3 in the presence of methanol-water mixture. FTIR results did not show significant changes in molecular structure of polypyrrole even though the concentration of AgNO3 and the solvents were changed. XPS results showed slight increase in the NO3- doping level in the samples from PPyAg_EG4 to PPyAg_EG1 with the increase in AgNO3 concentration. The conductivity measurements determined from CAFM indicated higher current densities of polypyrrole/Ag composite in the samples PPyAg_EG2and PPyAg_EG3 as compared to PPyAg_MW2 and PPyAg_MW3.