Preparation of polyaniline-polyvinyl alcohol-silver nanocomposite and characterization of its mechanical and antibacterial properties

2.1 Chemicals

Polyvinyl alcohol 98%, silver nanoparticles, ammonium persulfate, and HCl 37% were purchased from Merck Company of Germany. Polyaniline was prepared according to reference [4].

2.2 Preparation of polyaniline-polyvinyl alcohol composite

For preparing polyaniline-polyvinyl alcohol composite with wt% of 90–10%, 0.54 g of polyvinyl alcohol (90 wt%) was dissolved in 40 ml distilled water. Then, 0.06 g (10 wt%) polyaniline was added to the solution and put in a bath of Ultrasonic (Lbs2 model-made by FALC) for 30 min. It was stirred for 30 min. The above-mentioned method was used for preparing the composite of polyvinyl alcohol-polyaniline with the wt% of 80–20 wt%, and only the amount of the raw materials changed proportionate to the new wt%. The synthesized samples labeled as 8 and 9 are given in Table 1.

2.3 Preparation of polyvinyl alcohol-silver nanocomposite

Some 0.549 g (99 wt%) of polyvinyl alcohol was solved in 40 ml of distilled water. Then, 1 wt% of silver nanoparticles (0.006 g) was added to it, and put in a bath of Ultrasonic (Lbs2 model) for 30 min. It was stirred for 30 min. The above-mentioned method was used for preparing the nanocomposites of polyvinyl alcohol-silver including 1 wt%, 3 wt%, and 5 wt% of silver. The obtained samples labeled as 10, 11, and 12 are given in Table 1.

2.4 Preparation of polyvinyl alcohol-polyaniline-silver nanocomposite

Some 0.54 g (90 wt%) of polyvinyl alcohol was solved in 40 ml of distilled water. Then, 0.054 g of synthesized polyaniline (9 wt%) was added to the solution. After adding 0.006 g (1 wt%) of silver, the solution was stirred for 1 h, and the solution was put in an ultrasonic bath for 30 min. The above-mentioned method was used for preparing the nanocomposites of polyvinyl alcohol-polyaniline-silver including silver (1 wt%, 3 wt% and 5 wt%) and polyaniline (9 wt% and 19 wt%). The synthesized samples, labeled as 2–7, are given in Table 1.

2.5 Preparation of composite and nanocomposite films

After preparing the samples, as described in 2.2, 2.3 and 2.4, the obtained solutions were put inside the plate with a diameter of 10 cm, and after evaporation of the solution by oven drying at 50°C, the prepared film was removed from the plate.

2.6 Characterization

Fourier-transform infrared (FTIR) spectroscopy using Shimadzu 8400S (Japan) was used to record the FTIR spectra of silver nanoparticles, pure polyaniline, polyvinyl alcohol and the optimal nanocomposite film. Scanning electron microscopy (SEM) using LEO 1430VP (corporately made by Germany and England) was used to investigate the surface morphology of prepared films. Thermogravimetric analysis (TGA) for silver nanoparticles, pure polyaniline, polyvinyl alcohol and the optimal nanocomposite was obtained by Polymer Laboratories (PL)-TGA. The tensile property measurement was performed in a Zwick/Roell tester (Germany) at a crosshead speed of 50 mm/min at 25°C as ASTM D 638. The average values of three test results are reported.

2.7 Antibacterial test

The antibacterial properties of the prepared samples against standard Gram-positive bacteria (Lactobacillus and Staphylococcus aureus) as well as Gram-negative bacteria (Salmonella and Escherichia coli) were studied. In this method, first 100 ml of broth nutrient culture was mixed with a lump of Resazurin (Camlab Chemicals, CB4WE) and distributed in 2 ml culture holes. Broth nutrient has the same composition as the agar nutrient, but it lacks agar. Each of the polymers was added to cover the surface. Then, 0.5 McFarland bacteria were added to each of the holes. If the bacteria grow, Resazurin will lose its color as a result of the activity of the bacteria’s recovering enzyme called nicotinamide adenine dinucleotide. If the blue color is maintained, it indicates the maintenance of the antibacterial property, or it shows that the bacteria have not grown. After incubation at a temperature of 37°C for 24 h, the optical densities of the samples were measured using spectrophotometry, and the respective chart is drawn.

3.1 FTIR spectroscopy

The FTIR spectrum of silver nanoparticles is shown in the curve in Figure 1A. The adsorption band at 3447 cm⁻¹ is related to the O-H stretching vibration, and the adsorption band at 1550 cm⁻¹ is assigned to the C=C bond [15].

Figure 1:

Fourier-transform infrared (FTIR) spectrum of (A) silver, (B) polyaniline, (C) polyvinyl alcohol and (D) sample 5.

In the FTIR spectrum of pure polyaniline in the curve in Figure 1B, the absorption peaks at 1490 cm⁻¹ and 1580 cm⁻¹ are attributed to the C-C stretching mode of benzenoid and quinoid rings in polyaniline chains, respectively. The peaks at wavenumbers of 1293 cm⁻¹ and 3600 cm⁻¹ are assigned to the C-N and N-H stretching bonds of the secondary amine group, respectively [13].

In the FTIR spectrum of polyvinyl alcohol in the curve in Figure 1C, the peak in the adsorption area of 3442 cm⁻¹ is related to the stretching vibration of O-H, and the peak in 2925 cm⁻¹ belongs to the stretching vibration of aliphatic C-H. Moreover, the peaks at 1740–1640 cm⁻¹ are related to the stretching vibrations of C=O and C-O bonds left from acetate groups in polyvinyl alcohol (in reaction to the preparation of alcohol polyvinyl from polyvinyl acetate) [16].

In the curve in Figure 1D, the FTIR spectrum for nanocomposite of polyvinyl alcohol- polyaniline-silver (sample 5) is shown. It clearly reveals the major peaks associated with three components of the nanocomposite. In the analysis of the FTIR spectrum of this nanocomposite, the adsorption bands at 1398 cm⁻¹ and 1540 cm⁻¹ belong to the C-C stretching mode of benzenoid and quinoid rings in polyaniline chains. Moreover, the adsorption peak observed at 2935 cm⁻¹ indicates the stretching vibration of O-H of silver and polyvinyl alcohol. The obtained results show that the three components of the nanocomposite exist in a compatible mode in the structure of the nanocomposite.

3.2 SEM with energy dispersive X-ray analysis

In Figure 2, the SEM images of the nanocomposite film of polyvinyl alcohol-polyaniline- silver (sample 5) are shown at different magnified sizes. As observed in the images, the nanoparticles of silver were immersed in the polymer matrix.

Figure 2:

Scanning electron microscopy (SEM) images of sample 5 (A) 5 μm and (B) 2 μm.

Figure 3 shows the energy dispersive X-ray (EDX) spectrum of the nanocomposite of polyvinyl alcohol-polyaniline- silver (sample 5). Attached Table in figure 3 gives the element composition of the nanocomposite. On examining the EDX spectrum, the amounts of the elements (carbon, oxygen, nitrogen, silver, etc.) and their wt% in sample 5 of the nanocomposite are shown.

Figure 3:

Energy dispersive X-ray (EDX) pattern and chemical composition of sample 5.

According to this spectrum, 2.17% of silver was identified in the nanocomposite. Moreover, the distribution of the elements in one point of the nanocomposite is shown in Figure 4, which proves the good distribution of elements in this synthesized material.

Figure 4:

Scanning electron microscopy (SEM) image and energy dispersive X-ray (EDX) dot-mapping of sample 5.

3.3 Results of antibacterial test

According to Section 2.9, blue Resazurin was used to study the antibacterial properties of the prepared samples. The results are shown in Figure 5. After incubation at a temperature of 37°C for 24 h, it was observed that samples 6, 7, 11, and 12 had antibacterial effects on Lactobacillus, S. aureus, Salmonella and E. coli bacteria.

Figure 5:

Changes of samples color in Resazurin antibacterial test.

According to the results, the antibacterial property of the samples is improved by increasing the wt% of the nanoparticle. In other words, the samples with high wt% of silver nanoparticles show stronger antibacterial properties than the others. Of course, because of the darkness of the polymer samples, the color change was not visible with the naked eye. Therefore, using spectrophotometry, the color change in Resazurin was evaluated, and the results are given in the form of a graph (Figure 6). As an antibiotic, ampicillin (with a concentration of 100 μg in 1 ml) was used for negative and positive control without adding the polymer samples. Moreover, according to Figure 6, it can be concluded that the antibacterial effects were more effective on Gram-negative bacteria than the others. Although a precise mechanism has not been offered for justifying the antibacterial effects of the nanoparticles, the reported results indicate that the nanoparticles of silver show the highest antibacterial property against E. coli bacteria, and they were less effective against S. aureus, which is in line with the obtained results [17]. In other words, Gram-positive bacteria of S. aureus showed more resistance than the Gram-negative bacteria of E. coli. This may be due to the difference in the cell walls of the Gram-positive and Gram-negative bacteria.

Figure 6:

Antibacterial effects of the prepared samples according to Table 1.

3.4 Results of TGA test

The TGA curves of silver nanoparticles, pure polyaniline, polyvinyl alcohol and the nanocomposite of polyvinyl alcohol-polyaniline-silver (sample 5) under a heating rate of 10°C min⁻¹ are shown in Figure 7. The results show that silver nanoparticles are very stable and weight loss is not observed in the temperature range of 50–700°C. The results indicate that the initial mass loss of polyaniline at temperature ranges of 200–300°C is due to removal of solvent or dopant anions and decomposition of oligomers. Also, the weight loss occurring around 400°C is due to the decomposition of the polyaniline chains.

Figure 7:

Thermogravimetric analysis (TGA) curve of polyaniline, polyvinyl alcohol, silver and sample 5.

In the TGA graph of polyvinyl alcohol, there is an intense weight loss (80%) between 200°C and 450°C which happens due to the decomposition of the polymer chains and destruction of the polymer structure. A comparison of the TGA graph of the nanocomposite of polyaniline- polyvinyl alcohol-silver with the TGA graph of the pure polyaniline, polyvinyl alcohol and pure silver nanoparticles shows that up to 250°C, the weight loss pattern is similar to polyvinyl alcohol and is very steep. It should be mentioned that 70% of this sample includes polyvinyl alcohol. From 250°C to 450°C, the weight loss of the nanocomposite falls considerably, and the nanocomposite shows good thermal resistance. Above a temperature of 450°C, the nanocomposite has better thermal resistance in comparison with the pure polymers. It seems that the silver nanoparticle (with 3 wt%) has improved the thermal resistance of the nanocomposite.

3.5 Results of tensile test

The tensile measurements of polyvinyl alcohol-polyaniline-silver nanocomposites were performed according to ASTM (D-638-08). Figure 8 shows stress-strain curves of the films of polyvinyl alcohol-polyaniline-silver nanocomposites. The experimental data extracted from stress-strain curves of the samples are given in Table 2.

Figure 8:

Stress-strain curves of the prepared samples according to Table 1.

Based on the data in Table 2, the tensile strength of pure polyvinyl alcohol (sample 1) is 33.63 MPa. The prepared polyvinyl alcohol-polyaniline composite by adding 10 wt% and 20 wt% of polyaniline to pure polyvinyl alcohol (samples 8 and 9) leads to a sudden fall in tensile strength. This is because of very weak mechanical properties and non-processability of polyaniline. With an increase in the wt% of polyaniline from 10 wt% to 20 wt%, there would be greater fall in tensile strength.

Moreover, the prepared polyvinyl alcohol-silver nanocomposites by adding 1 wt%, 3 wt% and 5 wt% of silver to pure alcohol polyvinyl (samples 10–12) causes higher tensile strength, especially in sample 11 with a wt% of 3 for silver. Therefore, in this set of nanocomposites, polyvinyl alcohol-silver nanocomposite with a wt% of 3 for silver nanoparticles is the optimal nanocomposite, and the nanocomposites with wt% of 1 and 5 are in the next ranks.

Based on the values given in Table 2, in each set of the three-part nanocomposites with similar wt% of silver nanoparticles, the highest rate of tensile strength was recorded for the nanocomposites with polyvinyl alcohol wt% of 87–90 and polyaniline wt% of 8–10. The decrease in the wt% of polyvinyl alcohol or increase in the wt% of polyaniline caused a fall in the values of tensile strength. Therefore, the optimal values for the wt% of polyvinyl alcohol and polyaniline in the prepared nanocomposites are at the range of 87–90 and 8–10, respectively. Moreover, the comparison of the values of tensile strength in Table 2 shows that the highest rate of tensile strength was recorded for the nanocomposite with a wt% of 3 for silver nanoparticles, and the nanocomposites with 1 wt% and 5 wt% are in the next ranks. Therefore, the optimal value for silver nanoparticles is 3 wt%, and the nanocomposite of sample 5 is considered as the optimal state.

Based on the values recorded in Table 2, the trend of changes for the values of elongation at break of the composites of polyvinyl alcohol-polyaniline, the nanocomposites of polyvinyl alcohol-silver, and the nanocomposites of polyvinyl alcohol-polyaniline-silver are similar to the values of tensile strength. The highest rate of elongation at break was related to the samples in which the wt% of polyvinyl alcohol was the highest.

The comparison of the values in Young’s modulus also shows that all of the obtained samples, except the nanocomposites of polyvinyl alcohol-silver, have less Young’s modulus than pure polyvinyl alcohol. The decrease in Young’s modulus implies the increase in the flexibility of the obtained films. Conversely, the high rate of Young’s modulus indicates the hardness and roughness of the film, and such a film resists against elasticity and elongation.