The chemical oxidative polymerization of o-aminophenol and o-toluidine in IM HCl solution produces the PTSA-doped copolymer poly (o-amino phenol-co-o-toluidine) by the following chemical reaction.
Copolymerization reaction of poly (o-amino phenol-co-o-toluidine)
Step 1 (NH4)2S2O8 2NH4+ + S2O82-
3.Characterization of synthesized copolymer pOAPOT
3.1 UV-Visible Spectroscopy
The UV-Visible spectra of the pOAPOT in solution DMSO recorded at room temperature are shown in Figure 1. In this spectrum, there are two absorption bands. The absorption peak at 272 nm represents the ?-?* transitions of the benzenoid ring and 397 nm shows the donor-acceptor interaction of quinonoid rings. (Y. Wei et al., 1990)
3.2 FTIR Spectroscopy
For IR studies Perkin-Elmer spectrum RX1 FTIR was used. The spectra are shown in Figure 2. The band at 3380cm-1, indicating characteristic free N-H stretching vibration, which suggests that the presence of a secondary amino group (-NH-) a small shoulder band at 3301 cm-1 corresponds to the hydrogen bonded N-H vibrations. (Kar, Pradip et al., 2010) The peak around 3062cm-1 represents the CH stretching vibration of the aromatic ring. The peak at 2921cm-1 represents the presence of methyl C-H stretching vibrations and it indicates the presence of OT in the copolymer chain. The IR absorption bands at 1474-1615cm-1 are associated with the aromatic ring stretching. The 1584cm-1 and 1489cm-1 is assigned to the quinonoid and benzenoid rings. The weak peak at 1385cm-1 is attributed to the C-N stretching vibrations in the quinonoid-imine units and the peak at 1302cm-1 is because of the C-N stretching vibrations in the alternative units of quinonoid-benzenoid-quinonoid rings. ( D. Kumar, 2000) The peaks at 1117cm-1and 879cm-1 are assigned to the C-H in the plane and out-of-plane bending vibrations of 1,2,4 tri substitution. (Chuanxiang Chen et al., 2008) The peak at 567cm-1 represents the presence of PTSA as a dopant in the polymer.
3.3 X-ray diffraction studies
The XRD pattern for pOAPOT is given in Figure 3. The band at 2? values at12 .19,14.39,15.79,16.78,17.79,19.59,25.99,26.79 and 27.99 are the characteristics peaks of the weak Van der Waals distances. The broad peaks indicate the amorphous nature of the copolymer. (Mei-Rong Huang et al.,2001)
3.4 Thermal Analysis
Thermal stability of the copolymer was tested from TGA. Thermogram of copolymer pOAPOT is measured from 0-1200?C under Nitrogen atmosphere and presented in Figure 4. It shows the three-stage decomposition pattern. In the first stage, the weight loss is started from 30?C to 110?C is due to loss of water molecules present in the copolymer. The second stage between 140?C to 380?C is due to release of dopant molecule and oligomers and the weight loss after 380?C to 1200?Ccorresponds to decomposition of copolymer backbone units. (Abdel-Rahim, S.S., et al.,2006) The TGA curve indicates that pOAPOT possess good thermal stability.
3.5 Corrosion studies
3.5.1 AC impedance studies
The Nyquist representation of the impedance response of the iron electrode in 1M and 0.5M HCl without and with pOAPOT are given in the Figure 5 and Figure 6. Nyquist graphs are diminished semicircles with increasing diameter indicates that heterogeneity, roughness on the electrode surface, due to the formation of protective layer on the metal surface. (Amin, M. A et al., 2009) The experimental data are fitted with the standard Randle’s equivalent circuit containing double layer capacitance (Cdl) parallel to charge transfer resistance (Rct), in series with solution resistance (Rs). Figure 7 shows the standard Randle’s equivalent circuit model.( Hazwan Hussin, M et al., 2016) In Nyquist plots, there is a continuous increase in the diameter of the semicircle with the increase of inhibitor concentration is related to the inhibitor film formation on the metal surface. (U. Rammelt et al., 2001) The charge transfer resistance (Rct) values, the double layer capacitance (Cdl) values, and surface coverage values are presented in the Table 1. From the table, it can be seen that the Rct increase and Cdl decreases. This trend is observed due to the formation of a passive layer on the electrode surface, with the decrease of dielectric charge. (M. A. Quraishi et al., 2010) This shows an increase in surface coverage by the inhibitor molecules leading to an increase in inhibition efficiency.