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Fourier transform infrared spectra of transition metal ion-containing polyanilines synthesized in different reaction conditions
Spectrochimica Acta Part A 66 (2007) 37–41
Fourier transform infrared spectra of transition metal ion-containing polyanilines synthesized in different reaction conditions Chunming Yang ∗ , Chunyan Chen, Yue Zeng College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, PR China Received 31 December 2005; received in revised form 9 February 2006; accepted 13 February 2006
Abstract Fourier transform infrared spectroscopy has been employed to investigate the detailed chain structure changes during the chemical oxidative polymerisation of aniline in different reaction conditions including different polarity reaction medium, reaction temperature, reactants molar ratio and in the presence of different transition metal ions such as Fe2+ , Co2+ , Ni2+ and Cu2+ . The results show that stronger polarity reaction medium and lower reaction temperature were beneficial to obtain 1,4-para-disubstituted linear chain structure polyaniline with higher electrical conductivity. The higher oxidation degree polyaniline contained more linear chain structure than that in lower oxidation degree. Polyanilines containing Fe2+ and Cu2+ had more linear chain structure than that containing Co2+ and Ni2+ . These observations were in accordance with experimental measurements of electrical conductivity. © 2006 Elsevier B.V. All rights reserved. Keywords: Polyaniline; Transition metal ions; FT-IR spectroscopy; Characterization
1. Introduction Among all the promising electrical conducting polymers, polyaniline exhibits unique advantage both for research and application due to its easy preparation, high conductivity and good environmental stability [1–5]. Since 1970s, a great number of efforts have been devoted to its structure nature, doping and conductivity mechanism, physico-chemical and electrochemical properties associated with its application in electroactive materials [6], sensor components [7,8], anti-corrosion paintings [9,10], secondary battery electrode materials [11] and many other related fields [12,13]. Polyanilines containing some transition metal ions can be used as novel synthetic metal catalyst in some oxidation systems [14,15]. In our previous paper [16], we reported the synthesis of polyanilines containing Fe2+ , Co2+ , Ni2+ and Cu2+ by oxidizing the complex of aniline with corresponding metal chloride salt in solution. And the preliminary results were given by FT-IR spectroscopy to determine the molecular structure of resulting polyanilines. Here, we mainly employ FT-IR spectra data to elucidate the detailed chain structure changes of the polyanilines synthesized
∗
Corresponding author. Tel.: +86 731 8872285; fax: +86 731 8872531. E-mail address: [email protected] (C. Yang).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2006.02.016
under different reaction conditions such as reaction medium, the molar ratio of reactants, reaction temperature and time, and try to correlate these observations with corresponding electrical conductivity measurements. The aim is to provide some useful information to obtain novel polyanilines containing some transition metal ions with electrical conductivity. 2. Experimental 2.1. Reagents and materials Aniline was vacuum distilled twice in the presence of zinc powder prior to use. (NH4 )2 S2 O8 ·2H2 O, CuCl2 ·2H2 O, FeCl2 ·4H2 O, CoCl2 ·2H2 O, NiCl2 ·2H2 O, hydrochloric acid (36%), ammonia (28%), ethanol and acetone were analytical reagents and were used as received. 2.2. Synthesis of polyanilines Polyaniline hydrochloric acid salt (PAn–HCl) and its emeraldine base (EB) were prepared according to the literature procedure [17]. Polyanilines containing transition metal ions were synthesized as follows: 5 ml distilled aniline was added into
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200 ml solution containing a quantitative amount of transition metal chloride. The solution was stirred for 2 h. Then, a certain amount of oxidant (NH4 )2 S2 O8 was dissolved in 40 ml of the same solvent and added dropwise into the above solution in 30 min. The reaction was carried out with constant stirring for 24 h and the precipitate was filtered and washed thoroughly with 95% ethanol and acetone. The resulting polyaniline product was vacuum dried at 60 ◦ C for 24 h. 2.3. Characterization of polyanilines The FT-IR spectra for all samples were obtained on a 510P Nicolet FT-IR spectrophotometer in KBr pellets in the range of 4000–400 cm−1 with a resolution of 1 cm−1 . The electrical conductivities of polyaniline samples were measured at room temperature with dry pressed pellets (Ø12.7 mm × 2 mm, 1 MPa) by the standard four-point probe method. 3. Results and discussion 3.1. FT-IR spectra of polyanilines synthesized in different media As shown in Fig. 1A, spectrum (a) corresponds to the emeraldine base synthesized according to the standard litera-
Fig. 1. (A) FT-IR spectra of EB (a) and polyanilines synthesized in different media: (b) in 1:9 EtOH/H2 O; (c) in 3:7 EtOH/H2 O; (d) in 6:4 EtOH/H2 O; (e) in 9:1 EtOH/H2 O. (B) FT-IR spectra in the range of 1000–1200 cm−1 . (C) FT-IR spectra in the range of 600–900 cm−1 .
ture method [17]. Spectra (b–e) correspond to the polyanilines synthesized in 1:9, 3:7, 6:4 and 9:1 (v/v) EtOH/H2 O reaction medium, respectively. It can be found that emeraldine base shows five main characteristic absorption peaks at 1592, 1497, 1302, 1165 and 828 cm−1 , attributed to the νN=Q=N , νN–B–N , νC–N , the characteristic mode of N=Q=N (where Q and B stand for the quinonoid and benzenoid unit, respectively) and C–H out-of-plane bending on 1,4-para-disubstituted benzene ring, respectively [18]. It is worth to notice that the FT-IR spectrum of polyaniline obtained in the different polarity reaction medium is greatly different: (i) the absorption peak at 1165 cm−1 , which was referred to by Chiang and MacDiarmid [19] as a measure of the degree of electron delocalization on polyaniline molecular chain, changes tremendously with the increase of the polarity of reaction medium; (ii) the absorption bands corresponding to different substitution mode on benzene ring change very quickly with the change of polarity of the reaction medium. As shown in Fig. 1B, it is very clear that the peak at 1165 cm−1 gradually decreased with the increase of polarity of reaction medium from 9:1 to 6:4 (v/v) EtOH/H2 O, while the peak at 1145 cm−1 gradually increased concomitantly. When the polarity of solution was increased to 1:9 (v/v) EtOH/H2 O, the peak at 1165 cm−1 disappeared completely while the peak at 1145 cm−1 becomes the strongest. Therefore, we can conclude that the electrical conductivity for these samples should increase from (e) to (b) according to the electron delocalization degree on their molecular chains [19]. As a matter of fact, the conductivities for these samples were measured to be 8.0 × 10−5 , 1.0 × 10−4 , 1.65 × 10−3 and 3.36 × 10−3 S cm−1 , respectively. It clearly indicates that stronger polar reaction medium is beneficial to obtain higher conductivity polyaniline. On the other hand, substitution mode on the benzene ring of polyaniline also changes accordingly with the change of reaction medium polarity as shown in Fig. 1C. When the polarity of medium is very weak (in 9:1, v/v, EtOH/H2 O), three absorption peaks appear at 827, 752 and 694 cm−1 in spectrum (e), another middle strong absorption peak can be observed at 1040 cm−1 [20] at the same time as shown in Fig. 1B. It indicates that 1,2-ortho-disubstituted and/or 1,2,4-trisubstituted modes on the benzene ring of polyaniline may dominate in the weaker polarity reaction medium. With the increase of medium polarity from (e) to (b), the absorption peak at 694 cm−1 was found to become smaller gradually and the peak at 752 cm−1 also decrease and almost disappear at spectrum (b). Meanwhile, the peak corresponding to 1040 cm−1 also decreased accordingly. However, the peak at 827 cm−1 becomes stronger gradually. These observations suggest that 1,4-para-disubstitution mode on benzene ring of polyaniline dominate with the increase of polarity of reaction medium. It means that the linear chain structure is overwhelming over the branch chain structure in polyaniline molecule chains. This conclusion also greatly supports that the linear chain structure is more beneficial to electrical conductivity of polyaniline than other branch chain structures.
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Fig. 2. (a) FT-IR spectra of EB and polyanilines synthesized under different temperature: (b) 0 ◦ C; (c) 25 ◦ C; (d) 35 ◦ C; (e) 45 ◦ C; (f) 55 ◦ C.
3.2. FT-IR spectra of polyanilines synthesized under different temperature Fig. 2 displays the FT-IR spectra of polyaniline synthesized under different temperature. It can be clearly found that the peaks at 750 and 694 cm−1 became smaller gradually and disappeared in the end with the decrease of temperature from 55 ◦ C (spectrum (f)) to 0 ◦ C (spectrum (b)). While the absorption peak at 827 cm−1 gradually red-shifted to 800 cm−1 . At the same time, the peaks at 1165 cm−1 disappeared very quickly and the peak at 1145 cm−1 strengthened and became the strongest peak in the whole spectrum. It means that the electrical conductivity of resulting polyaniline at different temperature should increase with the decease of reaction temperature. This is in accordance with the actual measured results: 1.23 × 10−4 (at 55 ◦ C), 2.64 × 10−4 (at 45 ◦ C), 1.46 × 10−2 (at 35 ◦ C), 2.42 × 10−2 (at 25 ◦ C) and 2.72 × 10−2 S cm−1 (at 0 ◦ C), respectively. 3.3. FT-IR spectra of polyanilines synthesized according to different reactants ratio As shown in Fig. 3, the FT-IR spectrum of the resulting polyaniline was different when the reactants ratio (aniline:CuCl2 :oxidant) was varied according to the following order 4:1:1; 4:1:2; 4:1:4; 4:2:4; 4:4:4 and 4:8:4. With the increase of amount of oxidant (ammonium persulfate) from 4:1:1 (b) to 4:1:4 (d), the absorption peak at 750 cm−1 became smaller and
Fig. 3. FT-IR spectra of EB (a) and polyanilines synthesized according to different reactants ratios (aniline:CuCl2 :oxidant = 4:1:1 (b); 4:1:2 (c); 4:1:4 (d); 4:2:4 (e); 4:4:4 (f); 4:8:4 (g)).
disappeared very quickly. The peak at 697 cm−1 also became weaker. However, the absorption peak at 827 cm−1 became stronger and the peak at 850 cm−1 disappeared very quickly at the same time. It indicates that the amount of linear chain structure corresponding to the 1,4-para-disubstitution mode in polyaniline began to be dominating while the amount of branch chain structure corresponding to the 1,2-ortho-disubstitution and/or 1,2,4-trisubstitution mode was becoming less quickly. Meanwhile, with the increase of amount of oxidant, the oxidation degree in polyaniline molecule was found to increase significantly because the peak at 1592 cm−1 corresponding to νN=Q=N was increased very quickly accompanying by obvious decrease of absorption peak at 1497 cm−1 corresponding to νN–B–N . Furthermore, the characteristic absorption peak at 1145 cm−1 corresponding to electron delocalization degree in polyaniline molecule was increased accordingly. The results of electrical conductivity measurement also verify the increase of conductivity. Sample (b) does not show any electrical conductivity in the range of measurement and sample (c) has a conductivity
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PAn–Fe and PAn–Cu, whereas the absorption strength is almost the same at 800 and 820 cm−1 for PAn–Co and PAn–Ni, but there appears a middle strong peak at 1040 cm−1 and a weak peak at 694 cm−1 for the two samples. It may means that there exists 1,2-ortho-disubstitution and/or 1,2,4-trisubstitution mode in PAn–Co and PAn–Ni. Thus, the amount of branch chain structure is more than the amount of linear chain structure. Therefore, we can conclude that the electrical conductivity of PAn–Co and PAn–Ni should be lower than that of PAn–Fe and PAn–Cu. This is also in accordance with the experimental results of conductivity measurement: 5.13 × 10−2 , 2.72 × 10−2 , 1.24 × 10−2 and 5.73 × 10−3 S cm−1 for PAn–Fe, PAn–Cu, PAn–Co and PAn–Ni, respectively. However, the conductivity is about two to three orders of magnitude lower than that of PAn–HCl salt (5.5 S cm−1 ). This may be attributed to the complexation of polyaniline with the transition metal ions. It also demonstrates that the doping ability of transition metal salt (as a Lewis acid) is weaker than strong protonic acid such as hydrochloric acid. 4. Conclusions
Fig. 4. FT-IR spectra of polyanilines containing different transition metal ions: (a) EB; (b) PAn–HCl; (c) PAn–Fe; (d) PAn–Co; (e) PAn–Ni; (f) PAn–Cu.
of ca. 1.0 × 10−4 S cm−1 , while sample (d) has a conductivity of 3.38 × 10−3 S cm−1 . When the ratio of aniline over oxidant was fixed, the effect of changing amount of transition metal salt on the FT-IR spectrum of resulting polyaniline was not obvious, but the peak at 1040 cm−1 , as shown in Fig. 3 spectrum (e–g), was found to disappear gradually with the increase of amount of transition metal salt.
The FT-IR spectra study of polyanilines containing transition metal ions demonstrates that stronger polarity reaction medium and lower reaction temperature were beneficial to obtain polyanilines with higher electrical conductivity. During the chemical oxidative polymerisation of aniline in the presence of transition metal ion Fe2+ , Co2+ , Ni2+ and Cu2+ , there may exist a chain structure transition process from 1,2-ortho-disubstitution and/or 1,2,4-trisubstitution mode to 1,4-para-disubstitution mode. The more the amount of linear chain structure, the higher the electrical conductivity of resulting polyanilines. Furthermore, the electrical conductivity of polyanilines containing different transition metal ion is different due to its different complexation ability with polyaniline molecule chains. Acknowledgement
3.4. FT-IR spectra of polyanilines containing different transition metal ions Fig. 4 gives the FT-IR spectra of polyanilines containing different transition metal ions such as Fe2+ (c), Co2+ (d), Ni2+ (e) and Cu2+ (f). Emeraldine base (a) and its emeraldine hydrochloric acid salt PAn–HCl (b) samples also show here together for comparison. As shown in Fig. 4, from spectrum (c–f), all the four polyanilines containing transition metal ions have similar FT-IR spectra. When complexing with transition metal ions, the absorption peak corresponding to the νN=Q=N shifts 10–15 cm−1 towards lower wave number; the characteristic mode of N=Q=N also shifts from 1165 to 1138–1142 cm−1 and apparently becomes much broader than in EB. Furthermore, the band corresponding to the C–H out-of-plane bending on 1,4-disubstituted benzene ring appears at 827 and 824 cm−1 for EB and PAn–HCl salt, respectively. While this band splits into two peaks at 820 and 800 cm−1 for polyanilines containing transition metal ions. The peak at 800 cm−1 is stronger than at 820 cm−1 for both
The authors acknowledge the financial support of the Natural Science Foundation of Hunan Province, China, under Grant No. 02JJY3036. References [1] A. Pron, F. Genoud, C. Menardo, M. Nechtschein, Synth. Met. 24 (1988) 193. [2] Y. Cao, A. Andreatta, A.J. Heeger, P. Smith, Polymer 30 (1989) 2305. [3] N. Gospodinova, L. Terlemezyan, P. Mokreva, K. Kossev, Polymer 34 (1993) 2434. [4] A.G. MacDiarmid, A.J. Epstein, Synth. Met. 69 (1995) 85. [5] J. Stejskal, P. Kratochv´ıl, A.D. Jenkins, Polymer 37 (1996) 367. [6] A.F. Diaz, J.A. Logan, J. Electroanal. Chem. 111 (1980) 111. [7] M. Kaempgen, S. Roth, J. Electroanal. Chem. 586 (2006) 72. [8] S.K. Dhawan, D. Kumar, M.K. Ram, S. Chandra, D.C. Trivedi, Sens. Actuators B: Chem. 40 (1997) 99. [9] S.R. Moraes, D. Huerta-Vilca, A.J. Motheo, Prog. Org. Coat. 48 (2003) 28. [10] J.L. Lu, N.J. Liu, X.H. Wang, J. Li, X.B. Jing, F.S. Wang, Synth. Met. 135–136 (2003) 237. [11] T. Nakajima, T. Kawagoe, Synth. Met. 28 (1989) 629.
C. Yang et al. / Spectrochimica Acta Part A 66 (2007) 37–41 [12] S.T. McGovern, G.M. Spinks, G.G. Wallace, Sens. Actuators B: Chem. 107 (2005) 657. [13] Y. Long, Z. Chen, J.L. Duvail, Z. Zhang, M. Wan, Phys. B: Condens. Matter 370 (2005) 121. [14] T. Hirao, M. Higuchi, I. Ikeda, Y. Ohshiro, J. Chem. Soc., Chem. Commun. 2 (1993) 194. [15] T. Hirao, M. Higuchi, B. Hatano, I. Ikeda, Tetrahedron Lett. 36 (1995) 5925.
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[16] C. Yang, C. Chen, Synth. Met. 153 (2005) 133. [17] A.G. MacDiarmid, J.C. Chiang, A.F. Richter, N.L.D. Somasiri, A.J. Epstein, in: L. Aleacer (Ed.), Conducting Polymers, Reidel, Dordrecht, 1986, p. 105. [18] R. Singh, V. Arora, R.P. Tandon, S. Chandra, N. Kumar, A. Mansingh, Polymer 38 (1997) 4897. [19] J.C. Chiang, A.G. MacDiarmid, Synth. Met. 13 (1986) 193. [20] J.S. Tang, X.B. Jing, B.C. Wang, F.S. Wang, Synth. Met. 24 (1988) 231.
Fourier transform infrared spectra of transition metal ion-containing polyanilines synthesized in different reaction conditions Chunming Yang ∗ , Chunyan Chen, Yue Zeng College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, PR China Received 31 December 2005; received in revised form 9 February 2006; accepted 13 February 2006
Abstract Fourier transform infrared spectroscopy has been employed to investigate the detailed chain structure changes during the chemical oxidative polymerisation of aniline in different reaction conditions including different polarity reaction medium, reaction temperature, reactants molar ratio and in the presence of different transition metal ions such as Fe2+ , Co2+ , Ni2+ and Cu2+ . The results show that stronger polarity reaction medium and lower reaction temperature were beneficial to obtain 1,4-para-disubstituted linear chain structure polyaniline with higher electrical conductivity. The higher oxidation degree polyaniline contained more linear chain structure than that in lower oxidation degree. Polyanilines containing Fe2+ and Cu2+ had more linear chain structure than that containing Co2+ and Ni2+ . These observations were in accordance with experimental measurements of electrical conductivity. © 2006 Elsevier B.V. All rights reserved. Keywords: Polyaniline; Transition metal ions; FT-IR spectroscopy; Characterization
1. Introduction Among all the promising electrical conducting polymers, polyaniline exhibits unique advantage both for research and application due to its easy preparation, high conductivity and good environmental stability [1–5]. Since 1970s, a great number of efforts have been devoted to its structure nature, doping and conductivity mechanism, physico-chemical and electrochemical properties associated with its application in electroactive materials [6], sensor components [7,8], anti-corrosion paintings [9,10], secondary battery electrode materials [11] and many other related fields [12,13]. Polyanilines containing some transition metal ions can be used as novel synthetic metal catalyst in some oxidation systems [14,15]. In our previous paper [16], we reported the synthesis of polyanilines containing Fe2+ , Co2+ , Ni2+ and Cu2+ by oxidizing the complex of aniline with corresponding metal chloride salt in solution. And the preliminary results were given by FT-IR spectroscopy to determine the molecular structure of resulting polyanilines. Here, we mainly employ FT-IR spectra data to elucidate the detailed chain structure changes of the polyanilines synthesized
∗
Corresponding author. Tel.: +86 731 8872285; fax: +86 731 8872531. E-mail address: [email protected] (C. Yang).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2006.02.016
under different reaction conditions such as reaction medium, the molar ratio of reactants, reaction temperature and time, and try to correlate these observations with corresponding electrical conductivity measurements. The aim is to provide some useful information to obtain novel polyanilines containing some transition metal ions with electrical conductivity. 2. Experimental 2.1. Reagents and materials Aniline was vacuum distilled twice in the presence of zinc powder prior to use. (NH4 )2 S2 O8 ·2H2 O, CuCl2 ·2H2 O, FeCl2 ·4H2 O, CoCl2 ·2H2 O, NiCl2 ·2H2 O, hydrochloric acid (36%), ammonia (28%), ethanol and acetone were analytical reagents and were used as received. 2.2. Synthesis of polyanilines Polyaniline hydrochloric acid salt (PAn–HCl) and its emeraldine base (EB) were prepared according to the literature procedure [17]. Polyanilines containing transition metal ions were synthesized as follows: 5 ml distilled aniline was added into
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200 ml solution containing a quantitative amount of transition metal chloride. The solution was stirred for 2 h. Then, a certain amount of oxidant (NH4 )2 S2 O8 was dissolved in 40 ml of the same solvent and added dropwise into the above solution in 30 min. The reaction was carried out with constant stirring for 24 h and the precipitate was filtered and washed thoroughly with 95% ethanol and acetone. The resulting polyaniline product was vacuum dried at 60 ◦ C for 24 h. 2.3. Characterization of polyanilines The FT-IR spectra for all samples were obtained on a 510P Nicolet FT-IR spectrophotometer in KBr pellets in the range of 4000–400 cm−1 with a resolution of 1 cm−1 . The electrical conductivities of polyaniline samples were measured at room temperature with dry pressed pellets (Ø12.7 mm × 2 mm, 1 MPa) by the standard four-point probe method. 3. Results and discussion 3.1. FT-IR spectra of polyanilines synthesized in different media As shown in Fig. 1A, spectrum (a) corresponds to the emeraldine base synthesized according to the standard litera-
Fig. 1. (A) FT-IR spectra of EB (a) and polyanilines synthesized in different media: (b) in 1:9 EtOH/H2 O; (c) in 3:7 EtOH/H2 O; (d) in 6:4 EtOH/H2 O; (e) in 9:1 EtOH/H2 O. (B) FT-IR spectra in the range of 1000–1200 cm−1 . (C) FT-IR spectra in the range of 600–900 cm−1 .
ture method [17]. Spectra (b–e) correspond to the polyanilines synthesized in 1:9, 3:7, 6:4 and 9:1 (v/v) EtOH/H2 O reaction medium, respectively. It can be found that emeraldine base shows five main characteristic absorption peaks at 1592, 1497, 1302, 1165 and 828 cm−1 , attributed to the νN=Q=N , νN–B–N , νC–N , the characteristic mode of N=Q=N (where Q and B stand for the quinonoid and benzenoid unit, respectively) and C–H out-of-plane bending on 1,4-para-disubstituted benzene ring, respectively [18]. It is worth to notice that the FT-IR spectrum of polyaniline obtained in the different polarity reaction medium is greatly different: (i) the absorption peak at 1165 cm−1 , which was referred to by Chiang and MacDiarmid [19] as a measure of the degree of electron delocalization on polyaniline molecular chain, changes tremendously with the increase of the polarity of reaction medium; (ii) the absorption bands corresponding to different substitution mode on benzene ring change very quickly with the change of polarity of the reaction medium. As shown in Fig. 1B, it is very clear that the peak at 1165 cm−1 gradually decreased with the increase of polarity of reaction medium from 9:1 to 6:4 (v/v) EtOH/H2 O, while the peak at 1145 cm−1 gradually increased concomitantly. When the polarity of solution was increased to 1:9 (v/v) EtOH/H2 O, the peak at 1165 cm−1 disappeared completely while the peak at 1145 cm−1 becomes the strongest. Therefore, we can conclude that the electrical conductivity for these samples should increase from (e) to (b) according to the electron delocalization degree on their molecular chains [19]. As a matter of fact, the conductivities for these samples were measured to be 8.0 × 10−5 , 1.0 × 10−4 , 1.65 × 10−3 and 3.36 × 10−3 S cm−1 , respectively. It clearly indicates that stronger polar reaction medium is beneficial to obtain higher conductivity polyaniline. On the other hand, substitution mode on the benzene ring of polyaniline also changes accordingly with the change of reaction medium polarity as shown in Fig. 1C. When the polarity of medium is very weak (in 9:1, v/v, EtOH/H2 O), three absorption peaks appear at 827, 752 and 694 cm−1 in spectrum (e), another middle strong absorption peak can be observed at 1040 cm−1 [20] at the same time as shown in Fig. 1B. It indicates that 1,2-ortho-disubstituted and/or 1,2,4-trisubstituted modes on the benzene ring of polyaniline may dominate in the weaker polarity reaction medium. With the increase of medium polarity from (e) to (b), the absorption peak at 694 cm−1 was found to become smaller gradually and the peak at 752 cm−1 also decrease and almost disappear at spectrum (b). Meanwhile, the peak corresponding to 1040 cm−1 also decreased accordingly. However, the peak at 827 cm−1 becomes stronger gradually. These observations suggest that 1,4-para-disubstitution mode on benzene ring of polyaniline dominate with the increase of polarity of reaction medium. It means that the linear chain structure is overwhelming over the branch chain structure in polyaniline molecule chains. This conclusion also greatly supports that the linear chain structure is more beneficial to electrical conductivity of polyaniline than other branch chain structures.
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Fig. 2. (a) FT-IR spectra of EB and polyanilines synthesized under different temperature: (b) 0 ◦ C; (c) 25 ◦ C; (d) 35 ◦ C; (e) 45 ◦ C; (f) 55 ◦ C.
3.2. FT-IR spectra of polyanilines synthesized under different temperature Fig. 2 displays the FT-IR spectra of polyaniline synthesized under different temperature. It can be clearly found that the peaks at 750 and 694 cm−1 became smaller gradually and disappeared in the end with the decrease of temperature from 55 ◦ C (spectrum (f)) to 0 ◦ C (spectrum (b)). While the absorption peak at 827 cm−1 gradually red-shifted to 800 cm−1 . At the same time, the peaks at 1165 cm−1 disappeared very quickly and the peak at 1145 cm−1 strengthened and became the strongest peak in the whole spectrum. It means that the electrical conductivity of resulting polyaniline at different temperature should increase with the decease of reaction temperature. This is in accordance with the actual measured results: 1.23 × 10−4 (at 55 ◦ C), 2.64 × 10−4 (at 45 ◦ C), 1.46 × 10−2 (at 35 ◦ C), 2.42 × 10−2 (at 25 ◦ C) and 2.72 × 10−2 S cm−1 (at 0 ◦ C), respectively. 3.3. FT-IR spectra of polyanilines synthesized according to different reactants ratio As shown in Fig. 3, the FT-IR spectrum of the resulting polyaniline was different when the reactants ratio (aniline:CuCl2 :oxidant) was varied according to the following order 4:1:1; 4:1:2; 4:1:4; 4:2:4; 4:4:4 and 4:8:4. With the increase of amount of oxidant (ammonium persulfate) from 4:1:1 (b) to 4:1:4 (d), the absorption peak at 750 cm−1 became smaller and
Fig. 3. FT-IR spectra of EB (a) and polyanilines synthesized according to different reactants ratios (aniline:CuCl2 :oxidant = 4:1:1 (b); 4:1:2 (c); 4:1:4 (d); 4:2:4 (e); 4:4:4 (f); 4:8:4 (g)).
disappeared very quickly. The peak at 697 cm−1 also became weaker. However, the absorption peak at 827 cm−1 became stronger and the peak at 850 cm−1 disappeared very quickly at the same time. It indicates that the amount of linear chain structure corresponding to the 1,4-para-disubstitution mode in polyaniline began to be dominating while the amount of branch chain structure corresponding to the 1,2-ortho-disubstitution and/or 1,2,4-trisubstitution mode was becoming less quickly. Meanwhile, with the increase of amount of oxidant, the oxidation degree in polyaniline molecule was found to increase significantly because the peak at 1592 cm−1 corresponding to νN=Q=N was increased very quickly accompanying by obvious decrease of absorption peak at 1497 cm−1 corresponding to νN–B–N . Furthermore, the characteristic absorption peak at 1145 cm−1 corresponding to electron delocalization degree in polyaniline molecule was increased accordingly. The results of electrical conductivity measurement also verify the increase of conductivity. Sample (b) does not show any electrical conductivity in the range of measurement and sample (c) has a conductivity
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PAn–Fe and PAn–Cu, whereas the absorption strength is almost the same at 800 and 820 cm−1 for PAn–Co and PAn–Ni, but there appears a middle strong peak at 1040 cm−1 and a weak peak at 694 cm−1 for the two samples. It may means that there exists 1,2-ortho-disubstitution and/or 1,2,4-trisubstitution mode in PAn–Co and PAn–Ni. Thus, the amount of branch chain structure is more than the amount of linear chain structure. Therefore, we can conclude that the electrical conductivity of PAn–Co and PAn–Ni should be lower than that of PAn–Fe and PAn–Cu. This is also in accordance with the experimental results of conductivity measurement: 5.13 × 10−2 , 2.72 × 10−2 , 1.24 × 10−2 and 5.73 × 10−3 S cm−1 for PAn–Fe, PAn–Cu, PAn–Co and PAn–Ni, respectively. However, the conductivity is about two to three orders of magnitude lower than that of PAn–HCl salt (5.5 S cm−1 ). This may be attributed to the complexation of polyaniline with the transition metal ions. It also demonstrates that the doping ability of transition metal salt (as a Lewis acid) is weaker than strong protonic acid such as hydrochloric acid. 4. Conclusions
Fig. 4. FT-IR spectra of polyanilines containing different transition metal ions: (a) EB; (b) PAn–HCl; (c) PAn–Fe; (d) PAn–Co; (e) PAn–Ni; (f) PAn–Cu.
of ca. 1.0 × 10−4 S cm−1 , while sample (d) has a conductivity of 3.38 × 10−3 S cm−1 . When the ratio of aniline over oxidant was fixed, the effect of changing amount of transition metal salt on the FT-IR spectrum of resulting polyaniline was not obvious, but the peak at 1040 cm−1 , as shown in Fig. 3 spectrum (e–g), was found to disappear gradually with the increase of amount of transition metal salt.
The FT-IR spectra study of polyanilines containing transition metal ions demonstrates that stronger polarity reaction medium and lower reaction temperature were beneficial to obtain polyanilines with higher electrical conductivity. During the chemical oxidative polymerisation of aniline in the presence of transition metal ion Fe2+ , Co2+ , Ni2+ and Cu2+ , there may exist a chain structure transition process from 1,2-ortho-disubstitution and/or 1,2,4-trisubstitution mode to 1,4-para-disubstitution mode. The more the amount of linear chain structure, the higher the electrical conductivity of resulting polyanilines. Furthermore, the electrical conductivity of polyanilines containing different transition metal ion is different due to its different complexation ability with polyaniline molecule chains. Acknowledgement
3.4. FT-IR spectra of polyanilines containing different transition metal ions Fig. 4 gives the FT-IR spectra of polyanilines containing different transition metal ions such as Fe2+ (c), Co2+ (d), Ni2+ (e) and Cu2+ (f). Emeraldine base (a) and its emeraldine hydrochloric acid salt PAn–HCl (b) samples also show here together for comparison. As shown in Fig. 4, from spectrum (c–f), all the four polyanilines containing transition metal ions have similar FT-IR spectra. When complexing with transition metal ions, the absorption peak corresponding to the νN=Q=N shifts 10–15 cm−1 towards lower wave number; the characteristic mode of N=Q=N also shifts from 1165 to 1138–1142 cm−1 and apparently becomes much broader than in EB. Furthermore, the band corresponding to the C–H out-of-plane bending on 1,4-disubstituted benzene ring appears at 827 and 824 cm−1 for EB and PAn–HCl salt, respectively. While this band splits into two peaks at 820 and 800 cm−1 for polyanilines containing transition metal ions. The peak at 800 cm−1 is stronger than at 820 cm−1 for both
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