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camphor sulfonic acid in formic acid medium and their blends with polyamide-6 by in situ polymerization
Synthetic Metals 159 (2009) 1491–1495
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Synthesis of polyaniline/camphor sulfonic acid in formic acid medium and their blends with polyamide-6 by in situ polymerization Anderson R.A. Schettini a , Rosa C.D. Peres b , Bluma G. Soares c,∗ a b c
Centro Tecnológico do Exército, Grupo de Química, Avenida das Américas, 28.705 Guaratiba, Rio de Janeiro 23020-470, RJ, Brazil Instituto de Química, Universidade Federal do Rio de Janeiro, Centro de Tecnologia Bl A, Rio de Janeiro 21945-970, RJ, Brazil Instituto de Macromoléculas, Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Bl. J, Rio de Janeiro 21945-970, RJ, Brazil
a r t i c l e
i n f o
Article history: Received 16 April 2008 Received in revised form 26 March 2009 Accepted 7 April 2009 Available online 22 May 2009 Keywords: Polyaniline Polyamide 6 In situ polymerization Formic acid Cyclic voltammetry
a b s t r a c t Blends of polyaniline doped with camphor sulfonic acid (PAni.CSA) and polyamide 6 (PA6) were prepared for the first time by the in situ polymerization of aniline in the presence of a solution of PA6 in formic acid. The conductivity values of the blends prepared by both solution cast and in situ polymerization are in the range useful for electrostatic charge dissipation materials. The conductivity of the blend containing 10% of PAni.CSA was higher, when the in situ polymerization process was used, indicating the formation of conducting pathways in a higher extension. Most of the blends prepared by the in situ polymerization presented only one irreversible peak in the cyclic voltammetry, which is attributed to the presence of defects along the PAni.CSA chains. These defects may be caused by the presence of formic acid as the solvent, as observed by Fourier transformer infrared spectroscopy of the pure polyaniline prepared in the presence of formic acid. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Blends of polyaniline (PAni) with thermoplastics constitute a very useful approach for the development of conducting materials with good processability and good mechanical properties. Several methods of preparation of such blends have been proposed and include thermal and solution processing of PAni-based blends as well as the in situ polymerization of aniline within the insulating polymer [1]. Polyamide–PAni blends are usually prepared by solution processing [2–7] or by the in situ polymerization of aniline on the surface of the polyamide substrate as a film or fiber [8–10]. The main drawback of the last method is that the conductivity of the material is restricted to the substrate surface because the diffusion of the aniline monomer and the other ingredients is difficult. This method is however very useful for fibers and thin films. The solution processing results in more homogeneous conducting composites. However, it is important to identify a solvent in which both the conducting polymer and the host polymer are soluble. Some reports in the literature have employed sulfuric acid [2–4], formic acid [6–7], and m-cresol [5] to prepare these blends by the
∗ Corresponding author. Tel.: +55 51 212562 7207. E-mail addresses: [email protected] (A.R.A. Schettini), [email protected] (R.C.D. Peres), [email protected] (B.G. Soares). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.04.006
solution casting process. Formic acid is an interesting medium for PAni–polyamide blend preparation because it is able to dissolve both components, it is not an expensive solvent and it can be easily removed from the medium. In addition, it can act as a protonating agent for polyaniline. Indeed, some few papers in the literature deal with the chemical polymerization of aniline and their derivatives in formic acid medium [11,12]. To the best of our knowledge there is no study related to the in situ polymerization of aniline in a solution of polyamide in formic acid. In this work, we report for the first time the preparation of PAni–polyamide conducting composites by the in situ polymerization of aniline within a polyamide solution in formic acid, using camphor sulfonic acid as the protonating agent. The direct polymerization of aniline in this system was also investigated in terms of electrical and electrochemical properties.
2. Experimental 2.1. Materials Aniline (Ani) from Merck (Brazil) was distilled under reduced pressure before use. Ammonium peroxydisulfate (APS) (Vetec – Brazil), camphor sulfonic acid (CSA) (Across Organics) and formic acid (98%) (Nuclear – Brazil) were used without purification. Polyamide 6 (PA6) beads were supplied by Radici Plásticos Ltda, São Paulo – Brazil.
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2.2. Synthesis of polyaniline
Table 1 Conductivity and crystallinity degree of polyaniline prepared in different media.
Polyaniline doped with CSA (PAni.CSA) was synthesized by one step route using water or formic acid as the solvent. In a typical procedure 2.3 g (0.01 mol) of CSA were dissolved in 100 mL of the solvent. 0.93 g (0.01 mol) of aniline was added and the solution was cooled until 0 ◦ C with constant stirring (350 rpm) for 15 min. Then, an aqueous solution of 2.28 g (0.01 mol) of APS in 5 mL deionized water was slowly added. The reaction was performed at 0 ◦ C for 6 h. The product was precipitated into methanol, filtered, washed several times with methanol and dried under reduced pressure for 24 h. Polyaniline doped with formic acid (PAni.HCOOH) was prepared in a similar procedure, but without the presence of CSA.
Properties
PAni./CSA in water
PAni./CSA in formic acid
PAni.HCOOH
Conductivity (S cm−1 ) Crystallinity (%)
3.30 × 10−1 22
1.07 × 10−1 36
9.04 × 10−3 24
2.3. Blend preparation PAni.CSA/PA6 blends were prepared by solution processing in m-cresol (physical blends) and by in situ polymerization of aniline in a formic acid solution of PA6 at different polymer concentration. For blends prepared by solution procedure, PA6 and PAni.CSA prepared in aqueous medium were dissolved in m-cresol at a total solid percentage of 5 wt%. The medium was stirred for 48 h in order to obtain an homogeneous solution. The solution was casted into teflon sheet and the solvent was evaporated. For blends prepared by in situ polymerization, 2.32 g (0.01 mol) of CSA and different amounts of PA6 were dissolved into 100 mL of concentrated formic acid. The medium was cooled at 0 ◦ C and 0.93 g (0.01 mol) aniline was added under stirring. Then, an aqueous solution of 2.28 g (0.01 mol) of APS was slowly added and the reaction medium was kept at 0 ◦ C under constant stirring for 6 h. The product was precipitated into methanol, filtered, washed and dried.
2.4. Characterization The conductivity of the PAni.CSA samples and their blends was measured by the four-probe method with a Keithley 6517 electrometer, according to literature [13]. The samples were prepared in discs with a diameter of 1–2 cm and a thickness of 0.5 mm, by pressing the dried and milled powder of the samples at room temperature and pressure of 80 kgf/cm2 for 5 min. The conductivity of the physical blends was measured from the film casted from the m-cresol solution. The X-ray diffraction was recorded on a XRD Rigaku, model ´˚ The 2 Miniflex using Cu-K␣ radiation of wavelength of 1.5408 A. range was 5–45◦ . The Fourier transform infrared (FTIR) spectra were obtained from the KBr pellets containing the corresponding powder samples and recorded in a Nicolet FTIR spectrometer Magna IR 760. The cyclic voltammetry measurements were carried out with a Galvanostate/Potenciostate PGStat 30 – Autolab. For the electrochemical cell we employed a standard compartment cell. A Pt foil was the counter-electrode, the reference electrode Ag/AgCl, saturated KCl electrode and a Pt foil recovered with PAni or blend film (area = 4 cm2 ) as working electrode. The electrolyte was an aqueous solution HCl 0.1 mol L−1 /KCl 0.1 mol L−1 . It was scanning five cycles with a sweep rate of 30 mV s−1 in a range of −200 mV a 900 mV. All measurements were carried out at 25 ◦ C. For these measurements, the films were obtained by casting the m-cresol solution. The evaporation of solvent was under 80 ◦ C in an oven by 1 h. The films showed good adherence to Pt foil except PAni obtained in formic acid without CSA.
3. Results and discussion 3.1. Properties of PAni.CSA prepared in different media Before studying the characteristic of PAni/PA6 blends, it was decided to investigate the main features of PAni.CSA prepared with the same procedure as that used for the blends. PAni.CSA was prepared by one step polymerization of aniline in the presence of CSA as the protonating agent and APS as the oxidant agent. The effect of the reaction medium (water or formic acid) on the electrical conductivity and crystallinity degree of PAni.CSA is compared in Table 1. Polyaniline prepared in the presence of formic acid, without the addition of CSA (PAni.HCOOH) was also included for comparison. PAni.CSA prepared in water presented a little higher conductivity than that prepared in formic acid. PAni.HCOOH also presented an acceptable conductivity confirming the protonating action of the formic acid, but the value was not as high as those observed for PAni.CSA. The crystallinity degree was determined from X-ray diffractograms, illustrated in Fig. 1. PAni.CSA prepared in formic acid presented higher crystallinity degree and XRD pattern with sharp crystalline diffraction peak at 2 around 25◦ and other diffraction peaks at 2 = 7◦ , 13◦ and 19◦ (curve B). This WAXS pattern is very similar to that observed for the PAni.HCOOH (curve C) and also to those reported in the literature for PAni doped with chloridric acid (PAni.HCl) or with formic acid [14]. The higher crystallinity degree of PAni.CSA prepared in formic acid (PAni.CSA-HCOOH) may be attributed to the higher ability of this solvent in dissolving the PAni.CSA chains. The better polymer chain–solvent interaction contributes for an extended coil conformation, which facilitates the macromolecular rearrangements required for the formation of the crystalline regions in higher extent. FTIR spectroscopy constitutes a powerful tool for the evaluation of various intrinsic oxidation states of polyaniline [15]. It has been reported that the oxidation levels can be qualitatively estimated by the intensity ratio of the absorption peaks at ∼1600 cm−1 (quinoid
Fig. 1. X-ray diffractograms of (A) PAni.CSA obtained in aqueous medium, (B) PAni.CSA obtained in formic acid medium and (C) PAni.HCOOH.
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Fig. 2. FTIR of PAni.CSA obtained in aqueous medium, in formic acid medium and PAni.HCOOH. Fig. 3. Effect of the composition on the conductivity of PAni.CSA/PA6 blends obtained by (A) physical blend and (B) in situ polymerization in HCOOH.
ring stretching – Q) and ∼1500 cm−1 (benzenoid ring stretching – B) [16]. In order to evaluate the effect of the reaction conditions on the oxidation level of the PAni chains, the protonated polyaniline samples were deprotonated by treating the samples with 1 M aqueous solution of ammonium hydroxide for 48 h. The FTIR spectra of the neutral form of the polyaniline samples (emeraldine base) are compared in Fig. 2. The Q/B ratio of the neutral form of PAni.CSA obtained in aqueous medium (PAni.CSA-H2 O) (a/b peak ratio) corresponded to 0.93, which was close to the ratio of an ideal emeraldine base (50% oxidized). The presence of formic acid as the solvent decreased the Q/B ratio to a value corresponding to 0.83, indicating a little higher concentration of benzenoid units. Another important aspect in the FTIR spectra is the presence of an absorption at around 810 cm−1 (peak c) assigned to C–H out of plane bending of 1,4-substituted ring [15]. The ratio between the absorption at 810 cm−1 (peak c) and 1600 cm−1 (peak a) was used to compare the molecular structure of the PAni chains. The dedoped form of PAni.HCOOH (curve c) displayed the lowest ratio value (c/a ratio = 0.46), indicating a higher extent of 1,2-substituted benzenic ring, which can be associated to a high degree of defects in the polyaniline chain. The dedoped form of PAni.CSA in water and in formic acid presented similar c/a ratios (0.65 and 0.62, respectively), suggesting that the CSA as the dopant decreased the possibility of 1,2-dissubstitution. 3.2. PAni.CSA/polyamide 6 blends The effect of the methodology on the conductivity of PAni.CSA/PA6 blends is summarized in Table 2 and illustrated in Fig. 3. The conductivity of the blends increased with the PAni.CSA concentration. For blends containing PAni.CSA concentration lower than 10%, the in situ polymerization resulted in higher conductivity values because the formed PAni.CSA chains are more dispersed inside the PA6 matrix. For blends containing higher amount of
PAni.CSA, the conductivity of the in situ polymerized blends were lower probably because of the greater influence of the medium on the conductivity of the PAni.CSA formed in situ. 3.3. Cyclic voltammetry The inhomogeneity and defects in the polymer chain of PAni affect the electrochemical properties of PAni, since the oxidation and reduction processes depend on the relaxation of the polymer chain and on the charge/mass transport through the material. The effect of the blend preparation procedure on the electrochemical response of PAni.CSA samples was evaluated by cyclic voltammetry. Five cycles were scanned with a sweep rate of 30 mV s−1 . Fig. 4 illustrates the third cycle of PAni.CSA/PA6 blends with different composition and Table 3 summarizes the voltage values corresponding to the anodic and cathodic peaks. PAni.CSA prepared in aqueous medium (Fig. 4A left side) exhibits the two characteristic redox systems: the first peak system (Ep anodic = 403 mV and Ep cathodic = 10 mV) belongs to the oxidation/reduction of leucoemeraldine to emeraldine, while the second (Ep anodic = 725 mV and Ep cathodic = 354 mV) is due to the oxidation/reduction of emeraldine to pernigraniline. Comparing to pure PAni.CSA, the physical blends of PA6/PAni.CSA presented a shift of the first peak system towards lower potential values, whereas the second peak system was shifted towards higher potential values. This indicates that the presence of PA6 in the blend facilitates the formation of emeraldine but difficults the formation of pernigraniline. Physical blend containing 10% of PAni.CSA presented only one irreversible peak system (Fig. 4D left side) indicating that the electroactivity was lost. The factors affecting the broadening and shifting of the peak may be as follows: (i) the degradation of PAni backbone, (ii) the low ionic diffusivity to maintain charge neutrality due to the modification of PAni microstructure by the gradual elim-
Table 2 Conductivity values of PA6/PAni.CSA blends as a function of the preparation procedure. Conductivity (S cm−1 )
Blend components (wt%) PA6
PAni.CSA
Physical blends
In situ blends (in HCOOH)
In situ blends (in HCOOH.H2 O)
90 70 50 30 10
10 30 50 70 90
4.8 × 10−6 2.8 × 10−4 3.0 × 10−3 4.6 × 10−3 2.2 × 10−2
2.4 × 10−5 1.3 × 10−4 7.6 × 10−4 7.7 × 10−4 1.5 × 10−2
5.1 × 10−6 1.2 × 10−4 3.0 × 10−4 4.8 × 10−4 4.5 × 10−3
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Fig. 4. Cyclic voltammetry of PAni.CSA/PA6 blends obtained by physical blend and in situ polymerization as a function of the composition: PAni.CSA/PA6 = 100:0 (A); 90:10 (B); 50:50 (C); 10:90 (D).
ination of dopant ions from the film matrix, and (iii) the decrease in electronic conductivity due to the deprotonation of the polymer backbone [17]. Blends prepared by the in situ polymerization (right side in Fig. 4) presented only one irreversible peak system, except for the 50:50 wt% blend (Fig. 4C right side). Besides that, during the electrochemical measurements it was noted that the electrolytic
solution changed from incolor to yellow after the third voltammetric cycle. The absence of well defined electrochemical response in these blends indicates that the relaxation processes of the polymer chain, and the ionic diffusion, are not occurring satisfactorily leading to the material degradation during redox switching [18]. These results may be due to the presence of defects along the PAni.CSA chain, as observed by FTIR analysis.
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Table 3 Electrochemical data of PAni.CSA obtained in different media, determined from cyclic voltammetry. Blend composition (wt%)
Anodic peaks (mV)
Cathodic peaks (mV)
Q− (×10−1 ) (C)
PAni.CSA
PA6
Peak 1
Peak 2
Physical blendsa 100 90 70 50 30 10
0 10 30 50 70 90
403 305 318 283 359 –
725 757 750 735 698 530
10 −41 −27 17 149 –
354 491 486 515 447 349
3.978 2.774 2.270 1.961 1.594 0.4708
−3.955 −2.758 −2.265 −1.959 −1.595 −0.4539
In situ blendsb 100 90 70 50 30 10
0 10 30 50 70 90
– – – 401 415 –
750 769 574 747 728 611
56 −51 −117 −44 71 149
– – – 232 398 –
3.973 7.604 9.445 4.755 3.862 0.5020
−3.892 −7.534 −9.373 −4.670 −3.740 −0.5128
a b
Peak 1
Q+ (×10−1 ) (C)
Peak 2
PAni.CSA used in physical blends was prepared in aqueous medium. PAni.CSA and their in situ blends with PA6 were performed in formic acid medium.
The oxidation and reduction charges in all blends (Table 3) exhibited very close values, indicating similar concentration of oxidation and reduction sites. For comparative effect in terms of conductivity and electroactivity phenomena, PAni.CSA and PAni.CSA/PA6 blends were prepared by the in situ polymerization using HCOOH diluted in water in a concentration of 70% (HCOOH.H2 O). PAni.CSA.HCOOH.H2 O presented a decrease of an order in conductivity (6.0 × 10−2 S cm−1 ) but a similar electrochemical behavior (not shown here) as that observed with PAni.CSA.HCOOH (only one irreversible peak system). PAni.CSA.HCOOH.H2 O/PA6 blends presented lost of electroactivity in all compositions. Moreover, the conductivity decrease in compositions containing PA6 concentration lower than 70% (Table 2), comparing to physical blends. For blends containing higher amount of PA6 the conductivity were the same. 4. Conclusions From the results obtained in this work, one can conclude that: • The synthesis of PAni.CSA in the presence of formic acid resulted in PAni sample with higher crystallinity value and conductivity comparable to the PAni.CSA obtained in aqueous medium. • It is possible to prepare blends of PAni.CSA with polyamide 6 by in situ polymerization of aniline inside a formic acid solution of PA6. Except for the blend containing 10% of PAni.CSA, the conductivity values were lower in blends obtained by the in situ polymerization when compared to those obtained by physical blends, using the solution processing in m-cresol. • It was expected a better dispersion of the conductive phase in insulating matrix in blends prepared by in situ polymerization. It is known that the polymerization of a monomer in a polymer matrix is a useful approach for the in situ preparation of new materials. By this procedure, more intimate mixing of the two components is possible [19]. Besides that, in solution blending
is important identify a good solvent in which both phases are soluble in order to avoid the segregation of conducting phase by solvent evaporation. • In addition, the presence of the formic acid in the synthesis of PAni.CSA and their in situ polymerized blends with PA6 resulted in the electroactivity loss, as indicated by the presence of only one irreversible peak system. Acknowledgements We are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico-CNPq, Coordenac¸ão de Aperfeic¸oamento de Pessoal de Ensino Superior-CAPES, Fundac¸ão de Amparo a Pesquisa do Estado do Rio de Janeiro-FAPERJ for the financial support. References [1] A. Pud, N. Ogurtsov, A. Korzhenko, G. Shapoval, Prog. Polym. Sci. 28 (2003) 1701. [2] C.H. Hsu, H. Shih, S. Subramoney, A.J. Epstein, Synth. Metals 101 (1998) 677. [3] Q.H. Zhang, Z.C. Sun, J. Li, X.H. Wang, H.F. Jin, X.B. Jing, F.S. Wang, Synth. Metals 102 (1999) 1198. [4] Q. Zhang, H. Jin, X. Wang, X. Jing, Synth. Metals 123 (2001) 481. [5] Q. Zhang, X. Wang, Y. Geng, D. Chen, X. Jing, J. Polym. Sci., Phys. Ed. 40 (2002) 2531. [6] D. Abraham, A. Bharathi, S.V. Subramanyam, Polym. Commun. 37 (1996) 5295. [7] M. Zagórska, E. Taler, I. Kulszewicz-Bajer, A. Prón, J. Niziol, J. Appl. Polym. Sci. 73 (1999) 1423. [8] K.W. Oh, K.H. Hong, S.H. Kim, J. Appl. Polym. Sci. 74 (1999) 2094. [9] S.W. Byun, S.S. Im, Polymer 39 (1998) 485. [10] S.W. Byun, S.S. Im, Synth. Metals 69 (1995) 219. [11] A. Gök, B. Sari, J. Appl. Polym. Sci. 84 (2002) 1993. [12] A. Dan, P.K. Sengupta, J. Appl. Polym. Sci. 91 (2004) 991. [13] E.M. Girotto, I.A. Santos, Quim. Nova 25 (2002) 639. [14] K.K. Chaudhari, D.S. Kellar, J. Appl. Polym. Sci. 62 (1996) 15. [15] E.T. Kang, K.G. Neoh, K.L. Tan, Prog. Polym. Sci. 23 (1998) 277. [16] S. Davied, Y.F. Nicolau, F. Melis, A. Revillon, Synth. Metals 69 (1995) 175. [17] J.M. Ko, I.J. Chung, Synth. Metals 68 (1995) 233. [18] A.G. Bedekar, S.F. Patil, R.C. Patil, K. Vijayamohanan, Mater. Chem. Phys. 48 (1997) 76. [19] J. Anand, S. Palaniapan, D.N. Sathyanarayana, Prog. Polym. Sci. 23 (1998) 993.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Synthesis of polyaniline/camphor sulfonic acid in formic acid medium and their blends with polyamide-6 by in situ polymerization Anderson R.A. Schettini a , Rosa C.D. Peres b , Bluma G. Soares c,∗ a b c
Centro Tecnológico do Exército, Grupo de Química, Avenida das Américas, 28.705 Guaratiba, Rio de Janeiro 23020-470, RJ, Brazil Instituto de Química, Universidade Federal do Rio de Janeiro, Centro de Tecnologia Bl A, Rio de Janeiro 21945-970, RJ, Brazil Instituto de Macromoléculas, Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Bl. J, Rio de Janeiro 21945-970, RJ, Brazil
a r t i c l e
i n f o
Article history: Received 16 April 2008 Received in revised form 26 March 2009 Accepted 7 April 2009 Available online 22 May 2009 Keywords: Polyaniline Polyamide 6 In situ polymerization Formic acid Cyclic voltammetry
a b s t r a c t Blends of polyaniline doped with camphor sulfonic acid (PAni.CSA) and polyamide 6 (PA6) were prepared for the first time by the in situ polymerization of aniline in the presence of a solution of PA6 in formic acid. The conductivity values of the blends prepared by both solution cast and in situ polymerization are in the range useful for electrostatic charge dissipation materials. The conductivity of the blend containing 10% of PAni.CSA was higher, when the in situ polymerization process was used, indicating the formation of conducting pathways in a higher extension. Most of the blends prepared by the in situ polymerization presented only one irreversible peak in the cyclic voltammetry, which is attributed to the presence of defects along the PAni.CSA chains. These defects may be caused by the presence of formic acid as the solvent, as observed by Fourier transformer infrared spectroscopy of the pure polyaniline prepared in the presence of formic acid. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Blends of polyaniline (PAni) with thermoplastics constitute a very useful approach for the development of conducting materials with good processability and good mechanical properties. Several methods of preparation of such blends have been proposed and include thermal and solution processing of PAni-based blends as well as the in situ polymerization of aniline within the insulating polymer [1]. Polyamide–PAni blends are usually prepared by solution processing [2–7] or by the in situ polymerization of aniline on the surface of the polyamide substrate as a film or fiber [8–10]. The main drawback of the last method is that the conductivity of the material is restricted to the substrate surface because the diffusion of the aniline monomer and the other ingredients is difficult. This method is however very useful for fibers and thin films. The solution processing results in more homogeneous conducting composites. However, it is important to identify a solvent in which both the conducting polymer and the host polymer are soluble. Some reports in the literature have employed sulfuric acid [2–4], formic acid [6–7], and m-cresol [5] to prepare these blends by the
∗ Corresponding author. Tel.: +55 51 212562 7207. E-mail addresses: [email protected] (A.R.A. Schettini), [email protected] (R.C.D. Peres), [email protected] (B.G. Soares). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.04.006
solution casting process. Formic acid is an interesting medium for PAni–polyamide blend preparation because it is able to dissolve both components, it is not an expensive solvent and it can be easily removed from the medium. In addition, it can act as a protonating agent for polyaniline. Indeed, some few papers in the literature deal with the chemical polymerization of aniline and their derivatives in formic acid medium [11,12]. To the best of our knowledge there is no study related to the in situ polymerization of aniline in a solution of polyamide in formic acid. In this work, we report for the first time the preparation of PAni–polyamide conducting composites by the in situ polymerization of aniline within a polyamide solution in formic acid, using camphor sulfonic acid as the protonating agent. The direct polymerization of aniline in this system was also investigated in terms of electrical and electrochemical properties.
2. Experimental 2.1. Materials Aniline (Ani) from Merck (Brazil) was distilled under reduced pressure before use. Ammonium peroxydisulfate (APS) (Vetec – Brazil), camphor sulfonic acid (CSA) (Across Organics) and formic acid (98%) (Nuclear – Brazil) were used without purification. Polyamide 6 (PA6) beads were supplied by Radici Plásticos Ltda, São Paulo – Brazil.
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2.2. Synthesis of polyaniline
Table 1 Conductivity and crystallinity degree of polyaniline prepared in different media.
Polyaniline doped with CSA (PAni.CSA) was synthesized by one step route using water or formic acid as the solvent. In a typical procedure 2.3 g (0.01 mol) of CSA were dissolved in 100 mL of the solvent. 0.93 g (0.01 mol) of aniline was added and the solution was cooled until 0 ◦ C with constant stirring (350 rpm) for 15 min. Then, an aqueous solution of 2.28 g (0.01 mol) of APS in 5 mL deionized water was slowly added. The reaction was performed at 0 ◦ C for 6 h. The product was precipitated into methanol, filtered, washed several times with methanol and dried under reduced pressure for 24 h. Polyaniline doped with formic acid (PAni.HCOOH) was prepared in a similar procedure, but without the presence of CSA.
Properties
PAni./CSA in water
PAni./CSA in formic acid
PAni.HCOOH
Conductivity (S cm−1 ) Crystallinity (%)
3.30 × 10−1 22
1.07 × 10−1 36
9.04 × 10−3 24
2.3. Blend preparation PAni.CSA/PA6 blends were prepared by solution processing in m-cresol (physical blends) and by in situ polymerization of aniline in a formic acid solution of PA6 at different polymer concentration. For blends prepared by solution procedure, PA6 and PAni.CSA prepared in aqueous medium were dissolved in m-cresol at a total solid percentage of 5 wt%. The medium was stirred for 48 h in order to obtain an homogeneous solution. The solution was casted into teflon sheet and the solvent was evaporated. For blends prepared by in situ polymerization, 2.32 g (0.01 mol) of CSA and different amounts of PA6 were dissolved into 100 mL of concentrated formic acid. The medium was cooled at 0 ◦ C and 0.93 g (0.01 mol) aniline was added under stirring. Then, an aqueous solution of 2.28 g (0.01 mol) of APS was slowly added and the reaction medium was kept at 0 ◦ C under constant stirring for 6 h. The product was precipitated into methanol, filtered, washed and dried.
2.4. Characterization The conductivity of the PAni.CSA samples and their blends was measured by the four-probe method with a Keithley 6517 electrometer, according to literature [13]. The samples were prepared in discs with a diameter of 1–2 cm and a thickness of 0.5 mm, by pressing the dried and milled powder of the samples at room temperature and pressure of 80 kgf/cm2 for 5 min. The conductivity of the physical blends was measured from the film casted from the m-cresol solution. The X-ray diffraction was recorded on a XRD Rigaku, model ´˚ The 2 Miniflex using Cu-K␣ radiation of wavelength of 1.5408 A. range was 5–45◦ . The Fourier transform infrared (FTIR) spectra were obtained from the KBr pellets containing the corresponding powder samples and recorded in a Nicolet FTIR spectrometer Magna IR 760. The cyclic voltammetry measurements were carried out with a Galvanostate/Potenciostate PGStat 30 – Autolab. For the electrochemical cell we employed a standard compartment cell. A Pt foil was the counter-electrode, the reference electrode Ag/AgCl, saturated KCl electrode and a Pt foil recovered with PAni or blend film (area = 4 cm2 ) as working electrode. The electrolyte was an aqueous solution HCl 0.1 mol L−1 /KCl 0.1 mol L−1 . It was scanning five cycles with a sweep rate of 30 mV s−1 in a range of −200 mV a 900 mV. All measurements were carried out at 25 ◦ C. For these measurements, the films were obtained by casting the m-cresol solution. The evaporation of solvent was under 80 ◦ C in an oven by 1 h. The films showed good adherence to Pt foil except PAni obtained in formic acid without CSA.
3. Results and discussion 3.1. Properties of PAni.CSA prepared in different media Before studying the characteristic of PAni/PA6 blends, it was decided to investigate the main features of PAni.CSA prepared with the same procedure as that used for the blends. PAni.CSA was prepared by one step polymerization of aniline in the presence of CSA as the protonating agent and APS as the oxidant agent. The effect of the reaction medium (water or formic acid) on the electrical conductivity and crystallinity degree of PAni.CSA is compared in Table 1. Polyaniline prepared in the presence of formic acid, without the addition of CSA (PAni.HCOOH) was also included for comparison. PAni.CSA prepared in water presented a little higher conductivity than that prepared in formic acid. PAni.HCOOH also presented an acceptable conductivity confirming the protonating action of the formic acid, but the value was not as high as those observed for PAni.CSA. The crystallinity degree was determined from X-ray diffractograms, illustrated in Fig. 1. PAni.CSA prepared in formic acid presented higher crystallinity degree and XRD pattern with sharp crystalline diffraction peak at 2 around 25◦ and other diffraction peaks at 2 = 7◦ , 13◦ and 19◦ (curve B). This WAXS pattern is very similar to that observed for the PAni.HCOOH (curve C) and also to those reported in the literature for PAni doped with chloridric acid (PAni.HCl) or with formic acid [14]. The higher crystallinity degree of PAni.CSA prepared in formic acid (PAni.CSA-HCOOH) may be attributed to the higher ability of this solvent in dissolving the PAni.CSA chains. The better polymer chain–solvent interaction contributes for an extended coil conformation, which facilitates the macromolecular rearrangements required for the formation of the crystalline regions in higher extent. FTIR spectroscopy constitutes a powerful tool for the evaluation of various intrinsic oxidation states of polyaniline [15]. It has been reported that the oxidation levels can be qualitatively estimated by the intensity ratio of the absorption peaks at ∼1600 cm−1 (quinoid
Fig. 1. X-ray diffractograms of (A) PAni.CSA obtained in aqueous medium, (B) PAni.CSA obtained in formic acid medium and (C) PAni.HCOOH.
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Fig. 2. FTIR of PAni.CSA obtained in aqueous medium, in formic acid medium and PAni.HCOOH. Fig. 3. Effect of the composition on the conductivity of PAni.CSA/PA6 blends obtained by (A) physical blend and (B) in situ polymerization in HCOOH.
ring stretching – Q) and ∼1500 cm−1 (benzenoid ring stretching – B) [16]. In order to evaluate the effect of the reaction conditions on the oxidation level of the PAni chains, the protonated polyaniline samples were deprotonated by treating the samples with 1 M aqueous solution of ammonium hydroxide for 48 h. The FTIR spectra of the neutral form of the polyaniline samples (emeraldine base) are compared in Fig. 2. The Q/B ratio of the neutral form of PAni.CSA obtained in aqueous medium (PAni.CSA-H2 O) (a/b peak ratio) corresponded to 0.93, which was close to the ratio of an ideal emeraldine base (50% oxidized). The presence of formic acid as the solvent decreased the Q/B ratio to a value corresponding to 0.83, indicating a little higher concentration of benzenoid units. Another important aspect in the FTIR spectra is the presence of an absorption at around 810 cm−1 (peak c) assigned to C–H out of plane bending of 1,4-substituted ring [15]. The ratio between the absorption at 810 cm−1 (peak c) and 1600 cm−1 (peak a) was used to compare the molecular structure of the PAni chains. The dedoped form of PAni.HCOOH (curve c) displayed the lowest ratio value (c/a ratio = 0.46), indicating a higher extent of 1,2-substituted benzenic ring, which can be associated to a high degree of defects in the polyaniline chain. The dedoped form of PAni.CSA in water and in formic acid presented similar c/a ratios (0.65 and 0.62, respectively), suggesting that the CSA as the dopant decreased the possibility of 1,2-dissubstitution. 3.2. PAni.CSA/polyamide 6 blends The effect of the methodology on the conductivity of PAni.CSA/PA6 blends is summarized in Table 2 and illustrated in Fig. 3. The conductivity of the blends increased with the PAni.CSA concentration. For blends containing PAni.CSA concentration lower than 10%, the in situ polymerization resulted in higher conductivity values because the formed PAni.CSA chains are more dispersed inside the PA6 matrix. For blends containing higher amount of
PAni.CSA, the conductivity of the in situ polymerized blends were lower probably because of the greater influence of the medium on the conductivity of the PAni.CSA formed in situ. 3.3. Cyclic voltammetry The inhomogeneity and defects in the polymer chain of PAni affect the electrochemical properties of PAni, since the oxidation and reduction processes depend on the relaxation of the polymer chain and on the charge/mass transport through the material. The effect of the blend preparation procedure on the electrochemical response of PAni.CSA samples was evaluated by cyclic voltammetry. Five cycles were scanned with a sweep rate of 30 mV s−1 . Fig. 4 illustrates the third cycle of PAni.CSA/PA6 blends with different composition and Table 3 summarizes the voltage values corresponding to the anodic and cathodic peaks. PAni.CSA prepared in aqueous medium (Fig. 4A left side) exhibits the two characteristic redox systems: the first peak system (Ep anodic = 403 mV and Ep cathodic = 10 mV) belongs to the oxidation/reduction of leucoemeraldine to emeraldine, while the second (Ep anodic = 725 mV and Ep cathodic = 354 mV) is due to the oxidation/reduction of emeraldine to pernigraniline. Comparing to pure PAni.CSA, the physical blends of PA6/PAni.CSA presented a shift of the first peak system towards lower potential values, whereas the second peak system was shifted towards higher potential values. This indicates that the presence of PA6 in the blend facilitates the formation of emeraldine but difficults the formation of pernigraniline. Physical blend containing 10% of PAni.CSA presented only one irreversible peak system (Fig. 4D left side) indicating that the electroactivity was lost. The factors affecting the broadening and shifting of the peak may be as follows: (i) the degradation of PAni backbone, (ii) the low ionic diffusivity to maintain charge neutrality due to the modification of PAni microstructure by the gradual elim-
Table 2 Conductivity values of PA6/PAni.CSA blends as a function of the preparation procedure. Conductivity (S cm−1 )
Blend components (wt%) PA6
PAni.CSA
Physical blends
In situ blends (in HCOOH)
In situ blends (in HCOOH.H2 O)
90 70 50 30 10
10 30 50 70 90
4.8 × 10−6 2.8 × 10−4 3.0 × 10−3 4.6 × 10−3 2.2 × 10−2
2.4 × 10−5 1.3 × 10−4 7.6 × 10−4 7.7 × 10−4 1.5 × 10−2
5.1 × 10−6 1.2 × 10−4 3.0 × 10−4 4.8 × 10−4 4.5 × 10−3
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Fig. 4. Cyclic voltammetry of PAni.CSA/PA6 blends obtained by physical blend and in situ polymerization as a function of the composition: PAni.CSA/PA6 = 100:0 (A); 90:10 (B); 50:50 (C); 10:90 (D).
ination of dopant ions from the film matrix, and (iii) the decrease in electronic conductivity due to the deprotonation of the polymer backbone [17]. Blends prepared by the in situ polymerization (right side in Fig. 4) presented only one irreversible peak system, except for the 50:50 wt% blend (Fig. 4C right side). Besides that, during the electrochemical measurements it was noted that the electrolytic
solution changed from incolor to yellow after the third voltammetric cycle. The absence of well defined electrochemical response in these blends indicates that the relaxation processes of the polymer chain, and the ionic diffusion, are not occurring satisfactorily leading to the material degradation during redox switching [18]. These results may be due to the presence of defects along the PAni.CSA chain, as observed by FTIR analysis.
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Table 3 Electrochemical data of PAni.CSA obtained in different media, determined from cyclic voltammetry. Blend composition (wt%)
Anodic peaks (mV)
Cathodic peaks (mV)
Q− (×10−1 ) (C)
PAni.CSA
PA6
Peak 1
Peak 2
Physical blendsa 100 90 70 50 30 10
0 10 30 50 70 90
403 305 318 283 359 –
725 757 750 735 698 530
10 −41 −27 17 149 –
354 491 486 515 447 349
3.978 2.774 2.270 1.961 1.594 0.4708
−3.955 −2.758 −2.265 −1.959 −1.595 −0.4539
In situ blendsb 100 90 70 50 30 10
0 10 30 50 70 90
– – – 401 415 –
750 769 574 747 728 611
56 −51 −117 −44 71 149
– – – 232 398 –
3.973 7.604 9.445 4.755 3.862 0.5020
−3.892 −7.534 −9.373 −4.670 −3.740 −0.5128
a b
Peak 1
Q+ (×10−1 ) (C)
Peak 2
PAni.CSA used in physical blends was prepared in aqueous medium. PAni.CSA and their in situ blends with PA6 were performed in formic acid medium.
The oxidation and reduction charges in all blends (Table 3) exhibited very close values, indicating similar concentration of oxidation and reduction sites. For comparative effect in terms of conductivity and electroactivity phenomena, PAni.CSA and PAni.CSA/PA6 blends were prepared by the in situ polymerization using HCOOH diluted in water in a concentration of 70% (HCOOH.H2 O). PAni.CSA.HCOOH.H2 O presented a decrease of an order in conductivity (6.0 × 10−2 S cm−1 ) but a similar electrochemical behavior (not shown here) as that observed with PAni.CSA.HCOOH (only one irreversible peak system). PAni.CSA.HCOOH.H2 O/PA6 blends presented lost of electroactivity in all compositions. Moreover, the conductivity decrease in compositions containing PA6 concentration lower than 70% (Table 2), comparing to physical blends. For blends containing higher amount of PA6 the conductivity were the same. 4. Conclusions From the results obtained in this work, one can conclude that: • The synthesis of PAni.CSA in the presence of formic acid resulted in PAni sample with higher crystallinity value and conductivity comparable to the PAni.CSA obtained in aqueous medium. • It is possible to prepare blends of PAni.CSA with polyamide 6 by in situ polymerization of aniline inside a formic acid solution of PA6. Except for the blend containing 10% of PAni.CSA, the conductivity values were lower in blends obtained by the in situ polymerization when compared to those obtained by physical blends, using the solution processing in m-cresol. • It was expected a better dispersion of the conductive phase in insulating matrix in blends prepared by in situ polymerization. It is known that the polymerization of a monomer in a polymer matrix is a useful approach for the in situ preparation of new materials. By this procedure, more intimate mixing of the two components is possible [19]. Besides that, in solution blending
is important identify a good solvent in which both phases are soluble in order to avoid the segregation of conducting phase by solvent evaporation. • In addition, the presence of the formic acid in the synthesis of PAni.CSA and their in situ polymerized blends with PA6 resulted in the electroactivity loss, as indicated by the presence of only one irreversible peak system. Acknowledgements We are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico-CNPq, Coordenac¸ão de Aperfeic¸oamento de Pessoal de Ensino Superior-CAPES, Fundac¸ão de Amparo a Pesquisa do Estado do Rio de Janeiro-FAPERJ for the financial support. References [1] A. Pud, N. Ogurtsov, A. Korzhenko, G. Shapoval, Prog. Polym. Sci. 28 (2003) 1701. [2] C.H. Hsu, H. Shih, S. Subramoney, A.J. Epstein, Synth. Metals 101 (1998) 677. [3] Q.H. Zhang, Z.C. Sun, J. Li, X.H. Wang, H.F. Jin, X.B. Jing, F.S. Wang, Synth. Metals 102 (1999) 1198. [4] Q. Zhang, H. Jin, X. Wang, X. Jing, Synth. Metals 123 (2001) 481. [5] Q. Zhang, X. Wang, Y. Geng, D. Chen, X. Jing, J. Polym. Sci., Phys. Ed. 40 (2002) 2531. [6] D. Abraham, A. Bharathi, S.V. Subramanyam, Polym. Commun. 37 (1996) 5295. [7] M. Zagórska, E. Taler, I. Kulszewicz-Bajer, A. Prón, J. Niziol, J. Appl. Polym. Sci. 73 (1999) 1423. [8] K.W. Oh, K.H. Hong, S.H. Kim, J. Appl. Polym. Sci. 74 (1999) 2094. [9] S.W. Byun, S.S. Im, Polymer 39 (1998) 485. [10] S.W. Byun, S.S. Im, Synth. Metals 69 (1995) 219. [11] A. Gök, B. Sari, J. Appl. Polym. Sci. 84 (2002) 1993. [12] A. Dan, P.K. Sengupta, J. Appl. Polym. Sci. 91 (2004) 991. [13] E.M. Girotto, I.A. Santos, Quim. Nova 25 (2002) 639. [14] K.K. Chaudhari, D.S. Kellar, J. Appl. Polym. Sci. 62 (1996) 15. [15] E.T. Kang, K.G. Neoh, K.L. Tan, Prog. Polym. Sci. 23 (1998) 277. [16] S. Davied, Y.F. Nicolau, F. Melis, A. Revillon, Synth. Metals 69 (1995) 175. [17] J.M. Ko, I.J. Chung, Synth. Metals 68 (1995) 233. [18] A.G. Bedekar, S.F. Patil, R.C. Patil, K. Vijayamohanan, Mater. Chem. Phys. 48 (1997) 76. [19] J. Anand, S. Palaniapan, D.N. Sathyanarayana, Prog. Polym. Sci. 23 (1998) 993.