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lignin blends: FTIR, MEV and electrochemical characterization
European Polymer Journal 38 (2002) 2213–2217 www.elsevier.com/locate/europolj
Polyaniline/lignin blends: FTIR, MEV and electrochemical characterization Paula C. Rodrigues a, Maurıcio P. Cant~ ao b, Paulo Janissek b, Paulo C.N. Scarpa b,z, Alvaro L. Mathias c, Luiz P. Ramos a, Maria A.B. Gomes a,* a
c
Centro de Pesquisa em Quımica Aplicada, Departamento de Quımica, Universidade Federal do Paran a, P.O. Box 19081, 81531-990 Curitiba, PR, Brazil b Instituto de Tecnologia para o Desenvolvimento, LACTEC, P.O. Box 19067, 81531-990 Curitiba, PR, Brazil Departamento de Engenharia Quımica, Universidade Federal do Paran a, P.O. Box 19081, 81531-990 Curitiba, PR, Brazil Received 18 December 2001; received in revised form 8 April 2002; accepted 15 April 2002
Abstract Blends of polyaniline (emeraldine base) and Eucalyptus grandis kraft lignin were prepared by casting method. The maximum amount of lignin that could be used for blending was 36% (w/w); beyond that, fractile films were produced. The IR spectra of the blends indicated that interactions occurred between polyaniline and lignin. Cyclic voltammetry measurements showed peaks that were readily attributed to the oxidation/reduction of polyaniline and a new oxireduction peak due to oxidation/reduction of sites created during interaction of two polymers. Scanning electronic microscopy showed that all blends were homogeneous. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polyaniline; Kraft lignin; Cyclic voltammetry; FTIR; MEV
1. Introduction Polyaniline is one of the most studied conducting polymers known to date because it is relatively cheap, easy to synthesise and very stable under a wide variety of experimental conditions. Like other classes of conducting polymers, polyaniline is also easy to handle and can be readily processed into polymeric blends [1]. Lignin is a tri-dimensional natural polymer composed primarily of phenylpropane units linked together by a wide variety of chemical bonds (linkages) (C–C or C– O–C). It represents the second largest carbon source on Earth and is produced industrially by the pulp and paper industry, particularly when alkaline processes such as * Corresponding author. Tel.: +55-41-361-3470; fax: +55-41361-3186. E-mail address: [email protected] (M.A.B. Gomes). z In memoriam.
the widely acclaimed kraft process is used. In Brazil Eucalyptus grandis is the most important hardwood used for pulp and paper production. There has been an increasing amount of work devoted to the utilisation of lignin in polymeric blends. Blends with PVC [2], polyolefins [3] and polyvinyl alcohol [4] have been already produced. Our recent contribution in this area was related to the thermal, dynamic mechanical and XPS characterisation of polyaniline/lignin blends [5]. We verify that the lignin interacts with the polyaniline through their hydroxilic and carbonyl groups, forming the binary blend. The lignin can be also utilised as natural charge, forming a ternary system with other polymers. Therefore, lignin has gradually gone beyond its simple use as a solid fuel for heat generation. This present study investigates other aspects of blends of polyaniline/lignin (Pani/Lig), we in particular the morphology of the blends as well as in their electrochemical behaviour and infrared spectra.
0014-3057/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 2 ) 0 0 1 1 4 - 3
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P.C. Rodrigues et al. / European Polymer Journal 38 (2002) 2213–2217
2. Experimental Polyaniline was synthesised following a literature method [6]. The kraft lignin was supplied by ENCE (Empresa Nacional de Celulosas S/A, Madrid, Spain). Both polymers were solubilized in N-methyl-1-pyrrolidone (NMP) and the maximum lignin concentration in the blend was 36% (w/w). Beyond this, the resulting polymeric film developed cracks and defects as well as losing the mechanical resistance. For the assembly of the working electrode, the NMP solution was poured onto a platinum plate and the solvent evaporated at 60 °C until dryness. The total surface area available at the tip of the working electrode to the electrolytes corresponded to 0.09 cm2 . A two compartment cell having 1 M H2 SO4 as the electrolyte was used, where a platinum foil acted as the contra-electrode and the reference was a normal hydrogen electrode (NHE). The cyclic voltammetry and open circuit potential (Voc ) measurements were carried out with a Potentiostat/Galvanostat EG&G-PAR, model 273. Polymeric films were also prepared onto a KBr crystal for FTIR measurements, which were collectively carried out using a Bomem MB 100 FTIR Spectrophotometer. A Philips scan electronic microscopic model XL30 was used to analyse the morphology of the films, with acceleration tension of the 10 kV. The conductivity measurements of polymeric films were carried out using the HP 4339A high impedance meter (Hewlett–Packard) to which an HP 16008B resistivity cell was adapted. For each measurement, a potential of 10 V was applied for 60 s to the polymeric films.
3. Results and discussion Fig. 1 shows the IR spectra of Pani and Pani/Lig 36% blend films. To the Pani film (emeraldine base) and lignin films (not shown) characteristic absorptions were found, being all in accordance with the results described in the literature [7–11]. The following bands were observed to polyaniline: (a) N–H (H free) at 3383 cm1 , N–H (H bonded) at 3592 cm1 and N–H stretching at 3178 cm1 ; (b) C–H vibration of aromatic ring at 3037 cm1 ; (c) band at 1672 cm1 due to C@O from NMP solvent and/or C@N from quinoneimine group; (d) C@C at 1595 cm1 (quinoid, Q), at 1500 and 1402 cm1 (benzenic); (e) C–N from aromatic amines at 1380, 1308 and 1230 cm1 ; (f) N@Q@N at 1167 cm1 ; and (g) C–H of 1,4-disubstituted aromatic ring at 1111, 1010 and 833 cm1 . The following bands were observed to lignin: (a) the absorbance of OH bands at 3352, 1323, 1217 and 1033 cm1 ; (b) b-O-4 ether bond band at 1117 cm1 ; (c) methoxy group band at 2939, 2881, 1460 and 1425 cm1 ;
Fig. 1. IR Spectra for (a) Pani film and (b) Pani/Lig 36% film.
(d) the C@C vibration of aromatic ring at 1514 cm1 ; (e) the band at 1603 cm1 which is characteristic for quinoid structure; and (f) the carbonyl group at 1664 and 1720 cm1 . The Pani/Lig 36% blend showed a spectrum more similar to Pani that lignin. The –NH– vibration at 3383 cm1 (H free) was less evident in this blend, indicating the possibility of an interaction between lignin and polyaniline through amine hydrogen. A new absorption at 1146 cm1 , of low intensity, was detected for the blend and it could be an indication of the existence of interaction between the two polymers. According to Tang et al. [9] the presence of this particular band can be related to the vibrational mode of protonated amines generated during the acid doping process of Pani (Q@NHþ –B or B–NHþ –B). We have attributed this to the interaction of O–H groups (coming from carboxylic acids) present in lignin, with the nitrogen sites present in polyaniline forming hydrogen bond type between the two polymers. We have confirmed this hypothesis through XPS measurements [5].
P.C. Rodrigues et al. / European Polymer Journal 38 (2002) 2213–2217
Measurements of open circuit potentials (Voc ) were made for all polymer films and the Voc values remained constant after 1 h of immersion in H2 SO4 solution. The observed Voc value for Pani film was þ0.76 V, which was in perfect agreement with that described by Wrighton and coworkers [12] for the emeraldine salt, is a clear demonstration that the film was in its conducting form. There was a decrease in the potential value of the polymeric film as the amount of lignin was increased to 36%. The Voc value of the Pani/Lig 36% film (Voc ¼ þ0:65 V) was very similar to that observed for lignin film. Fig. 2 shows the cyclic voltammogram of the Pani film. To the Pani film two redox pairs were observed. The first redox pair (peaks a0 and a) was attributed to the leucoemeraldine–emeraldine transition, whereas the second (peaks b0 and b) was attributed to the emeraldine–pernigraniline transition. However, during the second voltammetric cycle, two intermediate peaks of relatively low intensity were observed between 0.6 and 0.9 V (peaks d). These peaks have been associated to the degradation of Pani at potentials higher than 0.9 V (vs NHE), when benzoquinone [13,14] and a range of insoluble products are produced. These latter degradation products have been identified as polymeric chains containing quinoneimine end groups and ortho-coupled polymeric chains which, in general, form insoluble deposits that remain adhered to the surface of the electrode [14,15]. The cyclic voltammogram for lignin film does not present oxi-reduction peaks, only an exponential increase in current was observed in potential values higher than 0.8 V and lower than 0.4 V. This increase in current is attributed to the evolution reaction of O2 and H2 respectively. The current values observed are about 1000 times smaller than what is observed for the Pani film, indicating that the lignin film shows little conductivity.
Fig. 2. Cyclic voltammetry for Pani film. 1 M H2 SO4 , v ¼ 50 mV/s.
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Fig. 3. Cyclic voltammetry for Pani/Lig 36% (w/w) blend. 1 M H2 SO4 , v ¼ 50 mV/s.
Fig. 3 shows the corresponding cyclic voltammograms for the Pani/Lig 36% blend. In the first cycle, two oxidation peaks were observed along with three reduction peaks. The first redox pair (peaks a0 and a) shows peak potential values similar to the Pani film, being these processes attributed to the leucoemeraldine–emeraldine transition. Peak b in this cyclic voltammogram was rather assymetric and displayed a superior current density when compared to the former peak a. This peak assymetry and/or broadening was probably associated to the overlap of two distinct oxidation phenomena: the emeraldine–pernigraniline transition and the oxidation of electroactive sites in lignin. As a much higher current density was observed within 0.7 V, one additional peak was assigned (peak d0 ) along with the cathodic scan. On the other hand, in the second voltammetric cycle a decrease occurs in the anodic current associated with peaks a and b and defined one new oxidation process (peak d). The current related to this peak have a density current intensity about five times higher than what observed to Pani film. We suggest that the peak d is related with a oxidation process of sites that were generated due the interaction between lignin and polyaniline. A direct comparison between Figs. 2 and 3 provides the following observations: (1) in Pani (Fig. 2), peak b has the same current density as peak a and it is narrower than the corresponding peak b in Pani/Lig 36%; (2) the capacitive region in Pani is greater than that observed for Pani/Lig 36%; (3) peak a in Pani/Lignin 36% is wider than peak a in Pani; and (4) the current density for peak d for Pani/Lig 36% is higher than observed for Pani film. All of the cyclic voltammetric measurements performed in Pani/Lig blends were similar to those shown in Fig. 3, with variations occurring only with the values assumed at peak potentials (Table 1) and current density for peak d. The DEp values for the three redox processes
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P.C. Rodrigues et al. / European Polymer Journal 38 (2002) 2213–2217
Table 1 Anodic and cathodic peak potentials for polymeric films d and d0 peaks
b and b0 peaks
% (w/w) Lignin
a and a0 peaks Epa =V
Epc =V
Epa =V
Epc =V
Epa =V
Epc =V
0 7 11 15 19 23 36
0.47 0.55 0.55 0.57 0.57 0.47 0.50
0.26 0.22 0.23 0.22 0.23 0.28 0.28
0.76/0.82 0.87 0.86 0.89 0.89 0.83 0.90
0.72/0.80 0.67 0.66 0.64 0.64 0.70 0.69
1.09 1.12 1.08 1.12 1.08 1.07 1.06
1.03 0.97 0.93 0.94 0.92 1.00 0.96
Table 2 DEp values for polymeric films 0
0
0
% (w/w) Lignin
DEpa;a =V
DEpd;d =V
DEpb;b =V
0 7 11 15 19 23 36
0.21 0.33 0.32 0.35 0.34 0.19 0.22
0.04/0.08 0.20 0.20 0.25 0.25 0.13 0.21
0.06 0.15 0.15 0.18 0.16 0.07 0.10
(Table 2) increase in about 100 mV with the increase in lignin concentration in the blend (from 7% to 19%). To the lignin concentrations of 23% and 36% the DEp values are similar to what was observed in the polyaniline film. The increase in DEp values reflect a decrease in the blend electrical conductivity. The measuring of volumetric electrical conductivity (for deprotonated films) indicated that the blends with lignin content of up to 19% have their values at about 1:0 1017 S/cm. On the other hand, the Pani/Lig 23% and Pani/Lig 36% films have higher conductivity values, similar to the ones shown on polyaniline films (about 1:0 1015 S/cm). Then, the increase in the blend electrical conductivity reflects a smaller DEp value. Consecutive cyclic voltammetry measurements totalizing 100 cycles were performed between 0.1 and 0.65 V to verify the influence of the lignin content on the leucoemeraldine–emeraldine transition. It was observed that the peak potential, the peak current and the voltammogram form are practically not altered. So, this transition is not affected by the addition of lignin and the blend presents stability similar to the polyaniline. The morphology the lignin film surface was characterised by the presence of cracks spread in all directions and also by the presence of globules (Fig. 4). This globular structure is also revealed in the film crack. On the other hand, the surface as well as the Pani films cracks and of the blends were smooth. The absence of globular structures in the blends may be an indicative of the miscibility between the two polymers.
Fig. 4. SEM for lignin film. (a) Surface and (b) fracture.
4. Conclusions The Pani/Lig 36% infrared spectrum suggests an interaction between the two polymers through the decrease in the band related to an –NH– (H-free) and of the presence of a band at 1146 cm1 , that is attributed to the protonated amines vibration. The lignin film did not show redox processes in the cyclic voltametry assays. However, when lignin was added to the polyaniline a new oxi-reduction processes
P.C. Rodrigues et al. / European Polymer Journal 38 (2002) 2213–2217
between 0.7 and 0.9 V were defined. This is a indicate of the presence of interaction between lignin and polyaniline. The difference between peak potentials for the three redox processes varies with the lignin concentration in the polymeric blend. This variation is in accordance with the electrical conductivity values observed for the blends. Morphological analysis of the lignin film showed the presence of globules, which are absent in both Pani films and Pani/Lig blends. It is an indication that both polymers are miscible. References [1] Anand J, Palaniappan S, Sathyanarayana DN. Conducting polyaniline blends and composites. Prog Polym Sci 1998; 23:993–1018. [2] Feldman D, Banu D, El-raghi S. Rigid poly(vinyl chloride)-organosolv lignin blends for applications in building. J Appl Polym Sci 1996;61:2119–28. [3] Vasile C, Downey M, Wong B, Macovean MM, Pascu MC, Choi JH, Sung C, Baker W. Polyolefins/lignosulfonates blends. II Isostatic polypropylene/epoxy-modified lignin blends. Cellulose Chem Technol 1998;32:61–88. [4] Corradini E, Pineda EAG, Hechenleitner AAW. Ligninpoly (vinyl alcohol) blends studied by thermal analysis. Polym Degrad Stab 1999;66:199–208. [5] Rodrigues PC, Munaro M, Garcia CM, Souza GP, Abbate M, Schreiner WH, Gomes MAB. Polyaniline/lignin blends: thermal analysis and XPS. Eur Polym J 2001;37:2217–23.
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[6] Ahmad N, MacDiarmid AG. Inibition of corrosion of steels with the exploitation of conducting polymers. Synth Met 1996;78:103–10. [7] Hu H, Cadenas JL, Saniger JM, Nair PK. Electrically conducting polyaniline-poly (acrylic acid) blends. Polym Int 1998;45:262–70. [8] Chen S-A, Lee H-T. Polyaniline-acid dopant interaction in the presence of 1-methyl-2-pyrrolidone. Synth Met 1992; 47:233–8. [9] Tang J, Jing X, Wang B, Wang F. Infrared spectra of soluble polyaniline. Synth Met 1988;24:231–8. [10] Nada A-AMA, EL-Sakhawy M, Kamel SM. Infra-red spectroscopic study of lignins. Polym Degrad Stab 1998; 60:247–51. [11] Nada A-AMA, Yousef MA, Shaffei KA, Salah AM. Infrared spectroscopy of some treated lignins. Polym Degrad Stab 1998;62:157–63. [12] Paul EW, Ricco AJ, Wrighton MS. Resistence of polyaniline films as a function of electrochemical potential and the fabrication of polyaniline-based microelectronic devices. J Phys Chem-US 1985;89(8):1441–7. [13] Stilwell DE, Park S-M. Electrochemistry of conductive polymers. Part II. Electrochemical studies on growth properties of polyaniline. J Electrochem Soc 1988; 135(9):2254–62. [14] Stilwell DE, Park S-M. Electrochemistry of conductive polymers. Part III. Some physical and electrochemical properties observed from electrochemically grown polyaniline. J Electrochem Soc 1988;135(10):2491–6. [15] Genies EM, Lapkowski M, Penneau JF. Cyclic voltammetry of polyaniline: interpretation of the middle peak. J Electroanal Chem 1988;249:97–107.
Polyaniline/lignin blends: FTIR, MEV and electrochemical characterization Paula C. Rodrigues a, Maurıcio P. Cant~ ao b, Paulo Janissek b, Paulo C.N. Scarpa b,z, Alvaro L. Mathias c, Luiz P. Ramos a, Maria A.B. Gomes a,* a
c
Centro de Pesquisa em Quımica Aplicada, Departamento de Quımica, Universidade Federal do Paran a, P.O. Box 19081, 81531-990 Curitiba, PR, Brazil b Instituto de Tecnologia para o Desenvolvimento, LACTEC, P.O. Box 19067, 81531-990 Curitiba, PR, Brazil Departamento de Engenharia Quımica, Universidade Federal do Paran a, P.O. Box 19081, 81531-990 Curitiba, PR, Brazil Received 18 December 2001; received in revised form 8 April 2002; accepted 15 April 2002
Abstract Blends of polyaniline (emeraldine base) and Eucalyptus grandis kraft lignin were prepared by casting method. The maximum amount of lignin that could be used for blending was 36% (w/w); beyond that, fractile films were produced. The IR spectra of the blends indicated that interactions occurred between polyaniline and lignin. Cyclic voltammetry measurements showed peaks that were readily attributed to the oxidation/reduction of polyaniline and a new oxireduction peak due to oxidation/reduction of sites created during interaction of two polymers. Scanning electronic microscopy showed that all blends were homogeneous. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polyaniline; Kraft lignin; Cyclic voltammetry; FTIR; MEV
1. Introduction Polyaniline is one of the most studied conducting polymers known to date because it is relatively cheap, easy to synthesise and very stable under a wide variety of experimental conditions. Like other classes of conducting polymers, polyaniline is also easy to handle and can be readily processed into polymeric blends [1]. Lignin is a tri-dimensional natural polymer composed primarily of phenylpropane units linked together by a wide variety of chemical bonds (linkages) (C–C or C– O–C). It represents the second largest carbon source on Earth and is produced industrially by the pulp and paper industry, particularly when alkaline processes such as * Corresponding author. Tel.: +55-41-361-3470; fax: +55-41361-3186. E-mail address: [email protected] (M.A.B. Gomes). z In memoriam.
the widely acclaimed kraft process is used. In Brazil Eucalyptus grandis is the most important hardwood used for pulp and paper production. There has been an increasing amount of work devoted to the utilisation of lignin in polymeric blends. Blends with PVC [2], polyolefins [3] and polyvinyl alcohol [4] have been already produced. Our recent contribution in this area was related to the thermal, dynamic mechanical and XPS characterisation of polyaniline/lignin blends [5]. We verify that the lignin interacts with the polyaniline through their hydroxilic and carbonyl groups, forming the binary blend. The lignin can be also utilised as natural charge, forming a ternary system with other polymers. Therefore, lignin has gradually gone beyond its simple use as a solid fuel for heat generation. This present study investigates other aspects of blends of polyaniline/lignin (Pani/Lig), we in particular the morphology of the blends as well as in their electrochemical behaviour and infrared spectra.
0014-3057/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 2 ) 0 0 1 1 4 - 3
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P.C. Rodrigues et al. / European Polymer Journal 38 (2002) 2213–2217
2. Experimental Polyaniline was synthesised following a literature method [6]. The kraft lignin was supplied by ENCE (Empresa Nacional de Celulosas S/A, Madrid, Spain). Both polymers were solubilized in N-methyl-1-pyrrolidone (NMP) and the maximum lignin concentration in the blend was 36% (w/w). Beyond this, the resulting polymeric film developed cracks and defects as well as losing the mechanical resistance. For the assembly of the working electrode, the NMP solution was poured onto a platinum plate and the solvent evaporated at 60 °C until dryness. The total surface area available at the tip of the working electrode to the electrolytes corresponded to 0.09 cm2 . A two compartment cell having 1 M H2 SO4 as the electrolyte was used, where a platinum foil acted as the contra-electrode and the reference was a normal hydrogen electrode (NHE). The cyclic voltammetry and open circuit potential (Voc ) measurements were carried out with a Potentiostat/Galvanostat EG&G-PAR, model 273. Polymeric films were also prepared onto a KBr crystal for FTIR measurements, which were collectively carried out using a Bomem MB 100 FTIR Spectrophotometer. A Philips scan electronic microscopic model XL30 was used to analyse the morphology of the films, with acceleration tension of the 10 kV. The conductivity measurements of polymeric films were carried out using the HP 4339A high impedance meter (Hewlett–Packard) to which an HP 16008B resistivity cell was adapted. For each measurement, a potential of 10 V was applied for 60 s to the polymeric films.
3. Results and discussion Fig. 1 shows the IR spectra of Pani and Pani/Lig 36% blend films. To the Pani film (emeraldine base) and lignin films (not shown) characteristic absorptions were found, being all in accordance with the results described in the literature [7–11]. The following bands were observed to polyaniline: (a) N–H (H free) at 3383 cm1 , N–H (H bonded) at 3592 cm1 and N–H stretching at 3178 cm1 ; (b) C–H vibration of aromatic ring at 3037 cm1 ; (c) band at 1672 cm1 due to C@O from NMP solvent and/or C@N from quinoneimine group; (d) C@C at 1595 cm1 (quinoid, Q), at 1500 and 1402 cm1 (benzenic); (e) C–N from aromatic amines at 1380, 1308 and 1230 cm1 ; (f) N@Q@N at 1167 cm1 ; and (g) C–H of 1,4-disubstituted aromatic ring at 1111, 1010 and 833 cm1 . The following bands were observed to lignin: (a) the absorbance of OH bands at 3352, 1323, 1217 and 1033 cm1 ; (b) b-O-4 ether bond band at 1117 cm1 ; (c) methoxy group band at 2939, 2881, 1460 and 1425 cm1 ;
Fig. 1. IR Spectra for (a) Pani film and (b) Pani/Lig 36% film.
(d) the C@C vibration of aromatic ring at 1514 cm1 ; (e) the band at 1603 cm1 which is characteristic for quinoid structure; and (f) the carbonyl group at 1664 and 1720 cm1 . The Pani/Lig 36% blend showed a spectrum more similar to Pani that lignin. The –NH– vibration at 3383 cm1 (H free) was less evident in this blend, indicating the possibility of an interaction between lignin and polyaniline through amine hydrogen. A new absorption at 1146 cm1 , of low intensity, was detected for the blend and it could be an indication of the existence of interaction between the two polymers. According to Tang et al. [9] the presence of this particular band can be related to the vibrational mode of protonated amines generated during the acid doping process of Pani (Q@NHþ –B or B–NHþ –B). We have attributed this to the interaction of O–H groups (coming from carboxylic acids) present in lignin, with the nitrogen sites present in polyaniline forming hydrogen bond type between the two polymers. We have confirmed this hypothesis through XPS measurements [5].
P.C. Rodrigues et al. / European Polymer Journal 38 (2002) 2213–2217
Measurements of open circuit potentials (Voc ) were made for all polymer films and the Voc values remained constant after 1 h of immersion in H2 SO4 solution. The observed Voc value for Pani film was þ0.76 V, which was in perfect agreement with that described by Wrighton and coworkers [12] for the emeraldine salt, is a clear demonstration that the film was in its conducting form. There was a decrease in the potential value of the polymeric film as the amount of lignin was increased to 36%. The Voc value of the Pani/Lig 36% film (Voc ¼ þ0:65 V) was very similar to that observed for lignin film. Fig. 2 shows the cyclic voltammogram of the Pani film. To the Pani film two redox pairs were observed. The first redox pair (peaks a0 and a) was attributed to the leucoemeraldine–emeraldine transition, whereas the second (peaks b0 and b) was attributed to the emeraldine–pernigraniline transition. However, during the second voltammetric cycle, two intermediate peaks of relatively low intensity were observed between 0.6 and 0.9 V (peaks d). These peaks have been associated to the degradation of Pani at potentials higher than 0.9 V (vs NHE), when benzoquinone [13,14] and a range of insoluble products are produced. These latter degradation products have been identified as polymeric chains containing quinoneimine end groups and ortho-coupled polymeric chains which, in general, form insoluble deposits that remain adhered to the surface of the electrode [14,15]. The cyclic voltammogram for lignin film does not present oxi-reduction peaks, only an exponential increase in current was observed in potential values higher than 0.8 V and lower than 0.4 V. This increase in current is attributed to the evolution reaction of O2 and H2 respectively. The current values observed are about 1000 times smaller than what is observed for the Pani film, indicating that the lignin film shows little conductivity.
Fig. 2. Cyclic voltammetry for Pani film. 1 M H2 SO4 , v ¼ 50 mV/s.
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Fig. 3. Cyclic voltammetry for Pani/Lig 36% (w/w) blend. 1 M H2 SO4 , v ¼ 50 mV/s.
Fig. 3 shows the corresponding cyclic voltammograms for the Pani/Lig 36% blend. In the first cycle, two oxidation peaks were observed along with three reduction peaks. The first redox pair (peaks a0 and a) shows peak potential values similar to the Pani film, being these processes attributed to the leucoemeraldine–emeraldine transition. Peak b in this cyclic voltammogram was rather assymetric and displayed a superior current density when compared to the former peak a. This peak assymetry and/or broadening was probably associated to the overlap of two distinct oxidation phenomena: the emeraldine–pernigraniline transition and the oxidation of electroactive sites in lignin. As a much higher current density was observed within 0.7 V, one additional peak was assigned (peak d0 ) along with the cathodic scan. On the other hand, in the second voltammetric cycle a decrease occurs in the anodic current associated with peaks a and b and defined one new oxidation process (peak d). The current related to this peak have a density current intensity about five times higher than what observed to Pani film. We suggest that the peak d is related with a oxidation process of sites that were generated due the interaction between lignin and polyaniline. A direct comparison between Figs. 2 and 3 provides the following observations: (1) in Pani (Fig. 2), peak b has the same current density as peak a and it is narrower than the corresponding peak b in Pani/Lig 36%; (2) the capacitive region in Pani is greater than that observed for Pani/Lig 36%; (3) peak a in Pani/Lignin 36% is wider than peak a in Pani; and (4) the current density for peak d for Pani/Lig 36% is higher than observed for Pani film. All of the cyclic voltammetric measurements performed in Pani/Lig blends were similar to those shown in Fig. 3, with variations occurring only with the values assumed at peak potentials (Table 1) and current density for peak d. The DEp values for the three redox processes
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P.C. Rodrigues et al. / European Polymer Journal 38 (2002) 2213–2217
Table 1 Anodic and cathodic peak potentials for polymeric films d and d0 peaks
b and b0 peaks
% (w/w) Lignin
a and a0 peaks Epa =V
Epc =V
Epa =V
Epc =V
Epa =V
Epc =V
0 7 11 15 19 23 36
0.47 0.55 0.55 0.57 0.57 0.47 0.50
0.26 0.22 0.23 0.22 0.23 0.28 0.28
0.76/0.82 0.87 0.86 0.89 0.89 0.83 0.90
0.72/0.80 0.67 0.66 0.64 0.64 0.70 0.69
1.09 1.12 1.08 1.12 1.08 1.07 1.06
1.03 0.97 0.93 0.94 0.92 1.00 0.96
Table 2 DEp values for polymeric films 0
0
0
% (w/w) Lignin
DEpa;a =V
DEpd;d =V
DEpb;b =V
0 7 11 15 19 23 36
0.21 0.33 0.32 0.35 0.34 0.19 0.22
0.04/0.08 0.20 0.20 0.25 0.25 0.13 0.21
0.06 0.15 0.15 0.18 0.16 0.07 0.10
(Table 2) increase in about 100 mV with the increase in lignin concentration in the blend (from 7% to 19%). To the lignin concentrations of 23% and 36% the DEp values are similar to what was observed in the polyaniline film. The increase in DEp values reflect a decrease in the blend electrical conductivity. The measuring of volumetric electrical conductivity (for deprotonated films) indicated that the blends with lignin content of up to 19% have their values at about 1:0 1017 S/cm. On the other hand, the Pani/Lig 23% and Pani/Lig 36% films have higher conductivity values, similar to the ones shown on polyaniline films (about 1:0 1015 S/cm). Then, the increase in the blend electrical conductivity reflects a smaller DEp value. Consecutive cyclic voltammetry measurements totalizing 100 cycles were performed between 0.1 and 0.65 V to verify the influence of the lignin content on the leucoemeraldine–emeraldine transition. It was observed that the peak potential, the peak current and the voltammogram form are practically not altered. So, this transition is not affected by the addition of lignin and the blend presents stability similar to the polyaniline. The morphology the lignin film surface was characterised by the presence of cracks spread in all directions and also by the presence of globules (Fig. 4). This globular structure is also revealed in the film crack. On the other hand, the surface as well as the Pani films cracks and of the blends were smooth. The absence of globular structures in the blends may be an indicative of the miscibility between the two polymers.
Fig. 4. SEM for lignin film. (a) Surface and (b) fracture.
4. Conclusions The Pani/Lig 36% infrared spectrum suggests an interaction between the two polymers through the decrease in the band related to an –NH– (H-free) and of the presence of a band at 1146 cm1 , that is attributed to the protonated amines vibration. The lignin film did not show redox processes in the cyclic voltametry assays. However, when lignin was added to the polyaniline a new oxi-reduction processes
P.C. Rodrigues et al. / European Polymer Journal 38 (2002) 2213–2217
between 0.7 and 0.9 V were defined. This is a indicate of the presence of interaction between lignin and polyaniline. The difference between peak potentials for the three redox processes varies with the lignin concentration in the polymeric blend. This variation is in accordance with the electrical conductivity values observed for the blends. Morphological analysis of the lignin film showed the presence of globules, which are absent in both Pani films and Pani/Lig blends. It is an indication that both polymers are miscible. References [1] Anand J, Palaniappan S, Sathyanarayana DN. Conducting polyaniline blends and composites. Prog Polym Sci 1998; 23:993–1018. [2] Feldman D, Banu D, El-raghi S. Rigid poly(vinyl chloride)-organosolv lignin blends for applications in building. J Appl Polym Sci 1996;61:2119–28. [3] Vasile C, Downey M, Wong B, Macovean MM, Pascu MC, Choi JH, Sung C, Baker W. Polyolefins/lignosulfonates blends. II Isostatic polypropylene/epoxy-modified lignin blends. Cellulose Chem Technol 1998;32:61–88. [4] Corradini E, Pineda EAG, Hechenleitner AAW. Ligninpoly (vinyl alcohol) blends studied by thermal analysis. Polym Degrad Stab 1999;66:199–208. [5] Rodrigues PC, Munaro M, Garcia CM, Souza GP, Abbate M, Schreiner WH, Gomes MAB. Polyaniline/lignin blends: thermal analysis and XPS. Eur Polym J 2001;37:2217–23.
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