Synthesis and characterization of fluorine-substituted polyanilines

European Polymer Journal 37 (2001) 1767±1772

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Synthesis and characterization of ¯uorine-substituted polyanilines b,*  A. Cihaner a, A.M. Onal b

a Department of Industrial Engineering, Atõlõm University, 06836 Ankara, Turkey Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey

Received 27 July 2000; received in revised form 5 March 2001; accepted 7 March 2001

Abstract Poly(2-¯uoroaniline), P2FAN, poly(3-¯uoroaniline), P3FAN, and poly(4-¯uoroaniline), P4FAN, have been synthesized from ¯uorine substituted aniline monomers in aqueous acidic medium using potassium dichromate as oxidizing agent. Characterization of polymer products has been carried out using FTIR, and NMR spectroscopic techniques. Thermal analysis of poly¯uoroaniline powders was also investigated using di€erential scanning calorimetry and thermogravimetric analysis. To compare the structural properties of the polymers, poly¯uoroanilines were also synthesized using ammonium peroxydisulfate as oxidizing agent. Poly¯uoroanilines synthesized by chemical oxidation were doped by using iodine and the change in the paramagnetic behavior was monitored by electron spin resonance spectroscopy. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Poly¯uoroanilines; Chemical polymerization; Doping; ESR spectroscopy

1. Introduction Among all classes of conducting polymer, the polyaniline (PAN) family of conjugated polymers is of much interest worldwide because of its stability in the presence of air and humidity, which makes PAN useful in various applications like energy storage and transfer, rechargable batteries [1±5], indicators [6], sensors [7] and electrodes for electrocatalytic reactions [8]. PAN can be synthesized both by chemical and electrochemical oxidations of aniline, and its transition from an insulating to a conducting state, as well as electrochromic e€ects, are known to depend on its oxidation state and pH [6,9,10]. The major disadvantage of PAN is its insolubility in common solvents and its infusibility. In order to en-

*

Corresponding author. Tel.: +90-312-2103188; fax: +90312-2101280.  E-mail address: [email protected] (A.M. Onal).

hance its solubility, alkyl substituents are introduced to the aromatic ring [11,12]. However, there are very few studies involving polymerization of anilines functionalized by an electron withdrawing group. Snauwaert and his coworkers [13] synthesized poly(haloanilines) by chemical and electrochemical oxidation of the monomer, but the polymers were not fully characterized. Diaz et al. [14] reported the synthesis of dihalogenated anilines using various oxidizing agents. Electrochemical polymerization of m-chloroaniline was reported by Anjoli et al. [15]. Kwon and coworkers [16] investigated the polymerization of ¯uorine substituted anilines using (NH4 )2 S2 O8 as oxidizing agent. Although they characterized the polymers by some spectroscopic techniques, it is not mentioned whether the poly¯uoroanilines synthesized were conducting or not. Herein, we report the synthesis of ¯uorine substituted PANs from 2-, 3- and 4-¯uoroaniline monomers using K2 Cr2 O7 as oxidizing agent. Furthermore, the spectroscopic (FTIR and NMR), thermal and electrical properties of the polymers were investigated.

0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 1 ) 0 0 0 6 2 - 3

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2. Experimental 2-, 3-, and 4-¯uoroaniline (Aldrich) were used without further puri®cation. Poly¯uoroanilines were synthesized by the oxidative polymerization of 2-¯uoroaniline (2FAN), 3-¯uoroaniline (3FAN) and 4-¯uoroaniline (4FAN) in aqueous HCl solution using potassium dichromate as the oxidant [9,10]. 11.5 mmol of monomer and 2 mmol of K2 Cr2 O7 were dissolved in 10 ml of 1.5 M HCl solution, separately. The potassium dichromate solution was added dropwise to the stirred monomer solution at room temperature. The black precipitate was ®ltered, after a 1 h reaction period, and then washed with 1.0 M HCl solution until the ®ltrate was clear. The polymers were dried under vacuum at 80°C for 48 h. In the preparation of poly¯uoroanilines using (NH4 )2 S2 O8 , a common procedure was followed [17±19]. The FTIR spectra were obtained from KBr pellets by using a Nicolet DX 510 FTIR spectrometer. 1 H-NMR and 19 FNMR spectra of samples in d-DMSO were recorded on a Bruker DPX-400-NMR spectrometer. A DSC 910S/TA instrument and Perkin Elmer TGA systems were used for di€erential scanning calorimetry (DSC) and thermogravimetric (TGA) analyses, respectively. The conductivity measurements were carried out on compressed pellets by using a two probe technique at room temperature. A Varian E12 electron spin resonance (ESR) spectrometer was used for investigating the paramagnetic behavior of the polymers before and after iodine doping.

3. Results and discussion

in appreciable amounts, especially in the case of poly(2-¯uoroaniline) P2FAN and poly(3-¯uoroaniline) P3FAN. Furthermore, the polymer yield is also a€ected by the position of the substituent. The higher yield obtained in the case of P2FAN can be explained due to a reduction in the degree of ortho-coupling which will lead to a regular head to tail coupling and increase the polymer yield. The higher yield obtained in the case of poly(4-¯uoroaniline) P4FAN can be explained in terms of elimination of the substituent during the polymerization reaction. However, the highest elimination ratio never approaches to 100% [13]. This e€ect has been observed previously, allowing the typical head to tail polymerization to occur. It is also found that polymer powders obtained by using two di€erent oxidizing agents were soluble in common solvents. Hence, the presence of an electronegative group on the aromatic ring enhances the solubility of the PANs. 3.1. Polymer characterization Fig. 1 compares the FTIR spectra of poly¯uoroanilines obtained using potassium dichromate as oxidizing agent. The spectra for polymers are very similar to each other with characteristic absorptions. The doublet peak belonging to ±NH2 at approximately 3460 and 3376 cm 1 , in the spectra of monomers, turned into a broad singlet peak at approximately 3225 cm 1 . This proves that the polymerization occurs through the ±NH2 group. Strong absorptions at about 1575 and 1500 cm 1 are typical base imine stretchings [10]. The assignments of various peaks present in the spectra of poly¯uoroanilines

Polymer powders were prepared by the polymerization of ¯uorine substituted anilines by using K2 Cr2 O7 as oxidizing agent in 1.5 M HCl solution. For the sake of comparison of the structural properties of the polymers, poly¯uoroanilines were also synthesized following a common procedure used for synthesizing PAN [17±19]. Polymer yields obtained from the polymerization of ¯uorine substituted anilines by using two di€erent oxidizing agents are tabulated in Table 1. The yields obtained by using (NH4 )2 S2 O8 is in accordance with literature values [17±19]. As seen from Table 1, the use of K2 Cr2 O7 increased the polymer yields Table 1 Yields obtained for poly¯uoroanilines synthesized by using ammonium peroxydisulfate and potassium dichromate Oxidants (NH4 )2 S2 O8 K2 Cr2 O7

%Yields P2FAN

P3FAN

P4FAN

3 44

4 22

33 55

Fig. 1. FTIR spectra of (a) P2FAN, (b) P3FAN and (c) P4FAN.

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Table 2 Results from FTIR spectra for poly¯uoroanilines synthesized with potassium dichromate and ammonium peroxydisulfate 1

Assignments

Wave numbers, cm P2FAN

P3FAN

P4FAN

NH stretching

3222 (3242) 1610,1504 (1578,1509) 826,756 (812,755) 1139 (1151) 1261,1302 (1263,1301,1332) 1451 (1451) 970 (940)

3225 (3245) 1577,1493 (1575,1497) 840,780 (832,778) 1137 (1137) 1258,1304 (1265,1304,1326) 1406 (1436) 965 (1042)

3224 (3234) 1571,1502 (1569,1509) 826 (832,728) 1154 (1148) 1230,1305 (1291,1355) 1411 (1411) 1042 (1042)

Ring stretching CH out of plane bend CH in plane bend CNC stretching C@C benzenoid diamine Fluorine substituents

Numbers in parentheses indicate the observed FTIR absorption frequencies for poly¯uoroanilines obtained using ammonium peroxydisulfate as the oxidant.

synthesized using potassium dichromate and ammonium peroxydisulfate are presented in Table 2. For polymers, a small peak corresponding to the carbonyl group is also observed. A close inspection of FTIR spectra of polymers clearly shows that polymerization mainly proceeds via 1,4 addition and the presence of quinoid units was also noted. The 1 H-NMR spectra for poly¯uoroanilines synthesized with potassium dichromate are shown in Fig. 2. In general, the 1 H-NMR spectra for polymers exhibit two or three distinct multiplets for aromatic protons between 6.7 and 8.2 ppm. These peaks are shifted up®eld from the typical 6.8 ppm (aromatic protons) observed in PAN, due to the strong electron withdrawing in¯uence of ¯uorine on the aromatic ring [17]. The amine protons appear as a broad peak between 3 and 4 ppm or 3 and 5 ppm. Also, some phenyl ring protons appear near 5.8 and 6.4 ppm due to the carbonyl group and protons of the quinoid structure at the end of the polymer backbone. In accordance with the 1 H-NMR spectra, the presence of ¯ourine groups was also con®rmed by 19 F-NMR. A comparison of the 19 F-NMR spectra of ¯uorosubstituted anilines with their polymers synthesized using potassium dichromate shows the appearance of new signals in addition to the original signals of monomers. The results of 19 F-NMR spectra are listed in Table 3. The 19 F-NMR spectrum for P2FAN consists of several peaks between 118 and 131 ppm together with a peak at 148 ppm, while P3FAN and P4FAN have multiplets centered at 110 and 115 ppm. The presence of more than one peak in these spectra suggests di€erent environments (such as end groups) for ¯uorine in the polymer [16]. Links between the monomer units mainly involve the para position, but ortho and even meta sites

Fig. 2. 1 H-NMR spectra of (a) P2FAN, (b) P3FAN and (c) P4FAN.

are also likely to get inserted into the chain framework [13,20].

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Table 3 Results from 19 F-NMR spectra of poly¯uoroanilines synthesized with potassium dichromate Monomer

ppm

Polymer

ppm

2FAN 3FAN 4FAN

136 113 127

P2FAN P3FAN P4FAN

148, 118±131 111.5, 112, 113, 114.5 109, 113±119

Although our FTIR data suggest 1,4 addition for the polymerization, in the case of 4FAN, this position is blocked by a halogen. For 1,4 addition to take place this position should be vacated by a dehalogenation reaction. However, our 19 F-NMR data clearly show the presence of a ¯uorine group along the polymer chain. Thus, three possibilities are likely to occur as suggested by Snauwaert and his coworkers [13]: site blocking, formation of an sp3 site or group elimination, maintaining the sp2 hybridization and so the p-electron delocalization. DSC thermograms of poly¯uoroanilines are shown in Fig. 3. The DSC thermogram of P2FAN exhibits an exothermic trend centered at 195°C indicating decomposition. On the other hand, P3FAN and P4FAN exhibit an endothermic trend centered at about 280°C and 240°C indicating decomposition of the polymer, respectively, [16]. TGA of PAN±HCl shows three major weight losses at around 100°C, 200°C, and 500°C that are assigned to removal of H2 O, HX, and decomposition of the polymer, respectively [17]. Also, the TGA of emeraldine base PAN shows a decomposition temperature (de®ned as P 10% weight loss) around 450°C: up to 5% of the weight loss can be attributed to the loss of associated water [21]. On the other hand, the TGA of the poly¯u-

Fig. 4. TGA thermograms of (a) P2FAN, (b) P3FAN and (c) P4FAN.

oroanilines (Fig. 4) has no distinctive weight loss due to water evaporation, consistent with the hydrophobicity of the halogen groups, but they also show a relatively lower decomposition temperature near 250°C. These observations are in accordance with the DSC results. Percent mass losses of polymers were obtained from TGA thermograms and the results are shown in Table 4. As seen from Table 4, P2FAN has the highest thermal stability. 3.2. Doping and ESR investigation

Fig. 3. DSC thermograms of (a) P2FAN, (b) P3FAN and (c) P4FAN.

Although P2FAN, P3FAN and P4FAN are synthesized in aqueous acidic medium, they were all found to be nonconducting except P4FAN. Therefore, this result indicates that poly¯uoroanilines are in their base form even though they were each polymerized in an acidic medium [16]. In order to impart electrical conductivity

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Table 4 % Mass loss of polymers at various temperatures Polymer

% Mass loss 200°C

400°C

600°C

800°C

P2FAN P3FAN P4FAN

2.68 2.68 3.35

13.33 22.11 23.45

25.71 38.19 37.19

± 45.24 42.86

to the polymers, obtained by using potassium dichromate as oxidizing agent, I2 is used as dopant. Iodine doping was achieved under vacuum at 50°C. The resistance values measured before and after I2 doping are given in Table 5. As seen from Table 5, the resistance of all polymers decreases as the duration of I2 doping increases. ESR spectra of polymers were also recorded during I2 doping, and the results are shown in Fig. 5. An inspection of Fig. 5 reveals that the ESR signal intensity also increases during I2 doping. The mechanism of I2 doping for PAN was explained by Zeng et al. [10]. According to their mechanism, I2 doping mostly caused the conversion of some of the benzenoid units to the quinoid units, and polarons (radical±cations) are formed as a consequence of an intramolecular redox reaction in the quinoid units.

Table 5 Variation of resistance as a function of I2 doping period at 50°C Doping period (Day)

Resistance P2FAN

P3FAN

P4FAN (kX)

Before doping 1 2 3 4 5

200 MX < 200 MX < 93 MX 13.8 MX 6.4 MX 400 kX

200 MX < 127 MX 75 MX 900 kX 525 kX 187 kX

500 142 85 70 28 11

Fig. 5. ESR spectra of P4FAN before and after the doping process.

Fig. 6. FTIR spectra of P4FAN (a) before the doping process and (b) after doping process with iodine for ®ve days.

Thus, the increase in the ESR signal intensity is evidence for an increase in the number of polarons. Fig. 6 compares FTIR spectra of P4FAN before and after I2 doping. The increase in the quinoid peak intensity and slight decrease in the benzenoid peak intensity can be explained in terms of interaction of I2 with benzenoid units.

4. Conclusion In this work, soluble polymers were synthesized from ¯uorine substituted anilines by chemical oxidation. Higher polymer yields were obtained when potassium dichromate was used as the oxidant. It was found that the position of halogen on the aromatic ring a€ects the polymer yield in appreciable amounts. Polymers obtained by using two di€erent oxidizing agents were found to be nonconducting except for P4FAN. FTIR results showed that polymerization mainly occurs via 1,4 addition. However, 19 F-NMR results indicated the existence of ¯uorine groups along the polymer chain. Thus, ortho and even meta sites are also likely to get inserted into the chain framework. During I2 doping, a decrease in the resistance of polymers and an increase in the ESR signal intensity were observed indicating an increase in the number of polarons. It was shown that I2 doping also increases quinoid peak intensity.

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