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On the action of ozone on undoped and doped alkyl and N-alkyl-substituted polyanilines
Polymer Degradation and Stability 75 (2002) 99–106 www.elsevier.com/locate/polydegstab
On the action of ozone on undoped and doped alkyl and N-alkyl-substituted polyanilines Franco Cataldo* Soc. Lupi arl, Chemical Research Institute, Via Casilina 1626/A, 00133 Rome, Italy Received 17 May 2001; received in revised form 12 July 2001; accepted 4 August 2001
Abstract N-Methyl- and N-ethyl-substituted polyaniline (PANI) as well as 2-ethyl- and 3-ethyl-substituted PANI have been ozone treated in CHCl3 solutions. All the polyamines have been studied in their undoped state (emeraldine-base) and doped with camphorsulphonic acid (CSA). The reactions have been followed by electronic spectroscopy and in all cases radical cations (or polarons) were found to be the early reaction intermediate. Alkyl-substituted-PANI show higher reaction rate constants with O3 in comparison to unsubstituted PANI. In CHCl3 solution 3-ethyl-PANI reacts with O3 slower than cis-1,4-polyisoprene. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Alkyl-substituted polyaniline; Ozone treatment; Radical cation; Undoped and doped state; Antiozonant activity
1. Introduction In previous work [1], we have examined the reaction between O3 and emeraldine-base type polyaniline (PANIEB). Since we have very recently prepared several other PANI derivatives having alkyl-substituted phenyl rings or N-alkyl-substitution, we have extended the study of the reaction with O3 to these polymers. As general rule, alkyl-substituted PANI are characterized by improved solubility in organic solvents in comparison to unsubstituted PANI but have significantly lower electrical conductivity [2]. Because of the easier processing properties in comparison to PANI, the substituted derivatives may find interesting applications in the near future in advanced sectors of technology [2], thus also in this case it is interesting to assess the stability of these molecules toward O3 attack. Concerning the evaluation of PANI-EB as possible antiozonant for diene rubber, we have shown that PANIEB reacts too slowly with O3 to be able to protect diene rubber from degradation [1]. Moreover, its high molecular weight should limit its migration in a cured rubber matrix further limiting its protection activity. However, poly-
* Tel.: +39-06-205-5084; fax: +39-06-205-0800. E-mail address: cdcata@flashnet.it (F. Cataldo).
meric PANI-type antiozonants also present advantages over common N,N-disubstituted p-phenylenediamines (PPDs) such as simpler production processes at lower costs [1], low-to-negligible volatility losses during mixing in rubber compound due to the higher molecular weight and finally, lower extractability by acid rain [1]. By using AM1 calculations, as detailed in [3], we have shown [4] that appropriately substituted PANI should show a significantly improved reactivity with O3 in comparison to unsubstituted PANI. In fact, we have found that there is a correlation between the enthalpy of formation of a radical cation of the PPDs and their antiozonant effect in rubber compounds [3]. Since we have demonstrated that radical cations (or polarons) are also formed as the first reaction products when PANI-EB reacts with O3 [1], the same AM1 calculation may be applied to PANI and substituted PANIs. The results of our calculations are shown in Table 1. From these data, it appears that alkyl-substitution of the phenyl rings of PANIs is able to enhance the reactivity with O3 leading the PANIs to the same reactivity level of the PPDs. On the other hand N-alkyl substitution should not exert any enhancement on the reactivity toward O3 for methyl, allyl and ethynyl groups while an enhancement is expected for N-ethyl-PANI-EB. The present work is the account of our study on the ozone reactivity of the substituted PANIs.
0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(01)00208-7
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F. Cataldo / Polymer Degradation and Stability 75 (2002) 99–106
Table 1 Antiozonant activity: correlations between calculations and experimental facts Molecule
Enthalpy of formation of the monomeric unit (KJ/mol)
Enthalpy of formation of the radical cation (KJ/mol)
Antiozonant activity (rating)
Fatigue crack formation (rating)
Antioxidant activity (rating)
77PD 3,5-Dimethyl-PANI-EB 2,6-Dimethyl-PANI-EB IPPD 2-Ethyl-PANI-EB 3-Ethyl-PANI-EB 6PPD N-Ethyl-PANI-EB PANI-EB N-Methyl-PANI-EB N-Allyl-PANI-EB (6DDP)3-TRIAZINE DTPD 3-Ethyl-PANI-EB DPPD DNPD
175.69 26.66 35.89 125.46 32.09 7.01 142.61 72.83 89.12 104.05 178.28 196.13 356.19 318.09 419.86 580.04
570.97 767.57 768.42 773.75 777.03 779.82 792.42 816.49 843.56 845.89 924.06 962.20 1023.77 1073.88 1093.45 1253.16
100
70
42
95
100
70
70
92
70
60 50
75 65
60 70
35 0
50 0
70 100
2. Experimental All substituted PANIs used in this work, namely Nmethyl-PANI-EB, N-ethyl-PANI-EB, 2-ethyl-PANI-EB and 3-ethyl-PANI-EB were prepared as detailed previously [5]. Doping with camphorsulphonic acid (CSA) was done by grinding an equimolar mixture of substituted-PANI and CSA in the solid state and dissolving the mixture in CHCl3. The cis-1,4-polyisoprene used in this work was a Cariflex sample from Shell. All ozone treatments were conducted by bubbling O3 generated electrochemically as already reported [1], into the substituted-PANI solutions. In the present case up to 12% O3 by weight over O2 was delivered into the solutions under study. O3 was generated at an average rate of 2105 mol/min. As usual [1,3], the electronic spectra were recorded after appropriate times on a Shimadzu spectrophotometer UV160A. The FTIR spectra on ozone treated samples were recorded with a Perkin-Elmer 1710 spectrometer on films grown on KBr plates.
3. Results and discussion 3.1. Ozone treatment of N-methyl PANI-EB and N-ethyl-PANI-EB N-Methyl-PANI-EB (about 8105 M) was treated with O3 bubbled in CHCl3 solution at a rate of 2105 mol/min. The evolution of the electronic spectrum in 150 s is shown in Fig. 1. The original band at 600 nm (Fig. 1A) due to the quinoneimine units of the polymer is strongly reduced in intensity after only 10 s ozone treatment (Fig. 1B) and disappears completely (Fig. 1C) after 25 s. As in the case of PANI-EB [1], the ozone treatment of this N-alkyl-substituted PANI causes the
immediate formation of radical cations or polaron structures as suggested by the growth of the band at 834 nm (Fig. 1B) which shifts to 786 nm after 25 s ozone treatment (Fig. 1C) and by another band in the near infrared, at 1000 nm (Fig. 1B) which shifts later to 1027 nm (Fig. 1C). All of these experimental facts are in agreement with the behaviour of unsubstituted PANI with O3 [1]. It is interesting to note that after doping with CSA the radical cation of N-methyl-PANI-EB appears in the near infrared at 1058 nm [2] while after the O3 treatment it appears as two bands at about 800 nm (as in the case of unsubstituted PANI) and at > 1000 nm as in the case of CSA doping. However, also in the case of unsubstituted PANI [1], the difference spectra (ozonate–pristine) show a broad band with an intense tail extending from 800 to the near infrared > 1100 nm. The radical cation band shows longer persistence in the case of PANI-EB while it appears with considerably lower persistence in the case of N-methyl-PANI-EB. In fact, after 60 s ozone treatment, the band at about 1000 nm disappears almost completely, while the band originally at 834 nm becomes just a shoulder at 738 nm (Fig. 1D) and after 150 s ozone treatment is a negligible shoulder at 630 nm (Fig. 1E). An almost completely analogous spectral evolution is shown by N-ethyl-PANI-EB (8.0105 M) (see Fig. 2). Ozone treatment completely destroys the band at 607 nm (Fig. 2A) and the radical cation band appears at 1040 with two other peaks at 770 and 600 nm (Fig. 2B). For comparison, it is interesting to remember that Nethyl-PANI-EB when doped with CSA shows the radical cation band at 1058 nm [2]. By continuing the ozone treatment of N-ethyl-PANI-EB, as in the case of its lower homologue discussed above, there is the disappearance of the band in the near infrared at 1030–1040 nm (Fig. 2D). Ozone treatment for 510 s completely destroys all the
F. Cataldo / Polymer Degradation and Stability 75 (2002) 99–106
Fig. 1. Electronic spectra of N-methyl-PANI-EB in CHCl3: (A) pure N-methyl-PANI-EB in CHCl3; (B) after 10 s ozone treatment; (C) after 25 s ozone treatment; (D) after 60 s ozone treatment; (E) after 150 s ozone treatment.
transitions in the visible and in the NIR part of the spectrum. Only an absorption peak at 250 nm survives and is due to the phenyl rings which survived to the ozone treatment. In both N-methyl-PANI-EB (Fig. 1A) and N-ethylPANI-EB (Fig. 2A), the original peak, at 313 and 315 nm, respectively, disappears gradually as function of the ozone treatment time, exactly as happens for the unsubstituted PANI-EB [1]. By using the Baude notation [3],
101
Fig. 2. Electronic spectra of N-ethyl-PANI-EB in CHCl3: (A) pure Nethyl-PANI-EB in CHCl3; (B) after 30 s ozone treatment; (C) after 70 s ozone treatment; (D) after 200 s ozone treatment; (E) after 500 s ozone treatment.
the peak at 313–315 nm of the original PANI is the Bband which originates from the ! mixed with the n! transition. Therefore the disappearance of the peak at about 310 nm is an indication of the ozone attack on the nitrogen atoms of the PANIs whose first act is the formation of the radical cation. Only the E2
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F. Cataldo / Polymer Degradation and Stability 75 (2002) 99–106
2-Ethyl-PANI-EB (4.2105 M) is readily soluble in CHCl3 and reacts very quickly with O3. As shown in Fig. 3A and B, 5 s ozone treatment causes the reduction of the intensity of the quinoneimine band at 587 nm (which shifts to 576 nm) and the growth of the band at 794 nm: the radical cation band. The spectral evolution is very
close to that discussed for PANI ozone treatment [1]. After 10 s ozone treatment, the quinoneimine band has almost completely disappeared while the polaron peak appears at 779 nm (Fig. 3C). Only 15 s ozone treatment is sufficient to completely destroy the polaron band and the B-band originally located at 307 nm (Fig. 3D). 3-Ethyl-PANI-EB (3.3105 M) reacts in a similar way to the 2-ethyl-substituted PANI-EB as illustrated in Fig. 4. There is the destruction of the quinoneimine band at 568 nm and the growth of the polaron band at 780 nm (Fig. 4B) after 10 s of O3 treatment. After 90 s ozone treatment, the polaron band has completely
Fig. 3. Electronic spectra of 2-ethyl-PANI-EB in CHCl3; (A) pure 2ethyl-PANI-EB in CHCl3; (B) after 5 s ozone treatment; (C) after 10 s ozone treatment; (D) after 15 s ozone treatment.
Fig. 4. Electronic spectra of 3-ethyl-PANI-EB in CHCl3: (A) pure 3ethyl-PANI-EB in CHCl3; (B) after 10 s ozone treatment; (C) after 20 s ozone treatment; (D) after 90 s ozone treatment;.
band at about 250 nm survives which is due exclusively to the benzene ring ! transition. 3.2. Ozone treatment of 2-ethyl-PANI-EB and 3-ethylPANI-EB
F. Cataldo / Polymer Degradation and Stability 75 (2002) 99–106
disappeared and only a shoulder at 560 nm remains (Fig. 4D). From these semiquantitative studies, we can trace a first conclusion. The reaction speed of PANIs with O3 seems to be as follows (from faster to slower):
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methyl-PANI-EB is predicted in (Table 1) to be as reactive as PANI-EB while the experimental results suggest a reactivity closer to that of N-ethyl-PANI-EB. PANI-EB is predicted to have a reactivity closer to that of N-substituted PANI-EB while the experimental facts suggest a significantly weaker reactivity.
½2-Ethyl-PANI-EB ¼ ½3-ethyl-PANI-EB> ½N-methyl-PANI-EB ¼ ½N-ethyl-PANI-EB>>> ½PANI-EB which seems to be in good agreement with the data in Table 1 concerning the enthalpy of formation of the radical cation, although some exceptions can be observed: N-
Fig. 5. Electronic spectra of N-methyl-PANI-CSA in CHCl3: (A) pure N-methyl-PANI-CSA in CHCl3; (B) after 5 s ozone treatment; (C) after 15 s ozone treatment; (D) after 45 s ozone treatment.
Fig. 6. Electronic spectra of N-ethyl-PANI-CSA in CHCl3: (A) pure Nethyl-PANI-CSA in CHCl3; (B solid line) after 14 s ozone treatment; (B dotted line) after 30 s ozone treatment (C) after 60 s ozone treatment; (D) after 90 s ozone treatment; (E) after 150 s ozone treatment.
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3.3. Ozone treatment of CSA doped samples of substituted PANI In the previous work on PANI-EB [1], we avoided study of the ozone treatment of doped PANI samples. In the present work we have ozone treated CHCl3 solutions of CSA-doped substituted PANI. Fig. 5A shows the electronic spectrum of N-methylPANI-CSA (about 103 M, 3 ml) with the polaron (radical cation) band induced by the dopant at 1058 nm.
Fig. 7. Electronic spectra in CHCl3: (A) pure 2-ethyl-PANI-CSA in CHCl3; (B) after 12 s ozone treatment; (C) after 25 s ozone treatment; (D) after 45 s and 75 s ozone treatment; (E) pure 3-ethyl-PANI-CSA in CHCl3; (F) after 10 s ozone treatment; (G) after 27, 45 and 75 s ozone treatment.
Fig. 8. FTIR spectra of ozone treated PANI (film on KBr plates): (A) N-methyl-PANI-EB; (B) N-ethyl-PANI-EB; (C) 2-ethyl-PANI-EB; (D) 3-ethyl-PANI-EB.
F. Cataldo / Polymer Degradation and Stability 75 (2002) 99–106
The action of O3 for 5 s causes the growth of a shoulder at 780 nm and a shift of the radical cation band to 1018 nm (Fig. 5B). At this stage the spectrum resembles that of Fig. 1C derived from the ozone treatment of the undoped N-methyl-PANI-EB. After 15 s ozone treatment of Nmethyl-PANI-CSA, only a weak band at 685 nm remains and also a significant reduction in the optical density of the band at 312 nm can be observed (Fig 5C). The spectrum may be compared with that of Fig. 1C. Fig. 6 shows the spectral evolution of N-ethyl-PANICSA (about 103 M, 25 ml) under the action of O3. The original polaron band due to CSA doping located at 1067 nm shifts to 1040 nm under the action of O3 and a growth of a band at 713 can be observed in Fig. 6B. Further ozone treatment (Fig. 6C and D) causes a reduction of the polaron band and a blue shift of the peaks. It is interesting to note the analogy between the absorption spectrum of Fig. 6C and that of Fig. 2B and C. The complete ozone treatment is reached in 150 s (Fig. 6E) and the resulting spectrum compares well with that of the corresponding undoped sample (Fig. 2E). CSA-doped 2-ethyl-PANI (2.4103 M, 40 ml) shows a spectrum with two polaron peaks in the NIR: at 910 and 1091 nm (Fig. 7A). Ozone treatment destroys these peaks and causes the growth of a band at 770 nm (compare Fig. 7B with Fig. 3C). Further ozone treatment destroys the band at 770 nm as happens for the undoped sample. CSA-doped 3-ethyl-PANI (3.8103 M, 25 ml) shows the polaron band in the NIR portion of the spectrum (Fig. 7E) and as in the case of the 2-substituted compound after ozone treatment its absorption spectrum becomes comparable to that of the ozone treated undoped sample (Fig. 7G vs Fig. 4D).
105
From these qualitative data, it appears that the doping with camphorsulphonic acid slightly slows the reaction between the polyamine and the O3. This is quite explainable because doping implies protonation of the polyamine chain with formation of polarons along the polymer chain due to internal redox reaction [2]. Thus O3 may encounter some difficulty to react especially in the early reaction stages because it must destroy the complex between the PANI and the CSA before reacting. We can exclude CSA interference with the ozone treatment reaction because it is a fully saturated ketone. Instead the acidity of CSA may modify or affect some reaction mechanism during the O3 attack. 3.4. FTIR spectra of the over-treated substituted-PANIs Ozone treatment of PANI-EB causes the growth of ketone and aldehydic groups [1]. Fig. 8 shows that exactly the same situation occurs with the ozone treatment of both N-alkyl (Fig. 8A and B) and alkyl-substituted PANI-EB (Fig. 8C and D). In all cases three intense bands are observed at 1740, 1700 and 1650 cm1. Inevitably all these bands imply both chain scission and ring opening reactions. The presence of ether and/or peroxidic groups is suggested by the intense band at 1100 cm1. 3.5. Assessment of the possible antiozonant activity of 3-ethyl-PANI-EB A solution of cis-1,4-polyisoprene in CCl4 (150 mg/50 ml) was used for the viscometric study of the possible antiozonant activity of 3-ethyl-PANI-EB which appeared
Fig. 9. Efflux time as function of the ozone treatment time of a cis-1,4-polyisoprene (IR) solution in CCl4 (150 mg/50 ml) through an Ostwald viscosimeter. Circles inidicate the pure IR solution, asterisks indicate the solution containing 2.8% by weight of 3-ethyl-PANI-EB.
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F. Cataldo / Polymer Degradation and Stability 75 (2002) 99–106
to be the among the most promising molecules to be tested for the protection of diene rubber. The usual procedure [1,6–8] has been adopted also in this case consisting in the measurement of the efflux time of a rubber solution with and without the protecting agent, as function of the ozone treatment time. 3-Ethyl-PANI-EB was used at a level of 2.8% over the rubber present in solution. The results are shown in Fig. 9. Despite the good premises, the experimental evidence demonstrates that the alkyl substitution of PANI does not lead to the desired results in terms of ozone protection of diene rubber.
4. Conclusions This study has shown that alkyl-substituted PANI-EB with substitution either in the phenyl ring or in the nitrogen atom leads to an increase in the reaction speed with O3 in comparison to the unsubstituted PANI-EB. It has been found that the reaction rate with O3 decreases according to the following order: ½2-Ethyl-PANI-EB ¼ ½3-ethyl-PANI-EB> ½N-methyl-PANI-EB ¼ ½N-ethyl-PANI-EB>> ½PANI-EB:
The general reaction mechanism with O3 is comparable to that of PANI-EB [1] and the simpler PPDs [3], always involving radical cation formation as the early reaction step. Alkyl-substituted PANIs seem to react more slowly with O3 when doped with CSA in comparison to their reactivity in the undoped state. 3-Ethyl-PANI-EB is not able to protect polyisoprene rubber from O3 in a CCl4 solution.
References [1] Cataldo F. On the action of ozone on polyaniline Polym Degrad Stab 2002;75:93. [2] Cataldo F. Synthesis of alkyl and N-alkyl-substituted polyanilines. A study on their spectral properties and thermal stability. Submitted for publication. [3] Cataldo F. A study on the reaction between N-substituted pphenylenediamines and ozone: experimental results and theoretical aspects in relation to their antiozonant activity. Eur Polym J, in press. [4] Cataldo F. Oral presentation at The International Rubber Chemicals, Compounding and Mixing Conference, 3–4 April 2001, Brussels, Belgium. [5] Cataldo F, Maltese P. Polym Adv Technol 2001;12:293. [6] Cataldo F, Ori O. Polym Degrad Stab 1995;48:291. [7] Cataldo F. Polym Degrad Stab 1996;53:51. [8] Cataldo F. Polym Degrad Stab 1998;60:233.
On the action of ozone on undoped and doped alkyl and N-alkyl-substituted polyanilines Franco Cataldo* Soc. Lupi arl, Chemical Research Institute, Via Casilina 1626/A, 00133 Rome, Italy Received 17 May 2001; received in revised form 12 July 2001; accepted 4 August 2001
Abstract N-Methyl- and N-ethyl-substituted polyaniline (PANI) as well as 2-ethyl- and 3-ethyl-substituted PANI have been ozone treated in CHCl3 solutions. All the polyamines have been studied in their undoped state (emeraldine-base) and doped with camphorsulphonic acid (CSA). The reactions have been followed by electronic spectroscopy and in all cases radical cations (or polarons) were found to be the early reaction intermediate. Alkyl-substituted-PANI show higher reaction rate constants with O3 in comparison to unsubstituted PANI. In CHCl3 solution 3-ethyl-PANI reacts with O3 slower than cis-1,4-polyisoprene. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Alkyl-substituted polyaniline; Ozone treatment; Radical cation; Undoped and doped state; Antiozonant activity
1. Introduction In previous work [1], we have examined the reaction between O3 and emeraldine-base type polyaniline (PANIEB). Since we have very recently prepared several other PANI derivatives having alkyl-substituted phenyl rings or N-alkyl-substitution, we have extended the study of the reaction with O3 to these polymers. As general rule, alkyl-substituted PANI are characterized by improved solubility in organic solvents in comparison to unsubstituted PANI but have significantly lower electrical conductivity [2]. Because of the easier processing properties in comparison to PANI, the substituted derivatives may find interesting applications in the near future in advanced sectors of technology [2], thus also in this case it is interesting to assess the stability of these molecules toward O3 attack. Concerning the evaluation of PANI-EB as possible antiozonant for diene rubber, we have shown that PANIEB reacts too slowly with O3 to be able to protect diene rubber from degradation [1]. Moreover, its high molecular weight should limit its migration in a cured rubber matrix further limiting its protection activity. However, poly-
* Tel.: +39-06-205-5084; fax: +39-06-205-0800. E-mail address: cdcata@flashnet.it (F. Cataldo).
meric PANI-type antiozonants also present advantages over common N,N-disubstituted p-phenylenediamines (PPDs) such as simpler production processes at lower costs [1], low-to-negligible volatility losses during mixing in rubber compound due to the higher molecular weight and finally, lower extractability by acid rain [1]. By using AM1 calculations, as detailed in [3], we have shown [4] that appropriately substituted PANI should show a significantly improved reactivity with O3 in comparison to unsubstituted PANI. In fact, we have found that there is a correlation between the enthalpy of formation of a radical cation of the PPDs and their antiozonant effect in rubber compounds [3]. Since we have demonstrated that radical cations (or polarons) are also formed as the first reaction products when PANI-EB reacts with O3 [1], the same AM1 calculation may be applied to PANI and substituted PANIs. The results of our calculations are shown in Table 1. From these data, it appears that alkyl-substitution of the phenyl rings of PANIs is able to enhance the reactivity with O3 leading the PANIs to the same reactivity level of the PPDs. On the other hand N-alkyl substitution should not exert any enhancement on the reactivity toward O3 for methyl, allyl and ethynyl groups while an enhancement is expected for N-ethyl-PANI-EB. The present work is the account of our study on the ozone reactivity of the substituted PANIs.
0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(01)00208-7
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F. Cataldo / Polymer Degradation and Stability 75 (2002) 99–106
Table 1 Antiozonant activity: correlations between calculations and experimental facts Molecule
Enthalpy of formation of the monomeric unit (KJ/mol)
Enthalpy of formation of the radical cation (KJ/mol)
Antiozonant activity (rating)
Fatigue crack formation (rating)
Antioxidant activity (rating)
77PD 3,5-Dimethyl-PANI-EB 2,6-Dimethyl-PANI-EB IPPD 2-Ethyl-PANI-EB 3-Ethyl-PANI-EB 6PPD N-Ethyl-PANI-EB PANI-EB N-Methyl-PANI-EB N-Allyl-PANI-EB (6DDP)3-TRIAZINE DTPD 3-Ethyl-PANI-EB DPPD DNPD
175.69 26.66 35.89 125.46 32.09 7.01 142.61 72.83 89.12 104.05 178.28 196.13 356.19 318.09 419.86 580.04
570.97 767.57 768.42 773.75 777.03 779.82 792.42 816.49 843.56 845.89 924.06 962.20 1023.77 1073.88 1093.45 1253.16
100
70
42
95
100
70
70
92
70
60 50
75 65
60 70
35 0
50 0
70 100
2. Experimental All substituted PANIs used in this work, namely Nmethyl-PANI-EB, N-ethyl-PANI-EB, 2-ethyl-PANI-EB and 3-ethyl-PANI-EB were prepared as detailed previously [5]. Doping with camphorsulphonic acid (CSA) was done by grinding an equimolar mixture of substituted-PANI and CSA in the solid state and dissolving the mixture in CHCl3. The cis-1,4-polyisoprene used in this work was a Cariflex sample from Shell. All ozone treatments were conducted by bubbling O3 generated electrochemically as already reported [1], into the substituted-PANI solutions. In the present case up to 12% O3 by weight over O2 was delivered into the solutions under study. O3 was generated at an average rate of 2105 mol/min. As usual [1,3], the electronic spectra were recorded after appropriate times on a Shimadzu spectrophotometer UV160A. The FTIR spectra on ozone treated samples were recorded with a Perkin-Elmer 1710 spectrometer on films grown on KBr plates.
3. Results and discussion 3.1. Ozone treatment of N-methyl PANI-EB and N-ethyl-PANI-EB N-Methyl-PANI-EB (about 8105 M) was treated with O3 bubbled in CHCl3 solution at a rate of 2105 mol/min. The evolution of the electronic spectrum in 150 s is shown in Fig. 1. The original band at 600 nm (Fig. 1A) due to the quinoneimine units of the polymer is strongly reduced in intensity after only 10 s ozone treatment (Fig. 1B) and disappears completely (Fig. 1C) after 25 s. As in the case of PANI-EB [1], the ozone treatment of this N-alkyl-substituted PANI causes the
immediate formation of radical cations or polaron structures as suggested by the growth of the band at 834 nm (Fig. 1B) which shifts to 786 nm after 25 s ozone treatment (Fig. 1C) and by another band in the near infrared, at 1000 nm (Fig. 1B) which shifts later to 1027 nm (Fig. 1C). All of these experimental facts are in agreement with the behaviour of unsubstituted PANI with O3 [1]. It is interesting to note that after doping with CSA the radical cation of N-methyl-PANI-EB appears in the near infrared at 1058 nm [2] while after the O3 treatment it appears as two bands at about 800 nm (as in the case of unsubstituted PANI) and at > 1000 nm as in the case of CSA doping. However, also in the case of unsubstituted PANI [1], the difference spectra (ozonate–pristine) show a broad band with an intense tail extending from 800 to the near infrared > 1100 nm. The radical cation band shows longer persistence in the case of PANI-EB while it appears with considerably lower persistence in the case of N-methyl-PANI-EB. In fact, after 60 s ozone treatment, the band at about 1000 nm disappears almost completely, while the band originally at 834 nm becomes just a shoulder at 738 nm (Fig. 1D) and after 150 s ozone treatment is a negligible shoulder at 630 nm (Fig. 1E). An almost completely analogous spectral evolution is shown by N-ethyl-PANI-EB (8.0105 M) (see Fig. 2). Ozone treatment completely destroys the band at 607 nm (Fig. 2A) and the radical cation band appears at 1040 with two other peaks at 770 and 600 nm (Fig. 2B). For comparison, it is interesting to remember that Nethyl-PANI-EB when doped with CSA shows the radical cation band at 1058 nm [2]. By continuing the ozone treatment of N-ethyl-PANI-EB, as in the case of its lower homologue discussed above, there is the disappearance of the band in the near infrared at 1030–1040 nm (Fig. 2D). Ozone treatment for 510 s completely destroys all the
F. Cataldo / Polymer Degradation and Stability 75 (2002) 99–106
Fig. 1. Electronic spectra of N-methyl-PANI-EB in CHCl3: (A) pure N-methyl-PANI-EB in CHCl3; (B) after 10 s ozone treatment; (C) after 25 s ozone treatment; (D) after 60 s ozone treatment; (E) after 150 s ozone treatment.
transitions in the visible and in the NIR part of the spectrum. Only an absorption peak at 250 nm survives and is due to the phenyl rings which survived to the ozone treatment. In both N-methyl-PANI-EB (Fig. 1A) and N-ethylPANI-EB (Fig. 2A), the original peak, at 313 and 315 nm, respectively, disappears gradually as function of the ozone treatment time, exactly as happens for the unsubstituted PANI-EB [1]. By using the Baude notation [3],
101
Fig. 2. Electronic spectra of N-ethyl-PANI-EB in CHCl3: (A) pure Nethyl-PANI-EB in CHCl3; (B) after 30 s ozone treatment; (C) after 70 s ozone treatment; (D) after 200 s ozone treatment; (E) after 500 s ozone treatment.
the peak at 313–315 nm of the original PANI is the Bband which originates from the ! mixed with the n! transition. Therefore the disappearance of the peak at about 310 nm is an indication of the ozone attack on the nitrogen atoms of the PANIs whose first act is the formation of the radical cation. Only the E2
102
F. Cataldo / Polymer Degradation and Stability 75 (2002) 99–106
2-Ethyl-PANI-EB (4.2105 M) is readily soluble in CHCl3 and reacts very quickly with O3. As shown in Fig. 3A and B, 5 s ozone treatment causes the reduction of the intensity of the quinoneimine band at 587 nm (which shifts to 576 nm) and the growth of the band at 794 nm: the radical cation band. The spectral evolution is very
close to that discussed for PANI ozone treatment [1]. After 10 s ozone treatment, the quinoneimine band has almost completely disappeared while the polaron peak appears at 779 nm (Fig. 3C). Only 15 s ozone treatment is sufficient to completely destroy the polaron band and the B-band originally located at 307 nm (Fig. 3D). 3-Ethyl-PANI-EB (3.3105 M) reacts in a similar way to the 2-ethyl-substituted PANI-EB as illustrated in Fig. 4. There is the destruction of the quinoneimine band at 568 nm and the growth of the polaron band at 780 nm (Fig. 4B) after 10 s of O3 treatment. After 90 s ozone treatment, the polaron band has completely
Fig. 3. Electronic spectra of 2-ethyl-PANI-EB in CHCl3; (A) pure 2ethyl-PANI-EB in CHCl3; (B) after 5 s ozone treatment; (C) after 10 s ozone treatment; (D) after 15 s ozone treatment.
Fig. 4. Electronic spectra of 3-ethyl-PANI-EB in CHCl3: (A) pure 3ethyl-PANI-EB in CHCl3; (B) after 10 s ozone treatment; (C) after 20 s ozone treatment; (D) after 90 s ozone treatment;.
band at about 250 nm survives which is due exclusively to the benzene ring ! transition. 3.2. Ozone treatment of 2-ethyl-PANI-EB and 3-ethylPANI-EB
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disappeared and only a shoulder at 560 nm remains (Fig. 4D). From these semiquantitative studies, we can trace a first conclusion. The reaction speed of PANIs with O3 seems to be as follows (from faster to slower):
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methyl-PANI-EB is predicted in (Table 1) to be as reactive as PANI-EB while the experimental results suggest a reactivity closer to that of N-ethyl-PANI-EB. PANI-EB is predicted to have a reactivity closer to that of N-substituted PANI-EB while the experimental facts suggest a significantly weaker reactivity.
½2-Ethyl-PANI-EB ¼ ½3-ethyl-PANI-EB> ½N-methyl-PANI-EB ¼ ½N-ethyl-PANI-EB>>> ½PANI-EB which seems to be in good agreement with the data in Table 1 concerning the enthalpy of formation of the radical cation, although some exceptions can be observed: N-
Fig. 5. Electronic spectra of N-methyl-PANI-CSA in CHCl3: (A) pure N-methyl-PANI-CSA in CHCl3; (B) after 5 s ozone treatment; (C) after 15 s ozone treatment; (D) after 45 s ozone treatment.
Fig. 6. Electronic spectra of N-ethyl-PANI-CSA in CHCl3: (A) pure Nethyl-PANI-CSA in CHCl3; (B solid line) after 14 s ozone treatment; (B dotted line) after 30 s ozone treatment (C) after 60 s ozone treatment; (D) after 90 s ozone treatment; (E) after 150 s ozone treatment.
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3.3. Ozone treatment of CSA doped samples of substituted PANI In the previous work on PANI-EB [1], we avoided study of the ozone treatment of doped PANI samples. In the present work we have ozone treated CHCl3 solutions of CSA-doped substituted PANI. Fig. 5A shows the electronic spectrum of N-methylPANI-CSA (about 103 M, 3 ml) with the polaron (radical cation) band induced by the dopant at 1058 nm.
Fig. 7. Electronic spectra in CHCl3: (A) pure 2-ethyl-PANI-CSA in CHCl3; (B) after 12 s ozone treatment; (C) after 25 s ozone treatment; (D) after 45 s and 75 s ozone treatment; (E) pure 3-ethyl-PANI-CSA in CHCl3; (F) after 10 s ozone treatment; (G) after 27, 45 and 75 s ozone treatment.
Fig. 8. FTIR spectra of ozone treated PANI (film on KBr plates): (A) N-methyl-PANI-EB; (B) N-ethyl-PANI-EB; (C) 2-ethyl-PANI-EB; (D) 3-ethyl-PANI-EB.
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The action of O3 for 5 s causes the growth of a shoulder at 780 nm and a shift of the radical cation band to 1018 nm (Fig. 5B). At this stage the spectrum resembles that of Fig. 1C derived from the ozone treatment of the undoped N-methyl-PANI-EB. After 15 s ozone treatment of Nmethyl-PANI-CSA, only a weak band at 685 nm remains and also a significant reduction in the optical density of the band at 312 nm can be observed (Fig 5C). The spectrum may be compared with that of Fig. 1C. Fig. 6 shows the spectral evolution of N-ethyl-PANICSA (about 103 M, 25 ml) under the action of O3. The original polaron band due to CSA doping located at 1067 nm shifts to 1040 nm under the action of O3 and a growth of a band at 713 can be observed in Fig. 6B. Further ozone treatment (Fig. 6C and D) causes a reduction of the polaron band and a blue shift of the peaks. It is interesting to note the analogy between the absorption spectrum of Fig. 6C and that of Fig. 2B and C. The complete ozone treatment is reached in 150 s (Fig. 6E) and the resulting spectrum compares well with that of the corresponding undoped sample (Fig. 2E). CSA-doped 2-ethyl-PANI (2.4103 M, 40 ml) shows a spectrum with two polaron peaks in the NIR: at 910 and 1091 nm (Fig. 7A). Ozone treatment destroys these peaks and causes the growth of a band at 770 nm (compare Fig. 7B with Fig. 3C). Further ozone treatment destroys the band at 770 nm as happens for the undoped sample. CSA-doped 3-ethyl-PANI (3.8103 M, 25 ml) shows the polaron band in the NIR portion of the spectrum (Fig. 7E) and as in the case of the 2-substituted compound after ozone treatment its absorption spectrum becomes comparable to that of the ozone treated undoped sample (Fig. 7G vs Fig. 4D).
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From these qualitative data, it appears that the doping with camphorsulphonic acid slightly slows the reaction between the polyamine and the O3. This is quite explainable because doping implies protonation of the polyamine chain with formation of polarons along the polymer chain due to internal redox reaction [2]. Thus O3 may encounter some difficulty to react especially in the early reaction stages because it must destroy the complex between the PANI and the CSA before reacting. We can exclude CSA interference with the ozone treatment reaction because it is a fully saturated ketone. Instead the acidity of CSA may modify or affect some reaction mechanism during the O3 attack. 3.4. FTIR spectra of the over-treated substituted-PANIs Ozone treatment of PANI-EB causes the growth of ketone and aldehydic groups [1]. Fig. 8 shows that exactly the same situation occurs with the ozone treatment of both N-alkyl (Fig. 8A and B) and alkyl-substituted PANI-EB (Fig. 8C and D). In all cases three intense bands are observed at 1740, 1700 and 1650 cm1. Inevitably all these bands imply both chain scission and ring opening reactions. The presence of ether and/or peroxidic groups is suggested by the intense band at 1100 cm1. 3.5. Assessment of the possible antiozonant activity of 3-ethyl-PANI-EB A solution of cis-1,4-polyisoprene in CCl4 (150 mg/50 ml) was used for the viscometric study of the possible antiozonant activity of 3-ethyl-PANI-EB which appeared
Fig. 9. Efflux time as function of the ozone treatment time of a cis-1,4-polyisoprene (IR) solution in CCl4 (150 mg/50 ml) through an Ostwald viscosimeter. Circles inidicate the pure IR solution, asterisks indicate the solution containing 2.8% by weight of 3-ethyl-PANI-EB.
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to be the among the most promising molecules to be tested for the protection of diene rubber. The usual procedure [1,6–8] has been adopted also in this case consisting in the measurement of the efflux time of a rubber solution with and without the protecting agent, as function of the ozone treatment time. 3-Ethyl-PANI-EB was used at a level of 2.8% over the rubber present in solution. The results are shown in Fig. 9. Despite the good premises, the experimental evidence demonstrates that the alkyl substitution of PANI does not lead to the desired results in terms of ozone protection of diene rubber.
4. Conclusions This study has shown that alkyl-substituted PANI-EB with substitution either in the phenyl ring or in the nitrogen atom leads to an increase in the reaction speed with O3 in comparison to the unsubstituted PANI-EB. It has been found that the reaction rate with O3 decreases according to the following order: ½2-Ethyl-PANI-EB ¼ ½3-ethyl-PANI-EB> ½N-methyl-PANI-EB ¼ ½N-ethyl-PANI-EB>> ½PANI-EB:
The general reaction mechanism with O3 is comparable to that of PANI-EB [1] and the simpler PPDs [3], always involving radical cation formation as the early reaction step. Alkyl-substituted PANIs seem to react more slowly with O3 when doped with CSA in comparison to their reactivity in the undoped state. 3-Ethyl-PANI-EB is not able to protect polyisoprene rubber from O3 in a CCl4 solution.
References [1] Cataldo F. On the action of ozone on polyaniline Polym Degrad Stab 2002;75:93. [2] Cataldo F. Synthesis of alkyl and N-alkyl-substituted polyanilines. A study on their spectral properties and thermal stability. Submitted for publication. [3] Cataldo F. A study on the reaction between N-substituted pphenylenediamines and ozone: experimental results and theoretical aspects in relation to their antiozonant activity. Eur Polym J, in press. [4] Cataldo F. Oral presentation at The International Rubber Chemicals, Compounding and Mixing Conference, 3–4 April 2001, Brussels, Belgium. [5] Cataldo F, Maltese P. Polym Adv Technol 2001;12:293. [6] Cataldo F, Ori O. Polym Degrad Stab 1995;48:291. [7] Cataldo F. Polym Degrad Stab 1996;53:51. [8] Cataldo F. Polym Degrad Stab 1998;60:233.