Structural and thermal characterization of PTSA doped polypyrrole–polytetrahydrofuran graft copolymer

Synthetic Metals 140 (2004) 69–78

Structural and thermal characterization of PTSA doped polypyrrole–polytetrahydrofuran graft copolymer ¨ zdileka, Jale Hacalog˘lua,*, Levent Topparea, Yusuf Yag˘cıb Ceren O a

Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey b Department of Chemistry, Istanbul Technical University, 80626 Istanbul, Turkey

Received 28 September 2002; received in revised form 13 December 2002; accepted 19 December 2002

Abstract The thermal degradation processes of p-toluene sulfonate (PTS) doped polypyrrole (PPY) grown from the pyrrole moiety located at both the ends of polytetrahydrofuran (PTHF) have been studied by direct pyrolysis mass spectrometry (DPMS) to gain structural information. To get a better insight, pyrolysis analysis on corresponding homopolymers, PTHF and PPY, have also been carried out. The DPMS data were in accordance with TGA results indicating a significant decrease in thermal stability of the copolymer with respect to both PPY and the matrix polymers. The data also pointed out the degradation of matrix polymer, PTHF during the electrochemical synthesis of polypyrrole on a PTHF coated anode. Although decomposition of matrix polymer occurred, the growth of pyrrole from the pyrrole moieties located at chain ends of PTHF was also confirmed. Thus, it has been proved that a low molecular weight copolymer of polypyrrole involving short chains of PTHF as the end groups has been synthesized. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Polypyrrole–polytetrahydrofuran graft copolymer; Pyrolysis mass spectrometry; Conducting polymers

1. Introduction Among various conductive polymers, polypyrrole (PPY) is one of the most extensively studied [1–3]. Yet the applications are still limited due to their poor mechanical properties. The problems encountered in processing are tried to be overcome by modifying the structure of the monomer or by preparing block and graft copolymers of polypyrrole with certain insulating polymer matrices having good mechanical characteristics. Several works on graft copolymerization of pyrrole have been reported in the literature [4–9]. Pyrrole– styrene graft copolymers have been prepared, first by reacting the pendant chloromethyl groups on a styrene–p-chloromethylstyrene random copolymer with potassium pyrrole and then electrochemically polymerizing pyrrole onto the pendant pyrrole moiety [4,5]. Stanke et al. synthesized pyrrole–methyl methacrylate copolymers by successfully attaching a pyrrole moiety onto a copolymer of methyl methacrylate and 2-bromoethyl methacrylate and chemically polymerizing pyrrole thereon [6]. Geissler et al. have *

Corresponding author. Tel.: þ90-312-210-5148; fax: þ90-312-210-1280. E-mail address: [email protected] (J. Hacalog˘lu).

reported the electropolymerization of pyrrole in the presence of poly(N-vinylcarbazole) in methylene chloride to yield graft copolymers [7,8]. In our early studies, we have attempted to grow polypyrrole by electrochemical polymerization of pyrrole from the pyrrole moiety located at one or two chain ends of polytetrahydrofuran. Enhancements in the mechanical properties were obtained though conductivities of the products and PPY were in the same order of magnitude with pristine PPY (0.2–0.9 S/cm). [9]. However, since polypyrrole and its copolymers are highly cross-linked insoluble materials, the characterization of graft copolymers was remained as a formidable problem. Pyrolysis mass spectrometry techniques and their applications to polymer studies have been discussed in a number of recent studies [10–15]. In our previous direct pyrolysis mass spectrometry (DPMS) study on polypyrrole–polytetrahydrofuran, we determined that the matrix polymer polytetrahydrofuran with pyrrole moiety located only at one end degraded during the electrochemical polymerization of polypyrrole and the sample was not actually a graft copolymer [16]. In this work, we report the direct pyrolysis mass spectrometry results on the sample of polypyrrole grown from the pyrrole moieties located at both chain ends of polytetrahydrofuran (PTHF).

0379-6779/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00022-5

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2. Experimental

by deionized water and CH2Cl2 several times, dried under high vacuum and maintained under nitrogen atmosphere.

2.1. Syntheses 2.2. Characterization Living polymerization of tetrahydrofuran (THF; Mn ¼ 13,750) and introduction of the pyrrole group at both chain ends of the resultant polymer (PTHF) have formerly been reported [9]. Polypyrrole (PPY) and Polypyrrole–polytetrahydrofuran (PPY–PTHF) graft copolymer were electrochemically prepared in a conventional three-compartment cell with two platinum foils (1.5 cm2 each) as the working and counter electrodes. A Ag/Agþ electrode was utilized as the reference electrode. For the synthesis of graft copolymer, pyrrole was electrochemically polymerized onto a PTHF pre-coated anode. A constant potential of 1.1 V was supplied via a Wenking POS 73 potentiostat for about 2 h for the synthesis of both pristine PPY and PPY–PTHF graft copolymer. Syntheses were carried out under nitrogen atmosphere in deionized water containing 0.02 M pyrrole and 0.05 M sodium p-toluene sulfonate (NaPTS). Samples were washed

Direct insertion probe mass spectrometry system consists of a 5973 HP quadrupole mass spectrometer covering a mass range of 10–700 amu coupled to a SIS direct insertion probe (Tmax ¼ 445 8C). Two different heating rates were employed, namely 10 8C/min (ramp rate) and 900 8C/min (ballistic rate). Pyrolysis mass spectra were recorded using 70 and 19 eV electrons to differentiate the extent of dissociative ionization in the ion source. Spectra were acquired at a scan rate of 2 scans/s.

3. Results and discussions The main purpose of this study is to characterize PPY– PTHF graft copolymer. For this purpose, the thermal degradation behavior and products of polytetrahydrofuran with

Fig. 1. (a) The TIC curve of PTHF recorded with a heating rate of 10 8C/min. (b) The mass spectrum at the TIC maximum.

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pyrrole moities located at both the chain ends, PTHF, polypyrrole and the graft copolymer obtained by electropolymerization of pyrrole on the pyrrole end groups and PPY–PTHF, have been studied by direct pyrolysis mass spectrometry under the same experimental conditions. 3.1. Polytetrahydrofuran The pyrolysis mass analysis of the pure matrix, polytetrahydrofuran (PTHF) with pyrrole moiety at both the chain ends, at a heating rate of 10 8C/min revealed that its decomposition starts above 300 8C and occurs in a broad temperature range. The maximum product yield was observed at 440 8C which is in agreement with TGA results [9]. The total ion current curves recorded at a heating rate of 10 8C/min and the mass spectrum recorded at 440 8C are given in Fig. 1. The related mass spectral data are summarized in Table 1. The base peak in the mass spectrum obtained at this temperature is at m/z 71, which corresponds to one hydrogen deficient tetrahydrofuran monomer. Peaks due to oligomers containing up to six repeating units are also present. Relative intensities of the peaks are reduced as the number of monomer units present in the corresponding fragment increases. The spectra contain fragments due to cleavages at C–O, and C–C bonds and loss of side groups. Decreasing

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ionization energy to 19 eV did not cause any significant change in the shape and position of the total ion current, and TIC curve. Yet the base peak is shifted to m/z 129 due to the dimer. Furthermore, significant increase in the relative intensities of high-mass fragments is detected in the low energy mass spectra. Similar product peaks are present in the spectra recorded during pyrolysis with a ballistic heating rate, namely 900 8C/min. There exists a slight increase in the relative intensities of high-mass fragments as expected due to general degradation trends at fast heating rates. To get a better insight on the thermal decomposition mechanism, single ion pyrograms were studied. In Fig. 2, we report the evolution profiles of representative fragments evolved during the direct pyrolysis at a heating rate of 10 8C/ min. The evolution profiles are nearly isochronous with the TIC curve. It is clear that the main thermal decomposition occurs by homolytic chain scissions of the labile C–O bond generating several oligomers. Though fragments generated by C–C bond scissions are also identified, in general they have relatively low abundance, thus, it may also be proposed that the cleavages at C–C bonds seem to be a less important degradation pathway. Another point of interest may be the fact that the loss of end groups yielding fragments bearing the pyrrole unit occurs slightly at lower temperatures. These results lead us to propose that polymer degrades via random

Table 1 The characteristic and/or intense peaks in the pyrolysis mass spectra corresponding to the maxima the TIC curves of PTHF, PPY and PPY–PTHF recorded during heating at a rate of 10 8C/min Assignment

m/z

PTHF 70 eV; 441 8C

H2O H2S CO2, CH2CH2O SO2 C5H5 C4H4N or C4H3NH C4H7O C4H8O C4H8OH C4H4NCH2 or SO3 C4H4NCH3 C4H8OCH2 C4H8OCH3 C7H7 C6H5CH3 C4H4NCH2CH2 C4H8OCHCH2, C4H2(OH)2NH C4H8OCH2CH2 C4H8OCH2CH2CH3 C4H4NCH2CH2CH2CH2 C4H8OCH2CH2CHCH2 C4H8OCH2CH2CH2CH3 DTHF–H (C4H8O)2, DTHF CH3C6H4SO3H, (C4H8O)2CH2CH2 (C4H8O)2C4H7O (C4H8O)4H

18 34 44 64 65 66 71 72 73 80 81 86 87 91 92 94 99 100 115 123 128 129 143 144 172 215 289

PPY–PTHF 19 eV; 441 8C

32 108 36 3 3 3 1000 77 504 12 18 33 48 16 4 8 25 149 64 15 90 662 136 16 4 15 1

579 31 256 26 51 25 28

37 120 49 118 41 1000 517 71

70 eV; 353 8C 401 129 48 262 147 27 3 1 4 1 2 8 6 1000 572 2

1

19 eV; 357 8C 115 55 11 78 12 5 1

2

375 1000 3 1

1

70 eV; 410 8C 175 267 62 218 117 23 2 4 2 2 9 6 1000 564 2

19 eV; 410 8C 376 92 202 35 17

900 1000

1 2 2

0.5 11

157

PPY

41

22

19

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Fig. 2. The time–temperature resolved profiles of some selected ions obtained in DPMS of PTHF.

cleavage or a mixed random cleavage and unzipping mechanism as in the case of PTHF with pyrrole moiety located only at one end [16]. 3.2. Polypyrrole In our previous studies, we published the DPMS results of polypyrrole [17,18]. Yet, in order to characterize the graft

copolymer, it is necessary to discuss, at least shortly, the thermal behavior and degradation products of polypyrrole (PPY) under the same experimental conditions. The total ion current, TIC curve of PPY shows a single broad peak starting just above 250 8C and with a maximum at 410 8C when the heating rate was 10 8C/min (Fig. 3a). In the pyrolysis mass spectrum recorded at this temperature, the base peak is at m/z 91, corresponding to the tropylium

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Fig. 3. (a) The TIC curve of PPY recorded with a heating rate of 10 8C/min. (b) The mass spectrum at the TIC maximum.

ion, C7H7þ (Fig. 3b). The most striking feature in the spectrum is that though nearly all of the intense peaks originate from the dopant, PTS, a completely different fragmentation pattern was generated compared to that of p-toluene sulfonic acid (PTSA). (It has been assumed that ptoluene sulfonate ion evaporate in the protonated form.) In general, the PPY based peaks were weak and only moderate monomer and very weak dimer peaks can be identified. The presence of peaks at m/z 26 and 39 are associated with the cleavage of the pyrrole ring. Furthermore, evolution of H2O and CO2 even in the high temperature range indicates the oxidation of the polymer sample [19]. The results obtained with a heating rate of 900 8C are quite similar. Only small enhancements in the relative intensities of the fragments attributed to toluene, PTSA, H2O, CO2, SO2, and H2S have been noted in the corresponding 19 eV pyrolysis mass spectra. Thus, the lack of high-mass products peaks, the weak monomer peak and decomposition of the pyrrole ring, under different experimental conditions (different heating rates and ionization energies) have been associated with a net work structure. In Table 1, the data related to the

characteristic and intense peaks due to the evolved products during pyrolysis of PPY at a heating rate of 10 8C/min and the mass spectral data for p-toluene sulfonic acid are also included for comparison. In Fig. 4, the temperature–time resolved single ion curves of H2Sþ at m/z 34, SO2þ at m/z 64, C4H3NHþ at m/z 66, C7H7þ at m/z 91, and CH3C6H4SO3Hþ at m/z 172, recorded at the heating rate of 10 8C/min are given. The discrepancies in the pyrograms of single ions are reasonable for a sample containing more than one component. The presence of blocks with different thermal stabilities along the polymer chain is also another possible cause. In our case, it seems as if both the possibilities exist. The most abundant fragment yielding tropylium ion have a higher evolution profile (maximum is at 410 8C) compared to that of the dopant molecular ion at m/z 172 (maximum 360 8C). The trends in single ion pyrograms of dopant based fragments indicate that the interaction between the dopant and the pyrrole is most probably not unique. Pyrolysis removes the dopant that has been adsorbed or physically bound more readily. The evolution of toluene, SO2, and H2S at higher temperatures

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Fig. 4. The time–temperature resolved profiles of some selected ions obtained in DPMS of PPY.

has been associated with blocks involving chemically bound dopant that degrades before evolution [17,18]. 3.3. PPY–PTHF graft copolymer The TIC curve of PPY–PTHF shows a broad peak with a maximum at 350 8C significantly lower than those detected

for the corresponding homopolymers (Fig. 5a). The related TGA value is 338 8C. Such a high temperature shift indicates a significant decrease in thermal stability and in molecular weight. The pyrolysis mass spectrum recorded at this temperature appears to be dominated by PPY based fragment peaks (Fig. 5c). The fragments peaks associated with PTHF blocks, though very weak, are also present in the spectrum

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Fig. 5. (a) The TIC curve of PPY–PTHF recorded with a heating rate of 10 8C/min. The mass spectra at: (b) 200 8C and (c) 350 8C.

(Fig. 5b). The data related to diagnostic peaks of each component of the graft copolymer are also given in Table 1. Surprisingly, the time–temperature resolved ion curves of fragments originating from both components exhibit quite similar trends indicating a similar thermal behavior for both polypyrrole and polytetrahydrofuran blocks. This is contrary to our previous results obtained for the sample prepared by polymerizing pyrrole on the pyrrole moiety located at only

one end of the PTHF chain [16]. Furthermore, the discrepancies in the single ion pyrograms observed in the case of PPY is not noted for those of corresponding fragments arising from PPY blocks of the copolymer. In Fig. 6, the time–temperature resolved ion curves of characteristic matrix based fragments obtained for the copolymer at a heating rate of 10 8C/min are given. These are C4H7Oþ at m/z 71, C4H4NCH2CH2þ at m/z 94, and C4H4NCH2þ at

76

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Fig. 6. The time–temperature resolved profiles of some selected ions attributed to degradation of PTHF obtained in DPMS of PPY–PTHF.

m/z 80. PPY related fragments namely, H2Sþ at m/z 34, SO2þ at m/z 64, C7H7þ at m/z 91 and CH3C6H4SO3Hþ, PTSA at m/z 172 obtained at the same heating rate are shown in Fig. 7. For comparison, the time–temperature resolved ion curve for M–Hþ ion of the pyrrole at m/z 66 due to C4H4NHþ or C4H4Nþ is included in both the figures. Note that pyrograms of PTSA molecular ion at m/z 172 generated from PPY and PPY–PTHF coincide nearly exactly. During the pyrolysis of PPY, evolution of the PTSA molecule at lower temperatures than nearly all the degradation products was associated with relatively weak physical interaction between the dopant and the host polymer, PPY. It

may be expected that the evolution of PTSA that was weakly bound to pyrrole ring or to graft copolymer should be nearly identical. However, it is obvious that, besides that of PTSA molecule, all other products appear at lower temperatures when generated during the pyrolysis of the copolymer. Actually, the fragmentation pattern observed in the pyrolysis mass spectra of the copolymer, is quite similar to what was observed for pure PPY. Thus, a somewhat similar interaction, both physical and chemical, between the dopant and polypyrrole units can also be assigned for the copolymer. Then the lower temperature evolution of PPY based fragments can then only be attributed to lower thermal stability

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Fig. 7. The time–temperature resolved profiles of some selected ions attributed to degradation of PPPY obtained in DPMS of PPY–PTHF.

of the PPY blocks in the copolymer which is probably due to the presence of shorter polymer chains compared to PPY. The matrix, PTHF based fragments show more drastic discrepancies with respect to those evolved from the corresponding homopolymer, PTHF. They are not only generated at lower temperatures but also significantly diminished. Therefore, it may be suggested that degradation of the matrix, PTHF with pyrrole moities at both the ends, also

occurred similar to the case for which matrix bears pyrrole moiety at only one end during electropolymerization process [16]. To reinforce this idea, the residue solution, which remained after the electropolymerization of pyrrole onto the PTHF matrix, was analyzed by DPMS for the presence of any decomposed PTHF chains. Since PTHF is insoluble in aqueous electrolytic medium, the absence of PTHF based

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fragments from the mass spectrum is normally expected. However, in the analysis of the residual solution, up to six repeating units of PTHF were detected. Thus, it is proved that the matrix was subjected to degradation during electrolysis at least to a certain extent and the degradation products fell into the solution. The data obtained at a ballistic heating rate also supports the above mentioned proposal. The evolution of degradation products is shifted to initial stages. The PTHF based peaks are weak, yet they show similar trends to those of PPY based peaks. The corresponding 19 eV direct pyrolysis mass spectrometry results are in accordance with the high-energy, 70 eV data. Enhancements in the relative intensities of the fragments attributed to toluene, PTSA, H2O, CO2, SO2, H2S and dimer of tetrahydrofuran were noted indicating formation of these species mainly during the pyrolysis process. Nevertheless, it is clear that the polymer synthesized using a matrix with pyrrole moieties located at both the ends involves some groups that degraded yielding diagnostic products of PTHF. The trends in single ion pyrograms indicated that the sample, produced during the electrochemical synthesis of polypyrrole from the pyrrole moities located at both the ends of PTHF, involves both PPY and PTHF units with comparable stabilities. This is unlike the case where the matrix with a pyrrole moity located at only one end was used. The evolution of matrix based products occurred readily at very initial stages for that sample [16]. It may be suggested that the probability of attack of oxidized pyrroles to the pyrrole moities located at chain ends of PTHF should increase by a factor of 2 when both the ends are caped. This should in turn lead to a reduction in the number of attacks of oxidized pyrroles to the main chain of PTHF and should cause a decrease in the extent of degradation of the matrix during the electrochemical polymerization process. Thus, the possibility of obtaining a graft copolymer should also increase. Detection of diagnostic PTHF products in the region of main thermal degradation confirms the presence of PTHF chains. However, the decrease in the thermal stability of PTHF after electrolysis may only be related with a significant reduction in chain length. Actually, if PTHF chains were sufficiently long and if no degradation of matrix has occurred, then two peaks should be present in the TIC curve; two blocks, PPY and PTHF should behave separately resuming their thermal stabilities. Thus, present results confirm that as a result of degradation of matrix during electrochemical synthesis the resultant copolymer contains only short chains of PTHF being the end groups of long polypyrrole chains.

4. Conclusion The direct pyrolysis mass spectrometry, DPMS analysis indicated that a low molecular weight copolymer of polypyrrole involving short chains of polytetrahydrofuran (PTHF) has been synthesized during the electrochemical synthesis of polypyrrole on PTHF coated anode. Degradation of the matrix polymer, PTHF during the growth of pyrrole from the pyrrole moieties located at both the ends of polytetrahydrofuran have been determined. The DPMS data are in accordance with TGA results indicating a significant decrease in thermal stability of the copolymer with respect to both PPY and the matrix polymer.

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