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Composite Langmuir–Blodgett films containing polypyrrole and polyimide
Thin Solid Films 327–329 (1998) 127–130
Composite Langmuir–Blodgett films containing polypyrrole and polyimide M.P. Srinivasan*, F.J. Jing Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore
Abstract A composite Langmuir–Blodgett film made of polyimide and polypyrrole that is thermally stable at elevated temperatures is reported. The composite, comprising alternating monolayers of polypyrrole and polyimide derived from polyamic acid-octadecyl amine salt, was made by a two-step polymerisation; 3-n-octyl pyrrole was polymerised by spreading it on aqueous ferric chloride. After LB film deposition, the composite was subjected to thermal or chemical treatment in order to convert the polyamic acid precursor to polyimide. For both imidisation methods, infrared spectroscopy showed the presence of polypyrrole in the polyimide matrix up to a temperature of 250°C. 1998 Elsevier Science S.A. All rights reserved Keywords: Langmuir–Blodgett film; Polypyrrole; Polyimide
1. Introduction Conducting and semiconducting polymeric and organic systems have been investigated extensively in recent years on account of their potential applications in devices such as rechargeable batteries, membranes and electroluminescent diodes [1–3]. However, their poor mechanical properties and problems with processing have been major obstacles to their extensive use [4]. With a view towards improving these characteristics, many variations have been tried, such as introduction of alkyl substitutions to increase solubility, and preparation of these materials in composite structures. In particular, thermal and mechanical stability problems have been addressed in the form of composites containing robust polymers that serve as matrices. Polyethylene [5] poly (vinyl chloride) [6] poly (vinyl alcohol) [7] polystryene [8] and polyurethane [9] have been used. In addition, composites of polypyrrole with polyimide [10], arachidic acid [11] and polypropylene [12] have also been fabricated. These films were prepared by either electrochemical polymerisation of pyrrole on an electrode or by exposing substrates containing ferric chloride as an oxidant to pyrrole vapour. In all these cases, polypyrrole was prepared as a separate film attached to a substrate. The LB technique offers the possibility of constructing highly-ordered systems, and the particular advantage in * Corresponding author. Tel.: +65 772 2171; fax: +65 779 1936; e-mail: [email protected]
0040-6090/98/$ - see front matter PII S0040-6090 (98 )0 0613-0
constructing composites is the intimate nature of mixing that is possible at the molecular level. This can be exploited either in the form of a mixed Langmuir film containing both species or as an ordered molecular laminate formed by controlled sequential deposition of the two species. Such intimate interaction between the active groups and the matrix may give rise to more robust materials and also to other useful properties as far as composites are concerned. As part of an ongoing effort to synthesise and fabricate composites containing polyimide and other thermally-resistant polymers, this article reports preparation of composite LB films containing polyimide derived from polyamic acidoctadecylamine salt and poly (3-n-octyl pyrrole). The composite film was prepared by a two-step polymerisation. First, the pyrrole was polymerised as a Langmuir film spread on aqueous ferric chloride [13] and subsequently, the polyamic acid salt was imidised thermally to complete the processing.
2. Experimental 2.1. Materials Polyamic acid octadecyl amine salt (PAAS) was prepared by reacting pyromellitic anhydride (from BDH, recrystallised over acetic anhydride) and 4,4′-diaminodiphenyl ether (from TCI, recrystallised from a 1:1 (v./v.) mixture of methanol and tetrahydrofuran). 3-n-octyl pyrrole (OCPY) and octadecyl amine (ODA), both from TCI, were used as
1998 Elsevier Science S.A. All rights reserved
128
M.P. Srinivasan, F.J. Jing / Thin Solid Films 327–329 (1998) 127–130
received. All the solvents used for synthesis were dried over molecular sieves. Following the method of Kakimoto et al. [14] the spreading solution for the polyimide precursor was prepared as a 1:2 mixture of polyamic acid and octadecyl amine in a benzene–dimethyl acetamide solvent mixture (1:1 by volume). OCPY was prepared as a mixed solution with ODA (OCPY to ODA = 5:1) in chloroform. The two solutions were spread–one in each compartment–over a subphase of aqueous ferric chloride (0.1% by weight) held in a NIMA trough set up for alternate layer deposition. Substrates for IR spectroscopy were cover-glass slips treated with a solution of dichlorodimethyl silane in chloroform to make the surfaces hydrophobic. 2.2. Film preparation The films were deposited at a speed of 5 mm/min; a wait time of 10 min in air was found necessary to ensure that no peeling occurred on the downstroke. Surface pressures were held constant at 20 mN/m for the PAAS film and at 15 mN/ m for the OCPY–ODA film. It was found that downstroke through the PAAS monolayer and upstroke through the OCPY–ODA monolayer yielded films with transfer ratios close to 1. At least 80 layers (40 layers of each species) were deposited on the substrates. The as-deposited LB films had a greyish tint, indicative of polymerisation of the pyrrole. Subsequent to deposition, the films were characterised by infrared spectroscopy (Shimadzu FTIR-8001). Thermal imidisation was performed by heating in air at a temperature of 200°C. Chemical imidisation was performed by immersing the composite in a mixture of acetic anhydride and pyridine (2:1) and subsequently heating the substrate at 90°C for 1 h.
3.2. Infrared Characterisation Initially, cover-glass slips were employed as substrates in order to extend the range of infrared investigation to 1500 cm−1, since the standard 1-mm-thick microscope glass slides absorb strongly from 2000 cm−1 onwards. Subsequently, calcium fluoride plates were used, and also, the material was scraped off the substrate and scanned in the form of a KBr pellet. In the case of pure PAAS LB films, disappearance of the alkyl bands at 2920 cm−1 and 2860 cm−1, together with appearance of the 1720 cm−1 band are indications of imidisdation [15]. Fig. 3 shows the IR spectra of composites that have been thermally and chemically imidised. The gradual decrease in absorbance from 4000 cm−1 to 1500 cm−1 that is typical of polypyrrole [13] and the imide–carbonyl absorption band at 1720 cm−1 indicate the presence of both species. The persistence of the N–H absorption at 3320 cm−1 for the thermally-imidised sample suggests incomplete imidisation. Thus, the presence of alkyl bands may be due to the octyl substitutions on the pyrrole as well as residual octadecyl amine of the polyamic acid. The IR spectra after chemical imidisation also reveals the presence of both polypyrrole and the polyimide. As with thermal imidisation, the alkyl bands (2920 cm−1 and 2860 cm−1) persist even after the appearance of the carbonyl band at 1720 cm−1. Since imidisation is complete in this case, the continued presence of the alkyl band is indicative of contribution from the octyl substitutions on the pyrrole groups. These bands disappeared if the films were heated to temperatures beyond 250°C and the IR spectrum deteriorated at temperatures above 300°C, indicating loss of material or decomposition. Thus, in contrast to the reported decomposition of pure
3. Results and Discussion 3.1. Langmuir layer analysis When OCPY was spread over the ferric chloride subphase, the condensed Langmuir film occupied an area of only 1 nm per repeat unit. Therefore, a mixture of ODA and OCPY was spread in order to stabilise the Langmuir film. Fig. 1 shows the isotherms obtained as a function of composition. The area per molecule is based on that of a PAAS repeat unit. The shift to larger areas with increasing POCPY content indicates presence of both species at the air–water interface. In order to ensure that interaction between PAAS and ferric chloride did not interfere with the imidisation process, a PAAS LB film supported on a substrate was immersed in ferric chloride; subsequent infrared spectroscopy indicated little difference between the immersed sample and the control. Additionally, as shown in Fig. 2, the isotherms of PAAS on water and on aqueous ferric chloride showed no significant difference on account of the change in subphase.
Fig. 1. Isotherms of POCPY–ODA mixed Langmuir films on a subphase of aqueous ferric chloride. Figures in the box indicate ODA content.
M.P. Srinivasan, F.J. Jing / Thin Solid Films 327–329 (1998) 127–130
129
Current efforts are focused on characterising the electrical properties of the composite and in exploring the feasibility of using the composite as a gas sensor for hightemperature applications. Preliminary examination of the electrical behaviour suggests that the composite film approaches the insulating behaviour characteristic of the polyimide with increasing temperatures of imidisation, indicating that thermal treatment is accompanied by loss of conductivity. Therefore, the feasibility of using impedance changes for gas sensing are also being investigated.
4. Conclusions
Fig. 2. Isotherms of PAAS on pure water and aqueous ferric chloride.
POCPY at 150°C, [16] it was possible for POCPY to withstand temperatures of up to 250°C in the environment of a polyimide matrix, demonstrating that a thermally-robust composite can be designed by judicious choice of the materials and the processing methods. Investigations conducted on spun-coated films of the polypyrrole-polyimide composite [17] (thermogravimetric analyses, FTIR spectra, Soxhelet extractions, and scanning electron microscopy) have provided corroborative evidence of the robustness of the composite and of the existence of pyrrole groups in the form of polypyrrole.
A thermally-robust LB film composite consisting of polyimide and 3-n-octyl substituted pyrrole has been fabricated. The composite was prepared as a molecular laminate by alternate deposition of polyamic acid-octadecyl amine salt and poly (3-n-octyl pyrrole)-octadecyl amine mixture. Poly (3-n-octyl) pyrrole, the less stable of the constituents, was present in the films up to a temperature of 250°C. The infrared spectra of the pyrrole deteriorated at temperatures above 300°C.
Acknowledgements The authors would like to thank the National University
Fig. 3. IR spectra of POCPY-polyimide thermally imidised (‘THERMAL’) at 200°C and chemically imidised (‘CHEMICAL’).
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M.P. Srinivasan, F.J. Jing / Thin Solid Films 327–329 (1998) 127–130
of Singapore for funding for this project through the Academic Research Fund (RP 940612 and RP 950668) The authors also thank Dr. Babu Narayanswamy for his advice and assistance.
[6] [7] [8] [9] [10] [11]
References
[12]
[1] D. MacInnes, Jr., M.A. Druy, P.J. Nigrey, D.P. Naires, A.G. MacDiarmid, A.J. Heeger, J. Chem. Soc. Chem. Commun. (1989) 317. [2] M.R. Anderson, B.R. Mattes, H. Reiss, R.B. Kaner, Synth. Met. 41– 43, (1991) 1151. [3] T.A. Skotheim, (Ed.), Handbook of Conducting Polymers, Vol. 1, Marcel Dekker, New York, 1986. [4] J.M. Machado, F.E. Karasz, R.W. Lenz, Polymer 29 (1988) 1412. [5] H.V. Dijk, O. Aagaard, R. Schellekens, Synth. Met. 55 (1993) 1085.
[13] [14] [15] [16] [17]
M. Nakata, H. Kise, Polymer J. 25 (1993) 91. T. Ojio, S. Miyata, Polymer J. 18 (1986) 95. E. Ruckenstein, J.S. Park, J. Appl. Polymer Sci. 42 (1991) 925. X.T. Bi, Q.B. Pei, Synth. Met. 22 (1987) 145. B. Tieke, W. Gabriel, Polymer 31 (1990) 20. Y.H. Park, S.Y. Park, S.W. Nam, C.R. Park, Y.J. Kim, J. Appl. Polymer Sci. 60 (1996) 865. J. Yang, Y. Yang, J. Hou, X. Zhang, W. Zhu, M. Xu, M. Wan, Polymer 37 (1996) 793. K. Hong, M.F. Rubner, Thin Solid Films 160 (1988) 187. M. Kakimoto, M. Suzuki, T. Konishi, Y. Imai, M. Iwamoto, T. Hino, Chem. Lett. (1986) 823. K.F. Schoch, W.F.A. Su, M.G. Burke, Langmuir 9 (1991) 278. H. Masuda, S. Tanaka, K. Kaeriyama, J. Polymer Sci. Polymer Chem. Edn. 28 (1990) 1831. B. Narayanswamy, M.P. Srinivasan, J. Applied Polymer Science, submitted.
Composite Langmuir–Blodgett films containing polypyrrole and polyimide M.P. Srinivasan*, F.J. Jing Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore
Abstract A composite Langmuir–Blodgett film made of polyimide and polypyrrole that is thermally stable at elevated temperatures is reported. The composite, comprising alternating monolayers of polypyrrole and polyimide derived from polyamic acid-octadecyl amine salt, was made by a two-step polymerisation; 3-n-octyl pyrrole was polymerised by spreading it on aqueous ferric chloride. After LB film deposition, the composite was subjected to thermal or chemical treatment in order to convert the polyamic acid precursor to polyimide. For both imidisation methods, infrared spectroscopy showed the presence of polypyrrole in the polyimide matrix up to a temperature of 250°C. 1998 Elsevier Science S.A. All rights reserved Keywords: Langmuir–Blodgett film; Polypyrrole; Polyimide
1. Introduction Conducting and semiconducting polymeric and organic systems have been investigated extensively in recent years on account of their potential applications in devices such as rechargeable batteries, membranes and electroluminescent diodes [1–3]. However, their poor mechanical properties and problems with processing have been major obstacles to their extensive use [4]. With a view towards improving these characteristics, many variations have been tried, such as introduction of alkyl substitutions to increase solubility, and preparation of these materials in composite structures. In particular, thermal and mechanical stability problems have been addressed in the form of composites containing robust polymers that serve as matrices. Polyethylene [5] poly (vinyl chloride) [6] poly (vinyl alcohol) [7] polystryene [8] and polyurethane [9] have been used. In addition, composites of polypyrrole with polyimide [10], arachidic acid [11] and polypropylene [12] have also been fabricated. These films were prepared by either electrochemical polymerisation of pyrrole on an electrode or by exposing substrates containing ferric chloride as an oxidant to pyrrole vapour. In all these cases, polypyrrole was prepared as a separate film attached to a substrate. The LB technique offers the possibility of constructing highly-ordered systems, and the particular advantage in * Corresponding author. Tel.: +65 772 2171; fax: +65 779 1936; e-mail: [email protected]
0040-6090/98/$ - see front matter PII S0040-6090 (98 )0 0613-0
constructing composites is the intimate nature of mixing that is possible at the molecular level. This can be exploited either in the form of a mixed Langmuir film containing both species or as an ordered molecular laminate formed by controlled sequential deposition of the two species. Such intimate interaction between the active groups and the matrix may give rise to more robust materials and also to other useful properties as far as composites are concerned. As part of an ongoing effort to synthesise and fabricate composites containing polyimide and other thermally-resistant polymers, this article reports preparation of composite LB films containing polyimide derived from polyamic acidoctadecylamine salt and poly (3-n-octyl pyrrole). The composite film was prepared by a two-step polymerisation. First, the pyrrole was polymerised as a Langmuir film spread on aqueous ferric chloride [13] and subsequently, the polyamic acid salt was imidised thermally to complete the processing.
2. Experimental 2.1. Materials Polyamic acid octadecyl amine salt (PAAS) was prepared by reacting pyromellitic anhydride (from BDH, recrystallised over acetic anhydride) and 4,4′-diaminodiphenyl ether (from TCI, recrystallised from a 1:1 (v./v.) mixture of methanol and tetrahydrofuran). 3-n-octyl pyrrole (OCPY) and octadecyl amine (ODA), both from TCI, were used as
1998 Elsevier Science S.A. All rights reserved
128
M.P. Srinivasan, F.J. Jing / Thin Solid Films 327–329 (1998) 127–130
received. All the solvents used for synthesis were dried over molecular sieves. Following the method of Kakimoto et al. [14] the spreading solution for the polyimide precursor was prepared as a 1:2 mixture of polyamic acid and octadecyl amine in a benzene–dimethyl acetamide solvent mixture (1:1 by volume). OCPY was prepared as a mixed solution with ODA (OCPY to ODA = 5:1) in chloroform. The two solutions were spread–one in each compartment–over a subphase of aqueous ferric chloride (0.1% by weight) held in a NIMA trough set up for alternate layer deposition. Substrates for IR spectroscopy were cover-glass slips treated with a solution of dichlorodimethyl silane in chloroform to make the surfaces hydrophobic. 2.2. Film preparation The films were deposited at a speed of 5 mm/min; a wait time of 10 min in air was found necessary to ensure that no peeling occurred on the downstroke. Surface pressures were held constant at 20 mN/m for the PAAS film and at 15 mN/ m for the OCPY–ODA film. It was found that downstroke through the PAAS monolayer and upstroke through the OCPY–ODA monolayer yielded films with transfer ratios close to 1. At least 80 layers (40 layers of each species) were deposited on the substrates. The as-deposited LB films had a greyish tint, indicative of polymerisation of the pyrrole. Subsequent to deposition, the films were characterised by infrared spectroscopy (Shimadzu FTIR-8001). Thermal imidisation was performed by heating in air at a temperature of 200°C. Chemical imidisation was performed by immersing the composite in a mixture of acetic anhydride and pyridine (2:1) and subsequently heating the substrate at 90°C for 1 h.
3.2. Infrared Characterisation Initially, cover-glass slips were employed as substrates in order to extend the range of infrared investigation to 1500 cm−1, since the standard 1-mm-thick microscope glass slides absorb strongly from 2000 cm−1 onwards. Subsequently, calcium fluoride plates were used, and also, the material was scraped off the substrate and scanned in the form of a KBr pellet. In the case of pure PAAS LB films, disappearance of the alkyl bands at 2920 cm−1 and 2860 cm−1, together with appearance of the 1720 cm−1 band are indications of imidisdation [15]. Fig. 3 shows the IR spectra of composites that have been thermally and chemically imidised. The gradual decrease in absorbance from 4000 cm−1 to 1500 cm−1 that is typical of polypyrrole [13] and the imide–carbonyl absorption band at 1720 cm−1 indicate the presence of both species. The persistence of the N–H absorption at 3320 cm−1 for the thermally-imidised sample suggests incomplete imidisation. Thus, the presence of alkyl bands may be due to the octyl substitutions on the pyrrole as well as residual octadecyl amine of the polyamic acid. The IR spectra after chemical imidisation also reveals the presence of both polypyrrole and the polyimide. As with thermal imidisation, the alkyl bands (2920 cm−1 and 2860 cm−1) persist even after the appearance of the carbonyl band at 1720 cm−1. Since imidisation is complete in this case, the continued presence of the alkyl band is indicative of contribution from the octyl substitutions on the pyrrole groups. These bands disappeared if the films were heated to temperatures beyond 250°C and the IR spectrum deteriorated at temperatures above 300°C, indicating loss of material or decomposition. Thus, in contrast to the reported decomposition of pure
3. Results and Discussion 3.1. Langmuir layer analysis When OCPY was spread over the ferric chloride subphase, the condensed Langmuir film occupied an area of only 1 nm per repeat unit. Therefore, a mixture of ODA and OCPY was spread in order to stabilise the Langmuir film. Fig. 1 shows the isotherms obtained as a function of composition. The area per molecule is based on that of a PAAS repeat unit. The shift to larger areas with increasing POCPY content indicates presence of both species at the air–water interface. In order to ensure that interaction between PAAS and ferric chloride did not interfere with the imidisation process, a PAAS LB film supported on a substrate was immersed in ferric chloride; subsequent infrared spectroscopy indicated little difference between the immersed sample and the control. Additionally, as shown in Fig. 2, the isotherms of PAAS on water and on aqueous ferric chloride showed no significant difference on account of the change in subphase.
Fig. 1. Isotherms of POCPY–ODA mixed Langmuir films on a subphase of aqueous ferric chloride. Figures in the box indicate ODA content.
M.P. Srinivasan, F.J. Jing / Thin Solid Films 327–329 (1998) 127–130
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Current efforts are focused on characterising the electrical properties of the composite and in exploring the feasibility of using the composite as a gas sensor for hightemperature applications. Preliminary examination of the electrical behaviour suggests that the composite film approaches the insulating behaviour characteristic of the polyimide with increasing temperatures of imidisation, indicating that thermal treatment is accompanied by loss of conductivity. Therefore, the feasibility of using impedance changes for gas sensing are also being investigated.
4. Conclusions
Fig. 2. Isotherms of PAAS on pure water and aqueous ferric chloride.
POCPY at 150°C, [16] it was possible for POCPY to withstand temperatures of up to 250°C in the environment of a polyimide matrix, demonstrating that a thermally-robust composite can be designed by judicious choice of the materials and the processing methods. Investigations conducted on spun-coated films of the polypyrrole-polyimide composite [17] (thermogravimetric analyses, FTIR spectra, Soxhelet extractions, and scanning electron microscopy) have provided corroborative evidence of the robustness of the composite and of the existence of pyrrole groups in the form of polypyrrole.
A thermally-robust LB film composite consisting of polyimide and 3-n-octyl substituted pyrrole has been fabricated. The composite was prepared as a molecular laminate by alternate deposition of polyamic acid-octadecyl amine salt and poly (3-n-octyl pyrrole)-octadecyl amine mixture. Poly (3-n-octyl) pyrrole, the less stable of the constituents, was present in the films up to a temperature of 250°C. The infrared spectra of the pyrrole deteriorated at temperatures above 300°C.
Acknowledgements The authors would like to thank the National University
Fig. 3. IR spectra of POCPY-polyimide thermally imidised (‘THERMAL’) at 200°C and chemically imidised (‘CHEMICAL’).
130
M.P. Srinivasan, F.J. Jing / Thin Solid Films 327–329 (1998) 127–130
of Singapore for funding for this project through the Academic Research Fund (RP 940612 and RP 950668) The authors also thank Dr. Babu Narayanswamy for his advice and assistance.
[6] [7] [8] [9] [10] [11]
References
[12]
[1] D. MacInnes, Jr., M.A. Druy, P.J. Nigrey, D.P. Naires, A.G. MacDiarmid, A.J. Heeger, J. Chem. Soc. Chem. Commun. (1989) 317. [2] M.R. Anderson, B.R. Mattes, H. Reiss, R.B. Kaner, Synth. Met. 41– 43, (1991) 1151. [3] T.A. Skotheim, (Ed.), Handbook of Conducting Polymers, Vol. 1, Marcel Dekker, New York, 1986. [4] J.M. Machado, F.E. Karasz, R.W. Lenz, Polymer 29 (1988) 1412. [5] H.V. Dijk, O. Aagaard, R. Schellekens, Synth. Met. 55 (1993) 1085.
[13] [14] [15] [16] [17]
M. Nakata, H. Kise, Polymer J. 25 (1993) 91. T. Ojio, S. Miyata, Polymer J. 18 (1986) 95. E. Ruckenstein, J.S. Park, J. Appl. Polymer Sci. 42 (1991) 925. X.T. Bi, Q.B. Pei, Synth. Met. 22 (1987) 145. B. Tieke, W. Gabriel, Polymer 31 (1990) 20. Y.H. Park, S.Y. Park, S.W. Nam, C.R. Park, Y.J. Kim, J. Appl. Polymer Sci. 60 (1996) 865. J. Yang, Y. Yang, J. Hou, X. Zhang, W. Zhu, M. Xu, M. Wan, Polymer 37 (1996) 793. K. Hong, M.F. Rubner, Thin Solid Films 160 (1988) 187. M. Kakimoto, M. Suzuki, T. Konishi, Y. Imai, M. Iwamoto, T. Hino, Chem. Lett. (1986) 823. K.F. Schoch, W.F.A. Su, M.G. Burke, Langmuir 9 (1991) 278. H. Masuda, S. Tanaka, K. Kaeriyama, J. Polymer Sci. Polymer Chem. Edn. 28 (1990) 1831. B. Narayanswamy, M.P. Srinivasan, J. Applied Polymer Science, submitted.