Polyaniline complex with fullerene C60
Introduction
The preparation of composites based on conjugated polymers and buckminsterfullerene (C60) has opened a large area of scientific and technological interest. The photoinduced electron transfer from semiconducting polymers (as donors) to C60 and its derivatives (as acceptors) has been demonstrated in blends as well as in heterostructures prepared from these two materials. The latter interpenetrating phase-separated donor–acceptor composites appear to be ideal photovoltaic materials for the preparation of various types of optical devices such as switches, dynamic memory units, etc. Devices based on polythiophene derivatives show a photoresponse, which is competitive with UV-sensitized silicon photodiodes [1], [2]. Zakhidov et al. [3] and Araki et al. [4] have reported that doping the composites of conjugated polymer and C60 with alkali-metal atoms results in a granular superconductivity of the C60 component embedded in the polymer matrix. There is an indication of increasing the thermal stability of conjugated systems by the introduction of C60 and its derivatives [5], [6]. Water-soluble complex of C60 with poly(N-vinylpyrrolidone) was demonstrated to have a high antivirus activity [7]. The list of applications is far from being complete.
Composites of several conducting polymers, viz. polyvinylcarbazole [8], polyalkylthiophene [9], poly(p-phenylenevinylene) [10], [11], have been studied during past years. The charge transfer between the C60 and the donor fragments leads to the doping of polymers, thus increasing their conductivity by making them p-type semiconductors. The doping level reflects the extent of interaction between the components.
Among the conducting polymers, polyaniline (PANI) has attracted much attention because of its multiple electronic states, high conductivity that occurs upon doping, as well as for its easy and economic preparation, good environmental stability, and high application potential [12]. Wei et al. [13] were the first to report the doping of PANI with C60. Various authors have, however, noted the low doping degree of this polymer [13], [15]. In all the cases, the solution blending has been employed for preparation of PANI–C60 composites. Polyaniline base was dissolved in N-methylpyrrolidone (NMP) and C60 in chlorobenzene or toluene. The solutions were mixed and evaporated to produce films of PANI–C60 composite ready for further research. It is known, however, that C60 reacts with both primary and secondary amines [16]. Consequently, there is a chance that NMP, used as a solvent, has not performed as an inert medium. It could competitively react with C60 and this might be a reason for the low doping degree in PANI.
In the present work, two new ways of PANI–C60 composite preparation are investigated. These are based on the solid-state blending of the components and on the introduction of C60 into the reaction mixture used for the synthesis of PANI.
Section snippets
Materials
Aniline (Fluka, Switzerland) was purified by double distillation at reduced pressure. Ammonium peroxodisulfate and toluene-4-sulfonic acid monohydrate (TSA) (Fluka, Switzerland), were used as delivered. Polyaniline (emeraldine) hydrochloride was prepared by oxidation of aniline with ammonium peroxodisulfate and deprotonated to PANI base with ammonium hydroxide [17]. Buckminsterfullerene C60 (>98%) purified by HPLC was provided by the Ioffe Physico-Technical Institute, Russian Academy of
Preparation of PANI–C60 composites
The formation of PANI–C60 complex faces the problem to disperse both the components to a molecular level. The solubility of both the substances is limited. The only generally used solvent for PANI base, NMP, contains amino groups, which interact with C60. We have observed slow irreversible changes in the visible spectrum of C60 dissolved in NMP. The competitive reaction of C60 with NMP may therefore complicate the doping interaction of C60 with PANI base. That is why another method of PANI–C60
Conclusions
The composites were prepared by the solid-state blending of polyaniline base with fullerene C60 (composite I) or by the introduction of fullerene into the reaction mixture used for the preparation of PANI (composite II). Both the NMR and FTIR spectra suggest the interaction between fullerene and the imine groups of PANI base in the composites. X-ray diffractograms indicate a decrease in the size of fullerene crystallites in composite I, and the doping of PANI with fullerene and the formation of
Acknowledgements
Support of the Russian Federal Programme “Topical problems in physics of condensed matter (Polymer-2)”, Ministry of Education of the Czech Republic (Project VZ 113 2000) and of the Grant Agency of the Academy of Sciences of the Czech Republic (A4050907) is gratefully acknowledged.
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