pubs.acs.org/Macromolecules Published on Web 11/24/2010 r 2010 American Chemical Society 10406 Macromolecules 2010, 43, 10406–10413 DOI: 10.1021/ma101474j Oxidation of Aniline with Silver Nitrate Accelerated by p-Phenylenediamine: A New Route to Conducting Composites Patrycja Bober,* ,† Jaroslav Stejskal, † Miroslava Trchov a, † Jan Proke  s, ‡ and Irina Sapurina § † Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic, ‡ Faculty of Mathematics and Physics, Charles University Prague, 182 00 Prague 8, Czech Republic, and § Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg 199004, Russian Federation Received July 2, 2010; Revised Manuscript Received October 27, 2010 ABSTRACT: The reaction between two nonconducting compounds, aniline and silver nitrate, yields a composite of two conducting products, PANI and silver. While the oxidation of aniline with silver nitrate is slow and takes over several months, the addition of a small amount of p-phenylenediamine, 1 mol % relative to aniline, shortens the reaction time to a few hours and, with higher concentrations of p-phenylenediamine, even to tens of minutes. Nonconducting aniline oligomers, however, are also present in the oxidation products as a rule. The chemistry of individual oxidation pathways is discussed. Higher concentrations of p-phenylenediamine in the reaction mixture with aniline give rise to copolymers, poly[aniline-co-( p- phenylenediamine)]s, and their composites with metallic silver. p-Phenylenediamine alone can similarly be oxidized with silver nitrate to poly(p-phenylenediamine) composite with silver. Silver is present in the composites both as nanoparticles of ∼50 nm size and as larger objects. The composites have conductivity in the range of the order of 10 -3 -10 3 S cm -1 at comparable content of silver, which was close to the theoretical expectation, 68.9 wt %. The composites prepared in 1 M acetic acid always have a higher conductivity compared with those resulting from synthesis in 1 M nitric acid. The polymerizations of aniline accelerated with 1 mol % of p-phenylenediamine in 1 M acetic acid yield a composite of the highest conductivity, 6100 S cm -1 . At higher contents of p-phenylenediamine, poly[aniline-co-( p-phenylenediamine)] composites with silver have a conductivity lower by several orders of magnitude. The oxidation of p-phenylenediamine alone with silver nitrate in 1 M acetic acid also yields a conducting composite, its conductivity being 1750 S cm -1 . The semiconductor type of conductivity in polymers and the metallic type of conductivity in silver may compensate to yield composites with conductivity nearly independent of temperature over a broad temperature range. Introduction Polyaniline (PANI), probably the most studied conducting polymer, is currently prepared by the oxidation of aniline with ammonium peroxydisulfate (APS) in acidic aqueous media, 1 its typical conductivity being of the order of the units of S cm -1 . One of the strategies to increase the conductivity is based on the incorporation of noble metals, such as silver. When silver nitrate is used as the oxidant of aniline, a composite of PANI and silver is directly obtained 2-7 (Figure 1). Such composites combine the metallic conductivity of silver and the semiconductor charge transport in PANI. The latter component is expected to introduce materials features of polymers, especially their mechanical pro- perties. 8 Nonconducting aniline oligomers have often constituted part of the oxidation products. 4,6 Despite this fact, composite conductivities were high, 4,6 exceeding 1000 S cm -1 . The potential usefulness of such composites as new conducting materials is seen in the design of flexible electronics, conducting inks, sensors, electrodes, etc. The oxidation of aniline with APS in an acidic aqueous medium is completed within tens of minutes with the currently used concentrations of reactants, 1 and PANI is collected as a precipitate. Depending on the acidity conditions, the morphology of PANI can vary from granules, nanofibers, nanotubes, to microspheres. 9,10 The oxidations of aniline using silver nitrate are much slower, 4,6 and more than one month is needed to obtain an appreciable yield of polymer; this is hardly acceptable for the routine syntheses of conducting composites. The reaction between aniline and silver nitrate was acceler- ated by an increase in temperature to 250 °C, 11 with UV- irradiation 5,7,12,13 or γ-irradiation and sonication. 2,3 The absorp- tion maximum at 630-640 nm in the UV-vis spectra, which is typical of the PANI (emeraldine) base, 14 however, has often been suppressed or even absent. 2,3,15,16 This means that the oxidation products were composed mainly of nonconducting aniline oligo- mers, 17-19 rather than of PANI. 10 The conductivity of such composites was below 0.1 S cm -1 and, in spite of the presence of silver, even lower than the conductivity of PANI prepared by ordinary oxidation using APS, 1 ∼4 S cm -1 . Composites con- tained a large fraction of aniline oligomers. 4,6 Only in two cases, high conductivities exceeding 1000 S cm -1 have recently been reported. 4,6 It is well-known that the oxidation of aniline with APS is accelerated by small quantities of p-phenylenediamine (PDA) in both the chemical 20-24 and electrochemical 25-27 oxida- tions of aniline. This approach has successfully been applied in the present work to the oxidations of aniline with silver nitrate. The ability of aniline to copolymerize with PDA has been reported in several studies when APS was used as the oxidant 28-34 as well as in electrochemical preparations. 35,36 The formation of copolymers is also discussed in the present communication when the aniline and PDA have been used in comparable proportions and oxidized with silver nitrate to poly[aniline-co-(p-phenylene- diamine)]. *To whom correspondence should be addressed.