Characterization of Reaction Intermediate Aggregates in Aniline Oxidative Polymerization at Low Proton Concentration Zhongfen Ding,* ,† Timothy Sanchez, ‡ Andrea Labouriau, § Srinivas Iyer, ‡ Toti Larson, | Robert Currier, ⊥ Yusheng Zhao, # and Dali Yang* ,§ Materials Physics and Applications DiVision, Bioscience DiVision, Materials Science and Technology DiVision, Earth and EnVironmental Science DiVision, Chemistry DiVision, and Los Alamos Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ReceiVed: March 23, 2010; ReVised Manuscript ReceiVed: June 30, 2010 Aggregates of reaction intermediates form during the early stages of aniline oxidative polymerization whenever the initial mole ratio of proton concentration to aniline monomer concentration is low ([H + ] 0 /[An] 0 e 1.0). Detailed characterization is carried out on those aggregates. The intermediate aggregates show a UV-Vis absorption peak at around 410 nm when dispersed in aqueous solution, whereas the peak is centered on 370 nm when dissolved in an organic solvent such as N-methylpyrrolidone. The electronic band gap decreases when the intermediates aggregate to form a solid, and thus, the absorption peak is red-shifted. Gel permeation chromatography (GPC) shows the aggregates contain a major low molecular weight peak with a long tail. The oligoanilines with low molecular weights consistently show a UV-Vis absorption peak at around 370 nm. Mass spectrometry confirms that the intermediate aggregates contain mainly a component with mass number 363 (M + H + ), likely a tetramer. UV-Vis, GPC, mass spectrometry, NMR, FTIR, and XRD characterization results are presented and chemical structures for the tetramer are proposed. The major components of the intermediate aggregates are likely highly symmetric phenazine- and dihydrophenazine- containing structures. These particular organic compounds have not been identified before as intermediates. The aggregation and precipitation of the tetramers apparently stabilizes these intermediates. The aggregates are highly crystalline, as evidenced by powder X-ray diffraction. A new reaction mechanism for the formation of these intermediates is proposed. Introduction Structured materials made from the conducting polymer polyaniline (PANI) have unique properties associated with their high surface area and ease of processing. The applications include antistatic and anticorrosion coatings, 1-3 sensors, 4,5 supercapacitors, 6,7 microelectronics, 8,9 batteries, and gas storage. 10,11 Thus, the link between synthesis conditions and PANI morphol- ogy has been investigated intensely in recent years. Factors known to affect PANI morphology include reaction temperature, aniline concentration, acid concentration, reactant ratios, me- chanical disturbances, seeding strategy, and dopants. 12-15 Aniline oxidative polymerization reaction can be carried out at different acid conditions, ranging from pH ) 0 to pH ) 11. 16,17 In recent years, it was discovered that aniline polym- erization in aqueous solutions of a variety of weak acids or in water leads to PANI nanotube formation. 13,14,18,19 We recently showed that the initial proton-to-aniline mole ratio ([H + ] 0 /[An] 0 ) affects PANI nanomorphologies and found that the formation of detectable intermediate aggregates is directly related to PANI nanotube formation, 20 consistent with the observations by others. 21 We found that the [H + ] 0 /[An] 0 ratio is important in the polymerization kinetics due to aniline protonation (its conjugated acid has a pK a of 4.87). 22 Whether the aniline monomer is fully protonated appears to determine the initial reaction pathway. Protonated aniline is slow to lose an electron to form a radical cation, whereas free aniline loses the electron easily. When [H + ] 0 /[An] 0 > 1.0, the intermediate aggregates were not detectable by UV-Vis, as was the case for the traditional PANI synthesis in strong acids. 23 When [H + ] 0 /[An] 0 e 1.0, unprotonated aniline was oxidized quickly to form the inter- mediate aggregates that precipitated from the reaction suspen- sion, thus slowing down further reaction. 20 These intermediate aggregates can form tetragonal rods when aggregating slowly or platelets when aggregation is fast. 20 By tuning the initial [H + ] 0 /[An] 0 ratio, we slowed aggregation of the intermediates and demonstrated synthesis of nanotubes with rectangular cross sections. 24 Aniline polymerization in pH-static conditions 25 differs from our reaction system in which pH naturally decreases as reaction proceeds. At the initial stage, the pH drops as aniline is oxidized in our system. When [H + ] 0 /[An] 0 < 1.0, there is a very fast step during which intermediates form, followed by a decrease in the rate of intermediates formation. Under the pH-static condition, formation of the intermediates remains constant for a significant amount of time. Nevertheless, the pH condition for both the pH-static method and our simple synthesis system correlates well. 25 Specifically, [H + ] 0 /[An] 0 ) 1.0 corresponds to a point between pH ) 2 and pH ) 3 regarding the initial free aniline * To whom correspondence should be addressed. (Z.D.) Address: MPA- 11, MS D429, Los Alamos National Laboratory, Los Alamos, NM 87545. Fax: +1 505 665 4292. E-mail: zding@lanl.gov. (D.Y.) Address: MST-7, MS E549, Los Alamos National Laboratory, Los Alamos, NM 87545. Fax: +1 505 667 8109. E-mail: dyang@lanl.gov. † Materials Physics and Applications Division. ‡ Bioscience Division. § Materials Science and Technology Division. | Earth and Environmental Science Division. ⊥ Chemistry Division. # Los Alamos Neutron Scattering Center. J. Phys. Chem. B 2010, 114, 10337–10346 10337 10.1021/jp102623z  2010 American Chemical Society Published on Web 07/26/2010