The courses of aniline oxidation with ammonium peroxydisulfate in aqueous solutions of strong (sulfuric) and in weak (acetic) acids, followed by temperature and acidity changes, are different. In solutions of sulfuric acid, granular polyaniline (PANI) was produced; in solutions of acetic acid, PANI nanotubes were obtained. The external diameter of the nanotubes was 100-300 nm, the internal cavity 20-100 nm, and the length extended to several micrometres. The morphology of PANI, granular or tubular, depends on the acidity conditions during the reaction rather than on the chemical nature of the acid. PANI nanotubes were also produced when aniline was oxidized in the absence of any acid. The bulk conductivity of PANI prepared in solutions of acetic acid was 0.08-0.27 S cm −1 , depending on the acid concentration. Protonated PANI prepared in sulfuric and acetic acids were deprotonated with ammonium hydroxide to obtain PANI bases and the ammonium salt of the protonating acid. FTIR spectroscopy showed the differences in the molecular structure of the PANI bases. Irrespective of whether the polymerization was performed in solutions of sulfuric or acetic acid, PANI had hydrogen sulfate counter-ions only. The PANI morphology is thus not controlled by the nature of counter-ions. The acidity of the reaction medium determines the protonation of monomer, oligomer and polymer species. The chemistry of aniline oxidation is likely to be affected especially by the protonation of an intermediate in the pernigraniline form. It is proposed that, in the course of aniline oxidation, pH-dependent self-assembly of aniline oligomers predetermines the final PANI morphology.
Aniline was polymerized in the presence of ultrafine colloidal
silica particles (hydrodynamic
diameter 35 nm) in aqueous media and, given a sufficient silica
concentration, colloidally stable
polyaniline−silica particles were obtained. The particle size of
the resulting dispersions was determined
by both dynamic light scattering and disk centrifuge
photosedimentometry. The typical size of these
polyaniline−silica particles is in the range 300−600 nm and is
insensitive to the concentration of
components in the reaction mixture, temperature, or acidity of the
reaction mixture. Particles produced
at 0 °C are spherical and their shape becomes less defined when
polymerization proceeds at 25 °C. As
the silica size is increased, the composite particles become larger and
their raspberry morphology more
distinct. The electrical conductivity of a typical polyaniline
(37.8 wt%)−silica composite is 6.1 × 10-2
S
cm-1 at 25 °C. It grows with increasing
temperature and does not depend on the frequency in the
investigated range from 20 Hz to 1 MHz.
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