Results and discussion
3.1. Characterization of PANI pigments
The conductivity of PANI pigments was measured by the fourprobe
method and the values obtained were 0.76, 4 and 1.3 S cm−1
for PANI–BS, PANI–LS1 and PANI–LS2, respectively. Fig. 1 shows the
FTIR spectra of PANI doped with benzene sulfonic acid (PANI–BS)
and lignosulfonic acid (PANI–LS1 and PANI–LS2). The ring stretch
of the quinoid form N Q N is observed as a broad band centered
around 1570 cm−1, while the ring stretch of the benzenoid form
N–B–N appears as a broad band centered around 1490 cm−1. These
bands present similar intensities which agrees qualitatively to the
50% oxidized emeraldine form. The band at 1242 cm−1 is interpreted
as a C–N•+ stretching vibration in the polaron structure
[26]. The long absorption tail above 2000 cm−1 (only the beginning
is shown in Fig. 1) [27] and the shoulder around 1150 cm−1
(interpreted as a vibration mode of the –NH•+ structure [28]) can
also be assigned to vibration modes specific to protonated emeraldine.
A small band is seen at 1030 cm−1 for PANI–BS, PANI–LS1 and
PANI–LS2 which corresponds to the S O stretch. This is confirmed
by the absence of that band in an HCl doped polyaniline (also shown
in Fig. 1). The high absorbance of the 1030 cm−1 band for ES–LS1
is in line with its high conductivity and with the presence of two
sulfo-dopants (LS and PTS).