EPR and charge-transport studies of polyaniline
V. I. Krinichnyi
Institute of Chemical Physics in Chernogolovka of Russian Academy of Sciences, Chernogolovka, 142432 M.R., Russia
S. D. Chemerisov and Ya. S. Lebedev
N. N. Semenov Institute of Chemical Physics of Russian Academy of Sciences, 117338 Moscow, Russia
Received 10 December 1996; revised manuscript received 5 February 1997
The study of dc and microwave 140 GHz electrical conductivities using multifrequency electron-spin
resonance in undoped and HCl-doped polyaniline is reported. The accidental quasi-three-dimensional 3D
charge hopping between the pinned and mobile small polarons dominates the bulk conductivity of the emer-
aldine base form of polyaniline. The increase in mobility and the number of excitations upon light doping of
the polymer leads to the isoenergetic interpolaron charge hopping between the polaron and bipolaron states. 1D
variable-range hopping of a charge between conducting islands, which correlates with a superslow torsional
dynamics of the polymer chains, dominates bulk conductivity of heavily doped polyaniline at low tempera-
tures. Intrinsic microconductivity is determined by the interaction of the charge with the lattice phonons at high
temperatures. Following Epstein and MacDiarmid we propose that emeraldine salt of polyaniline represents a
1D disordered conducting compound consisting of metal-like islands of well coupled chains with 3D delocal-
ized charge carriers. S0163-18299700524-9
I. INTRODUCTION
The electronic and magnetic properties of disordered
quasi-one-dimensional 1D semiconductors have been ex-
tensively investigated over the past decades.
1–5
The organic
conducting polymers, the electrical conductivity of which
can be varied up to the metallic state by doping in the range
of more then ten orders of magnitude, is the most interesting
class of 1D materials.
2
In contrast to usual semiconductors, a
charge is transferred by the nonlinear topological excitations
formed in the chains as a result of Peierls instability,
3
namely, solitons in trans-polyacetylene trans-PA and po-
larons or bipolarons in poly( p -phenylenePPP and other
PPP-like polymer semiconductors.
4
The specific nature of
such carriers is the reason for unusual charge transport be-
havior of these organic conducting polymers.
Polyacetylene, the simplest conducting polymer, was
studied thoroughly.
5–7
To explain the experimental results on
the temperature, pressure, and frequency dependencies of
electrical conductivity of the lightly doped trans-PA, Kivel-
son proposed a model,
8
which assumes interchain transport
as charge hopping between neutral and charged soliton states
at isoenergetic levels. This model was then successfully used
by Epstein
6
for the interpretation of charge transfer in lightly
doped trans-PA samples. As the doping level increases,
isoenergetic charge hopping is replaced by tunneling or hop-
ping between neighboring highly conducting islands
9
in the
framework of the Sheng’s
10
and Mott’s variable-range
hopping
11
VRH models. The highest room-temperature
RT conductivity of 10
5
S/cm was achieved for iodine
doped and stretch-oriented trans-PA.
12
However, this value
is by one to two orders of magnitude lower than that pre-
dicted by Kivelson and Heeger for a metal-like clusters in the
polymer.
13
The electrical and magnetic properties of doped PPP-like
polymers are generally similar to those of trans-PA.
2,14,15
In
contrast with PA, these polymers do not possess a degenerate
ground state
16
and, therefore, they are not expected to ac-
commodate single solitons. However, Bre
´
das et al.
17
have
shown that soliton-antisoliton pairs in the form of polarons
and bipolarons could be stabilized in doped PPP. Moreover,
Kivelson proposed
18
that the isoenergetic charge transfer
might be important not only for it trans-PA, but also for
another conducting polymers possessing solitonlike excita-
tions. Indeed, Kuivalainen et al.
19
have shown that the above
mechanism plays an important role in both dc and micro-
wave 25 GHz conductivities of lightly doped PPP. As in
the case of trans-PA, the VRH was shown experimentally
see, e.g., Refs. 19–22 to be mainly applied also for an
interpretation of the conducting properties of different medi-
ally and highly doped PPP-like polymers.
In contrast with trans-PA and PPP-like conducting poly-
mers, the chains of polyaniline PANI contain nitrogen het-
eroatoms involved in a conjugation.
15
Moreover, benzene
rings of PANI can rotate or flip, modulating strong electron-
phonon interactions.
23
This results in somewhat of a differ-
ence in magnetic and charge-transport properties of PANI
compared with other conducting polymers. An analysis of
experimental data on the temperature dependencies of dc
conductivity, thermoelectric power, and Pauli-like suscepti-
bility allowed MacDiarmid, Epstein et al.
24
to show that the
emeraldine base form of PANI PANI-EB is a completely
amorphous insulator in which 3D granular metal-like clusters
are formed in the course of its transformation into the emer-
aldine salt form of the polymer PANI-ES. A more detailed
study of the complex microwave dielectric constant, EPR
linewidth, and electric field dependence of conductivity of
PANI-ES Refs. 20, 22, and 25 allowed them to conclude that
both chaotic and oriented PANI-ES consist of some parallel
chains strongly coupled into ‘‘metallic bundles’’ between
which 1D VRH charge transfer occurs and in which 3D elec-
tron delocalization takes place. The intrinsic conductivity of
PHYSICAL REVIEW B 15 JUNE 1997-II VOLUME 55, NUMBER 24
55 0163-1829/97/5524/1623312/$10.00 16 233 © 1997 The American Physical Society