Chemistry Reference
In-Depth Information
In field-effect transistors based on the thin films of poly(3-alkyl)thiophene
derivatives, high field-effect mobility has been observed because of the for-
mation of crystal-like grains [116]. The mobility depends strongly upon the
crystallinity of the thin films. Introduction of liquid crystallinity into conju-
gated polymers is expected to be effective for enhancing microscopic crys-
tallization of conjugated polymers to increase their carrier mobilities. It can
also cause anisotropy in various physical properties if macroscopic molecular
alignment is achieved.
Conjugated polymers having mesogenic groups in their side chains have
been synthesized by the Akagi group [117]. In particular, polyacetylenes hav-
ing mesogenic side chains exhibited a high anisotropy in electrical conduc-
tivity. In polyacetylenes bearing cyclohexylphenyl groups in the side chains,
macroscopically aligned film can be fabricated under application of a mag-
netic field. In the iodine-doped aligned film, conductivity along the main
chain was 1.5
10
-6
Scm
-1
while that perpendicular to the main chains was
×
10
-11
Scm
-1
[118].
Poly(9,9-dialkylfluorene) exhibits a nematic phase above 160
◦
Cand
a uniaxially aligned nematic film can be fabricated on a rubbed substrate.
This uniaxally aligned nematic state can be quenched by cooling it below
75
◦
C [119]. The hole mobility was determined to be 9
1.4
×
10
-3
cm
2
V
-1
s
-1
at room temperature by the time-of-flight method, while it was on the
order of 10
-5
cm
2
V
-1
s
-1
in the isotropic film fabricated by the spin-coating
method [119]. Carrier transport is remarkably enhanced by nematic ordering
of polyfluorene chains. Amplified spontaneous emission from this polymer
and the related compounds are now being studied [120].
The liquid crystallinity of poly(3-alkylthiophene) has been pointed
out [121]. Enhancement of crystallinity by molecular alignment in a liquid
crystal phase is prominent in poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-
b]thiophene) exhibiting a lamellar mesophase. When a thin film of the
polymer fabricated by the spin-coating method is annealed at 100
◦
C, do-
main size remarkably extends from several tens of nanometers to a several
micrometers, maintaining molecular alignment within domains because of
the thermal motion of the polymer chains. The hole mobility of the thin film
transistors based on this polymer reached 0.7 cm
2
V
-1
s
-1
[122].
×
4
Conclusion
Liquid-crystalline nanostructures formed by polymeric materials have great
potential to be applied for ion- and electron-conducting materials because ef-
ficient and low-dimensional conduction can be achieved in nano-segregated
LC phases. Recent progress in supramolecular chemistry and nanotechnology
enables the design of new materials for ion conductors. Nematic LC poly-
Search WWH ::
Custom Search