TeMPD films prepared without LPUV irradiation showed no photoalignment characteristics, whereas the 6FDA-TeMPD films irradiated with LPUV above 1 J/cm
2
exhibited distinct photoalignment. The LC molecules were oriented in the direction
of the LPUV irradiation. However, no significant difference was observed between
the photoalignments of the films with the highest and lowest photoreactivities under
LPUV irradiation higher than 1 J/cm
2 . Figures 14 and 15 show that the polymer
cohesiveness of the DMAc 100
C 6FDA-TeMPD film decreased at 1 J/cm
2 LPUV
irradiation. However, this film did show photoalignment characteristics. Therefore,
the photoalignment characteristics were determined by the chemical structure and
did not depend on polymer cohesiveness.
No difference was observed between the photoalignment characteristics of the
different photoreactive films. Moreover, the photoalignment sensitivity was determined by comparing the two 6FDA-TeMPD films irradiated at 0.5 J/cm
2 LPUV. The
THF 250
C 6FDA-TeMPD film exhibited photoalignment characteristics. However, the photoalignment exhibited by the DMAc 100
C 6FDA-TeMPD film was
low. The photoalignment sensitivity increased as the photoreactivity increased. In
addition, the photoalignment sensitivity did not depend on polymer cohesiveness but
rather on the residual solvent trace in the film. The 6FDA-TeMPD film prepared
under low volatility preparation conditions exhibited photoalignment characteristics
because the residual solvent disrupted the cross-linking polymer segments through
photoreaction. This result indicates that photoalignment sensitivity can be increased
by reducing the residual solvent in the polymer film.
Photoalignment Characteristics of CT Complex-Type Polyimides
with Tetramethyl Phenylene Diamine
In the 6FDA-TeMPD polyimide, the diamine component, TeMPD, generally has a
high electron-releasing property, whereas the acid component, 6FDA, has an
electron-accepting property. A CT transition can take place because of the transition
from TeMPD to the 6FDA site. In general, the behavior is known to occur because of
the formation of CTCs or CT interaction in polyimides. The CTCs are formed
between the polymer chains in 6FDA-TeMPD polyimide. Based on our previous
study, we expected that the CT interaction between polymer segments strongly
depended on the photoreaction upon LPUV irradiation.
However, no direct evidence for these polyimides formed CTCs has been presented
in the literature. The energies of the LUMO, e LUMO , of the acid component and those
of the HOMO, e HOMO , of the diamine component depend on CTC formation (Choi
et al. 2009). The e LUMO and e HOMO are measures of the electron-accepting and
electron-releasing properties observed during CTC formation, respectively. As the
e LUMO decreases and the e HOMO increases, it is easy to form CTCs because of the lower
transition energy between HOMO and LUMO. Based on the MO calculation, it can be
concluded that TeMPD shows high e HOMO in 6FDA-TeMPD polyimides (À8.71 eV).
In the current study, the photoalignment characteristics of TeMPD-based polyimides with various e LUMO of the acid component were systematically investigated.
512
S. Sato et al.
2
exhibited distinct photoalignment. The LC molecules were oriented in the direction
of the LPUV irradiation. However, no significant difference was observed between
the photoalignments of the films with the highest and lowest photoreactivities under
LPUV irradiation higher than 1 J/cm
2 . Figures 14 and 15 show that the polymer
cohesiveness of the DMAc 100
C 6FDA-TeMPD film decreased at 1 J/cm
2 LPUV
irradiation. However, this film did show photoalignment characteristics. Therefore,
the photoalignment characteristics were determined by the chemical structure and
did not depend on polymer cohesiveness.
No difference was observed between the photoalignment characteristics of the
different photoreactive films. Moreover, the photoalignment sensitivity was determined by comparing the two 6FDA-TeMPD films irradiated at 0.5 J/cm
2 LPUV. The
THF 250
C 6FDA-TeMPD film exhibited photoalignment characteristics. However, the photoalignment exhibited by the DMAc 100
C 6FDA-TeMPD film was
low. The photoalignment sensitivity increased as the photoreactivity increased. In
addition, the photoalignment sensitivity did not depend on polymer cohesiveness but
rather on the residual solvent trace in the film. The 6FDA-TeMPD film prepared
under low volatility preparation conditions exhibited photoalignment characteristics
because the residual solvent disrupted the cross-linking polymer segments through
photoreaction. This result indicates that photoalignment sensitivity can be increased
by reducing the residual solvent in the polymer film.
Photoalignment Characteristics of CT Complex-Type Polyimides
with Tetramethyl Phenylene Diamine
In the 6FDA-TeMPD polyimide, the diamine component, TeMPD, generally has a
high electron-releasing property, whereas the acid component, 6FDA, has an
electron-accepting property. A CT transition can take place because of the transition
from TeMPD to the 6FDA site. In general, the behavior is known to occur because of
the formation of CTCs or CT interaction in polyimides. The CTCs are formed
between the polymer chains in 6FDA-TeMPD polyimide. Based on our previous
study, we expected that the CT interaction between polymer segments strongly
depended on the photoreaction upon LPUV irradiation.
However, no direct evidence for these polyimides formed CTCs has been presented
in the literature. The energies of the LUMO, e LUMO , of the acid component and those
of the HOMO, e HOMO , of the diamine component depend on CTC formation (Choi
et al. 2009). The e LUMO and e HOMO are measures of the electron-accepting and
electron-releasing properties observed during CTC formation, respectively. As the
e LUMO decreases and the e HOMO increases, it is easy to form CTCs because of the lower
transition energy between HOMO and LUMO. Based on the MO calculation, it can be
concluded that TeMPD shows high e HOMO in 6FDA-TeMPD polyimides (À8.71 eV).
In the current study, the photoalignment characteristics of TeMPD-based polyimides with various e LUMO of the acid component were systematically investigated.
512
S. Sato et al.