Chemistry Reference
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polymer film: longer UV-exposure time generates more thermal heat in the film.
Thus, a high exposure dose, which needs relatively long exposure time, gives a
chance as well as a thermal energy to the polymer chains to mobilize. The mobile
polymers chains allow the coumarin side groups to reorient favorably to LPUVL
to some extent and involve in the photoreaction, even though they are initially in
the direction perpendicular to the electric vector of LPUVL, leading to a declina-
tion in both the dichroic ratio and optical retardation. Such declination in the opti-
cal retardation is always observed for films of polyvinyl derivatives with cin-
namoyl moiety which also exhibit relatively low T
g
s because of their flexible
polymer backbone [9-11, 17, 18].
3.3. LC alignment on films subjected to UV exposure
Figure 7 shows polar diagrams of LC cells fabricated with 6F-HAB-CI films sub-
jected to LPUVL of 0.25 J/cm
2
exposure dose through 0
°
(
θ
)- and 90
°
(
θ
)-exposure
in a sequential manner. The first 0
)-exposure generates a very weak anisotropy
in the polar absorbance diagram. The alignment of LC molecules is very weak
along the direction of
°
(
θ
-exposure enhances
slightly the development of anisotropy in the polar diagram, but changes the di-
rection of LC alignment to
ϕ
= 150
°
= 330
°
. The subsequent 90
°
ϕ
= 176
°
= 356
°
. The LC alignment is changed again
to
ϕ
= 135
°
= 315
°
by the subsequent 0
°
(
θ
)-exposure. The LC alignment is moved
again to the direction of
ϕ
= 173
°
= 353
°
by the subsequent 90
°
(
θ
)-exposure. Fur-
ther 0
.
In this case, the anisotropy in the polar diagram is very distinctive, indicating that
a relatively high degree of LC alignment had taken place along the direction of
°
(
θ
)-exposure turns again the direction of LC alignment to
ϕ
= 102
°
= 282
°
ϕ
= 102
. Thereafter, the direction of LC alignment varies less sensitively
with further sequential 0
°
= 282
°
)-exposures. Finally, the principal director
of LC alignment is settled along the direction of
°
(
θ
)- and 90
°
(
θ
ϕ
= 99
°
= 279
°
, regardless of the
history of the sequential 0
)-exposures.
The initially observed large variations in the director of LC alignment may be
attributed to the relatively low energy dose per UV-exposure. The exposure dose
of 0.25 J/cm
2
seems to be too low to induce the photoreaction of cinnamoyl side
groups in a high yield. In fact, 0.25 J/cm
2
is just half of the minimum exposure
dose (0.50 J/cm
2
) needed for the photoreaction of cinnamoyl moieties in the film
to give maximum dichroic ratio and optical retardation as aforementioned. Thus,
each exposure causes the directional photoreaction of cinnamoyl side groups to a
relatively low extent, leading to a degree of reorientation of polymer chains and
side groups in the film. This situation is directly reflected in the induction of the
alignment of LC molecules in the cell, consequently causing a strong dependency
of LC alignment on the history of the sequential 0
°
(
θ
)- and 90
°
(
θ
°
(
θ
)- and 90
°
(
θ
)-exposures.
This dependency is large at the early stage of the sequential 0
)-
exposures because the number of unreacted cinnamoyl moieties on the surface is
relatively high. Then, the dependency becomes small with increasing number of
°
(
θ
)- and 90
°
(
θ
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