Chemistry Reference
In-Depth Information
exposure with an energy of 1.50 J/cm
2
(see the CU-1.50-#1 in Figure 12).
These results provide us some information as follows. First, for the polymer
films treated by 90
)-rubbing, the LPUVL ex-
posure process does not seem to make any contribution to the LC alignment on
the film surface. This appears distinctively for the LC cell fabricated with 6F-
HAB-CI film. This might result from the preferential reorientation of the polymer
chains and side groups, including the photoaligned polymer chains and side
groups, along the rubbing direction through the 90
°
(
θ
)-exposure and subsequent 90
°
(
θ
)-rubbing process. That is,
some fractions of polymer chains and side groups are oriented first along the di-
rection with an angle of 97-99
°
(
θ
°
with respect to the polarization of LPUVL
through the 90
)-exposure process but they reorient along the rubbing direction
by the subsequent 90
°
(
θ
)-rubbing process. Conclusively, the LC alignment is con-
trolled mainly by the surface texture generated through the 90
°
(
θ
°
(
θ
)-rubbing proc-
ess.
Second, for the polymer film treated by 90
)-
exposure, the surface textures created by both processes may involve in a competi-
tive manner to induce the alignment of LC molecules on the film surface. Perhaps,
the microgrooves generated by the rubbing process are well maintained through
the subsequent 90
°
(
θ
)-rubbing and subsequent 90
°
(
θ
)-exposure of linearly polarized UV light. However, the pho-
tosensitive side groups lain parallel to the electric vector of the polarized actinic
UV light undergo photoreaction selectively by the 90
°
(
θ
)-exposure process, re-
gardless of whether they are localized on the microgrooved or non-microgrooved
surfaces of the film. Consequently, the 90
°
(
θ
)-exposure reorients the polymer
chains and side groups of the rubbed film surface in part towards the direction with
an angle of 97-99
(
°
θ
with respect to the rubbing direction. In comparison, the frac-
tions of the photoaligned polymer chains and side groups may be much smaller
than those of the non-photoaligned polymer chains and side groups, because of the
directionally selective photoreaction to the polarized light and its relatively low
quantum yield. Considering these factors (namely, the directionally maintained
microgrooves and the relatively high fraction of non-photoaligned polymer chains
including side groups), LC molecules on the film should be aligned preferentially
along the rubbing direction rather than the photoalignment direction.
Finally, mechanical rubbing process generally generates a relatively high
roughness as well as microgrooves along the rubbing direction on polymer films.
Of course, the surface roughness and the dimension of the directional micro-
grooves are dependent upon the rubbing density as well as the kind of velvet
cloth. On the contrary, LPUVL exposure process generates relatively smooth sur-
face in the polymer film. Considering these surface textures, the measured polar
diagrams indicate that the relatively high surface roughness consisting of direc-
tionally aligned microgrooves plays a major role in the alignment of LC mole-
cules in the cells, as compared to the photoaligned polymer chains and their re-
lated surface characteristics.
°
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