New photoalignable polyimides and their ability to control liquid-crystal alignment - Polyimides and Other High Temperature Polymers - page 112

ess has some shortcomings, such as dust generation, electrostatic problems, and
poor control of rubbing strength and uniformity. In order to overcome the short-
comings in the rubbing process, several photoinduced LC alignment concepts us-
ing linearly polarized ultraviolet light (LPUVL) irradiation have recently been
proposed [9-18]. These approaches have attracted considerable attention from
both academic and industrial fields because of the capability of the rubbing-free
production of LC aligning films. Nevertheless, these approaches still are not
workable for the mass production of LCDs because of a number of unsolved
problems, such as low thermal stability, low anchoring energy, low pretilt angle,
limited processibility with ultraviolet (UV) light exposure, and unavailability of
proper materials. Thus, there still remains a big challenge to deliver high per-
formance materials suitable for processing LC aligning films with eliminating the
rubbing step. A representative class of photoalignment materials is poly(vinyl
cinnamate) (PVCi) and its derivatives [9-19]. However, such polymers have rela-
tively very low glass transition temperature (T g ). Because of the low T g , even the
polymer chains in the film are oriented preferentially by exposure to LPUVL, but
they are still mobile so that such chain orientation may not be stable enough to-
wards environmental influences such as temperature, aging time, and so on. Fur-
thermore, their photoalignment mechanism has not been fully understood yet.
In this study, new photosensitive polyimides with cinnamate and coumarin side
groups were synthesized, which are stable both thermally and dimensionally be-
cause of their high T g . Their photoreactivity and photoalignment characteristics
were investigated, and UV-exposure and rubbing processibility were examined. In
addition, the alignment behavior of LC molecules was investigated on films
treated by various LPUVL exposures, rubbing, and their combinations.

2. EXPERIMENTAL

2.1. Synthesis
The polyimide containing hydroxyl side groups, 6F-HAB was synthesized from
polycondensation of purified 2,2'-bis-(3,4-dicarboxyphenyl)hexafluoropropane
dianhydride (6F) (Chriskev Company, USA) and 3,3'-hydroxy-4,4'-
diaminobiphenyl (HAB) (Chriskev Co.) as follows. Equivalent moles of 6F and
HAB monomers were dissolved together with two equivalent moles of isoquino-
line catalyst in dry N-methyl-2-pyrrolidone (NMP). The solution was gently
heated at 70

C for 2 h and then refluxed for 5 h. Thereafter, the reaction solution
was poured into a mixture of methanol and water (6:4 volume ratio) with vigorous
stirring, giving 6F-HAB polyimide in the form of precipitated powder. The pre-
cipitated powder was filtered and dried. The synthesized polyimide was dissolved
in dimethyl-d 6 sulfoxide (DMSO-d 6 ), and characterized using a proton nuclear
magnetic resonance ( 1 H NMR) spectrometer (Bruker Aspect 300 MHz). Its mo-
lecular weight was estimated using a gel permeation chromatography (GPC) sys-

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