Photo-sensitive polyimide containing methoxy cinnamate derivatives on photo-alignment of liquid crystal

Article abstract of DOI:10.1080/15421400903582881

Photosensitive polyimide containing methoxy cinnamate derivatives as a photo-alignment layer is investigated. 4-Methoxy cinnamic acid, 2-methoxy cinnamic acid and 2,5-dimethoxy cinnamic acid were irradiated and the absorption spectra were obtained with ir

Full text of DOI:10.1080/15421400903582881

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Photo-Sensitive Polyimide Containing
Methoxy Cinnamate Derivatives on
Photo-Alignment of Liquid Crystal
Su Young Kim a , Sung Eui Shin a & Dong Myung Shin a
a Department of Chemical Engineering, Hongik University, Seoul,
Korea
Version of record first published: 19 Apr 2010
To cite this article: Su Young Kim, Sung Eui Shin & Dong Myung Shin (2010): Photo-Sensitive Polyimide
Containing Methoxy Cinnamate Derivatives on Photo-Alignment of Liquid Crystal, Molecular Crystals
and Liquid Crystals, 520:1, 116/[392]-121/[397]
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Mol. Cryst. Liq. Cryst., Vol. 520: pp. 116=[392]–121=[397], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903582881
Photo-Sensitive Polyimide Containing Methoxy
Cinnamate Derivatives on Photo-Alignment of
Liquid Crystal
SU YOUNG KIM, SUNG EUI SHIN,
AND DONG MYUNG SHIN
Department of Chemical Engineering, Hongik University, Seoul, Korea
Photosensitive polyimide containing methoxy cinnamate derivatives as a photo-
alignment layer is investigated. 4-Methoxy cinnamic acid, 2-methoxy cinnamic acid
and 2,5-dimethoxy cinnamic acid were irradiated and the absorption spectra were
obtained with irradiation time. 4-methoxy cinnamic acid and 2-methoxy cinnamic
acid are observed isosbestic points at 258 nm and 250 nm, respectively and
2,5-dimethoxy cinnamic acid has no isosbestic pioint. 2-Methoxy cinnamic acid
was most photosensitive with 30 sec irradiation time. Polyimide containing methoxy
cinnamte derivatives was synthesized and the anisotropy of alignment layer was
induced by irradiation linearly polarized UV (LPUV). UV irradiation of a film
a of polyimide with 2-methoxy cinnamate proceeded predominantly by the photo-
isomerizaiton.
Keywords Liquid crystal display; methoxy cinnamic acid; photo-alignment;
polyimide
1. Introduction
Uniform alignment of liquid crystals (LCs) is required for a successful operation of
liquid crystal displays (LCDs). The interaction between polymer surfaces and liquid
crystals (LCs) determines the alignment properties of a liquid crystal at its interface
with a polymer layer. Polyimide is used as alignment layers because of their advan-
tageous properties, such as excellent optical transparency, heat resistance and dimen-
sional stability. Rubbed such PI film surfaces have been widely used for aligning LC
molecules.
However, the rubbing method has some problems, such as the generation of
electrostatic charges and dust formation. Rubbing free method for LC alignment,
photo-alignment is expected to achieve the high resolution LCDs [1–5].
Photo-reactive polymers were actively studied for stability of LCDs alignment
[6–10]. The most prominent materials of these are derivatives of cinnamate groups.
The polyimide films exposed to linearly polarized UV (LPUV) irradiation give
homogenous LC alignment perpendicular to the polarization direction of the light
Address correspondence to Prof. Dong Myung Shin, Department of Chemical Engineer-
ing, Hongik University, Sangsu-Dong, Mapo-Gu, Seoul 121-791, Korea. Tel.: (þ82)2-320-1652;
Fax: (þ82)2-320-1191; E-mail: shindm930@paran.com
116=[392]

Photo-Alignment of Liquid Crystal
117=[393]
[11–12]. Ichimura et al. suggested that the homogenous photo-induced LC alignment
at the cinnamate polymer film is produced by the polarization photochroism that
result from photo-isomerization of the cinnamoyl unit [13–18]. In this study, photo-
reactivity of methoxy cinnamate derivatives and the effect of photo-isomerization on
LCs alignment layer are studied.
2. Experimental Section
4-Methoxy cinnamic acid, 2-methoxy cinnamic acid and 2,5-dimethoxy cinnamic
acid were irradiated and the absorption spectra were obtained in chloroform.
Polymerization processes are shown in Figure 1 3,5-Diaminobenzly alcohol was
polymerized with 1,2,3,4-cycolbutanetetrcatboxylic dianhydride (CBDA) using
N-metyl-2-pyrrolidinon (NMP) as a solvent at 0ꢀC–10ꢀC for 12–24 hr to obtain
the polyamic acid. The polyamic acid was imidized at 210ꢀC for 1 hr.
The polyamic acid was spin coated onto indium-tin-oxide (ITO) coated
glass substrate with spin coater. The polyamic acid films were pre-baked on the
temperature-controlled hot plate at 90ꢀC for 30 min. The polyamic acid film sub-
strates were hard-baked in the oven at 210ꢀC for 1 hr to convert to the polyimide.
Figure 1. The polymerization of 3,5-diamino benzyl alcohol and CBDA and surface reaction
of polyimide film and 4-methoxy cinnamoyl chloride.

118=[394]
S. Y. Kim et al.
A mixture of 4-methoxy cinnamic acid and thionyl chloride was stirred under
reflux until the disappearance of the starting materials as evidenced by TLC about
4 hr. After reaction, the excess thionyl chloride was removed in vacuum. 2-Methoxy
cinnamoyl chloride and 2,5-dimethoxy cinnamoyl chloride were prepared using the
same method. The polyimide film reacted with methoxy cinnamoyl chloride in
dichloride methane (MC) using triethylamin as a catalyst by interfacial reaction.
Linearly polarized ultraviolet light (LPUV) irradiated for 30 min with 200 W
super pressure short arc mercury lamp equipped polarizer (LUMATEC model
SUV-DC-P, Glan-Taylor polarizer) on polyimide film.
Parallel LC cells were fabricated with the alignment layers and the cell gaps of
approximately 5 mm using polymer beads. The liquid crystal (LC) was obtained from
Merck (E-7 TN LC), and used as it was. In order to determine the direction of LC
alignment, the dichroic dye (Disperse Blue 14) was used. It was dissolved 1% into E-7
LC. The dichroic dye shows strong absorption at 655 nm. The absorption change as
a function of polarization angle was measured UV-visible spectroscopy (HP model
8453) by rotating the polarizer.
In addition antiparallel LC cells were assembled and the alignment behavior of
the LCs was characterized. Optical phase retardation was measured using an optical
setup with a photoelastic modulator (PEM) (model REM-100).
Figure 2. UV-visible absorption spectra of (a) 4-methoxy cinnamic acid, (b) 2-methoxy cin-
namic acid and (c) 2,5-dimethoxy cinnamic acid in chloroform irradiated with UV light with
irradiation time.

Photo-Alignment of Liquid Crystal
3. Result and Discussion
119=[395]
Figure 2 shows UV absorption spectra of 4-methoxy cinnamic acid and 2-methoxy
cinnamic acid and 2,5-dimethoxy cinnamic acid obtained by irradiation with UV
light. As shown in Figure 2 (a) and (b), 4-methoxy cinnamic acid has isosbestic point
Figure 3. Polar diagrams of absorptions of a linearly polarized visible light (655 nm wave-
length) measured from LC cells fabricated with polyimide containing (a) 4-methoxy cinna-
mate, (b) 2-methoxy cinnamate, and (c) 2,5-dimethoxy cinnamate film irradiated with
LPUV as a function of the angle of rotation of the LC cell.

120=[396]
S. Y. Kim et al.
Figure 4. PEM spectra of polyimide films with methoxy cinnamate.
at 268 nm and maximum peak is observed at 313 nm and 2-methoxy cinnamic acid
has isosbestic point at 250 nm and maximum peaks at 278 and 327 nm. 4-methoxy
cinnamic acid and 2-methoxy cinnamic acid maintain isosbestic points for 20 min
and 15 min, respectively. 2,5-Dimethoxy cinnamic acid has no isosbestic point and
mixumum peaks at 280 and 355 nm. This result suggests that the photo-reactions
of 4-methoxy cinnamic acid and 2-methoxy cinnamic acid mainly follow a single
reaction process which is the photo-isomerization. 4-Methoxy cinnamic acid,
2-methoxy cinnamic acid and 2,5-dimethoxy cinnamic acid reached to their
photo-stationary state within 35, 30 and 20 min respectively. 2-Methoxy cinnamic
acid was most photosensitive, the absorbance decrease drastically with 30 sec
irradiation time. It indicated that photo-isomerization is the major reaction pathway
for 2-methoxy cinnamic acid.
The photo-induced alignment direction was determined by measuring the
dichroic dye dissolved in a nematic LC (E7) and polar diagrams (Fig. 2) confirm
direction of alignment. Polyimide films exposed to linearly polarized UV (LPUV)
irradiation give homogenous LC alignment perpendicular to the polarization
direction of the light.
Figure 4 shows PEM spectra for the confirmation of optical anisotropy on
photo-aligned polyimide film. The range between the highest point and the lowest
point signifies the anisotropy of the polyimide containing methoxy cinnamate deri-
vatives. LC cell with polyimide containing 2-methoxy cinnamate showed biggest
contrast between the highest point and the lowest point. 4-methoxy cinnamate has
a slightly difference in contrast.
4. Conclusion
The photo-sensitivity of various photo-reactive methoxy cinnamic acid for
photo-alignment indicated that the 2-methoxy cinnamic acid reacted the fastest
among other derivatives under study. 2-Methoxy cinnamic acid which has twisted
charge transfer exited state enable to transfer electrons easier than others. The
photo-alignment layer was induced by irradiation with LPUV light and homogenous

Photo-Alignment of Liquid Crystal
121=[397]
LC alignment perpendicular to the polarization direction of the light was observed
by polar diagram. LC cell with polyimide containing 2-methoxy cinnamate has
broadest range of retardation. It indicated that 2-methoxy cinnamic acid is most
photo-sensitive and UV irradiation of a film of a polyimide with 2-methoxy cinna-
mate proceeded predominantly by the photo-isomerizaiton.
Acknowledgment
This work was supported by National Research Foundation of Korea Grant funded
by the Korean Government (2009-0071137).
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