Improvement of liquid crystal alignment using novel photo-activators

Article abstract of DOI:10.1166/jnn.2018.15476

Photo-activator is a kind of additive that can improve the anchoring energy by attacking some of the bonds of polyimide (PI). Photo-activators were synthesized from the reaction of cyclohexanone oxime with three different anhydrides, respectively. Each ac

Full text of DOI:10.1166/jnn.2018.15476

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Nanoscience and Nanotechnology
Vol. 18, 7207–7210, 2018
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Improvement of Liquid Crystal Alignment
Using Novel Photo-Activators
In-Hye Lee and Dong-Myung Shin∗
Department of Chemical Engineering, Hongik University, Mapo-gu, Seoul 04066, Korea
Photo-activator is a kind of additive that can improve the anchoring energy by attacking some of
the bonds of polyimide (PI). Photo-activators were synthesized from the reaction of cyclohexanone
oxime with three different anhydrides, respectively. Each activator generates different active radicals
when irradiated. These fragmented and activated radicals are responsible for the liquid crystal (LC)
alignment of PI film. The reactivity was confirmed through UV-Visible spectroscopy. All the three
photo-activators had characteristic bimodal-shaped absorption peaks at 270∼280 nm. The photo-
fragmentation reactions were completed within 1 min of UV irradiation, which implies that the acti-
vators are highly reactive to UV light. The short reaction time is very useful for liquid crystal display
(LCD) factory applications. The photo-activator using crotonic anhydride (CAP) showed the highest
surface anchoring energy, of 6ꢀ92 × 10−5 J/m2, compared to that of the other activators and that
obtained by rubbing methods; (1ꢀ11 × 10−5 J/m2ꢁ. This result was obtained due to resonance sta-
bilization from the allyl radicals of CAP. The photo-activator using acetic anhydride (AAP) reached
its maximum anchoring energy in less than 3 min of irradiation, which is the shortest optimum irra-
diation time. Considering the fact that this process does not require additional procedure and time,
the photo-activators can be considered an innovate additive.
Keywords: Liquid Crystal Display, Photo-Activator, Photo-Alignment, Anchoring Energy.
1. INTRODUCTION
methods include photo-dimerization, photo-isomerization,
and photo-decomposition.
Since the operation of LCD is based on LC alignment and
its variation in the panel, the electro-optical properties of
LCD panels are determined by the shape and character-
istics of LC.1ꢀ6–8ꢀ11 Improving the stability of LCD is an
important factor because it preserves the superior electro-
optical properties of the panel to maintain high definition
and performance.1ꢀ2ꢀ6–8ꢀ10ꢀ11 The uniform and stable LC
alignment is a prerequisite and can be achieved by induc-
ing anisotropy on the surface of the alignment layer. For
this purpose, the rubbing method was widely used until
recently.1ꢀ2ꢀ6–8ꢀ10ꢀ11
To overcome the limitations of mechanical rubbing,
photo-alignment has been proposed as a non-contact alter-
native. The anisotropic intermolecular interactions between
the alignment surfaces and LC in the photo-alignment
method are strong enough to hold the LC on the align-
ment surfaces.3 The photo-alignment methods are capable
of minimizing the pixels accompanied by the high defi-
nition of LCD and the division of pixels necessary for a
wide viewing angle. The commonly used photo-alignment
Photo-activation is a new concept in the photo-alignment
technique. To facilitate the photo-reactions, we use fast
fragmentation reactions employing photo-activators. The
reactive radicals can be used to orient the LC by the forma-
tion of alkyl groups on the PI surfaces. Because the manu-
facturing lead times are directly related with productivity in
the LCD process, along UV irradiation time can be a limita-
tion when compared with the rubbing method.4 Therefore,
in this paper, we propose novel photo-activators that can
react faster and improve the anchoring energy compared to
that obtained in the existing rubbing methods.
2. EXPERIMENTAL DETAILS
2.1. Synthesis of Photo-Activators
The three kinds of anhydride (0.011 mol), were respec-
tively dissolved in 30∼50 ml of hexane or dimethyl for-
mamide (DMF), with cyclohexanone oxime (0.01 mol),
followed by the addition of one drop of 57% perchlo-
ric acid. The mixture was stirred by shaking and left to
stand for 24∼28 hrs at 20∼23 ꢀC. At the same time,
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2018, Vol. 18, No. 10
1533-4880/2018/18/7207/004
doi:10.1166/jnn.2018.15476
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Improvement of Liquid Crystal Alignment Using Novel Photo-Activators
2.3. Measurement of Alignment Properties
Lee and Shin
The photo-activators were measured using a UV-Visible
spectroscope (Agilent 8453) to analyze the reactivity as
a function of UV irradiation time through observance of
1
its absorbance change. A H-NMR spectrometer (Bruker
Biospin 400 MHz FT-NMR spectrometer) was used with
d-CDCl3, and the data were compared with the graph of
MestRe-C and Chemdraw program.
Contact angle (phoenix 300, SEO), anchoring energy,
and pretilt angle measurements were carried out on the LC
cells. For contact angle, the PI and photo-activator were
dropped on a non-treated silicon wafer, or DI water was
dropped on the wafer coated with PI.
Figure 1. Synthetic schemes of photo-activators (AAP, HAP, and CAP)
from cyclohexanone oxime.
the round-bottom flasks were respectively wrapped in alu-
minum foil several times. The mixture was then diluted
with water and extracted with hexane, and the phase con-
taining the product was separated, and removed with a
small amount of water with 98.5% anhydrous sodium
sulfate. Finally, the mixture was filtered using a filter
paper to remove the precipitate. Thereafter, the soꢀlvent was
removed at a temperature not exceeding 25∼30 C.
When synthesizing AAP, DMF was used as the sol-
vent. The mixture was then diluted with ethyl acetate (EA)
to 10 times the volume of DMF, and washed with water
several times. Then, the solvent, EA, was removed at a
temperature not exceeding 25∼30 ꢀC.5 Figure 1 shows
the three synthesis reactions. To compare the alignment
properties according to the different structures of photo-
activators, three anhydrides were selected.
2.2. Fabrication of LC Cell
The photo-alignment solution was prepared in volume
proportions of NMP: ꢁ-butyrolactone: 2-butoxyethanol =
8:1:1. TN PI (JSR) (15 wt%) and photo-activator (5 wt%)
were then added to the above solution. These were then
coated on indium tin oxide (ITO) glass using a spin coater
(Midas 1200T) for 10 s at 1000 rpm and 20 s at 3000 rpm.
The coated substrates were pre-baked at 80 ꢀC for 2 min to
vaporize the solvent and then baked at 200 ꢀC for 15 min.
To align the LC, 1000 W intensity LPUV (66921,
Newport) and 310∼400 nm polarizer (1026819, Newport)
were used in parallel direction. After passing through the
polarizer, the UV intensity was 1.5 mW/cm2. If the time
required for the photo-alignment process exceeds 10 min,
it cannot be shown as an improvement in terms of time.
Hence, it was measured only up to 10 min.
Two substrates that were prepared in the same condi-
tions were placed in parallel direction and sealed using a
UV hardening agent (NOA 68) with 5.5 ꢂm spacer. The
LC material (ML-0643, ꢃꢄ = 6ꢅ9, ꢃn = 0ꢅ1023, Merck)
mixed with a chiral dopant (R-811, Merck) was injecꢀted
between the cell gap by the capillary method at 100 C.
Finally, all the edges of the cell were sealed with a UV
hardener to prevent the leakage of LC.
Figure 2. IR spectra of AAP, HAP, and CAP.
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Lee and Shin
Improvement of Liquid Crystal Alignment Using Novel Photo-Activators
Azimuthal anchoring energy was measured from the
angular difference between the force of the dopant and
photo-alignment.9ꢀ12 For pretilt angle, the polarizer rota-
tion method and extended Jones matrix were used.1ꢀ12 The
twist angle was set at 50ꢀ, and other conditions also used
same LC with anchoring energy when calculated.
3. RESULTS AND DISCUSSION
The structure of all the activators were confirmed from the
1
FT-IR and H-NMR spectra; shown in Figures 2 and 3,
respectively. In the IR spectra, the characteristic bonds
of each activator were examined. In particular, C
O
(1715 cm−1ꢆ and N–O (1515 cm−1 and 1345 cm−1ꢆ com-
monly appeared for all the activators.
Since all the activators had similar structures, the spectra
were compared for a common double peak: the cyclohex-
1
ane ring (2.37∼2.55 ppm) in the H-NMR spectrum. With
integration, peaks corresponding to the characteristic allyl
bond of CAP appeared at 5.88 ppm and 7.07 ppm at the
same area. Further, HAP showed a relatively strong peak
at 1.31∼1.6 ppm due to its large hexyl group. The peak at
7.25 ppm is characteristic of CDCl3.
UV-Visible spectroscopy was performed to observe
the absorbance changes, which indicate the reactivity
of the photo-activators to UV light. As shown in Figure 4,
the absorbance drastically declined within several seconds
in all the spectra. This implies that the photo-activators
Figure 4. Changes in the UV-Vis spectra of AAP, HAP, and CAP after
5 min of UV irradiation.
were highly reactive to UV and disintegrated into radi-
cals. The common bimodal peaks appeared at 270∼280 nm
and are indicated by dotted line. These peaks remained
even after irradiation. The characteristic peak appeared
due to the common N–O–C O–C bond in the photo-
activators. The photo-fragmentationof AAP and HAP com-
pleted within 1 min, while the CAP photo-fragmented in
5 min. Even when these activators were added to TN PI
at 5 wt%, the photo-fragmentation reactions completed
rapidly within 1 min. Interestingly, the photo-fragmentation
of CAP required the longest irradiation time because of the
relatively high stability of its allyl radicals.
During the experiment, we found the possibility that
surface roughness can affect the anchoring energy. Thus,
Figure 3. 1H-NMR spectra of AAP, CAP, and HAP.
J. Nanosci. Nanotechnol. 18, 7207–7210, 2018
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Improvement of Liquid Crystal Alignment Using Novel Photo-Activators
Lee and Shin
Table I. Maximum anchoring energy of photo-activators using TN PI
with UV irradiation time.
meaningful and useful in terms of productivity and effi-
ciency. AAP and HAP respectively showed their maximum
anchoring forces at 3 min and 5 min of irradiation. The
shortest irradiation time was achieved due to the methyl
radicals of AAP that can react faster than the other acti-
vators. Allyl radicals are more stable than methyl radicals
are; this indicates that prolonged irradiation makes the sur-
face of the films more random.
Maximum anchoring
Irradiation
time (min)
Activator
energy (×10−5 J/m2)
AAP
HAP
CAP
3.46
3.68
6.92
3
8
10
Furthermore, all the cells had similar pretilt angles of
8∼9ꢀ, which did not exceed 10ꢀ. When the cells were
treated by the rubbing method before photo-alignment, the
pretilt angles slightly decreased to 7∼9ꢀ.
we measured the contact angle. The contact angles for
each photo-activator and TN PI on silicon wafer were mea-
sured to observe the changes on the surface of the PI film.
The contact angle of DI water droplet on silicon wafer was
6.3104ꢀ and was used as a reference. The HAP demon-
strated the largest angle, of 21.459ꢀ, while AAP had the
smallest, contact angle of 8.765ꢀ. The large contact angle
of HAP resulted from its characteristic hydrophobic hexyl
groups.
When TN-PI-coated silicon wafers were used, after irra-
diation, all the contact angles remarkably increased due
to hydro-phobization of the surface. Carbon dioxide gas,
which generated from the irradiated photo-activators, evap-
orated from the surface of the PI films during photo-
irradiation. As a result, unsaturated C C bonds were
formed on the surface, which rendered the surface more
hydrophobic and rough. We found the possibility that frag-
mented parts were located on the surface of the PI films
and could affect the surface interactions with LC.
4. CONCLUSION
Three novel photo-activators were synthesized to improve
the LC properties and were characterized by FT-IR,
1H-NMR and UV-Vis. All the photo-activators demon-
strated fast degradation and high reactivity. Moreover, the
photo-activators exhibited similar pretilt angles of about
8∼9ꢀ when mixed with TN PI. Further, the contact angle
increased due to surface hydro-phobization through evap-
oration of carbon dioxide by UV irradiation. All the three
activators achieved maximum anchoring energy within
10 min. The CAP-containing PI films showed the high-
est surface anchoring force, of 6ꢅ92 × 10−5 J/m2, which
is around twice of that of the others. Considering the
UV irradiation time, and the average figures of the usual
photo-alignment process and rubbing process, the novel
photo-activators can be efficient photo-alignment additives,
sufficiently applicable to the LCD process.
As shown in Table I and Figure 5, CAP showed the
highest anchoring energy, of 6ꢅ92 × 10−5 J/m2, at 10 min
of irradiation. This is outstanding result as compared to
that obtained in the rubbing method, (1ꢅ11 × 10−5 J/m2ꢆ.
Moreover, shortening the irradiation time to 10 min is very
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Figure 5. Anchoring energy of photo-activators (AAP, HAP, and CAP).
Received: 9 June 2017. Accepted: 1 September 2017.
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