Liquid crystal alignment properties on polyamide films bearing a phenylenediacryloyl moiety in the main chain

Article abstract of DOI:10.1080/15421406.2012.687889

Soluble photosensitive polyamide, which has a photoreactive 1,4-phenylenediacryloyl (PDA) moiety in the main chain with biphenyl side groups, was synthesized with high molecular weights. The polymer produced high quality films through conventional spin-ca

Full text of DOI:10.1080/15421406.2012.687889

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Liquid Crystal Alignment Properties
on Polyamide Films Bearing a
Phenylenediacryloyl Moiety in the Main
Chain
Se Jin Kwon a , Boknam Chae b , Joo Young Kim a , Woo-Sik Jung a
Seung Woo Lee a
&
a School of Chemical Engineering and the BK21 Program , Yeungnam
University , Gyeongsan , 712-749 , Korea
b Pohang Accelerator Laboratory, Beamline Division , Pohang ,
790-784 , Korea
Published online: 02 Aug 2012.
To cite this article: Se Jin Kwon , Boknam Chae , Joo Young Kim , Woo-Sik Jung &
Seung Woo Lee (2012) Liquid Crystal Alignment Properties on Polyamide Films Bearing a
Phenylenediacryloyl Moiety in the Main Chain, Molecular Crystals and Liquid Crystals, 563:1, 1-9, DOI:
10.1080/15421406.2012.687889
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Mol. Cryst. Liq. Cryst., Vol. 563: pp. 1–9, 2012
Copyright © Yeungnam University
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.687889
Liquid Crystal Alignment Properties on Polyamide
Films Bearing a Phenylenediacryloyl Moiety in the
Main Chain
SE JIN KWON,1 BOKNAM CHAE,2 JOO YOUNG KIM,1
WOO-SIK JUNG,1 AND SEUNG WOO LEE1,∗
1School of Chemical Engineering and the BK21 Program, Yeungnam University,
Gyeongsan 712-749, Korea
2Pohang Accelerator Laboratory, Beamline Division, Pohang, 790-784, Korea
Soluble photosensitive polyamide, which has a photoreactive 1,4-phenylenediacryloyl
(PDA) moiety in the main chain with biphenyl side groups, was synthesized with high
molecular weights. The polymer produced high quality films through conventional spin-
casting and dry processing with relatively good thermal stability. The photochemical
reactions of the polyamide in the films were investigated by UV-Vis spectroscopy; the
polymer in the film demonstrated excellent photoreactivity to UV light. Linearly po-
larized ultra-violet light (LPUVL) irradiation induced anisotropic reorientations of
the polymer chains in the film as a result of direction-selective photoreaction of the
PDA moieties. The polyamide thin films were found to have excellent unidirectional
orientation ability as a result of photo-exposure with LPUVL; the direction-selective
photoreaction of the PDA moiety in the main chains induced alignment of nematic
liquid-crystals (LCs) on the surface. The pretilt behaviors of the LC molecules on the
LPUVL irradiated film surfaces were controlled by both the exposure dose of LPUVL
and the annealing temperature.
Keywords LCD; nematic liquid-crystal; phenyleneacryloyl moiety; photosensitive
polyamide; pretilt angle; unidirection alignment
Introduction
Several photoinduced liquid-crystal (LC) alignment concepts have been of wide interest
in both academia and the LC flat panel display (LCD) device industry because these
concepts can be used to prepare a rubbing-free LC alignment layer for LCD devices. Some
photoinduced LC alignment concepts have been reported [1–8] and reviewed [9,10]. The
photoinduced LC alignment concepts using linearly polarized ultraviolet light (LPUVL)
irradiation can be categorized into three classes of materials according to the photochemical
reaction responsible for the photoalignment, as follows [9,10].
The first class involves azobenzene dye containing materials [1–3,9,10]. Azobenzene
dyes undergo photoinduced isomerization from the trans-isomer to the cis-isomer and the
reverse. LC alignment behavior using LPUVL irradiation on polymer surfaces containing
∗Address correspondence to Prof. Seung Woo Lee, School of Chemical Engineering, Yeungnam
University, 214-1 Dae-Dong, Gyeongsan 712-749, Korea (ROK). Tel.: (+82)53-810-2516; Fax:
(+82)53-810-4631. E-mail: leesw1212@ynu.ac.kr
1

2
S. J. Kwon et al.
azobenzene side groups has been reported by several research groups. The second class
includes materials containing [2+2] photodimerizable moieties such as cinnamate [4],
styrylpyridine [5], chalcone [6], and coumarin [7] moieties. The photodimerization gener-
ates crosslinks in the polymer via cyclobutane ring formation, leading to insolubilization
of the polymer film. Poly(vinyl cinnamate) (PVCi), a representative photoalignment mate-
rial, and its derivatives in films have been reported to align LC molecules in the direction
perpendicular to the electric vector of the LPUVL when exposed to LPUVL. The third
class includes LC alignment induced by photodegradation caused by LPUVL irradiation
[8]. Hasegawa and coworkers reported photoalignment of polyimide by LPUVL at 257 nm
irradiation [11].
Nevertheless, these approaches still are not feasible for mass production of LCDs due
to several unsolved problems, including low thermal stability, low anchoring energy, low
pretilt angle, limited processibility with ultraviolet (UV) light exposure, and the lack of
availability of suitable materials. Thus, there remains a significant challenge to delivering
high performance materials suitable for LPUVL induced LC aligning films.
In this study, a novel photosensitive polyamide containing the p-phenylenediacryloyl
(PDA) moiety in the main chain with biphenyl groups as a side chain was synthesized
by solution polymerization, which exhibited good solubility and processibility. The pho-
toreactivity and photoalignment characteristics of the polyamide films were investigated
using spectroscopic methods. In addition, the alignment behavior and pretilt angle of LC
molecules were investigated on the polymer films treated with various LPUVL exposures.
In addition, the pretilt angles of LC molecules on the annealed films also were investigated.
Experimental
Materials and Measurements
3,5-diminobezoic acid, p-toluenesulfonic acid monohydrate (TsOH), di-tert-butylcarbonate
[(Boc)2], N,N-dicyclohexylcarbodiimide (DCC), 4-(dimethylamino)pyridine (DMAP), 1,4-
phenylenediacrylic acid, triphenylphospine (TPP), pyridine (Py), and 4-phenylphenol were
purchased from Aldrich Company and used without purification. N-Methyl-2-pyrrolidone
(NMP) was distilled over calcium hydride under reduced pressure and under nitrogen
atmosphere.
1H NMR spectra were obtained at room temperature with a Bruker AM 300 spec-
trometer. For the synthesized polyamide, the inherent viscosity was measured in NMP at
25◦C using an Ubbelohde suspended level capillary viscometer by a method described
in the literature [12]. The glass transition temperature (Tg) and degradation temperature
(Td) of each film were measured using a differential scanning calorimeter (DSC) (model
DSC 220CU, Seiko, Japan) and a thermogravimeter (model TGA7, Perkin-Elmer, USA),
respectively. UV-visible absorption spectra were obtained as a function of the exposure
dose using a Hewlett-Packard 8453 spectrophotometer with or without a dichroic polar-
izer (Oriel, model 27320). Optical phase retardation was measured using an optical set
up consisting of a photoelastic modulator (PEM) (Hinds Instruments, model PEM90), as
described elsewhere [13]. The LC alignment in the cell was examined by measuring the
absorption of the linearly polarized He-Ne laser beam (632.8 nm wavelength) as a function
of the rotational angle of the cell, thereby allowing the construction of polar diagrams. For
these measurements, the LC cell was installed perpendicular to the incident laser beam
direction. The pretilt angle, α, of the LC molecules was measured using a crystal rotation
apparatus.

LC Alignment Properties on Polyamide Films
3
Synthesis of 3,5-di-tert-butoxycarbonylaminobenzoic Acid
A solution of [(Boc)2] (5.2 g, 23.9 mmol) and dried NMP (10 ml) was added dropwise to
a stirred solution of 3,5-diaminobezoic acid (1.65 g, 10.9 mmol) and dried NMP (40 ml)
at 0◦C under nitrogen atmosphere. The reaction mixture was stirred overnight at room
temperature, poured into 500 ml of cold water, and extracted with ethyl acetate. The
combined organic layer was dried over magnesium sulfate, followed by evaporation and
drying for 24 h at 80◦C under vacuum. The solid obtained was recrystallized from ethyl
acetate and hexane to produce a 94% yield of 3,5-di-tert-butoxycarbonylaminobenzoic acid
(3.6 g, 10.2 mmol). 1H NMR (DMSO-d6, δ): 12.81 (s, 1H, -OH), 9.48 (s, 2H, -NH-), 7.87
(t, 1H, ArH), 7.70 (d, 2H, ArH), 1.48 (s, 18H, -C(CH3)3).
Synthesis of Biphenyl-3,5-diaminobenzoate
3,5-Di-tert-butoxycarbonylaminobenzoic acid (3.5 g, 9.9 mmol), 4-phenylphenol (2.09 g,
11.9 mmol), and DCC (2.5 g, 11.9 mmol) were dissolved in dry methylene chloride
along with DMAP catalyst (0.15 g, 1.23 mmol), and the reactant mixture was stirred at
ambient temperature for 12 h. After stirring, the reaction solution was filtered and the
solvent removed by rotary evaporation. The residue was purified using column chro-
matography [SiO2, MC/ethyl acetate (45:1 in volume)] to yield biphenyl-3,5-di-tert-
1
butoxycarbonylaminobenzoate (4.1 g, 8.1 mmol, yield 82%). H NMR (DMSO-d6, δ):
9.84 (s, 2H, -NH), 8.36–8.27 (m, 3H, -ArH-), 8.05–7.96 (m, 4H, -ArH-), 7.80–7.75 (m,
2H, -ArH-), 7.69–7.63 (m, 3H, -ArH-), 1.79 (s, 18H, -C(CH3)3). The obtained biphenyl-
3,5-di-tert-butoxycarbonylaminobenzoate (4.0 g, 7.9 mmol) and TsOH (33.2 g, 0.17 mol)
were dissolved in dry NMP and THF. The reaction mixture was stirred at room temperature
for 24 h, poured into 500 ml of cold water, and extracted with ethyl acetate three times. The
combined organic layer was dried over magnesium sulfate, followed by rotary evaporation.
The residue was purified using column chromatography [SiO2, ethyl acetate/hexane (3:2
in volume)] to yield biphenyl-3,5-diaminobenzoate (BP). The monomer was purified to
polymerization grade by recrystallization from ethanol (1.88 g, 6.2 mmol, yield 78%). 1H
NMR (DMSO-d6, δ): 7.74–7.67 (m, 3H, -ArH-), 7.51–7.46 (m, 2H, -ArH-), 7.40–7.38
(m, 2H, -ArH-), 7.31–7.28 (m, 2H, -ArH-), 6.65 (d, 2H, -ArH), 6.17 (s, 1H, -ArH), 5.34
(s, 4H, -NH2).
Synthesis of Poly[oxy(4-biphenyl-3,5-benzoate)oxy-1,4-phenylenediacryloyl]
(PBP-PDA)
PBP-PDA was synthesized from the direct polycondensation of PDA and BP as follows
[14]. A mixture of 1,4-phenylenediacrylic acid (1.08 g, 4.9 mmol), BP (1.50 g, 4.9 mmol),
40 mL of NMP/Py (v/v 5 4/1), and TPP (2.59 g, 9.9 mmol) was stirred at room temperature
under nitrogen for 2 h. The reaction mixture was heated to 100◦C for an additional 12 h.
Thereafter, the reaction solution was poured into hot methanol with vigorous stirring,
producing PBP-PDA polyamide as a powder precipitate with 92% yield.
LC Cell Fabrication
The polyamide solution was spin-cast onto indium tin oxide (ITO) glasses for optical retar-
dation and LC cell assembly. The polyamide films adhered to glass slides were irradiated
by LPUVL at various exposure doses. Here, the LPUVL exposure was conducted with the
film positioned with a tilt angle of 0◦, where the tilt angle is the angle between the film

4
S. J. Kwon et al.
plane and the propagation plane of the LPUVL. Some of the polyamide films irradiated
with LPUVL at 1.5 J/cm2 were thermally annealed in a cumulative step manner from
room temperature to 150◦C using the regime, 35◦C/10 min, 60◦C/10 min, 90◦C/10 min,
120◦C/10 min, and 150◦C/10 min, and then they were cooled back to room temperature.
LC cell assemblies were constructed from the annealed films. Using these polymer films,
LC cells were assembled and filled with 4-pentyl-4-biphenylcarbonitrile (5CB) containing
1.0 wt% of a dichroic dye (Disperse Blue 1) using a capillary technique.
Results and Discussion
Photoreactive PBP-PDA polyamide containing a PDA moiety in the main chain was synthe-
sized using PDA and BP monomer from direct polycondensation, modifying the procedure
reported previously (see Scheme 1) [14]. The obtained polymer was characterized by 1H-
NMR. Figure 1 shows the 1H NMR spectrum of the synthesized PBP-PDA polyamide. The
BP monomer had its characteristic proton peak of amine groups at 5.34 ppm while the
PBP-PDA polyamide did not reveal any proton peak originating from the amine groups
of the BP monomer and showed a characteristic amide proton peak at 10.61. Measured
inherent viscosity of the polyamide in NMP at 25◦C using an Ubbelhode-type viscometer
was 1.21 dL/g. Considering the 1H NMR spectra and inherent viscosity, it can be inferred
that the polyamide was successfully synthesized with a reasonably high molecular weight.
Glass transition (Tg) and degradation temperatures (Td) were measured using DSC and
TGA. Investigation of PBP-PDA revealed a 283◦C Td and a 113◦C Tg.
Scheme 1. Synthetic scheme and chemical structure of the BP monomers and the photoreactive
PBP-PDA polyamide.
Figure 2 shows UV-vis spectra measured from PBP-PDA polyamide films irradiated
with UV light at various exposure doses. The PBP-PDA polymer exhibited an absorption
maximum at 353 nm (= λmax). This UV absorption might originate from the photosensitive
PDA group in the polymer. The absorption peak intensity at λmax = 353 nm decreased
drastically with increasing exposure dose up to 6.0 J/cm2 and then remained nearly constant
with further increasing exposure doses. The absorption peak λmax = 353 shifted to 348 nm
at 0.25 J/cm2, and blue-shift increased with increasing exposure dose. This might be due

LC Alignment Properties on Polyamide Films
5
Figure 1. 1H NMR spectrum of the photoreactive PBP-PDA polyamide.
to photoreactions, such as [2+2] cycloaddition and/or trans-cis isomerization of the PDA
chromophore in the films.
Egerton et al. reported that the PDA moiety in the polymer films undergoes photodimer-
ization through a hydrolysis experiment of UV irradiated PPDA, which is a polyester of
PDA with 1,4-bis(2-hydroxyethoxy)cyclohexane [15]. Vargas et al. [17] and Ikeda et al.
[18] also reported that polymer films containing the PDA moiety in the main chain be-
come crosslinked photochemically via [2+2] cycloaddition. Moreover, the absorption peak
λmax = 353 shift to 348 nm was attributed to shortening of conjugation length as a result of
the reaction of the double bond converting into a cyclobutane ring.
For the polymer films exposed to the linearly polarized UV light, the dichroic ratio was
monitored as a function of UV-exposure dose. Here, the dichroic ratio is estimated from
the following equation:
Dichroic ratio = (A − A// )/(A + A// )
(1)
⊥
⊥
where A and A// are the absorbances of the polymer film at λmax as measured by a linearly
⊥
polarized probing UV light perpendicular to and parallel with, respectively, the electric
vector of LPUVL used in the exposure. As shown in Fig. 3(A), PBP-PDA polyamide
films exposed to LPUVL consistently revealed a positive dichroic ratio for 0.0–3.0 J/cm2.
Figure 2. UV spectra of photoreactive PBP-PDA polymer films irradiated with unpolarized UV light
(260–380 nm) at varying exposure doses.

6
S. J. Kwon et al.
Figure 3. (A) Dichroic ratios and retardation measured from PBP-PDA films irradiated by LPUVL
with various exposure energies; (a) dichroic ratio (•), (b) retardation (◦). (B) Polar diagram of
transmitted light intensity [(in-plane birefringence) × (phase)] taken from the optical phase retardation
measurement of a PPDA-C4BZ film irradiated with LPUVL at 1.5 J/cm2 as a function of the angle
of rotation of the film.
The dichroic ratio increased gradually with increasing exposure dose to reach 0.051 at
3.0 J/cm2. The obtained dichroic ratio values are similar to those of previous reported results
[20–22]. This indicates that PDA moieties located parallel to the electric vector of LPUVL
were consumed more rapidly than those positioned perpendicularly. The direction-selective
consumptions of PDA moieties might take place via [2+2] cyclo-addition. In addition, the
low values of dichroic ratio might come from the remaining disordered PDA moieties in
the polymer film.
Optical birefringence was measured using a PEM system with a He-Ne laser
(632.8 nm). Fig. 3(B) shows a representative polar diagram of the transmittance with respect
to angle of rotation of LPUVL-exposed PBP-PDA films from optical phase-retardation mea-
surements, which was measured for the polyamide film irradiated with LPUVL at 1.0 J/cm2.
The optical axis of the LPUVL-exposed polyamide film in this study lie along the 90–270◦
direction, which is at an angle of 90◦ with respect to the polarization direction of LPUVL
(0–180◦). The measured optical birefringences are plotted in Fig. 3(A) as a function of
exposure dose. For the PBP-PDA film, the optical birefringence increased sharply with
increasing exposure dose, reaching 0.026 at 0.5 J/cm2. Then, the birefringence remained
constant with further increase of exposure dose. Here, the UV-exposed polyamide films
exhibited a positive in-plane birefringence generated by exposure to LPUVL. The positive
birefringence might come from two major factors. (1) Polymer chains reorientation induced
by selective [2+2] photodimerization by LPUVL irradiation. This means that photodimer-
ization of PDA moieties in the main chain requires movement of the polymer chains [13].

LC Alignment Properties on Polyamide Films
7
Thus, the reorientation of polymer chains causes positive birefringence of LPUVL irradi-
ated films [5]. (2) The remaining trans-isomer of the PDA moieties has a relatively large
polarization along its longer axis compared to its products from direction-selective photore-
action. This means that PDA moieties located parallel to the electric vector of the LPUVL
were consumed selectively by [2+2] photodimerization. As a result of photoreaction, the
PDA moieties positioned perpendicular to the electric vector of LPUVL remained greater
than the parallel ones. It turns out that such remaining trans-isomers of PDA moieties
contribute to creation of positive birefringence. Together these factors may contribute to
the generation of positive birefringence. However, these factors may compete against each
other, meaning, the overall birefringence may be controlled by one predominant factor or
a combination of these factors.
Polar diagrams constructed from the LC cells fabricated with LPUVL irradiated films
of the PBP-PDA polyamide were measured. The polar diagram of LCs on the surface
of the PBP-PDA polyamide at 1.5 J/cm2 with LPUVL exposed films is shown in Fig. 4.
Similar polar diagrams were obtained for other LC cells fabricated from the polyamide
films irradiated with LPUVL at various exposure doses. For an LC cell with a PBP-PDA
polyamide film, the LC molecules were aligned along the perpendicular direction with
respect to the electric vector of LPUVL, independent of LPUVL exposure dose. These
results suggest that the selective photoreaction by LPUVL irradiation induced preferential
LC molecule alignment. Using polyesters containing a PDA moiety in the main chains,
Kawatsuki et al. [19] and Song et al. [20] reported that the perpendicular LC alignment was
caused by the interaction between the LCs and the nonphotoreacted PDA group that was
perpendicular to the electric vector of LPUVL. Considering these results, it is reasonable
that the LC alignment is primarily controlled by the remaining PDA moiety after LPUVL
exposure. And, polymer chain reorientation by selective photodimerization may be a minor
factor in LC alignment.
Pretilt angle for the PBP-PDA polyamide was measured for the LC cells fabricated from
films irradiated with various exposure doses of LPUVL by means of the crystal rotation
method. For PBP-PDA polyamide films, pretilt angles varied in the range of 1.8–4.2◦,
depending on the exposure dose of LPUVL irradiation. As shown in Fig. 5(A), pretilt
Figure 4. Polar diagrams of the absorption of linearly polarized visible light (632.8 nm wavelength)
by LC cells fabricated with PBP-PDA films irradiated with LPUVL at 1.5 J/cm2 as a function of the
angle of rotation of the LC cells.

8
S. J. Kwon et al.
Figure 5. Pretilt angle variations of LCs measured for LC cells fabricated from PBP-PDA films.
(A) The PBP-PDA films were exposed to LPUVL at various exposure doses. (B) The PBP-PDA films
were irradiated with LPUVL at 1.5 J/cm2 and annealed using a stepped heating regimen (45◦C/10 min,
60◦C/10 min, 90◦C/10 min, 120◦C/10 min, and 150◦C/10 min).
angle initially increased with increasing exposure dose but leveled off above 1.5 J/cm2.
Pretilt angle of the PBP-PDA polyamide was also measured for the LC cells fabricated
from films irradiated with 1.5 J/cm2 of LPUVL, which were subsequently annealed at
various temperatures. The obtained pretilt angle as a function of annealing temperatures is
presented in Fig. 5(B). Pretilt angle of LC cells using annealed polyamide films increased
with increasing annealing temperature. These pretilt angle variations can be understood as
due to the changes in the orientations of the biphenyl side groups in the film surfaces that
result from thermal annealing. Furthermore, all cells fabricated from annealed films reveal
polar diagrams very similar to that observed for the cell of non-annealed films, independent
of annealing.
Conclusions
A photosensitive polyamide with a photoreactive PDA moiety in the main chain and
biphenyl side groups was synthesized by direct solution polycondensation. The polymers
produced high quality films through conventional spin-casting and dry processing with

LC Alignment Properties on Polyamide Films
9
respective Tg and Td values of 102 and 283 ◦C. Overall, the polyamide was synthesized at
reasonably high molecular weights. The polymer film has an excellent photoreactivity to
UV light. The dichroic ratio of the film due to linearly polarized UV light increased with
increasing exposure dose. However, birefringence of the film reached the maximum values
at 1.0 J/cm2 exposure dose. By irradiation of LPUVL, the PDA moiety in the polymer main
chain undergoes selective[2+2] photodimerizationand induces reorientationof the polymer
main chains. For the synthesized polyamide film, the direction-selective photoreaction of
the PDA moiety in the main chains and the unreacted PDA moiety are believed to induce the
alignment of LC molecules on the surface. The direction of LC alignment is perpendicular to
the LPUVL. The pretilt behaviors of the LC molecules on the LPUVL irradiated polyamide
films were controlled by both the exposure dose of LPUVL and the annealing temperature.
In conclusion, this novel PBP-PDA polyamide is a good candidate material for applications
in the LCD industry.
Acknowledgments
This research was supported by Yeungnam University research grants in 2009 and a Human
Resources Development Program of Korea Institute of Energy Technology Evaluation
and Planning (KETEP) grant (No. 20104010100580) funded by the Korean Ministry of
Knowledge Economy.
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