A novel generation of photoactive comb-shaped polyamides for the photoalignment of liquid crystals

Article abstract of DOI:10.1002/pola.26831

A new approach to the synthesis of photoactive comb-shaped homo- and copolyamides containing azobenzene, cinnamate, and coumarin side groups for photoalignment of liquid crystals was elaborated. Photooptical properties and photoorientational ability of these polymers with respect to liquid crystals were studied. It was shown that polarized UV irradiation of all spin-coated polyamides leads to orientation of liquid crystalline molecules deposited on the polyamide thin films. The synthesized polymers containing cinnamate and coumarin side groups as well as azobenzene-containing cyano- and nitro-substituted polymers demonstrated good orientation ability in relation to liquid crystals displaying photoinduced planar orientation with high dichroism values within the range of 0.68-0.72. Contrary to the above-mentioned polyamides, azobenzene-containing fluorosubstituted polymers induced a homeotropic orientation of liquid crystals. It was shown that the synthesized photoactive polyamides can be considered as promising photoalignment materials for application in display technology, photonics, and other smart optical devices.

Full text of DOI:10.1002/pola.26831

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A Novel Generation of Photoactive Comb-Shaped Polyamides for the
Photoalignment of Liquid Crystals
Alexander Ryabchun,1 Alexey Bobrovsky,1 Sung-Ho Chun,2 Valery Shibaev1
1Chemistry Department of Moscow State University, Moscow 119991, Russia
2LG Chem, Ltd 104-1, Moonji-dong, Yuseong-gu, Daejeon 305-380, Korea
Correspondence to: A. Ryabchun (E-mail: ryabchunmsu@gmail.com)
Received 12 February 2013; accepted 29 May 2013; published online 4 July 2013
DOI: 10.1002/pola.26831
ABSTRACT: A new approach to the synthesis of photoactive
comb-shaped homo- and copolyamides containing azoben-
zene, cinnamate, and coumarin side groups for photoalign-
ment of liquid crystals was elaborated. Photooptical properties
and photoorientational ability of these polymers with respect
to liquid crystals were studied. It was shown that polarized UV
irradiation of all spin-coated polyamides leads to orientation of
liquid crystalline molecules deposited on the polyamide thin
films. The synthesized polymers containing cinnamate and
coumarin side groups as well as azobenzene-containing cyano-
and nitro-substituted polymers demonstrated good orientation
ability in relation to liquid crystals displaying photoinduced
planar orientation with high dichroism values within the range
of 0.68–0.72. Contrary to the above-mentioned polyamides,
azobenzene-containing fluorosubstituted polymers induced a
homeotropic orientation of liquid crystals. It was shown that
the synthesized photoactive polyamides can be considered as
promising photoalignment materials for application in display
technology, photonics, and other “smart” optical devices.
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KEYWORDS: azobenzene groups; azo polymers; comb-shaped
polyamides; liquid-crystalline polymers (LCPs); liquid crystals;
photoactive
groups;
photoalignment;
polyamides;
poly
(succinimide)
surface) containing photosensitive groups, such as azoben-
zene, cinnamate, or coumarin,6,11–13 are also very often used
for the same aims. The operating principle of these materials
is based on the noncontact (as opposed to mechanical rub-
bing of polyimide coatings) activation of the aligning layers
by irradiation with polarized UV light. Irradiation induces
angular-selective photochemical reactions and a small anisot-
ropy of surface free energy at the surface of photoalignment
layer, which causes orientation of the LC molecules in a cer-
tain direction.
INTRODUCTION The creation of orientational (alignment)
materials for liquid crystalline (LC) displays remains one of
the more important problems of the current LC display tech-
nology. Today, there are many ways of orienting of liquid
crystal molecules, such as rubbed polyimide coating,
obliquely deposited metal coatings, surfactants, surface gra-
tings, etc.1–3 To obtain a planar uniaxial orientation of liquid
crystals in displays, the rubbed polyimide layers are usually
used; however, they have a number of drawbacks are usually
used. These disadvantages include electrostatic charges gen-
eration, fine dust particles formation, etc.4 Because of the
rapid development of the LC display manufacturing, the poly-
imide coatings (rubbing technology) no longer satisfy the
needs of the industry. Hence, a large number of research
groups and laboratories have focused their attention on the
study and development of some new materials and methods
for LC alignment.5,6 One of the most promising areas in this
field involves developing of new photoalignment layers (or
photocontrollable “command surfaces”). Usually, these layers
represent low-molar-mass molecules adsorbed on the surface
or chemically connected to it.7–10 In addition, some polymers
of different structures (linear, comb-shaped, grafted on the
There are a number of photoreactions and photoprocesses
that can be used for the LC-photoalignment: (1) photoorien-
tation and photoselection (caused, for example, by E/Z-isom-
erization), which is typical for azobenzene14–17 and
cinnamate18 derivatives; (2) angular selective [212]-photo-
cycloaddition (dimerization) (stilbene,19 cinnamate,20 and
coumarin21 derivatives, benzylidenephtalimidine, and benzy-
lideneacetophenone22); (3) photodegradation,23,24 allowing
to create orientation by means of degradation products.
A distinctive feature of photoalignment layers is the ability
to control some orientation parameters: the anchoring
Additional Supporting Information may be found in the online version of this article.
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FIGURE 1 Schemes of orientation of side photoactive groups under the UV irradiation and alignment of liquid crystals on the pho-
toalignment layer: (a) initial unoriented layer of polymer; (b) oriented layer of polymer; and (c) alignment of low-molar-mass liquid
crystal deposited on the polymer layer.
energy, pretilt angle, and alignment direction. In the case of
reversible photoreactions (i.e., E-Z isomerization of azoben-
zenes), the aligning direction can be changed repeatedly by
irradiation with UV light via variation of polarization plane
direction.4,9,25
groups: polyacrylates, polymethacrylates, polyvinyl esters,
etc. These polymers are not the best as photoaligning materi-
als, as far as they are partially soluble in liquid crystals, they
have low glass transition temperatures, and as a result, they
do not keep the LC orientation.9 A growing interest in new
LC-photoalignment substances with advanced properties
motivated us to develop a novel type of such materials with
high thermostability. In this respect, one of such types of
polymers close to polyimides (normally used for LC-
alignment by rubbing) is polyamides because of ability to
form intra- and intermolecular hydrogen bonds stabilizing
the material.
Figure 1 schematically illustrates the principle of liquid crys-
tals alignment by means of photochromic azobenzene-
containing polymer. Polarized light induces uniaxial orienta-
tion of photochromic side groups, which are aligned in the
direction perpendicular to the polarization plane of the light
as is shown in Figure 1(a,b). Such irradiated layer can work
as “command surface” and orient nematic LC-director (n)
of low-molar-mass liquid crystal in the same direction
[Fig. 1(c)].
This study is the first attempt to synthesize photosensitive
polyamides (polyaspartic acid amides). The proposed method
is based on the polymer-analogous ring-opening reaction of
Over the past few decades, new photoalignment materials
and methods have been actively developed. Many laborato-
ries engaged in LC science have begun to apply the photoa-
lignment layers for orientation of LC molecules; there is
even a commercial example of such uses by Sharp, Corp.26
poly(succinimide) (PSI) with aliphatic amines (Fig. 2). A sim-
27
ilar approach was described in Refs.
and 28. For the syn-
thesis of comb-shaped polyamides with side aliphatic chains,
the authors of these articles modified PSI by various ali-
phatic amines to produce amphiphilic biodegradable deriva-
tives of polyaspartic acid for medical applications. We have
changed the chemistry of the reactants by using instead of
The commonly used polymers as photoalignment materials
are the comb-shaped polymers with photochromic side
FIGURE 2 The main idea of the synthesis of the side chain photoactive polyamides by polymer analogous reaction of PSI with
photoactive amines.
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usual aliphatic amines the amines with photoactive groups
chemically attached to the opposite tails of them. The basic
idea of this method is shown in Figure 2. Thus, as a result of
the described reaction, photoactive comb-shaped polyamides
were obtained. It is noteworthy that this reaction occurs
with ring opening in a position and in b one (see Fig. 2).
Typically, the product of b-opening predominates than the a-
opening29; for clarity sake, Figure 2 shows only the mono-
mer unit for b-opening reaction.
Thus, this study can be roughly subdivided into two parts
devoted to the (i) synthesis and (ii) study of LC photoalignment
properties of polyamides. The synthetic section contains the
description of photochromic amines synthesis, obtaining of PSI,
and its chemical modification by photoactive amines.
EXPERIMENTAL
Synthesis of Low-Molar-Mass Compounds
Amines containing azobenzene and coumarin terminal frag-
ments were synthesized according to scheme given in Figure 3.
As photoactive side groups, azobenzene, cinnamate, and cou-
marin derivatives were used. The chemical structures of ter-
minal substituents (see “-R” in Fig. 2) of the synthesized
side-chain polyamides are listed in Table 1. Among them sev-
eral azobenzene-containing polyamides with different sub-
stituents in benzene ring (-CN, -NO2, and -F), two coumarin
derivatives differ by the position of a linkage to an aliphatic
spacer, and a number of cinnamate derivatives with various
substituents in benzene ring (-H, -F, and -OCH3) were
obtained. It should be stressed that, on the basis of the pro-
posed synthetic approach, a successful attempt to get copo-
lyamide was also realized.
Synthesis of tert-Butyl-N-(3-brompropyl)-carbamate (1)
To a cooled (ice bath) and stirred suspension of (3-bromo-
propyl)amine (2.50 mol), di-tert-butyl dicarbonate (Boc2O)
(2.50 mol), and dichloromethane (1.25 L), triethylamine
(3.00 mol) was added dropwise over 3 h. After stirring for 1
day, dichloromethane was added and the solution was
washed with 1M KHSO4, water, and brine, and the mixture
was dried over anhydrous Na2SO4 and concentrated in vac-
uum. Compound (1) was isolated as clear light-yellow oil,
with the yield ꢀ94%.
TABLE 1 Structural Formulas of the Photoactive Side Groups of the Synthesized Polyamides and Copolyamide
Abbreviated Name of Polyamide
Spacer Length, n
6
Structural Formulas of Photoactive Side Groups
P6Cin
P6CinF
6
6
3
P6CinOMe
P3Coum-6
P3Coum-7
3
P3AzoCN
3
3
P3AzoNO2
P3AzoCN2
P3AzoF
3
3
P3AzoF2
3
6
Copolyamide
; ꢀ30 mol %
; ꢀ70 mol %
6
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FIGURE 3 Scheme of synthesis of the aliphatic amines with terminal azobenzene and coumarin fragments.
General Procedure of (3-i) Synthesis
slowly added dropwise during 1 h under intensive stirring.
After 2 h, the precipitate was filtered off, washed several
times by 5% aqueous solution of potassium carbonate,
evaporated, and dried in vacuum at room temperature. As a
result, the powder (4-i) was obtained, with the yield ꢀ87%.
4.2 mmol tert-buthyl-N-(3-brompropyl)-carbamate (1), 3.8
mmol (2-i) (the azobenzene derivatives was synthesized
according to standard approach of azo-coupling reactions,
see Supporting Information) and 6 mmol of anhydrous
potassium carbonate were placed into the round-bottom
flask. Then, 20–30 mL of the anhydrous acetone was added,
and the mixture was refluxed during 24 h. Reaction was con-
trolled by thin layer chromatography (eluent: mixture of
chlorophorm/methanol, 10/1). After completion of the reac-
tion, the precipitate was filtered off and washed three times
with acetone. Solvent was evaporated and residue dried in
vacuum. The obtained powder of (3-i) was recrystallized
from ethanol and dried on air, with the yield ꢀ91%.
General Procedure of (7-j) Synthesis
The synthesis of amine (7-j) was carried out in two stages
(Fig. 4). At the first stage, compound (6-j) was obtained accord-
ing to standard method of ester preparation as follow: dicyclo-
hexylcarbodiimide (1.4 mmol) was added to a solution of
pentafluorophenol (1 mmol), cinnamic acid derivative (5-j) (1.2
mmol), and 4-dimethylaminopyridine (0.1 mmol) in anhydrous
tetrahydrofuran (THF) (30 mL) cooled with ice water for 1.5 h.
After adding all dicyclohexylcarbodiimide, the reaction mixture
was kept at room temperature for 24 h. The progress of the
reaction was monitored by thin layer chromatography. After
completion of the reaction, the precipitate of dicyclohexylurea
General Procedure of (4-i) Synthesis
Substance (3-i) was suspended in small amount of ethylace-
tate, and then 2 mL of concentrated hydrochloric acid was
FIGURE 4 Scheme of synthesis of the aliphatic amines with cinnamate terminal groups.
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was carried out three times. The obtained precipitate con-
taining polymer P3AzoCN was filtered and dried in vacuum
ꢁ
at 90 C. Yield, 0.65 g (70%).
Polyamides P6Cin, P6CinF, P6CinOMe, P3Coum-6, P3Coum-7,
P3AzoNO2, P3AzoCN2, P3AzoF, P3AzoF2, and Copolyamide
were obtained by the same procedure as P3AzoCN. 1H NMR
spectra of synthesized polyamides are presented in Supporting
Information (Fig. S1). In the case of Copolyamide, mixture of
corresponding amines (30 mol % azo- and 70 mol %
cinnamate-containing amine) was applied for PSI modification.
According to NMR data, composition of Copolyamide corre-
sponds to the initial mixture of amines.
SCHEME 1 Synthesis of poly(succinimide).
was filtered off, and THF was evaporated. The product (6-j)
was purified by column chromatography (eluent: mixture of
chloroform /methanol, 10/1).
Samples Preparation
To characterize the LC-alignment properties of polyamides,
the sandwich-like glass cells with a gap of 10 lm prede-
termined by Teflon spacers were prepared. Before assem-
bling of cells, the glass substrates were spin coated (2000
rpm, 1 min) by DMF solution of tested polyamide (ꢀ0.2
wt %) with following prebacking at 80 ꢁC for 10 min. The
thickness of coatings was about 10 nm. The obtained cells
At the second stage, 10% THF solution of (6-j) was slowly
added to 10-fold excess of hexamethylendiamine in THF.
After 1 h, the reaction mixture was poured in chloroform,
and the formed precipitate was filtered out. The residue was
washed five times with water for complete removal of hex-
amethylendiamine traces. Then, combined organic layer was
dried over Na2SO4 and evaporated. The yield of the last stage
was ꢀ80%.
were irradiated using
a linear polarized polychromatic
light of ultrahigh pressure mercury lamp during 10 min.
At the last stage after illumination, the cells were filled
with the LC mixture containing nematic MLC6816 from
Merck. To estimate the degree of LC-orientation, 0.1 wt %
of dichroic dye ASh253 (dichroic dye ASh253 was pro-
vided to us by Dr. A. Shimkin and Dr. V. Shirinyan, to
whom we express our thanks) was added (absorbance
maxima is 614 nm in MLC6816 host).
Polymer Synthesis
Synthesis of PSI
The thin layer of paste consisting of D,L-aspartic acid and
85%ꢁ phosphoric acid (50 wt %) was heated for 2–3 h at
190 C in a vacuum (Scheme 1). The resulting foamed glassy
mass was dissolved in a minimum amount of dimethylforma-
mide (DMF) and precipitated to the water. The resulting pre-
cipitate was filtered and washed with water (then with
methanol) to remove the trace of phosphoric acid. After this,
the precipitate was dried in vacuum at 100 ꢁC for 8 h. The
yield of the final product was 90%.
Structure of ASh-253
Synthesis of Polyamide P3AzoCN
The solution of 2.5 mmol of PSI in 5 mL of dry DMF was
placed into the glass ampoule, and then the solution of 2.6
mmol of amine (4-1) in 5 mL of dry DMF under stirring dur-
ing 10 min was added (Scheme 2). Then, argon was purged
through the reaction mixture during 30 min. Aftꢁer that, the
ampoule was sealed and placed in oil bath at 50 C. After 24
h, ampoule was opened, and DMF was removed using rotor
evaporator. The residue was dissolved in chloroform with
the addition of 5 vol % of methanol to obtain solution with
the concentration of ꢀ10%; after that, hexane was added
until the finishing of precipitate formation. Reprecipitation
For optical investigations, the thin films of tested polyamides
were prepared on quartz substrates by spin coating oꢁf DMF
solution (ꢀ0.1 g/mL) with following prebacking at 80 C.
Physicochemical and Photooptical Investigations
The phase transitions of the polymers were studied by dif-
ferential scanning calorimetry (DSC) with a Perkin Elmer
DSC-7 thermal analyzer (a scanning rate of 10 K/min).
SCHEME 2 Synthesis of azobenzene-containing homopolyamide P3AzoCN.
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The polarizing microscope investigations were performed
using a Mettler TA-400 thermal analyzer and a LOMO P-112
polarizing microscope.
with cinnamate terminal fragments, since cinnamic acid
derivatives do not have active phenol fragments, which are
necessary for Williamson reaction. Therefore, an alternative
two-step reaction for their synthesis was developed by us
(Fig. 4). At the first stage, the esterification reaction of penta-
fluorophenol and a cinnamic acid derivative (5-j) was carried
out. Pentafluorophenol was chosen as a good “leaving” group
in nucleophilic reaction. The resulting compound (6-j) was
reacted with a 10-fold excess of hexamethylenediamine to
obtain monosubstituted diamine (7-j). All amines, (4-i) and
(7-j), were obtained according to the general procedures
Chemical structure of obtained substances was verified by
NMR spectroscopy; TMS was used as standard and DMSO-d6
and deuterochlorophorm were used as the solvent. 1H NMR
was registered using «Bruker Avance-400» spectrometer
with the frequency 400 MHz..
The photochemical properties were studied using a special
instrument equipped with a DRSh-350 ultrahigh pressure
mercury lamp (polychromatic light) and MBL-N-457 diode
laser (457 nm, CNI Laser). To prevent heating of the samples
due to IR irradiation of the lamp, water filter was used. To
obtain plane-parallel light beam, quartz lens was used. The
intensity of light was equal to ꢀ15 mW/cm2 for lamp and
ꢀ2 W/cm2 for laser as measured by LaserMate-Q (Coherent)
intensity meter. Spectral measurements were performed
using Unicam UV-500 spectrophotometer.
1
given in Experimental section; their H NMR spectra are pre-
sented in Supporting Information.
Let us briefly discuss PSI synthesis. PSI is commonly used
for obtaining of poly(aspartic acid), which is expected to be
one of the promising water-soluble and biodegradable poly-
mers in medicine. There are many different routes for PSI
preparation.29,30 The bulk acid-catalyzed polycondensation of
D,L-aspartic acid was chosen for PSI producing. Reaction
scheme (1) and method description were presented above.
1H NMR spectrum of DMSO-d6 solution of PSI is shown in
Figure 4. The signals at 2.7, 3.2, and 5.3 ppm assigned to the
methylene and methine protons of the main chain of PSI,
respectively, were observed. Molecular mass characteristics
of prepared PSI were measured by capillary viscosimetry.
Degree of polymerization (p) calculated from the equations
p 5 3.52 3 (gre)1.56 and [g] 5 1.48 3 1022 3 M0.64 (for
poly(b-benzyl-DL-aspartate).31 The degree of polymerization
of PSI was equal to 280, that is, Mw ꢀ 27 kDa.
The study of photoorientation process and quality of LC-
photoalignment verification were performed using polarized
UV/visible spectroscopy. For this purpose, the angular
dependence (with a step-width of 10ꢁ) of the polarized light
absorbance was measured using a photodiode array UV/visi-
ble spectrometer TIDAS (J&M) equipped with rotating polar-
izer (Glan–Taylor prism controlled by computer program).
The dichroism values were calculated from the spectra
using:
D ¼ ðA 2A Þ=ðA 1A Þ;
(1)
?
?
jj
jj
It is well known that PSI is very reactive compound in
respect to organic and inorganic bases including aliphatic
amines.29 To obtain the comb-shaped polyamides, the
polymer-analogous nucleophilic ring-opening reaction of PSI
under the action of aliphatic amines with terminal photoac-
tive groups was developed.
where A and A are polarized absorbance parallel and per-
k
?
pendicular to the orientation of dye molecules, respectively.
RESULTS AND DISCUSSION
Synthesis
As far as all polyamides were prepared by the same proce-
dure, let us consider a concrete example of cyanoazobenzene-
containing polyamide P3AzoCN synthesis (see Reaction 2). H
The idea behind the proposed method of photochromic poly-
amides synthesis consists in chemical modification of PSI by
low-molar-mass amines containing photochromic fragments.
This method opens a great possibility to synthesize a variety
of side chain homo- and copolyamides by applying amines
with varied structure. Furthermore, not only the photoactive
amines but also any functionalized ones can be used.
1
NMR spectrum of P3AzoCN is characterized by the new sig-
nals at 1.9 and 3.9 ppm (Fig. 5). In addition, the ring opening
of succinimide units resulted in the appearance of a new
signals at 4.4–4.9 ppm assigned to the methine proton of the
backbone units. The disappearance of signal at 5.3 ppm con-
firms that the modification of PSI comprise 100%. 1H NMR
spectra of all others synthesized polyamides are presented in
Figure S1 (see Supporting Information).
To obtain amines with azobenzene and coumarin terminal
fragments, we used the three-step synthetic scheme shown
in Figure 3. At the first stage, the protection of amino-group
of 3-brompropylamine by the action of di-tert-butyl dicar-
bonate (Boc2O) was carried out. After that, the substrate (2-
i) (azo dye or coumarin) containing phenol fragment was
linked to the substance (1) to form ether (3-i) (Williamson
ether synthesis). At the final stage, the removal of the Boc
protecting group led to pure, uncharged amine (4-i).
All the synthesized polyamides represented themselves
either white or orange powders readily soluble in DMF,
DMSO, NMP, and mixture of chloroform and methanol. As far
as the modification degree of PSI was around 100%, the
molecular weight of the final polymers can be estimated
from the degree of polymerization of the original PSI. It was
found that Mw values of all synthesized polymers laid in the
range between 90 and 113 kDa.
The proposed method for azobenzene- and coumarin-
containing amines is not suitable for the synthesis of amines
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1
FIGURE 5 H NMR spectra of PSI and P3AzoCN (solvent: DMSO-d6, 25 ꢁC).
Phase Behavior of Polyamides
phase with the marble texture (Figs. 6 and S2 in Supporting
Information). The nematic phase is stable up to the decom-
position of polyamides (250–270 ꢁC). It should be empha-
sized that polyamide P3AzoCN2 containing additional
cyanogroup in comparison with P3AzoCN does not form any
LC phase, because an existence of additional cyano-
substituent in ortho-position decreases aromatic fragment
anisometry. Copolyamide also does not form mesophase
because it contains only 30 mol % of azobenzene fragments,
which is not enough to form LC phase.
It is noteworthy that all polyamides having the identical
chemical structure of the polymer backbone are character-
ized approximately the same values of glassꢁ transition tem-
peratures, which is in the range of 160–170 C (according to
DSC data).
All cinnamoyl- and coumarin-containing polymers (as
revealed by polarizing optical microscopy method) are amor-
phous substances and they do not display any birefringence
even after a prolonged annealing.
Optical Properties of Polyamides
Owing to the presence of the rod-like mesogenic azobenzene
fragments, the polyamides P3AzoCN, P3AzoNO2, P3AzoF,
and P3AzoF2 demonstrate LC phase formation. According to
the polarized optical microscopy they display a nematic LC
The absorbance characteristics of photoalignment materials
play a significant role in their perspective of photooptical
application. Figure 7 shows the absorption spectra of thin
films of the synthesized polyamides prepared on quartz
substrates. As can be seen from Figure 7(a), red shift of
absorption peak is taking place for azobenzene-containing
polyamides in a series of their derivatives with substituents:
F, 2F, CN, NO2, and 2CN.
For polyamides containing derivatives of cinnamic acid [Fig.
7(b)], bathochromic shift of the spectra is observed with its
increasing in the following substituent order: F, H, and OCH3.
The observed spectral changes are associated with an
increase in the electron-donor character of the substituents
in the considered series of polyamides.
The spectrum of copolyamide (Table 1) has two absorption
bands corresponding to the side fragments of p-methoxycin-
namic acid (kmax ꢀ 294 nm) and azobenzene (kmax ꢀ 365
nm) [Fig. 7(b)]. The variation of the structure in coumarin
series results in significant change in the absorption spectrum.
So polymer P3Coum-7 is characterized by one absorption
peak, but P3Coum-6 has two maxima [Fig. 7(c)]. The presence
of two peaks can be attributed to keto-enol tautomerism of
FIGURE 6 Polarized optical microscopy image of polyamide
P3AzoCN.
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The main optical properties relating to absorbance bands of
polyamides are summarized in Table 2. It is clearly seen that
all tested polymers have a good absorbance properties
(extinction coefficient > 104) in the UV region, which make
it possible to use Hg lamps for photoalignment layers
activation.
Photoorientation Properties of Azobenzene-Containing
Polyamide Films
As discussed in Introduction, LC-photoalignment properties of
photochromic polymers are associated with angularly selective
photoprocesses, particularly, photoorientation phenomenon in
azobenzene-containing polymers. This phenomenon leads to
an appearance of photoinduced dichroism and birefringence
in polymer films. To investigate photoorientation processes,
the thin layers of azobenzene-containing polyamides prepared
by spin-coated method were illuminated by polarized laser
light (457 nm, ꢀ2 W/cm2). Polarized light induces uniaxial
orientation of photochromic side groups, which are aligned in
the direction perpendicular to the polarization plane of the
light, as is schematically shown in Figure 1.
The polarized absorbance spectra and polar diagrams for
P3AzoNO2 [Fig. 8(a,b)] demonstrate an orientation of side
chain azobenzene fragments before and after irradiation. As
is seen in Figure 8(b), the direction of photochromic frag-
ments is perpendicular to electric vector (polarization plane)
of the laser beam. The same processes are observed for
other azobenzene-containing polyamides.
Figure 9 shows the kinetics of the photoinduced dichroism
growth during irradiation. Maximal achievable dichroism val-
ues for all azobenzene-containing polyamides are quite small
(0.1020.33) when compared with azobenzene-containing
polyacrylates studied earlier.33,34 Low dichroism values could
be explained by competitive out-of-plane (homeotropic)
chromophores alignment during light action.35
Introduction of additional ortho-substituents (polyamides
PAzoCN2, PAzoF2, and PAzoNO2) results in decrease of
dichroism values in comparison with mono-para-substituted
chromophores (Fig. 9), which is associated with decreasing
in chromophore anisometry.
It should be pointed out that occurrence of the photoorienta-
tion phenomena in the synthesized polyamide films under
polarized light action is warranted for their good LC-
photoalignment properties.
LC-Photoalignment Properties of Polyamides
For investigation of LC-photoalignment properties of synthe-
sized polyamides, the LC cells were filled with MLC6816
doped by dichroic dye ASh-253 (see detail in Experimental
section). Figure 10(a) shows the absorption spectra of polar-
ized light of the cell filled with LC mixture, which was
aligned by polyamide layer of P6CinOMe. As can be seen
from Figure 9(a), rotation of the sample (inset photographs)
relative to the plane of polarization of detecting beam light
leads to dramatically change of the absorption spectrum.
FIGURE 7 Absorbance spectra of (a) azobenzene-, (b)
cinnamate-, and (c) coumarin-containing polyamide films spin
coated on quartz substrate.
7-oxy-substituted coumarin. In the case of 6-oxy-substituted
coumarin (polyamide P3Coum-6), such tautomerism is not
possible, and there exists only one resonance form.32
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TABLE 2 LC-Photoalignment Properties of Polyamides
Absorbance
Type of Polymers
Abbreviation
Maximum (nm)
Dichroism
Alignment directiona
Cinnamic acid-containing polyamides
P6Cin
272
0.72
0.63
0.72
0.62
0.69
0.68
0.69
0.70
0
Perpendicular
Perpendicular
Perpendicular
Parallel
P6CinF
278
P6CinOMe
P3Coum-6
P3Coum-7
P3AzoNO2
P3AzoCN
P3AzoCN2
P3AzoF
294
Coumarin-containing polyamides
Azobenzene-containing polyamides
278/346
325
Parallel
355
Perpendicular
Perpendicular
Perpendicular
Homeotropic
Homeotropic
Perpendicular
353
375
336
P3AzoF2
Copolyamide
345
0
Cinnamic acid and azobenzene based copolyamide
294/365
0.70
a
Alignment direction with respect to polarization plane of UV light.
This phenomenon indicates the presence of a uniaxial orien-
tation of liquid crystal molecules that is more clearly demon-
strated in Figure 10(b).
Polar diagram shows the direction of the preferred orienta-
tion of the LC molecules in the cell. One can see that the ori-
entation of liquid crystals is perpendicular to the plane of
polarization of UV light, which was used to activate photoa-
lignment coating before filling a cell with liquid crystal.
Moreover, the estimated value of the dichroism (D) corre-
sponding to absorbance of dichroic dye (at 614 nm) was
equal to 0.72. It corresponds to a good uniaxial orientation
of liquid crystal in the cell. For comparison, the dichroism
values of the similar cell aligned by rubbed polyimide coat-
ing (which is used in display technology) was 0.69.
FIGURE 8 Polarized absorbance spectra (a) and polar diagrams
(b) for polymer PAzoNO2 before and after 60 s of laser irradia-
tion (457 nm, I 5 2 W/cm2).
FIGURE 9 Dichroism growth kinetics for different azobenzene-
containing polyamides. Photoorientation processes were
induced by 457 nm laser irradiation (ꢀ2 W/cm2).
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containing polyamides P3Coum-7 and P3Coum-6, which
induces an LC orientation in the direction parallel to the
polarization plane of UV light. Such behavior is typical for
this class of compounds and is determined by cyclobutane
adducts formed by the axis-selective photodimerization.
However, opposite situation can be realized in some cases.4
The second group is fluorine-containing azobenzene polya-
mides P3AzoF and P3AzoF2, which stimulate an orientation
in a direction perpendicular to the plane of the LC cell, that
is, homeotropic one. Furthermore, these materials induce
homeotropic orientation even without exposure by UV light.
As was shown in previous section, the investigation of pho-
toorientation processes under the action of laser irradiation
(457 nm) in thin layers of polyamides revealed that their
dichroism values (D) corresponding to maxima were small
and lay between 0.05 and 0.33 (Fig. 9). It is noteworthy that
even such small dichroism values provide a good orientation
of the LC molecules deposited on such coatings due to small
surface energy anisotropy required for effective LC
orientation.36
CONCLUSIONS
A novel series of comb-shaped polyamides with photoactive
side groups were synthesized by means of chemical modifica-
tion of PSI. Their phase behavior, optical, and photooptical
properties and LC-photoalignment ability were studied. It was
shown that irradiation of polyamides layers with polarized UV
light leads to orientation of LC molecules deposited on them.
The relatively simple synthetic approaches developed in this
work, high thermal stability and good photoalignment proper-
ties of the prepared polyamides, give opportunity to consider
them as promising materials for application in display tech-
nology, optics, photonics, and so on.
ACKNOWLEDGMENTS
FIGURE 10 Polarized absorbance spectra of the cell (insert
photograph) prepared using polyamide P6CinOMe as photoa-
lignment layer and filled by nematic mixture MLC6816 doped
with blue dichroic dye (a). Corresponding polar diagram of
absorbance. Before filling, cell was irradiated by polarized UV
light for 10 min (b).
This research was supported by the Russian Foundation of Fun-
damental Research (11-03-01046-a, 12-03-31115-mol_a, 13-
03-00648-a) and by the Grant of LG-Chem.
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