Spacer length influence on the electrooptic effect of methacrylic azopolymer series

Article abstract of DOI:10.1080/15421400802463068

Methacrylic azopolymers with different spacer lengths in side chains have been synthesized. The spectra and kinetics of the electrooptic effect which arises in azopolymer films after their irradiation by linearly polarized light are investigated. The photo-induced optical anisotropy in the films appears due to a change of the concentration ratio of trans- and cis-isomers of azobenzene fragments. This explains the nature of the electrooptic effect. Owing to a mobility increment of azobenzene fragments by means of the spacer extension, the electrooptic effect becomes more pronounced, and its relaxation time decreases.

Full text of DOI:10.1080/15421400802463068

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Spacer Length Influence on
the Electrooptic Effect of
Methacrylic Azopolymer Series
O. N. Nadtoka a , N. A. Davidenko a , S. L.
Studzinsky a & V. G. Syromyatnikov a
a Taras Shevchenko Kyiv National University,
Macromolecular Chemistry Department , Kyiv,
Ukraine
Published online: 10 Jun 2010.
To cite this article: O. N. Nadtoka , N. A. Davidenko , S. L. Studzinsky & V. G.
Syromyatnikov (2008) Spacer Length Influence on the Electrooptic Effect of
Methacrylic Azopolymer Series, Molecular Crystals and Liquid Crystals, 497:1, 261/
[593]-267/[599], DOI: 10.1080/15421400802463068
To link to this article: http://dx.doi.org/10.1080/15421400802463068
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Mol. Cryst. Liq. Cryst., Vol. 497, pp. 261=[593]–267=[599], 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802463068
Spacer Length Influence on the Electrooptic Effect
of Methacrylic Azopolymer Series
O. N. Nadtoka, N. A. Davidenko, S. L. Studzinsky,
and V. G. Syromyatnikov
Taras Shevchenko Kyiv National University, Macromolecular
Chemistry Department, Kyiv, Ukraine
Methacrylic azopolymers with different spacer lengths in side chains have been
synthesized. The spectra and kinetics of the electrooptic effect which arises in azo-
polymer films after their irradiation by linearly polarized light are investigated.
The photo-induced optical anisotropy in the films appears due to a change of the
concentration ratio of trans- and cis-isomers of azobenzene fragments. This
explains the nature of the electrooptic effect. Owing to a mobility increment of azo-
benzene fragments by means of the spacer extension, the electrooptic effect becomes
more pronounced, and its relaxation time decreases.
Keywords: azopolymer; electro-optical modulator; fragment mobility, information
medium; photoisomer
1. INTRODUCTION
Azobenzene-containing polymers are widely studied due to their
potential application in many fields which include the optical data
storage, liquid crystals displays, and holographic surface relief
gratings [1–5]. The photoresponsive properties of the polymers are
based on the trans-to-cis and cis-to-trans photoisomerizations of
azobenzene which lead to considerable changes in their molecular
shape and dipole moments [6,7].
Induced polarization changing is possible under the external
thermal or mechanical action (heating, light illumination, external
electric field). In the external electric field, the rotation of azobenzene
isomers takes place. The mechanism of the electric field influence is
Address correspondence to N. A. Davidenko, Taras Shevchenko Kyiv National
University, Macromolecular Chemistry Department, 60, Volodymyrs’ka Str., Kyiv
01033, Ukraine. E-mail: daviden@ukrpack.net
261=[593]

262=[594]
O. N. Nadtoka et al.
determined by the appearance of directive forces which influence the
dipole moments of azofragments [8]. Nevertheless up to date, the issue
about the influence of a spacer length (cluster of atoms which connects
side azobenzene fragments with the main chain) on electrooptic
properties of methacrylic polymer films was not investigated. This
question is important for the creation of new electrooptic holographic
media. Therefore, the goal of our exploration was the generation of
azopolymers with different spacer lengths and the investigation of
electrooptic properties of films of such azopolymers.
2. EXPERIMENTAL
2.1. Materials
The general structure of methacrylic azopolymers is shown in Chart 1.
All the chemicals were purchased from commercial suppliers and used
without further purification unless otherwise noted. Standard distil-
lation procedures were applied. The structures of all the precursors
1
and final products were confirmed by solution H NMR spectroscopy.
All 1H NMR spectra were taken in CDCl3. The synthesis of all
polymers was similar, and only the steps toward making methacrylic
azopolymers are detailed.
Monomer Synthesis
The synthetic route for the target azomonomers (M) is shown on
Scheme 1. The corresponding monomers were synthesized by general
methods. The azocompound (0.06 mole) and triethylamine (9.0 ml)
were dissolved in THF (200 ml). The solution was kept in an ice bath
for 10 min. A solution of distilled methacryloyl chloride (6.0 ml, 0.06
mole) was added slowly to the above mixture. After the addition of
methacryloyl chloride, the resulting mixture was stirred at room tem-
perature overnight. Then the solution was poured into distilled water
(1 l), and the obtained residue was filtered and air-dried. Recrystalliza-
tion of monomers was carried out in ethanol.
CHART 1 Investigated azopolymers.

The Electrooptic Effect of Methacrylic Azopolymer Series
263=[595]
SCHEME 1 Azomonomer synthesis.
For the purpose of the insertion of an alkyl spacer into the azomo-
nomer structure, the azocompound was alkylated by bromine deriva-
tive of alcohols over potassium carbonate. The obtained product was
methacrylated as stated above (Scheme 1).
The azomonomers obtained in this way are characterized by the
good solubility in DMF, acetone, chloroform, and dioxane.
40-Methacryloxy-4-Nitroazobenzene (M1)
Orange crystals; yield 69%; mp 145 C (by DSC). H NMR (CDCl3), d
(ppm): 8.43 (d, 2H, Ph-H ortho to NO2), 8.07 (d, 2H, Ar), 8.03 (d, 2H,
ꢀ
1
=
=
Ar), 7.40 (d, 2H, Ar), 6.34 (s, 1H, CH2), 5.91 (s, 1H, CH2), 2.05 (s,
3H, ꢁCH3). UV-vis (Ethanol) kmax: 360, 485 nm. Elem. Anal. Calcd
for C16H13O4N3: C, 61.74%, H, 4.18%; N, 13.50%. Found: C,
61.70%, H, 4.16%; N, 13.52%.
40-Methacryloxyethyloxy-4-Nitroꢀazobenzene (M2)
1
Orange crystals; yield 56%; mp 130 C (by DSC). H NMR (CDCl3), d
(ppm): 8,00 (d, 2H, Ar), 8,37 (d, 2H, Ar), 7,62 (d, 2H, Ar), 7,13(d, 2H,
=
=
Ar), 6.4 (s, 1H, CH2), 5.8 (s, 1H, CH2), 2.15 (s, 3H, ꢁCH3), 4,10
(m, 2H, ArOCH2), 1,72 (m, 2H, OCH2). UV-vis (Ethanol) kmax: 360,
490 nm. Elem. Anal. Calcd for C18H17O5N3: C, 60.85%, H, 4.79%; N,
11.83%. Found: C, 60.88%, H, 4.81%; N, 11.85%.
40-Methacryloxyhexyloxy-4-Nitroazobenzene (M3)
ꢀ
1
Orange crystals; yield 50%; mp 72 C (by DSC). H NMR (CDCl3), d
(ppm): 8,02 (d, 2H, Ar), 8,34 (d, 2H, Ar), 7,68 (d, 2H, Ar), 7,10 (d,
=
=
2H, Ar), 6.30 (s, 1H, CH2), 5.95 (s, 1H, CH2), 2.20 (s, 3H, ꢁCH3),

264=[596]
O. N. Nadtoka et al.
TABLE 1 Properties of Azopolymers
ꢀ
Code
X, mol.%
Tg, C
kMax, nm
Mn
Mw
Mw=Mn
AP1
AP2
AP3
20
20
20
111
109
102
350
355
362
6100
7020
8000
8240
9760
11120
1.35
1.39
1.39
4.10 (m, 2H, ArOCH2), 3,9 (m, 2H, HOCH2) 1, 72 (m, 4H, H2) UV-vis
(Ethanol) kmax: 365, 490 nm. Elem. Anal. Calcd for C22H25O5N3: C,
61.74%, H, 4.18%; N, 13.50%. Found: C, 61.70%, H, 4.16%; N, 13.52%.
Polymerization
Copolymers were synthesized by free-radical polymerization of the
corresponding azomonomers with MMA in toluene. The polymerization
was carried out in a 10wt.% toluene solution of monomer with AIBN
as a free radical initiator (1 wt.% of monomers) at 80ꢀC for more than
30 h. Copolymers were isolated from the reaction solution by precipi-
tation into the excess of methanol followed by reprecipitation from
toluene into methanol and then dried at 20ꢀC overnight. The syn-
thetic work is described in more details in [12].
1
The synthesized azopolymers were characterized by H NMR spec-
troscopy. The obtained results are in agreement with the proposed
structures.
The phase transitions were studied by differential scanning calor-
imetry (DSC) using a Perkin Elmer DSC-2 instrument equipped with
a IFA GmbH processor at a scan rate of 20ꢀ K=min. The calorimetry
calibration was carried out according to the well-known recent recom-
mendations [13–15] using sapphire and quartz as standards.
Copolymers glass-transition temperatures (Tg) and chain-length
characteristics are represented in Table 1. All polymers are character-
ized by high Tg (above 100ꢀC). The insertion of the alkyl spacer leads to
a decrease of Tg as a result of the appearance of unconfined space and
due to an increase in the macromolecule mobility. As for the azopoly-
mer chain, its length is moderate, as seen from Table 1. We explain it
by the ability of azogroups to play a role of ‘‘traps’’ for radicals, and, as
a result, the role of the chain termination and chain transfer reactions
increases extremely.
2.2. Films
The samples for investigation were prepared as structures which have the
free surface of a polymer film: glass substrate layer – electroconductive

The Electrooptic Effect of Methacrylic Azopolymer Series
265=[597]
layer SnO2: In2O3 – polymer film. The sample preparation method is
described in [8]. The thickness of polymer films was 2–3 mm.
2.3. Methods
In the samples, we measured the optical density (D) spectra for
k ¼ 350–900 nm and the quantity dIE ¼ ðIE ꢁ I0Þ=I0, where I0 and IE
are the intensities of monochromatic light transmissed through the
sample with system of two polarizers before and after switching on
the external electric field, respectively. A sample was placed between
two polarizers, with the angle between the planes of polarization being
equal to 90ꢀ. Values of dIE were measured after the prolonged (60 min)
irradiation of the sample through a polarizer with UV-Vis light
(440 nm, normal incidence). The electric field intensity E ¼ 1 108 V=m
in the film was generated using the corona electric discharge as
FIGURE 1 Curves of dependence D=Dmax (1,2), and dIE (10,20) on k in samples
with films AP1 (1, 10), AP3 (2, 20).

266=[598]
O. N. Nadtoka et al.
FIGURE 2 Dependence dIE on t after the switching-on and switching-off of
the external electric field E in a sample with film AP3, which previously
was irradiated by a linearly polarized light with k ¼ 440 nm. Curve was
obtained for k ¼ 650 nm. The time moments of switching off and on the electric
field E are pointed by the vertical arrows.
described in [8]. The value of dIE was determined as a function of k. All
measurements were carried out at room temperature.
3. RESULTS AND DISCUSSION
The absorption of AP1-AP3 films lies in the range k < 550 nm (Fig. 1).
It strongly depends on the optical excitation of azobenzene groups and
is almost not sensitive to the presence of a ‘‘spacer’’ in the polymer
structure. The influence of E on the light transmission becomes
significant when the sample was previously irradiated with polarized
light in the absorption range of azobenzene groups (k < 550 nm). The
largest change in I0 under the switching-on of E was observed when
the light passed throught a polarizer for a long time period (t > 40 min)
(Fig. 1). After the long exposure by a linearly polarized light with
k < 550 nm, the influence of the external electric field on the light
transmission intensifies in the long-wavelength range of absorption
of the compound under study (in the wavelength range k ¼ 400–
750 nm). The kinetics of the light intensity change from I0 to IE before
and after the switching-off of the electric field (Fig. 2) can be described
by the simple relations I(t) ¼ I0 þ (IE ꢁ I0)(1–exp(ꢁ t=sE)) and I(t) ¼
I0 þ (IE ꢁ I0)(exp(ꢁ t=sE)), where sE is the time constant. In series
AP1-AP3, the value of jdIEj increases.

The Electrooptic Effect of Methacrylic Azopolymer Series
267=[599]
4. CONCLUSIONS
In the same way as in [8], we consider that, under the action of a linear
polarized light from the range of azochromophore trans-isomer absorp-
tion in AP1-AP3, the trans-cis-isomerization of azofragmensts occurs,
and the concentration ratio of fragments with different dipole
moments changes. In the external electric field, the photoinduced
dipole moments change their orientation relatively to the polarization
direction, which leads to a change of the transmission of the polymer
films. The ‘‘spacer’’ influence in azopolymer series at the transition
from AP1 to AP3 is revealed in increasing the mobility of azofrag-
ments, and, at the same time, the electrooptic effect intensifies. The
results of the present work could be used for the elaboration of new
electrooptic modulators and media for the polarization holography
with informational properties controlled by an external electric field.
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