Photoinduced spatial orientational order in methacrylic azopolymers

Article abstract of DOI:10.1080/15421400701229487

The combination of the transmission null ellipsometry and the total absorption method is applied to study the 3D orientational configurations of azochromophores in polymethacrylates with azobenzene side groups. The transformation of the initial orientation due to the photoexcitation of azochromophores is investigated. Under irradiation, if the reorientation mechanism of the photoinduced ordering prevails, the azochromophores are reoriented perpendicularly to the polarization direction of the exciting light, Eex. In the case where the photoselection ordering mechanism dominates, the 3D distribution of azochromophores in the saturation state of irradiation is isotropic due to a strong exhaustion of the number of anisotropic trans isomers. The observed regularities were earlier described for several other classes of photosensitive polymers, and so they may be common rules for the photoordering in photochromic media.

Full text of DOI:10.1080/15421400701229487

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Photoinduced Spatial
Orientational Order in
Methacrylic Azopolymers
Oksana Nadtoka a , Vladimir Syromyatnikov a , Lidiya
Olkhovik a , Oleg Yaroshchuk b & Tetyana Bidna b
a Taras Shevchenko Kyiv National University , Kyiv,
Ukraine
b Institute of Physics, NASU , Kyiv, Ukraine
Published online: 22 Sep 2010.
To cite this article: Oksana Nadtoka , Vladimir Syromyatnikov , Lidiya Olkhovik ,
Oleg Yaroshchuk & Tetyana Bidna (2007) Photoinduced Spatial Orientational Order in
Methacrylic Azopolymers, Molecular Crystals and Liquid Crystals, 468:1, 63/[415]-76/
[428], DOI: 10.1080/15421400701229487
To link to this article: http://dx.doi.org/10.1080/15421400701229487
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Mol. Cryst. Liq. Cryst., Vol. 468, pp. 63=[415]–76=[428], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701229487
Photoinduced Spatial Orientational Order
in Methacrylic Azopolymers
Oksana Nadtoka
Vladimir Syromyatnikov
Lidiya Olkhovik
Taras Shevchenko Kyiv National University, Kyiv, Ukraine
Oleg Yaroshchuk
Tetyana Bidna
Institute of Physics, NASU, Kyiv, Ukraine
The combination of the transmission null ellipsometry and the total absorption
method is applied to study the 3D orientational configurations of azochromophores
in polymethacrylates with azobenzene side groups. The transformation of the
initial orientation due to the photoexcitation of azochromophores is investigated.
Under irradiation, if the reorientation mechanism of the photoinduced ordering
prevails, the azochromophores are reoriented perpendicularly to the polarization
direction of the exciting light, Eex. In the case where the photoselection ordering
mechanism dominates, the 3D distribution of azochromophores in the saturation
state of irradiation is isotropic due to a strong exhaustion of the number of aniso-
tropic trans isomers. The observed regularities were earlier described for several
other classes of photosensitive polymers, and so they may be common rules for
the photoordering in photochromic media.
Keywords: azopolymer; photoinduced anisotropy; polymethacrylate; 3D orientational
order
1. INTRODUCTION
Azobenzene and its derivatives have attached a much attention over
the past few decades because of a number of fascinating features of
These studies were carried out within the framework of the project ‘‘Ordering regula-
rities and properties of nano-composite systems’’ of the NAS of Ukraine. Partially, they
were also supported by INTAS (project No 03-51-5448).
Address correspondence to Oksana Nadtoka, Kyiv National Taras Shevchenko Uni-
versity, Volodymyrs’ka St., 64, Kyiv 01033, Ukraine. E-mail: oksananadtoka@ukr.net
63=[415]

64=[416]
O. Nadtoka et al.
these compounds. They were initially used for the preparation of
paints, because of a rich spectrum of colors that can be obtained
depending on the chemical structure of azochromophores. Further
investigations revealed the strong photochromism of azobenzene deri-
vatives. It was found that this effect in azobenzene derivatives can be
accompanied by the novel phenomenon that is known as photoinduced
optical anisotropy (POA) and can be detected in dichroism and
birefringence measurements [1]. This feature makes azopolymers
rather attractive for a number of photonic applications such as polar-
ization holography, optical memory, integrated optical circuits, and
liquid crystal (LC) aligning substrates, etc. [2,3].
On the microscopic level, POA in azopolymers is explained by the
orientational ordering of azochromophores. The widely accepted model
of this photoorientation is based on the specific photochemistry (trans-
cis isomerization) and the strong absorption dichroism of azochromo-
phores in the trans configuration. According to theory of Dumont
[4,5], the mechanism of POA is determined by the molecular extinc-
tions (along the long and short molecular axes) of trans and cis
azochromophores depending on the wavelength of excitation, lifetime
of cis isomers, and the coefficient of rotational diffusion. M. Dumont
selected two elementary mechanisms. The first mechanism, photose-
lection or angular hole burning, is realized if the lifetime of cis isomers
is long, while their molecular extinction is low, so that azochromo-
phores undergo only one or few cycles of isomerization over the
irradiation period. Under these conditions, if the rotational diffusion
rate is not very high, the anisotropic distribution of azochromophores
appears due to the angularly selective conversion of trans chromo-
phores into the less anisotropic (isotropic in the first approximation)
cis form. The strongest conversion is observed for the fragments
oriented along the polarization direction of the exciting light, Eex.
So, due to the photoselection, azochromophores will be mainly aligned
perpendicularly to Eex. If photoproducts are thermally and photoche-
mically stable, the hole burning saturation leads to a total depletion
of the trans form and so to the isotropy of an azopolymer film. In the
opposite case, if the lifetime of cis isomers is short and=or their mol-
ecular extinction is high, azochromophores undergo many isomeriza-
tion cycles over the excitation period. The change of the molecular
shape leads to a change of the molecular orientation. Due to the many
cycles of isomerization, a random-walk rotation of an azochromophore
appears. The chromophores accidentally aligned perpendicularly to Eex
will be excluded from the further rotation, because their absorption is
minimized. Thus, the photoinitiated random-walk rotation leads to the
accumulation of azochromophores in the direction perpendicular to the

Photoinduced Order in Methacrylic Azopolymers
65=[417]
polarization of the exciting light. This mechanism is known as photo-
reorientation or angular redistribution. More generally, through both
the photoselection and photoreorientation, the system tends to mini-
mize the probability of the optical excitation. The final distribution
of azochromophores is a result of the competition between the above
ordering process and the thermal diffusion in the ground state, which
tends to establish the isotropy.
In the present research, we elucidate the ordering regularities in
azogroups contained in polymethacrylates (poly-AzoMA). This is the
most popular class of azopolymers quite attractive for practical uses
because of the enhanced thermal stability, excellent film-forming
properties, relatively simple synthesis, etc. In spite of this popularity,
the studies of the photoinduced anisotropy in these polymers were
mainly restricted to the 2D case, which cannot give a complete idea
of the ordering peculiarities. Regarding the 3D consideration, we note
only a few studies based on different variations of the prism coupling
method [4,6,7] and works of Spiess with coworkers [8] used the total
absorption method briefly described in the following section. In
addition, the 3D order in a polymer of this class has been studied in
our previous works [9–11]. In the present paper, we apply the trans-
mission null ellipsometry supported by the modified total absorption
method to investigate the 3D order in a big series of azopolymers based
on polymethacrylate. As we earlier proved, this combination is effec-
tive practical tool for the study of orientational molecular distribu-
tions. We consider the induced orientational order as a function of
the molecular structure, supramolecular organization, and irradiation
conditions. The regularities obtained are compared with those earlier
established for other kinds of azopolymers.
2. EXPERIMENTAL
2.1. Materials
The general structure of poly-AzoMA is shown in Chart 1. All the
chemicals were purchased from commercial suppliers and used with-
out further purification unless otherwise noted. Standard distillation
procedures were applied. The structures of all the precursors and final
1
1
products were confirmed by H NMR spectroscopy of solutions. All H
NMR spectra were taken in CDCl3. The synthesis of all polymers was
similar, and only the steps toward making poly-AzoMA are detailed.
Monomer synthesis. The synthesis route for the target azomono-
mers (M) is shown in Scheme 1. The corresponding monomers were
synthesized by general methods. The azocompound (0.06 mol) and

66=[418]
O. Nadtoka et al.
CHART 1 Objects of Investigation (p-AzoMA).
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 mol) was added slowly to the above mixture.
After the addition of methacryloyl chloride, the resulting mixture
SCHEME 1 Synthesis of methacrylic azomonomers.

Photoinduced Order in Methacrylic Azopolymers
67=[419]
was stirred at room temperature overnight. Then the solution was
poured into distilled water (1 L), and the obtained residue was filtered
and air-dried. Recrystallization of monomers was carried out in ethanol.
40-methacryloxy-4-nitroazobenzene (M1). Orange crystals;
ꢀ
1
yield 69%; mp 145 C (by DSC). H NMR (CDCl3), d (ppm): 8.43 (d,
2H, PhꢁH ortho to NO2), 8.07 (d, 2H, Ar), 8.03 (d, 2H, 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-nitroazobenzene (M2). Orange
ꢀ
1
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). Orange
crystals; yield 50%; mp 72ꢀC (by DSC). 1H 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), 4.10 (m,
2H, ArOCH2), 3.9 (m, 2H, HOCH2) 1.72 (m, 4H, H2). UV-vis (ethanol)
k
max: 365365, 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%.
40-methacryloxy-4-buthoxycarbonylazobenzene (M4). Yellow
crystals; yield 60%; mp 63ꢀC (by DSC). 1H NMR (CDCl3), d (ppm):
7.89 (d, 2H, Ar), 7.73 (d, 2H, Ar), 7.93 (d, 2H, Ar), 6.94 (d, 2H, Ar),
=
=
6.34 (s, 1H, CH2), 5.90 (s, 1H, CH2), 2.10 (s, 3H, ꢁCH3), 4.17 (m,
2H, CH2) 1.72 (m, 9H, C4H9). UV-vis (ethanol) kmax: 363, 495 nm.
Elem. Anal. Calcd for C21H22O4N2: C, 67.38%, H, 5.88%; N, 7.65%.
Found: C, 67.40%, H, 5.90%; N, 7.61%.
4’-methacryloxy-4-pentoxyazobenzene (M5). Yellow crystals;
yield 65%; mp 72ꢀC (by DSC). 1H NMR (CDCl3), d (ppm): 7.85 (d,
2H, Ar), 7.65 (d, 2H, Ar), 7.74 (d, 2H, Ar), 6.91 (d, 2H, Ar), 6.30 (s,
=
=
1H, CH2), 5.84 (s, 1H, CH2), 2.08 (s, 3H, ꢁCH3), 4.2 (m, 2H, CH2)
1.72 (m, 9H, C4H9). UV-vis (ethanol) kmax: 378, 500 nm. Elem. Anal.
Calcd for C21H24O3N2: C, 71.59%, H, 6.82%; N, 7.95%. Found: C,
71.62%, H, 6.80%; N, 7.93%.
Polymerization. Homopolymers were synthesized by free-radical
polymerization in toluene. The polymerization was carried out in a
10-wt.% toluene solution of the monomer with AIBN as a free radical
initiator (10 wt.% of the monomer) at 80ꢀC over more than 30 h. Poly-
mers were isolated from the reaction solution by precipitation into the

68=[420]
O. Nadtoka et al.
excess of methanol followed by reprecipitation from toluene into meth-
anol and then dried at 20ꢀC overnight. The synthesis is described in
more details in a separate paper [12].
1
The synthesized azopolymers were characterized by H NMR spec-
troscopy. The obtained results were 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
an IFA GmbH processor at a scan rate of 20ꢀ K=min. The calorimetry
calibration was carried out according to the well-known recent rec-
ommendation [13–15] using sapphire and quartz as the standard.
2.2. Films
The polymers were dissolved in DMF (2 wt%) and filtered by using a
0.2 mm Teflon filters. The prepared solution was spin coated on the
plates of fused quartz allowing us to measure the polymer absorption
in a UV range. The films were dried at 90ꢀC for 1 h. The thickness of
the films was measured by a profilometer. It was varied within 300–
1000 nm.
The photo-ordering processes were initiated with the polarized UV
light from a high-pressure mercury lamp. The light propagation direc-
tion corresponded to the z-axis, while the light polarization was always
chosen along the x-axis of the Cartesian coordinate system with the x-
and y-axes parallel to the verges of a rectangular polymer film, the z-
axis being normal to this film (Fig. 1). The line at 365 nm was selected
with an interference filter. The light was directed normally to the
films. The irradiation was carried out stepwise. After each irradiation
step, the photoinduced order was evaluated. The time period between
the irradiation and measurements was about 10 min to ensure the
relaxation of cis isomers (at least their short living fraction).
2.3. Methods
The key method of our studies was the transmission null ellipsometry
(TNE). This is a modified version of the Senarmon method extended
for measurements of the out-of-plane retardation. The obtained in-
plane and out-of-plane retardation values (ðny ꢁ nxÞd and ðnz ꢁ nxÞd,
Fig. 1) allowed us to judge about the ellipsoid of the refractive index
nij and, finally, about the orientational configuration of azochromo-
phores assuming that the directions of maximal values of nij corre-
spond to the maximally populated directions of azochromophores.
The details of this method can be found in our previous works [9,10].

Photoinduced Order in Methacrylic Azopolymers
69=[421]
FIGURE 1 The transformations of the 3D orientational distributions of trans-
azochromophores in the poly-AzoMA films due to k ¼ 365 nm irradiation. (a)
before irradiation; (b) biaxial transient distribution; (c) oblate distribution
with the symmetry axis parallel to Eex (P1, P2, P3, P4, photosaturation state);
(d) isotropic distribution (P5, photosaturation state).
As a complementary characterization method, we also perform 2D
dichroism measurements in the UV=Visible spectral range. The optical
densities, Dx and Dy corresponding to the x and y in-plane polariza-
tions, are measured with a probe beam propagating perpendicularly
to the sample. The third out-of-plane component Dz, can be estimated
by the total absorption (TA) method, if the total absorption, Dtotal
¼
Dx þ Dy þ Dz, is the same for all measuring steps. This is true, for
instance, if the photoinduced cis isomers quickly relax back to the
trans form and the loss of azochromophores due to a photodegradation
is negligible. The Dtotal can be easily obtained, if the sample is uniaxial
at some instant of time t0 with an in-plane orientation of the
anisotropy axis, say y. Then
Dtotal ꢂ Dxðt0Þ þ Dyðt0Þ þ Dzðt0Þ ¼ 2Dxðt0Þ þ Dyðt0Þ
ð1Þ
Since we assumed that the number of azobenzene units in the trans
configuration remains constant at each instant of time t, Dz(t) can be
estimated as
DzðtÞ ¼ Dtotal ꢁ DxðtÞ ꢁ DyðtÞ;
ð2Þ

70=[422]
O. Nadtoka et al.
where Dx(t) and Dy(t) are the experimentally measured in-plane
components.
Then, the diagonal terms of the tensor of orientational order Sij can
be estimated. For example:
1
Dx ꢁ 2 ðDy þ DzÞ
Sxx
¼
:
ð3Þ
Dx þ Dy þ Dz
The components Syy and Szz can be obtained by cyclic permutation in
expression (3).
In the original version of TAM, Diði ¼ x; y; zÞ corresponded to the
maximum of the vibration bands (IR spectral range) characteristic of
azochromophore. In our version, the absorption in the UV=Vis range
is measured. We operated with the Diði ¼ x; y; zÞ values corresponding
to the ppꢃ absorption maximum of trans chromophores.
The UV=Vis absorption measurements were carried out by using
a diode array spectrometer S2000 from Ocean Optics Co. The samples
were set normally to the testing light from a low-intensity deuterium
lamp. A Glan-Thompson prism was used to polarize the probe beam.
RESULTS AND DISCUSSION
First of all, we measured the UV=Vis absorption spectra of azopoly-
mers before and after the irradiation with the unpolarized UV light
(365 nm, normal incidence). These measurements confirmed that the
major photoreaction is the trans-cis isomerization revealed itself in a
strong depletion of the ppꢃ band and an increase of the npꢃ band which
ꢃ
is much more intense for cis isomers. The kpp values corresponding to
a maximum of the ppꢃ absorption band are shown in Chart 1. As
usually, a batochromic shift of the ppꢃ band is observed by substituting
the alkoxy chain by a polar NO2 group in the chromophore’s tail (push-
pull chromophores). In spite of this shift, the 365-nm excitation wave-
length corresponds in all polymers to the effective adsorption within
the ppꢃ absorption band.
With regard for the results on the 3D ordering, two types of the
ordering behavior can be selected. We describe them by an example
of P1 and P5, representatives of these two types. The values of
the in-plane, ðny ꢁ nxÞd, and the out-of-plane ðnz ꢁ nxÞd retardation
for the films of P1 and P5 are presented, respectively, in Figure 2a
and Fig. 3a. The data correspond to successive exposure doses (the
x polarization of the UV irradiation). Several conclusions from
these curves can be drawn. First of all, before the irradiation,

Photoinduced Order in Methacrylic Azopolymers
71=[423]
FIGURE 2 Dependence of the absorption components (a) and phase retar-
dation components (b) of polymer P1 on irradiation time under conditions:
the wavelength was 365 nm, the intensity was 5 mW=cm2, x polarization).
azochromophores demonstrate a slight preference to the out-of-plane
alignment (nz > nx ¼ ny, Fig. 1a). This rule was true for all homologues
studied. The irradiation, however, induces different kinds of a trans-
formation of this initial order in two different polymer classes we
selected. This difference is especially clear for the structures realized
in the saturated states of the irradiation reached for all polymers. In
the P1 film, the irradiation results in an oblate order with the x order-
ing axis (nx < ny ¼ nz, Fig. 1c). In contrast, in the P5 film, the isotropic
order is formed (nx ¼ ny ¼ nz, Fig. 1d). In the general case, the transi-
ent orientational structures are biaxial (nx ¼ ny ¼ nz, Fig. 1b). In P5
films, between the biaxial and isotropic orientations, the uniaxial
out-of-plane alignment (nz > nx ¼ ny, Fig. 1a) can be observed as a

72=[424]
O. Nadtoka et al.
FIGURE 3 The kinetics of the absorption components Dx; Dy (a) and phase
retardation components ðny ꢁ nxÞd and ðnz ꢁ nxÞd (b) for successive expouse
doses for polymer P5. Irradiation is with kex ¼ 365 nm (I ¼ 5 mW=cm2, x polar-
ization).
result of the faster degradation of the in-plane anisotropy ðny ꢁ nxÞd
compared with the out-of-plane anisitropy ðnz ꢁ nxÞd. The Dx and Dy
absorption components as functions of the exposure time measured
for P1 and P5 are presented in Figure 2b and Figure 3b. The behavior
of the Dx(t) and Dy(t) curves in Figure 2b is typical of the reorientation
mechanism: an increase of Dy(t) and a simultaneous decrease of Dx(t)
(xkEex) is an evidence for the redistribution of azochromophores with
increase of their concentartion in the y direction perpendicular to
the light polarization Eex [4,5]. In turn, the simultaneous decrease of
Dx(t) and Dy(t) curves is typical of the angular photoselection [4,5].
The conversion of trans isomers in the long living cis form firstly in

Photoinduced Order in Methacrylic Azopolymers
73=[425]
FIGURE 4 Kinetics of Sxx, Syy, Szz for successive exsposure doses (polymer
P1) calculated from the data of Fig. 2 b according to formula (3).
the light polarization direction Eex and then in all other spatial
directions may explain the isotropic order realized in the P5 film at
high irradiation doses.
As we discussed in Introduction, the reorientation mechanism may
be an evidence for the short lifetime of cis isomers. If it is really so, the
total absorption of azochromophores Dtotal can be practically kept con-
stant after successive irradiation steps. In conjunction with a fact of
the uniaxial ordering in a stationary state (ny ¼ nz), this allows us to
apply the TA method to calculate the Dz component. The calculated
Dz(t) curve is shown in Figure 2b.
The Di(i ¼ x,y,z) valueswereused to calculate the diagonal components
ˆ
Sxx, Syy, and Szz of the order parameter tensor S according to (3). The Sxx(t),
Syy(t), and Szz(t) curves are shown in Figure 4. The uniaxial oblate distri-
bution of azochromophores with the x ordering axis, realized in
the stationary state of irradiation, is characterized by the scalar
S ꢂ Sxx ¼ ꢁ0:07. The initial order of azochromophores (uniaxial ordering
along the film normal) is also characterized by a scalar: S ꢂ Szz ¼ 0:06.
The latter value indicates that the initial out-of-plane ordering is rather
weak. The transient structures between the uniaxial structures in the
initial state and in the stationary state of the excitation are biaxial and
ˆ
so can be described at least with three (diagonal) elements of S.
Note again, that the TA method cannot be applied for P5 because of
the strong exhaustion of trans chromophores possible for rather stable

74=[426]
O. Nadtoka et al.
cis isomers. This automatically excludes the conditions needed for the
TA method.
Qualitatively, the ordering peculiarities of P1 inhere in the series
P2, P3, P4, i.e., in all homologues containing an acceptor group
(push-pull azochromophore). The transformation of the 3D order
under irradiation for these polymers is schematically presented in
Fig. 1 as transformation way I. This general tendency does not depend
on the length of the side-chain spacer as well as on the concentration
of azochromophores in co-polymers. At the same time, the mentioned
molecular variations influence the induced birefringence and dichro-
ism which determine the ordering rate of azochromophores. These
quantitative regularities will be considered in the forthcoming publi-
cations.
The ordering features of P5 are presented as transformation way II
in Fig. 1: on the initial stage of irradiation, the biaxial alignment is
observed, which transforms, with increase in the irradiation dose, into
a spatially isotropic distribution.
The general tendencies of the photoinduced ordering allows us to
conclude that the photoinduced order in poly-AzoMA is mainly influ-
enced by the molecular photoordering through the photoselection
and reorientation mechanisms described above. The clear evidence
for it is the fact that the realized orientational configurations of azo-
chromophores reflect the photoexcitation geometry. Energetically,
these configurations can be not profitable (as in our experimental
case), counter to a self-organization of anisotropic molecules aimed
at the achivement of a uniaxial prolate order to minimize the free
energy of a polymer. So, in strong contrast to azobenzene polyesters
earlier studied [16], the self-ordering under photoirradiation in poly-
AzoMA at the ambient temperatures is rather weak. This can be
caused by rather high temperatures of the glass transition
(Tg > 100ꢀC), which testifies to a deep ‘‘freezing’’ of p-AzoMA chains,
so that they cannot be involved in the processes of self-organization
stimulated by UV light. Contrary to this, the glass transition tempera-
tures in azopolyesters are low (Tg < 20ꢀC). So both azochromophores
and polyester chains participate in the collective processes of self-
organization. The ‘‘freezing’’ of polymer chains in poly-AzoMA poly-
mers can also explain why the liquid crystallinity, as a particular
feature of the self-organization, does not influence substantially the
order in these polymers. According to [17], the contribution of the
instrinsic self-organization to the orientational order in poly-AzoMA
can be strongly increased if the film is irradiated at a temperature a
little lower than Tg. Thus, the competition of the molecular photoorie-
nation and the self-organization in the 3D ordering can be achieved for

Photoinduced Order in Methacrylic Azopolymers
75=[427]
elevated temperatures close to 100ꢀC. This can be a rather interesting
subject for the future studies.
Since a light ordering factor at ambient temperatures strongly
prevails, the induced orientation depends rather on the molecular
photochemistry than on the self-ordering processes. If the photoreor-
ientation mechanism dominates, the oblate order with the symmetry
axis parallel to Eex is formed. In the opposite case, if the photoselection
mainly influences the order, a spatially isotropic alignment is realized.
CONCLUSIONS
Thus, studying the 3D orientational ordering in the azogroups con-
taining polymethacrylates, we revealed the regularities earlier estab-
lished for other kinds of azopolymers [16], as well as the polymers
containing a cinnamoyl photosensitive group [18]. The main rule says
that, under photoexcitation, the initial orientational distribution (fre-
quently, it is uniaxially ordered due to the self-ordering of photosensi-
tive groups) is transformed into some other uniaxially ordered
distribution (in the case of the photoreorientation mechanism) or the
isotropic distribution (in the case of photoselection). The transient
structures from the initial to the final uniaxial (isotropic) distributions
are biaxial. In the case of photoselection, the transient uniaxial home-
otropic alignment can be observed if the in-plane depletion rate of
trans chromophores is substantially higher than the out-of-plane
one. Before the irradiation, the azochromophores in the studied poly-
AzoMA demonstrate the preference to the out-of-plate alignment that
can be caused by the self-ordering in the diluted polymers under the
film formation. The final photoinduced alignment is a result of the con-
curring photoorientation and the self-organization. The latter factor in
poly-AzoMA at ambient temperatures is weak, presumably, because of
high Tg. This does not allow the collective self-organization processes
under photoexcitation to play an important role, for example, in azopo-
lyesters [19]. The regularities observed may predict the alignment
tendencies in other kinds of photopolymers showing POA.
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