On biaxiality of photoinduced structures in azopolymer films
Spatial orientational structures in the layers of liquid crystalline polymalonates are investigated using UV absorption method and the results of the null ellipsometry technique. Preferential in-plane alignment of azobenzene fragments and in-plane reorientation under irradiation with polarized UV...
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Інститут фізики конденсованих систем НАН України
2001
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irk-123456789-1197592017-06-09T03:04:53Z On biaxiality of photoinduced structures in azopolymer films Kiselev, A. Yaroshchuk, O. Zakrevskyy, Yu. Tereshchenko, A. Spatial orientational structures in the layers of liquid crystalline polymalonates are investigated using UV absorption method and the results of the null ellipsometry technique. Preferential in-plane alignment of azobenzene fragments and in-plane reorientation under irradiation with polarized UV light are established. The components of the order parameter tensor of azobenzene fragments are estimated for the initial state and after different doses of irradiation. The uniaxial as well as biaxial order of the azobenzene chromophores are detected. The biaxiality is observed in the intermediate stages of irradiation, whereas the uniaxial order is realized in the saturated state. The proposed theory takes into account biaxiality of the induced structures and describes well the experimental dependencies of the order parameter components on the time of irradiation. Шляхом використання методу УФ поглинання та результатів еліпсометричних вимірювань вивчені просторові орієнтаційні структури в шарах рідкокристалічних полімерів. Встановлено, що під дією поляризованого УФ випромінювання азобензольні фрагменти орієнтуються переважно в площині. Отримано оцінку для компонентів тензора параметра порядку як у початковому стані, так і після різних доз опромінювання. Виявлено як одновісне так і двовісне впорядкування азобензольних хромофорних груп. Причому двовісність спостерігається у проміжній стадії опромінювання, тоді як одновісний стан реалізується в режимі насичення. Запропонована теорія враховує двовісність індукованих структур і добре описує експериментальні залежності компонентів параметра порядку від часу опромінювання. 2001 Article On biaxiality of photoinduced structures in azopolymer films / A. Kiselev, O. Yaroshchuk, Yu. Zakrevskyy, A. Tereshchenko // Condensed Matter Physics. — 2001. — Т. 4, № 1(25). — С. 67-75. — Бібліогр.: 15 назв. — англ. 1607-324X PACS: 61.30.Gd, 42.65.-k, 78.66.Qn DOI:10.5488/CMP.4.1.67 http://dspace.nbuv.gov.ua/handle/123456789/119759 en Condensed Matter Physics Інститут фізики конденсованих систем НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| description |
Spatial orientational structures in the layers of liquid crystalline polymalonates
are investigated using UV absorption method and the results of the
null ellipsometry technique. Preferential in-plane alignment of azobenzene
fragments and in-plane reorientation under irradiation with polarized UV
light are established. The components of the order parameter tensor of
azobenzene fragments are estimated for the initial state and after different
doses of irradiation. The uniaxial as well as biaxial order of the azobenzene
chromophores are detected. The biaxiality is observed in the intermediate
stages of irradiation, whereas the uniaxial order is realized in the saturated
state. The proposed theory takes into account biaxiality of the induced
structures and describes well the experimental dependencies of the order
parameter components on the time of irradiation. |
| format |
Article |
| author |
Kiselev, A. Yaroshchuk, O. Zakrevskyy, Yu. Tereshchenko, A. |
| spellingShingle |
Kiselev, A. Yaroshchuk, O. Zakrevskyy, Yu. Tereshchenko, A. On biaxiality of photoinduced structures in azopolymer films Condensed Matter Physics |
| author_facet |
Kiselev, A. Yaroshchuk, O. Zakrevskyy, Yu. Tereshchenko, A. |
| author_sort |
Kiselev, A. |
| title |
On biaxiality of photoinduced structures in azopolymer films |
| title_short |
On biaxiality of photoinduced structures in azopolymer films |
| title_full |
On biaxiality of photoinduced structures in azopolymer films |
| title_fullStr |
On biaxiality of photoinduced structures in azopolymer films |
| title_full_unstemmed |
On biaxiality of photoinduced structures in azopolymer films |
| title_sort |
on biaxiality of photoinduced structures in azopolymer films |
| publisher |
Інститут фізики конденсованих систем НАН України |
| publishDate |
2001 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/119759 |
| citation_txt |
On biaxiality of photoinduced structures in azopolymer films / A. Kiselev, O. Yaroshchuk, Yu. Zakrevskyy, A. Tereshchenko // Condensed Matter Physics. — 2001. — Т. 4, № 1(25). — С. 67-75. — Бібліогр.: 15 назв. — англ. |
| series |
Condensed Matter Physics |
| work_keys_str_mv |
AT kiseleva onbiaxialityofphotoinducedstructuresinazopolymerfilms AT yaroshchuko onbiaxialityofphotoinducedstructuresinazopolymerfilms AT zakrevskyyyu onbiaxialityofphotoinducedstructuresinazopolymerfilms AT tereshchenkoa onbiaxialityofphotoinducedstructuresinazopolymerfilms |
| first_indexed |
2025-07-08T16:32:59Z |
| last_indexed |
2025-07-08T16:32:59Z |
| _version_ |
1837097162293379072 |
| fulltext |
Condensed Matter Physics, 2001, Vol. 4, No. 1(25), pp. 67–75
On biaxiality of photoinduced
structures in azopolymer films
A.Kiselev 1 , O.Yaroshchuk 2 , Yu.Zakrevskyy 2 ,
A.Tereshchenko 2
1 Chernigiv Technological University,
95 Shevchenko Str., 14027 Chernigiv, Ukraine
2 Institute of Physics of the National Academy of Sciences of Ukraine,
46 Nauki Ave., 03028 Kyiv, Ukraine
Received August 1, 2000
Spatial orientational structures in the layers of liquid crystalline polymalo-
nates are investigated using UV absorption method and the results of the
null ellipsometry technique. Preferential in-plane alignment of azobenzene
fragments and in-plane reorientation under irradiation with polarized UV
light are established. The components of the order parameter tensor of
azobenzene fragments are estimated for the initial state and after different
doses of irradiation. The uniaxial as well as biaxial order of the azobenzene
chromophores are detected. The biaxiality is observed in the intermediate
stages of irradiation, whereas the uniaxial order is realized in the saturat-
ed state. The proposed theory takes into account biaxiality of the induced
structures and describes well the experimental dependencies of the order
parameter components on the time of irradiation.
Key words: azopolymer, photo-induced anisotropy, spatial orientation
PACS: 61.30.Gd, 42.65.-k, 78.66.Qn
1. Introduction
The photoinduced anisotropy (POA) in azopolymer films is a well-known phe-
nomenon that is extremely promising for applications in systems for optical storage
and computing, telecommunication, holography etc. [1]. Besides, photooriented poly-
mer layers are excellent aligning substrates for liquid crystals [2]. The microscopic
origin of POA in azopolymers is assumed to be a photoinduced orientational order-
ing of azobenzene moieties in the direction perpendicular to polarization direction
of the exciting light E. Mechanisms of such orientational ordering are described
elsewhere [1,3]. As far as the spatial case is concerned, there are many directions
perpendicular to E. The variety of orientational 3D structures (uniaxial, biaxial,
splayed with different spatial orientation of the principal axes) can be observed de-
c© A.Kiselev, O.Yaroshchuk, Yu.Zakrevskyy, A.Tereshchenko 67
A.Kiselev et al.
pending on many factors such as chemical structure, method of film preparation,
irradiation conditions and so on. The study of the 3D orientational structures in
polymer films was restrained with the lack of effective experimental methods. The
known approaches [4,5] are associated with a limited field of application and strong
approximations. For the sake of simplicity, in the experimental studies [1,6,7] and
even in theoretical calculations [3] uniaxial ordering had been assumed. The first at-
tempt to estimate the biaxiality was by Wiesner et al. [5] that used the results of IR
(infrared) absorption. The weak point of the approach [5] is that it can be applied
only to the special orientational configuration and requires to known the original
orientational structure. We used this method to process the data of UV (ultraviolet)
absorption. On the other hand, the method of null ellipsometry is complementary
to the UV spectroscopic measurements. By using it we estimated [8,9] the type of
the 3D orientational configuration before irradiation as well as after subsequent pe-
riods of irradiation. These results combined with the UV absorption measurements
allowed us to estimate the components of the order parameter tensor of azobenzene
chromophores. Our theoretical approach to the photoinduced ordering in azopoly-
mers is based on kinetic rate equations extended to take into consideration the
biaxiality of the induced structures. As a result, we computed the order parameter
components of azobenzene units for different irradiation doses. The predictions of
the theory are in good agreement with the data obtained experimentally.
2. Experimental
2.1. Samples
We used the poly[octyl(4-hexyloxy-4’-nitro)azobenzenemalonate] which synthesis
is described in [10]. The polymer was solved in dichloroethane and spincoated on
the quartz slabs. The prepared films were kept at room temperature for 24 h for the
solvent to evaporate.
To induce anisotropy in the films, we used irradiation by a Hg lamp. The intensity
of the actinic light was about 4.0 mW. A Glan-Thomson polarizer was applied for the
polarization of UV light. We used normal incidence of the actinic light in our studies.
The irradiation was provided in several steps followed by absorption measurements.
2.2. Method
The UV/Vis (ultraviolet/visible) absorption measurements were carried out us-
ing a PC conjugated spectrometer KSVU (LOMO, Russia). The wavelength of the
testing light was tuned to the absorption maximum of the azobenzene fragments.
The testing beam was polarized and directed normally to the film plane. Absorption
components in the direction of the exciting light polarization E and perpendicularly
to E were measured. We denote them as Dx and Dy, respectively. The out-of-plane
absorption component, Dz, was estimated by making use of the method proposed
in [5]. The latter assumes that the sample has uniaxial structure with in-plane posi-
tion of the axis of anisotropy at the instant of time t0. It implies thatDz(t0) = Dx(t0)
68
On biaxiality of photoinduced structures in azopolymer films
and the total (spatial) absorption can be estimated as follows
Dtotal = Dx(t0) +Dy(t0) +Dz(t0) = 2Dx(t0) +Dy(t0) . (1)
When the number of trans azobenzene units does not change considerably, the total
absorption is constant and the value of Dz at the instant of time t can be determined
from the following equation:
Dz(t) = Dtotal −Dx(t)−Dy(t) , (2)
where Dx(t) and Dy(t) are experimentally measured parameters.
2.3. Results and discussion
The UV/Vis spectra of the studied compound have an intensive absorption band
with the maximum at λt = 377 nm corresponding to the ππ∗ transition of trans
azobenzene fragments. The wavelength of the testing beam was tuned to this value.
After switching off the actinic light, the spectrum revealed changes that became
stationary for approximately 5 min. In order to have the azobenzene units relaxed
to the stationary state, the components Dx and Dy were measured 15 min after each
irradiation period.
In order to estimate a lifetime of cis isomers, we have measured the relaxation
of the spectral changes at λt = 377 nm after irradiation with non-polarized light.
The incidence directions of both actinic and testing light were approximately nor-
mal to the film. It was found that the relaxation curve contains two components
with characteristic times of 0.5 s and 4 min, respectively. The first value can be
attributed to cis -trans transition of azobenzene chromophores, whereas the second
one corresponds to orientational relaxation of the units.
The experimentally measured absorption components Dx and Dy for different
irradiation doses are presented in figure 1a. The kinetics of Dx and Dy is typical for
the reorientation mechanisms of azobenzene units [3]. Both curves Dx(t) and Dy(t)
reveal saturation. As it was shown earlier shown by null ellipsometry method [8,9],
the saturated state of the film of the studied polymer is fitted as uniaxial structure
with the in-plane orientation of the axis of anisotropy. Taking it into account and
additionally assuming that lifetime of cis form is about 0.5 s we arrive at the con-
clusion that, in our experimental case, the method described above can be applied
to estimate Dz. The values of Dz calculated by means of equations (1)–(2) are also
presented in figure 1a. Dependencies Dx(t), Dy(t) and Dx(t) show that the photoin-
duced ordering is mainly due to the in-plane reorientation of azobenzene fragments
in the y direction. In addition, slight reorientation from x to z direction is observed.
The orientational structure in general case is described by the tensor Sij, which is
diagonal when the coordinate axes directed along the principal axes of the film. The
diagonal elements Sxx ≡ Sx, Syy ≡ Sy and Szz ≡ Sx are related to the absorption
components Dx, Dy and Dz [5]. For example,
Sx =
2Dx − (Dy +Dz)
2(Dx +Dy +Dz)
. (3)
69
A.Kiselev et al.
0 10 20 30 40 50 60
Time, min
-0.2
0
0.2
0.4
O
rd
er
p
ar
am
et
er
sSx
Sy
Sz
0 10 20 30 40 50 60
Time, min
0.5
1
A
bs
or
ba
nc
e,
a
.u
. Dy
Dx
Dz
(a) (b)
Figure 1. Dependencies of (a) the principle absorption coefficients and (b) com-
ponents of the order parameter on irradiation time.
The components Sy and Sz can be obtained by cyclic permutation in the expres-
sion (3). The values of Sx, Sy and Sz calculated using equation (3) are presented in
figure 1b. It is seen that the initial stage of irradiation is characterized by biaxiality,
whereas the photosaturated stage by is characterized uniaxial orientation with the
order parameter about 0.7. According to [9], the uniaxiality of the saturated state
could be caused by mesomorphic properties of azobenzene fragments. The action
of actinic light then can be considered as a factor stimulating the selforganization
peculiar to mesophases [11]. From this point of view, the realized structure is a result
of photoreorientation and selfassembling processes in the film.
3. Theory
In this section we briefly discuss our theoretical approach to the kinetics of the
photoinduced reorientation. Starting from the master equations, we then specify the
rates of the transitions involved. The final kinetic equations for the order parameters
are derived after making assumptions regarding the form of the angular redistribu-
tion probabilities. In addition, this phenomenological model includes a set of order
parameters that characterizes main chain orientation of the polymer.
70
On biaxiality of photoinduced structures in azopolymer films
3.1. Master equations
In what follows we shall assume that the dye molecules in the ground state
are of the trans form with the orientation of the molecular axis defined by the
unit vector n̂. The latter is specified by the polar, θ, and azimuthal, φ, angles:
n̂ = (sin θ cos φ, sin θ sinφ, cos θ).
Angular distribution of the trans molecules is characterized by the distribution
function Ftr(n̂). Molecules in the excited state have the cis conformation and the
corresponding distribution function is Fcis(n̂). Then for the number of trans and cis
molecules we have
Ntr =
∫
Ftr(n̂) dn̂, Ncis =
∫
Fcis(n̂) dn̂, N = Ntr +Ncis , (4)
respectively, and N is the total number of molecules.
Introducing the normalized distribution functions f1(n̂) ≡ Ftr/N and f2(n̂) ≡
Fcis/N , we write the kinetic rate equations of the following general form [12,13]:
∂fi
∂t
=
[
dfi
dt
]
Diff
+
2
∑
j=1
∫
[Wij(n̂, n̂
′) fj(n̂
′, t)−Wji(n̂
′, n̂) fi(n̂, t) , ] dn̂
′ , (5)
where i = 1, 2. The first term on the right hand side of equation (5) is due to
rotational diffusion of molecules in trans (i = 1) and cis (i = 2) conformations, so
that the terms proportional to Wii can be incorporated into the diffusion term. The
remaining part of equation (5) accounts for trans-cis (terms proportional to W21)
and cis-trans (terms proportional to W12) transitions.
The trans-cis transition is stimulated by the incident UV-light quasiresonant to
the corresponding transition. Assuming that the electromagnetic wave is linearly
polarized along the x–axis, the rate of the transition can be written as follows [3,14]:
W21(n̂, n̂
′) = Γtr(n̂, n̂
′)Ptr(n̂
′) , (6)
Ptr(n̂
′) = Φtr→cis
∑
i,j
σ
(tr)
ij (n̂′)EiE
∗
j = qtI(1 + u n2
x) , (7)
where σ(tr)(n̂) is the tensor of absorption cross section for a trans molecule oriented
along n̂: σ
(tr)
ij = σ
(tr)
⊥ δij + (σ
(tr)
|| − σ
(tr)
⊥ )ni nj , u ≡ (σ
(tr)
|| − σ
(tr)
⊥ )/σ
(tr)
⊥ and Φtr→cis is
the quantum yield of the process; Γtr(n̂, n̂
′) describes the angular redistribution of
molecules excited in the cis state; I is the pumping intensity.
For the cis-trans transition along the same line of arguments we have
W12(n̂, n̂
′) = (γc + qcI) Γc(n̂, n̂
′), qc ≡ Φcis→transσ
(cis) , (8)
where γc ≡ 1/τc, τc is the lifetime of cis molecule and the anisotropic part of
absorption cross section is disregarded, σ
(cis)
|| = σ
(cis)
⊥ ≡ σ(cis).
71
A.Kiselev et al.
3.2. Order parameters
The following three order parameters are of our primary concern:
Sx = 〈 3n2
x − 1 〉/2 ≡ −S0/2 +
√
3/2S2, (9a)
Sy = 〈 3n2
y − 1 〉/2 ≡ −S0/2−
√
3/2S2, (9b)
Sz = 〈 3n2
z − 1 〉/2 ≡ S0 , (9c)
where 〈Ψ〉 ≡
∫
Ψf1 dn̂.
We can now expand the distribution functions fi(n̂) in spherical functions and
derive equations for the corresponding components. The next step involves truncat-
ing the system and computing the kernels of angular redistribution. Omitting the
details, in the simplest case, we have the following system of kinetic equations:
∂n
∂t
= −(γc + qcI)n+ q̄tI(1− n+ ū Sx) , (10a)
∂Si
∂t
= −q̄tI
[
∑
j=0,2
wij Sj − v(1− n)(δi,0 −
√
3/8 δi,2)
]
− (q̄tI +Dt)(Si − S
(p)
i ) + (γc + qcI)ΓcCi , (10b)
∂Ci
∂t
= −(γc + qcI +Dc)Ci , (10c)
∂S
(p)
i
∂t
= γp (Si − S
(p)
i ) , i = 0, 2 , (10d)
where n ≡ Ncis/N , q̄t ≡ qt(1 + u/3), ū ≡ 2u/(3 + u), v ≡ ū [10π]−1/2, w22 = −w00 =
ū/7, w02 = −2w20 =
√
6w00; Dc ≡ 6D
(cis)
r and Dt ≡ 6D
(tr)
r are rotational diffusion
constants of cis and trans molecules, respectively. Additional order parameters C i
and S
(p)
i characterize ordering of intermediate states of cis molecules and ordering
of polymer backbone [15], respectively.
3.3. Steady state
Equations (10) yield the uniaxial stationary state with
S(st)
x = S(st)
z = S
(st)
0 ⇐⇒ S
(st)
2 =
√
3/2S
(st)
0 .
The result is
S
(st)
0 =
αv
w00 +
√
3/2w02 + βv
, 1− nst = α− βS
(st)
0 , (11)
where
α =
γc + qcI
γc + (qc + q̄t)I
, β =
ūq̄tI
γc + (qc + q̄t)I
.
72
On biaxiality of photoinduced structures in azopolymer films
0 1 2 3 4 5
γc, (= 1/τc)
-0.2
0
0.2
0.4
Sx=Sz
Sy
Figure 2. Stationary value of Sz vs γc ≡ 1/τc computed from equation (11). The
lifetime of cis molecules is scaled by q̄tI. The marked point corresponds to the
experimental steady state value of Sz, S
(st)
z ≈ −0.267.
According to figure 1b, experimental data give S
(st)
z = S
(st)
y ≈ −0.267. Assuming
that u = 19, σ(cis) ≈ σ
(tr)
⊥ , and qt/qc ≈ 30 (see [15]), as it is shown in figure 2, the
relevant steady state order parameters can be calculated from equation (11).
Referring to figure 2, it is seen that the experimental value of Sz corresponds
to γc ≈ 2.68 q̄tI. From equation (11) we can now calculate nst: nst ≈ 0.14. Accord-
ing to [3,14], the mechanism of reorientation in our case, where the fraction of cis
molecules in the photosteady state is found to be relatively small, corresponds to
angular redistribution.
4. Conclusions
We have demonstrated that the UV absorption method supplemented by the
null ellipsometry technique is an effective tool for the investigation of the features of
spatial ordering in azopolymer films. This method is shown to detect spatial rear-
rangement of chromophores under irradiation. In addition, it enables us to calculate
the components of the order parameter tensor for the original structures of the films
as well as for structures at various stages of irradiation.
73
A.Kiselev et al.
Our theoretical approach to the kinetics of photoinduced reorientation is based on
master equations with allowance for biaxiality. Predictions of the simple phenomeno-
logical model proposed in this paper are in good agreement with experimental data
as far as the kinetics of order parameters and the photosteady state are concerned.
Although, for brevity, the details of the underlying theoretical considerations
have been almost completely left aside in this paper, it should be emphasized that
angular redistribution is of crucial importance in our approach. Moreover, in order
to account for long term stability of the photoinduced isomerization, a more sophis-
ticated treatment would involve the development of an effective medium theory. We
plan to publish a more comprehensive discussion of this theory elsewhere.
Acknowledgment
We gratefully acknowledge financial support from U.S. Civilian Research & De-
velopment Foundation (grant UP1–2121B).
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74
On biaxiality of photoinduced structures in azopolymer films
Про двовісність фотоіндукованих структур в
азополімерних плівках
О.Кисельов 1 , О.Ярощук 2 , Ю.Закревський 2 ,
О.Терещенко 2
1 Чернігівський технологічний університет,
14027 Чернігів, вул. Шевченка, 95
2 Інститут фізики НАН України, 03028 Київ, пр. Науки, 46
Отримано 1 серпня 2000 р.
Шляхом використання методу УФ поглинання та результатів еліпсо-
метричних вимірювань вивчені просторові орієнтаційні структури в
шарах рідкокристалічних полімерів. Встановлено, що під дією поля-
ризованого УФ випромінювання азобензольні фрагменти орієнтую-
ться переважно в площині. Отримано оцінку для компонентів тензо-
ра параметра порядку як у початковому стані, так і після різних доз
опромінювання. Виявлено як одновісне так і двовісне впорядкування
азобензольних хромофорних груп. Причому двовісність спостерігає-
ться у проміжній стадії опромінювання, тоді як одновісний стан реалі-
зується в режимі насичення. Запропонована теорія враховує двовіс-
ність індукованих структур і добре описує експериментальні залеж-
ності компонентів параметра порядку від часу опромінювання.
Ключові слова: азополімер, фотоіндукована анізотропія,
просторова орієнтація
PACS: 61.30.Gd, 42.65.-k, 78.66.Qn
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