Photo-induced deformations in azobenzene-containing side-chain polymers: molecular dynamics study
We perform molecular dynamics simulations of azobenzene containing side-chain liquid crystalline polymer subject to an external model field that mimicks the reorientations of the azobenzenes upon irradiation with polarized light. The smectic phase of the polymer is studied with the field applied p...
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irk-123456789-1212872017-06-14T03:07:59Z Photo-induced deformations in azobenzene-containing side-chain polymers: molecular dynamics study Ilnytskyi, J. Saphiannikova, M. Neher, D. We perform molecular dynamics simulations of azobenzene containing side-chain liquid crystalline polymer subject to an external model field that mimicks the reorientations of the azobenzenes upon irradiation with polarized light. The smectic phase of the polymer is studied with the field applied parallel to the nematic director, forcing the trans isomers to reorient perpendicularly to the field (the direction of which can be assosiated with the light polarization). The coupling between the reorientation of azobenzenes and mechanical deformation of the sample is found to depend on the field strength. In a weak field the original smectic order is melted gradually with no apparent change in the simulation box shape, whereas in a strong field two regimes are observed. During the first one a rapid melting of the liquid crystalline order is accompanied by the contraction of the polymer along the field direction (the effect similar to the one observed experimentally in azobenzene containing elastomers). During the slower second regime, the smectic layers are rebuilt to accomodate the preferential direction of chromophores perperdicular to the field. 2006 Article Photo-induced deformations in azobenzene-containing side-chain polymers: molecular dynamics study / J. Ilnytskyi, M. Saphiannikova, D. Neher // Condensed Matter Physics. — 2006. — Т. 9, № 1(45). — С. 87–94. — Бібліогр.: 25 назв. — англ. 1607-324X PACS: 31.15.Qg, 64.70.Md, 64.70.Nd DOI:10.5488/CMP.9.1.87 http://dspace.nbuv.gov.ua/handle/123456789/121287 en Condensed Matter Physics Інститут фізики конденсованих систем НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
| language |
English |
| description |
We perform molecular dynamics simulations of azobenzene containing side-chain liquid crystalline polymer
subject to an external model field that mimicks the reorientations of the azobenzenes upon irradiation with polarized
light. The smectic phase of the polymer is studied with the field applied parallel to the nematic director,
forcing the trans isomers to reorient perpendicularly to the field (the direction of which can be assosiated with
the light polarization). The coupling between the reorientation of azobenzenes and mechanical deformation
of the sample is found to depend on the field strength. In a weak field the original smectic order is melted
gradually with no apparent change in the simulation box shape, whereas in a strong field two regimes are
observed. During the first one a rapid melting of the liquid crystalline order is accompanied by the contraction
of the polymer along the field direction (the effect similar to the one observed experimentally in azobenzene
containing elastomers). During the slower second regime, the smectic layers are rebuilt to accomodate the
preferential direction of chromophores perperdicular to the field. |
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Article |
| author |
Ilnytskyi, J. Saphiannikova, M. Neher, D. |
| spellingShingle |
Ilnytskyi, J. Saphiannikova, M. Neher, D. Photo-induced deformations in azobenzene-containing side-chain polymers: molecular dynamics study Condensed Matter Physics |
| author_facet |
Ilnytskyi, J. Saphiannikova, M. Neher, D. |
| author_sort |
Ilnytskyi, J. |
| title |
Photo-induced deformations in azobenzene-containing side-chain polymers: molecular dynamics study |
| title_short |
Photo-induced deformations in azobenzene-containing side-chain polymers: molecular dynamics study |
| title_full |
Photo-induced deformations in azobenzene-containing side-chain polymers: molecular dynamics study |
| title_fullStr |
Photo-induced deformations in azobenzene-containing side-chain polymers: molecular dynamics study |
| title_full_unstemmed |
Photo-induced deformations in azobenzene-containing side-chain polymers: molecular dynamics study |
| title_sort |
photo-induced deformations in azobenzene-containing side-chain polymers: molecular dynamics study |
| publisher |
Інститут фізики конденсованих систем НАН України |
| publishDate |
2006 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/121287 |
| citation_txt |
Photo-induced deformations in azobenzene-containing side-chain polymers: molecular dynamics study / J. Ilnytskyi, M. Saphiannikova, D. Neher // Condensed Matter Physics. — 2006. — Т. 9, № 1(45). — С. 87–94. — Бібліогр.: 25 назв. — англ. |
| series |
Condensed Matter Physics |
| work_keys_str_mv |
AT ilnytskyij photoinduceddeformationsinazobenzenecontainingsidechainpolymersmoleculardynamicsstudy AT saphiannikovam photoinduceddeformationsinazobenzenecontainingsidechainpolymersmoleculardynamicsstudy AT neherd photoinduceddeformationsinazobenzenecontainingsidechainpolymersmoleculardynamicsstudy |
| first_indexed |
2025-07-08T19:32:12Z |
| last_indexed |
2025-07-08T19:32:12Z |
| _version_ |
1837108438961750016 |
| fulltext |
Condensed Matter Physics 2006, Vol. 9, No 1(45), pp. 87–94
Photo-induced deformations in azobenzene-containing
side-chain polymers: molecular dynamics study
J.Ilnytskyi∗1,3, M.Saphiannikova2, D.Neher1
1 Institute for Physics, University of Potsdam,
Am Neuen Palais 10, 14469 Potsdam, Germany
2 Leibniz Institute of Polymer Research,
Hohe Str. 6, 01069 Dresden, Germany
3 Institute for Condensed Matter Physics,
National Academy of Sciences of Ukraine,
1 Svientsitskii Str., 79011 Lviv, Ukraine
Received August 28, 2005, in final form January 23, 2006
We perform molecular dynamics simulations of azobenzene containing side-chain liquid crystalline polymer
subject to an external model field that mimicks the reorientations of the azobenzenes upon irradiation with po-
larized light. The smectic phase of the polymer is studied with the field applied parallel to the nematic director,
forcing the trans isomers to reorient perpendicularly to the field (the direction of which can be assosiated with
the light polarization). The coupling between the reorientation of azobenzenes and mechanical deformation
of the sample is found to depend on the field strength. In a weak field the original smectic order is melted
gradually with no apparent change in the simulation box shape, whereas in a strong field two regimes are
observed. During the first one a rapid melting of the liquid crystalline order is accompanied by the contraction
of the polymer along the field direction (the effect similar to the one observed experimentally in azobenzene
containing elastomers). During the slower second regime, the smectic layers are rebuilt to accomodate the
preferential direction of chromophores perperdicular to the field.
Key words: liquid crystals, azobenzene, polymers, molecular dynamics
PACS: 31.15.Qg, 64.70.Md, 64.70.Nd
1. Introduction
Azobenzene-containing polymer materials (here and further on we will use the term “azo-
polymers”) possess unique attractive features due to the trans-cis photoisomerization of the chro-
mophores. On absorbing a photon the planar trans isomer transforms into the bent and twisted cis
form, the latter undergoing reverse transformation either by the same mechanism or by thermally
induced back isomerization [1], see figure 1. The absorption bands for both photoisomerization
processes depend on the chemical nature of the substituents R, R′. If both bands overlap then the
cyclic trans-cis-trans photoisomerization can take place during illumination with single wavelength.
Figure 1. The photoisomerization cycle for azobenzene.
∗E-mail: iln@icmp.lviv.ua
c© J.Ilnytskyi, M.Saphiannikova, D.Neher 87
J.Ilnytskyi, M.Saphiannikova, D.Neher
Unsubstituted trans-azobenzene is a crystalline powder at room conditions with the melting
point at TM = 68−69◦C (some computer simulations relevant to its crystallisation can be found in
[2]). Its derivatives have been mostly studied either in solution (acting as dyes) or being dispersed
or attached to a polymer framework of various architecture (main-chain, side-chain, elastomeric,
dendritic). In the latter case the azobenzene groups act as typical mesogens and induce liquid
crystalline (LC) phases [3]. It has further been used as a photo-switchable component in block
copolymer system or as a dye dopant combined with other mesogens, e.g. 5CB, see [4,5].
Let us concentrate on two particular polymeric architectures, namely LC elastomers and LC
side-chain polymers. LC elastomers are rubbers with covalently attached mesogenic units. As the
result, there is a strong coupling between nematic order and mechanical stress of the underlying
polymeric framework [6]. This leads, for instance, to the remarkable property of nematic elastomers
to change their shape reversibly by up to 400% in the vicinity of their nematic-isotropic (NI) tran-
sition [7]. The theory of nematic networks is now well established [8,9]. In chromophore containing
LC elastomers the orientational order can be also affected by the light. A large mechanical de-
formation has been observed experimentally on azobenzene containing LC elastomers with both
covalently attached and dissolved azo dyes in response to the non-uniform illumination [10–12].
Unlike LC elastomers, the chains of LC side-chain polymers are not cross-linked and, therefore,
are more free to move. One of the new applications of side-chain azo-polymers is the fabrication
of sinusoidally modulated surface structures, known as surface relief gratings (SRG). These are
produced on thin polymer films using optical setups with various interference patterns similar to
the ones used for holographic recordings [13]. The process is all-optical and the SRG films can be
applied as diffractive elements and in optical storage media (see, reviews [14,15] and references
therein). The main peculiarity of this process is the nature of the force behind this photo-induced
large scale mass-transport that takes place in the amorphous state of a polymer. A number of
theories have been developed, namely, thermal gradients, diffusive mechanisms, pressure gradients
resulting from the isomerization, the superposition of the interactions between the azo dipoles and
the electric field of the light interference pattern, and a recent entropic theory (for more detail, see
review [15] and [16]). However, none of these is able to describe equally well all the combinations
of optical setups and different polymers being used. In fact, a clear microscopical picture of both
photo-induced reversible and irreversible changes in azo-polymers is still missing.
The aim of this study is not to perform an in-scale modelling of the photo-induced effects in azo-
polymers but rather to take a look at some possible microscopic mechanisms behind these. The idea
is to simulate the azo-polymer in “dark” and “light” areas and to analyse the mechanical response
of the system to the light. To this end we use molecular dynamics simulation with NPT ensemble
and anisotropic pressure control. This allows the simulation box to fluctuate independently in all
three dimensions while keeping the total pressure equal to the atmospheric one. The change in the
shape of the box reflects (with some limitations given below) that of the macroscopic sample and
can be seen as an indication of the mechanical response of the system to an external perturbation.
The simulations are performed on a side-chain architecture but we expect many similarities to the
elastomer system, particularly on a short time scale. The details of the model are presented in the
next section.
2. Model
Atomic-level modelling of the photo-induced deformations in azo-polymers faces a number of
difficulties. For instance, a length scale of the material deformation in SRG is of the order of
microns (typical film thickness is about 1µm, the grating periodicity is 1−5µm) and the recording
time ranges from seconds to tens of minutes. This length and time scale is clearly beyond the
current capabilities of atomistic molecular dynamics simulations, therefore one needs to find ways
to speed up the effect being considered. Another issue is the level of required chemical detailization
which also influences the speed of the simulations to a great extent. The SRGs are known to be
successfully recorded on a great variety of azobenzene containing chemical structures, which can
be seen as an indication that the molecular model is not needed to be chemically exact.
88
Photo-induced deformations in azobenzene polymers
Most types of azo-polymers used for SRG inscription can be split into two groups: the amor-
phous and LC systems. The terminology evidently originates from the phase in which a particular
polymer can be found at moderate temperatures, T < 100◦C. Typical amorphous systems have
a short spacer (typically, of 1–4 CH2 groups) which links the azobenzene to the polymer back-
bone, and, sometimes, a stiffer backbone. The glass transition temperatures Tg are normally above
100◦C. The LC systems, on the contrary, have a relatively long spacer (often of 10 or more CH2
groups) and, as a result, lower Tg. At the temperature interval T = 20◦ − 100◦C these are usually
found in the smectic phase.
The photo-induced effects are known to occur on much shorter time-scales in LC azo-polymers.
In particular, recordings of SRG within 0.5 s have been reported [5], as compared to tens of mi-
nutes for amorphous systems. For this reason we have chosen this particular architecture for our
simulations. The smectic phases inherent to LC azo-polymers may also shed some light on the
importance of the aggregation of azobenzenes [17–19], since the rearrangement of these aggregates
is expected to be on much shorter time scale in LC azo-polymers. We empolyed the semi-atomistic
modelling, in which the CHn groups of the alkyl chains are represented as united-atom Lennard-
Jones monomers and the azobenzene groups (including possible substituents) are considered to be
anisometric units interacting via the Gay-Berne potential [20] with the following parametrisation,
µ = 1, ν = 2, κ = 3, κ′ = 5. The bonded interactions include bond stretch, bond bending and
torsional terms (for more details, see [21]) with the force field developed for branched alkanes
[22]. Each polymer consists of a backbone of 39 monomer with 10 side-chains attached to it. Each
side-chain has a flexible spacer of 10 monomers terminated by the azo model unit (see, figure 2).
Figure 2. A single side-chain of azo-polymer.
The detailed study of the photoisomerization of azobenzene is beyond the scope of this paper,
therefore we employed a simplified statistical description of this effect. It is assumed that the
material contains azobenzene with substituents so that rapid cyclic trans-cis isomerization occurs
and the photostationary state is quickly established. The trans-cis photoisomerization rate is known
to be proportional to cos2(θ) (θ is the angle between the long axis of the isomer and the light
polarization) therefore trans isomers are to be found predominantly perpendicular to the light
polarization (where the probability for the photoisomerization is the lowest). This effect can be
modelled via an external model field, the energy of each i-th trans isomer in a field is
Ufield
i = F · P2(cos(θi)). (1)
Here the field strength is F > 0, θi is the angle between the long axis of the i-th trans azobenzene
and the prescribed direction (here and thereafter referred as the field direction), and P2(x) =
1/2(3x2 − 1) is the second Legendre polynomial. Unlike the electric or magnetic fields which align
the mesogens parallel to a given direction, the field defined via equation 1 forces trans azobenzenes
to be confined in the planes perpendicular to the field direction. The angular derivative of Ufield
i
with respect to θi is related to the torque applied to each azobenzene. The cis isomers are not
included in our model explicitly.
One should mention that our model lacks some important aspects of photoisomerization, par-
ticularly, the existence of cis isomers and the dynamical nature of the photostationary state. The
only effect taken into account is the reorientation of the trans isomers of the azobenzenes.
89
J.Ilnytskyi, M.Saphiannikova, D.Neher
3. Results
At first, our model side-chain LC polymer has been examined with respect to the formation
of LC phases. The initial configuration was built by packing 64 side chains (see, figure 2) into the
simulation box in some regular layered way (see, figure 3, on the left, here and thereafter we show
the snapshots of the polymer contained in one periodic box only). To avoid artificial splitting-up
of the polymer at the box edges along X axis, the backbones were positioned to interdigitate
throughout the box boundaries and the periodic boundary conditions have been employed (see,
figure 3, in the middle).
Figure 3. Initial geometrically packed configuration (on the left), backbones interdigitation along
the X axis (in the middle) and equlibrated configuration after simulation for 4 ns at atmospheric
pressure P = Patm = 1atm at T = 500 K (on the right).
This initial configuration was equilibrated then for the times of up to 4 ns at atmospheric pres-
sure P = Patm = 1 atm and at different temperatures T ranging from 200 K to 600 K. The molecular
dynamics was performed with the aid of the parallel program GBMOLDD [21,23] in an anisotropic
NPT ensemble with the timestep ranging from 1 fs to 1.5 fs. The Hoover barostat has been used to
fix the tensile stress at Pxx = Pyy = Pzz = Patm/3 = 1/3 atm, and the temperature has been con-
trolled by means of the Nose-Hoover thermostat. The ensemble chosen allows independent fluctuati-
ons for each box dimension and proved to be especially useful to simulate smectic and crystal phases
to avoid the incommesurability of the box dimensions with the characteristic pitch of the phase.
The model system revealed crystalline, smectic and isotropic phases with the transition tem-
perature from the smectic to the isotropic phase at TSI just above 500 K. At T = 500 K the system
still has weak smectic order (see,figure 3, on the right), but the proximity to TSI allows us to easily
perturb the delicate balance of internal interactions. This particular configuration equilibrated at
T = 500 K and Patm (the corresponding mass density is ρ = M/V = 0.63 g/cm3) was chosen to
study the response of the system on an external model field defined via equation 1 with various
strengths F . The field direction has been chosen to coinside with the nematic director of the original
smectic configuration (see figure 3, on the right) to maximize the reorientation effect.
The simulational setup described above has much in common with the experiments of Finkel-
mann’s group on the photoresponse of nematic elastomers [12] except the fact that our system is
built of non-crosslinked chains. However, on a short time scale the effect of chains diffusion can be
neglected and the model system can be affinely deformed. In this case splitting of the macroscopic
sample into a grid of adjacent cells will map the shape of each cell onto that of the whole sample.
One can consider the simulation box (which is able to fluctuate freely in shape) as one of such
cells. On a larger time scale, however, this does not hold due to chain diffusion.
To study the coupling of the sample shape with the LC order of the chromophores, we have
monitored the scaled box dimensions, sx = Lx/V 1/3, sy = Ly/V 1/3, sz = Lz/V 1/3 and the nematic
order parameter S2 = 〈P2(θi)〉 and analysed the snapshots of the systems in a course of the
simulations. Here, Lx, Ly, Lz are the box dimensions, V = LxLyLz is the volume of the box, θi is
90
Photo-induced deformations in azobenzene polymers
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Figure 4. Box shape parameter sz along the direction of the external model field vs time, weak
fields (left graph) and strong fields (right graph).
the angle between the long axis of the i-th chromophore with the nematic director and P2(x) is the
second Legendre polynomial. One should mention that in the presence of an external field one can
measure the nematic order either in respect to the field direction or in respect to the instantaneous
director of the chromophores. For our simulational setup, the former will only confirm that the
chromophores are indeed perpendicular to the field (the order parameter defined in this way will be
equal to −1/2), whereas the latter will also provide the level for chromophores self-organizations
(if any) in other directions. The usual procedure for the calculation of the order parameter is
as follows. At each time t the instantaneous order parameter tensor is evaluated first, then it is
diagonalised so that the maximum eigenvalue provides the value for the scalar order parameter S2.
All the simulations were performed at fixed temperature T = 500 K and atmospheric pressure Patm
while the box shape was allowed to fluctuate. The Z axis was chosen along the nematic director
of the original smectic phase. The model field has been applied in the same direction.
For a weak field strength, F = 0.1 · 10−20 J, no changes of the box shape were observed at
all times studied (up to 4ns, see figure 4, on the left). On the contrary, the case of stronger field
strengths, F > 1 · 10−20 J, reveals an analogy with the experiments on nematic elastomers. Here
a quick initial contraction of the box was observed along the field direction (see, figure 4, on
the right).
Figure 5. Snapshots of the model system at times t = 0 (left image) and t = 10ps (right image)
at a temperature T = 500 K, the light propagation is shown by a dotted arrow whereas the field
direction is designated as ~E, the field strength parameter is F = 5 · 10−20 J.
91
J.Ilnytskyi, M.Saphiannikova, D.Neher
This contraction was accompanied by the extension of the box in the other two dimensions,
so that no apparent change in the average material density has been observed. The side-by-side
comparison of the snapshots of the system at time t = 0 and that at t = 10 ps in a strong field
F = 5 · 10−20 J (see, figure 5) suggests that the contraction of the sample has its origin in steric
interactions. At time t = 0, in the smectic phase, the long axes of the azobenzenes are aligned
preferentially along the Z axis. After applying the external field in the same direction, this order
melts within ca. t = 10 ps as being energetically unfavourable and a new orientational order with
the azobenzenes perpendicular to the Z axis is started to be induced. This quick reorientation
happens on a time scale far less than that of the chains diffusion, so that, the voids emerging
around the azobenzenes cannot be filled. As a result, the material contracts rapidly along the Z
axis. One can conclude that the material demonstrates an elastic response to a strong external
field, similar to the elastomer system, on a short time scale.
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Figure 6. The orientational order parameter S2 for the chromophores vs time, weak fields (left
graph) and strong fields (right graph).
Figure 7. Self-organised smectic layers in planes perpendicular to the field direction ~E (directed
towards the observer) after the time t = 4ns and field strength F = 1 · 10−20 J (left image),
F = 5 ·10−20 J (middle image) and F = 10 ·10−20 J (right image), the light propagation is shown
by a dotted arrow.
92
Photo-induced deformations in azobenzene polymers
The shape of the simulation box on a longer time scale is of lesser interest as far as chain diffusion
smears out its resemblance to the shape of the macroscopic sample. However, the dynamics of the
orientational order parameter S2 is of interest. One should mention that the behaviour of the order
parameter in a weak field reveals only slow “melting” of the initial smectic phase (see, figure 6, on
the left). The time scale of this process is comparable with the chains diffusion kinetics and neither
changes of the box shape nor regrowing of new phases have been observed up to times of 4 ns (see,
figure 4, 6 on the left).
In the case of a stronger field, after an initial quick fall-down, the order parameter S2 grows
until it reaches a final new value, for instance S2 ∼ 0.8 for F = 5 · 10−20 J (see, figure 6). During
this process, all the chromophores reorient preferably perpendicularly to the field direction, and
self-organize into new smectic layers (see, figure 7).
One should mention that in a very strong field, F > 10 · 10−20 J, the structure of the ordered
phase is a polysmectic with many possible defects, as a result of quick formation of smectic clusters
with rather high interfacial energy.
To summarize the results, our simulations of a side-chain azo-polymer subjected to a external
model field, reproduce some of the features of real photo-active polymers observed previously in
experiment. In weak fields, only a photo-chemical smectic-isotropic transition is observed [24] with
no apperent change of the polymer shape. In stronger fields, on a short time scale, photo-induced
contraction of the sample in the direction of light polarization is observed (the effect similar to the
one in nematic elastomers [12]), whereas on a longer time scale the reorganisation of smectic layers
is observed (for more details, see. [25]).
4. Conclusions
This study considers molecular dynamics simulations of a model side-chain azo-polymer subject
to an external model field which approximates some aspects of the photoisomerization of trans
azobenzenes. In weak fields a photo-chemical smectic-isotropic transition is reproduced, while in
stronger fields, the short time scale behaviour is similar to the photoresponse of azobenzene-
containing elastomers. On longer time scale, reorganisation of smectic layers in strong fields is
observed.
The results obtained in this study confirm that the chromophore reorientation may play a
significant role in photo-induced effects in azo-polymers. In particular, both effects of reversible
contraction/extension of LC elastomers and SRG formation in LC side-chain polymers might be
of the same microscopic origin, as was already suggested before [16]. More detailed modelling is
needed to describe other aspects of photoisomerization which will be subject of subsequent studies.
5. Acknowledgements
The authors acknowledge the financial support under DFG Grant NE410/8–1 and computer
time offered by Computing Cluster of the Institute for Condensed Matter Physics, Lviv, Ukraine.
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PACS: 31.15.Qg, 64.70.Md, 64.70.Nd
94
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