Light-controllable azobenzene materials have a remarkable potential for micro- and nanotechnologies as patterning templates, sensors, micropumps and actuators. The photoisomerization between trans and cis states of azo-chromophores is the primary source of photodeformation in azo-polymers. The direction of deformation can be controlled by the light polarization. In our analytical and computer simulation studies, description of the light-induced anisotropy is simplified by applying effective orientation potential to the trans isomers orienting them perpendicular to the light polarization. Using coarse-grained modelling we proved that effective potential approximates well the reorientation of trans isomers under linearly polarized light. The proposed orientation approach is quite promising. It allows not only the explanation of the sign and magnitude of photodeformation in azo-polymers with diverse chemical architecture and topology, but also the prediction of new effects, such as appearance of biaxial deformation in liquid crystal (LC) azo-polymers. A rich behavior is predicted for two-component polymer networks containing azobenzenes and non-chromophoric LC mesogens. Whether such two-component network expands or contracts with respect to the light polarization, depends on the art of attachment of the mesogens to the network strands.
The light-induced deformation of two-component polymer networks with liquid crystalline (LC) mesogens and azobenzene chromophores attached covalently to the network strands is theoretically studied. It is shown that preferential reorientation of chromophores perpendicular to the polarization direction of the light E leads to the reorientation of the mesogens due to LC interactions between the chromophores and mesogens. Reorientation of both components under light illumination leads to the light-induced deformation of the polymer network. The sign of deformation (expansion / contraction with respect to the vector E) and the magnitude of deformation is determined by the orientation distribution of the mesogens and chromophores inside the network strands. The magnitude of deformation increases with increase of the volume fraction of chromophores and the strength of LC interactions between the components.
We present the theoretical and computer simulation studies of photo-mechanics in glassy azo-oligomers. Angular distributions of chromophores in respect to the backbones obtained at different temperatures served as input into a theoretical expression for the striction stress, which was found to be positive for the structure under investigation. The light-induced reorientation of typical propeller-like structures is shown to be a microscopic reason of the sample elongation. The azo-propellers work as nanoscopic actuators which convert the light energy into material deformation. This finding opens a way for prediction of photomechanical properties of glassy azo-compounds directly from their chemical structure.
Azobenzene elastomers due to their ability to change the shape under light irradiation have a fascinating potential for
technical applications serving as artificial muscles, sensors, microrobots, actuators, etc. The present study deals with the
effects of the orientation liquid-crystalline (LC) interactions between the chromophores on the light-induced deformation
of azobenzene elastomers. The orientation LC-interactions are taken into account in the framework of the mean-field
approximation. Interaction of the chromophores with the light is described by means of an orientation potential which
leads to preferable orientation of chromophores perpendicular to the light polarization direction. The statistics of network
strands is described in the framework of the Gaussian approach. We consider photo-mechanical properties of azobenzene
elastomers which are in the LC nematic state even at the absence of the light. It is shown that the light is able to induce a
phase transition form the uniaxial to biaxial state with preferable orientation of chromophores perpendicular to the light
polarization direction. The photomechanical behaviour is very sensitive to the chemical structure of network strands: an
azobenzene elastomers can elongate (contract) along the polarization direction of the light, if the chromophores are
oriented preferably perpendicular (parallel) to the backbones of network strands.
A microscopic theory is developed to describe light-induced deformation of azobenzene polymers of different chemical
structures: uncross-linked low-molecular-weight azobenzene polymers and cross-linked azobenzene polymers
(azobenzene elastomers) bearing azobenzene chromophores in their strands. According to the microscopic theory the
light-induced deformation is caused by reorientation of azobenzene chromophores with respect to the electric vector of
the linearly polarized light, E. Theoretical calculations of the order parameter of short azobenzene molecules (oligomers)
affected by the light show that the sign of the light-induced deformation (expansion / contraction along the vector E)
depends strongly on the chemical structure of the oligomers. The conclusion of the theory about different signs of the
light-induced deformation of low-molecular-weight azobenzene polymers is in an agreement with performed series of
molecular dynamics simulations. Using the microscopic theory it is shown that cross-linked azobenzene polymers
demonstrate the same light-induced deformation (expansion / contraction) as their low-molecular-weight analogues, i.e.
polymers consisting of short azobenzene molecules whose chemical structure is the same as chain fragments of the
We propose a microscopic theory of light-induced deformation of side-chain azobenzene polymers using a statistical
model which takes the chemical architecture of azobenzene macromolecules explicitly into account. The theory provides
the values of the light-induced stress larger than the yield stress. This result explains a possibility for the inscription of
surface relief gratings in glassy side-chain azobenzene polymers. We show that the photo-elastic behavior of azobenzene
polymers is very sensitive to their chemical structure. Depending on the chemical structure, a sample can be either
stretched or uniaxially compressed along the polarization direction of the linearly polarized light. For some chemical
structures, elongation of a sample demonstrates non-monotonic behavior with the light intensity and can even change its
sign (a stretched sample starts to be uniaxially compressed). These results are in agreement with experiments and recent
Surface relief gratings (SRG) made from azobenzene polymer films by holographic exposure with actinic light show remarkable density modifications in addition to the surface relief. The origin of the huge material transport is attributed to cooperative phenomena associated with the light induced trans-cis and cis-trans isomerization of azobenzene moieties and the subsequent changing of viscoelastic properties during illumination. In case of polydisperse red 1 methacrylate (pDR1m) films and using particular illumination conditions the amplitude of the density grating (DG) can be maximized whereas the amplitude of surface undulations keeps small. The capability of DG formation makes it possible to induce grating formation in the azobenzene polymer film through a thick polymer cover layer which is not affected by the actinic light. Using PMMA as cover layer the grating is located at the PMMA - pDR1m interface (interface grating) while the sample surface stayed almost flat. This concept can be used to prepare 3D mesoscopic crystals by stacking several PMMA/pDR1m bi-layers on top of each other. The interface grating is created in each bi-layer before it becomes covered by the next bi-layer. Patterning of the upper bi-layer takes place after careful positioning of the writing position with respect to the underlying one. These 3D multilayer gratings can be used as dispersive elements for optical light. Their structural performance can be probed by means of light diffraction similar to the x-ray Laue experiment.