Magnetic-field-induced stress in confined magnetoactive elastomers.

We present a theoretical approach for calculating the state of stress induced by a uniform magnetic field in confined magnetoactive elastomers of arbitrary shape. The theory explicitly includes the magnetic field generated by magnetizable spherical inclusions in the sample interior assuming a non-linear magnetization behavior. The initial spatial distribution of particles and its change in an external magnetic field are considered. This is achieved by the introduction of an effective demagnetizing factor where both the sample shape and the material microstructure are taken into account. Theoretical predictions are fitted to the stress data measured using a specifically designed experimental setup. It is shown that the theory enables the quantification of the effect of material microstructure upon introducing a specific microstructural factor and its derivative with respect to the extensional strain in the undeformed state. The experimentally observed differences between isotropic and anisotropic samples, compliant and stiff elastomer matrices are explained.

Theory of light-induced deformation of azobenzene elastomers: influence of network structure.

Azobenzene elastomers have been extensively explored in the last decade as photo-deformable smart materials which are able to transform light energy into mechanical stress. Presently, there is a great need for theoretical approaches to accurately predict the quantitative response of these materials based on their microscopic structure. Recently, we proposed a theory of light-induced deformation of azobenzene elastomers using a simple regular cubic network model [V. Toshchevikov, M. Saphiannikova, and G. Heinrich, J. Phys. Chem. B 116, 913 (2012)]. In the present study, we extend the previous theory using more realistic network models which take into account the random orientation of end-to-end vectors of network strands as well as the molecular weight distribution of the strands. Interaction of the chromophores with the linearly polarized light is described by an effective orientation potential which orients the chromophores perpendicular to the polarization direction. We show that both monodisperse and polydisperse azobenzene elastomers can demonstrate either a uniaxial expansion or contraction along the polarization direction. The sign of deformation (expansion/contraction) depends on the orientation distribution of chromophores with respect to the main chains which is defined by the chemical structure and by the lengths of spacers. The degree of cross-linking and the polydispersity of network strands do not affect the sign of deformation but influence the magnitude of light-induced deformation. We demonstrate that photo-mechanical properties of mono- and poly-disperse azobenzene elastomers with random spatial distribution of network strands can be described in a very good approximation by a regular cubic network model with an appropriately chosen length of the strands.

Light-induced deformation of azobenzene elastomers: a regular cubic network model.

We propose a microscopic theory of light-induced deformation of azobenzene elastomers bearing photosensitive azo-moieties in their strands. The theory is based on the orientation approach (Toshchevikov et al. J. Phys. Chem. B 2009, 113, 5032), in which the light-induced mechanical stress originates from reorientation of chromophores with respect to the electric vector of the light. A regular cubic network model built from freely jointed polymer chains bearing azobenzene chromophores is used to calculate the light-induced deformation of azobenzene elastomers under homogeneous light illumination. We show that the photomechanical behavior of azobenzene elastomers is very sensitive to their chemical structure: it depends on the orientation distribution of chromophores around the main chains which is defined by the chemical structure of spacers. Depending on the chemical structure, azobenzene elastomers demonstrate either expansion or uniaxial contraction along the electric vector of the light. The magnitude of the light-induced deformation depends on the degree of cross-linking: the larger is the degree of cross-linking, the smaller is the magnitude of deformation. Additionally, we discuss possible bending motions of azobenzene elastomers under inhomogeneous light illumination, when the light intensity changes inside the polymer due to the absorption of the light energy by the material.

Microscopic theory of light-induced deformation in amorphous side-chain azobenzene polymers.

We propose a microscopic theory of light-induced deformation of side-chain azobenzene polymers taking into account the internal structure of polymer chains. Our theory is based on the fact that interaction of chromophores with the polarized light leads to the orientation anisotropy of azobenzene macromolecules which is accompanied by the appearance of mechanical stress. It is the first microscopic theory which provides the value 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. For some chemical architectures, elongation of a sample demonstrates a nonmonotonic behavior with the light intensity and can change its sign (a stretched sample starts to be uniaxially compressed), in agreement with experiments. Using a viscoplastic approach, we show that the irreversible strain of a sample, which remains after the light is switched off, decreases with increasing temperature and can disappear at certain temperature below the glass transition temperature. This theoretical prediction is also confirmed by recent experiments.

Optical patterning in azobenzene polymer films.

Thin azobenzene polymer films show a very unusual property, namely optically induced material transport. The underlying physics for this phenomenon has not yet been thoroughly explained. Nevertheless, this effect enables one to inscribe different patterns onto film surfaces, including one- and two-dimensional periodic structures. Typical sizes of such structures are of the order of micrometers, i.e. related to the interference pattern made by the laser used for optical excitation. In this study we have measured the mechanical properties of one- and two-dimensional gratings, with a high lateral resolution, using force-distance curves and pulse force mode of the atomic force microscope. We also report on the generation of considerably finer structures, with a typical size of 100 nm, which were inscribed onto the polymer surface by the tip of a scanning near-field optical microscope used as an optical pen. Such inscription not only opens new application possibilities but also gives deeper insight into the fundamentals physics underlying optically induced material transport.

Formation mechanism and dynamics in polymer surface gratings.

We present the results of time-dependent x-ray and visible light (VIS) scattering measurements during formation of surface relief grating (SRG). These gratings are formed on polymer films containing azobenzene side groups during pulselike exposure with a holographic pattern of circularly polarized light at 488 nm. The SRG formation is accompanied by a density grating just below the film surface. Assuming viscoelastic flow, a change in polymer's elastic properties upon light exposure can explain the massive material transport. Finite element calculations reveal a dynamic model of grating formation characterized by different relaxation times. The simultaneous formation of a surface relief grating and of a density grating explains quantitatively the findings of the VIS experiment, but only qualitatively the findings of the x-ray measurements.