Orientation Approach to Directional Photodeformations in Glassy Side-Chain Azopolymers

Abstract

To make a polymer-based material photosensitive, it is usually modified by inclusion of azobenzene (azo) chromophores. Their interaction with the light leads to conversion of absorbed energy into mechanical work. The wavelengths ∼500 nm induce cyclic trans–cis isomerization, which results in preferred orientation of the trans-isomers perpendicular to light polarization. This causes reorientation of the polymer backbones to which the azos are attached and appearance of the light-induced stress that dictates a direction of the macroscopic deformation. The directional photodeformations can be explained by an orientation approach, in which the reorientation of azos is described by the effective orientation potential. Here, we show how to calculate the time-dependent orientation state of the polymer backbones and the light-induced stress tensor. For side-chain azopolymers, a tensile stress in the direction of light polarization is predicted. Implementing the stress in a viscoplastic material model of the finite element software ANSYS, we show that a square azopolymer post elongates along the electric field vector for the linearly polarized light and contracts along the propagation direction for the circularly polarized light. These results of viscoplastic material modeling are in accordance with the experiments on light-induced reshaping of microscaled square and cylinder posts. Hence, the orientation approach works rather well for homogeneous illumination. We discuss how this approach can be used to describe surface deformations induced by complex light interference patterns.

Photocontrollable Deformations of Polymer Particles in Elastic Matrix

“…We have also excluded the possible influence of reduction of scalar order parameter of LC polymers upon UV exposure since similar deformations of microparticles are produced by visible light generating little amount of Z ‐azobenzenes (Figure S1, Supporting Information). On the contrary, the experimental data agrees with the building up high local mechanical stresses in azo‐polymer upon polarized light illumination which was theoretically predicted by Saphiannikova and co‐workers and experimentally demonstrated by Santer and co‐workers . The concept assumes that the (re)orientation of azobenzene fragments perpendicularly to light polarization leads to the reorientation of polymer backbone due to its strong coupling with side‐groups which causes the buildup an anisotropic internal stress .…”

Modeling of Stripe Patterns in Photosensitive Azopolymers

“…In particular, it has been observed in study [29] that an epoxy based azopolymer square post deforms directionally in the presence of polarized light. In agreement with this experiment we predict that the post deforms in the direction of polarization for linearly polarized light and in radial direction for circularly polarized light [32]. Presently we would like to test our approach on directional deformations taking place in the upper layer of thin azopolymer films.…”

“…This orientation phenomenon has been called "angular hole burning effect" [30,31]. It can be described by introduction of the effective orientation potential, as has been shown in a series of our papers [19,32]. Based on this orientational picture, the generation of light-induced stress can be predicted in two ways.…”

“…The light-induced stress can then be estimated from the orientational order parameters using the Giesekus relation [33]. Secondly, the main chain can be modelled as an ensemble of rod-like Kuhn segments and for such an ensemble the light-induced stress can be calculated analytically from the effective orientation potential [19,32].…”

Solving an old puzzle: fine structure of diffraction spots from an azo-polymer surface relief grating

“…A possible explanation of such a deformation is the orientation model proposed by Saphiannikova et al 16,17 In this model the SRG formation is considered as a multiscale chain of several processes starting from the small scale motion of azobenzene molecules under cyclic trans-cis-trans isomerization, which causes the local alignment of azobenzene groups perpendicularly to the electrical field vector, followed by re-orientation of the polymer backbones they are connected to. 18,19 The process ends up generating strong internal, anisotropic stress and the subsequent macroscopic opto-mechanical deformation of the film, as manifested in the SRG. 20,21,22,23,24,25 During this process the formation of two gratings can be distinguished: a birefringence grating in the bulk due to the local alignment of azobenzene chromophores, and a surface grating as a result of topographical deformation.…”