Orientation Approach to Directional Photodeformations in Glassy Side-Chain Azopolymers

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 similar to 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.