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.