Computer Simulations of Microstructure

We carry out molecular dynamic simulations of photo-mechanics in glassy azo-oligomers and cross-linked azo-elastomers. We use a semi-atomistic model, in which hydro-carbon groups are represented as Lennard-Jones spheres and azobenzenes as Gay-Berne ellipsoidal particles. In accordance with the theory, a sample can either expand or contract along the polarization direction E depending on the chemical structure of macromolecules. Angular distributions of chromophores in respect to the backbones obtained at different temperatures serve 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 azo-compounds directly from their chemical structure. We plan to increase a predictive strength of our approach by extracting molecular conformations of azo-compounds from atomistic simulations.