Photo-induced deformation of azobenzene polymers: theory and simulations

A microscopic theory is developed to describe light-induced deformation of azobenzene polymers of different chemical structures: uncross-linked low-molecular-weight azobenzene polymers and cross-linked azobenzene polymers (azobenzene elastomers) bearing azobenzene chromophores in their strands. According to the microscopic theory the light-induced deformation is caused by reorientation of azobenzene chromophores with respect to the electric vector of the linearly polarized light, E. Theoretical calculations of the order parameter of short azobenzene molecules (oligomers) affected by the light show that the sign of the light-induced deformation (expansion / contraction along the vector E) depends strongly on the chemical structure of the oligomers. The conclusion of the theory about different signs of the light-induced deformation of low-molecular-weight azobenzene polymers is in an agreement with performed series of molecular dynamics simulations. Using the microscopic theory it is shown that cross-linked azobenzene polymers demonstrate the same light-induced deformation (expansion / contraction) as their low-molecular-weight analogues, i.e. polymers consisting of short azobenzene molecules whose chemical structure is the same as chain fragments of the elastomers.