Towards understanding photomigration: Insights from atomistic simulations of azopolymer films explicitly including...

The light-induced surface modification of a thin film of poly-(disperse orange-3-methylmethacrylate) is investigated computationally using atomistic molecular dynamics simulations specifically tailored to include photoisomerization dynamics. For a model surface consisting of a periodic pattern of alternating irradiated and dark spots, it is shown that repeated photoisomerization in the irradiated areas initially leads to a local temperature increase and a raised surface profile accompanied by a migration of molecules away from the bright spots. After switching off the light source and letting the system cool down, this leads to an inversion of the surface profile, i.e., dips in the bright spots and bumps in the dark spots. To separate the effect of photoisomerization from the pure heating effect, a second simulation is performed in which no photoisomerization is allowed to occur in the bright spots, but the equivalent amount of energy is introduced there locally in the form of heat. This also leads to a raised surface in these areas; however, no outward migration of molecules is observed and the surface pattern practically vanishes when the system is subsequently cooled back to room temperature.