Photomechanical surface patterning in azo-polymer materials

Azobenzene thin films undergo an unexplained spontaneous surface patterning when exposed to light intensity and/or polarization gradients. The elastic modulus of an azobenzene-polymer film is measured before and during laser irradiation using AFM indentation experiments. It is found that there is no significant change in elastic modulus with laser illumination, indicating that photosoftening can be neglected in these systems. In particular, this eliminates mechanisms that require photosoftening as candidate explanations for azo surface patterning. AFM measurements of patterning in azo-polymer thin films, irradiated at various temperatures, are compared to recent neutron reflectometry measurements of photomechanical effects in the same material. The magnitude and sign of the patterning exactly match the literature trend for photomechanical effects. This represents the first report of measuring both photoexpanded and photocontracted surface patterns in the same material, at different temperatures. These results are interpreted to mean that the unexplained surface mass transport phenomenon observed in the azobenzene system is in fact due to this newly identified photomechanical effect. Previous patterning results are discussed in terms of this explanation, and it is shown that the photomechanical effect can explain the vast majority of the literature results to date.