Light-Induced Modulation of Visco-Elastic Properties in Azobenzene Polymers via Bimodal AFM

Photo-induced isomerization of azobenzene molecules drives mass migrations in azopolymer samples. [1] The resulting macroscopic directional photo-deformation of the material morphology has found many applications in literature, although the fundamental mechanisms behind this mass transfer is still under debate. [2] Hence, it is of paramount importance to find quantitative observables that could drive the community towards a better understanding of this phenomenon. In this regard, azopolymer mechanical properties have been intensively studied, but the lack of a nanoscale technique capable of quantitative visco-elastic measurements has delayed the progress in the field. Here, we use bimodal atomic force microscopy (AFM) as a powerful technique for nanomechanical characterizations of azopolymers. With this multifrequency AFM approach, we are able to map the azopolymer local elasticity and viscosity. We find that, while in the illuminated region a general photo-softening is measured, locally the Young modulus and the viscosity depend upon the inner structuring of the illuminating light spot. We then propose a phenomenological model based on a light-induced expansion plus a local alignment of the polymer chains (directional hole-burning effect).[3]<br/>[1] P. Rochon, E. Batalla and A. Natansohn, Appl. Phys. Lett., 66, 136-138 (1995)<br/>[2] S. L. Oscurato, M. Salvatore, P. Maddalena and A. Ambrosio, Nanophotonics, 7, 1387-1422 (2018)<br/>[3] S. Chiodini, F. Borbone, S. L. Oscurato, P. D. Garcia and A. Ambrosio, Nanophotonics, 13, 229-238 (2024)