Surface Properties, Thermal Behavior, and Molecular Simulations of Azo-Polysiloxanes Under Light Stimuli. Insight into the Relaxation

Light-sensitive materials as azo-polymers have interesting applications in microelectronics, biology, or energy storage, due to their capacity to change their supramolecular ordering. Light-induced modifications result in i) changes in the azobenzene-groups dipole moment due to the trans-cis isomerization, ii) thermal backward cis-trans relaxation, and iii) re-ordering of the polymer chain conformation. Conjugation of these phenomena leads to mass transport in the solid state, which is still not well understood. Changes in the surface properties of non-substituted and p-CF3,-CN,-NO2-substituted azophenoxy-polysiloxane films, before and after photo-excitation, were examined. Molecular simulations were performed to evaluate the dipole-moments of the trans and cis p-substituted azobenzene groups and provide a general vision of the spatial arrangement of polymer chains. The role of trans-cis back relaxation and that of the polymer matrix relaxation on the surface and bulk of the materials was discussed. Surface free energy determination of the non-substituted azophenoxy-polysiloxane suggested that the relaxation process is dominated by the thermal slow back-isomerization of the cis fraction and not by the motion of a glassy polymer matrix involving both the surface and bulk. A less clear situation was presented by the semi-crystalline polysiloxanes with electron-withdrawing groups substituted in the para-position. This was attributed mainly to the similar values of the dipole-moment of trans and cis azo-group, the former being slightly more dipolar than the latter.