Resolution of multiple photo induced processes in azo polymeric liquid crystalline linear and star polymers

Azobenzene containing polymeric liquid crystals (azo-PLC) have been subject of intense research in the last two decades for the possibility they offer of an optical control of many physical properties; such optical absorption, birefringence, viscosity or volume. The photoinduced molecular relaxations in these materials, present usually a strong deviation from a first order kinetics due to the concomitance of several effects caused by illumination. In order to resolve the processes occurring during the optical pumping we carried out a detailed study by high resolution null ellipsometry, which in thin films allows a simultaneous and independent determination of both mechanical and optical properties (e.g. thickness and refractive index). In order to reduce the material heterogeneity, which broadens the relaxation times distribution, we studied a newly synthesized highly monodisperse azo-PLC (polydispersity index 1.08-1.17) and we characterized the samples as a function of polymeric chain length and of geometry, by studying both linear and star polymers. The characterization has been performed as a function of temperature above and below the glass transition and as a function of optical pump varying both power and wavelength. In the study in particular we quantified the shift of the glass transition temperature of the material under illumination, the dependence of volume variation as a function of polymeric chain length and we identified a critical density which separates two different molecular relaxation processes