Déformation photoinduite dans les films minces contenant des ...

Chapter 3. Microscopic mechanisms at the origin of the photo-induced deformation inazobenzene-containing thin films 108Note that the two mechanisms can be singled out by setting the polarization angle ϕ. Atϕ = 0 ◦ the polarization mechanism is inhibited, since η pol = 0; at ϕ = 35 ◦ the statisticalmechanism should vanish, since η I = 0. These two angles provide SRG formation of oppositespatial phase. At ϕ = 26 ◦ , the two mechanisms are expected to compensate eachother, since η I = η pol . No SRG growth should be observed in this condition. Furtherexperiments in these polarization configurations might confirm our interpretation.pastel-00527388, version 1 - 19 Oct 2010The weighted contribution of the polarization-dependent and the intensity-dependentmechanisms in sol-gel Si-DR1 and PMMA-DR1 stresses out the fundamental role ofthe matrix in matter migration processes. Both materials allow the polarization-drivenmechanism with almost the same efficiency. However, the statistical mechanism is inhibitedin the tridimensional mineral matrix of sol-gel thin films, while it is very efficient inthe linear polymer matrix of PMMA. It is most likely that the relative influence of bothmechanisms on the migration efficiency is also influenced by the polymer chain length.First indications in this sense have already been reported in ref. [26].The photomechanical response of a PMMA-DR1 thin film is found to be very differentfrom the one of the sol-gel Si-DR1 thin films. Indeed, the results that we have obtainedin PMMA-DR1 demonstrate the existence of two distinct mechanisms at the origin ofmatter migration phenomena. One is the mechanism observed in sol-gel Si-DR1 thinfilms, which is governed by the light polarization pattern. The other one does notexist in sol-gel Si-DR1 but prevails in our PMMA-DR1 material. Our results showthat it is related to the light intensity pattern and it is well described by a statisticalmigration process, along the molecule dipole axis. Although it is governed by theintensity pattern, the efficiency of this mechanism depends also on the polarizationthrough the absorption anisotropy of the molecules.