Molecular orientation and liquid crystal alignment properties of new cinnamate-based photocrosslinkable polymers

Isotropic thin films of three original phenyl substituted cinnamate-based p olymers, hereafter referred to as 'Para', 'Meta' and 'Metamet' have been ex posed to linearly polarized UV light and their photoinduced molecular orien tations have been studied. The resulting photocrosslinked anisotropic polym er films were characterized using UV, conventional and polarization modulat ion (PM) FTIR spectroscopies. From UV and PM-IR linear dichroism measuremen ts, at least two simultaneous orientation processes appear to play a key ro le in these phenyl substituted cinnamate-based systems. On the one hand, is omerization reactions deplete chromophores along the polarization direction (P) of the UV light and induce a preferential orientation of remaining 'tr ans'-isomers perpendicular to P; on the other hand, cycloaddition reactions lead to the formation of either head to head or head to tail photodimers a ligned preferentially along P in the 'Para' and to a lesser extent in the ' Meta' and 'Metamet' systems. These last results are related to the differen t liquid crystal alignment properties of the films, and the influences of t he chemical structure of the chromophores are discussed.