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

Isotropic thin films of three original phenyl substituted cinnamate-based polymers, here-after referred to as 'Para', 'Meta' and 'Metamet' have been exposed to linearly polarized UV light and their photoinduced molecular orientations have been studied. The resulting photocrosslinked anisotropic polymer films were characterized using UV, conventional and polarization modulation (PM) FTIR spectroscopies. From UV and PM-IR linear dichroism measurements, at least two simultaneous orientation processes appear to play a key role in these phenyl substituted cinnamate-based systems. On the one hand, isomerization reactions deplete chromophores along the polarization direction (P) of the UV light and induce a preferential orientation of remaining 'trans'-isomers perpendicular to P; on the other hand, cycloaddition reactions lead to the formation of either head to head or head to tail photodimers aligned preferentially along P in the 'Para' and to a lesser extent in the 'Meta' and 'Metamet' systems. These last results are related to the different liquid crystal alignment properties of the films, and the influences of the chemical structure of the chromophores are discussed.