Photoinduced orientation in azopolymers studied by infrared spectroscopy: Cooperative and biaxial orientation in semicrystalline polymers

A disperse red 19-containing semicrystalline azopolymer (pDR19T) with phenylene diacrylate (PD) groups within the main chain has been studied by polarization modulation and two-dimensional correlation infrared spectroscopy. The time dependence of the orientation function of azobenzene and PD groups has been followed during the orientation (linearly polarized laser on) and relaxation (laser off) periods. The results show a high degree of orientation of the different groups perpendicular to the direction of the laser polarization and a cooperative orientation of the PD groups with the azobenzene side chains. The azobenzene groups are not randomly oriented in the plane normal to the direction of the laser polarization leading to a biaxial orientation distribution. This photoinduced biaxiality has been quantified for pDR19T polymer and for amorphous copolymers containing azobenzene and methyl methacrylate structural units.