Laser-induced temperature distribution in photo-alignment of azobenzene liquid crystalline side chain polymers

A theoretical model has been worked out to describe a temperature field, which is established within a film of isotropic trans azobenzene liquid crystalline side chain (LCSC) polymers when a linearly polarized laser beam illuminates it, and contributes to the photo-induced alignment behavior of LCSC polymers at an ambient temperature below the glassy transition temperature (Tg). The distribution of the temperature field is analyzed and the mechanism that the mesogens align perpendicular to the electric vector of the light is discussed with this model. The minimum light intensity Ioff required to achieve the photo-induced phase transition of LCSC polymers at a certain temperature below Tg is calculated. A prediction of existence of an offset photo-alignment temperature Toff is made based on this theory model, while below this temperature no photo-induced phase transition can be observed when a light of certain intensity illuminates the sample.