Molecular reorientations and relaxation in photosensitive LCP's: Induced birefringence and polarized micro-Raman spectroscopy

The liquid crystalline side chain polymer azopolyacrylate (PA4) has interesting photoisomerization effects upon absorption of light by the azo-group in the side chain. The material also easily forms a glassy structure, with T-g approximately 2o degreesC . In this work we show that upon illumination with polarized blue laser light the molecules reorient perpendicularly to the polarization direction of the incident light. We demonstrate this on the microscopic scale by polarized micro-Raman spectroscopy of the illuminated zone. The reorientation process was followed in time as a function of incident light power and at different temperatures in the neighborhood of T-g, by monitoring the induced birefringence. Thus we were able to study molecular relaxation and its behavior in the neighborhood of the glass transition in this fragile glass, using light to take the system out of equilibrium.