Photoinduced Birefringence and Surface Relief Gratings in Novel Polyurethanes with Azobenzene Groups in the Main Chain

Three novel polyurethanes with V-shaped bisazo groups (PU1 and PU2) or rodlike monoazo groups (PU3) in the main chain were synthesized. On irradiation with a linearly polarized laser beam, birefringence was induced in the three films to the level of 0.035, 0.037, and 0.044 for PU1, PU2, and PU3, respectively. The stable birefringence obtained at T g − T = 33 °C was 71%, 46%, and 19% of that achieved at room temperature for PU1, PU2, and PU3, respectively. The relative stability of the photoinduced birefringence of PU3 decreased linearly with the increase of temperature, while that of PU1 and PU2 was constant even close to T g. Surface relief gratings with high diffraction efficiency were inscribed on these main-chain polymers. The photoinduced gratings are only partially erasable by heating above the T g of the polymers.