Photoorientation of a liquid crystalline polyester with azobenzene side groups. 1. Effects of irradiation with linearly polarized blue light
The photoorientation process in a polyester with 4-cyano-4'-alkoxyazobenzene side group and long methylene spacers in the side and the main-chain was studied as a function of irradiation with linearly polarized light of 488 nm under systematic variation of the power density and temperature. This model polymer is characterized by liquid crystallinity (g 24 S-X 26 S-A 34 n 47 i) and a strong aggregation tendency. The photoorientation is cooperative, i.e., the orientation of the photochromic side group induces the alignment of the ester unit (which is a part of the main-chain) and both methylene segments in the side- and main-chain. The very high values of the normalized linear dichroism up to 0.8 and the birefringence (above 0.3) are due to the interaction of photoorientation and thermotropic self-organization. The induction of anisotropy shows a pronounced dependence on the power density and the working temperature. This is related to the thermal properties of the LC polymer and the resulting ordering behavior. Surprisingly, the photoorientation process, is not restricted by J-aggregation of the azobenzene groups even at low power densities. The highest anisotropy at 27 T (slightly above T-g) is achieved by a low power density (1 mW/cm(2) and dose of 0.5 mW/cm(2)s). Higher power densities result in a decrease of the maximum dichroism and, finally, the initially induced small anisotropy is erased during continuing light exposure. At a constant high power density of about 700 mW/cm(2), the saturation value of the dichroism first increases with the irradiation temperature, then passes through a maximum of about 0.8 at 18 degreesC and finally the film is transferred to the isotropic state at 27 T. Both effects indicate a significant thermal effect of the laser beam on the polymer film. Taking into account the thermal properties of the investigated polymer it is demonstrated that the photoinduced anisotropy depends,,strongly on the irradiation conditions.