Synthesis and optical properties of polyureas with azoaromatic groups in the main chain

We report the synthesis and optical properties of two different polyureas containing azo units in the main chain. The polymers were synthesized from the reaction between a diisocyanate and a diaminoazo compound (4,4'-diaminoazobenzene) with good yield. These polyureas have glass transition temperatures around 197 degrees C and the films of these polymers show an absorption maximum at 380 nm due to the azo chromophore. Upon polarized laser irradiation at 488 nm, dichroism was optically induced in these main-chain azoaromatic polymers with a fairly high glass transition temperature. In addition we observed a degree of polar order by corona poling and a build up of second harmonic signal. It is conjectured that the induced order results mainly from an alignment of azo units through trans-cis-trans isomerization, as well as from the molecular rearrangement of the polyurea backbone. We further observed regularly-spaced surface relief gratings on the polyurea film upon exposure to an interference pattern of polarized argon laser beams. Large scale molecular migration in this high glass transition temperature (T-g) polymer at room temperature was achieved for the first time.