SYNTHESIS AND OPTICAL-PROPERTIES OF POLYUREAS WITH AZOAROMATIC GROUPSIN THE MAIN-CHAIN

We report the synthesis and optical properties of two different polyur eas containing azo units in the main chain. The polymers were synthesi zed 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 po lymers 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 gl ass 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 o f 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 ex posure to an interference pattern of polarized argon laser beams. Larg e scale molecular migration in this high glass transition temperature (T-g) polymer at room temperature was achieved for the first time.