Photochemically induced birefringence in polyurethanes containing donor-acceptor azobenzenes as photoresponsive moieties

Three kinds of photoresponsive polyurethanes containing slightly different azobenzene moieties [i.e., disperse red 19 (DR19), 2-position-substituted DR19, and 2,2-position-stibstituted DR19] were prepared, and their photoinduced birefringence behavior was examined. These polyurethanes showed nearly the same molecular weights, glass transition temperatures, viscosities, and absorption coefficients at the wavelength of the light used to induce the photoresponse, Irradiation of these polymer films with a linearly polarized Ar+ laser induced birefringence due to reorientation of the azobenzene moieties through multiple trans-cis-trans isomerization cycles. Despite the slight difference in the chemical structure of the azobenzene moiety, a large difference in the growth behavior of the photoinduced birefringence was observed among these polymers. Because of the small differences in the characteristics of these polymers, we concluded that the isomerization behavior of the azobenzene moiety was the main factor for the results obtained in this study. In addition, the saturated value of the induced birefringence differed to a great extent among these polymers. It was suggested that such a phenomenon could be related to the reorientation behavior induced by the introduction of two methyl groups. (C) 2002 Wiley Periodicals, Inc.