Effect of crosslinking on the photoinduced orientation of azo groups in thin polymer films

A series of amorphous copolymers containing disperse red 1 and crosslinkabl e acrylic groups were prepared. The crosslinked polymers were prepared in t hin films by thermal polymerization of the acrylic groups in the copolymers , The orientation induced by irradiation with a linearly polarized laser wa s measured as birefringence at several temperatures, and the effect of cros slinking on the photoinduced orientation was investigated, Crosslinking enh anced the stability of the photoinduced birefringence. In particular, cross linking helped to maintain the birefringence both at high temperatures and after the linearly polarized laser was turned off. The birefringence dynami cs was analyzed with biexponential curve fitting. Crosslinking influenced n ot only the birefringence levels but also its rate of growth. The growth ra te of the photoinduced birefringence decreased by crosslinking, whereas the relaxation was not significantly affected. Although crosslinking restraine d the mobility of the azo chromophores, a certain fraction could orient or move randomly even in highly crosslinked polymer networks. (C) 2001 John Wi ley & Sons, Inc.