Azo polymers for reversible optical storage .13. Photoorientation of rigid side groups containing two azo bonds

Copolymers of methyl methacrylate with a methacrylate containing a rigid group with two azo bonds (3RM) were prepared and their photoinduced birefringence levels and rates studied. Birefringence levels of 0.11 for the copolymer with 11.6 mol% azo structural units and 0.13 for the copolymer with 30.0 mol% azo structural units were found; this is higher than the birefringence inducible in a typical azo homopolymer containing a chromophore with only one azo group, poly{4'-[(2-(acryloploxy)ethyl)ethylamino]-4 nitroazobenzene} [poly(DR1A)]. The birefringence per azo structural unit for a copolymer containing 11.6 mol% 3RM is about five times that for a DR1A copolymer with similar azo content, because of the intrinsic structural properties of 3RM (high length/diameter ratio). Dichroism in both u.v. and visible regions of the spectrum contribute to the overall photoinduced birefringence. The rate of inducing birefringence in the 3RM copolymers is lower than in poly(DR1A) and the birefringence stability (91-96% of the induced birefringence is maintained after the writing laser is off) is much better than that for poly(DR1A) (about 80%). The good stability and slow birefringence growth rate are due to the lesser mobility of the larger side group. (C) 1997 Elsevier Science Ltd.