Molecular addressing? Selective photoinduced cooperative motion of polar ester groups in copolymers containing azobenzene groups

Amorphous copolymers with rigid azobenzene and ester side groups form films in which birefringence can be induced using linearly polarized light. When both the azobenzene and the ester groups are polar, cooperative motion occurs and high levels of birefringence can be achieved at relatively low azobenzene content. With less polar ester groups, this cooperative motion is significantly reduced, suggesting that the effect is not dictated by steric factors (as is the case in liquid crystalline copolymers), but by electric interaction between the side group dipoles. The differences in cooperative motion of two copolymer systems: poly{4'-[(2-(methacryloyloxy)ethyl)ethyl)ethylamino]-4-nitroazobenzene (DR1M)-co-4-nitrophenyl 4-[[2-[(2-methyl-1-oxo-2-propenyl)oxy]ethyl]oxy]benzoate (BEM)} (a polar azo/polar ester pair), and poly{DR1M-co-4-phenyl 4-[[2-[2-(2-methyl-1-oxo-2-propenyl)oxy]ethyl]oxy]benzoate (NBEM)} (a polar azo/less polar ester) have been investigated using birefringence measurements and time dependent infrared spectroscopy. Thus, the "molecular addressing" concept introduced by Anderle and Wendorff for liquid crystalline copolymers is real. Kinetic analysis using time-dependent infrared spectroscopy clearly shows that the azobenzene groups move first and are followed by the ester groups, which move to a much greater degree when they have similar polarity (BEM with DR1M).