THERMAL CIS-TRANS ISOMERIZATION RATES OF AZOBENZENES BOUND IN THE SIDE-CHAIN OF SOME COPOLYMERS AND BLENDS

Copolymers of 4'-[[2-(acryloyloxy)ethyl]ethylamino]-4-nitroazobenzene (DR1A) with methyl methacrylate (MMA) and of the respective methacrylate monomer (DR1M) with MMA were cast as films and subjected to a pump/relax procedure to analyze the thermal cis-trans isomerization behavior of the azo groups. Blends of polyDR1A with polyMMA were also used in a similar manner. All samples show a fast isomerization process due to strained cis isomers, trapped below the glass transition temperature of the film, followed by a slower isomerization process. The homopolymers, all blends, and the copolymers with very low azo concentrations can be analyzed by a monoexponential decay for the slow process. The isomerization of all other copolymers is analyzed as a sum of three monoexponential processes, in which each process is the isomerization of an azo-centered triad. The rate constants found for the individual processes indicate that isomerization is faster when more azo groups are neighboring the central azo group in the triad. This is proof for the neighboring effect on motion of the azo groups, even in these completely amorphous samples.