Counter-intuitive photo-modulation of the aqueous phase behavior of azo dye-functionalized polyacrylamides

Three sets of water-soluble non-ionic polymers, which are thermo- as well as photo-responsive, are synthesized by statistical copolymerization of hydrophilic substituted acrylamides with varying amounts of a polymerizable azobenzene dye. Their coil-to-globule phase transition of the lower critical solution (LCST) type in aqueous solution is investigated with respect to the nature of the <i>N</i>-substituents and the content of azo dye. The phase transition temperatures <i>T</i>_CP drop from >100 °C to <0 °C with increasing dye content. Irradiation of the copolymers with UV-vis light at 365 nm induces the <i>E</i>-to-<i>Z</i> ("<i>trans</i>-<i>cis</i>") isomerization of the azobenzenes, reaching an <i>E </i>: <i>Z</i> ratio of 21 : 79 in the photo stationary state, while three isosbestic points in the spectra suggest a clean interconversion. The thermal back reaction to the <i>E</i>-isomer is relatively slow, with a half-life of the order of 12 h. For the copolymers with a small azo dye content, the <i>E</i>-to-<i>Z</i> photo-isomerization induces an increase of the phase transition temperature <i>T</i>_CP, as it is expected due to the enhanced dipole moment of the <i>Z</i>-isomer. In contrast, the copolymers with higher azo dye contents behave counter-intuitively, <i>i.e.</i>, their <i>T</i>_CP values markedly lower upon UV-irradiation. Possible reasons, <i>e.g.</i> chromophore aggregation, are discussed. In addition, DLS reveals that, at all temperatures and in both the <i>E</i>- and the <i>Z</i>-state, small clusters are formed by the polymer chains, probably mediated by the hydrophobic azobenzene side groups. These coexist with large associates.