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 N-substituents and the content of azo dye. The phase transition temperatures T_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 E-to-Z ("trans-cis") isomerization of the azobenzenes, reaching an E : Z 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 E-isomer is relatively slow, with a half-life of the order of 12 h. For the copolymers with a small azo dye content, the E-to-Z photo-isomerization induces an increase of the phase transition temperature T_CP, as it is expected due to the enhanced dipole moment of the Z-isomer. In contrast, the copolymers with higher azo dye contents behave counter-intuitively, i.e., their T_CP values markedly lower upon UV-irradiation. Possible reasons, e.g. chromophore aggregation, are discussed. In addition, DLS reveals that, at all temperatures and in both the E- and the Z-state, small clusters are formed by the polymer chains, probably mediated by the hydrophobic azobenzene side groups. These coexist with large associates.