We explore water-soluble, multi-responsive polymers that combine thermo- and photo-responsiveness by functionalizing polyacrylamides with azobenzene. Those amphiphilic copolymers have the advantage that both triggers are non-invasive and independent of each other. Further, temperature and light are fast and easy to apply, allowing for facile back and forth switching, in contrast to invasive triggers such as changes in pH or salt concentration.
These copolymers show lower critical solution temperature (LCST) transitions that are modulated by the reversible, UV-light induced E-Z (trans-cis) isomerization of the azo dye. A priori, the higher dipole moment of the Z-form is expected to decrease the chromophore's hydrophobicity, thus increasing the phase transition temperature (T_T) of the polymers. Yet, against intuition and common wisdom, irradiation occasionally decreases T_T. Possibly, the azobenzene forms aggregates in the E-state, thus shielding the hydrophobic chromophores at least partially from the polar solvent, while in the Z-state, the chromophores remain isolated and fully exposed to the polar solvent molecules in the solution. Accordingly, the number of chromophores incorporated and their freedom of movement influences the modulation of T_T. To explore this phenomenon, we prepared sets of azobenzene-functionalized copolymers varying alkyl substituents, spacer groups between chromophore and polymer backbone, and the incorporated amount of dye. We found that the dye content not only influences the magnitude of the shift of T_T, but also its direction, i.e. whether T_T increases or decreases after irradiation. The systematic variation yielded a polymer that shows a decrease of 15 K after UV-light irradiation, significantly exceeding the light-induced modulation of T_T of around 5 K typically achieved.
The poster will present synthesis and characterization of the multi-responsive azobenzene functionalized copolymers, as well as their UV-light modulated switching behavior in water.

UV-light induced photo-isomerization between the partly shielded E-state and the exposed Z-state chromophores in an aqueous solution. The effect on the phase transition temperature T_T cannot be clearly predicted.

Maria Richter

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