Counterintuitive Photoresponsiveness of Thermoresponsive Polyacrylamides: Effects of Azobenzene Content and Amino Acid Spacers

Four sets of dual-responsive statistical copolymers are synthesized from N,N-dimethylacrylamide and acrylamide comonomers bearing photoresponsive 4-aminoazobenzene (AzBn) side groups linked by various amino acid spacers. In aqueous solution, the copolymers exhibit lower critical solution temperature (LCST) behavior modulated by the E-to-Z photoisomerization of AzBn. Their cloud point temperatures (TCP) decrease markedly with increasing AzBn content and, at contents of 8 mol % and above, are counterintuitively lower in the—a priori—more polar-Z-state, with photoinduced shifts reaching –16 °C. Dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) on three exemplary copolymers support the formation of compact, spherical single-chain nanoparticles (SCNPs), ascribed to hydrophobic association of AzBn. In most cases, photoisomerization induces only modest changes in global SCNP dimensions, but substantially alters the LCST behavior. These results indicate that the AzBn content primarily determines the overall photoresponsive trend, whereas spacer-dependent differences in local chemical and steric environment further modulate the thermoresponsive behavior.