Temperature-, Light-, and Host-Molecule-Responsive Polymers with UCST Behavior for Aqueous Sensing Applications

A light-sensitive moiety, azobenzene, is incorporated into polyacrylamide (PAM) to yield copolymers with upper critical solution temperatures (UCST) in aqueous solutions. The thermoresponsiveness of the copolymers can be tuned not only by light irradiation, which causes the transition between the trans- and cis-forms of the azobenzene, but also by addition of α-cyclodextrin (α-CD), which preferentially complexes with the trans-form of azobenzene. The UCST transition is a combined effect of hydrogen bonding between the amide groups of the PAM chains and the hydrophobic interaction between azobenzene units, the latter of which can be influenced by the photoisomerization of azobenzene and the host–guest interaction between azobenzene and α-CD. The strategy of incorporating switchable hydrophobicity may inspire designs of UCST polymers, which may have potential applications as drug delivery devices and catalyst carriers.