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Modulating Polymeric Amphiphiles Using Thermo- and pH-Responsive Copolymers with Cyclodextrin Pendant Groups through Molecular Recognition of the Lipophilic Dye

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journal contribution
posted on 06.05.2021, 20:45 by Shintaro Kawano, Jenni Lie, Ryusei Ohgi, Motohiro Shizuma, Masahiro Muraoka
A multiple-stimuli-responsive copolymer, i.e., poly­(N-isopropylacrylamide-co-acrylic acid) substituted with aminoethyl acrylamide-β-CD pendant groups (poly­(NIPAM-AA-β-CD)), was synthesized by free radical copolymerization. The copolymer has thermo-, pH-, and molecular-recognition segments. Its amphoteric structure comprises units with cationic and anionic species that promote ionic complexation by electrostatic interaction between the carboxylic anion and the amino cation at approximately pH 4. The ability of poly­(NIPAM-AA-β-CD) to dissolve a lipophilic dye, i.e., Nile red (NR), changed according to the pH and increased in a more acidic environment to efficiently incorporate NR in the β-CD cavity. However, the lower critical solution temperature of the NR-incorporated poly­(NIPAM-AA-β-CD) solution, which indicates its phase transition behavior, decreased markedly at pH 4.0 owing to ionic complexation and an increase in hydrophobicity resulting from the inclusion of NR. In a homogeneous mixture of water and acetone, a micellar-like assembly was produced in response to the molecular recognition of NR in the β-CD cavity following the evaporation of acetone at pH 5.8. These results indicate that polymeric amphiphiles can be modulated by host–guest interactions in the presence of a lipophilic dye to induce assembly and phase transition under precise pH control.