Stimulus-responsive polymers can be utilized as highly-functionalized materials in cosmetics. In order to make products that can be easily washed out by hot water, we focus on thermo-responsive polymers with upper critical solution temperatures (UCSTs). Poly(acrylamide-co-acrylonitrile) (poly(AAm-co-AN), Fig. 1a) was selected as a model polymer due to its simple chemical structure without permanent charges. First, we synthesized poly(AAm-co-AN)s with a different AAm/AN ratio. When AN was added immediately prior to the polymerization (to prevent its volatilization), the amount of AN units in the copolymer corresponded to the molar ratio of AN in feed. Poly(AAm-co-AN) containing a higher amount of AN units showed a higher UCST in water, which was in accordance with a previous report by Agewal et al. (Macromolecules, 2012, 45, 3910) Next, the functional mechanism of poly(AAm-co-AN) was investigated by fluorometry. The environment-sensitive DBD (N-{2-[(7-N,N-dimethylaminosulfonyl)-2,1,3-benzoxadiazol-4-yl](methyl)amino}ethyl-N-methylacrylamide, J. Phys. Chem. B, 2008, 112, 2829) was used as a labelling fluorophore. The maximum emission wavelength of DBD units hypsochromically shifts in a more hydrophobic environment. In our previous study on poly(N-isopropylacrylamide) with a lower critical solution temperature (ca 32 °C), we observed a drastic shift in the maximum emission wavelength from 577 to 550 nm along with the thermal response (J. Phys. Chem. B, 2008). In contrast, only a slight change in the maximum emission wavelength from 580 to 576 nm was observed in the response of DBD-labeled poly(AAm-co-AN) (poly(AAm-co-AN-co-DBD), Fig. 1a), suggesting that the microenvironment around the DBD units in the copolymer did not change around UCST (Fig. 1b). In fluorescence quenching titration using potassium iodide as a quencher, the DBD units in poly(AAm-co-AN-co-DBD) were quenched even below UCST. This result indicated that the iodide ion was accessible to the DBD units in the aggregates of the copolymer, indicating that poly(AAm-co-AN) aggregated loosely below UCST (Fig. 1c). In the aggregated form. it is likely that the dipole-dipole interactions between the cyano groups in AN units dominantly work as crosslinkers.

Fluorescent labeling method reveals the functional mechanism of poly(acrylamide-co-acrylonitrile)s with upper critical solution temperatures

256th National Meeting (2018)

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