Volume phase transitions of poly(acryloyl-l-proline methyl ester) gels in response to water–alcohol composition

The volume phase transitions of poly(acryloyl-l-proline methyl ester (A-ProOMe)) gels in mixtures of water and various alcohols (methyl, ethyl, propyl, and t-buthyl alcohols) were investigated. The poly(A-ProOMe) gels showed two swelling phases (first swell, 0–10 vol%; second swell, 50–80 vol%) and two shrinking phases (first shrink, 10–50 vol%; second shrink, 80–100 vol%) in the presence of each aqueous alcohol under alcohol concentrations ranging from 0 to 100 vol%. The swelling profiles at second swell and second shrink varied in terms of the solvent alcohols; this difference was quantitatively elucidated by taking into account the hydrophobicity of the solvents using dynamic hydration numbers as a parameter of the alcoholʹs hydrophobicity. The gels showed a shrinking phase in the region of high alcohol concentration (second shrink), which had not been observed in other hydrogels such as NIPAAm. Infrared spectra of the poly(A-ProOMe) revealed that the hydrogen bonding of the amide moiety of poly(A-ProOMe) plays a key role in the gelʹs sensitivity to the solvent composition, resulting in the sudden shrinking.