Lower Critical Solution Temperature-Type Phase Behavior of Poly( N -isopropylacrylamide) in Nonaqueous Organic Solvents

Poly(N-isopropylacrylamide) (PNIPAM) is a representative thermoresponsive polymer exhibiting lower critical solution temperature (LCST)-type phase transition in aqueous media; however, achieving LCST-type behavior in organic solvents has remained elusive. In this study, we realized LCST-type thermoresponsive behavior of PNIPAM in nonaqueous organic solvents comprising mixtures of aliphatic alcohols and hydrocarbon solvents. As a nonpolar environment, hydrophobic hydrocarbon is essential for promoting desolvation and phase separation of PNIPAM. A trace amount of alcohol stabilizes the solvation of PNIPAM chains at ambient temperature. Upon heating, the hydrogen bonds between PNIPAM and alcohol were cleaved, leading to desolvation and coil–globule transitions. Systematic experimental investigations, together with density functional theory (DFT) calculations, revealed that the moderate hydrogen-bonding ability of aliphatic alcohols and orientational constraint of hydrogen bonds at amide sites are critical for achieving LCST-type phase separation. This study, focusing on PNIPAM as a prototypical thermoresponsive polymer, lays a strong foundation for advancing research on LCST-type phase separation in nonaqueous systems.