Multitunable LCST and UCST Behaviors of Y-Junction-Bearing Polyacrylamides

Rational design of thermoresponsive polymers allows facile control over LCST/UCST and nano-object shapes. Despite tremendous progress, it remains a challenge to monitor chain conformations as the solvent isotope affects types of phase transitions. This study aims at addressing the challenge by construction of thermoresponsive Y-junction-bearing polyacrylamides with nontraditional intrinsic luminescence. With an increasing carbon number of substituents connecting with ester groups (x = 1, 2, 3, 4, 6, 8, 10, and 12), copolymers transform from hydrophilic (x = 1) to LCST (x = 2) and dual LCST/UCST (x ≥ 3) in H2O, while no UCST is available in D2O due to more stable amide–solvent hydrogen bonding and strengthened intra-/intermolecular interactions. Meanwhile, copolymer aqueous solutions can exhibit substituent/pH-dependent evolution orders of single, dual, and triple phase transitions, and asymmetric V-shaped evolution of LCST/UCST values with increasing pH is usually observed. Although 1H NMR analysis can only reveal the solvation status of subunits in the LCST region, fluorescence analysis allows to elucidate intra-/intermolecular hydrogen bonding interactions in LCST/UCST regions. Thermoinduced self-assembly in water renders sphere-to-nanocapsule-to-lamella-to-nanocapsule transitions. These important findings provide us with considerable insight into hydrogen bonding-related UCST formation by rational macromolecular design. Our study may open new avenues for regulating phase transitions and exploring solvent isotope effects.