Rational design of hydrogen bonds for driving thermo-responsive phase transition and assembly behavior of block copolymer in water

For some thermo-responsive polymers, the absence of upper critical solution temperature (UCST) behavior in water was attributed to the limited driving force of hydrogen bonding interactions, in which a hydrophobic interaction was required to provide a synergetic effect. This hydrogen bonding interaction was intensified in the present study by adjusting chain architectures, which endowed thermo-responsive polymers with reversible UCST behaviors in both water and electrolyte solutions. By incorporating acrylic acid (AAc) monomers into a block polymer of PEG-b-PAAm, cooperative zipper-type intra- and intermolecular hydrogen bonding interactions between PAAc and PAAm blocks were constructed, which provided a sufficient driving force for phase transition. The controlled enhancement of the UCST value by varying the sequence lengths was shown and further increased to approximate room temperature (26.6 °C) by introducing hydrophobic blocks. Spontaneous cooling induced the fast phase transition of soluble unimers to globular coacervate droplets, and then coacervate droplets fused gradually to yield liquid–liquid phase separation. This developed strategy represents a valuable addition to the design of UCST-type polymers, which sheds light on the future construction of new UCST-type polymers.