Thermoresponsive Gelation and Phase Transition of PEG/Cation Random Terpolymer Micelles in Water in the Presence of Salts

Amphiphilic random terpolymers bearing hydrophilic poly(ethylene glycol) (PEG) chains, quaternary ammonium cations, and hydrophobic dodecyl groups form size-controlled micelles in water. The terpolymer micelles show a lower critical solution temperature (LCST)-type solubility in water in the presence of salts. Focusing on the features, we herein investigated the thermoresponsive properties of the concentrated aqueous solutions of the PEG/cation random terpolymer micelles containing NaCl to find unique gelation and phase transition dependent on the PEG/cation composition. Upon heating, the PEG/cation (2/1, 1/1, and 1/2) terpolymer micelle solutions exhibited a two-step phase transition through transient gelation to viscous and turbid solutions via LCST-type phase separation. In contrast, a cation-rich PEG/cation (1/3) terpolymer micelle and a cation copolymer micelle formed gels in water at room temperature, where the former changed to a viscous and transparent solution upon heating. The rheological properties of the gels can be tuned by the PEG/cation ratio and degree of polymerization of the terpolymers. Small-angle neutron scattering measurements revealed that the transient gelation of the terpolymer micelle solutions occurred via the formation of intermicellar networks.