Self-assembly and salt-induced thermoresponsive properties of amphiphilic PEG/cation random terpolymers in water

Abstract

Herein, we report the self-assembly and salt-induced thermoresponsive properties of amphiphilic random terpolymers consisting of hydrophilic poly(ethylene glycol) (PEG) and quaternary ammonium cations, and hydrophobic dodecyl groups in water. The random terpolymers self-assembled into size-controlled multichain micelles in pure water or in water containing NaCl. The micelle size increased upon increasing the content of the cationic groups in the total hydrophilic monomer units (∼50 mol%) and turned larger in the presence of NaCl than that in pure water. More uniquely, the random terpolymer micelles showed lower critical solution temperature-type solubility in water containing salts such as NaCl, while the solutions of the polymer micelles in pure water were transparent even upon heating to over 90 °C. The cloud point (Cp) temperature of the aqueous polymer micelle solution was controlled by the concentration of NaCl or the composition of the terpolymers. The critical concentration of NaCl for thermoresponsive solubility depended on the PEG/cation composition of the terpolymers. For example, a PEG/cation/dodecyl (1/1/2) random terpolymer micelle exhibited thermoresponse in water containing more than 0.5 M NaCl; the Cp of the aqueous solution decreased from 86 °C to 59 °C upon increasing the concentration of NaCl from 0.5 M to 2.0 M. The Cp of their terpolymers increased upon increasing the content of quaternary ammonium cations.