Examination of phase transition behavior of ion group functionalized MPEG-b-PCL diblock copolymers

Thermogelling block copolymers are central to a variety of biomedical applications. Here, we examined the thermal phase transition behavior of the MPEG-b-PCL diblock copolymer (MC) with carboxyl (MC–COOH) and amine (MC–NH2) groups, and their salt forms (MC–COO−Na+ and MC–NH3+Cl−) at the chain ends of the PCL segment. All MC copolymers formed an opaque emulsion sol at room temperature when prepared as 20 wt% aqueous solutions. As the temperature increased from room temperature, a sol-to-gel transition was observed for MC, MC–COOH, and MC–NH2 copolymers, although not for their salt forms (MC–COO−Na+ and MC–NH3+Cl−). Introduction of a carboxyl and an amine group into the PCL segment decreased the crystallinity and hydrophobicity of the PCL block domains, which altered the onset temperature of gelation (the gel temperature range) and the maximum viscosity. We confirmed that the sol-to-gel phase transition behavior, which indicated the formation and destruction of a structured gel network of MC copolymers, depended on the crystallinity and hydrophobicity of the PCL domains in aqueous media.