Phase-transition characteristics of amphiphilic poly(2-ethyl-2-oxazoline)/poly(ε-caprolactone) block copolymers in aqueous solutions

Amphiphilic diblock and triblock copolymers of various block compositions based on hydrophilic poly(2-ethyl-2-oxazoline) (PEtOz) and hydrophobic poly(ε-caprolactone) were synthesized. The micelle formation of these block copolymers in aqueous media was confirmed by a fluorescence technique and dynamic light scattering. The critical micelle concentrations ranged from 35.5 to 4.6 mg/L for diblock copolymers and 4.7 to 9.0 mg/L for triblock copolymers, depending on the block composition. The phase-transition behaviors of the block copolymers in concentrated aqueous solutions were investigated. When the temperature was increased, aqueous solutions of diblock and triblock copolymers exhibited gel-sol transition and precipitation, both of which were thermally reversible. The gel-sol transition- and precipitation temperatures were manipulated by adjustment of the block composition. As the hydrophobic portion of block copolymers became higher, a larger gel region was generated. In the presence of sodium chloride, the phase transitions were shifted to a lower temperature level. Sodium thiocyanate displaced the gel region and precipitation temperatures to a higher temperature level. The low molecular weight saccharides, such as glucose and maltose, contributed to the shift of phase-transition temperatures to a lower temperature level, where glucose was more effective than maltose in lowering the gel-sol transition temperatures. The malonic acid that formed hydrogen bonds with the PEtOz shell of micelles was effective in lowering phase-transition temperatures to 1.0 M, above which concentration the block copolymer solutions formed complex precipitates.