MPEG-PCL diblock copolymers consisting of methoxy polyethylene glycol (MPEG, 750 g/mol) and poly(epsilon-caprolactone) (PCL) were synthesized by ring-opening polymerization. Aqueous, solutions of the synthesized diblock copolymers were prepared by dissolving the MPEG-PCL diblock copolymers at concentrations in the range of 0-20 wt %. When the PCL molecular weight was 3000 or greater, the polymer was only partially soluble in water. As the temperature was increased from room temperature, the diblock copolymer solutions showed two phase transitions: a sol-to-gel transition and a gel-to-sol transition. The sol-to-gel phase transition temperature decreased substantially with increasing PCL length. The sol-gel-sol transition with the increase in temperature was confirmed by monitoring the viscosity as a function of temperature. The temperature ranges of the phase transitions measured by the tilting method were in full agreement with those determined from the viscosity measurements. The maximum viscosity of the copolymer solution increased with increasing hydrophobicity of the diblock copolymer and with increasing copolymer concentration. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analyses revealed that the diblock copolymers exhibited crystalline domains that favored the formation of an aggregated gel because of the tight aggregation and strong packing interactions between PCL blocks. Scanning electron micrographs of the diblock copolymer solutions in the Sol state showed interconnected polyhedral pore structures, whereas those of the gel state revealed a fibrillar-like morphology. Atomic force microscope (AFM) studies of the Sol and gel surfaces showed that the Sol surface was covered with fine globular particles, whereas the gel surface was covered with particles in micron-scale irregular islets. These findings are consistent with uniform mixing of the diblock copolymer and water in the Sol state, and aggregation of PCL blocks in the gel state. In conclusion, we confirm that the MPEG-PCL diblock copolymer solution exhibited a sol-gel-sol transition as a function of temperature. (c) 2006 Wiley Periodicals, Inc.