Thermosensitive sol-gel transition behaviors of poly(ethylene oxide)/aliphatic polyester/poly (ethylene oxide) aqueous solutions

Abstract

The thermoreversible gelation of Pluronic [poly(ethylene oxide) (PEO)-polypropylene oxide (PPO)-PEO] aqueous solutions originates from micelle formation and micelle volume changes due to PEO-water and PPO-water lower critical solution temperature behavior. The micelle volume fraction is known to dominate the sol-gel transition behavior of Pluronic aqueous solutions. Triblock copolymers of PEO and aliphatic polyesters, instead of PPO, were prepared by hexamethylene diisocyanate coupling and dicyclohexyl carbodoiimide coupling. Through changes in the molecular weight and hydrophobicity of the polyester middle block, the hydrophobic-hydrophilic balance of each block was systematically controlled. The following aliphatic polyesters were used: poly(hexamethylene adipate) (PHA), poly(ethylene adipate) (PEA), and poly(ethylene succinate) (PESc). With the hydrophobicity and molecular weight of the middle block increasing, the critical micelle concentration at the same critical micelle temperature decreased, and the absolute value of the micellization free energy increased. The micelle size was rather insensitive to temperature but slightly decreased with increasing temperature. PEO-PHA-PEO and PEO-PEA-PEO triblock copolymers needed high polymer concentrations to form gels. This was ascribed to the tight aggregation of PHA and PEA chains in the micelle core due to strong hydrophobic interactions, which induced the contraction of the micelle core. However, because of the relatively hydrophilic core, a PEO-PESc-PEO aqueous solution showed gelation at a low polymer concentration. (C) 2003 Wiley Periodicals, Inc.