Role of PPO–PEO–PPO triblock copolymers in phase transitions of a PEO–PPO–PEO triblock copolymer in aqueous solution

Abstract

The effects of two poly(propylene oxide)–poly(ethylene oxide)–poly(propylene oxide) (PPO–PEO–PPO) triblock copolymers, Pluronics-R 10R5 (PPO8–PEO22–PPO8) and 25R4 (PPO19–PEO33–PPO19), on the thermal and rheological properties as well as the phase transitions of a poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymer (Pluronic F127, PEO100–PPO65–PEO100) in aqueous solution have been systematically studied. Both 10R5 and 25R4 could shift the critical micellization temperature of F127 to lower temperatures, showing a “salt-out like” effect. When F127 itself is unable to form a gel at low concentrations, 25R4 could assist micelles of F127 to be connected into a gel network. However, 10R5 is too short to be able to assist F127 to form a gel. At low contents of 10R5 or 25R4, F127 dominates its gelation but 25R4 could affect the hard gel–soft gel transition while10R5 showed a weaker effect on the hard gel–soft gel transition than 25R4. The rheological scaling law in the vicinity of the gel point was examined to obtain the critical relaxation exponent n and the critical gel strength Sg. n decreases and Sg increases with increasing 25R4 at low concentrations of F127. However, 10R5 and 25R4 affected n and Sg differently at high concentrations of F127.