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

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 S-g. n decreases and Sg increases with increasing 25R4 at low concentrations of F127. However, 10R5 and 25R4 affected n and S-g differently at high concentrations of F127. (C) 2015 Elsevier Ltd. All rights reserved.