Micellization of poly(ethylene oxide)-poly(propylene oxide) alternating multiblock copolymers in water

The micellization behaviors of two types of the poly(ethylene oxide) (PEO) - poly(propylene oxide) (PPO) alternating multiblock (AMB) copolymers in water, which have a different weight fraction of PEO (f(PEO)), have been investigated. Surface tension measurements reveal that the molecular area at air/water interface of the PEO-PPO AMB copolymers is significantly small. This indicates that the solubility of the unimer of PEO-PPO AMB copolymers in water is quite low and structural changes occurs when the unimer transfers from the aqueous media to the interface. The critical micelle concentration (CMC) and critical micelle temperature (CMT) have been determined by the fluorescent probe method with the dye solubilization. The CMC of the PEO-PPO AMB copolymer in water is lower by 100 times than that of the PEO-PPO-PEO triblock copolymer (Pluronics) having a similar f(PEO). The small angle X-ray scattering and the dynamic light scattering measurements suggest that the unimer of the PEO-PPO AMB copolymer forms a core-corona structure below CMT, and then a core-corona multimer micelle with the 14-18 nm of radius appears above CMT. The thermodynamic quantities of micellization of the PEO-PPO AMB copolymer indicates that the PPO blocks in the unimer are loosely assembled in the core and stabilized through the hydrophobic interaction. The difference in micellization between PEO-PPO AMB copolymers and Pluronic arises from the difference in the block arrangement.