Micellization of polyoxyalkylene block copolymers in formamide

The solution properties of amphiphilic molecules in nonaqueous polar solvents are important in the elucidation of the effects of solvent quality on self-assembly and also in practical applications where the use of water as a solvent is undesirable. We used small-angle neutron scattering (SANS) to investigate the micellization of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer (Pluronic P105: EO37PO58EO37) in formamide as a function of block copolymer concentration tin the range 1-20 wt %) and temperature (10-60 degrees C). Results on the micellization thermodynamics and the micelle structure are presented here. PEO-PPO-PEO block copolymers self-assemble in formamide into micelles upon increasing the temperature, indicating an endothermic micellization. The enthalpy and entropy of micellization in formamide are about 3 times smaller than the respective quantities in water. The radius of gyration of the unimers and the core radius and hard-sphere interaction radius of the spherical micelles were obtained by fitting appropriate models to the SANS scattering patterns. The degree of swelling of the block copolymer micelles by the solvent; was estimated on the basis of the different radii. The micelle radii and the corresponding number of block copolymers that participate in each micelle increase with increasing temperature. The amount of solvent in the micelle corona decreased with increasing temperature up to 15 degrees C above the critical micellization temperature and then remained constant (for the temperature range examined) at a value of about 1.5 solvent molecules per ethylene oxide segment.