Effects of molecular weight, temperature and salt on the self assembly of triblock copolymer solutions

The phase behavior of 5 wt% aqueous solutions of a relatively hydrophilic family of triblock copolymers of PEO and PPO, Pluronic F38, F68, F88, F98 and F108, all containing approximately 80% PEO in the molecular weight, at different temperatures and sodium carbonate concentrations were investigated by using small angle neutron scattering (SANS). We determined the thermodynamic parameters of micellization and detailed structural information at various stages of self assembly for these copolymers. Based on obtained structural details, we also proposed possible mechanisms of micellization and spherical-to-cylindrical micelle transformation. The study shows that temperature, polymer molecular weight and sodium carbonate concentration have strong influence on the phase behavior of the triblock copolymers. As the polymer molecular weight increases, the micelle aggregation number and the core and corona radii increase. However, the temperature at which the maximum of the volume fraction of the micelles decreases with increasing molecular weight. As expected, an increase in polymer molecular weight and/or the presence of carbonate ions decrease both the CMT and the temperature at which spherical-to-cylindrical micelle transformation occurs. The mechanisms of micellization and spherical-to-cylindrical micelle transformation are interpreted to be gradual dehydration of the copolymer chains upon the increase in either temperature and/or salt concentration. Progressive dehydration occurs in both the core and the corona at lower temperatures and progressive insertion of PEO segments from the corona into the core occurs at higher temperatures.