Micellization of polystyrene-b-poly(ethylene/butylene)-b-polystyrene triblock and polystyrene-b-poly(ethylene/propylene) diblock copolymers in n-octane

Static and dynamic light scattering and viscometry experiments were performed to study the micellization thermodynamics, micelle structural parameters, and micelle size distribution of a polystyrene-b-poly(ethylene/propylene) diblock copolymer, SEP2, and a polystyrene-b-poly(ethylene/butylene)-b-polystyrene triblock copolymer, SEBS2, in n-octane solutions. n-Octane is a selective solvent for the olefin blocks. Both copolymers have similar chemical composition and molar mass, but whereas SEP2 will form classical micelles, SEBS2 will have to form micelles with a corona constituted by loops of the middle poly(ethylene/butylene) block in order to bring the two end polystyrene blocks into the core and it is also conceivable that some of these end blocks become extended into solution. The influence of the different micelle architecture in the micellization process was analyzed. Standard thermodynamic functions of micellization were determined from light scattering measurements. All the functions obtained for SEBS2 are much smaller than those for SEP2. SEBS2 micelles have also lower association number and size than those for SEP2. Interestingly, an aggregation process of micelles was detected for SEBS2 solutions at relatively low concentration whereas in the SEPS solutions only micelles with a narrow size distribution were observed. The different behaviors observed for both copolymers confirm the possible structure suggested above for a triblock copolymer dissolved in a selective solvent of the middle block.