Reversible morphological transitions of poly styrene-b-polyisoprene micelles

Reversible morphological transitions of diblock copolymer micelles in dilute solutions were monitored by light scattering and atomic force microscopy for two different polymer samples near the calculated morphological boundaries. These transitions were induced solely by temperature changes. At 25 degrees C, a sample of polystyrene-b-polyisoprene diblock micelles with a polystyrene block of 20.6 kDa and a polyisoprene block of 6 kDa was observed to form cylindrical micelles in heptane, a selective solvent for polyisoprene. Upon heating to 35 degrees C, the sample adopted a spherical micelle morphology. When the sample was cooled back to 25 degrees C, cylindrical micelles were once again observed. In addition, a reversible transition from vesicles to cylindrical micelles, upon heating from 25 to 40 degrees C, was observed for a second diblock sample with the same polystyrene block (20.6 kDa) and a shorter polyisoprene block of 4.3 kDa. The change in morphology upon heating was found to be much faster then the reverse process upon cooling.