Micellar transformations of poly(styrene-b-isoprene) block copolymers in selective solvents

Abstract

The morphology of micelles formed from linear and cyclic poly(styrene-b-isoprene) copolymers (PS₁₆₆-b-PI₂₇₈) has been studied in solvents with different selectivity for the two block components: heptane and decane, good solvents for the PI block, and DMF, a good solvent for the PS block. Using small and ultra small angle X-ray scattering experiments, SAXS and USAXS, respectively, morphological changes were monitored as a function of the temperature. While micelles formed in DMF were stable and no morphological changes occurred as a function of the temperature, transformations from cylinder-like micelles to vesicles-like objects were observed in both decane and heptane above 60 °C for the linear PS-b-PI copolymer. The transition occurred through intermediate steps, where the coexistence between different micellar morphologies was observed. On the contrary, the morphology of the block copolymers with the cyclic architecture was found to be independent of temperature and concentration. For linear blocks, a demicellization was detected at higher temperatures (above 80 °C) and the process was fully reversible when the solution was cooled down to ambient temperature. These results are consistent with complementary dynamic light scattering (DLS) observations.