Unimolecular micelle formation of poly(methyl methacrylate)-graft-polystyrene in mixed selective solvents of acetonitrile/acetoacetic acid ethyl ether

Unimolecular micelle formation is investigated by means of light scattering for poly(methyl methacrylate)-graft-polystyrene copolymers with low grafted chain densities in dilute solution of mixed selective solvents. Molecular weights of poly(methyl methacrylate) (PMMA) backbone and polystyrene (PS) branch are about 6 x 10(6) and 9 x 10(3) g mol(-1), respectively, and the grafted chain density ranges from 6 to 17 wt % in terms of PS composition. The mixed solvents are marginally good solvents for PMMA and are nonsolvents for PS. From the results of static and dynamic light scattering measurements as a function of temperature and solvent composition, it is demonstrated that, at higher branch densities and lower temperatures, the rod like unimolecular micelles are formed, made of a few linearly connected flowers with small cores of associated PS branches and petals of PMMA backbone chain. The number of petals per flower increases with increasing branch density and decreasing solubility of solvents for PMMA. On the other hand, at the lowest branch density of 6 wt %, instead of unimolecular micelle formation, intermolecular associations take place, even at low copolymer concentration.