Atypical micellization of star-block copolymer solutions

The self-assembly of star-block copolymer (B(y)A(x))(n) in a selective solvent for its outer block is studied by using dissipative particle dynamics, where (B(y)A(x))(n) denotes a n-armed star with each arm comprising of y solvophobic B-segments and x solvophilic A-segments. The effects of arm number, arm length, block length ratio y/x, solvent quality, and component compatibility on mean aggregation number < p > are examined. Unusual micellization behaviors are observed. The total number of arms in a multimolecular micelle n(*) is invariant with the arm number but grows with the segment concentration phi, which is different from typical micellization of short-chain surfactants. For a given phi, multimolecular micelles are formed for stars with n < n(*)(phi) and the mean aggregation number follows < p >=n(*)/n. As n >= n(*), only unimolecular micelles exist in the solution. According to the n(*)-phi relation, the critical multimolecular micelle concentration is found to grow linearly with arm number. As solvent quality deteriorates or component compatibility increases, the total arm number within a micelle rises.