Effect of architecture on the micellization properties of block copolymers:: A2B miktoarm stars vs AB diblocks

Light-scattering and viscometric results are presented from micellar solutions of(PI)(2)PS (I2S) and (PS)(2)PI (S2I) three-miktoarm stars and a PSPI(SI) diblock copolymer in n-decane. The influence of architecture on the micellization properties of simple graft copolymers is investigated by keeping the overall molecular weights and compositions of the samples constant. All samples formed spherical micelles in n-decane, a selective solvent for polyisoprene. Aggregation numbers were found to increase in the order I2S < S2I < SI. Hydrodynamic radii of the micelles increased in the same order. The thickness of the corona is determined by the length of the soluble blocks, which they were found to be stretched to almost the same extent in all cases. The area of the core-corona interface per copolymer chain, A(c), depends on the architecture of the molecule, and it is larger in the case of I2S micelles. In the case of S2I, larger A(c) values were found compared to the SI reference sample, indicating that the PS arms are arranged in different ways in the two kinds of micelles. The presence of only one grafted chain per molecule can change considerably the micellar characteristics of complex block copolymers. A simple scaling theory is developed taking into account the free energy contributions from the core, the corona, and the interfacial region of the micelle in the different cases. Theoretical predictions agree qualitatively with the experimental results.