Effect of Architecture on the Micellar Properties of Amphiphilic Block Copolymers:  Comparison of AB Linear Diblock, AAB, and A₂B Heteroarm Star Block Copolymers

Amphiphilic AB, A 1 A 2 B, and A 2 B block copolymers, where A = polyisobutylene, B = poly(methyl vinyl ether), and the superscripts denote molecular weight asymmetry, with constant molecular weight and composition have been synthesized by living cationic polymerization. The influence of architecture on aqueous micellar properties of these block copolymers were investigated in the temperature range 20−30 °C by fluorescence spectroscopy and static and dynamic light scattering (SLS and DLS). The critical micelle concentration (cmc) measured at 23 °C increased in the order A 2 B < A 1 A 2 B < AB. The partition equilibrium constants, K v of pyrene, characteristic of hydrophobicity, increased in the opposite order of cmc. The hydrodynamic radii ( R h ) and aggregation numbers ( N agg ) of micelles remained approximately constant in the whole temperature range for A 1 A 2 B and A 2 B and below 25 °C for AB. At ∼25 °C, however, there was a sudden increase in both R h and N agg for AB. Below 25 °C both R h and N agg increased in the order AB < A 1 A 2 B < A 2 B. The particle size distribution for all block copolymers remained narrow in the whole temperature range. The results are discussed in terms of possible morphologies.