Effect of the Number of Arms on the Association of Amphiphilic Star Block Copolymers

Amphiphilic eight-arm star block copolymers with poly(methyl methacrylate) inner blocks and poly(acrylic acid) outer blocks were synthesized and their self-assembling properties were investigated in aqueous solutions. The critical aggregation concentrations of the stars were determined by steady-state fluorescence spectroscopy and static light scattering. The effects of salt, pH and the ionic strength on the self-assemblies have been investigated. Unlike in the earlier study on chemically similar four-arm stars, no transition in the morphology of aggregates was observed upon the addition of salt. The amphiphilic stars preferred forming spherical aggregates with low aggregation numbers at various solution conditions due to the steric crowding of the hydrophobic core as well as due to the remaining repulsion between the polyelectrolyte blocks in the outer shell. In addition to the experiments, computer simulations of the eight-arm amphiphilic star block copolymer with hydrophobic inner and hydrophilic outer blocks in a ratio 1: 1 were studied using a coarse-grained model. It was observed that the eight-arm stars formed spherical micelle-like aggregates both in salt-free solutions and in the presence of salt, which is in agreement with the experimental results.