Morphogenic effect of added ions on crew-cut aggregates of polystyrene-b-poly(acrylic acid) block copolymers in solutions

The morphology of crew-cut aggregates of amphiphilic block copolymers in dilute solutions can be controlled by the addition of ions in micromolar (HCl, NaOH, CaCl2, Ca(Ac)(2)) or millimolar (NaCl) concentrations. The copolymers are highly asymmetric polystyrene-b-poly(acrylic acid) diblocks, PS-b-PAA, in which the lengths of the insoluble PS blocks are much longer than those of the soluble PAA blocks. In addition to spherical, rodlike, and univesicular or lamellar aggregates, large compound vesicles (LCVs), a new morphology, can be obtained from a single block copolymer. The morphogenic effect of different added ions on the crew-cut aggregates can be ascribed to the changed repulsive interactions among the hydrophilic PAA segments, due to neutralization by NaOH, protonation by HCl, ion-binding or bridging by Ca2+, and electrostatic screening by NaCl, respectively. The formation of the LCVs may involve a secondary aggregation of individual Vesicles and a subsequent fusion process. Some features of the spontaneously formed LCVs may make them especially useful as drug delivering vehicles, and as models of biological cells.