| The morphological characteristics of highly crew-cut micelle-like aggregates of asymmetric polystyrene-b-poly(acrylic acid) copolymers PS-b-PAA, was studied by transmission electron microscopy and light scattering. The insoluble PS block contents in the copolymers ranged from 80 to 98 wt. %. The aggregates were formed by the water addition to the copolymer solution in N,N-dimethylformamide, and then were isolated in aqueous solutions.; Spherical micelles were formed when the PAA content in the copolymers was relatively higher. The PS core radii scaled as R{dollar}sb{lcub}rm core{rcub} sim{dollar} N{dollar}sb{lcub}rm PS{rcub} sp{lcub}0.4{rcub}{dollar} N{dollar}sb{lcub}rm PAA{rcub}sp{lcub}-0.15{rcub}{dollar}, where N{dollar}sb{lcub}rm PS{rcub}{dollar} and N{dollar}sb{lcub}rm PAA{rcub}{dollar} are the lengths of the PS and PAA blocks, respectively. The PAA corona shell dimensions were also studied. As the PAA content in block copolymers decreased, the morphology of the aggregate changed progressively from spheres to cylinders, to vesicles and lamellae, and eventually to compound micelles. The effects of the initial copolymer concentration and the addition of homopolystyrene on the aggregate morphology were also examined. The aggregate morphology appears to be controlled by a force balance mainly involving three aspects, i.e. the stretching of the PS chains in the core, the surface tension between the PS core and the solvent, and the inter-corona chain interactions.; The aggregate morphology can be controlled by adding ions in micro-molar (HCl, NaOH, CaCl{dollar}sb2{dollar} or Ca(Ac){dollar}sb2{dollar}) or milli-molar (NaCl) concentrations. In addition to the various morphologies obtained by changing the copolymer composition, large compound vesicles, 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 inter-corona chain repulsive interactions, due to neutralization, protonation, ion-binding, and screened electrostatic field, etc. The range of morphologies obtained in this study is believed to be unprecedented for a block copolymer self-assembly system in low molecular weight solvents. |
