Morphogenic effect of solvent on crew-cut aggregates of apmphiphilic diblock copolymers

It is shown that the morphologies and other characteristics of crew-cut aggregates of polystyrene-b-poly(acrylic acid) (PS-b-PAA) diblock copolymers are related to the nature of the initial common solvent in which the micelle-like aggregates are prepared. Polymer-solvent interactions determine the dimensions of both the core and the corona of the aggregates. Solubility parameters and dielectric constants of the solvents can be used to estimate the strength of the PS-solvent interaction (which influences the solvent content in the core) and the strength of PAA-solvent interaction (which influences the repulsion among corona chains). The closer the match between the solubility parameter of the solvent and that of the core forming block, the higher the solvent content of the core and the higher the degree of stretching of the core chains. The lower the polarity of the solvent;, the weaker the PAA-solvent interaction and the weaker the repulsive interactions among the corona chains; this increases the aggregation number and degree of stretching in the core. As the degree of stretching of PS chains in the cores increases and the repulsion among the corona decreases, the morphology of the aggregates can change progressively from spheres to cylinders, to vesicles, or to large compound micelles. In N,N-dimethylformamide, the core dimensions are smaller and the corona dimensions are larger than in tetrahydrofuran (THF) or dioxane, so spherical aggregates are favored. In THF, the solvent content in the core and the corona dimension are larger than in dioxane. Since an increase of solvent content in the core favors a morphological change away from spheres, but a larger corona dimension favors spheres, the final morphology is determined by a balance of these two factors. Relationships between the nature of common solvent and critical water content as well as degree of micellization are studied in detail. Finally, morphological changes for the same diblock copolymer in different solvents are explored further.