MORPHOGENIC EFFECT OF SOLVENT ON CREW-CUT AGGREGATES OF APMPHIPHILIC DIBLOCK COPOLYMERS

It is shown that the morphologies and other characteristics of crew-cu t aggregates of polystyrene-b-poly(acrylic acid) (PS-b-PAA) diblock co polymers are related to the nature of the initial common solvent in wh ich the micelle-like aggregates are prepared. Polymer-solvent interact ions determine the dimensions of both the core and the corona of the a ggregates. Solubility parameters and dielectric constants of the solve nts can be used to estimate the strength of the PS-solvent interaction (which influences the solvent content in the core) and the strength o f 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 co ntent 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-sol vent interaction and the weaker the repulsive interactions among the c orona chains; this increases the aggregation number and degree of stre tching in the core. As the degree of stretching of PS chains in the co res increases and the repulsion among the corona decreases, the morpho logy of the aggregates can change progressively from spheres to cylind ers, to vesicles, or to large compound micelles. In N,N-dimethylformam ide, the core dimensions are smaller and the corona dimensions are lar ger than in tetrahydrofuran (THF) or dioxane, so spherical aggregates are favored. In THF, the solvent content in the core and the corona di mension are larger than in dioxane. Since an increase of solvent conte nt in the core favors a morphological change away from spheres, but a larger corona dimension favors spheres, the final morphology is determ ined by a balance of these two factors. Relationships between the natu re of common solvent and critical water content as well as degree of m icellization are studied in detail. Finally, morphological changes for the same diblock copolymer in different solvents are explored further .