Micellization, aggregation, and gelation of polystyrene-block-poly(ethylene oxide) in cosolvents added with hydrochloric acid

Abstract

The micellization, aggregation, and gelation of the polystyrene-block-poly(ethylene oxide) block copolymer in solutions were quantitatively analyzed by small-angle X-ray scattering. The content and concentration of added hydrochloric acid influence the micellization, aggregation, and gelation of a polystyrene-block-poly(ethylene oxide) block copolymer in tetrahydrofuran/water and dimethylformamide/water cosolvents added with hydrochloric acid. The polystyrene-block-poly(ethylene oxide) block copolymer forms spherical micelles with screened Coulomb repulsions when dispersed in tetrahydrofuran/water cosolvents added with hydrochloric acid. The screened Coulomb repulsion arises from the poly(ethylene oxide) shells bound with protonated water and surrounded by unbalanced negative charges. If excessive water is introduced from hydrochloric acid, the non-solvent quality of tetrahydrofuran/water cosolvent predominantly causes aggregation of spherical micelles. Such screened Coulomb repulsions do not form for core-shell cylinders dispersed in dimethylformamide/water added with hydrochloric acid. Hydrochloric acid tends to form complexes with dimethylformamide. The length of core-shell cylinders can increase with the increased content of hydrochloric acid. Long core-shell cylinders favorably form three-dimensional networks through inter-cylinder entanglement. Furthermore, the absence of screened Coulomb repulsions makes entangled cylinders more compact to strongly trap the liquid phase. Thus, gels occurred in dimethylformamide/water cosolvents with added hydrochloric acid. Nevertheless, the content of added hydrochloric acid should be finely controlled. Adding a large amount of hydrochloric acid prohibits network formation and instead favors kinetically trapped rigid cylinders. This study has demonstrated that finely controlling solvent quality by additives provides a physical design strategy for fabricating cylindrical micelles with tunable properties in solutions. This should offer the fundamental understanding of self-assembly for potential applications of block copolymer micelles.