Amphiphilic Block-Random Copolymers: Mapping the Boundaries of Aqueous Self-Assembly

The structure-property relationships of polystyrene-b-poly(styrene-r-(acrylic acid)) block-random copolymers in aqueous media have been investigated. A library of copolymers with varied degree of polymerization of the hydrophobic polystyrene block, varied mol % acrylic acid (AA) in the hydrophilic block, and varied volumetric block ratios was designed. Rather counterintuitively, it was discovered that PS-b-(PS-r-PAA) block-random copolymers containing relatively low amounts of AA in the hydrophilic block disperse much more readily than those containing higher amounts of AA, despite the decrease in the hydrophilic monomer content. Dispersible copolymers were examined in terms of their critical aggregation concentration, aggregation number, and particle size of aggregates in relation to their structure. A change in interblock interaction parameters is proposed for the differences in dispersion behavior between copolymers containing relatively low and high amounts of AA in the stabilizing block. The findings of this study will not only inform their use as stabilizers in emulsion polymerization but shed light on using highly hydrophobic block copolymers to make small aggregates of a tunable size.