Amphiphilic Block Random Copolymers: Influence of pH and Ionic Strength on Aqueous Solution Properties

Abstract

Aqueous solution properties of polystyrene-b-poly-(styrene-r-acrylic acid), PS-b-P(S-r-AA), block−random copolymers (BRC) as a function of pH and ionic strength are reported using light scattering and surface tensiometry. Critical aggregation concentrations (CAC) are two orders of magnitude greater than similar PS-b-PAA block copolymers at pH >7.7; however, the CAC approaches that of block copolymers as the pH approaches the pK a of the BRC (pH = 7.0, pK a ∼6.3). Aggregation numbers (N agg ) between 5 and 26 are observed under alkaline conditions, about one to two orders of magnitude lower than those of PS-b-PAA block copolymers. Lowering the pH from 9.5 to 7.0 and increasing the ionic strength from 30 to 1000 mM reduced CAC and increased N agg due to a lower effective charge along the BRC chains. Analysis of the multichain aggregates using a core−corona spherical model showed that the PS-r-PAA blocks are compressed on the surface of the PS core due to the favorable mixing of the PS units. The BRCs are surface active above 0.1 g/L and slow to adsorb to interfaces. Results are related to previous work with emulsion polymerizations stabilized by BRC, suggesting that the structure of the aggregates and the slow adsorption kinetics play significant roles in the emulsion polymerization mechanism.