Aggregation and Critical Micelle Concentrations of Polystyrene-b-poly(sodium acrylate) and Polystyrene-b-poly(acrylic acid) Micelles in Organic Media

Abstract

Block copolymer micelles formed from polystyrene-b-poly(sodium acrylate) (PS-b-PANa) and polystyrene-b-poly(acrylic acid) (PS-b-PAA) were characterized in organic solvents. The critical micelle concentrations (cmc's) were determined for one block copolymer series in which PS blocks of 660 units were attached to either relatively short PANa or PAA blocks ranging in length from 2.6 to 18 units. The cmc's for these block copolymers in toluene, tetrahydrofuran (THF), and chloroform were found to range from ca. 1 × 10-7 to 5 × 10-9 M. The a and b constants in the relation log Ci = aNi1/3 + b, where Ci is the cmc for a monodisperse polymer and Ni is the insoluble block length were evaluated and correlated to the polymer−solvent interaction parameters for the present systems as well as for several other block copolymer micelles. The PS(660)-b-PANa(x) series was also investigated by size exclusion chromatography (SEC) in THF. Two peaks in the SEC chromatograms could be distinguished, one corresponding to micellized block copolymers and the other to unassociated chains. The position of the latter peak was the same as that of the PS homopolymer, which was also present. These micelles were also characterized by static and dynamic light scattering (SLS and DLS). The effect of the soluble (PS) block length on the aggregation was investigated by SLS in THF. For this study, three additional PS-b-PANa series containing PS block lengths of 190, 630, and 2300 units attached to PANa blocks ranging in lengths from 4.2 to 69 units were characterized. The aggregation numbers (Nagg) were found to be influenced significantly by the soluble PS block length (NPS). The Nagg and the calculated core radii (Rc) were found to scale with the block lengths as Nagg ∝ NPANa0.5NPS-0.6 and Rc ∝ NPANa0.5NPS-0.2.