HPLC fractionation and surface micellization behavior of polystyrene-b-poly(methyl methacrylate)

A polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) was fractionated by HPLC, and the morphology of surface micelles formed at the air/water interface was investigated. Individual PS and PMMA blocks were fractionated by normal-phase and reversed-phase HPLC, respectively, into three fractions each to obtain nine fractions of narrower distribution in molecular weight as well as in composition. Although the mother PS-b-PMMA was prepared by anionic polymerization and had a narrow molecular weight distribution as characterized by size exclusion chromatography (M-w = 72.8 kg/mol, M-w/M-n = 1.08, PS wt % = 73.7%), the nine fractionated samples showed significant variations in molecular weight (M-w = 63.1-79.5 kg/mol) and in composition (PS wt % = 66.5-76.3%). The fractionated block copolymers formed more uniform surface micelles than the unfractionated mother PS-b-PMMA and exhibited a large variation with respect to the composition. The PS-b-PMMA fraction of the lowest PS content (66.5 wt %) showed circular micelles only. Rod-shaped micelles started to appear with a slight increase in the PS content, but circular and rod-shaped micelles coexist over ca. 10% concentration range and no sharp phase boundary appears to exist. The rod-shaped micelles formed at low PS contents exhibit a structure of linearly connected circular micelles.