Micelle Formation of Diblock Copolymers of Styrene and Sulfonated Isoprene in Aqueous Solution

Asymmetric diblock copolymers of styrene (St) and sulfonated isoprene (SI) with a short St block and a much longer SI block were synthesized by sulfonation of diblock copolymers of St and isoprene, and their micellization in aqueous solution was investigated. Three block copolymers having different degrees of polymerization of St block (DP St ) and SI block (DP SI ) were employed, which include polymers with DP St = 40 and DP SI = 240 (St−SI-20), DP St = 60 and DP SI = 210 (St−SI-30), and DP St = 130 and DP SI = 820 (St−SI-207). Dry samples of these St−SI block copolymers could be directly dissolved in water, undergoing intermolecular self-association to form core−corona type micelles. The micellar mass, hydrodynamic size and its distribution, critical micelle concentration (cmc), and aggregation number were estimated by light scattering and fluorescence techniques. It was suggested that the micelles and unimers coexist at equilibrium with a well-defined cmc on the order of 10 -2 g/L, with St−SI-207 (with the longest SI block) showing a higher cmc than the other two polymers. The aggregation number was strongly dependent on the DP St /DP SI ratio, with St−SI-30 (with the highest DP St /DP SI ratio) showing a largest aggregation number of 480. The hydrodynamic size of the micelle was almost independent of the polymer concentration up to 1.0 g/L, ranging from 61 nm for St−SI-30 to 210 nm for St−SI-207 in 0.01 M NaCl. On increasing ionic strength, the hydrodynamic size of St−SI-207 decreases markedly because of a collapse of the SI blocks in the micelle corona. The equilibrium constant for the partition of pyrene probes between the aqueous bulk and micellar phases was found to be on the order of 10 5 M -1, reflecting effective solubilization of pyrene molecules in the cores of the St−SI block copolymer micelles. Protection of pyrene fluorophores from a bulk quencher (acrylamide) was found to be more effective in the St−SI-30 and St−SI-207 micelles than in the St−SI-20 micelle, attributable to a larger size of the micelle core.