Micellization of strongly segregated block copolymers

Abstract

Series of diblock copolymers of the type poly(styrene-b-4-vinylpyridine) (PS-P4VP) are synthesized via anionic polymerization. In toluene, a selective solvent for the PS block, well-defined micelles with narrow molecular weight distribution are formed. Size and shape of the micelles are characterized by static (SLS) and dynamic light scattering (DLS), and electron microscopy (EM). The aggregation number Z exhibits a scaling relation Z∝N1.93P4VPN−0.79PS, where N is the degree of polymerization of each block. We find this behavior close to a Z∼N2AN−0.8B limiting scaling law which appears to be a characteristic of strongly segregated diblock and triblock copolymer systems as well as low molecular weight cationic, anionic, and nonionic surfactants. A simple micellization model is developed that predicts aggregation numbers of surfactants, diblock, triblock, and graft copolymers. The corona dimension Dh as measured by DLS scales as Dh∝Z0.21N0.63PS in agreement with the theoretical prediction D∼Z1/5N3/5B for starlike molecules in good solvents. Neither unimers nor a critical micelle concentration are observed. Instead at very low concentrations the excess scattering intensity increases due to the formation of large particles.