Block copolymer self-assembly in solution: structure and dynamics.

Abstract

Block copolymers can self-assemble into micelles or vesicles when dispersed in a selective solvent. In this study, spherical micelles were formed by poly(styrene-bethylene- alt-propylene) (PS-PEP) in squalane, highly selective to PEP blocks, leading to PS cores and swollen PEP coronas. The micelle structure was characterized by dynamic light scattering (DLS) and small-angle x-ray scattering (SAXS). The experimental results provide a detailed picture of micelle structure and intermicelle interaction as a function of block copolymer molecular weight and composition, concentration, and temperature. Based on this structural information, the single molecular exchange kinetics between the spherical micelles in dilute solution was examined by time-resolved small-angle neutron scattering (TR-SANS). Two pairs of structurally matched partially protonated and deuterated micelles were prepared and each pair was blended to provide an initially isotopically segregated state in solution. The SANS intensity is directly related to the concentration of protonated chains in the micelle cores. Therefore, a reduction in the measured scattering intensity can be quantitatively correlated with the exchange of chains. This measurement was aimed at probing the dependence of molecular exchange kinetics on temperature, molecular weight, and concentration. The temperature dependence of the chain exchange rate R(t) can be explained based on the core block dynamics, while the documented quasi-logarithmic decay of R(t) is shown to be consistent with single chain exchange that is hypersensitive to the core degree of polymerization and therefore polydispersity. Complementary measurements were also conducted with concentrated solutions where the micelles pack onto a body-centered cubic lattice. Based on a first-principles model, the exchange kinetics are expected to be independent of micelle concentration. However, slower dynamics in ordered micelles were observed. These results suggest that contributions from factors other than core block dynamics can come into play in the exchange kinetics for ordered micelles.