Mechanisms of Chain Exchange in Block Copolymer Micelles

Abstract

Mechanisms of equilibration in block copolymer micelles were investigated in detail using time resolved small angle neutron scattering (TR-SANS). The model polymers used in this study were polystyrene-b-polyethylenepropylene (PS-PEP) diblock copolymers and corresponding triblock copolymers (PS-PEP-PS, PEP-PS-PEP). When dissolved in squalane, the polymers self assembled into spherical micelles with the PEP blocks forming the solvated coronas, and undiluted PS blocks as the micelle cores. Normal and selectively deuterated equivalent polymers with controlled molecular weight, narrow molecular weight distribution and composition were synthesized by anionic polymerization of styrene and isoprene followed by the selective saturation of the polyisoprene blocks. The structure of polymer micelles were characterized using dynamic light scattering (DLS) and small angle X-ray scattering (SAXS). A contrast matching strategy was employed for the TR-SANS experiments, where separately prepared deuterated and protonated micelles were mixed at equal volume fractions in a solvent containing 42 vol% h-squalane and 58 vol% d-squalane. Chain exchange reduces the mean contrast of the micelle cores in the solvent mixture, thus reducing the SANS scattering intensity, providing a method to characterize the dynamics of the process as a function of time. In this thesis, several aspects of chain exchange mechanisms were investigated. The hypothesis of hypersensitivity of chain exchange rate to the core block length, and the single chain exchange mechanism, were first tested and confirmed in the PS-PEP model micelle system. The chain exchange mechanisms in PEP-PS-PEP and PS-PEP-PS micelles were then investigated, and a remarkable effect of molecular architecture on the chain exchange rate is documented. In addition, this study explores the facilitating role of the corona chains in molecular exchange. It was found that adding PEP homopolymers of size comparable to the PEP blocks into dilute PS-PEP micelle solutions can significantly retard the chain exchange rate. Decreasing the corona block fraction in the PS-PEP polymers also reduced the chain exchange rate, and the concentration dependence of the chain exchange relaxation time constant. Finally, we extended our scope to chain exchange between micelles away from equilibration, i.e., micelle hybridization of two populations of PS-PEP micelles of different sizes. The results of this work suggested quantitatively different mechanisms when the micelle systems are away from equilibration, and a concentration effect was found, even when the micelles are still dilute.