Dynamic chain exchange of diblock copolymer micelles.

摘要

Two groups of matched diblock copolymers were synthesized by anionic polymerization for experiments measuring the rate of chain exchange of copolymer chains between molecularly dissolved unimers and micelles. They included polystyrene-b-poly(hydroxyethyl methacrylate) (PS-PHEMA), and poly(methyl methacrylate)-b-poly(methacrylic acid) (PMMA-PMAA). Each set of copolymers included an unlabelled polymer and a matched polymer labelled with a pyrene chromophore.;Micelles of PS-PHEMA in cyclohexane/THF and PMMA-PMAA in ethyl acetate/methanol were characterized by viscometry and dynamic light scattering to determine the aggregation numbers. PMMA-PMAA micelles were formed in solutions of 80% or greater ethyl acetate and PS-PHEMA formed micelles in solutions of 20 to 75% cyclohexane. The critical micelle concentrations (CMC) of PS-PHEMA in 20% cyclohexane/80% THF and of PMMA-PMAA in 85% ethyl acetate/15% methanol and in 90% ethyl acetate/10% methanol were determined by measuring an increase in the fluorescence lifetime of pyrene attached to the end of the core forming block upon the onset of micellization.;Two major goals of this research were to confirm the rate law and to measure the polymer chain exchange constant under various conditions. The kinetics experiments were conducted by mixing a micellar solution of the unlabelled copolymer with that of a solution of a matching pyrene labelled copolymer whose concentration was below its CMC. The incorporation of the labelled copolymer into micelles was following the variation in the fluorescence intensity of pyrene with time. The chain insertion rate constant ;Micelle chain exchange experiments were also attempted for PS-PHEMA in 20% cyclohexane. At room temperature the rate of polymer chain exchange was not detectable after 36 hours, however after heating at 50