Publications

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We show that olefin metathesis can be used in an extremely simple process to rapidly alter the morphology of self-assembled poly(butadiene-b-ethylene oxide) (PB-PEO) dispersions in situ. The addition of a water-insoluble Hoveyda-Grubbs catalyst to aqueous assemblies of PB-PEO leads to degradation of the hydrophobic PB block by well-established metathesis pathways and a concomitant change in the composition of the block copolymer. This phenomenon drives morphological transitions characterized by rapidly decreasing sizes of the self-assembled aggregates, the ultimate extent of which is readily controlled by catalyst concentration. Exemplary cases are presented in which transitions from worm-like micelles to spherical micelles or from vesicles to worm-like micelles can be accomplished within minutes.

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Self-assembled giant polymer vesicles prepared from double-hydrophilic diblock copolymers, poly(ethylene oxide)-b-poly(acrylic acid) (PEO-PAA) show significant degradation in response to pH changes. Because of the switching behavior of the diblock copolymers at biologically-relevant pH environments (2 to 9), these polymer vesicles have potential biomedical applications as smart delivery vehicles.

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Controlling the traffic of molecules and ions across membranes is a critical feature in a number of biologically relevant processes and highly desirable for the development of technologies based on membrane materials. In this paper, ion transport behavior of hybrid lipid/polymer membranes was studied in the absence and presence of ion transfer agents. A pH-sensitive fluorophore was used to investigate ion (H+/OH−) permeability across hybrid lipid/polymer membranes as a function of the fraction of amphiphilic block copolymer. It was observed that vesicles with intermediate lipid/polymer ratios tend to be surprisingly more permeable to ion transport than the pure lipid or pure polymer vesicles. Hybrid vesicle permeability could be further modulated with valinomycin, nigericin, or gramicidin A, which significantly expedite the dissipation of externally-imposed pH gradients by facilitating the transport of the rate-limiting co-ions (e.g. K+) ions across the membrane. For gramicidin A, ion permeability decreased with increasing polymer mole fraction, and the method of introduction of gramicidin A into the membrane played an important role. Strategies to incorporate biofunctional molecules and facilitate their activity in synthetic systems are highly desirable for developing artificial organelles or other synthetic compartmentalized structures requiring control over molecular traffic across biomimetic membranes.

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