Recent Research Developments

University of Minnesota graduate student Zhibo Li working in the laboratories of MRSEC investigators Tim Lodge and Marc Hillmyer and in collaboration with Ellina Kesselman and Yeshayahu Talmon from the Technion-Israel Institute of Technology discovered a set of remarkable micellar structures formed in dilute solutions of multicomponent block copolymers. Covalent connection of a hydrophobic polymer to a hydrophilic polymer gives an (AB) amphiphilic macromolecule that will spontaneously self-assemble in aqueous solution. The micelles formed are aggregates of these molecules with the hydrophobic portion on the inside of the micelle (protected from the water) and with the hydrophilic component as the water compatible corona. While a variety of micellar structures (spheres, cylinders, bilayers) can be formed using diblock copolymers, with only two components the structures simply consist of an inside and an outside. To further subdivide space within the hydrophobic core would be a first step toward mimicking the structural and functional complexity of eukaryotic cells with multiple functional units (nucleus, organelles, etc.) in an all-synthetic system.

To prepare a "multicompartment micelle" using block copolymers, Zhibo incorporated a third block (C) that is both hydrophobic and incompatible with the other hydrophobic block. In this way, the two hydrophobic materials are forced to share the same space inside the micelle, but do mix. To further "frustrate" the system, each of the blocks was connected at one junction point thus giving an ABC mikto (mixed) arm star polymer (Figure 1). By tying all three chains to a single juncture point, a remarkable array of multicompartment micellar structures were formed. A particular interesting structure is the segmented worm-like micelles that contain alternating disks of the two hydrophobic components. Direct evidence for these structures was obtained by cryo transmission electron microscopy as shown in Figure 2. The nature of the structures could be tuned by controlling the molecular parameters in the triblock copolymers. This "bottom-up" engineering of nanostructures is a particularly appealing approach for the preparation of controlled supramolecular assemblies. This study highlights the versatility of block copolymers in the design of self-assembled structures with unique features. From and applied perspective, these structures may have utility in many nanotechnological applications including drug-delivery, light-harvesting, and nanostructure templating.

Li, Z.; Kesselmann, E.; Talmon, Y.; Hillmyer, M. A.; Lodge, T. P. "Multicompartment Micelles from ABC Triblock Copolymer Miktoarm Stars" Science 2004, 306, 98.

Figure 1. Chemical structure and schematic representation of the ABC star block terpolymers.

Figure 2. Cryo-TEM images of 1 wt % aqueous solutions of four different ABC star structures with varying compositions (scale bars indicate 50 nm).

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