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Materials

Crystallite Seeds Shape Up Micelles

ACS Meeting News: Templated self-assembly process produces highly uniform cylindrical nanomaterials

by Stephen K. Ritter
August 23, 2010 | APPEARED IN VOLUME 88, ISSUE 35

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Credit: Nat. Chem.
TEM images show long cylindrical micelles before (top) and after (bottom) sonication, which breaks the micelles into 20-nm seed crystallites.
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Credit: Nat. Chem.
TEM images show long cylindrical micelles before (top) and after (bottom) sonication, which breaks the micelles into 20-nm seed crystallites.

By coupling organic, inorganic, and polymer chemistries, Ian Manners of the University of Bristol, in England, and coworkers have created a "crystalline-driven living self-assembly" process to more precisely control the dimensions of cylindrical micelles.

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Credit: Nat. Chem.
Highly uniform PFS/PDMS cylindrical micelles pose together in this TEM image.
8835news3a.jpg
Credit: Nat. Chem.
Highly uniform PFS/PDMS cylindrical micelles pose together in this TEM image.
[+]Enlarge
Credit: Nat. Chem.
Cylindrical micelle self-assembly leading to uniform-length materials starts from a nanocrystallite.
8835news3c.jpg
Credit: Nat. Chem.
Cylindrical micelle self-assembly leading to uniform-length materials starts from a nanocrystallite.

The fiberlike materials formed by the process are block copolymers consisting of a crystalline polymer core surrounded by a corona of pendant chains of a second polymer. They have potential applications as reinforcement additives for epoxy resins, as templating substrates for making nanomaterials, and in drug delivery and nanolithography.

Manners and coworkers previously developed procedures to make cylindrical micelles with controlled properties from polyferrocenylsilane (PFS) and polydimethylsiloxane (PDMS). Manners' team has now discovered that the trick to controlling uniform length of the micelles, leading to narrow molecular weight distributions, is to start with very small seed nanocrystallites (Nat. Chem., DOI: 10.1038/nchem.664). These nanocrystallites initiate formation of cylindrical micelles, Manners said, similar to the way small initiator molecules are used to make controlled-length polymers via living anionic polymerization—where a polymer chain will keep growing as long as there is monomer available to it.

The researchers first sonicated 5-μm-long PFS/PDMS micelle cylinders, fracturing them into 20-nm nanocrystallites. They then used the crystallites to make micelles from 200 nm up to 2 μm in length by varying the PFS/PDMS monomer-to-crystallite ratio, achieving average monodispersities of 1.03—nearly perfectly uniform lengths.

The crystalline-driven process is "an exciting outcome for controlled growth," says Karen L. Wooley of Texas A&M University, who specializes in preparing self-assembled block copolymer micelles. "This method extends far beyond typical supramolecular assembly by combining concepts from crystallization and polymerization processes, leading to well-defined and unusual nanostructures."

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