Polymers Polymers
Asymmetric micelle growth could improve computer circuits
A Canadian-British team has developed block copolymers that self assemble into rod-shaped micelles with a crystalline core and 'hairy' corona. Adding a different block copolymer will cause the micelle to grow from either end, creating a dumbbell shaped co-micelle (A). By cross-linking the end blocks and selectively dissolving the middle block, the researchers created daughter micelles (B) that grow from one end only (C). The process can be used to create asymmetrical diblock or triblock co-micelles (D) which themselves can assemble into ‘supermicelles’ (E). This type of hierarchical self-assembly could be used in computer circuits or other nanostructures.
By Tyler Irving
Posted October 2012
Imagine being able to ‘grow’ ultra-tiny computer circuits from electrically- conductive, self-assembling polymers. That dream is one step closer to reality after a Canadian-British collaboration created polymer micelles that grow in a single direction.
In the early 1990s Ian Manners, then a professor of chemistry at the University of Toronto, invented polyferrocenyldimethylsilane( PFS), a polymer with a number of unique properties. Now at the University of Bristol, Manners collaborates with another U of T chemistry professor, Mitch Winnik, on block copolymers that combine PFS with various other polymers. In the right solvent, a block copolymer of PFS and polydimethylsiloxane (PDMS) forms cylindrical micelles with a core composed of crystalline PFS and a corona made of PDMS; Winnik calls them ‘hairy rods.’ When a new block copolymer - this time a combination of PFS and polyisoprene (PI) - is added, it spontaneously assembles to both ends of the existing rods, forming a triblock co-micelle. The PI corona chains can be cross-linked by adding an appropriate chemical agent, thus ‘capping’ the ends of the co-micelle.
In their most recent paper, published in Science, the group was able to dissolve only the middle block, leaving the two capped ends. “I would never have believed that would be possible,” says Winnik, “but our post-doc Paul Rupar found a solvent combination that was just right.” The capped ends become seed micelles to which new block copolymers can be added, but only to one end. The group went on to create directionally-grown co-micelles containing multiple block copolymers, which themselves can assemble into even larger ‘super-micelles.’ This kind of hierarchical self- assembly mimics natural processes like the formation of plant and animal tissues.
If used with conductive polymers, the technique could one day allow the creation of complex structures like integrated circuits or organic photovoltaics. “From a chemistry perspective, we’re still taking baby steps,” cautions Winnik. “But in terms of learning how to control hierarchical assembly, I think this shows a path forward.”
Photo Credit: Paul Rupar
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