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Copolymers Self-Assemble Multiple Times To Form Complex Structures

ACS Meeting News: Scalable strategy yields structures on the micrometer length scale

by Celia Henry Arnaud
March 24, 2015


A self-assembly technique allows chemists to build polymeric structures on the micrometer length scale. The resulting structures could ultimately be used for molecular electronics and drug delivery applications.

Chemists usually struggle to assemble objects on these length scales. The new method is hierarchical, first assembling nanoscale structures that then organize into larger ones.

Ian Manners of the University of Bristol, in England; Mitchell A. Winnik of the University of Toronto; and coworkers developed the method by building on a previously reported one called living crystallization-driven self-assembly (CDSA), which produces individual micelles. In the new method, these micelles help build structures that can self-assemble into larger “supermicelles” (Science 2015, DOI: 10.1126/science.1261816). Manners presented the work today in a symposium sponsored by the Division of Polymer Chemistry at the American Chemical Society national meeting in Denver.

In CDSA, block copolymers in which one of the blocks forms a crystalline lattice self-assemble into cylindrical micelles. Manners reported that he can use one of those micelles to seed the growth of a larger block comicelle, which is a structure built from more than one type of micelle.

The researchers mix seed micelles with a different block copolymer in solution. The block copolymer adds to both ends of the seed micelle. The structure grows until the block copolymer has been used up. The ratio of the concentrations of the seed micelle and the block copolymer dictates the length of the comicelle.

By using a mix of copolymers with hydrophobic and hydrophilic shells, Manners and coworkers made block comicelles with alternating hydrophobic and polar segments. The process can be repeated multiple times. So far, the researchers have made structures with as many as 11 blocks.

When the researchers put the comicelles in various solvents, they assembled into different structures. Triblock comicelles with a polar-hydrophobic-polar structure stacked side by side in hydrophilic solvents such as isopropyl alcohol and end to end in nonpolar solvents such as hexane or decane. One-dimensional and 3-D superlattices formed in isopropyl alcohol from the end-to-end stacking of triblock comicelles with a hydrophobic-polar-hydrophobic structure.

“The 3-D structures are pretty exciting,” Manners said. “We should be able to make structures with periodicity on the order of the wavelength of light.” Such structures would be of interest as photonic crystals, he said.

The work “introduces a new approach to making materials that are structured on multiple length scales,” Timothy P. Lodge, a chemistry and materials science professor at the University of Minnesota, told C&EN. “The strategy is truly hierarchical, in the sense that there are two completely separate levels of self-assembly.” The strategy is particularly appealing, Lodge said, because it is scalable and because various final structures can be made from one type of intermediate building block.

In the future, Manners hopes to make structures that can conduct electricity or harvest light. “In principle, you can functionalize these structures very easily,” he said. By adding light-harvesting dyes, he said, the structures could generate current that then feeds into an external circuit.


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