Needle and thread molecules connecting materials in new ways
Determining details of attraction in mechanically-linked molecules allows chemists to fine-tune shapes, capabilities of supramolecules for improved and new polymers
Virginia Tech chemistry professor H.W. Gibson and his students have been able to take advantage of self assembly to create new chemical structures from mechanically-linked molecules. Gibson will give an invited talk in the Division of Polymer Chemistry at the 225th national meeting of the American Chemical Society March 23-27 in New Orleans.
The field of mechanically-linked chemistry uses a limited set of compounds as building blocks for supramolecules. Starting with crown ethers, which are rings (the ’hosts’), and rod-shaped molecules such as ammonium ions (the ’guests’), the chemists thread the rings with the rods to create molecules called pseudorotaxanes. Bulky groups are then added to the ends of the rods so they won’t unthread and these molecules are rotaxanes.
The attractive forces between the rings and rods is what allows them to self-assemble into pseudorotaxanes. "It’s an equilibrium process," says Gibson.
The researchers in Gibson’s lab studied these complexes using x-ray crystallography to understand the forces that hold the different compounds together. "The x-rays gave us the exact angles and distances, allowing us to determine the attractive forces," says Gibson.
Now the researchers are putting those molecular entities at the end of polymer chains, like spaghetti strands with selective host and guest species at the ends that will only complex with each other. "It’s like a needle and thread, but only a certain thread will pass through the eye of the needle," says Gibson. What also makes the new molecule interesting is that it is held together by a noncovalent bond, meaning it can be reversed.
"We are trying to create materials at low temperatures that have the properties of block copolymers," says Gibson. "If heated, they will loosen and come apart. Such materials are easier to process. Heat causes them to revert to smaller molecules, so the material has lower viscosity. It would take less pressure to force them into a mold and you could create very thin molds."
Compare that to a thermosetting polymer that must be heated to 200 degrees C to be processed. "If there is a problem, you have to throw it away because it is not soluble or meltable. The thread and needle material could simply be reprocessed because the molecules would separate at the noncovalent bond into the smaller units, then could be allowed to reform into the specific supramolecular thread and needle."
Joining chain polymers using pseudorotaxanes could also make it possible to tailor the materials joined in block copolymers to improve mechanical or other behavior. For instance, by selection of one of a series with host species at the chain ends (from column A) and one from a series of polymers with guest species at chain ends (from column B) a range of block copolymers could be assembled and tested very readily. These systems would be analogs of traditional covalent block copolymers such as impact resistant styrene-butadiene systems.
Gibson’s group has also connected up to 12 crown ethers to C60 fullerene (60 carbon atom buckeyball) and separately up to 12 guest species. These octopus-like 12-armed species can interact with the end-functionalized polymers described above to encapsulate the buckeyeball, thus isolating the fullerene core for various applications, such as catalysis and enhanced solubility.
Gibson will deliver the paper, " Self-assembly with molecular buildings blocks (Poly 611)," co-authored by Zhongzin Ge, Jason W. Jones, and Aurica Farcas, at 9:45 a.m., Wednesday, March 26 in the Hilton Riverside Grand Salon C18/C15.
Jones and Feihe Huang, Gibson’s Ph.D. students, will also deliver papers in the Organic Chemistry Division on related research results.
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Harry Gibson | EurekAlert!