image: This is a schematic of a magnetic tweezers measurement of a growing polymer (black string) tethered between a surface and a ruthenium-based catalyst that is anchored to a magnetic particle. The catalyst inserts new monomers (blue spheres), leading to the extension lengthening of the polymer. Extension vs. time trace of polymer growth shows wait-and-jump dynamics. view more
Credit: Courtesy of Cornell University
ADELPHI, Md. (Feb. 6, 2018) - Army research that resulted in the first real-time visualization of single polymer chain growth was named "Research of the Year" by Chemical and Engineering News.
This research helps the Army address ever-evolving threats by expanding its understanding of polymers, specifically how they grow and how to manipulate their growth kinetics to enable controlled structures with specified properties. This insight, researchers say, will enable advanced materials for the future.
Scientists at Cornell University, funded by the U.S. Army Research Laboratory's Army Research Office, focused their research on developing new analytical techniques for probing polymer dynamics and exploring how those dynamics can be manipulated to control polymer microstructure which, in turn, impacts the ensuing macroscopic properties.
"Polymers offer unparalleled opportunities for preparing materials with tailored structures and tunable physical properties. These molecular chains are ubiquitous in society, impacting every area of our daily lives from the fabrics in our clothing, to the plastics that comprise our food and drink packaging, to the composites that make up our automobiles and airplanes. In the military, polymers have been key to our wartime success," said Dr. Dawanne Poree, an Army Research Office program manager.
A 2007 article in the Army Logistician noted that it was the interwar discoveries of synthetic rubber such as neoprene for vehicle tires, and machine parts; polyamide, which is nylon used for ropes and parachutes; polyethylene, or the insulating material that enabled the development of radar, and polytetrafluoroethylene, a key element in Teflon, that led to the atomic bomb that enabled the allies to shoot, move and communicate with greater ease, reliability and lethality than the German and Japanese forces during World War II.
"Present-day, polymers are still vital to our military, serving as key components in Soldier protective systems, anti-corrosive coatings and medical supplies for wounded Soldiers," said Poree, who's based at ARO's Research Triangle Park, North Carolina.
Poree said this research offered a novel approach that had the potential to significantly transform how the kinetics and mechanism of polymerization catalysis can be studied as well as provide the fundamental knowledge needed to help devise strategies to manipulate polymerizations kinetics for microstructure control.
Catalytic polymerization is a key process in making synthetic polymers. In chain-growth polymerization, a chain grows from a catalyst continually to reach thousands of subunits. However, the real-time dynamics of chain growth was unknown until recently.
Combining magnetic tweezers, optical microscopy, and spectroscopic techniques, the research team was able to track a living, ring-opening polymerization in real-time and discovered, very surprisingly, that individual polymer chains do not grow steadily from the catalyst, but rather undergoes consecutive wait-and-jump steps.
With the help of molecular dynamics computer simulations, the researchers were able to attribute these wait-and-jump dynamics to conformational entanglements formed by newly incorporated monomers. More specifically, during the "wait" period, these polymer tangles, termed hairballs, form around the catalysts as new monomer units are added. The hairballs then randomly unravel, resulting in a "jump" in polymer molecular weight, and a new hairball then starts to form.
The researchers also found that the configurations of these entanglements play a key role in determining the polymerization rates and the dispersion among individual polymers, opening new opportunities to manipulate polymer conformation during synthesis to alter their dispersions.
Thus, this first-of-its-kind demonstration has the potential to transform how the kinetics and mechanism of polymerization catalysis can be studied as well as provide the fundamental knowledge needed to devise strategies to manipulate polymerizations kinetics for microstructural control to enable polymers with designer properties.
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This work was recently published in Science (Liu et al Science 2017, 358, 352-355; DOI: 10.1126/science.aan6837) and received a "Research of the Year" distinction from Chemical Engineering News as one of the most notable chemical research advances of the 2017.
The U.S. Army Research Laboratory is part of the U.S. Army Research, Development and Engineering Command, which has the mission to provide innovative research, development and engineering to produce capabilities that provide decisive overmatch to the Army against the complexities of the current and future operating environments in support of the joint warfighter and the nation. RDECOM is a major subordinate command of the U.S. Army Materiel Command.