MIT Researchers Develop Simple Technique to Produce Stronger Polymers

Researchers have found a new approach for reducing the number of loops (red) in a polymer. The method could offer an easy way for manufacturers of industrially useful materials such as plastics or gels to strengthen their materials. (Courtesy of the researchers)

Plastic, rubber, and a number of other useful materials are composed of polymers — long chains set in a cross-linked network. At the molecular level, these polymer networks have structural defects that weaken them.

Many years ago, MIT researchers were the first to measure specific types of these flaws, known as “loops,” which are caused when a chain in the polymer network binds to itself rather than to another chain. At present, the same researchers have discovered a simple way to decrease the number of loops in a polymer network and thus reinforce materials produced from polymers.

To accomplish this, the researchers just add one of the components of the polymer network very gradually to a large quantity of the second component. Using this method, they were able to reduce the number of loops in half, in many types of polymer network structures. This could provide an easy means for manufacturers of industrially useful materials such as gels or plastics to strengthen their materials.

Just by changing how fast you add one component to the other, you can improve the mechanical properties.

Jeremiah A. Johnson, Associate Professor of Chemistry, MIT

MIT graduate student Yuwei Gu is the paper’s first author. The paper appears in the Proceedings of the National Academy of Science the week of April 24.

Other authors are MIT associate professor of chemical engineering Bradley Olsen; MIT graduate student Ken Kawamoto; former MIT postdocs Mingjiang Zhong and Mao Chen; Case Western Reserve University Assistant Professor Michael Hore; Case Western Reserve graduate student Alex Jordan; and former MIT visiting professor and Case Western Reserve Associate Professor LaShanda Korley.

Controlling loops

In 2012, Johnson’s group came up with the first approach to measure the number of loops in a polymer network and confirmed those results with theoretical predictions from Olsen. The researchers discovered that the loops can make up approximately 9% to about 100% of the network, based on the concentration of polymer chains in the initial material and other factors.

A few years later, Johnson and Olsen formulated a way to calculate how much these loops deteriorate a material. In their newest work, they focused on reducing loop formation, and to attain this without altering the composition of the materials.

The goal we set for ourselves was to take the same set of precursors for a material that one would normally use, and, using the exact same precursors under the same conditions and at the same concentration, make a material with fewer loops.

Jeremiah A. Johnson, Associate Professor of Chemistry, MIT

In this research paper, the MIT researchers first focused on a type of polymer structure called as a star polymer network. This material has two varied building blocks: a star with four identical arms, referred as “B4,” and a chain referred as “A2.” Each molecule of A2 links to the end of one of the B4 arms. However, during the representative synthesis process, when everything is combined together at once, some of the A2 chains end up attaching to two of the B4 arms, developing a loop.

The research team noticed that if they incorporated B4 very gradually to a solution of A2, each of the B4 arms would swiftly react with a single molecule of A2, so there was less chance for A2 to develop loops.

After a few hours of gradually incorporating half of the B4 solution, they incorporated the second half all at once, and the star-shaped subunits combined together to develop a cross-linked network. This material, the researchers observed, had about half as many loops as the same material manufactured using the traditional synthesis method.

Based on how many loops were in the original material, this “slow then fast” approach can enhance the material's strength by as much as 600 %, Johnson says.

This very simple ingenious and powerful approach, based on slow crosslinker addition, diminishes the intramolecular cyclization and significantly increases mechanical properties of polymeric networks.

Krzysztof Matyjaszewski, Professor of Chemistry, Carnegie Mellon University

Better products

The MIT team also tried this method with four other types of polymer network synthesis reactions. They could not measure the number of loops for all of those types of polymers, but they did discover similar developments in the strength of the materials.

This method could potentially help to boost the strength of any material made from a gel or other cross-linked polymer, including membranes for water purification, adhesives made of epoxy, plastics, or hydrogels such as contact lenses.

Johnson’s lab is currently looking at how to apply this strategy to various materials, including gels used to grow cells for tissue engineering.

The National Science Foundation funded this research.

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