Surprise! Weaker bonds can make polymers stronger

Chemists significantly improved a polymer network's tear resistance by including weak linkers.

A group of scientists from MIT and Duke University have found a paradoxical method to strengthen polymers by adding a few weaker links to the substance.

The researchers discovered that by just utilizing a weaker form of crosslinker to attach parts of the polymer building blocks, they could boost the materials' resistance to ripping by up to tenfold when working with a class of polymer known as polyacrylate elastomers.

In addition to being often utilized in automobile components, these rubber-like polymers are frequently employed as the "ink" for 3D-printed products. Currently, the researchers are looking into the possibility of applying this methodology to different kinds of materials, such rubber tires.

"If you could make a rubber tire 10 times more resistant to tearing, that could have a dramatic impact on the lifetime of the tire and the amount of microplastic waste that breaks off," says Jeremiah Johnson, an MIT chemistry professor and one of the study's senior authors. The research was published in the journal Science today.

The fact that this method doesn't seem to change any of the polymers' other physical characteristics is a huge benefit.

Polymer engineers are skilled in strengthening materials, but doing so always requires altering a desirable aspect of the material. According to senior author and Duke University chemistry professor Stephen Craig, "the toughness enhancement here comes without any other significant change in physical properties, at least that we can measure. It is also achieved by replacing only a small portion of the total material."

Johnson, Craig, and Michael Rubinstein, a professor at Duke University and a senior author on the study, have worked together on this topic for a while. Shu Wang, an MIT postdoc who received his PhD from Duke, is the paper's primary author.

the gaping hole

Polymer networks comprised of acrylate strands kept together by connecting molecules are known as polyacrylate elastomers. These components can be put together in many ways to produce materials with various qualities.

A star polymer network is one design that these polymers frequently adopt. Two different types of building blocks are used to create these polymers: one is a star with four identical arms, and the other is a chain that serves as a linker. These linkers connect to the tips of each star's arm to form a network like a volleyball net.

Craig, Rubinstein, and MIT Professor Bradley Olsen collaborated on a research in 2021 to gauge the durability of these polymers. As predicted, they discovered that the material weakened when weaker end-linkers were utilized to bind the polymer strands together. When compared to the linkers that are often employed to unite these building blocks, those weaker linkers, which include cyclic molecules known as cyclobutane, may be broken with a lot less power.

Following up on that research, the scientists made the decision to look into a different kind of polymer network where the polymer strands are randomly cross-linked to one another rather of being united at the ends.

This time, the researchers discovered that the material became significantly more tear-resistant when weaker linkers were utilized to connect the acrylate building blocks together.

The reason for this, according to the researchers, is that the weaker links are randomly dispersed throughout the material as junctions between otherwise strong strands rather than being a part of the final strands themselves. Any fractures that form when this material is stretched past its breaking point attempt to bypass the stronger connections and instead pass through the weaker ones. As a result, more ties must be broken by the fracture than would be necessary if all of the bonds had the same strength.

Despite the fact that such links are weaker, Johnson claims that more of them must be broken since the fracture travels via the weakest bonds first, which results in a longer route.

enduring materials

Using this method, the researchers demonstrated that weaker linkers in polyacrylates created with stronger crosslinking molecules were nine to ten times more difficult to shred than weaker linkers in polyacrylates manufactured with other weaker linkers. Even though the weak crosslinkers made up just around 2 percent of the material's total composition, this result was nevertheless obtained.

Additionally, the researchers demonstrated that the material's other features, such as its resistance to degrading under heat, were unaffected by the composition change.

According to Johnson, it is highly uncommon for two materials to have the same structure and characteristics at the network level yet have tearing that differs by practically an order of magnitude.

The possibility of using this strategy to increase the hardness of other materials, such as rubber, is now being explored by the researchers.

"There's a lot to explore here about what level of enhancement can be gained in other types of materials and how to best take advantage of it," adds Craig.

The Center for the Chemistry of Molecularly Optimized Networks, which receives funding from the National Science Foundation, is where the group's work on polymer strength is housed. The goal of this center, under Craig's direction, is to investigate how the molecular characteristics of polymer networks impact their physical behavior.