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.
