Taking a shine to polymers: Fluorescent molecule betrays the breakdown
of polymer materials

Date:
November 24, 2020
Source:
Tokyo Institute of Technology
Summary:
Scientists have come up with a simple method to evaluate the
strength and performance of polymer materials. They hope that their
work will enable scientists and engineers to better evaluate the
polymers they work with, and eventually synthesize better ones.



FULL STORY ========================================================================== Nylon, rubber, silicone, Teflon, PVC -- these are all examples of
human-made polymers -- long chains of repeated molecular units that we
call monomers.

While polymers also exist in nature (think wool, silk, or even hair),
the invention of synthetic polymers, the most famous of which is plastic, revolutionized the industry. Light, stretchy, flexible, yet strong and resistant, synthetic polymers are one of the most versatile materials on
the planet, used in everything from clothing to building, packaging and
energy production. Since the very beginning of this new era in material engineering, understanding the influence of external forces on polymers' strength and stability has been crucial to evaluate their performance.


==========================================================================
When subjected to mechanical stress, the weak bonds that keep some
polymer chains together are overcome, and one inevitably breaks. When this happens, a free radical (a molecule with an unpaired electron, which is naturally unstable and very reactive, called a "mechanoradical" in this
case) is generated. By estimating the amount of free mechanoradicals
produced, we can infer the resistance of a material to the amount of
stress. While this phenomenon is well documented, scientists struggled
to observe it under ambient temperature in bulk state, because
mechanoradicals produced for polymers in bulk are not stable due to
their high reactivity toward oxygen and other agents.

Researchers from Tokyo Institute of Technology led by Professor Hideyuki
Otsuka decided to take up the challenge. In their study published in
Angewandte Chemie International Edition, they used a small molecule called diarylacetonitrile (H- DAAN) to capture the rogue free radicals. "Our
theory was that H-DAAN would emit a distinctive fluorescent light
when it reacts with the free radicals, which we could then measure to
estimate the extent of polymer breakdown," explains Prof Otsuka. "The
theory is simple; the higher the force exerted on the polymer, the more mechanoradicals are produced, and the more they react with H-DAAN. This
higher reaction rate results in more intense fluorescent light, changes
in which can easily be measured." The researchers now wanted to see how
this would work in practice. When polystyrene (in the presence of H-DAAN)
was subjected to mechanical stress via grinding, the H-DAAN acted as a
radical scavenger for polymeric mechanoradicals, and bound with them
to produce "DAAN* ," which has fluorescent properties. This caused a
visible yellow fluorescence to appear.

"More important, probably, is the clear correlation that we found
between fluorescence intensity and the amount of DAAN radicals generated
by the ground- up polystyrene, as we had predicted," reports Prof
Otsuka. "This means that it is possible to estimate the amount of DAAN
radicals generated in the bulk system just by measuring the fluorescence intensity." The implications of their findings are wide-ranging: by being
able to visually quantify how materials respond to different external
stimuli, they can test how suitable polymers are for various uses,
depending on the mechanical stress they will be expected to undergo. This method could prove to be an invaluable tool for scientists and engineers
as they strive to improve material performance and specificity.

This exciting research this shine light on the responses of polymers
to mechanical stress and illuminate the way forward in the research of
polymer mechanoradicals!

========================================================================== Story Source: Materials provided by Tokyo_Institute_of_Technology. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Hideyuki Otsuka, Takumi Yamamoto, Sota Kato, Daisuke Aoki.

Diarylacetnitrile as a Molecular Probe for the Detection
of Polymeric Mechanoradicals in the Bulk State via a Radical
Chain‐transfer Mechanism. Angewandte Chemie International
Edition, 2020; DOI: 10.1002/ anie.202013180 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/11/201124131134.htm

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