# Mechanochromic Break Points Control the Toughness of Entangled Polyphenylenes

## TL;DR

This paper shows how adding specific chemical units to a polymer can control its toughness and mechanical failure behavior.

## Contribution

The study introduces mechanochromic break points to predictably tune the mechanical failure of entangled polyphenylenes.

## Key findings

- Incorporating DFSN motifs into PmmpP significantly reduces strain at break in a predictable manner.
- In situ UV–vis and EPR spectroscopy reveal homolytic bond scission and radical formation during mechanical stress.
- The reversibility of bond scission suggests potential for self-healing materials with controlled failure.

## Abstract

Toughness engineering of a kinked polyphenylene (PmmpP) is demonstrated by using mechanochromic molecular
break points.
Varying amounts of thermally stable yet mechanically labile difluorenylsuccinonitrile
(DFSN) motifs incorporated into PmmpP allow to largely
tune mechanical failure of the specimen. While strain at break values
of pristine PmmpP reach up to 300%, an increasing
concentration of DFSN break points leads to a strongly decreasing
and predictable strain at break. Homolytic bond scission of DFSN and
formation of colored DFSN radicals is characterized by in
situ UV–vis spectroscopy, which allows us to discern
regions of necking and strain hardening during tensile testing. The
formation and lifetime of radicals is further probed by EPR spectroscopy,
suggesting reversibility of bond scission and thus the possibility
to design tough materials with predicted failure and self-healing
properties.