Mechanochromic Break Points Control the Toughness of Entangled Polyphenylenes

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.