Mechanophores with a Reversible Radical System and Freezing‐Induced Mechanochemistry in Polymer Solutions and Gels

Abstract

Visualization and quantitative evaluation of covalent bond scission in polymeric materials are highly important for understanding failure, fatigue, and deterioration mechanisms and improving the lifetime, durability, toughness, and reliability of the materials. The diarylbibenzofuranone-based mechanophore radical system enabled, through electron paramagnetic resonance spectroscopy, in situ quantitative evaluation of scission of the mechanophores and estimation of mechanical energy induced along polymer chains by external forces. The coagulation of polymer solutions by freezing probably generated force but did not cleave the mechanophores. On the other hand, cross-linking led to efficient propagation of the force of more than 80 kJ mol(-1) to some mechanophores, resulting their cleavage and generation of colored stable radicals. This mechanoprobe concept has the potential to elucidate other debated issues in the polymer field as well.

Mechanochromic Polymers That Turn Green Upon the Dissociation of Diarylbibenzothiophenonyl: The Missing Piece toward Rainbow Mechanochromism

“…The ultimate advantage of radical‐type mechanochromic polymers is not only the qualitative visualization of the mechanical stress, but also its quantitative evaluation by electron paramagnetic resonance (EPR) spectroscopy, both in solution and the solid state. We have already reported two radical‐type mechanochromic polymers that bear diarylbibenzofuranone (DABBF) or tetraarylsuccinonitrile (TASN) moieties (Figure a and b) . Homolytic cleavage of the central carbon‐carbon bond in the DABBF and TASN units in response to mechanical stress affords arylbenzofuranone (blue) and diarylacetonitrile radicals (pink), respectively.…”

Stress-induced colouration and crosslinking of polymeric materials by mechanochemical formation of triphenylimidazolyl radicals

“…36 For this purpose PU-1 as well as polyurethane containing solution-blended HABI were first swollen in DMF and then immersed into liquid N 2 . Despite the fast radical recombination of TPI radicals to form HABI in systems with limited translational diffusion, we could achieve stress-sensing with a weightfraction of only 4% 1 which is a considerable improvement compared to the radical systems reported in the literature 15,16 (a movie of the fading coloration of compressed PU-1 in mpg format is available in the ESI †). 2a and c it becomes clear that the reference polyurethane with HABI not covalently attached to the polymer backbone does not change its colour upon freezing and thus does not cleave the HABI-moiety through the application of mechanical stress.…”

“…[1][2][3][4][5] Commonly, this is achieved by incorporating mechanically activatable molecular moieties (mechanophores) into polymer architectures. The former was recently demonstrated by Otsuka and co-workers relying on the reversibly induced cleavage of the diarylbibenzofuranone moiety [15][16][17] while the latter is reported exploiting unspecific, ultrasound-induced polymer scission to unstabilized radicals in solution yielding end-capped, chainextended or grafted polymers. Amongst the motifs for stress-report, functional moieties changing their optical properties upon the application of stress are highly desirable as they allow the visualization of areas under heavy loading.…”

Dynamic covalent bond from first principles: Diarylbibenzofuranone structural, electronic, and oxidation studies

“…Imato et al achieved a unique set of conditions. Using as cross‐linker diarylbibenzofuranone (DABBF), the dimer of arylbenzofuranone (ABF, analog to widely used commercial product HP‐136), the polymer gel synthesized was able to undergo self‐healing under mild conditions at room temperature, due to the dynamic covalent bond in its structure.…”