Visualization and Quantitative Evaluation of Toughening Polymer Networks by a Sacrificial Dynamic Cross-Linker with Mechanochromic Properties

Abstract

A difluorenylsuccinonitrile-(DFSN)-based linker, whose central C−C bond is readily cleaved under mechanical stress to generate a relatively stable pink radical species, was introduced into polymer networks. DFSN-based cross-linked polymers exhibit improved mechanical properties as compared to those of the corresponding covalently cross-linked polymers owing to the energy dissipation induced by cleavage of the central DFSN bond. The toughening mechanism of DFSN-based elastomers is qualitatively visualized by the intensity of the pink color and can be quantitatively characterized by electron paramagnetic resonance. These results demonstrate that the extent of DFSN cleavage is the main factor improving the mechanical properties of the polymer networks.

Segmented Polyurethane Elastomers with Mechanochromic and Self‐Strengthening Functions

“…In the EPR spectrum of a compressed SPU‐1 film, several radicals were detected (Figure 4 a). We assumed that the obtained EPR spectrum was composed of CF radicals (one broad line) [17, 18] and radicals derived from methacrylate (9 + 5 lines), [34–36] and a simulation and fitting of the spectrum were conducted based on this assumption (Table S2, Figure S15) [37] . The resulting simulated spectrum matched well with the experimental data, thus supporting the notion that these radicals are present.…”

Mechanochromophore‐Linked Polymeric Materials with Visible Color Changes

“…DFSN cross‐linked PHMA exhibited a significantly higher fracture energy than the PHMA cross‐linked with hexamethylene dimethacrylate (HDMA), as the dissociation of DFSN molecules was able to dissipate the energy concentrated at the cross‐linked sites. [ 154,177 ]…”

Segmented Polyurethane Elastomers with Mechanochromic and Self‐Strengthening Functions

“…In the EPR spectrum of a compressed SPU-1 film, several radicals were detected (Figure 4 a). We assumed that the obtained EPR spectrum was composed of CF radicals (one broad line) [17,18] and radicals derived from methacrylate (9 + 5 lines), [34][35][36] and a simulation and fitting of the spectrum were conducted based on this assumption (Table S2, Figure S15). [37] The resulting simulated spectrum matched well with the experimental data, thus supporting the notion that these radicals are present.…”

“…To develop bulk-state self-strengthening polymers, we focused on the mechano-reactive molecule (mechanophore) [9][10][11][12][13][16][17][18][19][20][21][22][23][24][25][26] difluorenylsuccinonitrile (DFSN), [17,18] which exhibits relatively high stability but dissociates into pink cyanofluorene (CF) radicals in response to mechanical stimuli (Figure 1 b). The CF radical exhibits sufficient tolerance toward oxygen to be used in air without any special treatment.…”