Energy Dissipation and Mechanoresponsive Color Evaluation of a Poly(n-hexyl Methacrylate) Soft Material Enhanced by a Mechanochromic Cross-Linker with Dynamic Covalent Bonds

Abstract

A rigid and brittle cross-linking structure was introduced into the flexible poly­(n-hexyl methacrylate) (PHMA) network by the mechanochromic cross-linker difluorenylsuccinonitrile-containing methacrylate (DFMA), whose central C–C bond acted as a dynamic covalent bond and could generate pink radicals when fractured. PHMA with DFMA showed a remarkable hysteresis loop and mechanical enhancement. After deformation, the reassociation of dynamic covalent bonds and the reorganization of the network structure slowed down the recovery of the polymer to its initial state. The correlations between tension stimulation, energy dissipation, and mechanoresponsive color change were discussed. Under stress, the polymer changed from light gray to pink. The broad distribution of red channel intensity under large deformation detected on the surface confirmed that the rupture of dynamic covalent bonds occurred evenly throughout the polymer and suppressed stress concentration. The color showed a strong dependence on stress, which started to appear at around 1.5 to 2.0 MPa. The incorporation of DFMA promised mechanical enhancement and noncontact stress detection ability of the PHMA soft material.

Mechanochromophore‐Linked Polymeric Materials with Visible Color Changes

“…DFSN with bis‐methylacryloyl functional groups has been cross‐linked into poly( n ‐hexyl methacrylate) (PHMA) via free radical polymerization. [ 154 ]…”

“…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 ]…”

“…DFSN with bis-methylacryloyl functional groups has been cross-linked into poly(n-hexyl methacrylate) (PHMA) via free radical polymerization. [154] Four steps were required to synthesize HABI-diol (Scheme 5D). [78] HABI-diol was readily incorporated into PU backbone, whereas the synthesis of HABI-PMA required the oxidative coupling step to be carried out after TPI was polymerized with MA, due to an incompatibility of the dynamic equilibrium of HABI and TPI radicals under the conditions of living radical polymerization.…”

“…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] The transmission and distribution of applied force differ for both linear and cross-linked polymers. For example, linear DABBF-PU dissolved in 1,4-dioxane did not show any color change when frozen in liquid nitrogen, whereas cross-linked DABBF-PU swollen with 1,4-dioxane switched to blue during freezing.…”

Heterocyclic Mechanophores in Polymer Mechanochemistry

“…The weak bonds are defined as chemical bonds with a bond-dissociation energy (BDE) of less than 72 kcal mol -1 ; they are generally found in acyclic structures. These include covalent bonds such as diazo, 23,47 C-S, [48][49][50] C-C (in tetraarylsuccinonitrile), 25,28,[51][52][53][54][55][56] C-O, 50,57,58 S-S, [59][60][61][62] (Figure 2a), and coordinative bonds such as Ru-carbene, Ag-carbene, or Cucarbene bonds, 16,63 Fe-Cp (in ferrocene), [64][65][66] Pt-acetylide, 67 Fe-pyridine, 68 Cu-naphthalene, 69 Pd-phosphine, 70 and Pdcarbene (Figure 2b). 71 Covalent bonds in strained ring structures have lower BDEs than the analogous linear structures and can respond to mechanical forces.…”