Mechanical Performance and Visual Fracture Warning Function of Mechanochromic Stimuli-Recovery Polymer Networks

Abstract

Stimuli-recovery polymer networks with enhanced mechanical performance were designed and synthesized through UV-curing photo-polymerization. Thanks to a mechanochromic cross-linker difluorenylsuccinonitrile-containing dimethacrylate (DFMA), poly(stearyl methacrylate-co-N,N-dimethyl acrylamide) (P(SMA−DMAA)) networks visualized the stress, showing improvements in toughness and higher energy dissipation. The molar ratios determined the transition temperatures, crystal structures, and mechanical performance of the polymer networks. A more efficient and scientific analysis based on achromatic gray-scale colorspace was first proposed to evaluate the mechano-responsive color quantitatively. Uniform evaluation criteria were expected to be established based on the method. The stress distribution and dissociation of dynamic covalent bonds were recorded precisely and expressed clearly on gray-scale color maps, providing a clear warning for when materials were threatening to break. Mechanochromic P(SMA−DMAA) networks showed a prolonged recovery at room temperature. While under heat stimulation, they presented excellent recovery ability with 90% strength and 95% energy. Additionally, the mechano-responsive color changes repeated, showing a similar changing trend to that in the first cycle. The mechanochromic stimuli-recovery P(SMA−DMAA) networks had enhanced mechanical performance and a reliable visual fracture warning function.

Mechanochromic elastomers with different thermo- and mechano-responsive radical-type mechanophores

“…Since the radicals generated from DFSN are pink and relatively stable in a network structure, they have been used to evaluate energy dissipation properties under stress via not only the change in coloration, but also the quantitative evaluation of the radicals using electron paramagnetic resonance (EPR) measurements. [20][21][22][23][24] Moreover, it can be expected that the generated radicals are able to spontaneously recombine under an ambient atmosphere and reform the original dimer in an appropriate matrix with high mobility, thus leading to the ability to repeatedly dissipate stress. [25][26][27][28] Although there are several examples of cross-linked polymers with dynamic covalent bonds (DCBs) at the cross-linking points, attention has hitherto been focused on their selfhealing and reprocessing properties, and there are only a few examples in which their sacrificial bonding properties have been evaluated.…”

“…Since the radicals generated from DFSN are pink and relatively stable in a network structure, they have been used to evaluate energy dissipation properties under stress via not only the change in coloration, but also the quantitative evaluation of the radicals using electron paramagnetic resonance (EPR) measurements. 20–24 Moreover, it can be expected that the generated radicals are able to spontaneously recombine under an ambient atmosphere and reform the original dimer in an appropriate matrix with high mobility, thus leading to the ability to repeatedly dissipate stress. 25–28…”

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, 48–50 C–C (in tetraarylsuccinonitrile), 25,28 , 51–56 C–O, 50 57 58 S–S, 59 60 61 62 (Figure 2a ), and coordinative bonds such as Ru–carbene, Ag–carbene, or Cu–carbene bonds, 16 63 Fe–Cp (in ferrocene), 64 65 66 Pt–acetylide, 67 Fe–pyridine, 68 Cu–naphthalene, 69 Pd–phosphine, 70 and Pd–carbene (Figure 2b ). 71 Covalent bonds in strained ring structures have lower BDEs than the analogous linear structures and can respond to mechanical forces.…”

Determining Entanglement Molar Mass of Glassy Polyphenylenes Using Mechanochromic Molecular Springs

“…In terms of force sensing, a further drawback of two-state MCPs is their limited ability to differentiate between forces of different magnitude. To some extent, this aspect can be addressed by multicolor systems − or the calibration of signal intensity, enabling stress mapping. , Mechanochromism based on simple conformational changes is known but has received much less attention. − …”

“…To some extent, this aspect can be addressed by multicolor systems 25−27 or the calibration of signal intensity, enabling stress mapping. 28,29 Mechanochromism based on simple conformational changes is known but has received much less attention. 30−35 Previously, we designed an ortho-functionalized diphenyldiketopyrrolopyrrole (oDPP) derivative acting as a mechanochromic donor−acceptor (DA) torsional spring and incorporated it into the kinked polyphenylene poly(meta,meta,paraphenylene) (PmmpP) with outstanding mechanical toughness.…”