Facile mechanochemical cycloreversion of polymer cross-linkers enhances tear resistance

Abstract

The mechanical properties of covalent polymer networks often arise from the permanent end-linking or cross-linking of polymer strands, and molecular linkers that break more easily would likely produce materials that require less energy to tear. We report that cyclobutane-based mechanophore cross-linkers that break through force-triggered cycloreversion lead to networks that are up to nine times as tough as conventional analogs. The response is attributed to a combination of long, strong primary polymer strands and cross-linker scission forces that are approximately fivefold smaller than control cross-linkers at the same timescales. The enhanced toughness comes without the hysteresis associated with noncovalent cross-linking, and it is observed in two different acrylate elastomers, in fatigue as well as constant displacement rate tension, and in a gel as well as elastomers.

Bioinspired Strong and Tough Poly(ε-caprolactone)/Graphene Nanodot Composite Films via Weak Hydrogen Bonds: Implications for Thermal–Mechanical Properties

“…The centered-GNDs nanoconfinement can block the motion and failure of the PCL molecular chains based on the multiple H-bond effects, thus resulting in the high strength and modulus . In addition, the expansion of the large-deformation PCL chains can generate high malleability while maintaining the original stiffness . Nevertheless, microcracks may be created in this step.…”

Naphthopyran molecular switches and their emergent mechanochemical reactivity

“…5 Recent research has also unveiled remarkable capabilities for tailoring the macroscopic mechanical properties of materials by controlling the molecular level force sensitivity of individual mechanophores within a polymer network. 6,7 On the other hand, mechanical force can be leveraged as an external stimulus to effect desired chemical reactions with spatial and temporal precision. 8–10 In an ever-growing collection of more than 100 different mechanophores developed to date, 11 judicious structural design has enabled a diverse range of force-responsive functions including catalyst activation, 12,13 conductivity switching, 14 mechanically triggered chemiluminescence, 15 and cargo release, 16–18 among many others.…”

Sacrificial Mechanical Bond is as Effective as a Sacrificial Covalent Bond in Increasing Cross-Linked Polymer Toughness

“…16,17 Sacrificial bonds whose dissociation either fractures a polymer backbone or releases hidden length have been used successfully to increase polymer toughness. 2,3,18,19 A mechanical bond, that keeps the ring of a rotaxane threaded through its axle and maintains the two macrocycles of a catenane interlocked, is considered a distinct type of chemical interaction. 20−22 Depending on the relative sizes of its wheel and the stopper, the mechanical bond of a rotaxane can be broken without affecting any of its covalent bonds.…”

“…Here, we report experimental and computational data, suggesting that a sacrificial mechanical bond is as effective at increasing the tensile toughness of an elastomer as a wellestablished sacrificial covalent bond. We synthesized and characterized the mechanical properties of 6 poly(methyl acrylate)s shown in Figure 2 whose cross-links contained only sacrificial mechanical bonds in the form of a dethreadable rotaxane, P [2]Me; only sacrificial covalent bonds of mechanochromic difluorenylsuccinonitrile (DFSN), 44 P[3] t Bu and P DFSN ; both types of sacrificial bonds, P [3]Me; or neither (P [2] t Bu, and P Alk ). Across this series, the mechanical sacrificial bond yielded material (P[2]Me) with the highest fracture energy, whereas both types of sacrificial bonds were comparably effective in increasing the material's fracture strain and stress, compared to analogs lacking sacrificial bonds.…”

“…Across this series, the mechanical sacrificial bond yielded material (P[2]Me) with the highest fracture energy, whereas both types of sacrificial bonds were comparably effective in increasing the material's fracture strain and stress, compared to analogs lacking sacrificial bonds. A comparison of materials containing both DFSN and a rotaxane (P [3]Me and P[3] t Bu) demonstrates that competition between failure of sacrificial mechanical and covalent bonds can be controlled predictably to favor either one or the other as the primary dissipative mechanism. Finally, our data suggest that even short cross-linker sliding distances of <3 nm measurably improve material failure properties.…”