Toughening elastomers using mussel-inspired iron-catechol complexes

Abstract

Materials often exhibit a trade-off between stiffness and extensibility; for example, strengthening elastomers by increasing their cross-link density leads to embrittlement and decreased toughness. Inspired by cuticles of marine mussel byssi, we circumvent this inherent trade-off by incorporating sacrificial, reversible iron-catechol cross-links into a dry, loosely cross-linked epoxy network. The iron-containing network exhibits two to three orders of magnitude increases in stiffness, tensile strength, and tensile toughness compared to its iron-free precursor while gaining recoverable hysteretic energy dissipation and maintaining its original extensibility. Compared to previous realizations of this chemistry in hydrogels, the dry nature of the network enables larger property enhancement owing to the cooperative effects of both the increased cross-link density given by the reversible iron-catecholate complexes and the chain-restricting ionomeric nanodomains that they form.

Bioinspired Functional Gradients for Toughness Augmentation in Synthetic Polymer Systems

“…Inspiration can be taken from nature, wherein several materials have this ability . The byssus of marine mussels, for example, is used as a holdfast that anchors to rocky surfaces in the hostile intertidal zone . The byssal thread ( Figure A,B) is comprised of a soft, collagenous fiber‐bundle core and a hard, 5–10 µm‐thin protective cuticle.…”

“…The byssal thread ( Figure A,B) is comprised of a soft, collagenous fiber‐bundle core and a hard, 5–10 µm‐thin protective cuticle. With this combination of stiffness ( E ≈ 800 MPa) and extensibility (ε > 100%), the thread allows for significant degree of tensile movement, preventing from being torn off under the high loads from waves and wind. The previous works have presented that this protective coating strategy can be endowed by (but not limited to) granular microstructure of the cuticle and catechol‐iron coordination chemistry .…”

“…With this combination of stiffness ( E ≈ 800 MPa) and extensibility (ε > 100%), the thread allows for significant degree of tensile movement, preventing from being torn off under the high loads from waves and wind. The previous works have presented that this protective coating strategy can be endowed by (but not limited to) granular microstructure of the cuticle and catechol‐iron coordination chemistry . Inspired by this natural protective coating strategy, we here desire to mimic the mitigation of core‐cuticle delamination to quantify its possible viability to increase overall material toughness through a simple gradient layering technique in a most common synthetic polymer (acrylate polymer).…”

“…Dual physically cross‐linked (DPC) hydrogels of acrylic polymers with primary cross‐links of laponite nanoparticles and secondary cross‐links of Fe 3+ (ferric) ion coordination complexes were also introduced, which significantly improved both elastic modulus and extensibility of DPC hydrogels over non‐DPC . However, these methods involve multi‐step processes of network penetration, wherein a soft network is swollen in aqueous solution, followed by cross‐linking with another polymer network . These approaches, therefore, are limited to hydrogels or polymers that swell in water such as polyethylene glycol (PEG)‐based networks .…”

“…However, these methods involve multi‐step processes of network penetration, wherein a soft network is swollen in aqueous solution, followed by cross‐linking with another polymer network . These approaches, therefore, are limited to hydrogels or polymers that swell in water such as polyethylene glycol (PEG)‐based networks . Additionally, high localization of stress in similar polymer matrices often causes failures in areas of localization and asymmetric cross‐linkage, endowing high initial rigidity but resulting in premature fracture at only fractions of maximum strain .…”