Tough and strong bioinspired nanocomposites with interfacial cross-links

Strength and toughness are two mechanical properties that are generally mutually exclusive but highly sought-after in the design of advanced composite materials. There has only been limited progress in achieving both high strength and toughness in composite materials. However, the fundamental underlying mechanics remain largely unexplored, especially at the nanoscale. Inspired by the lamellar structure of nacre, here a layered graphene and polyethylene nanocomposite with tunable interfacial cross-links is studied via coarse-grained molecular dynamics simulations in order to achieve both high strength and toughness. Our simulations indicate that, as the cross-link density increases from 0 to about 25%, strength and toughness of the nanocomposite experience a surprising 91% and 76% increase respectively. This strengthening mechanism can be well explained by the extent of increased nonbonded contacts between polymer chains (van der Waals interaction) during the stretch and exceptional stretchability of each polymer chain (dihedral interaction) due to interfacial cross-links by comparing nanocomposites with and without cross-links. As the strength of cross-links increases, both mechanical strength and toughness of graphene-based polymer nanocomposite increase as expected. This may be attributed to the intra-chain bond and angle interactions among polymer chains, which may be negligible for nanocomposites with weak cross-links but play a key role in enhancing both strength and toughness for nanocomposites with strong cross-links. Overall, our findings unveil the fundamental mechanism at the nanoscale for tough-and-strong polymer composites via interfacial cross-linking as well as offer a novel way to design bioinspired nanocomposites with targeted properties via tunable interfacial cross-linking.