Osaka, Japan – Your shock-absorbing sneaker soles are likely made of polyurethane, a highly elastic and tough polymer. The ability of these elastomers to absorb impact without breaking is extremely important for practical applications, and while multiple strategies exist for enhancing elastomer toughness, each has its limitations. However, achieving synergistic toughening by integrating all three mechanisms within a single material remains challenging.
Now, researchers at the University of Osaka have overcome these limitations by developing a multipath synergistic strategy to toughen elastomers. This exciting discovery will be reported in Nature Communications.
Elastomers are polymers that are exceptionally elastic, i.e., they can deform strongly under external stress and revert to their original shape when the stress is removed. However, traditional elastomers are not very tough, as microscopic cracks can cause the material to tear.
Consequently, strategies are employed to enhance the toughness of elastomers by dissipating energy. That is, during deformation, the polymer absorbs mechanical energy and dissipates it by converting it into other forms of energy.
To reduce the likelihood of tears, three types of energy dissipation strategies can be employed.
- Molecular sliding – Rotaxane molecules are incorporated into the elastomer, which slide and rotate under an external force, redistributing stress across the network and preventing breakage.
- Force-induced bond scission – Molecules are embedded in elastomers with "sacrificial" bonds that break under an applied stress, delaying damage to the elastomer.
- Chain entanglement – Molecular design is used to introduce structurally well-defined chain entanglements, which allow chains to slide and rearrange tension across the network when stress occurs.
Individual energy-dissipation strategies provide only a limited improvement in elastomer toughness. Although multiple mechanisms have been incorporated into a single material, achieving synergistic toughening by activating them sequentially as the applied stress increases remains challenging.
"We integrated three energy dissipation pathways that become activated in sequence under increasing stress to prevent failure of the elastomer," explains lead author Xue Li. "Thus, we synergistically combined three toughening mechanisms."
In this study, the authors introduced ring molecules with sacrificial bonds into an elastomer. Under applied stress, ring sliding occurs in the elastomer first to absorb force. As the stress increases, the rings cleave to form linear chains. Under even higher stress, the linear chains entangle with other chains, maintaining network connectivity and dissipating energy via chain slippage.
This novel strategy can be used to create materials that are both soft and durable, with uses such as tires, gloves, and adhesives. The superior toughness of these materials translates into improved service life and reliability.
