Failsafe Interstitial Design Enables Decoupled Control of Stiffness and Toughness

Many structural materials suffer from a trade-off between stiffness and toughness because the same molecular interactions that form their rigid frameworks also restrict the plastic deformation needed to resist fracture. Here we demonstrate a failsafe design principle in which flexible polymers are embedded within the interstitial spaces of a rigid molecular framework. The framework maintains high stiffness under elastic loads, while the hidden polymers remain dormant until a local framework failure occurs. At that point, they activate as a "stretch margin", accommodating crack-tip plasticity without compromising the structural integrity of the framework. The length of this stretch margin is governed by interstitial polymer content: a lower ring loading leaves longer hidden chains that can be drawn out further, enlarging the plastic zone and enhancing toughness. At the same time, Young's modulus can be independently tuned through chemical modification of the framework, which alters the strength of intermolecular interactions that hold the rigid structure together. By treating interstitial spaces as reservoirs for a hidden, flexible phase, this approach establishes a molecular design strategy that decouples stiffness and toughness─two properties long believed to be inseparably linked. This concept highlights interstitial space engineering as a route to organic materials that combine rigidity with resistance to fracture, expanding the design space for high-performance structural polymers.