Rigid Particle-Induced Anomalous Softening in Elastomer Composites

Abstract

Particle-filled soft composites generally stiffen as particle loading increases, imposing a trade-off between functionality and compliance. Here, we show that this trade-off can be overcome by designing particles to actively direct gelation rather than act as passive fillers. In a sol-rich elastomer matrix, sol-phase kinetics redirect network formation towards particle-mediated assembly, yielding anomalous softening despite increasing particle content. This response is governed by topological reconfiguration of the load-bearing network during gelation rather than by filler loading alone. A structure-based model captures the composition-dependent onset of this behaviour, and the same response is reproduced across distinct particle systems. The resulting materials combine functionality with softness, energy dissipation, and recoverability under large deformation. These findings establish topology-guided gelation as a general strategy for engineering compliant functional composites for soft robotic actuation, conformal interfaces, and other systems that require both deformability and embedded material functionality.