Low-Strain Reinforcement as a Volume-Expansion Effect in Filled Elastomers with Glassy Interphases

Filling elastomers with nanoparticles such as carbon black can increase modulus by orders of magnitude, enabling technologies from actuators to tires. Despite their century-long importance, a mechanistic model of reinforcement has remained elusive: why does adding non-cohesive filler produce such a dramatic modulus increase? A prevailing hypothesis holds that attractive filler surfaces immobilize nearby polymer, forming "glassy bridges" that cement particles into a load-bearing network. Using molecular dynamics simulations of a model filled elastomer, we find that glassy interphases do not sufficiently raise the extensional modulus. Instead, they chiefly amplify the same reinforcement mechanism observed in composites that lack such interphases. Reinforcement arises from competition between coexisting filler and elastomer networks for control of the material's volume. By driving expansion, the filler network activates contributions from the elastomer's bulk modulus-1000× larger than its Young's modulus-thereby substantially increasing resistance to deformation. Our results also suggest a practical experimental diagnostic for detecting glassy bridging. Taken together with prior work, these findings reframe and unify the picture of low-strain reinforcement in filled elastomers, opening routes to manufacturing ultra-tough elastomeric nanocomposites.