On the origin of heating-induced softening and enthalpic reinforcement in elastomeric nanocomposites

TL;DR

This paper uncovers that heating-induced softening and reinforcement in elastomeric nanocomposites arise from a volume-competition mechanism between elastomer and nanoparticle networks, challenging the glassy bridge hypothesis.

Contribution

It introduces a novel volume-competition theory explaining reinforcement and softening in elastomer nanocomposites, supported by molecular simulations and predictive modeling.

Findings

Simulations show enthalpic softening results from volume competition.

The theory predicts the inversion of the modulus-temperature relationship.

Reinforcement is driven by a volume-competition mechanism in co-continuous networks.

Abstract

Despite a century of use, the mechanism of nanoparticle-driven mechanical reinforcement of elastomers is unresolved. A major hypothesis attributes it to glassy interparticle bridges, supported by an observed inversion of the variation of the modulus E(T) on heating -- from entropic stiffening in elastomers to enthalpic softening in nanocomposites. Here, molecular simulations reveal that elastomer enthalpic softening can instead emerge from a competition over preferred nonequilibrium volumes between elastomer and nanoparticulate networks. A theory for this competition accounting for softening of the bulk modulus on heating predicts the simulated E(T) inversion, suggesting that reinforcement is driven by a volume-competition mechanism unique to co-continuous systems of soft and rigid networks.