  Nanoparticles can greatly enhance the mechanical response of elastomeric
polymers essential to a wide range of applications, yet their precise molecular
mechanisms of high-strain reinforcement remain largely unresolved. Here we
show, based on molecular dynamics simulations, that high-strain reinforcement
emerges from an interplay between granular nanoparticulate compressive behavior
in the normal direction and polymer incompressibility. This feedback loop,
which is initiated by a mismatch in the Poisson ratios of nanofiller and
polymer, invokes a contribution from the polymer's bulk modulus to the
elongational stress, while the tendency of the polymer to contract in the
normal direction maintains a near-jammed filler state. This effect persists
even once the direct filler elongational contribution becomes dissipative after
the 'Payne effect' yield. These results indicate that direct particle-particle
contact effects, even in the absence of potential augmenting mechanisms such as
glassy polymer bridges, can drive the mechanical reinforcement effects typical
of experimental systems.
