Molecular underpinnings of the mechanical reinforcement in polymer nanocomposites

Equilibrium molecular dynamics simulations on amorphous polymers filled with solid nanoparticles show that mechanical reinforcement results from the formation of a long-lived transient polymer-particle network only over a narrow range of parameter space. In these cases it is necessary that (i) the interfacial zone occupy significantly less volume than the bulk region and (ii) particle-polymer interactions must be strong enough that the relaxation time for the small fraction of adsorbed monomers is much longer than that characterizing the neat polymer. In all other cases, reinforcement will appear to be particle driven since there is no clear demarcation between the adsorbed segments and the bulk polymer. However, the apparent size of the particle will be larger because of the adsorbed segments. Both reinforcement mechanisms occur for systems that do not easily equilibrate, leading us to stress the importance of starting states and processing history which is reminiscent of glassy systems.