Molecular-dynamics simulations of mechanical reinforcement in filled elastomers

Np zzT coarse-grained molecular-dynamics computer simulations have been performed of a bead-rod polymer film confined between two crystalline substrates; the latter mimick the surfaces of the filler particles in a composite material. Different film thicknesses and different polymer-substrate interaction strengths have been simulated. The layer-resolved analysis of the polymer segmental orientational dynamics shows a tremendous slowing down of the mobility upon approach of the polymer-substrate interface, while the middle-layer monomer dynamics exhibits rather fast bulk-like behavior. Upon decreasing the film thickness the mobility gradients become overlapping. The polymer films were further deformed uniaxially perpendicular to the film, and their elastic moduli were extracted. A reinforcement was found compared to the bulk elasticity, increasing with decreasing film thickness (i.e., with an increase of the filler volume fraction) and with increasing strength of the adsorption to the substrates. We connect this effect with the slowing down of the segmental mobility in the proximity of the fillers.