Structural and dynamic origins of Payne effect reduction by steric stabilization in silica reinforced poly(butadiene) nanocomposites

Silica nanoparticles in polybutadiene nanocomposites cause significantly hindered polymer dynamics in the form of a glassy layer, which can extend more than 10 nm from the nanoparticle surface. We present experiments for a well-defined series of these nanocomposites to contrast the contributions of free chain ends versus hindered dynamics on the non-linear rheological effects associated with energy loss in transport. Quasi-elastic neutron scattering (QENS) experiments using isotopic labeling and interfacially-active polymers enabled quantification of the glassy layer's thickness, which we relate to the strain softening (Payne effect), energy dissipation and the previously established nanostructure of the materials. We explored the thickness of the glassy layer from near the bulk glass transition temperature up to 363 K, in the presence or absence of nanoparticles and interfacially active polymers. Although the glassy layer becomes thinner with increasing temperature, it remains significant (> 3 nm) even when the sample is more than 180 K above the bulk glass transition temperature. This layer can account for the substantial levels of reinforcement, up to a factor of 20, and energy dissipation observed in silica filled rubbers beyond values expected from particle volume fraction arguments alone. Since the interfacially-active polymers simultaneously inhibit nanoparticle aggregation and introduce polymer chain ends to the interparticle region, these experiments provide a direct test of whether energy loss in reinforced nanocomposites is attributable to “glassy bridges” between particles or chain ends. We show that while chain ends may be significant at low strain, below 1%, the percolation of the glassy layer of hindered polymer dynamics between silica particles dominates the Payne effect at high strain (~10% or more) and that this contribution can be dramatically reduced by sufficient steric stabilization to the nanoparticles.