Linear-Viscoelastic Energy Dissipation At the Filler-Matrix Interface in Silica-Rubber Composites

Understanding the reinforcement of rubber by particulate fillers has posed a challenge to academics and industrialists for many years. Whilst the stiffening of rubbers by rigid particulates is reasonably well understood in terms of hydrodynamics and filler network dynamics, the understanding of the modification of rubber viscoelastic behaviour upon introduction of fillers remains incomplete.

Given the hydrodynamic assumption that there is no slippage of polymer at the filler-matrix interface, and that the dynamics of the matrix are homogeneous; at strains small enough so as not to induce Fletcher-Gent (Payne) non-linearities, such as filler structure breakdown, the viscoelastic response (tanδ) of filled rubbers should be equal to that of the unfilled rubber. We find this holds for rubbers filled with glass beads.

However, examining the linear-viscoelastic properties of model precipitated silica-filled rubbers - developed to control experimental variables - at suitably small strains using custom built equipment we find this is not the case. A significant dissipation of energy (G’’) can be quantified at the filler-matrix interface.

Interpretation of this dissipation as a physical mechanism can be based on concepts of polymer nano-confinement or polymer slippage at the interface – both invalidate the basic hydrodynamic assumptions.

Silanes are used to improve adhesion to the silica surface resulting in a reduction in interfacial energy dissipation – contrary to current understanding of T_g shifts in nano-confined polymers.

As such, interfacial slippage is identified as the most likely cause of energy dissipation in these composites. From creep test data within the linear-viscoelastic region over a range of temperatures, retardation spectra are calculated giving an estimate of slippage activation energies.