Mechanisms of Particulate Reinforcement of Elastomers at Small Strains

Understanding the reinforcement of rubber by particulate materials is a perennial issue within the associated academic and industrial communities. In this experimental study we examine the reinforcement of model elastomers filled with various carbon blacks and precipitated silicas under the simplest mechanical conditions: within the small strain linear viscoelastic region and with minimum strain history. By removing the complexity of filler structure breakdown effects a clear picture of the phenomenology of filler reinforcement at small strain becomes apparent. We compare these experimental results to a series of elastomers reinforced with micrometer scale glass beads; where the reinforcement is attributed predominantly to hydrodynamic effects. The observed deviations highlight several potential non-hydrodynamic contributions to the phenomenology of elastomer reinforcement by commercial filler particles. We examine the nature of the polymer dynamics in the filled elastomers using scanning calorimetry, broadband dielectric spectroscopy and dynamic mechanical analysis. No significant deviation from unfilled behaviour is observable meaning that the observed reinforcement cannot be attributed the presence of a layer or gradient of polymer interface with retarded dynamics near to the filler surface. We then examine the non-hydrodynamic temperature dependence of the small strain elastic and viscous moduli. By considering particle filled elastomers both above and below the filler networking volume fraction threshold it is apparent that a temperature dependence of the interface between filler and polymer must play a role in defining the observed bulk mechanical properties. We explore the possible physics of this mechanism in terms of an interfacial polymer slippage phenomenon. Finally we consider why, for filled elastomers containing a percolated network of particulates, such a dramatic stiffening effect is observed immediately after the glass transition prior to the onset of the observed temperature dependence. Flocculation and filler networking are considered in the context of the temperature dependence of the Payne Effect.