Control of Dynamic Composites through Filler Surface Chemistry

The addition of hard fillers to polymeric networks allows for enhancement of mechanical properties, generally at the expense of extensibility. In the case of filled elastomers (such as tires), the hard particles cause damage to the underlying network when strained, resulting in severe mechanical hysteresis in cyclic loading experiments (the Mullins effect). As such, dynamic networks, which are able to heal damage through exchange reactions, are a promising candidate for composite matrices. This work investigates the influence of tunable dynamic bonds at the surface of silica particles in the presence of a fixed, complementary dynamic network matrix. The surface chemistry, composed of benzalcyanoacetamide Michael acceptors, undergoes room temperature, catalyst-free dynamic exchange with thiols with equilibrium constants (K_eq) that can be manipulated by the electronic nature of the acceptor. Increasing the K_eq of the particle surface relative to the dynamic matrix was found to promote the overall reinforcement of the composites, while also influencing the phase separation behavior of the matrix. Critically, tensile experiments reveal that ambient dynamic exchange allows for the recovery of network damage as a function of waiting time between loading cycles.