Dispersion and functionalization as interfacial drivers in next-generation rubber composites.

Elastomers are indispensable across applications ranging from tires to soft robotics, prized for their elasticity, resilience, and durability. Despite this central role, progress in elastomer composites has been guided largely by empirical compounding, with insufficient emphasis on the interfacial energetics that govern filler wetting, dispersion, and matrix adhesion. This Review reframes the filler-matrix interface through the concept of surface free energy (SFE), a thermodynamic construct encompassing two key components: London dispersive forces and specific acid-base interactions. We highlight how strategic modulation of these components, through nanostructuring, chemical functionalization, silane coupling, plasma or ozone treatments, and electrochemical modification, enables controlled tailoring of interfacial energetics. This framework provides an integrative rationale for the long-standing success of conventional fillers (e.g., carbon black, silica, clay) and extends naturally to advanced nanofillers (e.g., CNTs, graphene, MXenes), where engineered surface energetics serve as important interfacial drivers of composite performance by regulating filler-matrix compatibility, while operating in concert with filler morphology, dispersion state, network formation, and curing behavior. By consolidating these insights, this Review presents an SFE-centered, thermodynamics-informed interfacial perspective that organizes existing principles of surface energetics, filler functionalization, dispersion, and interphase formation into a coherent design logic for elastomer composites. This perspective supports a transition from empirical compounding toward more rational and design-oriented strategies for improving composite reliability and multifunctional performance.