Hierarchical morphology and interfacial dynamics in silane-free rubber nanocomposites: a SAXS-guided approach toward sustainable high-performance tire materials.

Developing sustainable, high-performance elastomers for tire applications has become a growing priority for the chemical industry, driven by environmental mandates and the functional demands of modern transportation. In response, additive engineering is increasingly employed to replace conventional silane coupling agents (SCAs), which raise environmental concerns and constrain optimization of the rolling resistance (RR)-wet grip (WG) trade-off. A central challenge in this domain lies in elucidating how interfacial modifiers reconfigure filler architecture and influence macroscopic properties. In this study, we introduce a novel small angle X-ray scattering (SAXS)-guided analytical framework that integrates a mass-fractal model with a gel-like network model to resolve the hierarchical three-tiered structure of poly(ethylene glycol) (PEG)-modified, silica-filled tire compounds. This hybrid model enables the quantitative extraction of cluster radius and-critically-the contribution of occluded rubber domains, a morphological feature often suggested visually but seldom structurally characterized. In contrast to a widely used SCA, which enhances filler dispersion via covalent silica-rubber linkages, PEG induces greater filler aggregation and occluded rubber formation through hydrogen bonding, while simultaneously promoting interfacial slippage under dynamic strain. These coexisting mesoscale features-quantified via SAXS and directly linked to dynamic mechanical properties-result in a 40% reduction in RR, a 14% enhancement in WG, and 81% higher stiffness relative to the SCA-modified system. This mechanistic breakthrough diverges from conventional dispersion-centric frameworks and establishes PEG as a viable SCA-free alternative. More broadly, this work demonstrates a transferable, structure-informed strategy for the design of next-generation high-performance, environmentally friendly rubber nanocomposites.