We show that a bound layer composed of short and long chains can be exploited to regulate the elastic moduli of bulk polymer nanocomposites at same particle loadings and dispersion states. The bound layer thickness on particles with high coverage of long chains is reduced with oscillatory deformation in a model attractive nanocomposite system. Reversibility of the bound layer is, thus, possible for the short chains in the interphase. Compositional dynamic heterogeneity in the interphase has subsequent effects on the fragility of composites. With increasing amount of adsorbed long chains, the fragility index systematically improves from moderate to high values. Our results suggest that the interphase layer between adsorbed chains and free matrix directly governs the reinforcement in poly(methyl methacrylate)–silica nanocomposites and can be dynamically altered under large shear.