While it is known that the properties of polymer nanocomposites are largely dominated by the interfacial layer around nanoparticles, the molecular parameters controlling the interfacial layer structure and dynamics remain unknown. In this work we combine small-angle X-ray scattering, differential scanning calorimetry, and broadband dielectric spectroscopy to analyze the dependence of the interfacial layer thickness, l(int), on polymer rigidity defined through the characteristic ratio, C-infinity. This analysis revealed a value of C-infinity similar to 5-7, beyond which l(int) increases substantially with C-infinity. Moreover, l(int), grows upon approaching the glass transition temperature from above, and the rate of this growth seems to correlate with polymer fragility. Most important, our analysis revealed that lint is comparable to the characteristic length scale of dynamic heterogeneities in the studied materials. These results provide new understandings of molecular parameters controlling the interfacial layer and are important not only for the field of polymer nanocomposites but also for the fields of thin polymer films and dynamics of soft matter in general.