ABSTRACTWith particular focus on the characterization of polymer–nanoparticle interaction, the effects of polymer molecular weight (Mn) on confined crystallization of the interfacial region in poly(ethylene oxide) (PEO)/silica (SiO2) composites were investigated, employing differential scanning calorimetry, dynamic light scattering, and proton nuclear magnetic resonance relaxation techniques. Dependence of crystallization on Mn in PEO/SiO2 composites is found to be closely related to the microstructure changes in adsorbed polymer. PEO adsorbs in a manner that produces a tightly bound layer (amorphous) with constant thickness, independent of Mn, and creates a loosely bound layer with flattened structure for short PEO chains but coiled conformation for long chains. The contribution of this Mn‐dependent layer to crystallization grows with Mn, showing a transition from non‐crystalline phase to the formation of a crystallization peak (−25 to −33 °C), and then to crystallization together with non‐adsorbed PEO. The comprehensive description of the interfacial region should be a helpful tool in the preparation and modification of polymer nanocomposites. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1608–1616