With particular focus on the characterization of polymer-nanoparticle interaction, the effects of polymer molecular weight (M-n) 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 M-n 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 M-n, and creates a loosely bound layer with flattened structure for short PEO chains but coiled conformation for long chains. The contribution of this M-n-dependent layer to crystallization grows with M-n, showing a transition from non-crystalline phase to the formation of a crystallization peak (-25 to -33 degrees 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. (C) 2017 Wiley Periodicals, Inc.