Shortly after processing, Polyethylene/Polypropylene (PE/PP) multilayer films demonstrate an increase in tensile modulus and other mechanical properties when the individual layer thickness is below 0.5 mu m. Subsequent annealing at 60 degrees C for 16 h brings the properties of all other samples to similar values. WAXD characterization of the layered films identified a prevalence of mesophase in the thicker PP layers. In samples with increased layer numerosity or subjected to annealing, WAXD detected its conversion to crystalline phase that correlates with improved mechanical properties. SSNMR and DSC detailed the defective nature of iPP crystallites. Comonomers, detected by NMR in the commercial polymers used for the films, are the source of tunable disorder that dictates the formation of the PP mesophase and the low temperature of conversion to the mechanically stronger defective phase. Soft intrafilm layer interfaces instead enable nucleation and localized polymer chain rearrangement even without annealing. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 520-531