Heterogeneous lamellar structures near the polymer/substrate interface

We report the structural heterogeneity of recrystallized linear low-density polyethylene (LLDPE) films (25, 50, and 100 nm in thickness) in the direction normal to the surface, based on in situ grazing incidence small-angle X-ray scattering (GISAXS) and X-ray diffraction (GID) measurements. The GID results have clarified the presence of the edge-on lamellae at the surfaces and in the interior of the LLDPE films prepared on Si substrates as thin as 25 nm in thickness. However, the degree of the crystallinity for the 25 nm thick film was almost half of those for the 50 and 100 nm thick films, while the melting temperature (T _m) for all the films remained unchanged relative to the bulk (T _m = 117 °C). Moreover, the GISAXS results for the 25 nm thick film indicate the structural heterogeneity in the direction normal to the surface: (i) At the polymer/air interface, the presence of the disordered edge-on lamellae which lack well-defined long periods even at T ≪ T _m; (ii) At the polymer/substrate interface, the persistence of a substrate-bound edge-on lamellar layer even at T ≫ T _m; (iii) In between the two interfacial layers, the existence of the well-ordered edge-on lamellae with the long periods. These heterogeneous structures can be explained as a consequence of the nucleation initiated at the topmost surface of the substrate-bound lamellar layer.