Heterogeneous Lamellar Structures Near the Polymer/Substrate Interface

Abstract

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.

Blocked crystallization in capped ultrathin polymer films studied by molecular simulations

“…Figure illustrates the crystalline morphology for two films with different interfacial interactions ( B / E c = 0 and −2.8, respectively) at 200 000 MC cycles. Edge‐on crystals with stems in parallel with the film surface can be observed in the film with an interfacial interaction of B / E c = 0, which is consistent with the experimental findings . Using molecular dynamics simulations, Luo et al also observed the crystalline stems aligned parallel to substrates .…”

“…Besides, Asada et al . demonstrated the occurrence of heterogeneous nucleation initiated from the polymer–substrate interface . Figure depicts the changes in the number of crystalline bonds in the interfacial regions (the Z = 2 and Z = 5 layers) during crystallization for systems with different interfacial interactions.…”

“…On the other hand, excessive interfacial interaction can also bring some negative effects on polymer crystallization. First, the activation energy for diffusion is increased due to the reduction of interfacial segmental mobility, resulting in decline of the crystallization rate . Besides, Napolitano and colleagues further proposed that chain adsorption by the substrates can increase the entropic barrier required for the formation of crystalline structures, reducing the polymer crystallization rate .…”

“…Asada et al . also observed the presence of heterogeneous edge‐on lamellae in linear low‐density polyethylene films . In addition, the polymer crystallization rate fell significantly due to strong polymer–substrate interaction, or sometimes polymer crystallization was inhibited in long experimental time scales …”

“…For semicrystalline polymers, the nanoscale confinement can cause dramatic changes of polymer crystallization behaviors. Variations of film thickness or interfacial interaction can lead to the appearance of different crystalline morphologies. For instance, Mellbring et al .…”