In situ investigation of annealing effect on lamellar stacking structure of polyethylene thin films by synchrotron grazing-incidence small-angle and wide-angle X-ray scattering

We have investigated lamellar stacking structure of melt-crystallized and annealed high-density polyethylene (HDPE) thin films, with a thickness of ca 400 nm prepared on silicon wafers, using synchrotron grazing-incidence small-angle and wide-angle X-ray scattering (GISWAXS) measurements at the BL40B2 beamline in SPring-8. In-situ measurements of GISWAXS were carried out for the films in a stepwise annealing process under vacuum. Scattering peaks relating to the long period, the average distance between stacked crystalline lamellae, were measured only in the in-plane direction near the Yoneda peak of the grazing-incidence small-angle X-ray scattering patterns. On the other hand, the orthorhombic (110) and (200) reflections of oriented HDPE crystals were measured in the out-of-plane direction of the grazing-incidence wide-angle X-ray scattering patterns. It was revealed that crystalline lamellae were stacked in a parallel direction to the film surface and the long period increased from ca 25 nm to ca 32 nm in a stepwise annealing process. Within a lamella, molecular chains were found to be packed regularly and the chain axis (the c axis) was relatively oriented parallel to the film surface.