Investigation of molecular and lamellar aggregation structure of crystalline polymer thin films by synchrotron GIXD and GISAXS measurements

In this stuay, molecular and meso-scale structures sucn as molecular/Tamellar orientation ana lamellar stacking distance were investigated for HDPE thin films by synchrotron 2D-grazing-incidence wide-angle X-ray diffraction/small-angle X-ray scattering (SR-2D-GIXD/GISAXS) simultaneous measurements. A sample used in this study was additive free high-density polyethylene (HDPE). HDPE films with a thickness of ca. 400 nm were prepared onto Si wafers by a dip-coating method. The obtained films on the wafers were isothermally crystallized at a crystallization temperature of 373K from the melt. Afterward, some of the obtained films were treated at an annealing temperature (Ta) of 378K, 383K or 388K for 24 hours or annealed at each Ta in a stepwise heating from ca. 300K. In-situ and ex-situ measurements of 2D-GIXD/GISAXS for melt-crystallized and annealed films were carried out with imaging-plates at the BL40B2 beamline of SPring-8. The wavelength of incident X-rays was 0.15 nm and camera length was ca. 110 mm for GIXD and ca. 2180 mm for GISAXS. On the basis of 2D-GIXD patterns for HDPE thin films during a stepwise annealing, it was implied that the a axis of orthorhombic unit cell relatively oriented in the out-of-plane direction. Strong scatterings near Yoneda peak in the in-plane direction of 2D-GISAXSpatterns were detected for the films. The peak position of the scattering corresponds to the long period, the average distance between stacked crystalline lamellae. By annealing the film, the peak position of the scattering shifted to the lower q_y range in the in-plane direction. These results indicated that crystalline lamellae were stacked in the parallel direction to the film surface and the long period became longer from ca. 25 nm to ca. 35 nm by annealing. In a lamella, chains were regularly packed and the chain axis (the c axis) was relatively oriented parallel to the film surface.