Accelerated aging-induced variation of polypropylene (PP) structure studied by two-dimensional (2D) small-angle X-ray scattering (SAXS) correlation spectroscopy

Submolecular-level variation of polypropylene (PP) caused by aging treatment is studied by two-dimensional (2D) small-angle X-ray scattering (SAXS) in conjunction with 2D correlation spectroscopy. PP samples were prepared by a T-die extrusion method and then probed by 2D SAXS. Direction-depending SAXS profile data were obtained by extrusion the SAXS intensities along the direction from 90 degrees (i.e. direction of extrusion) to 120 degrees on the 2D SAXS image. 2D correlation spectra derived from the SAXS profiles of the PP samples revealed the detailed orientations of the lamellae in the polymer system. For example, the development of asynchronous correlation peaks indicates that significant portion of the lamellae in the initial PP sample aligns to the direction of the extrusion. These lamellae tend to go through subsequent crystallization to increase the thickness during the cooling process, while the polymer chain inclined to the direction of the extrusion does not undergo such sufficient level of crystallization. It was also revealed that the aging treatment based on the heating well above glass transition temperature of the PP (ca. 0 degrees C) essentially provides additional crystalline growth especially to the inclined lamellae. (C) 2020 Elsevier B.V. All rights reserved.