Revealing the detailed structure in flow-induced crystallization of semicrystalline polymers

We systematically investigate the detailed structure in the flow-induced crystallization of a lightly cross-linked high-density polyethylene as a model semicrystalline polymer sample by combining the small-angle X-ray scattering (SAXS) and the spherical harmonic expansion (SHE) method. The SHE divides the two-dimensional SAXS pattern into several components according to the deformation geometry, and allows extraction of the most relevant information. Employing the first two anisotropic components in the expansion and a comprehensive model, we determine the crystalline morphological parameters, such as the long period, the lamellar diameter and thickness, and their polydispersities. In particular, we find that the lamellar diameter exhibits bimodal distributions at high strains. Lamellae with similar diameters tend to gather rather than to randomly distribute with others, suggesting the existence of heterogeneity in the semicrystalline structure. Moreover, we observe the strong polydispersities of the lamellar structure at low strains. The structural heterogeneity and polydispersities could be related to the inhomogeneities in crystal growth and nucleation processes.