Oligo-cyclic loading-induced evolution of stress distribution and apparent amorphous modulus in lamellar stacks of high-density polyethylene

Abstract

Assessing the resistance of high-density polyethylene (HDPE) against earthquake-like loads involves understanding the changes in structure and properties induced by oligo-cyclic loading at various length scales. To study the evolution of stress distribution and intrinsic properties within lamellar stacks from pristine to oligo-cyclic loading pre-conditioned materials, simultaneous in-situ SAXS/WAXS measurements were performed. During the elastic deformation of each pristine and preconditioned sample, crystal strain was tracked using the in-situ WAXS technique. Based on the established elastic tensor of the crystalline structure in polyethylene, we calculated the microscopic stress values within crystalline lamellae. In the pristine sample, lamellar stacks exhibit closely series-like coupling in the equatorial region and parallel-like coupling in the polar region of the spherulite. In the pre-conditioned sample, stress is primarily concentrated in the intra-fibrillar region, where the crystalline and amorphous phases are series-coupled, and strong strain concentration occurs in the inter-fibrillar region. By combining the local strain in the amorphous layer within the lamellar stacks in the equatorial region of the spherulite and the intra-fibrillar region with series-coupled lamellar stacks, measured by in-situ SAXS tests, the apparent amorphous modulus at the lamellar stack scale can be determined. This modulus changes from 71 to 106 MPa in the equatorial region of pristine spherulites to a notable 2000–7000 MPa in the intra-fibrillar region under the influence of oligo-cyclic pre-loading. Importantly, this apparent modulus is affected by both crystallization conditions and molecular structure, with molecular parameters exerting the primary influence.