Abstract
Deformation instabilities, such as microbuckling or lamellar fragmentation due to slip localization, play a very important role in the deformation of semicrystalline polymers, although it still not well explored. Such instabilities often appear necessary to modify the deformation path and facilitate strain accommodation in an energy-minimizing manner. In this work, microbuckling instability was investigated using partially oriented, injection-molded (IM) samples of high-density polyethylene, deformed by a plane-strain compression. Deformed samples were probed by SEM, X-ray (small- and wide-angle X-ray scattering: SAXS, WAXS), and differential scanning calorimetry (DSC). It was found that microbuckling instability, followed quickly by the formation of lamellar kinks, occurred in high-density polyethylene (HDPE) at a true strain of about e = 0.3–0.4, mainly in those lamellar stacks which were initially oriented parallel to the compression direction. This phenomenon was observed with scanning electron microscopy, especially in the oriented skin layers of IM specimens, where a chevron morphology resulting from lamellae microbuckling/kinking was evidenced. Macroscopically, this instability manifested as the so-called “second macroscopic yield” in the form of a hump in the true stress–true strain curve. Microbuckling instability can have a profound effect on the subsequent stages of the deformation process, as well as the resulting structure. This is particularly important in deforming well-oriented lamellar structures—e.g., in drawing pre-oriented films of a semicrystalline polymer, a process commonly used in many technologies.
Highlights
Semicrystalline polymers demonstrate a high ability to deform plastically, which appears to be one of their most essential properties, and has been extensively studied
Wide-angle X-ray scattering (WAXS), and small-angle X-ray scattering (SAXS) results, we found microbuckling instability occurring in PE samples with various structures around the true strain of ecrit = 0.2–0.4 and manifesting macroscopically as a low and wide hump observed in the true stress–true strain curve beyond the yield point, which can be considered as the second yield point
We reported in our previous papers [10,11,15] a hump in the form of a very low and broad maximum related to the second yield in unoriented samples of several grades of polyethylene, which was observed around e ≈ 0.3–0.4
Summary
Semicrystalline polymers demonstrate a high ability to deform plastically, which appears to be one of their most essential properties, and has been extensively studied (see reviews—e.g., [1,2,3,4]). The deformability of semicrystalline polymers can largely be attributed to their unique morphology, which at a basic level consists of thin alternating crystalline and amorphous layers, tightly linked together by chains crossing the interface. At temperatures above the glass transition, the rubber-like amorphous layers appear very soft and compliant compared to the crystalline layers; both components must deform cooperatively to satisfy the material integrity condition [4]. The deformation of a semicrystalline polymer turns out to be a complex process, involving various mechanisms active at different stages. The non-crystallographic mechanism—strain-induced “melting-recrystallization” transition—has been suggested by some
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