Abstract
The structure, morphology, and mechanical properties of two compression-molded grades of ultrahigh-molecular-weight polyethylene (UHMWPE) and, for comparison, one conventional linear polyethylene (HDPE) were studied. Compression molding resulted in some preferred orientation of lamellae in the compression direction in UHMWPE samples, while no preferred orientation in HDPE. The mean crystal thickness estimated from the size distribution agrees better with those obtained from small-angle X-ray scattering (SAXS) and mechanical yield data than the thickness determined from the melting peak temperature. Microscopic examination of microtomed and etched UHMWPE samples showed that the lamellae are in the form of platelets with the width and length in the range of 300–700 nm. The lamellae radiate from primary nuclei forming small embryonal spherulites; their radial growth ends at 0.3–0.7 μm from the center. There is no evidence of branching and secondary nucleation from those primary lamellae. Because the lamellae are radially ordered, there is no parallel stacking of lamellae. Samples were subjected to deformation by plane-strain compression at a constant true strain rate. In axial UHMWPE samples, where lamellae were preferentially oriented along the loading direction, the second yield was clearly observed. The second yield was found to be related to the deformation instability leading to kinking of lamellae oriented initially along the loading direction. Kinking was clearly shown by SAXS and microscopic observation of microtomed and etched samples. No cooperativity of kinking was observed because the lamellae are arranged in small spherulites and not parallel in stacks. The stress–strain curves were fitted with model curves assuming crystal plasticity and network elasticity in the amorphous component. The effective density of the molecular network within the amorphous phase depended on the molecular weight of UHMWPE.
Highlights
Ultrahigh-molecular-weight polyethylene (UHMWPE) is a well-known polyethylene grade possessing excellent mechanical properties including a high toughness, high abrasion, and wear resistances as well as excellent friction characteristics compared to any other polymer materials
The melting peaks for both UHMWPEs are at slightly lower temperature than the peak of HDPE 06 (Tm ∼ 136.0 °C). These data can indicate that HDPE is characterized by thicker crystals that melt at higher temperature than the crystals in both UHMWPEs
Melting of HDPE and melting of UHMWPEs are significantly different, which result in different thickness distributions of lamellar crystals: HDPE exhibits a significant fraction of crystals thicker than 23 nm
Summary
Ultrahigh-molecular-weight polyethylene (UHMWPE) is a well-known polyethylene grade possessing excellent mechanical properties including a high toughness, high abrasion, and wear resistances as well as excellent friction characteristics compared to any other polymer materials. The evolution of the crystalline texture observed upon deformation is a continuous process resulting from active crystallographic slip systems, interlamellar shear, and associated crystal rotations.[16,17,32] The yield point indicates the transition from elastic to plastic behavior and it is the onset of cooperative activity of the crystallographic slip mechanism.[33] The yield stress is directly related to the critical resolved shear stress of the easiest slip system active in the material. In addition to the shape of the true stress−true strain curve, the yielding and ultimate behavior of UHMWPE are of considerable theoretical and practical importance They are relevant to the theoretical development of micromechanical wear and fracture models as well as to the development of finite element models to predict accurately the multiaxial stress state, including increased equivalent von Mises stresses, in the cup of the hip or knee joint replacement (see, for example, refs 1 and 40). Maintaining the constant true strain rate during the test and determination of the true stress−true strain curve are relatively easy in the plane-strain compression.[39,41,42]
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