The physical properties of ultrahigh molecular weight polyethylene (UHMWPE) are generally highly dependent on its molecular weight. However, in our study, it was found that two UHMWPE samples of similar molecular weight, SLL‐5 and GUR 4150, have significantly different impact strengths, with the Charpy impact strength of GUR 4150 being almost 3.4 times that of SLL‐5. To reveal the reasons, the structure–property relations of these UHMWPE materials were investigated. Morphologies of the nascent particles and impact fracture surfaces, the melting behavior, rheological behavior, and three‐phase (crystalline, amorphous, and interphase) contents were characterized by scanning electron microscopy, differential scanning calorimetry, advanced rotary rheometer, and Raman spectroscopy, respectively. It was observed that no significant differences in the crystal structures of SLL‐5 and GUR 4150, but GUR 4150 had smaller nascent particles sizes and a lower degree of entanglement when compared with those of SLL‐5. Accordingly, a mechanism to clarify the significant difference in the impact strengths of GUR 4150 and SLL‐5 was developed. More importantly, this work may be useful for improving the preparation technologies and industrial applications of UHMWPE. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 632–641

The crystallization behaviors and mechanical properties of both linear isotactic polypropylene (iPP) and long chain branching polypropylene (LCBPP) were investigated with differential scanning calorimetry, polarized optical microscopy, and wide-angle X-ray diffraction. The results indicated that the long chain branching (LCB) structure could act as nucleating agent accelerating the nucleation but impeding the crystal growth of LCBPP, and eventually increased the crystallization peak temperature and decreased the spherulite size of LCBPP. Moreover, with the presence of LCB structure, b-form polypropylene was generated in LCBPPs under injection molding conditions, and the content of b-form increased with the increasing of LCB structure content. Benefiting from the combined effect of the LCB structure and b-form crystal, the LCBPPs revealed not only outstanding toughness but also desired stiffness. The impact strength, flexural modulus and tensile strength of HMSPP3 were increased by, respectively, 260.0, 54.7, and 17.2 % than those of linear iPP. These results are helpful to develop new types of high performance polypropylene.

Although it is generally believed that the extraordinarily high molecular weight plays a great role in the excellent wear resistance of ultrahigh-molecular-weight polyethylene (UHMWPE), the mechanism behind this effect remains poorly understood. In this study, we investigated the wear behavior of UHMWPE with respect to its microstructures, measured in terms of its crystallinity, lamellar thickness and crystallite size, entanglement, interphase fractions, and surface roughness. From these structure− property relationships, we conclude that the high wear resistance of UHMWPE can be attributed to its higher degree of entanglement and its high fraction content of interphase domains; the other parameters were either not significant or not relevant. Accordingly, we propose a mechanism in which the more greatly entangled molecular chain networks of UHMWPE protect the surface and subsurface layers from damage under stresses during sliding wear.

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