Recently, the use of high density polyethylenes (HDPE) with different molecular weight distributions to produce large dimension water pipes with stable mechanical properties, that can replace aging metallic pipes for water transportation, has gained increasing scientific and engineering interests.  In this study, the isothermal crystallization behavior of bimodal and unimodal HDPE samples with broad molecular weight (MW) distributions under shear were studied <i>in situ</i> by synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. The application of shear led to elevated initial crystallization temperatures for both HDPE samples, and accelerated the crystallization rates. However, the final crystallinity values of the sheared samples were found to be lower than those of un-sheared samples. The bimodal sample was less shear sensitive, and its low MW portion resulted in higher crystallinity; whereas the branched structure in unimodal sample appeared to result in a lower crystallization rate. Lamellar structure was formed in both samples with or without shear. Under shear, the lamellar structure was formed earlier, and long period of the dominant lamellar structure decreases drastically and that of the minor lamellar structure increases slightly, during the crystallization process in both samples. The dominant long period of the bimodal resin is larger than that of the unimodal resin.  A molecular model was proposed to correlate the structure and property in these two samples.

Shear induced crystallization of bimodal and unimodal high density polyethylene

253rd National Meeting (2017)

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