Shear Induced Crystallization Of Bimodal And Unimodal High Density Polyethylene

High-density polyethylene(HDPE)pipes are light,strong,extremely tough and very durable,and therefore have widespread applications in water pipeline to replace aging pipeline.To produce high quality HDPE pipeline,it is necessary to understand the crystallization behavior of bimodal and unimodal HDPE under shear field.Shish-kebab structure is considered as the fundamental form in crystalline polymers induced by flow fields.Previous studies aforementioned provide valuable knowledge about shear-induced crystallization behavior of crystalline polymers.However,detailed investigations about how molecular weight distributions affect the crystallization behavior are rare.In this paper,synchrotron radiation X-ray diffraction and scattering techniques were applied to intensively study crystallinity,lamellae orientation,long period and orientation of HDPE.The effect of polymer molecular structure on the condensed structure of HDPE were analyzed.This study may be useful for the selection of a suitable polyethylene resin and the preparation of large diameter water-conveying plastic pipes with stable mechanical properties.The crystallization behavior of unimodal HDPE with different molecular weight distributions(MWD)was studied in this thesis.It was shown that HDPE with a wide MWD has a higher orientation under the shear field and can form a shish-kebab structure due to the orientation and extended high molecular weight(M_W)population.We also compared the crystallization behavior of bimodal and unimodal HDPE with different MWDs and short branched chain distributions.The bimodal HDPE sample which has high M_W,broad MWD and short chain branch(SCB)at high Mw regions has higher degree of crystallinity and a lower lamellar orientation than the unimodal HDPE sample which has medium Mw,narrower MWD and SCB at high Mw regions.Therefore,under the low shearing rate,high M_W molecular chains are not easy to disentangle and have a low degree of orientation.MWD and branched chain structure have a great influence on the crystallization behavior of HDPE at different shear temperatures and shear rates.The bimodal HDPE has a low orientation due to the increase in the content of short-chain branches and the increase in high-end fractions.Under low shear rates,high M_W chains are not easy to disentangle and have a low degree of orientation.The bimodal HDPE with typical double peak characteristics has relatively higher crystallinity and lower crystalline orientation and lamellar orientation than the unimodal sample.The bimodal HDPE contained a bimodal M_W and composition distribution with the low M_W components having less short chain branches,and the high M_W components having more SCBs.The weight average Mw of the bimodal HDPE was higher than that of the unimodal HDPE.The unimodal HDPE sample contained a unimodal but broad molecular weight distribution with a constant SCB value between those of the low and high M_W components in bimodal HDPE.Discrete primary and secondary lamellae were found in both HDPE,whereas the unimodal one exhibited faster secondary lamellar growth.Shear-induced crystallization resulted in higher lamellar orientation in the unimodal HDPE,but higher crystallinity in the bimodal.The large fraction of medium to high M_W components in the unimodal HDPE was found to be effective in crystallization under deformation,leading to stronger shear-induced crystalline orientation.