Effect of molecular weight distribution on rheological, crystallization and mechanical properties of polyethylene-100 pipe resins

摘要

In this article, a series of commercial bimodal polyethylene-100 (PE100) resins were synthesized to clarify the effect of molecular weight distribution (MWD) on rheological, crystallization and mechanical properties of bimodal PE. The GPC results show that the three GC100S samples with similar MWD merely differ in the ratio of low molecular weight (MW) fraction which gradually ascends from 50 % to 58 %. Rheological measurements were performed to show that the low MW fraction which has the function of dilution and lubrication can obviously decline the melt elasticity and increase the onset shear rate of shark-skin failure, which dramatically improve the processability of GC100S. Furthermore, isothermal crystallization and stepwise segregation crystallization were studied by differential scanning calorimetry (DSC). It appears that the introduction of homopolymerized low molecular fraction could remarkably promote the crystallization capacity and form thick lamellae, which eventually improve the rapid crack propagation (RCP) resistance of PE100 pipe. On the other hand, uniaxial tensile test was used to evaluate slow crack growth (SCG) resistance of PE100 pipes. It shows that the concentration of intercrystalline tie molecules declines with the increasing ratio of low MW fraction, which slightly deteriorates the SCG resistance.