An Investigation on Slow Crack Growth Resistance of Polyethylene Based on Strain Hardening Test

ABSTRACT Slow crack growth (SCG) is the dominant failure mechanism in polyethylene (PE) pipes during long‐term service. SCG leads to premature fractures without any visible warnings. An in‐depth understanding of the relationship between microstructure and SCG resistance is crucial for the design and development of PE resins. In this work, the strain hardening modulus (SHM) is used to rank the SCG resistance of five PE resins. The effects of molecular and morphological properties on SHM are systematically investigated. The results show that an increase in molecular weight (MW), polydispersity index (PDI), and short chain branch (SCB) content is generally favorable for improving the SHM. However, a single molecular parameter cannot solely determine SCG resistance due to differences in molecular weight distribution (MWD) and short chain branch distribution (SCBD). Moreover, longer SCBs favor the enhancement of SHM. For unimodal resins, the SHM increases with decreasing crystallinity and lamella thickness because lower crystallinity or lamella thickness corresponds to a greater number of tie molecules (TMs). In contrast, the tie‐molecule density of bimodal resins is less affected by SCB content due to their inverse SCBDs. Therefore, an increase in crystallinity or lamella thickness strengthens the bonds between the TMs and the crystals when comparing resins with similar tie‐molecule densities, leading to a higher SHM. Three positive effects of long chains with SCBs on enhancing SHM are proposed, integrating the TM density, MWD, and SCBD spectra. Long chains with SCBs are found to contribute the most to SHM and SCG resistance.