Molecular-Weight-Dependent Puncture Mechanics and Structural Evolution of High-Density Polyethylene Revealed by In Situ Synchrotron SAXS/WAXD and USAXS

The influence of molecular weight distribution on puncture mechanics and structural evolution in high-density polyethylene (HDPE) was investigated using a model system in which the number-average molecular weight (Mn) was kept approximately constant, while the fraction of high-molecular-weight chains was progressively increased, leading to a systematic increase in weight-average molecular weight (Mw) and broadening of the molecular weight distribution. A self-developed in situ synchrotron puncture device enabled simultaneous measurements of mechanical response and multiscale structural evolution via small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD) and Ultrasmall Angle X-ray Scattering (USAXS) techniques. The results show that cavity initiation, crystal transformation, and accelerated late-stage damage all occur at similar displacements, indicating that the key structural events during puncture are primarily displacement-triggered. Compared with samples containing a lower fraction of high-molecular-weight chains, samples with higher Mw and broader molecular weight distribution show lower peak puncture force but sustain substantially larger deformation and dissipate more energy prior to failure. In situ scattering results further show that the low-Mw samples undergo more intense cavitation, and a larger extent of orthorhombic-to-monoclinic transformation, indicating that their internal load-bearing structures destabilize more readily. By contrast, the high-Mw samples preserve a more stable oriented amorphous network because of stronger long-chain entanglement and tie-chain connectivity, which suppresses the growth of localized damage. Therefore, increasing the fraction of high-molecular-weight chains, accompanied by higher Mw and broader molecular weight distribution, improves load-path continuity and structural tolerance to damage. This effect mitigates local stress concentration, delays cavity growth and crack penetration, and transforms the failure mode from brittle local tearing to more ductile global rupture. These findings identify the essential role of molecular-weight architecture in governing puncture resistance in HDPE and provide a mechanistic basis for designing polyethylene materials with improved puncture performance.