Control of nano-cavitation in semi-crystalline polymer nanocomposites during uniaxial tension: In situ synchrotron X-ray study

The cavitation behavior of polypropylene-silica (PP-silica) and high density polyethylene-silica (HDPE-silica) composites with varying silica content during uniaxial tension was investigated via in situ X-ray techniques. The introduction of silica was found to significantly influence the mechanical properties of the materials. Small -angle X-ray scattering (SAXS) was employed to determine the cavitation onset strain and to observe the evolution of cavity structures. Through model fitting, the size evolution of nanoscale cavities at the onset of cavitation was quantitatively characterized. Scanning electron microscopy (SEM) was utilized to examine the morphology before and after stretching, revealing that cavity structures primarily resulted from internal defects in the polymer matrix and interfacial debonding between silica and the polymer. The long axis of the nanoscale cavities at the onset of cavitation was perpendicular to the tensile direction, and further deformation led to the coalescence into micron-sized cavities with the long axis parallel to the tensile direction. Wide -angle X-ray scattering (WAXS) was used to characterize the microscale deformation process of crystals, showing that during stretching, the lamellae reoriented with the angle between their normal direction and the tensile direction gradually decreasing. Several sets of transition strains representing the plastic deformation were used to compare the microscale and macroscale deformation, elucidating the sequence and differentiation between crystal plastic deformation, cavitation, and yield in the two polymer materials. Intriguingly, both PP-silica and HDPE-silica composites exhibited optimal silica additions and initial structural parameters for regulating cavitation behavior. These findings offer significant guidance for the control of internal structures in porous separators in industrial production.