Evolution of Free-Volume Pores during Stretch-Induced Instability of the Interlamellar Amorphous Phase in Semicrystalline Polymers

Above the glass transition temperature, the stretch-induced instability of the amorphous phase in semicrystalline polymers is usually accompanied by cavitation. However, the early-stage mechanism of cavitation remains unclear and remains to be debated, owing to the hierarchical structure and the limitations of time-resolved experimental methods in probing the pores, particularly at the subnanoscale. In this study, we chose hard-elastic polypropylene (HEPP) as a model system to simplify the structural hierarchy of semicrystalline polymers and enable a focused investigation on the lamellar stack scale. Benefiting from our newly developed positron annihilation lifetime spectroscopy (PALS) apparatus with a super-high counting rate, combined with a specifically designed tensile machine, we successfully tracked the real-time evolution of subnanoscale free-volume pores in HEPP during continuous tensile loading. During macroscopic yield, we directly observed the growth (increase in average size), coalescence (reduction in number density), and redistribution (transition from unimodal to bimodal size distributions) of the free-volume pores. By integrating in situ PALS data with in situ small-angle X-ray scattering (SAXS) and volumetric strain measurements, we find that our stress-induced microphase separation is a more suitable explanation for the deformation instability of amorphous phase and volume increase in HEPP compared to other traditional models. In this nonequilibrium process, the voids on the scale of tens of nanometers originate from pre-existing free-volume pores in the amorphous phase and represent the ultimate state of the free-volume-rich microphases within unstable lamellar clusters.