Retrospecting the relationship between interlamellar amorphous structure and intrinsic modulus of semicrystalline polymers via in-situ positron annihilation lifetime spectroscopy and in-situ X-ray scattering

The structure-property relationship of the interlamellar amorphous phase in semicrystalline polymers has been a fundamental yet controversial topic in polymer physics for several decades. The intricate hierarchical structure and limited availability of experimental techniques pose significant challenges in quantitatively characterizing the evolution of the amorphous structure and establishing its connection to the mechanical behavior. In this work, the unannealed and annealed isotactic polypropylene hard-elastic films composed of idealized series-arranged lamellar stacks are studied to simplify the mechanical coupling between the two phases. By utilizing the synchrotron-based in-situ small and wide-angle X-ray scattering (SAXS/WAXS) techniques, the apparent modulus and overall volumetric strain of the amorphous phase are estimated. Moreover, with our significant progress in the counting rate of positron annihilation lifetime spectroscopy (PALS), the minute-scale structure evolution of amorphous phase (free-volume pores) during the tensile process is now feasible to be captured. Combing in-situ X-ray scattering and in-situ PALS results, in the elastic regime, it is found that triaxial stress in the amorphous phase leads to the expansion of free-volume pores (about 23% volume increase for the annealed sample, 35% volumetric strain for the unannealed one) and density decrement within occupied volume by overcoming Van Der Waals force. Our quantitative analysis also reveals that the annealing process not only enhances the crystallinity and aspect ratio of lamellae but also weakens the intrinsic modulus and chain packing density of amorphous phase.