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• iPP film crystallized via the mesophase exhibits improved photo-oxidation stability in comparison with melt-crystallized iPP film. • iPP with nodular morphology has higher transparency and lower O 2 diffusivity than the iPP with lamellar morphology. • The relationship between enhanced photo-oxidation stability and nodular morphology of iPP has been established. Photo-oxidation degradation behaviors of isotactic polypropylene (iPP) films with different crystalline morphology were investigated by artificially accelerated aging test. The α form iPP with nodular morphology was obtained by quenching the melt and subsequent annealing process, and its photo-oxidation degradation behavior was compared with that of the melt-crystallized α form iPP sample. Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), X-ray scattering, dynamic mechanical analysis (DMA) and atomic force microscopy (AFM) were utilized to characterize the microstructural variations of these two kinds of iPP films during photo-oxidation degradation process. The results showed that iPP film of nodular morphology exhibits improved photo-oxidation stability compared with that of lamellar and spherulitic structure. UV-Vis spectra and O 2 diffusivity test of unexposed and aged samples demonstrated that the iPP film of nodular morphology has higher transparency and lower O 2 diffusivity than that of lamellar morphology. Combining the microstructural analysis and surface morphology observation, the improved photo-oxidation stability of "nodular" iPP was correlated with its higher UV light transparency, lower O 2 diffusion coefficient as well as the slower segmental mobility in the amorphous phase. [ABSTRACT FROM AUTHOR]

• SAXS and AFM techniques were employed to investigate the evolution of lamellar structure and exposed surface morphology of photo-oxidized iPP. • Chemi-crystallization exists in both unstabilized and stabilized iPP, but its extent is lower in the latter. • In addition to chemi-crystallization, the loss of amorphous region induced by "UV light etching" is another probable factor contributing to the increase of crystallinity at the early degradation stage of iPP. • A model was proposed to describe the microstructural evolution of iPP film during photo-oxidation degradation. Photo-oxidation degradation behavior of isotactic polypropylene (iPP) has been extensively investigated, however, there are still some ambiguous points, e.g., the origin of improved crystallinity at the early stage of photo-oxidation, the evolution of condensed state structure during the process of UV light irradiation, etc. In the present work, iPP film samples, unstabilized and stabilized with a specific reactive hindered amine light stabilizer (RHALS), were exposed to UV irradiation for accelerated aging tests. The microstructural evolution of the samples was systematically investigated by combination of Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). The results show that chemi-crystallization exists both in unstabilized and stabilized iPP, but the chemi-crystallization extent is much lower in the stabilized iPP. In addition to chemi-crystallization, the loss of amorphous region induced by "UV light etching" is another probable factor contributing to the increase of crystallinity at the early stage of photo-oxidation. AFM results provide direct evidence that with the progress of photo-oxidation, the degradation gradually propagates from amorphous region to crystalline region, thus inducing the destruction of lamellar structure. On the basis of above results, a comprehensive model is summarized for the structure evolution of iPP film during photo-oxidation and will be of meaningful reference to the degradation investigation of other semi-crystalline polymeric materials. [ABSTRACT FROM AUTHOR]