Probing into the microstructural evolution of isotactic polypropylene during photo-oxidation degradation

Abstract

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.