Structure evolution of α- and β-polypropylenes upon UV irradiation: A multiscale comparison

The effects of UV irradiation on neat and β-nucleated isotactic polypropylenes have been studied at the molecular, morphological and microscopic levels. Commercially available isotactic polypropylene (PP) was modified by a specific β-nucleating agent based on N,N′-dicyclohexylnaphthalene-2,6-dicarboxamide. Compression-moulded plates were prepared from both starting (α-PP) and nucleated (β-PP) polypropylenes and exposed to UV irradiation. The exposure time varied from 0 to 240 h. Molecular degradation and the evolution of supermolecular structure were determined using infrared spectroscopy, wide-angle X-ray scattering and differential scanning calorimetry; surface cracking was observed by light microscopy. Lower degradability was found in β-PP as compared to α-PP; an increase in crystallinity on short-time exposure followed by crystallinity decrease with prolonged exposure was detected in both samples. The β-phase content in the crystalline portion of β-PP remained stable during UV irradiation under given irradiation conditions. Re-melting experiments have shown that the crystallization ability of both α-PP and β-PP markedly decreased with exposure time. Lower degradability of β-PP as compared to α-PP has been ascribed to higher light absorbance resulting from the specific morphology of β-PP.