Structural and morphological changes during UV irradiation of the crystalline helical form of syndiotactic polypropylene

Syndiotactic polypropylene films, in the most usual crystallographic modification (form I), were exposed to accelerated weathering in a UV device at 46°C for increasing times. A different series of films characterized by the same structural organization were annealed at 45°C to discern the thermal effects from those due to UV irradiation. The influence of UV irradiation and thermal treatments on the structural changes in terms of phase composition, crystallite sizes, and chain conformations was investigated. The analyzed samples were crystallized from the melt at 25°C, obtaining the disordered crystalline form I, with chains in helical conformations, in large amounts, and a small fraction of mesophase with chains in trans-planar conformations. Diffractometric and thermal analysis have shown that UV radiation causes a strong increase in the crystallinity degree within the first 170 h of treatment. Such an increase mainly occurs by an increase in the coherent crystalline domain size in the direction perpendicular to the (200) planes. Spectroscopic data (FTIR) confirm that the increase in the crystallinity degree occurs not only by a strong reduction of the amorphous phase and a consistent increase of the conformational order of the chains in helical conformation but also by a reduction of the domains with chains in the trans-planar conformation. The trans-planar domains, very small and arranged in the amorphous matrix, are attached by UV radiation together with the amorphous phase. The observed phenomena are to be attributed to the effects of chain scission due to the photooxidation. Samples only annealed at 45°C show only small effects of physical aging within the first 10 h of treatment.