Structural and morphological changes during UV irradiation of the trans-planar form of syndiotactic polypropylene

Abstract

Syndiotactic polypropylene films, obtained in the trans-planar mesophase, were exposed to accelerated weathering in a UV device at 45 °C for increasing times. A different series of films, characterized by the same structural organization, was annealed at 45 °C for the same times, 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 studied. Diffractometric and thermal analyses show that UV radiation causes a strong increase of helical crystallinity (189% with respect to the initial value), leaving the trans-planar mesophase unchanged. The mesophase domains have a higher rigidity and hinder the oxygen diffusion slowing down the photooxidative degradation. The chain scission in the amorphous phase produces a higher mobility, allowing crystallization. The crystallinity also increases in the thermally treated samples, but the increase is limited to the first 48 h and reaches about 89% of the initial value. Both in the thermally treated and UV irradiated samples the crystallite coherence length perpendicular to the 200 planes increases with time. The amorphous chains do crystallize at the beginning near the already formed crystals, thus increasing their dimensions. The carbonyl index shows an induction period of 48 h, before a rapid increase which apparently is inversely proportional to the percentage of mesomorphic and crystalline phases of the irradiated samples, indicating that the photooxidative reactions took place mainly in the amorphous region. A comparison with data already published on the photooxidative behavior of samples crystallized in form I shows that the presence of compact domains in trans-planar mesophase led to an increase in the induction period and to a greater stability of the material in the first 150 h of UV treatment.