Chemical and morphologial modifications of irradiated linear low density polyethylene (LLDPE)

Films of linear low density polyethylene (LLDPE) produced with the spherilene technology were exposed to accelerated weathering through UV irradiation at 60 degreesC for increasing times. A different series of films were only annealed at 60 degreesC, to differentiate the thermal effects from those due to the UV irradiation. FT-IR analysis was used to investigate the change of the chemical structure. It was found that, following the initial production of hydroperoxides, the degradation is dominated by the formation of carbonyl and vinyl species. This process proceeds slowly up to 150 h of irradiation and afterwards it accelerates, completely degrading the material. The samples exposed more than 150 h are mechanically very fragile and soon fail. Structural analysis of the irradiated samples showed that there is a perfection of the elementary cell of polyethylene, principally along the a axis, and a consistent increase of crystallinity. The first effect was found also in thermally treated samples, whereas the second is more consistent in the irradiated samples. Also the dimension of the crystals increase much more for the irradiated samples than for the annealed ones. All the effects are mostly evident for samples after 150 h of irradiation, that is when the degradation is accelerated. The chain scission due to the photo-oxidation makes the amorphous chains more mobile and free for further crystallization, and this happens mainly when the degradation occurs simultaneously in many chains. (C) 2001 Elsevier Science Ltd. All rights reserved.