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

Films of linear low density polyethylene (LLDPE) produced with the spherile ne 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 d ue to the UV irradiation. FT-IR analysis was used to investigate the change of the chemical structure. It was found that, following the initial produc tion of hydroperoxides, the degradation is dominated by the formation of ca rbonyl and vinyl species. This process proceeds slowly up to 150 h of irrad iation and afterwards it accelerates, completely degrading the material. Th e samples exposed more than 150 h are mechanically very fragile and soon fa il. Structural analysis of the irradiated samples showed that there is a pe rfection of the elementary cell of polyethylene, principally along the a ax is, 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 m ore 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 man y chains. (C) 2001 Elsevier Science Ltd. All rights reserved.