Characterization of polyethylene films drawn by machine direction orientation and consequences on their photooxidation

The effect of uniaxial drawing by Machine Direction Orientation (MDO) on polyethylene films was first studied by complementary analytical methods. XRD and polarized FTIR spectroscopy revealed the transformation of the initial spherulitic structure of PE into microfibrils composed of small crystalline blocks connected by amorphous chains oriented along the drawing direction. No stress-induced crystallization was observed upon MDO processing, but the permeability to oxygen was reduced by 30 % owing to this morphology change. In a second time, the effect of photooxidation on undrawn and MDO-drawn films was studied. There was no difference in the photooxidation kinetics and stoichiometry between them. A slight reorganization of the polymer structure due to physical ageing was observed before any photooxidation, but no change in orientation occurred upon photochemical ageing. Finally, the oxygen permeability of undrawn and MDO-drawn films decreased owing to the changes brought by photooxidation, especially the formation of polar functions within the initially apolar polyethylene and the occurrence of chemicrystallization.