The degradation of polymers has a great influence on their characteristics. Two kinds of processes may be distinguished, physical and chemical. Both are strongly linked. The degradation may be for example induced by UV irradiation, thermal ageing, physico-chemical attacks (water, oxygen, etc.), and mechanical stress. To investigate the thermal ageing effect on the large deformation stress-strain behaviour of PE100, tensions tests were conducted on this material samples submitted to a temperature of 90°C during different hold times. These tests show that thermal ageing leads to an increase of the mechanical properties like the Young Modulus, the yield stress and the stress at break. We show in our work, by different investigations techniques like X-rays diffraction, Transform Infrared Spectroscopy (FTIR), that this increase of mechanical properties is not due to a chemical changes but is due to an increase of the rate of the crystallinity commonly designated by the post-crystallization phenomena.

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