Extensibility of the Inter-Lamellar Amorphous Layer and the Mechanical Behaviour of Polyethylene

Abstract

The mechanical behaviour of semicrystalline polymers is dependent upon the property of both the amorphous and crystalline phases and their eventual interactions. In this context, heat shrinkable films have been evidenced as an interesting material model to investigate the influence of the amorphous phase on the mechanical behaviour of semicrystalline polymers, as upon heating these materials only show changes on the extensibility of the amorphous phase (macromolecular conformational state). The effect of the amorphous phase on the mechanical behaviour can therefore be studied independently. In this work are investigated the mechanical properties of a heat shrinkable polyethylene films as processed and after annealing at 60 °C (for 15 min). The morphology of the film before and after the annealing treatment was characterised by 2D-SAXS patterns. The tensile tests were performed at 50 mm/min and room temperature (23 °C) in different directions respectively to the longitudinal film direction, LD (0, 30, 45, 60 and 90º). The results show that the conformational state of the amorphous phase affects the tensile modulus and the deformation capabilities of the films, namely in the transverse machine direction. No influence on the sustained stress level was observed. Furthermore, the essential work of fracture is determined at large extent by the conformational state of the amorphous phase. This study suggests the high importance of the amorphous network on the initial strain levels and on deformation capabilities of the lamellar structure loaded transversely to the crystalline phase orientation.