Actuation mechanism of drawn polyethylene evaluated by structural change during cyclic stretching/shrinking

Abstract

Structural change during cyclic stretching/shrinking of drawn film of linear low-density polyethylene (LLDPE) was analyzed using a combination of in-situ X-ray and NMR measurements. Small-angle X-ray scattering (SAXS) for the original drawn LLDPE film before stretching exhibits a split-layer-line pattern, indicating lamellar tilt. Stretching the film transforms the pattern into the straight-layer-line one, and subsequent shrinking regains the original split-layer-line one. Solid-state 1H-NMR measurements detected three components having different relaxation times for both originally shrunk and subsequently stretched states. Among these components, the intermediate one exhibits reversible change in relaxation time, synchronizing the stretching/shrinking cycles. This component is attributed to tie molecules, bridging the rigid components of crystalline lamellae and accompanying a small amount of the mobile component consisting of unrestricted amorphous chains. Stretching and shrinking such tie molecules induces reversible transformation of the microscopic lamellar arrangements, which causes macroscopic actuation of drawn LLDPE.