Constrained and free uniaxial stretching induced crystallization of polyethylene film: A comparative study

Constrained and free uniaxial stretching induced crystallization of high density polyethylene (HDPE) film were studied with in situ synchrotron radiation small and wide-angle X-ray scattering (SR-SAXS, SR-WAXS). According to the initial structure after stretching, as well as the structural evolution, three characteristic regions can be defined in strain space for both stretching modes, while the strain boundaries between different regions are different for the two stretching modes. Region I is located at low strain levels where completely twisted lamellae are induced. Region II is in an intermediate strain level, which induces the formation of partially twisted lamellae with relatively large lateral size (defined as quasi-micro-fibrils). Region III with large strain produces flat lamellae with small lateral dimensions (micro-fibrils). During the crystallization process, a new type of lamellar stack with smaller long period forms in regions II and III while no new types of lamellae appear in region I for both stretching modes. Along the strain space, the scenario of constrained stretching delays the transition from region I to region II, as well from region II to region III. Also, the draw ratio windows of region I and region II are enlarged by constrained stretching, which is more favorable for film processing.