Real-Time Investigation of the Structural Evolution of Polyethylene during Tenter-Frame Biaxial Stretching

The online structure development during tenter-frame biaxial stretching of linear low-density (LLDPE), high-density polyethylene (HDPE), and their 50/50 blend was investigated using synchrotron Wide-Angle X-ray Scattering (WAXS). During stretching, crystallite reorientation, crystallographic slip phenomena activated at yielding, and lamellar fragmentation at higher strains are responsible for a decrease in crystallinity. These processes induce a morphological transformation from the initial semisolid spherulitic morphology into an oriented fibrillar network with lamellae in edge-on orientation, and the b- and c-axes constrained within the film plane, as confirmed by ex-situ WAXS and SAXS. These phenomena are more important for HDPE, which exhibits higher susceptibility to biaxial stretching. Upon cooling, the morphology developed during stretching is preserved, as newly crystallized material grows epitaxially on the preexisting structure. These findings highlight the higher structural sensitivity of HDPE during semisolid biaxial stretching compared with LLDPE, providing unique information to understand and control the biaxial processing of polyethylene.