From Molecular Entanglement Network to Crystal-Cross-Linked Network and Crystal Scaffold during Film Blowing of Polyethylene: An in Situ Synchrotron Radiation Small- and Wide-Angle X-ray Scattering Study

Combining a homemade film blowing machine and an in situ synchrotron radiation source with small- and wide-angle X-ray scattering (SAXS and WAXS) capability, an investigation of film blowing of polyethylene (PE) has been studied. From the die exit to the positions above the frost line, four zones defined with different structural features are observed with SAXS and WAXS measurements. In zone I, precursor and crystal structures emerge from the polymer entanglement network during cooling and extension, which lead to the formation of a deformable crystal-cross-linked network at the boundary between zones I and II. The occurrence of the crystal-cross-linked network enhances the effective chain stretching during further deformation in zone II. Crystallization is largely accelerated, which generates crystals with high orientation. Further increasing the crystallinity results in the deformable crystal-cross-linked network transforming into a nondeformable crystal scaffold at the frost line (the boundary between zones II and III), which stabilizes the bubble and prevents further deformation. In zones III and IV, the scaffold and the entire sample are gradually filled up by crystals, respectively. Interestingly, increasing the take-up ratio (TUR) does not influence the critical crystallinity (chi(I-II)) for the formation of the deformable crystal-cross-linked network, while the crystallinity (chi(f)) at the frost line or for the formation of nondeformable scaffold does vary with TUR. This suggests that the former (chi(I-II)) is mainly controlled by molecular parameters, while the latter (chi(f)) is determined by both processing and molecular parameters of PE material.