Probing Flow-Induced Precursor Structures in Blown Polyethylene Films by Synchrotron X-rays during Constrained Melting

Structural changes during constrained melting of blown polyethylene (PE) films were followed by in-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. Results indicated that thermally stable flow-induced crystallization precursor structures (i.e., shish-kebabs) can be revealed above the nominal melting point determined by differential scanning calorimetry (DSC). Two samples were investigated: (1) linear low-density polyethylene (LLDPE) with M̄ w of 116 kg/mol and (2) a binary blend of 95 wt % LLDPE and 5 wt % high-density polyethylene (HDPE). HDPE had a bimodal molecular weight distribution, containing 80 wt % of low molecular weight (LMW) component ( M̄ w = 99 kg/mol) and high molecular weight (HMW) component ( M̄ w = 1100 kg/mol). Even though the blend contained only a low concentration of HMW−HDPE chains ( c = 1 wt %), above its overlap concentration ( c * = 0.5 wt %), the initial film showed significantly higher lamellar orientation, and the high-temperature film showed a much more intense shish-kebab scaffold than LLDPE. The Ruland method for meridional streak analysis was used to analyze the kebab diameter and orientation during melting; the Vonk procedure for single lamella scattering was used to analyze the corresponding kebab thickness change. The temperature dependence of kebab thickness for both LLDPE and HDPE/LLDPE blend was used to determine the equilibrium melting temperature for PE ( T m ° = 142.6 °C), which showed a very reasonable agreement with the literature value.