Behavior of polyethylene in a variable temperature density gradient column

Polymer morphology (phase size and phase density) of slow cooled and quenched polyethylene (HDPE, LDPE, and LLDPE) has been characterized over a range of temperatures. The characterization methodology includes variable-temperature density gradient column (VT-DGC), small-angle x-ray scattering (SAXS), wide-angle x-ray diffraction (WAXD) and differential scanning calorimetry (DSC). Using a novel technique, a VT-DGC was prepared and cycled over a range of service temperatures (20-60 °C) for 5 cycles to investigate the changes of slow cooled and quenched HDPE, LDPE and LLDPE. A significant change in bulk density was present in each sample between the first cycle and subsequent cycles. Morphological analysis was performed using both the two-phase and three-phase models. The two-phase model showed that, for a particular sample, the thickness of the crystalline and amorphous phases varied very little within the experimental temperature range. Using the three-phase model, differences in the interfacial layer thickness were measured and observed to be significant compared to the amorphous and crystalline phase changes. The amorphous and crystalline densities of all samples varied less than 2%. Overall, significant difference in crystalline density was observed between HDPE, LDPE and LLDPE due to molecular structure.