Phase Boundaries and Crystallization of Polyethylene in n-Pentane and n-Pentane + Carbon Dioxide Fluid Mixtures

The liquid-liquid and fluid-solid phase boundaries and the densities of 5 wt % solutions of polyethylene in n-pentane and in n-pentane (85 wt %) + carbon dioxide (15 wt %) fluid mixtures were determined over a temperature range from ~360 to 400 K at pressures up to 52 MPa. The measurements were conducted using a variable-volume view cell equipped with sensing elements to monitor the changes in the internal volume and in the transmitted light intensity as the pressure or the temperature of the system is changed. Polyethylene crystals were formed at selected temperatures below the fluid-solid phase boundary along a series of selected constant-pressure paths. They were analyzed by scanning electron microscopy (SEM) and by differential scanning calorimetry (DSC) at ambient conditions for differences in morphological features and thermal properties. Depending on the temperature, the pressure, and the crystallization time, crystallinity levels changed from 65 to 80 %. The crystals that are produced from these high-pressure solutions all showed multiple melting transitions. The majority of the DSC scans at 10 K/min heating rate show a small melting peak at ~395 K and two additional, more-distinct peaks in the temperature range from 399 to 403 K. The multiple transitions were, however, observed to collapse to a single melting peak at 404 K in the second heating scans. Microscopic evaluations show that the morphologies are prevailingly dominated by ellipsoid-shaped folded-lamellar units 10-20 (mu)m in longer dimension that agglomerate. Unique, long (50-100 (mu)m) strands of stacked lamellar structures appear to form at ~40 MPa.